Tires are just about the biggest limiting factor in any form of racing. The size, style, and treadwear rating all limit how much tire is on the ground and how much it’s sticking to the ground. We try to put larger wheels on there to allow for a larger tire and more surface area touching the road to increase traction.
We expect a lot from a set of wheels. They are expected to last for thousands of extremely hard miles on the track, numerous sets of tires, stay true after a flat, and most importantly – look good while doing it. We caught up with Forgeline President David Schardt who helped explained everything that goes into a set of wheels and tires.
“There are a lot of factors that usually go unseen from the casual racer. These factors effect how we produce a wheel and ensure that the wheel performs flawlessly,” explained Schardt. These factors include what the intended usage is, weight of the vehicle, how much downforce the vehicle is seeing while at speed, the type of tire that’s mounted on the wheel, and what the wheel itself is made out of. “When selecting rims and tires, most people just look at the weight rating from a static standpoint of sitting in their garage,” explained Schardt. But there’s much more to a set of wheels than meets the eye.
Forging Wheels With Forgeline
To dive a bit deeper into the making of a wheel, Forgeline has to know what kind of materials to choose for the wheel. This material has to have an acceptable level of strength to withstand forces it will or might see. The material also has to have a specific gravity, meaning a light material like carbon fiber or a heavy material such as steel. On top of that, they look at elongation, which is how much the material would stretch before see failure. Forgeline also looks at fatigue resistance where the wheel must hold up under extreme stress time and time again. Finally, they look at more obvious factors such as cost and how hard it is to manufacture. When looking at all these different factors, different materials perform better and worse in each category. The overall winner is 6061 forged aluminum, which is strong, flexible, light and cost effective.
Once the material has been chosen, the design of the wheel is taken into consideration to handle even more factors. “One of the biggest stresses to your wheels is your tires,” explained Schardt. Tires cause friction with the road surface, which causes stress on the wheels. “With today’s modern tires, there’s a friction coefficent present anywhere from 0.7g to 1.2g, depending on how sticky the tire is, or what the tread wear rating states,” continued Schardt. To put that simply, the stickier the tire, the more stress that wheel is going to see.
When choosing a tire for your car, there are a lot of different things to consider. Width, diameter, price, availability and treadwear rating are some of the biggest concerns when looking to purchase. One specification that most don’t think about on their performance vehicle is load rating. On a trailer or your tow rig this is something that you’d consider, but why is it important on your performance ride? We all know the lower the treadwear rating, the softer the tire, and the more traction we get. But that can have catastrophic consequences if you’re not careful.
To put this into perspective, say your car weighs 3,000 pounds and each tire can handle 1,000 pounds, meaning all four tires can handle 4,000 pounds. That’s more than enough to carry the weight of the vehicle right? “No that’s not always the case. When you have a softer, stickier tire that grips the road that means the forces put on that tire going around a corner are going to increase, which means the force that the wheel sees also increases,” explained Schardt. What this boils down to, is the stickier the tire, the lower the weight rating is on your wheels.
Putting Down The Force
When you add aero modifications to your car to increase downforce, that will add even more force pushing down on the tires and wheels,” said David. You can see how quickly you could over load your tires and wheels, causing a catastrophic failure.
If you don’t think downforce is something to consider, look at Top Fuel dragsters. The wings hanging off the back of these rocket ships can produce between 4,000 to 8,000 pounds of downforce! Imagine if your car had the same wings and saw 300+MPH that’s equivalent another car sitting on the trunk lid – that’s why you need to remember about the weight capacity of tires and wheels. Now this is an extreme example, but is still something to consider especially when pushing 150+ MPH on a road course.
Downforce in a numerical form is hard to calculate since it’s only seen when the car is at speed. There is equipment out there to measure this but it usually isn’t terribly accurate, and with shifts in wind, speed, and air density the amount of downforce will constantly change.
Now there are some pretty fancy calculations that you can do to ensure that your tires and wheels are still going to be safe and perform with the additional traction and downforce the car is seeing.
Head Scratchin’ Math
When Forgeline creates a wheel, they use the following calculation to ensure the wheel performs: S=L[(R*u)+d]/12. In this equation, “S” represents the number of foot pounds of side load on the actual axle. “L” is the total load the wheel sees, including static weight and downforce. “R” is the radius of the tire in inches. “u” is the coefficient of the tire or the G-forces the car is showing, and finally “d” is the offset of the wheels in inches. Your head is probably spinning with the variations and trying to comprehend this. Don’t worry though, Forgeline has already taken this into consideration when building your wheel.
When you choose your tire, there is a little bit of thought that needs to happen on your end to help prevent against a wheel or tire failure. In a worse case scenario of the stickiest tires out there, race tires, the wheel load rating drops in half. “If our wheel was used on a car that weighed roughly 3,000 lbs and had a perfect 50/50 weight distribution with normal DOT tires, the weight rating of 1,300 lbs. for the wheel would be more than enough. But if you installed a racing slick where the capable G-force goes way up in the previous equation, the weight rating for that wheel would only be 663 lbs, meaning they could see failure,” explained Schardt.
For most of us, we are running a 200 tread wear tire, or close to it. In this calculation, that 1300 pound weight rating would drop down to 956 pounds because of the added G-force of those tires alone. If your car is seeing 100 pounds of downforce, then you’d need to subtract that down to only 856 pounds. You can see how with a race tire and some basic aerodynamic mods, you could easily overload the wheels or tires.
Whenever you’re buying a set of wheels, any reputable manufacture will have a load rating on the wheel. This rating is for normal street driving, with normal street rated tires, ones with approximately 550 or higher tread wear rating. In the case of Forgeline, they make it easy by applying a decal with this rating, along with the ratings for other tire types right onto the wheel itself. With this information and knowing the weight of your car and it’s intended usage, you’ll have a wheel that will perform flawlessly for years to come.