The Importance of Performance Tires
If there’s one thing I wish I had understood earlier in my racing life, it’s the importance of performance tires.
In this post I want to focus on why performance tires make such a radical difference at the track. To fully understand this concept you need to know the following information.
- The differences between road tires and racing tires
- Optimal tire operating temperatures
- How contact patch affects traction
- Tread block squirm and how to alleviate it
So let’s jump right in.
#1. The Differences Between Road Tires And Racing Tires
I remember when I started racing in autocross. I was driving a 1997 Mazda Miata M Edition and finishing mid-pack. So, instead of taking responsibility for my losing skill level, I convinced myself that the car needed some upgrades to win.
Note: Blaming the car is a huge trap to fall in to. It’s so easy to believe that having a better car will make you a better driver. And while that’s partially true (that’s why it’s so easy to believe), a better car will only make you a slightly faster poor driver. The best way to be faster is to learn more about vehicle dynamics. The only exception to this is performance tires. But we’ll get to that.
So I made a list of upgrades that I thought would make my car faster. The car especially lousy under braking and I decided that the first thing I needed to do was spend a lot of money on Brembo pads and rotors. Unfortunately, they didn’t help at all. Looking back now it’s obvious that the car only needed two things to be more competitive:
- A more skilled driver
- Performance tires
Normal street tires, which is what I was using on the Miata at the time, have some defining characteristics.
- They are formed with a hard rubber compound. This gives the street tire a very long life span and allows for adequate traction under any normal operating condition.
- They have many built-in recesses in the tread. The strategically placed grooves help eject water away from the tire to reduce the possibility of hydroplaning and losing control of the vehicle.
- They typically have a relatively low optimal operating temperature.
Performance Tires have their own characteristics
- They are typically made from a much softer rubber compound. A softer compound is able to produce more grip, as the tire is more flexible and can conform more effectively to the driving surface. This soft rubber also makes the tire more sticky, which further increases grip. Imagine the difference between sliding the eraser end of a pencil across the table instead of the writing end.
- With less concern for driving around on the road, performance tires often have fewer recesses cut into the tread to shed water. While this may not be helpful during a rainstorm, it increases the surface area of the contact patch (we’ll talk about that in a minute).
- They also have higher operating temperatures. This allows the tires to provide maximum grip under load for longer periods before deterioration begins to set in.
Let’s get a little more specific.
#2. How Optimal Operating Temperature Affects Performance
Consider the purpose of a street tire. It’s goal is to withstand several thousands miles of use before wearing to the point of replacement. The vast majority of street tires only see highway use (as opposed to performance use), meaning that the maximum temperatures they are exposed to are minimal.
There are two main factors that cause heat in a street tire:
- The friction between the tire and driving surface
- The flexing of the tire as it continually rotates and conforms to the driving surface
During normal use, street tires heat up to temperatures usually between 70-90 degrees F. As such, their optimal temperature ranges don’t need to be much higher than that.
So what happens when you exceed that optimal temperature range? This is where it starts getting interesting.
Because street tires aren’t designed to be used in a performance application, they generally make poor racing tires. Since autocrossing can generate heat signatures between 120-150 degrees F (that’s typically what I see on with the BRZ), a street tire can not withstand these conditions for long.
When their optimal temperatures have been exceeded, street tires will begin to deteriorate quickly. This means that rubber can be used up rapidly or simply chunk out. In this condition, tires are far beyond their ability to produce any decent amount of grip. At this point the tires are just being destroyed.
Avoiding this condition is a performance tire’s specialty.
Having been designed for aggressive use, performance tires have a much higher optimal temperature range. This number can often reach as high as 200 degrees F. This characteristic allows performance tires to be used harder for longer periods without breaking down.
As a side note, the highest temperature I have recorded on my Dunlop Direzza ZII Star Spec tires is 149 degrees F. At this temperature I have produced some of my fastest times.
But, there’s always a down side to everything. . .
The soft rubber compound of a performance tire is great. It produces larger amounts of grip and can withstand hard use for longer periods.
But here’s the problem.
Soft rubber compounds are very susceptible to heat cycles. This means that each time a performance tire completes a cycle of heating up and cooling down it will become less effective.
As a tire is exposed to more and more cycles the soft compound gradually begins to harden up. As you can imagine, this results in a loss of grip over time.
The best way to avoid heat cycling your race tires is to bring them to the track on a separate set of wheels. The more you drive them on the road, the more heat cycles they experience.
Tire Rack has a great article explaining how heat cycling your tires at their initial usage can extend the life of your tires. Click HERE to read their article.
#3. Contact Patch
I know I’ve said it several times now. . .but it’s always going to be true! It doesn’t matter how tuned or powerful your car is – there are only four small places where it touches the ground.
The place where a tire contacts the driving surface is called the contact patch. This is a high-priority consideration for your race car. The goal is to create the most efficient contact patch possible. Without that you’ll be leaving a lot of traction and speed on the table.
This is another huge characteristic that differentiates street and performance tires. Look at the different tread patterns below. The one on the left is an all season street tire. The one on the right is a performance tire.
You can clearly see that the street tire has an intricate network of recesses that help shed water. The problem is that wherever there is a recess, there isn’t any rubber. And where there’s no rubber, there’s no potential for grip.
The tire on the right, a Dunlop Direzza ZII Star Spec, has many sections of continuous rubber that help maximize the effectiveness of the contact patch. When more rubber is touching the driving surface there is a much greater potential for traction.
More traction will allow a vehicle to accelerate, corner, and brake more efficiently – or faster. This highlights the importance of performance tires.
My personal notes on the Dunlop:
- Even though the Dunlop has fewer channels to evacuate water away from the contact patch, its performance on wet surfaces is surprising. While nothing will be as good as a dry surface, I’ve experienced excellent racing results in the pouring rain on sweeping corners up to 50 MPH.
- The difference between street tires and performance tires is roughly 1.5-2 seconds on a 40 second autocross course. This may not seem like much, but 2 seconds can easily be the difference between the winner and 8th place.
- The Dunlop has allowed my BRZ to corner at an impressive 1.2 Gs during a combination of braking and corner forces. You can read my review of the Dunlop Direzza ZII Star Spec by clicking HERE.
#4. Tread Squirm And What You Can Do About It
Tires are actually part of the suspension system. As various vertical loads are placed on a tire, the tire will slightly change shape. The video below is an awesome example of how tires react under various loads.
As higher loads are distributed to a tire, the sidewalls will begin to bulge outward. This causes a momentarily lower ride height on that corner of the vehicle.
This is the same effect has a spring under a higher load.
The last components that forces travel through before hitting the driving surface are the tire tread blocks. These little blocks are the raised rubber treads that are surrounded by recesses.
As lateral forces (corning forces) are applied to the vehicle, these tread blocks experience such large forces that they actually bend. Rather than being straight like we normally view them, they have no choice but to lay over under the forces acting on them.
This is called tread block squirm.
There are two problems with tread block squirm.
- When tread blocks are bent the tire is losing some of its ability to grip. Imagine wearing shoes that bend during side-to-side movements. They wouldn’t be nearly as effective as a pair of stiff high-tops.
- The bending generates friction which generates heat. Tires heat up quickly enough as it is, and the added heat can cause a tire to exceed its optimal temperature range and deteriorate.
You can probably imagine that a taller tread block is more likely to squirm. This is because their height gives them mechanical advantage to bend more easily.
What Can We Do About Tread Squirm?
Many modern tire manufactures have taken steps to alleviate squirm. BFG, Hancook, Hooiser, Kuhmo, and Toyo all produce performance tires with shallow tread depths in the 4/32nds of an inch range.
This is a pretty simple solution: make the tread blocks shorter, make it harder to bend them.
But many performance tires, like the Dunlop we talked about earlier, are still made with a tread depth of 8/32nds of a inch. Unfortunately, this allows for greater tread block squirm.
So here’s the thing:
If you want to improve the performance of a tire that has a larger tread depth, shave the tire.
Shaving tires is a lost art. The practice used to be common before radials become popular, when tires had to be shaved after they were mounted to the car. At that time, tires were made with excess rubber that was shaved off later to make them perfectly round.
But this practice still plays a large role in the motorsport world. Shaving the tread from 8/32nds to 4/32nds will have a huge effect on a vehicles ability to grip during lateral weight transfer (cornering). There are a number of other considerations that coincide with tire shaving:
- Shaving four tires can result in weight reduction of several pounds. You may not think of it initially, but removing 4/32nds of rubber across four 7 inch wide tires all the way around their circumferences will make your tires much lighter. This is an excellent side effect of increasing grip.
- Your tires can actually last longer. This is also a bit counter-intuitive since you’re removing treads, but think about it: by removing tread block squirm you are alleviating excess heat buildup from a bending rubber block. Keeping the tires cooler will help them fight deterioration.
- Shaving tires lowers your ride height. You may not see a huge difference here, but any time a vehicle is lowered the center of gravity follows suit. And where center of gravity is lower, you have the potential to minimize lateral weight transfer and therefore have better cornering.
- Tires are the best upgrade you can apply to a vehicle
- Keeping your tires inside their optimal operating temperature range will help keep them from breaking down
- A contact patch with a higher surface area of rubber will generate more grip
- Shorter tread blocks help keep tires cool and produce more traction
Man, I wish I had been privy to importance of performance tires when I first started out. But that’s why I’m posting this information here:
So you can learn from my mistakes and find success faster than I did.