Traction vs Inflation Pressure

Status
Not open for further replies.
C

CapriRacer

Joined
Feb 22, 2003
Messages
4,484
Location
Somewhere in the US
In the Aug 2013 issue of Grassroots Motorsports Magazine, they performed a tire test on the new Bridgestone RE-11 – an Autocross tire. They compared the tire to a Goodrich g-Force Rival.

I note with interest that a prelude to the actual test was a skipdpad evaluation to determine the best inflation pressure for each tire. They started at 40 psi and dropped 4 psi until they maxed out. This portion was not well documented and I have lots of questions concerning their procedure – particularly since it contradicts what many have been saying about the relationship of traction and inflation pressure (and I would not be one of those!)

Also of interest was that the 2 tires got different results – 32 psi for the B.S. and 27 psi for the BFG.

I am going to ask Grassroots Motorsports for some details about this portion of the test - and thought I would post this here to start a conversation on the subject.
 
Thanks. Looking forward to more info on this.

For me, it makes sense that traction would decrease as pressure is increased. Isn't that why most people increase pressure in hopes of improving MPG? The improved MPG comes from the fact that the tires grip less. Or am I oversimplifying things here?
 
QP...you're quite right...

Among the variables to consider...MAX. PSI on the side-wall

...adding some air, as much as 15-20% PSI over door-jamb specs isn't severe, could improve the ride and road manners (cornering, braking, response to road conditions) of some cars (like the Camry)...but if that much added pressure approaches that particular tire's MAX PSI, I would be wary of going even that high...
 
Last edited:
Originally Posted By: Quattro Pete
Thanks. Looking forward to more info on this.

For me, it makes sense that traction would decrease as pressure is increased. Isn't that why most people increase pressure in hopes of improving MPG? The improved MPG comes from the fact that the tires grip less. Or am I oversimplifying things here?


No, you're oversimplifying things. Rolling losses in tires come from mostly from hysteresis, which is the deformation and resulting reformation of both the tread and the sidewall; increasing pressure will reduce hysteresis, which is why it increases FE.

There are compounds that have both low rolling resistnace and good grip. For motorsport, both are obviously a huge consideration--and those compounds have trickled down. Truth is, some of best-gripping compounds also have very low Crr.

You're right in the sense that higher pressures reduce skin friction, but that actually plays a vanishingly small role in overall Crr.
 
But I thought Tactical Driver (and his cohort, AstroTurf) had PROVED that hyperinflation rested in better grip! I remember that we were all morons who don't understand that 65+ PSI, well over sidewall max, made all tires perform better....and now, you're shattering that illusion with actual Testing?!?!

Sigh ...
 
The 4 cyl circle track guys that come to autocross for the first time, know that lower pressures increases traction on the oval. So they drop the front tire pressures to try to minimize understeer and end up getting more. But autocross is anything but a static skid pad test, so I find to minimize understeer I end up running the fronts a bit harder, like 36psi cold and the rears at 24-26psi. I think the lower rear pressure allows the car to have some drift angle plus also allows some play so the momentum of changing direction help break the rear end loose.
For autocross, I only tuned tire pressures to where the car "feels right", you don't have enough runs to start comparing tire pressures influence on times.
Also tire size, dynamic camber changes, car weight are all factors in the best pressure for static grip. Throw in all the dynamic changes on an autocross course, picking the "best" tire pressures might be tough to nail down to a 4-6 psi range.
 
Originally Posted By: JOD
No, you're oversimplifying things. Rolling losses in tires come from mostly from hysteresis, which is the deformation and resulting reformation of both the tread and the sidewall; increasing pressure will reduce hysteresis, which is why it increases FE.

OK, but if you have less deformation/reformation happening as a result of higher PSI, then by the same token you have worse traction exactly because the tire resist to deform itself to "hug" the road better.

Quote:

There are compounds that have both low rolling resistnace and good grip.

Sure, but we are not discussing/comparing different compounds here, or at least I don't think we are. We are taking one single compound/tire, and playing around with its PSI to see how it affects traction.
 
Originally Posted By: Quattro Pete

OK, but if you have less deformation/reformation happening as a result of higher PSI, then by the same token you have worse traction exactly because the tire resist to deform itself to "hug" the road better.



Not necessarily. Traction really isn't about "hugging the road" as much as maximizing the size and cross-section of the contact patch. And that's a pretty complex issue, of which tire pressure is just one factor. Pressure that is too low can have an even worse effect that pressure that is too high as excessive sidewall flex results in uneven (and unpredictable) loading. Also, keep in mind that traction over the course of an entire event isn't static. Lower pressures which localize contact to the outside of the tire on a track are going to result in higher temps, and ultimately less grip.

Keep in mind, maximizing contact patch area doesn't substantively lower fuel economy or rolling resistance.
 
Originally Posted By: JOD
Keep in mind, maximizing contact patch area doesn't substantively lower fuel economy or rolling resistance.


Sure, nobody here suggested that it's substantial, but those Grassroots Motorsports guys must have found it worthwhile to make some adjustments for their purposes.
 
I see I have generated some comments - and it is clear that there are many myths still floating around about tires, inflation pressure, grip, and rolling resistance. Of course, that was the purpose of my post.

Yes, also part of my purpose in starting this thread was the result of the "conversation" Astro14 referred to. At the time, I found it very interesting that the TD apparently had the resources to perform a test and not only hadn't, but seemed to think he didn't need to. Very sad, because it would have been very illuminating to have actual data.
 
I think in the end...

Since no consumer tire is designed be mated to a specific, mass produced car/model or suspension design, (other than in the most generalized terms), finding the optimal PSI for tire/car combination is as much (if not more) a result of personal on-the-road testing for traction/cornering etc. under various conditions and driving habits, as it is of "objective" tire testing...
 
When I take my car to the track, particularly in the summer, I take the pressure down to 28-30psi. At the end of my session they're at 36psi (and the tires are wonderfully sticky to the touch).
The new Potenza RE11A is the best extreme performance summer tire (for street and track) available. I hope to have a set this summer.
 
Last edited:
The way to do it without a skid pad and accelerometer and such is to just put some white shoe polish on the sidewall/shoulder, when the pressure is so low the tire starts folding over and trying to run on the sidewall(the shoe polish rubs off), you up the pressure till it stops folding over and you'll be pretty close.
 
Last edited:
Originally Posted By: default
The way to do it without a skid pad and accelerometer and such is to just put some white shoe polish on the sidewall/shoulder, when the pressure is so low the tire starts folding over and trying to run on the sidewall(the shoe polish rubs off), you up the pressure till it stops folding over and you'll be pretty close.
You can measure tire temp across the tread. Hot spots indicate an uneven distribution.
 
Last edited:
BOTH of the above two posts are correct, and they point out the MAIN reason for going higher pressure in the front tires of a FRONT engined car in an autocross scenario, namely, sidewall 'rollover'.

This is especially true when using, or limited by the rules, to a street type, over 160 treadwear, softer sidewalled, non-R compounded tire, on a camber/toe/caster limited (by the rules or by front suspension design) vehicle.

The stiffer the sidewall, the less pressure needed due to less/no rollover, and the more stick/adhesion there is (given the same compounding, and front suspension geometry of course).
This, along with heat buildup, is why R compound tire manufacturers suggest lower pressures for their product in competition use, than for street tires used in that scenario, and why they perform better with these lower (static/cold) pressures.
wink.gif


Road racing/open tracking requires even lower pressures yet, due to extended heat buildup from longer duration events, and also from generally more 'open', less tight, and larger radius turns.
 
Originally Posted By: default
The way to do it without a skid pad and accelerometer and such is to just put some white shoe polish on the sidewall/shoulder, when the pressure is so low the tire starts folding over and trying to run on the sidewall(the shoe polish rubs off), you up the pressure till it stops folding over and you'll be pretty close.


...or just carefully look at the dirt pattern when you stop and see how close it gets to the sidewall....a couple of PSI can be all that is needed to keep the wear from rubbing the sidewall...in Kitacam's case, where 29psi is on the doorjamb, 33psi still gets dirt to the sidewall, 34/35psi and the dirt no longer curls to the edge.
 
Last edited:
One of the reasons I asked the question was because doing the dirt across the face of the tread or the shoepolish on the sidewall doesn't tell me if that is the optimum for grip - and those guys said they actually tested the tire to get the value.

It would have been interesting if they had used shoepolish and correlated that to the answer they got - but they didn't.

I remain skeptical of using those methods as definitive - even though I have used that method myself - basically because it was the only indicator I had.
 
Originally Posted By: CapriRacer
One of the reasons I asked the question was because doing the dirt across the face of the tread or the shoepolish on the sidewall doesn't tell me if that is the optimum for grip - even though I have used that method myself - basically because it was the only indicator I had.


Adding @ 2psi air at a time over the doorjamb spec and testing the tire's performance is how I figure out the optimal grip...and it does seem that matching the dirt across the face seems to give me the best...and THAT meant that the left side tires needed 1psi more than the right side tires...odd but true..
 
Status
Not open for further replies.

Similar threads

DriveHard
Calling Capri Racer - inflation pressure on tire size change
Replies
15
Views
2K
jadatis
L
Tire over inflation
2
Replies
26
Views
2K
zzyzzx
M
tire load/ pressure
Replies
8
Views
1K
CapriRacer
JeffKeryk
2 Milton pencils vs Jaco vs Milwaukee Inflator Test
Replies
9
Views
1K
JeffKeryk
L
So I got the Michelin Defender 2
2 3 4
Replies
71
Views
26K
daves87rs
Back
Top