cgfen

several inet resources say

To Reduce Understeer Increase Front tire Pressure in 2.5 lb increments.

this comes from

http://www.rennsportsystems.com/1-e.html

This does not make much sense to me.
Why do reference materials say that raising front tire pressure decreases understeer (push)?
Understeer means the car's turning radius is greater than desired? Right?
In my experience AXing my 4S, i get less understeer when i decrease tire pressure from >32 to approximately 28.
The tires seem to get better traction and the car tracks a tight radius better.

Is it a sidewall stiffness thing, or contact patch thing or ????????

help a guy out?

cheers

Craig

Steve Weiner-Rennsport Systems

Hi Craig:

Context,....its all about context.

To answer your question,...its both. Every tire has its optimal pressure (on a given wheel) to keep the tread flat on the ground and the sidewalls from rolling under. Underinflation generates understeer due to the above phenomenon and overinflation reduces the contact patch to increase understeer.

The context here is that one really needs to optimize tire pressures by reading tire temps with a tire pyrometer and adjusting pressures and camber so the across-the-tread temps are within 20 degrees and the tread temps are inside the tire's working range. This latter item is indigenous to each tire and the manufacturer can supply those figures.

Soooooo,..borrow-buy one of those instruments and get busy looking at your tire temps (hot off the track!!) to see where you are at. Naturally, this applies to race tires as these are made to work in specific temperature ranges.

jwade944

Higher front tire pressures = higher front spring rate. This means less weight transfer on the front and more weight transfer on the rear. Since the rear tires are loaded less equally, there is less overall grip on the rear and thus there is less "push". You are correct that the contact patch on the front will be smaller and there may be a tradeoff on braking vs cornering. It's something to try, but may or may not be the best solution for your car and conditions.

SundayDriver

Both are correct. Any deviation from optimum pressure will reduce grip on that end, so going either higher or lower from ideal will work. HOWEVER, if you lower the pressure, you are also softening the spring rate which leads to less understeer and now you have two factors working against each other so you may not get the results you expect. That, of course, if based on you being at the ideal pressure for max grip on that end.

Changing spring rate does NOT change weight transfer (except for a very tiny component related to roll angle). Weight transfer is a function of wheelbase or track, CG location and g forces.

paradisenb

Could someone explain the relationship between tire pressure and spring rates?
Spring rate is compression and rebound but spring rate is the sum of more than just the springs themselves?

Matt Marks

Without getting into the minutia of spring rate vs. wheel rate (I'll leave that to the engineers), assume that you have a 500 lb spring. It's connected via the sterring knuckle or hub to the control arm or trailing arm. When the whole suspension moves, it's working with this 500 lb spring.

However, the tire is obvisouly flexible - so as the suspension compresses, the tire acts as an additional spring over and above the 500 lb spring in this example. Actual spring rate of the tire will vary according to the stiffness of the sidewall construction, and more importatly by the air pressure in the tire. Increase the air pressure and you've increased the spring rate - so effectively you've got 500 lbs of spring plus or minus the tire's spring rate at a given pressure that are hanging on the end of the main spring. For production car racing, I believe that this is largely ignored for calculation purposes as the stiffness of the tires is effectively a constant front to rear - as you are most likely using the same brand, model, and compound on both front and rear. However, in Formula cars, it is a huge factor.

Assuming you're in the correct temperature as noted above, moving the pressures provides you the ability to make fairly "gross" changes in your spring rates at each corner. Moving a couple of psi in either direction will probably let you alter the tire's spring rate a max of 50 to 100 lbs - useful - but with the caveat that the tire's spring action is undamped, which creates its own drawbacks. Watch the F1 onboard cameras as the cars pound the curbs in the chicanes, and you can see the front tires moving in a high frequence "wubba wubba wubba" fashion - completely uncontrolled.

RedlineMan

Craig;

Think of graphing tire performance; pressure on the bottom (X), and performance up the side (Y). The graph will look roughly like a pyramid. On the left side of the peak, you are underinflated; the tire overheats from flexing too much and working too hard, and has no mechanical grip (it is too floppy, or has low spring rate). On the right of the peak, it ceases to stay warm enough because it is stiffer and not flexing as much, and is too stiff to provde nuanced mechanical grip (too high a spring rate). The sweet spot is somewhere near the peak of the pyramid where proper pressure for that driver and circumstance give optimal heat in the rubber and support for the tire.

To say that you always add or always subtract air to get an effect now can be seen to be wrong. Using this graph analogy, you can see that which path you take to optimal air pressure depends on which side of the peak you are on currently.

I find this analogy helps people figure out the theory, particularly where it concerns tuning one end of the car relative to the other. Beyond the theory, they need to create their own experience scale; starting at a pressure known to be a tad low, and then adding progressively until they feel performance improve, peak, and then fall off. Now, they have their parameters, and the understanding to know where they are.

Unfortunately - but understandably - many people feel most comfortable at a pressure that is really below the tire's peak performance threshold. The tire will feel resilient and forgiving, but will actually not be up to full potential (speed) in terms of mechanical grip and temp control. Getting over that hump to where the higher pressure feels right is part driving skill (car control), and part guts (mental determination).

TheOtherEric

+100000000000. It never ceases to amaze me that people say to increase front pressure for more front grip. Like RM explained, if you're already high, more pressure will make it WORSE. Overinflate your street front tires sometime and you'll understand quickly.

Great thread. More people need to understand this issue.

SundayDriver

I'll expand a bit on the good answers already provided. What matters, in terms of spring, is the rate acting upon/through the contact patch. So you have a spring, a tire that is flexible, as well as deflection/compliance in the suspension. If you have solid bushings or heim joints, then you can pretty much ignore the latter item.

Tires deflect - if you put an additional 1000 lbs through the tire, it will deflect and move the center downwards. That is an equivalent spring rate.

A big item, that many do not think about is wheel rate, rather than spring rate. A spring that moves less than 1:1 with wheel travel, is a softer effective spring. Taken to an extreme, if I have a 1000 lb/in but it is connected horizontal to the suspension travel, it does not compress at all with suspension motion. You may have a 1000 lb/in spring rate, but you have a 0 wheel rate. Wheel rate is your spring rate times the motion ratio squared. For a coilover at a 45 degree angle, this halves the spring rate. So a lot of people that think they have huge spring rates, really have a much softer setting than they thought.

Many formula cars and sports racers use pushrod suspension that increases the travel. On my car, the motion ratios (F&R) are almost 1:1 so my spring rate is pretty close to the wheel rate.

fly2low

There is SO much to learn!!

IcemanG17

In my limited experience the general rule is that lowering pressure INCREASES grip.....raising pressure is the opposite......so increasing pressure in the front would decrease grip to the front which should make understeer worse

Of course this is within proper ranges.....way to little or too much causes other problems....so 5 psi would not have more grip than 30psi....

mark kibort

sure, if you are terribly under inflated, then this is true. but at optimal pressure for the application, lowering pressure could increase grip and increasing pressure could lower it. (as was said, to a point) so, if you are under inflated and are pushing, then increasing pressure would increase grip.

Mk

RedlineMan

Hey;

Try thinking on the pyramid analogy some more.

If you are underinflated, you will lower tire temps and raise mechanical grip by adding air. You are now approaching the peak.

If you are over inflated already, you will achieve more of the same by adding air, but the result will be LESS grip. You are now falling off the wrong side!

Thinking...

George A

I thought if you were under inflated, it would cause excessive movement of the tire carcass, thus increasing tire temps. Never mind, I reread you statement and you are correct.

Also, my understanding is that it's more like a bell curve than a pyramid. Finally, like mentioned above, I've found out that I get better grip by lowering the pressures but it's eventually negated once the tire overheats.

G.

the90

I went to the Claude Rouelle seminar a little while ago, and from what I remember the pressure distribution in the contact patch, as well as the shape of said contact patch and how these two variables changed as a function of inflation pressure, had as much if not more to do with handling then did the vertical (and lateral, and longitudinal) spring rates.