The car is awd. What kind of difference does a higher psi compared to a lower psi make on snow? ice? concrete?
Winter tire psi reccomendations?
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Within reason, lower psi = better traction on snow and ice.
Contrary to what many would tell you, using 3 to 5 psi more than what is listed on the vehicle placard has benefits - assuming you are using the same size as listed on the placard.
If you are using a different size than what is listed on the placard, then the pressure has to be recalculated. The calculation is not difficult but it requires that use of tire load tables - and finding those is difficult. But I have them and can do the calculation.
Now, back to the discussion on pressure.
Wny more psi?
Because pavement has much, much, much more traction than snow or ice. If you can penetrate through the snow to reach the pavement, you will get better traction. To do than you need a smaller footprint - and that means higher pressure.
Since normally you aren't riding on unpenetrable snow or on an ice rink, more pressure is the better recommendation.
If you are using a different size than what is listed on the placard, then the pressure has to be recalculated. The calculation is not difficult but it requires that use of tire load tables - and finding those is difficult. But I have them and can do the calculation.
Now, back to the discussion on pressure.
Wny more psi?
Because pavement has much, much, much more traction than snow or ice. If you can penetrate through the snow to reach the pavement, you will get better traction. To do than you need a smaller footprint - and that means higher pressure.
Since normally you aren't riding on unpenetrable snow or on an ice rink, more pressure is the better recommendation.
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Also, lower ambient temperature means lower pressure. In case of winter tires, which are softer than summer tires, slightly increased pressure improves sidewall rigidity. In driving school I was taught to run 0.2 to 0.3 bar higher pressure in the winter (presuming the same tire size).
Lowering tire pressure would be a bad idea. The tire will be less stable due to reduced sidewall stiffness. In order to increase the contact patch one would have to reduce tire pressure considerably. Driving on snow and ice is not like crawling off-road over gravel and sand!
Lowering tire pressure would be a bad idea. The tire will be less stable due to reduced sidewall stiffness. In order to increase the contact patch one would have to reduce tire pressure considerably. Driving on snow and ice is not like crawling off-road over gravel and sand!
So you saying that in order to get to the pavement thru snow easier you should have more concentrated presure point in your tires, which could be achieved by putting more pressure in. But what about times when you drive on packed snow during the snofall where chances to get a contact with the pavement are minimal? What should your pressure be than?
Also can you look up my long question about Nokian tires in the "Nokians vs, Blizzaks vs. Winter Sport M3" topic below. See if you can give me yout insight on the situation, from logical point of view. Thank You.
I usually leave the pressure the same in winter as in summer. I want enough tread in contact with the road so the studs on the edge can do their thing. I also go with narrower tires in the winter whenever I can.
If/when the local roads get rough from the buildup of packed snow, I drop the pressure by a few psi to soften it a bit. It's usually very cold by that time, so the shocks are pretty stiff.
For really deep snow, I'd think that wider tires and/or lower pressures would be better.
If/when the local roads get rough from the buildup of packed snow, I drop the pressure by a few psi to soften it a bit. It's usually very cold by that time, so the shocks are pretty stiff.
For really deep snow, I'd think that wider tires and/or lower pressures would be better.
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For passenger cars and other vehicles that are driven on snowy roads, slush, dry, wet etc during the winter, narrower tires are of advantage.
If you drive on deep snow like picturted below, then lower pressure and big fat tires are best:
Yes, that is exactly what I am saying.
This is exactly where the disargeement on this subject is. If the measure tire traction on snow or ice, lower pressure is better.
But clearly, if there is any possibility that the tire might get down to the pavement, you want to encourage that. Traction on pavement is order of magnitudes greater than it is on snow or ice.
I usually post if I have something significant to add. I don't remember that I could add anything to the discussion, but I'll look again.
How often do you drive through deep snow, slush, and ice in Charlotte?
I drive through it often; lower air pressure, or at least what the door placard says, is the way to go.
My G0d, 5 psi more, in winter, to gain traction? That's really not even funny but dangerous. Hitting a shaded corner on a road going 50 mph where there is snow/frost/ice buildup...
On my mom's car, last year when there was FEET of snow on our street, dropping the air pressure from 30 psi to 25 psi just so she could get around, meant the difference between being stranded and actually moving. SERIOUSLY. Of course, I bumped it back up to about 29 psi. 35 psi would have made the car sit and spin. I've even helped neighbors get unstuck on my hill with the good ol tire pressure gauge...
I drive through it often; lower air pressure, or at least what the door placard says, is the way to go.
My G0d, 5 psi more, in winter, to gain traction? That's really not even funny but dangerous. Hitting a shaded corner on a road going 50 mph where there is snow/frost/ice buildup...
On my mom's car, last year when there was FEET of snow on our street, dropping the air pressure from 30 psi to 25 psi just so she could get around, meant the difference between being stranded and actually moving. SERIOUSLY. Of course, I bumped it back up to about 29 psi. 35 psi would have made the car sit and spin. I've even helped neighbors get unstuck on my hill with the good ol tire pressure gauge...
Quote:
......How often do you drive through deep snow, slush, and ice in Charlotte?.......
Well, for one, I wasn't born in Charlotte. I spent most of my life in the midwest. In that part of the world, it is not likely that the roads are snow covered.
In your case - Colorado - the situation is obviously different.
Plus, I've flavored my recommendations based on the testing I have seen.
So the question becomes - What should we communicate to the average joe, especially considering we don't know where he lives.
To my mind, more people experince the slush / light snow than the deep powder. So I'd rather they used more pressure - and then if they get stuck, lower the pressure to get unstuck.
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I have seen several recommendations from tire and vehicle venders; these suggest a several lb increase. Perhaps due to the higher weight of the tire, extra heat produced at speed, potential better traction on ice and snow. They don't say why but these could be the reason. I always add a few lb if the temperature trend is downward.
Richard
Richard
Originally Posted By: CapriRacer
Wny more psi?
Because pavement has much, much, much more traction than snow or ice. If you can penetrate through the snow to reach the pavement, you will get better traction. To do than you need a smaller footprint - and that means higher pressure.
Since normally you aren't riding on unpenetrable snow or on an ice rink, more pressure is the better recommendation.
Sorry but this is wrong. There is indeed a benefit to a skinnier tire creating better winter on road traction, which it does, but increasing psi will not achieve this. I'll explain why.
Example, two identical tires, Bridgestone W965 10 ply studless winter tires.
I'm running 235/85-16's, 40/50psi fr/rr. I also tried the same tire in a 265/75-16 10 ply a few years back, just to see the difference. Same psi, same truck, I had less traction everywhere. Not dangerous, but less since it had a wider print and less contact pressure for the surface area.
Would I have gained traction cranking up the psi to narrow the tire so it would match the 235/85-16 profile? Nope, it would make it worse. The tire would be even less flexible, providing less bite and friction. Even though we think raising the outer edges will narrow the tread and provide more ground pressure, you're still using the full width of the tire through bumps, road imperfections, cornering etc, but now we've just reduced the tire's ability to flex/conform and create friction. Just because it's skinnier now doesn't mean it's making more traction. A skinnier tire with proper psi makes more traction.
If you have outer edges of your tread blocks not making contact, your psi is too high. Drop until you achieve a full tread print, this will give you the best flexablilty for your tire to create friction/traction. If you have to drop psi to the point handling is compromised, you're either running too heavy a tire (too many plies) or it's just too wide.
I started out with my truck at 55/55 fr/rr, and slowly dropped to the point where I got the best traction without compromising handling or the stuff I haul around being a work truck.
The only reason manufacturers recommend increasing psi is the fact that air becomes more dense in the winter. 35psi in the tire shop at 20c may only be 28psi at -20c. That's when people complain of poor milage, handling feels wierd and may be compromised if a quick lane change is needed when running dry pavement in a city that doesn't get much snow if any, but still has sub-zero temps occasionally.
The condition described initially of biting though the snow to the pavement, this is achieved just from proper psi, not over-inflation. And proper I mean the measured footprint the tread makes, not what is on the door tag.
My two cents, cheers!!
Wny more psi?
Because pavement has much, much, much more traction than snow or ice. If you can penetrate through the snow to reach the pavement, you will get better traction. To do than you need a smaller footprint - and that means higher pressure.
Since normally you aren't riding on unpenetrable snow or on an ice rink, more pressure is the better recommendation.
Sorry but this is wrong. There is indeed a benefit to a skinnier tire creating better winter on road traction, which it does, but increasing psi will not achieve this. I'll explain why.
Example, two identical tires, Bridgestone W965 10 ply studless winter tires.
I'm running 235/85-16's, 40/50psi fr/rr. I also tried the same tire in a 265/75-16 10 ply a few years back, just to see the difference. Same psi, same truck, I had less traction everywhere. Not dangerous, but less since it had a wider print and less contact pressure for the surface area.
Would I have gained traction cranking up the psi to narrow the tire so it would match the 235/85-16 profile? Nope, it would make it worse. The tire would be even less flexible, providing less bite and friction. Even though we think raising the outer edges will narrow the tread and provide more ground pressure, you're still using the full width of the tire through bumps, road imperfections, cornering etc, but now we've just reduced the tire's ability to flex/conform and create friction. Just because it's skinnier now doesn't mean it's making more traction. A skinnier tire with proper psi makes more traction.
If you have outer edges of your tread blocks not making contact, your psi is too high. Drop until you achieve a full tread print, this will give you the best flexablilty for your tire to create friction/traction. If you have to drop psi to the point handling is compromised, you're either running too heavy a tire (too many plies) or it's just too wide.
I started out with my truck at 55/55 fr/rr, and slowly dropped to the point where I got the best traction without compromising handling or the stuff I haul around being a work truck.
The only reason manufacturers recommend increasing psi is the fact that air becomes more dense in the winter. 35psi in the tire shop at 20c may only be 28psi at -20c. That's when people complain of poor milage, handling feels wierd and may be compromised if a quick lane change is needed when running dry pavement in a city that doesn't get much snow if any, but still has sub-zero temps occasionally.
The condition described initially of biting though the snow to the pavement, this is achieved just from proper psi, not over-inflation. And proper I mean the measured footprint the tread makes, not what is on the door tag.
My two cents, cheers!!
good post, going to invest in some skinnier snow tires the next time around
If nothing else, now is a good time to be checking your tire pressures, as the cooler temps are dropping those pressures.
Crazy Mike:
You're not so crazy after all. Good post.
But I think you may have misunderstood.
Originally Posted By: CrazyMike
....Sorry but this is wrong. There is indeed a benefit to a skinnier tire creating better winter on road traction, which it does, but increasing psi will not achieve this......
The increased pressure is not to narrow up the footprint - that isn't the way it works except at high pressure / low load situations.
The purpose of the increased pressure is to increase the pressure within the footprint.
Besides, the pressure recommendation is based on the idea that you are using the same size tire - not comparing different sizes.
Originally Posted By: CrazyMike
.......I'm running 235/85-16's, 40/50psi fr/rr. I also tried the same tire in a 265/75-16 10 ply a few years back, just to see the difference. Same psi, same truck, I had less traction everywhere.......
Yes, you have a wider footprint, but the load on the tire is the same, so the percent of it's rated load would be less. This complicates matters a great deal
First, you were using the same rims? The LT235/85R16 should be using a 6 1/2" rim and the LT265/75R16 should be using a 7 1/2" rim, so to get the equivalent situation you need to be using different rims.
Second, to get the equivalent loading situation, you needed to use less pressure - 34 / 43.
And this doesn't take into account that you didn't do back to back tests.
Tire traction testing is very difficult because of the variability. It takes many runs with the traction vehicle to get reliable data. And while I have no doubt about your experience, I have seen quite different results from highly controlled tests - enough to make me know that doing tire traction tests as little as a few hours apart is fruitless.
But I get the sense that you believe the inflation pressure equals the footprint pressure. Not true. Even though this seems intuitively true, it isn't.
And I should point out one more thing:
Pickup trucks have a unique problem when it comes to tire loading. The pressure needed to support the load when the truck is fully loaded - this is the placard value - is quite a bit higher than the equivalent situation when the bed is empty.
Cars have this problem, but 1) they are rarely used fully loaded, and 2) The problem is much less severe than with a pickup.
This adds even further complication to the situation.
Bottom line - I think the pressure increase is a valid recommendation, but it gets a little dicey when we start talking unloaded pickup trucks.
Footnote: Yeah!!!! An edit button!!!!
You're not so crazy after all. Good post.
But I think you may have misunderstood.
Originally Posted By: CrazyMike
....Sorry but this is wrong. There is indeed a benefit to a skinnier tire creating better winter on road traction, which it does, but increasing psi will not achieve this......
The increased pressure is not to narrow up the footprint - that isn't the way it works except at high pressure / low load situations.
The purpose of the increased pressure is to increase the pressure within the footprint.
Besides, the pressure recommendation is based on the idea that you are using the same size tire - not comparing different sizes.
Originally Posted By: CrazyMike
.......I'm running 235/85-16's, 40/50psi fr/rr. I also tried the same tire in a 265/75-16 10 ply a few years back, just to see the difference. Same psi, same truck, I had less traction everywhere.......
Yes, you have a wider footprint, but the load on the tire is the same, so the percent of it's rated load would be less. This complicates matters a great deal
First, you were using the same rims? The LT235/85R16 should be using a 6 1/2" rim and the LT265/75R16 should be using a 7 1/2" rim, so to get the equivalent situation you need to be using different rims.
Second, to get the equivalent loading situation, you needed to use less pressure - 34 / 43.
And this doesn't take into account that you didn't do back to back tests.
Tire traction testing is very difficult because of the variability. It takes many runs with the traction vehicle to get reliable data. And while I have no doubt about your experience, I have seen quite different results from highly controlled tests - enough to make me know that doing tire traction tests as little as a few hours apart is fruitless.
But I get the sense that you believe the inflation pressure equals the footprint pressure. Not true. Even though this seems intuitively true, it isn't.
And I should point out one more thing:
Pickup trucks have a unique problem when it comes to tire loading. The pressure needed to support the load when the truck is fully loaded - this is the placard value - is quite a bit higher than the equivalent situation when the bed is empty.
Cars have this problem, but 1) they are rarely used fully loaded, and 2) The problem is much less severe than with a pickup.
This adds even further complication to the situation.
Bottom line - I think the pressure increase is a valid recommendation, but it gets a little dicey when we start talking unloaded pickup trucks.
Footnote: Yeah!!!! An edit button!!!!
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Again, good post, and yes I dreamed of an edit button too, my typing is lame!!
On with the discussion.....
Quote:
The increased pressure is not to narrow up the footprint - that isn't the way it works except at high pressure / low load situations.
The purpose of the increased pressure is to increase the pressure within the footprint
This is a bit of a contradiction. Increasing psi to only increase pressure within the footprint will not occur, you need to reduce the surface area of contact, or increase weight. Tire pressure alone will not do this. Let's say we have a perfectly balanced car, a 3,000lb car with 200 sq inches of tire/road contace area at 35psi, this puts you with 750lbs of load per 50 sq inches of contact. With no reduction of surface area, running 40psi is still 3,000lbs, 750lbs per 50 sq inches. All we've done is reduce the tire's ability to conform to the terrain.
If we reduce surface area, as in the edges of the tires are raised, we've increased lbs per sq inch, increasing contact pressure, but now with a tire providing less friction/traction since it is now less pliable. A tire running lower psi will be much more pliable and provide better traction.
Quote:
Besides, the pressure recommendation is based on the idea that you are using the same size tire - not comparing different sizes.
Not necessarrily in all cases. Ford uses a standardized psi recommendation, as do other manufacturers I'm sure, so I do see it quite viable including different sizes :
Windstar 15/16"= 35psi
Freestar 16/17"= 35psi
Freestyle 17/18"= 35psi
Focus 15/16" = 35psi
1/2 ton F-Series 16/17/18/20" = 35psi
Super Duty's non-dually 16/17/18/20= 55fr/70rr psi
Quote:
First, you were using the same rims? The LT235/85R16 should be using a 6 1/2" rim and the LT265/75R16 should be using a 7 1/2" rim, so to get the equivalent situation you need to be using different rims.
The truck came with Firestone Steeltex AT's, 265/75-16 10 plys, on the factory 16x7 wheel you saw in the pic. I see no issue using a 7" wide wheel with both sizes, you can get either tire from the factory with this same size wheel. IMO it's vital to keep as many variables the same between tires when comparing, it eliminates factors that may effect performance.
Quote:
Second, to get the equivalent loading situation, you needed to use less pressure - 34 / 43.
I still would have more surface area and less pressure, less traction. The only way to match the 235's traction would be from adding weight to match the lbs per sq inch.
Quote:
And this doesn't take into account that you didn't do back to back tests.
This actualy was back to back, same day, same conditions. I am great friends with the manager and assist manager at the local big chain tire shop, we have done many comparisons between brands over the years, different vehicles. When we test these tires, I get priority for change-overs, and we keep the miles low and do not abuse them, they do get sold. Other shops have even done the same with us, so we've actually covered a pretty big volume of tires just being a bunch of guys curious about which tire keeps our kids the safest in the winter.
I may even be considered by Nokian for R&D in real world applications (my vehicles/fleet) for central BC, I am very excited about this, as you can probly tell, heheh.
Quote:
Tire traction testing is very difficult because of the variability. It takes many runs with the traction vehicle to get reliable data. And while I have no doubt about your experience, I have seen quite different results from highly controlled tests - enough to make me know that doing tire traction tests as little as a few hours apart is fruitless.
I agree, and dissagree. The more data, reliable data, the better, absolutely. But considering I'm just a Canuck in the deep frozen north, heheh, I don't have the means so I/we work with what is available. It is very easy to run 3-4 different sets of tires over an 8 hour period, as long as the weather is cooold and the conditions don't change. Sometimes the resuls vary so much you don't need to record the results, sometimes you need a stopwatch, or the tape. Either way, I absolutely agree a controlled enviroment with as much data is ideal, but I don't agree if you can't access this format of data collection it is fruitless.
Quote:
But I get the sense that you believe the inflation pressure equals the footprint pressure. Not true. Even though this seems intuitively true, it isn't.
Actually, this it what I sense you believed.
Quote:
The purpose of the increased pressure is to increase the pressure within the footprint.
Isn't that what you've stated here, you're increasing psi with the belief you're increasing footprint pressure.
Quote:
Pickup trucks have a unique problem when it comes to tire loading. The pressure needed to support the load when the truck is fully loaded - this is the placard value - is quite a bit higher than the equivalent situation when the bed is empty.
Cars have this problem, but 1) they are rarely used fully loaded, and 2) The problem is much less severe than with a pickup.
This is why trucks, like my crew cab for example, are so ideal to find the traction limitations of tires, compounds, widths, psi, etc. It is sooooo easy to get to the limits, and adjust accordingly to maximize the available traction. I've had cars that would of been useless for test pigs, just by design they found traction so well. Where you would take a sports car, or mini-van, throw it in a controlled enviroment for data collection, and need the recorded data to decipher the results, a huge Super Duty with a light rear end throws them out at you.
Quote:
Bottom line - I think the pressure increase is a valid recommendation, but it gets a little dicey when we start talking unloaded pickup trucks.
I'll share a story of a trip with you, it pretty much nips the raising psi theory in the bud, IMO anyway.
It was Jan/06, My family and I flew down to Vancouver BC to spend a week with my sister and her kids. The airport car rentals were dishing out 4wd SUV's at huge prices, I took a Freestar with 235/60-16 Goodyear Integrity, junky/old design/technology, cheap.
Anyways, Budget thought I was insane with the warnings of snowfall on the lower mainland, I don't care. I drive 6 months a year in gross winter conditions, I almost sickly enjoy driving in Vancouver when it snows, everyone panicking, I love it!
We get to our hotel, and decide to drive over to my sister's after we check in, well the snow hits, and it's coming down hard. In Vancouver they don't get feet of snow, it' a greasy, wet, layer of slop, slippery as snot. Well we pile in the van, I stop at a 7-11 to get some juice and snacks for the kids if we get trapped somewhere in a lineup of smashed cars, and I also get myself a tire guage.
We cut through some odd roads, avoiding the main flows of grief-stricken Vancouverites that have no clue how to drive in snow, and I head to the worst hill in the area, it's the shortest way to get to my sisters from the Executive hotel on north road, we're going up Galiardy Way on Burnaby mountain. I figured this would be a clean shot, no one would dare drive this way, man was I wrong.
By this time there's buses stalled out, other vans, cars, and lunatics in suv's running 4wd to the floor, bouncing off curbs, flailing wildly. I lock the tranny on 2nd for better tractability, and slooowly crawl around the majority of stranded people, until the slope degree increases. The JunkYears provide nothing on this thin layer of greasy cheese. A few more attempts, nada.
No one is close to me, the suvs are way up the hill flailing all over, I get out and drop the psi. All the tires were set at 35psi, I put them to 26psi....... sidewalls got a nice pliable bulge, they don't look rock hard anymore, let's see what happens......
....... I roll back a few feet so I'm not starting on a polished patch, 2nd gear, I crawl up and away, spinning slightly through the polished spots, and I actually have enough traction I'm gaining speed too. I ascend the hill so well I catch the 4wd suv's smashing the curb and sliding everywhere, I stay back and let the unexperienced ruin their rims and sidewalls. We eventually get to my sister's, I drop off the kids and wife, and I'm sent out for for food/supplies if the city ends up in total chaos for a few days.
I do contribute my experience in winter conditions a major factor, but I couldn't make the hill at 35psi where 26psi did, that cannot be ignored. It is not data from a controlled test enviroment, but it is years of data contributed from experience, experimentation, and the desire to maximize all available traction to keep my family the safest. I'm all for just as much psi you need for a full footprint and no more to keep it pliable.
Hey, this place would really be boring if we all agreed on everything, right?
Cheers,
Not-so-CrazyMike
On with the discussion.....
Quote:
The increased pressure is not to narrow up the footprint - that isn't the way it works except at high pressure / low load situations.
The purpose of the increased pressure is to increase the pressure within the footprint
This is a bit of a contradiction. Increasing psi to only increase pressure within the footprint will not occur, you need to reduce the surface area of contact, or increase weight. Tire pressure alone will not do this. Let's say we have a perfectly balanced car, a 3,000lb car with 200 sq inches of tire/road contace area at 35psi, this puts you with 750lbs of load per 50 sq inches of contact. With no reduction of surface area, running 40psi is still 3,000lbs, 750lbs per 50 sq inches. All we've done is reduce the tire's ability to conform to the terrain.
If we reduce surface area, as in the edges of the tires are raised, we've increased lbs per sq inch, increasing contact pressure, but now with a tire providing less friction/traction since it is now less pliable. A tire running lower psi will be much more pliable and provide better traction.
Quote:
Besides, the pressure recommendation is based on the idea that you are using the same size tire - not comparing different sizes.
Not necessarrily in all cases. Ford uses a standardized psi recommendation, as do other manufacturers I'm sure, so I do see it quite viable including different sizes :
Windstar 15/16"= 35psi
Freestar 16/17"= 35psi
Freestyle 17/18"= 35psi
Focus 15/16" = 35psi
1/2 ton F-Series 16/17/18/20" = 35psi
Super Duty's non-dually 16/17/18/20= 55fr/70rr psi
Quote:
First, you were using the same rims? The LT235/85R16 should be using a 6 1/2" rim and the LT265/75R16 should be using a 7 1/2" rim, so to get the equivalent situation you need to be using different rims.
The truck came with Firestone Steeltex AT's, 265/75-16 10 plys, on the factory 16x7 wheel you saw in the pic. I see no issue using a 7" wide wheel with both sizes, you can get either tire from the factory with this same size wheel. IMO it's vital to keep as many variables the same between tires when comparing, it eliminates factors that may effect performance.
Quote:
Second, to get the equivalent loading situation, you needed to use less pressure - 34 / 43.
I still would have more surface area and less pressure, less traction. The only way to match the 235's traction would be from adding weight to match the lbs per sq inch.
Quote:
And this doesn't take into account that you didn't do back to back tests.
This actualy was back to back, same day, same conditions. I am great friends with the manager and assist manager at the local big chain tire shop, we have done many comparisons between brands over the years, different vehicles. When we test these tires, I get priority for change-overs, and we keep the miles low and do not abuse them, they do get sold. Other shops have even done the same with us, so we've actually covered a pretty big volume of tires just being a bunch of guys curious about which tire keeps our kids the safest in the winter.
I may even be considered by Nokian for R&D in real world applications (my vehicles/fleet) for central BC, I am very excited about this, as you can probly tell, heheh.
Quote:
Tire traction testing is very difficult because of the variability. It takes many runs with the traction vehicle to get reliable data. And while I have no doubt about your experience, I have seen quite different results from highly controlled tests - enough to make me know that doing tire traction tests as little as a few hours apart is fruitless.
I agree, and dissagree. The more data, reliable data, the better, absolutely. But considering I'm just a Canuck in the deep frozen north, heheh, I don't have the means so I/we work with what is available. It is very easy to run 3-4 different sets of tires over an 8 hour period, as long as the weather is cooold and the conditions don't change. Sometimes the resuls vary so much you don't need to record the results, sometimes you need a stopwatch, or the tape. Either way, I absolutely agree a controlled enviroment with as much data is ideal, but I don't agree if you can't access this format of data collection it is fruitless.
Quote:
But I get the sense that you believe the inflation pressure equals the footprint pressure. Not true. Even though this seems intuitively true, it isn't.
Actually, this it what I sense you believed.
Quote:
The purpose of the increased pressure is to increase the pressure within the footprint.
Isn't that what you've stated here, you're increasing psi with the belief you're increasing footprint pressure.
Quote:
Pickup trucks have a unique problem when it comes to tire loading. The pressure needed to support the load when the truck is fully loaded - this is the placard value - is quite a bit higher than the equivalent situation when the bed is empty.
Cars have this problem, but 1) they are rarely used fully loaded, and 2) The problem is much less severe than with a pickup.
This is why trucks, like my crew cab for example, are so ideal to find the traction limitations of tires, compounds, widths, psi, etc. It is sooooo easy to get to the limits, and adjust accordingly to maximize the available traction. I've had cars that would of been useless for test pigs, just by design they found traction so well. Where you would take a sports car, or mini-van, throw it in a controlled enviroment for data collection, and need the recorded data to decipher the results, a huge Super Duty with a light rear end throws them out at you.
Quote:
Bottom line - I think the pressure increase is a valid recommendation, but it gets a little dicey when we start talking unloaded pickup trucks.
I'll share a story of a trip with you, it pretty much nips the raising psi theory in the bud, IMO anyway.
It was Jan/06, My family and I flew down to Vancouver BC to spend a week with my sister and her kids. The airport car rentals were dishing out 4wd SUV's at huge prices, I took a Freestar with 235/60-16 Goodyear Integrity, junky/old design/technology, cheap.
Anyways, Budget thought I was insane with the warnings of snowfall on the lower mainland, I don't care. I drive 6 months a year in gross winter conditions, I almost sickly enjoy driving in Vancouver when it snows, everyone panicking, I love it!
We get to our hotel, and decide to drive over to my sister's after we check in, well the snow hits, and it's coming down hard. In Vancouver they don't get feet of snow, it' a greasy, wet, layer of slop, slippery as snot. Well we pile in the van, I stop at a 7-11 to get some juice and snacks for the kids if we get trapped somewhere in a lineup of smashed cars, and I also get myself a tire guage.
We cut through some odd roads, avoiding the main flows of grief-stricken Vancouverites that have no clue how to drive in snow, and I head to the worst hill in the area, it's the shortest way to get to my sisters from the Executive hotel on north road, we're going up Galiardy Way on Burnaby mountain. I figured this would be a clean shot, no one would dare drive this way, man was I wrong.
By this time there's buses stalled out, other vans, cars, and lunatics in suv's running 4wd to the floor, bouncing off curbs, flailing wildly. I lock the tranny on 2nd for better tractability, and slooowly crawl around the majority of stranded people, until the slope degree increases. The JunkYears provide nothing on this thin layer of greasy cheese. A few more attempts, nada.
No one is close to me, the suvs are way up the hill flailing all over, I get out and drop the psi. All the tires were set at 35psi, I put them to 26psi....... sidewalls got a nice pliable bulge, they don't look rock hard anymore, let's see what happens......
....... I roll back a few feet so I'm not starting on a polished patch, 2nd gear, I crawl up and away, spinning slightly through the polished spots, and I actually have enough traction I'm gaining speed too. I ascend the hill so well I catch the 4wd suv's smashing the curb and sliding everywhere, I stay back and let the unexperienced ruin their rims and sidewalls. We eventually get to my sister's, I drop off the kids and wife, and I'm sent out for for food/supplies if the city ends up in total chaos for a few days.
I do contribute my experience in winter conditions a major factor, but I couldn't make the hill at 35psi where 26psi did, that cannot be ignored. It is not data from a controlled test enviroment, but it is years of data contributed from experience, experimentation, and the desire to maximize all available traction to keep my family the safest. I'm all for just as much psi you need for a full footprint and no more to keep it pliable.
Hey, this place would really be boring if we all agreed on everything, right?
Cheers,
Not-so-CrazyMike
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Mike,
Another thoughtful and thought provoking post. Keep this up and BITOG might give you a gold star!! (Hint - I'm still waiting!!)
OK, let's start with the inflation pressure vs footprint pressure:
If we were to measure the size of the footprint of a loaded tire, then increase the inflation pressure, the footprint will get smaller. Most of the time it is the length of the footprint that changes - but occassionally, especially under extreme circumstances, the width will change.
Since the footprint got smaller, but the load did not change, the footprint pressure goes up. However, you can not use the inflation pressure to calculate the footprint pressure (unless you have a graph that tells you what this relationship is.)
To be clear, the footprint pressure and the inflation pressure pretty much go hand in hand, but they aren't the same value.
I liked your story - but I think you've missed an important point. If you have a situation where you don't get down to the pavement, then lower pressure is better. This applies to mud and ice as well, since what is generating traction is almost 100% dependent on the amount of edges in contact with whatever surface you are driving on.
However, pavement has a much, much higher level of traction and if there is even a chance that you will break through the snow and be able to reach pavment, you should go there. Unfortunately, once you have picked a pressure, the only way to go is down (unless you carry a compressor with you.)
And as you so clearly point out, driver skill level is probably the most deciding factor when it comes to winter driving.
I could argue further about some of the other stuff, but I think we have points we agree on. But I think the major difference between our positions is the issue about snow coverage.
Persoanlly, I think more uninformed / low skilled drivers live in cities, and in the lower 48, and these folks are most likely to encounter slush and loose snow, and the idea of penetrating through that to get to pavement is better. Once they get into a situation where the lower pressure is helpful, they just aren't prepared to deal with the levels of traction the lower pressure provides. Put a different way, at 0.10 coefficients of friction, it takes quite a bit of experience to be able to take advantage of even a 25% increase in traction.
And that's usually the folks that ask these kinds of questions. So I tend to think that the "ONE ANSWER" that these folks will get and remember ought to be the higher pressure answer.
Another thoughtful and thought provoking post. Keep this up and BITOG might give you a gold star!! (Hint - I'm still waiting!!)
OK, let's start with the inflation pressure vs footprint pressure:
If we were to measure the size of the footprint of a loaded tire, then increase the inflation pressure, the footprint will get smaller. Most of the time it is the length of the footprint that changes - but occassionally, especially under extreme circumstances, the width will change.
Since the footprint got smaller, but the load did not change, the footprint pressure goes up. However, you can not use the inflation pressure to calculate the footprint pressure (unless you have a graph that tells you what this relationship is.)
To be clear, the footprint pressure and the inflation pressure pretty much go hand in hand, but they aren't the same value.
I liked your story - but I think you've missed an important point. If you have a situation where you don't get down to the pavement, then lower pressure is better. This applies to mud and ice as well, since what is generating traction is almost 100% dependent on the amount of edges in contact with whatever surface you are driving on.
However, pavement has a much, much higher level of traction and if there is even a chance that you will break through the snow and be able to reach pavment, you should go there. Unfortunately, once you have picked a pressure, the only way to go is down (unless you carry a compressor with you.)
And as you so clearly point out, driver skill level is probably the most deciding factor when it comes to winter driving.
I could argue further about some of the other stuff, but I think we have points we agree on. But I think the major difference between our positions is the issue about snow coverage.
Persoanlly, I think more uninformed / low skilled drivers live in cities, and in the lower 48, and these folks are most likely to encounter slush and loose snow, and the idea of penetrating through that to get to pavement is better. Once they get into a situation where the lower pressure is helpful, they just aren't prepared to deal with the levels of traction the lower pressure provides. Put a different way, at 0.10 coefficients of friction, it takes quite a bit of experience to be able to take advantage of even a 25% increase in traction.
And that's usually the folks that ask these kinds of questions. So I tend to think that the "ONE ANSWER" that these folks will get and remember ought to be the higher pressure answer.
'Snow' and 'ice' are kind of like saying 'drink' and 'food', pretty general terms. The fresh powdery snow when it's cold is different than the fresh slushy snow when it's not cold, the deep fresh snow is different than the packed stuff at the intersections that has frozen hard and been polished down during the night. The hard ice that all season tires can get around on is different than the soft ice close to freezing that has a layer of water on top of it. Small cars with skinny tires, heavy pickups with wide tires, everything in between, and cocktails of different types of snow on different types of ice, or ice on snow.
Going from one afternoon to the next conditions can change dramatically, so you gotta pay attention to the road conditions and change your tactics on how to get adequate traction. I notice that I need to pay a lot more attention with my truck that weighs about 7k lbs empty than I did with a Honda Civic, but properly prepared I can also go more places in the truck. At some point it's best to stay home if your vehicle isn't capable, as a coworker found out last year when his AWD Subaru with square link chains on all of his studded snow tires still ended up in a ditch.
That being said I'll air down my studded snow tires top get a larger tire patch when traction suffers, as by then I've already tried higher contact pressure (normal running) and it didn't work, and if that doesn't work I'll chain up.
Going from one afternoon to the next conditions can change dramatically, so you gotta pay attention to the road conditions and change your tactics on how to get adequate traction. I notice that I need to pay a lot more attention with my truck that weighs about 7k lbs empty than I did with a Honda Civic, but properly prepared I can also go more places in the truck. At some point it's best to stay home if your vehicle isn't capable, as a coworker found out last year when his AWD Subaru with square link chains on all of his studded snow tires still ended up in a ditch.
That being said I'll air down my studded snow tires top get a larger tire patch when traction suffers, as by then I've already tried higher contact pressure (normal running) and it didn't work, and if that doesn't work I'll chain up.
Originally Posted By: CrazyMike
I'll share a story of a trip with you, it pretty much nips the raising psi theory in the bud, IMO anyway.
It may be a data point arguing for lowered pressure in that situation, but it hardly counts as a definitive "nip in the bud," considering the anecdotal nature of the story.
I'd be really interested to see the different test numbers for a vehicle's friction coefficients and the corresponding forces possible on deep snow with low and high inflation pressures.
How big of a difference is it really? If it's the difference between a person with a size 9 foot slipping around on an ice rink and a person with a size 10 foot, well I don't think I'd want to really bother with the pressures either way, not when getting some crampons would be the only real noticeable difference...
I'll share a story of a trip with you, it pretty much nips the raising psi theory in the bud, IMO anyway.
It may be a data point arguing for lowered pressure in that situation, but it hardly counts as a definitive "nip in the bud," considering the anecdotal nature of the story.
I'd be really interested to see the different test numbers for a vehicle's friction coefficients and the corresponding forces possible on deep snow with low and high inflation pressures.
How big of a difference is it really? If it's the difference between a person with a size 9 foot slipping around on an ice rink and a person with a size 10 foot, well I don't think I'd want to really bother with the pressures either way, not when getting some crampons would be the only real noticeable difference...
Don't conduse coefficient of friction with total friction. Higher tire pressure / lower contact patch size / higher contact patch pressure will work for more passenger vehicles in more conditions, but it only works until it doesn't :^) Deep snow typically seesm to call for lower contact patch, looking at vehicles like 'snow cats' that are designed to try to ride on top of snow, and snow shows. 'Soft ice' with water on it seems to call for higher contact patch in order to minimize hydroplaning, but some vehicles that nornally run at high pressures may benefit from lower tire pressure in order to get something like more studs in contact. 'Hard ice' seems to benefit from getting more tire in contact with the ice, so lower pressure may be better.
http://www.tc.gc.ca/tdc/summary/14000/14005e.htm
Measuring tires for harmonized friction measurements of runway surfaces and prediction of aircraft wheel braking
....Coefficient of friction decreases with increased vertical load (tire contact pressure).
.....The effects of braking rate and contact pressure have by far the most significant effect on friction values. Because of the effects of tire contact pressure, friction force values increase by decreasing the contact area or increasing the load on the tire....
4.7 Load/Contact Pressure
In the 1998 tests the ITTV was used in a series of tests using a 22 x 6 aircraft tire (the same as the nose wheel tire on the Falcon 20). The series consisted of running variable loads (455 kg to 2270 kg, 1000 to 5000 lb.) on an ice surface and a packed snow surface. Figure 17 shows a clear drop in friction as the load is increased on both surfaces. Figure 18 shows the same data where friction force rather than friction is plotted against load. Here the force increases with load and is reaching a constant level at higher loads. It appears that at about 4000 lb. (1818 kg) the shear strength of the snow is reached and there is no further increase in friction force.
Figure 18 Friction force (shear) versus tire load on the ITTV
These tests show the importance of load (contact pressure) on this tire and thus the need to have similar characteristics for all of the tires being used if one is to model one tire and test with another. In the case of the smaller, lighter loaded ground friction tires, we have seen that increased contact pressure has increased the friction, while for the larger, much higher loaded aircraft tire we see the opposite. This finding was expected since the aspect ratio of the aircraft tires is nearly constant with load, while the ground test tires have a constant width with a change in length and thus a changing aspect ratio with load
http://www.ctre.iastate.edu/pubs/crossroads/94measurement.pdf
The Measurement and Theory of Tire Friction on Contaminated Surfaces
COMPOSITE WINTER SURFACES
We introduce the composite surface classification depicted in Figure 3.
The braked wheel can displace all or a large part of a layer of slush, fresh snow or drifting snow that has a fluid powder character. This gives rise to contaminant displacement drag forces on the wheel and varying levels of fluid lift and fluid lubrication as some of the fluid contaminant gets trapped under the tire. Compression may occur with the trapped snow to build a thin layer of new snow base in the track of the wheel. When there is sand applied to the surface (likely before the snow base in Figure 3), it will interact with the tire and raise the friction force experienced. Even in cases without fresh snow/drifting snow, the snow base may be sufficiently soft for the tire to shear off snow crystals during braked wheel rolling and create small amounts of powder to sustain a partial planing condition.
Surface Temperature
Surface temperature is observed to raise friction level on winter contaminated surfaces as temperature falls. The mechanisms producing that effect are many. A major effect is believed to be a rise in friction due to the increased shear strength and the absence of free water, which could act as a lubricant.
PRELIMINARY RESULTS FOR NORTH BAY TESTS
That contact pressure is a very strong influence, in fact in the simple correlation there is a very strong influence of contact pressure on the multiplier constant (correlation is R2=0.82).
http://www.tc.gc.ca/tdc/summary/14000/14005e.htm
Measuring tires for harmonized friction measurements of runway surfaces and prediction of aircraft wheel braking
....Coefficient of friction decreases with increased vertical load (tire contact pressure).
.....The effects of braking rate and contact pressure have by far the most significant effect on friction values. Because of the effects of tire contact pressure, friction force values increase by decreasing the contact area or increasing the load on the tire....
4.7 Load/Contact Pressure
In the 1998 tests the ITTV was used in a series of tests using a 22 x 6 aircraft tire (the same as the nose wheel tire on the Falcon 20). The series consisted of running variable loads (455 kg to 2270 kg, 1000 to 5000 lb.) on an ice surface and a packed snow surface. Figure 17 shows a clear drop in friction as the load is increased on both surfaces. Figure 18 shows the same data where friction force rather than friction is plotted against load. Here the force increases with load and is reaching a constant level at higher loads. It appears that at about 4000 lb. (1818 kg) the shear strength of the snow is reached and there is no further increase in friction force.
Figure 18 Friction force (shear) versus tire load on the ITTV
These tests show the importance of load (contact pressure) on this tire and thus the need to have similar characteristics for all of the tires being used if one is to model one tire and test with another. In the case of the smaller, lighter loaded ground friction tires, we have seen that increased contact pressure has increased the friction, while for the larger, much higher loaded aircraft tire we see the opposite. This finding was expected since the aspect ratio of the aircraft tires is nearly constant with load, while the ground test tires have a constant width with a change in length and thus a changing aspect ratio with load
http://www.ctre.iastate.edu/pubs/crossroads/94measurement.pdf
The Measurement and Theory of Tire Friction on Contaminated Surfaces
COMPOSITE WINTER SURFACES
We introduce the composite surface classification depicted in Figure 3.
The braked wheel can displace all or a large part of a layer of slush, fresh snow or drifting snow that has a fluid powder character. This gives rise to contaminant displacement drag forces on the wheel and varying levels of fluid lift and fluid lubrication as some of the fluid contaminant gets trapped under the tire. Compression may occur with the trapped snow to build a thin layer of new snow base in the track of the wheel. When there is sand applied to the surface (likely before the snow base in Figure 3), it will interact with the tire and raise the friction force experienced. Even in cases without fresh snow/drifting snow, the snow base may be sufficiently soft for the tire to shear off snow crystals during braked wheel rolling and create small amounts of powder to sustain a partial planing condition.
Surface Temperature
Surface temperature is observed to raise friction level on winter contaminated surfaces as temperature falls. The mechanisms producing that effect are many. A major effect is believed to be a rise in friction due to the increased shear strength and the absence of free water, which could act as a lubricant.
PRELIMINARY RESULTS FOR NORTH BAY TESTS
That contact pressure is a very strong influence, in fact in the simple correlation there is a very strong influence of contact pressure on the multiplier constant (correlation is R2=0.82).
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