When adding performance modifications to your car, the obvious number one reason to do so is to make the car faster. By faster, I mean accelerate faster, stop faster, and turn faster. The modification list for many shadetree tuners almost always includes a big set of wheels, and a big brake kit to show off behind them. That’s when you have to ask yourself if the result you are actually interested in is to look cool rather than to go faster. If increased performance is the goal that a sports car build is going for, a huge factor that is often overlooked is unsprung weight.
Effectively by subtracting any weight, you are increasing the power to weight ratio. To achieve a favorable power-to-weight ratio, some tuners simply add power, which is generally speaking very easy. Others prefer the opposite, by subtracting weight, which is not as easy (and effects the balance of the car). On paper, a 3600lb car with a Horsepower to Pound ratio of 1:13 and a 2400lb car with a ratio of 1:13 should be equally as fast. In the real world, the lighter car is working with some advantages.
The much heavier car will require more tire surface area, and more braking force. By nature, this means that the heavier car will have bigger wheels, fatter tires, larger brakes… and as a result, more unsprung weight. Unsprung weight, in simple terms, is one of the most detrimental things to performance that you can add to a vehicle, besides that sweet intake vortex eBay turbo, or the under-hood open air filter that attaches to it.
So what IS unsprung weight, professor?
Well I’m glad you asked. Your vehicle and everything in it is considered sprung weight. It’s weight that is able to be controlled by the motion and geometry of the suspension. Everything on the OTHER side of the suspension is called unsprung weight (makes sense right?), and includes your wheels, tires, brakes, and spindle. Certain cars with rear axles are also considered to be unsprung. The end result is mass that cannot be controlled by the suspension, and is subject to the forces of the road independently of the car.
Example: When barreling down a bumpy road, even if your suspension is tuned perfectly and is theoretically able to react to all surfaces changes immediately. If you have a significant amount of unsprung mass, then you will still find that the tires themselves will be too heavy to change direction and effectively soak up all the bumps. The result can be moments where the tire is not touching the surface, or not able to achieve maximum grip.
Wheels, tires, and brake rotors are especially important to focus on, because not only are these unsprung, they are also rotational mass, which also absorbs torque. By having bigger wheels, you also increase the gearing of the car, requiring more torque still to achieve the same amount of Go.
Have any real life examples?
Of course. Look at any purpose built race car. You will notice they all have something noticeable in common, the wheels are usually very thin and spindly, while the tires are low profile. These wheels are often made of extremely light and strong alloys, usually aluminum, titanium, and in the highest ranks, magnesium, and they carry a price tag to match. Because they are susceptible to corrosion, and can actually explode in some cases (magnesium powder is what makes sparklers work), magnesium wheels don’t often make it onto street cars. However in top league racing applications such as Formula 1 or WRC Rally, magnesium wheels are still used to help reduce the, you guessed it, unsprung weight. There’s a reason why Newman-Haas isn’t rolling on Dayton 22’s. Koenigsegg even went to the great length of figuring out how to make wheels out of carbon fiber.
Lightweight racing wheels aren’t designed to look cool, they are designed to use as little mass as possible to keep weight down, while still staying strong. Any civil engineer that designs spans or bridges can tell you this is a complicated fight. A quick look on Tirerack.com shows a selection of wheels for the BMW E36. Sortable by weight, you can see the Kosei K4R in 17×8 comes in at 15.5lbs, while the Sport Edition A7 in 17×7.5 comes in at a whopping 27lbs! That’s an addition of nearly 50lbs in unsprung weight.
Some sources claim unsprung weight can be quantified by multiplying it with some arguable number to come up with the equivalent of sprung weight. Enkei says (allegedly) every pound of unsprung weight is equal to 20lbs of sprung weight, which sounds a little preposterous. A more conservative calculation puts it closer to 1.5. Even still, that’d be the equivalent of putting nearly 70lbs of dead weight in your trunk for your next track day, not counting the rotational mass. If you are racing a 2600lb E30, that’s an utterly game changing performance hit.
Top league racers are also fond of replacing stock components in the spindle with lightweight tubular pieces, and brakes with lightweight racing brakes. If you have ever held a racing Wilwood brake caliper next to a stock brake of the same size, you will notice a tremendous difference in weight.
Mercedes-Benz recently released a press release noting the difference in their new two-piece brake rotor design. The idea is to reduce the massive amount of weight on some of these brake rotors (we know they are heavy, we ship them), but in doing so the rotors are weaker and should not be picked up by the rotor hat. Also if they are dropped, they have a higher risk of cracking. You can read about Mercedes’s new Lightweight Rotors HERE. Although they are more of a hassle to deal with, Mercedes Benz realizes that this is a necessity to reduce the amount of unsprung weight on their performance vehicles. In doing so, they are addressing the not-so-secret performance killer in their cars. Unsprung weight.
In conclusion, and since I spent precious time not describing car parts we sell, I’ll take a tight left turn here. Enjoy a random smattering of car parts your car may need, so give them a thought and thanks for shopping!
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