VaSteve

Don't you also have to factor in the fact that you start out with lower pressure to account for the increase as the tires heat up? I have always heard that you don't want to come off at higher than 40 psi, hot. Is that the peak of the graph?


This sounds like trying to maintain that perfect balance of sweetener in the tea when the wait staff keeps filling your half-full glass - a never ending calculation!

RedlineMan

Hey;

I don't claim to have any advanced knowledge of this stuff. Curves, plots, graphs, bells, pyramids. I was just trying to offer a visual reference to a moving data target. Visuals to reinforce the conceptual.

Tires do indeed gain most of their temp through mechanical motion (flex), not surface friction. You may indeed scrub off track junk when weaving on a warmup lap, but sliding the tires does not heat them. Flexing them does.

Interesting analogy, Steve. Real world and, er... grasp-able! Most R tires do like to be around 40 hot. That is enough data for most people. Using a pyrometer will certainly give you a much better idea of whether you are hitting good temp ranges. It also gives you good alignment feedback. The downside is that using one as accurately as possible takes some help in the pit lane. Surface temps drop off so fast that "old" data becomes highly questionable.

cgfen

ahhhhh, thank you. This makes complete sense to me. When i asked about SW stiffness and contact patch, i also nearly asked about " what is the baseline or reference pressure"
spot on Steve. Thanks again


This is also an excellent point, though not as obvious as the above. I'll have to do some thinking about this to better understand it.


this helps quite a bit.
thank you
i look forward to exploring the skills / guts equation.

Thanks to all that took the time to comment.

blue skies
Craig

TraqGear

+1

Geo

This is not correct. First of all, spring rates and tire pressures are different animals. Secondly, spring rates and tire pressures have zero effect on weight transfer. Higher spring rates will speed up the weight transfer, but the weight transfer remains the same. Higher tire pressures may have a very slight effect on the speed of the weight transfer, but that is likely minuscule compared with the effect on contact patch.

jwade944

I probably used the wrong terms, but I am envisioning a car making a left hand turn. If weight is transferred from the left to the right wheels then some deflection must occur to both sides of the suspension. A portion of that deflection occurs in the springs and a portion in the tires. By raising the front tire pressure, you are forcing more of that deflection into the front springs which allows the front sway bar deflection to increase. This allows the front swaybar to transfer more force back to the front left side and help balance the transfer of forces on the front more equally. It seems this would also act to "twist" the rear and affect the loading of the rear tires. Granted the magnitude of these changes is small, but the overall change is to improve the grip of the front at the expense of the rear and that would improve understeer. Note that I have never designed a suspension and rarely make adjustments to my car's as I have found that bettering my driving is the answer to most of my handling problems.

Geo

The load transfer remains the same. The rate at which it transfers is what changes.

rsrdan

this is a great question ...,but guys he's driving a STOCK 4S..and assuming he's on street tires,the graft is missleading.....now rear wheel drive/race tires.its a different story A Tire Pyrometer is the only way!! learn the pyrometer,learn it !!

Professor Helmüt Tester

To throw a curve into this discussion, Der Professor offers this:

http://www.scca-enterprises.com/upda...nical_Info.pdf

(this data is for a "short track asphalt slick")

Tires do have an "effective spring rate". The construction of the tire can and does affect the "wheel rate" of a suspension. While it's rare to see it published is such a user-friendly way, this data exists for every tire. Changing tires, and even just changing air pressure in a tire, can dramatically affect total spring rates of a suspension.

Now...the question is: "Is this spring rate in series, or in parallel ?" (you electronic geeks will understand this)

Chew on that, kiddies....

SundayDriver

I disagree. At best, small changes in pressure (2-3 psi) with stiff sidewall, low profile tires, have a very minimal affect on contact patch. With radial tires, there is almost no affect.

mark kibort

The nascar guys say that 1psi change in tire pressure is the same as a spring change of 50lbs.

you can get more or less weight transfer that can be good or bad, depending on the situation, with changes in spring and tire pressures.

simple example. soft springs in front, race car coming down in speed in a hard braking zone from 140mph. the softer the front springs, the more the weight transfer. the same things happens in a turn. too soft, you can get too much weight tranfer and lift both inner turn tires around a turn (or lift one of the tires)

I do think the tire pressure is a small factor as part of the suspension (as far as adjustments) , but the pressure does effect grip, which then can effect suspension. Hard to generalize here as there are so many different types of cars, tires, suspension set ups, geometries, etc etc.

mk

Geo

Absolutely incorrect. Spring rates and tires pressures do NOT affect weight transfer.

Load (weight) transfer = acceleration x (mass x height)/wheelbase (or track depending upon whether you're calculating longitudinal or lateral load xfer). No where in that calculation does spring rate come into play.

cgfen

well i don't have a tire pyrometer, but i do have a thermocouple probe and reader that reads between +60 - ~+400 dgrees F accurately.
I'm going to assume i can make that work.

how "deep" into the rubber do you insert the probe? Or, do you take a surface temperature measurement?
I would think that a surface measurement would be prone to error due to rapid cooling.

thanks

Craig

ronbo56

I managed to tumble off Mr. Hajny's pyramid, fortunately without serious consequences. Back before I learned the value of a dedicated track car, I drove a very nice car up to a track day at the Glen. I was carrying pressures that I knew were low for the track, but I wanted a compliant ride for the trip up. Somehow I forgot to goose the pressures before my first run. Very bad understeer. Second session was a little better, if only because the tires were hotter, but the handling was still pretty bad.

After lunch I remembered that I had never corrected the street pressures for the track. So I added a couple of pounds (the tires were now cool again). And then a couple more. And then maybe one or two more for luck. I'm sure everyone can guess the result: too much pressure --> too small a contact patch --> too little suspension compliance --> even more push!

I can't prove this scientifically, but I suspect that there is a difference between underinflation understeer and overinflation understeer. The former is annoyingly mushy, but I think it helps keep speeds down. The latter is just scary, and I believe is more likely to introduce the driver to Mr. Armco.

Now that I'm slinging around the much less expensive sheet metal, I take pressures and temps much more seriously, and to much more satisfying effect.

gbaker

Not to complicate matters further, but we are talking a two-mass system. The spring rate of the tire cannot directly equate to the spring rate of the spring. Probably a good rule-of-thumb though.