Ultra High Performance Tires(UHP) and Fuel Economy losses?

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quote:
Originally posted by rpn453:
Good link, but I've yet to be convinced that the fuel economy gains resulting from tread reduction are due to the reduced mass of the tread.
Click to expand...
Capriracer: after another read, I think I'm finally convinced that you're right. More mass is more material that needs to cyclically deform and return to its original shape, which would create more heat and therefore more wasted energy. If an empirical formula is used to determine rolling resistance of a tire, mass would certainly be a factor and I think, with all other variables being equal, that increased mass would always increase rolling resistance (at least, up to the point that the rubber becomes thick enough to be modeled as a solid rubber wheel).

I still believe tread design can potentially have a greater effect on fuel economy than tread mass. But in the case of "filling in the rain grooves" of a smooth-tread tire to convert it to a full slick, I can finally see that the increased mass is the cause of the reduced fuel economy.

[ March 02, 2006, 06:52 PM: Message edited by: rpn453 ]
 
"....I think I'm finally convinced that you're right....."

Oh, my!! You had some doubt? (hehe)

"....I still believe tread design can potentially have a greater effect on fuel economy than tread mass...."

Look at it like this: The belt is extremely stiff compared to the tread rubber, so even the large amount of tread mass will have little effect on the deflection compared to the effect the belt has. Therefore, if the same amount of tread mass is used, the largest contributor to heat generation (and therefore rolling resistance) is going to be the amount of mass and not how flexible the individual tread elements are.

Don't get me wrong, a tire with lots of small tread elements will have a higher RR than one that doesn't, but compared to the flexing the overall tread is making, the individual tread elements flex very little.
 
Thanks for the additional info, CapriRacer. But there must be some point when tread design has a more significant effect on fuel economy than just mass (I threw in "potentially" to cover myself on that statement!). Maybe that point only comes with extremely aggressive, knobby mud tires that are unsuitable for on-road use?
 
".... But there must be some point when tread design has a more significant effect on fuel economy than just mass...."

I'm sure there is a point, but it must be WAY, WAY out there, because the stiffness of the belt is orders of magnitude more than tread rubber.

One of the things that stiffens the belt is inflation pressure - which is one of the reasons inflation pressure has such a pronounced effect on RR.

What inflation pressure does is press against the belt and resist inward (radial) deflection. Another way of looking at this is that inflation pressure pre-tensions the belt(circumferentially).

Conversely, rubber is highly extensible, so the same amount of stretch produces less tension force compared to steel. The net effect is that the tread rubber is a very minor player when it comes to overall package. This is not to say that the type of tread rubber doesn't have a measureable affect on RR, nor to say that small tread elements won't adversely affect RR, but these factors are overwhelmed by the effect the belt has.

So, you may ask, wouldn't it be better for RR if they used something stiffer in the belt? And wouldn't this also help tread wear and cornering?

Yup!!! Which is why steel is the material of choice. Early radials used fabric belts - 4 of them! Glass has been tried (shatters in cold weather!) Kevlar (Its strength is good for its weight, but you still have to use more of it to get the same stiffness.)

Why not more steel? There's an upper limit to the amount of rubber penetration into the steel cable and this affects the rubber to belt wire adhesion. This was one of the early lessons learned in the steel belted radial story. Anyone remember the Firestone 500, which was followed by the 721? The 721 cable was to address this penetration issue. Didn't work as well as was hoped, because rubber penetration was only part of the problem. Rubber chemistry was the other part.

Pretty much every tire manufacturer has settled in the same vicinity with regard to belt wire. So difference between RR of tires is more a function of tread compound than tire construction, simply because the construction differences are minor. Probably the only major construction difference is cap plies (nylon overlays - whatever you'd like to call them). These add considerable mass compared to the amount of stiffness.

But nylon has one remarkable property - it shrinks when heated. So a tire generating heat will have it's belt restricted the more heat is generated.

Nevertheless, adding nylon to a tire hurts RR.
 
I think you are also getting worse mpg because of the reduced diameter of the wheel. you went to wider tires, but also with a lower profile. if you calculate out the actual diameter of the whole wheel, you will find that it is shorter now. for short trips, this will lead to a higher mpg, but for long drives, this will drastically reduce your mpg because you have to rev higher for longer periods of time. if you drive on the highway a lot, i think this could account for at least some of your reduced mpg. next time you change wheels, make sure the diameter stays the same, and see if your mpg changes again... however long that takes
 
IF diameter size dropped, don't have a tire size guide handy, but I'd about bet it did. Not only would decreased diameter cause drop in mpg, but would also cause speedo to be out of calibration. This would show more miles traveled than actual, in turn making mileage seem worse than actual. IMO

Bob
 
The decreased diameter would cause a drop in REAL mpg, but it shouldn't affect the calculated mpg, since the miles on the odometer are measured by tire revolutions, not actual distance traveled. Am I wrong here?
 
I noticed that I lost some fuel economy when I went to wider tires.

My switch was quite drastic, going from 215/60/16 to 245/50/16. This from all-season touring to UHP.
 
That is also partially due to your tire width being reduced. if you calculated it, the sidewall difference is 6.5mm. that isn't that much of a difference, but that is just diameter, i'm not sure exactly how much of a circumference difference that would account for. but i think when you are traveling highway distances, it all adds up.
 
On Speedometer Calibration ... according to Garmin C330 GPS reciever it seems to be off 1-2mph at 60mph? GPS will say 58mph, instrument cluster 60.

I have not tried my TeleNav GPS phone yet.

On 215/60/16 vs 235/55/16 & 245/50/16 ... my Blizzak WS50 were 215's and I at least think Fuel Economy improved.

Air pressure? Are you saying air the tire to the maximum sidewall manafactures recommendation?

How does air pressure in general impact ride quality? My experiance in LT Truck (Load range D, E) have high pressures supposedly resulting in a jarring ride.

Door jab factory decal says oem tires at 30psi , service manual charts vehicle sustained speed to recommended inflation.
 
you want to set the air pressure to the appropriate level, not just the highest written on the tire sidewall.

your observation is correct. higher pressure causes a stiffer ride, but in general gives a more precise feel to turn-in and a feel in 'response'

i generally inflate my tires to a bit higher than what is on the door. this is because i drive on the spirited side, so overinflated is better for cornering and longevity of the tires.
 
gnef does it the same way I do. A few extra pounds over the VEHICLE placard, not the sidewall max inflation pressure. The ride is a little more harsh, but not overly so. Steering response is better, and mileage is better (measurable on very long trips). Not to mention that handling is improved because the sidewall doesn't flex as much in the corners. Gives a nice, "connected" feel to the car, too.

Dave
 
quote:
Originally posted by outrun:
I went from an all season T rated touring Goodyear Eagle GA to a wider UHP Z-Rated (W) Michelin Pilot Sport A/S . . . The handling and wet grip is amazing, but this must come at a cost of higher friction and rolling resistance.
Click to expand...
I just installed a set of those Pilot Sport A/S on my car and the grip is certainly amazing compared to the Goodyear RS-As that the car came with. These are the grippiest tires I've ever driven with on a FWD car! They're about 1/2" wider than the GYs (in the same size - 205/50R17). I don't know if I'll be able to notice any fuel economy difference though since my driving habits are inconsistent.
 
There may be other factors involved, too, not tire related, nor tuneup factors. This is not to say that the new tires haven't lowered the MPG to some degree-- it would only make sense that a wider, lower profile tire would have more RR.

In my car i've noticed that switching from ARCO and Unocal to Chevron has drastically increased my MPG on the highway (almost 10%), while all other factors remained the same.

I'd recommend changing lubes to a full synthetic to help regain the lost MPG, too. I prefer a extended drain brand like Redline or Amsoil.

Just changing from a semi-syn to full syn in my 04 Audi S4 netted .5mpg on the highway. Not bad for a car with 385hp and 175mph potential.
 
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