[stretch]

Suspension tuning is all about optimizing the grip of the tires. The best upgrade you can get for your car is the tires! They are the most important performance part of ANY car and are critical to the suspension. I am certainly not(!!!) a tire expert, but I'd like to share a few things [I think] I've learned.


An Introduction to Tires: Slip Angles

You may know the basics of how a tire works, but there's more to it than just rubber rolling along the ground. Tires are elastic and deform, and that's part of how they grip. However, the nature of how this happens is very important. (Feel free to skip this topic if you're already familiar.)

Here is a drawing showing what the tread of a tire does during a left turn:



The angle between the direction the tire is pointing and the direction the tire is moving is called the slip angle. No matter how gentle you turn, accelerate, or brake, the tire will be generating a slip angle- be it small or large. The larger the slip angle, the more grip that tire will have- to a certain point. A tire generating 100lbs of cornering force may be doing so at a one degree slip angle, while the same tire generating 200lbs of cornering force may be doing so at a two degree slip angle. The curve goes up almost linearly until it reaches a peak, and then grip tapers off beyond that. At that point, the tire is being pushed too hard and will wear and heat very quickly instead of gripping. Some tires taper off very rapidly while others taper more gradually- these tires are said to feel very "progressive" and predictable when driven at the limit.



Different tires deform different amounts and thus reach their peak amounts of grip at various slip angles. A street tire might see its peak grip at 15 degrees of slip, but a racing tire might see its peak grip at five degrees of slip or less. Wider, more aggressive tires deforms less and thus work best at smaller slip angles. Beyond this threshold, the tire can lose grip very quickly. This is why "drifting" a car is usually very slow- the tires are well beyond their optimal slip angles for grip.

This is also where we get the definitions of understeer, oversteer, and neutral steer. A car that understeers will have greater slip angles in the front than the rear. A car that oversteers will have greater slip angles in the rear, and a neutral vehicle will have similar slip angles at all four tires.

Beneath the tire, a high slip angle causes a few things to happen. The tread will try to bend so with the direction of the slip angle since it is trying to grip the asphalt your are slipping along. Look at the illustration below where the tread slips off towards the trailing edge; we want to delay this as much as possible for optimal grip. This shows tread deformation in the contact patch:


The goal here is really to have a short enough contact patch that the tread can conform along the slip angle. We want to delay that drop off near the trailing edge of the contact patch. Wider tires accomplish this with a shorter contact patch whereas racing compound tires are optimized for very low slip angles.


Coefficients of Friction and Load: The Meat and Potatoes of Suspension Tuning

Let's focus on that peak grip for a moment. This happens when the tire is at its ideal temperature, perpendicular to the ground, at it's ideal slip angle (as shown above), and many other ideals. A graph of this potential grip might look like this:



This is a graph showing the peak coefficient of friction for a tire over its range of load, and it is perhaps the absolute most critical relationship to understand when examining vehicle dynamics. While this graph might not represent the tires on your car, one rule governing all tires remains: the relationship between load on a tire and the friction (grip) it creates is not linear!

What does this mean? It means the more weight we put on a tire, the lower its coefficient of friction.

Look more closely: a tire carrying 400lbs will be able to resist 600lbs of lateral force. Since 600/400 = 1.5, this is enough for 1.5g's of cornering force! On the other hand, a tire carrying 1400lbs resists 1400lbs of lateral force- enough for only 1g of cornering force. Obviously, 1.5g's is a much more impressive figure!

We can graph this change in coefficient of friction, which is essentially the ratio of the X and Y axis from the previous chart. Here's what we get:


So, while putting more load on a tire does increase the total grip of that one tire, it does so disproportionately with the amount of vertical load on it. Thus, grip decreases relative to the vertical load on it. Knowing this, we also know the traction gained at that tire will not be greater than the traction lost from the tires that weight has been transferred from! This means:

• Peak grip exists when all four tires are evenly loaded.
• Reducing weight transfer (via a wider track or lower center of gravity) can increase the mechanical grip of your tires.
• A lighter car will have more total grip than a heavier car when on the same set of tires.

Using this graph, we can predict how much grip a car will have, on which end it has the most grip, and finally whether a car will understeer or oversteer. We just need to know how much load will be on each of the car's tires while cornering. If you haven't already, use my suspension calculator to see how weight transfers around a car:
Weight transfer, spring frequency, damper, body roll calculator... and more!



Notice how installing a larger rear sway bar increases load transfer in the rear but reduces load transfer in the front. This is why a rear sway bar creates oversteer; it helps evenly load the front tires (for optimal grip) while unevenly loading the rear tires (for less grip). However, since the STI is a front heavy car, the rear tires are less saturated to begin with, and the overall grip of the car increases. Increasing the front bar size has the opposite effect (though can still increase grip in camber-challenged cars, a separate topic), while installing firmer but equal sized front and rear bars keeps things pretty similar to stock (though again, will still greatly benefit camber challenged cars).

This rule is applicable all over the place, so I'll state it one more time for emphasis: your car will have the most total grip when your four tires are as evenly loaded as possible. This keeps your average coefficient of friction at its highest.

Even load distribution is difficult to achieve in a front-heavy car which is why mid-engine cars enjoy so much success. It's also why so few cars can even come close to stopping as quickly as a Porsche 911: since the car has most its weight in the rear, the tires become more evenly loaded under braking than its comparatively front-heavy rivals. It's not the brake pads or discs; it's the weight distribution that gives Porsches the ability to stop so short. Evenly loaded tires produce more overall grip.

Ever wonder why a stock STI understeers under trail braking? Some of it comes from the stiff bump stops up front, and still more comes from the anti-dive geometry in the suspension. However, a lot comes from the front tires being overwhelmed under braking: all the weight shifts forward on an already front-heavy car, and the tires don't like this.

If we could use a tire that does not saturate as easily, that would be very beneficial. If a tire could handle the heavy loads present when driving at the limit without a substantial drop-off in its coefficient of friction, there would be numerous performance benefits. Braking would improve, cornering, and predictability would be removed while under-and-oversteer characteristics would be minimized.

Unfortunately, tire manufacturers do not provide graphs detailing their grip versus load. So, it's left to the buyer to experiment and learn the characteristics of their tires and tune for them based on feel. However, I've found a few guidelines that seem to work and I'd like to share them. These are merely clues as to which tires will resist a drop in their coefficient of friction longer.


Guidelines for Choosing a Tire That Doesn't Saturate Easily:

1) Load rating is important.

While a tire's load rating is more a safety guideline for DOT regulations, it does give a user a vague guess of how much load that tire is meant to handle. A tire with a low load rating will tend to saturate earlier because it isn't designed to bear lots of weight. Low profile and/or reduced diameter tires can be a problem here. Larger diameter, wider tires generally have much higher load ratings.

Be careful though, changing the width and especially diameter have many other effects on the suspension, some good and others not. Going to a larger diameter tire may increase the tire's load rating substantially but also raise the car, causing increased overall weight transfer. Also keep in mind that the load rating of a tire changes with its inflation pressure, which I'll mention shortly.

2) Aggressive Tires Handle Aggressive Loads:

Not all load ratings are equal: a racing compound tire tends to have a much higher saturation point than a street tire. Racing tires work best at much lower slip angles than street tires. This means less heat generation and less propensity to overwhelm the tread.

Notice how aggressive tires have a much narrower "ideal" range of grip, whereas street tires allow a nice progressive slip. This makes aggressive tires a bit harder to control at the limit (but with higher peak potential):


I believe the ability of R-compounds to handle heavy loads without their coefficient of friction substantially dropping off is one reason autocrossers can get away with running huge front bars without understeer.


3) Shorter contact patches are better. (Get a wide tire.)

If a skinny tire and a wide tire are at the same exact tire pressure, they should have the exact same contact patch area. Think about it. If you inflate your tires to 32psi, than no matter what tire you run, there should only be 32 inches of pressure per square inch of tread. That's what psi means!

So, what's with the trend for wider tires? They change the shape of the contact patch. Tire Rack has these two images showing the difference between wide and skinny tires and their respective contact patches:



A wider tire will have a shorter contact patch, and this is important for a variety of reasons. A shorter contact patch means less lateral tread deformation, the latter of which leads to a lower slip angle as shown here:


The lower the slip angle, the longer the tread will be able to deform along that angle. This improves grip but also, in my experience, gives the tire a greater ability to handle huge loads without its coefficient of friction dropping rapidly. It's harder to saturate a wider tire.

Just remember that wider tires often necessitate a wider wheel, too. (More on that later.)


4) Higher tire pressures can help.

Higher tire pressures give you a smaller contact patch. This is both bad and good for grip at the same time. I did this experiment and found the results interesting:

I was having terrible mid-corner understeer at one autocross and decided to raise the tire pressure in my front tires. Doing so decreased mid-corner understeer but increased corner exit understeer.

You want as much rubber on the ground as possible, right? This is true up until the tire reaches its saturation point. When a tire becomes saturated, it is no longer making the most of its contact patch. Thus, you want to run tire pressures just high enough to prevent the tire from reaching its saturation point.

When I put more air in my tires, I raised their saturation point and the tires were able to grip more mid-corner when they were overloaded. However, on corner exit (when the front tires have little weight on them), saturation was not a problem so they lost grip from having a smaller contact patch.

Thus, there is no one "ideal" tire pressure. It depends on what you're doing. Drag racers run at very low pressures because they need not worry about lateral tread deformation. Higher tire pressures can fix one problem but cause another, and if you find yourself stuck with a problem perhaps you simply need wider or different tires. Your goal should be a tire that works well even at lower tire pressures.


5) Stiff sidewalls aren't always best.

This is complex. You want as large of a contact patch as possible, right? A narrow tire with stiff sidewalls will work against this goal as it reaches its saturation point and doubly so if the tire is off camber. Think about it: a stiff sidewall will prevent the contact patch from growing uniformly by resisting the necessary bend in the carcass that allow the contact patch to balloon. When this happens, the center and edges of the contact patch get a disproportionate amount of load on them, overwhelming the rubber in those areas. This causes the tire's coefficient of friction to drop.

Since we want a short contact patch, this isn't always a huge problem, however we also want weight on the contact patch to be uniformly distributed. Thus, control over the size of the contact patch is best regulated with tire pressures since air pressure is uniformly distributed. For this to happen, some sidewall flex must be allowed, and the narrower the narrower the tire, the more flex it'll need.

I made this ugly, not-to-scale graphic to illustrate why stiff sidewalls can be bad. In the wide tire, the sidewall need not deflect much so no problem exists, but in the narrow tire, it leads to an uneven distribution of load in the contact patch (illustrated by darker shades of gray):


The most recent trend in high and max performance street tires has been towards slightly softer sidewalls, and perhaps this is the reason? Stiff sidewalls make great sense when the tires are extremely wide and camber is well controlled, but with new cars being heavier and softer than their predecessors, stiff sidewalls might hurt more than they help. I believe the stiff sidewalls in the stock tires are one reason they are so tire pressure sensitive.

I don't mean to imply stiff sidewalls are bad; on a properly set up car they are very desirable. They provide immediacy and lateral stability. I don't think they are automatically better for every car, though.

If running one of the more modern tires with a softer sidewall, be warned: you will want to mount them on the widest wheels possible to prevent the sidewall from rolling over in turns. This happens when the sidewall cannot support the lateral force the tread is generating, and it flexes so much that the side of the tire touches the asphalt. If this happens, your are losing a lot of grip. You have no choice but to raise your tire pressure, change tires, or change wheels. Wider wheels act as a natural brace for tires (since lateral forces can be more easily pushed onto the outside lip of the wheel), and this adds greatly to lateral stability.


Other Considerations:

Remember that the above outlines assume the tire is able to operate at its peak grip. A tire that is not perfectly perpendicular to the ground (not off camber) will not be at its peak level of grip. Here is a graph illustrating this:



Likewise, tires must be in their ideal operating temperature. Street tires can overheat, whereas racing tires might not heat up enough. Furthermore, tires might heat up at different rates, giving much more grip to one end of the car compared with the other. Here is a graph illustrating this:



Finally, remember that tires differ greatly from one another! Remember that your other suspension modifications play a large roll in what is ideal for your car, especially things like overall vehicle weight, wheel size, suspension geometry modifications, and suspension travel. Driving style is another huge variable. This post is hardly a complete authority on tire decisions, so please, do what you think feels best on your car with you driving!


Closing thoughts:

It absolutely amazes me that some users here have "neutral" cars despite running huge front sway bars which should be completely saturating the front tires. It just goes to show that a suspension must be designed for the tires, and that a setup designed for wide, sticky rubber might feel horrible on street tires. Some tires can take very heavy loads while others cannot.

I have found that the best upgrade on a stock STI is a rear sway bar to make the car feel neutral, but legions of other autocrossers have settled into just the opposite: a huge front bar with the stock rear. I tried this and got understeer and lots of front tire wear. Even at low speeds I can feel the front slip angle being much larger than the rear, and I've got excessive front tire wear to show for it.

The "big front bar" crowd has the opposite goal of wanting the rear tires as evenly loaded as possible so that the inside rear tire doesn't lose traction on corner exit. This too is important, but means lots of load transfer at the front tires (which decreases grip). Every measure should be taken to compensate for this, including tires that can handle the inevitably huge amounts of load on the front tires.

A lot of people debate whether 245/45/17 is a better tire size than 245/40/17. It's a complex debate, for sure. The larger diameter tires have a much higher load rating, but being larger they also raise the car and cause more overall weight transfer. Is the pro worth the con? As always, there are a slew of other variables!

Like most of the stuff I write here, I don't claim to be an expert. I'm only posting here to get people thinking. Please, comment and share your experiences!


More reading (and the source for some of these images):

Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire
Car Bibles : The Wheel and Tyre Bible Page 2 of 2
Physics of Racing Series
Don Palmer's Driving Handbook
How to Make Your Car Handle - Google Book Search

[lackskill]

oh good god.

[pharo]

You are the Unabomber of iwsti.com haha

[PowerLabs]

Excellent post! Thank you for taking the time to post this.

Also, I recommend "How to make your car handle" for those looking for more in-depth information on the subject.

[Turninconcepts.com]

Hey Stretch -

I just briefly scanned through this (I'll read it more thoroughly in bed tonight) and have a couple of questions -

On G force loading - how did you come up with the 1.0 and 1.3 numbers for a given load on the tire?

On contact patch - did you account for camber loss due to compression of suspension or did you assume flat contact with max. contact patch at all times?

As for contact patch - I know that this is going to come up from folks so I make this suggestion - explain contact patch and what happens and how it's affected depending upon width, and weight on the tire.

All - granted tires are a pretty easy subject on the surface (hey, it's just a rubber doughnut) it's a very very complex subject to get into once you start to look at the details.

[stretch]

Originally Posted by Turninconcepts.com

On G force loading - how did you come up with the 1.0 and 1.3 numbers for a given load on the tire?

Whoops, the 1.3 is wrong! But since a "pound" is a unit of measure dependent on the Earth's gravity, it is easy to calculate "G's" in lbs. Creating 1000lbs of force from 500lbs of load will create 2g's of grip, whereas 500lbs of force from 500lbs of load is exactly 1g of grip. I edited the original post.

On contact patch - did you account for camber loss due to compression of suspension or did you assume flat contact with max. contact patch at all times?

Yup, this is a narrow-sighted article. I know there's a lot more to tackle, and I'm actually trying to work that stuff into my suspension calculator. (It's difficult.)

[JAYMAC10]

I find your comments on BIG FSB interesting because as you said, just about everyone on IWSTI seems to agree with the 27-29mm Front Sway and 24mm Rear Sway is the way to go!

Seeing how Prodrive just came out with their new suspension setup which only option is a Rear sway bar while keeping the Stock Front seems to confirm your findings!

Great job Stretch. I always love to read what you have to say! Even if it takes me an hour...

EDIT: I just installed 24mm rear and 24mm front sways and Prodrive Springs. Going around my usual corners I experienced Oversteer for the very first time!

[Turninconcepts.com]

Originally Posted by stretch

Yup, this is a narrow-sighted article. I know there's a lot more to tackle, and I'm actually trying to work that stuff into my suspension calculator. (It's difficult.)

Originally Posted by jaydow11

I find your comments on BIG FSB interesting because as you said, just about everyone on IWSTI seems to agree with the 27-29mm Front Sway and 24mm Rear Sway is the way to go!

This is why I brought up the contact patch size due to camber loss. To be fair I threw a very complicated wrench at Stretch with that one (if he can dodge a wrench he can dodge a ball). Keep in mind a lot of this above has to do with ideal contact patch on the ground. As we all know that's not going to happen with the front setup on these cars.

Now the question becomes what bar to use to keep the ideal contact patch on the ground. Here's the variables that will go into it that will make it very very very complicated:

ride height
suspension compression
roll couple
tire loading due to springs
tire loading due to struts
bushing flex
weight transfer if trail braking
steering angle
weight transfer if accelerating
slip angle

[ConradSTI]

oh crap my brain just melted. No this is seriously good stuff for all to consider. I guess this is why I have made the decision to have 2 front sways (27/29 & 22mm) and one 24mm adjustabe rear . I should be able to get a feel for just about any combo and find what works best for my driving needs.

[stretch]

Originally Posted by Turninconcepts.com

This is why I brought up the contact patch size due to camber loss. To be fair I threw a very complicated wrench at Stretch with that one (if he can dodge a wrench he can dodge a ball). Keep in mind a lot of this above has to do with ideal contact patch on the ground. As we all know that's not going to happen with the front setup on these cars.

Well, obviously having the tire perpendicular to the ground is important. It is increasingly important with wider tires and/or tires with stiffer sidewalls. However, quantifying its importance is as difficult as knowing a tire's peak slip angle, peak coefficient of friction for a given load, etc: tire manufacturers simply don't publish that data. It's up to the driver to know what they're feeling in the car, then weigh the importance of each.

I'm not arguing that big front sway bars are bad if it came across that way. I can't argue with other peoples' successes. I'm just showing the effects of weight transfer on tires and trying to explain how to predict what the load versus grip graph might look like for your tires (since manufacturer's don't supply that data). There are a lot of other "wrenches" you could throw at me too, and I suspect we'll get into some of them later on in this thread.

I used to race FWD and I've spun out at the very first turn of an autocross. I had the gas to the floor throughtout the turn. Why'd I spin? I couldn't heat up the rear tires whereas I put heat in the fronts from the launch. So, there are ALWAYS other variables to consider!

Nonetheless, I updated the original post again to briefly discuss camber.

Now the question becomes what bar to use to keep the ideal contact patch on the ground. Here's the variables that will go into it that will make it very very very complicated:

ride height
suspension compression
roll couple
tire loading due to springs
tire loading due to struts
bushing flex
weight transfer if trail braking
steering angle
weight transfer if accelerating
slip angle

Don't forget the importance of scrub radius, ackerman angle, polar moment of inertia, and more!

This is interesting: http://www.smithees-racetech.com.au/ackerman.html

[Turninconcepts.com]

Oh, some other things to think about in regards to contact patch. For loose traction situations, as in snow, gravel, dirt/sand, a narrower contact patch is better than wide.

Also, in regards to hydroplaning, a higher tire pressure will perform better than a low tire pressure (something to think about if it rains during an autoX). This is something that pilots having to land on wet runways learned long ago.

[Real Deal]

Originally Posted by pharo

You are the Unabomber of iwsti.com haha

i dont think even the unabomber gets this detailed and complex

thanks stretch for all the info you share with us

[GoodTimes]

Dam stretch.

[twiSTies]

It stinks like suspension in here!

[SubieNewbie]

Here he goes again. Nice thread Stretch!