5 Things You Didn't Know About Wheels and Tires

Slip Angles

Tires are made of rubber, and rubber deforms as you go through a corner. Slip angles are the difference between the direction the wheel is traveling and the way the wheels are pointing. In drifting, you see huge slip angles whereas in a sport like Formula 1 you often see very small slip angles. The reason why this is important lies in the way a tire produces grip. Seen in the graph above we have data from a street tire vs. racing tire. A racing tire produces much higher peak grip but operates in a very narrow window of slip angle. The street tire on the bottom doesn’t have as steep of a ramp, meaning it is easier to get up to peak grip and the curve is much broader meaning it is much more forgiving. This is why street tires are recommended for people new to racing. 

PRO TIP: Noise is a form of feedback with tires. Racing slicks tend to make less noise so it is harder to detect where the limit is.

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Wheel Weight

Wheels are not just there to hold the tire. They are an active part of tuning your suspension. This is referred to as unsprung weight. So why is it important? The lighter the mass of your wheel and tire package, the less your shocks and springs have to work to control the tire. Think of it like weight lifting. You don’t work as hard to curl 10lbs as you do to curl 50lbs. In the past when I raced SCCA C-Stock Autocross one of the best modifications I made was  SSR Competition wheels. The 16” wheels weighed around 11lbs which were a big saving over the OEM equipment. Remember that savings are multiplied by not just one wheel, but all four!

PRO TIP: Wheels can get very expensive very quickly, so research heavily before making a purchase. Cheap wheels can be light but are often cast instead of forged. Lighter cars stress components less so you can get away with it, but a good forged wheel is worth the extra money for added strength and piece of mind.

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Wheel Width

Let’s assume we have a Mazda Miata making 200whp with 225mm racing slicks. This is a pretty common setup to see at the track. Now let’s assume we have three different sized wheels: 15x7, 15x8, and 15x9. Without changing the tire size at all, we will see net gains in lap time by going up with each size. Rubber deflects and acts like a spring. Because of this, a small amount of stretch actually benefits the tire. The reason this occurs is that it allows less deformation which keeps the contact patch flat. We are not talking about the stance nation crap that plagues the automotive industry, but a small amount will help. 

PRO TIP: There are 25.4mm to an inch. You may see a pattern emerging. If a 205mm tire fits best on an 8-inch wide wheel that means the wheel is about 203mm wide. If a 225mm tire fits best on a 9-inch wide wheel that means the wheel is about 228.6mm wide. As you go up in size it becomes harder to find custom sizes. You also find issues with clearance so there is some modification required of body work.

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Tire Size

In general, you want the widest tire you can stuff under your fenders without clearance issues. If you’ve ever looked at a 125cc racing kart you might notice how huge the rear tires are compared to the size of the actual kart, or the rear tires of a Formula Atlantic car (massive). People sometimes have the misconception that a really wide tire is somehow going to be too much weight for a smaller car to move. I’ve actually heard this criticism at local club races! It’s simply not the case. A lot of racing tires are actually very light for their size and when paired with a lightweight wheel (unsprung weight) the net gain is massive.

PRO TIP: Size matters!  

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Treadwear (TW) is not absolute

This is a common one to hear at the race track and on forums. Due to the differences in manufacturing processes and techniques, you actually can’t compare two different companies and their treadwear ratings and think it is going to tell you about which is better. The only way to know which tire works better is to try them. Furthermore, the way two tires are tested is crucial. Track temperature and even wind at certain tracks are major factors in terms of lap times. Driving in similar conditions yields the best results. 

PRO TIP: Watch the fast guys at race tracks and see what they are using. Ask them questions to get the winning answers.

>>Join the conversation about Wheels and Tires right here in the S2ki Forum!