Dyno Runs - The effects of wheel/tire weights on wheel horsepower
 

Test:
Davenport Motorsports (Custom Performance Parts and Service : Davenport Motorsports) of Canada, wanted to see the dyno effects of running different wheels on cars. They took a factory 2012 Camaro SS and ran 3 dyno runs. They ran the first run with a set of aftermarket wheels, the 2nd run with a set of stock factory wheels and the 3rd run with a set of HRE P45S wheels, all in 20” sizes.

These results highlight the effects of rotational inertia on drive-train losses (the hp lost between the engine crank and the ground). Wheels and tires contribute to drive-train losses as energy is used to spin up the wheels (and decelerate the wheels under braking). From the dyno chart you can see the effect of replacing factory wheels with lighter HRE wheels and see the negative effects of installing heavier aftermarket wheels.

Results:
1. (Blue curve) Factory wheels: 20”x9.0” with Pirelli 275/40-20 tires weighing 68 lbs combined per rear wheel. – Max hp: 371 hp, Max Torque: 375 ftlbs - (Baseline)
2. (Red curve) Aftermarket wheels: 20”x9.0” with Pirelli 275/40-20 tires weighing 72 lbs combined per rear wheel – Max hp: 369 hp, Max Torque: 373 ftlbs - (A [-] loss of 2 hp and 2 ftlbs)
3. (Green curve) HRE wheels: 20”x11.0” with Nitto 315/35-20 tires weighing 60 lbs combined per rear wheel – Max hp: 380hp, Max Torque: 384 ftlbs - (A [+] gain of 8 hp and 9 ftlbs and that is running a 2” wider wheel/tire combo)

Conclusion:
The engine obviously still cranks out the same amount of hp and torque, the lighter HREs simply waste less of it before it gets to the ground. Also interesting to note is that the gains are not just peak gains, but gains across the entire rev range. If they had done a braking test, we would have seen similar results as the rotational inertia effects also have a significant effect on how much energy is used to stop the wheel/tire combo vs. stopping the car. We talk about these effects all the time and focus on designing lightweight wheels with low rotational inertia, but it isn’t every day that you get to see real hard data showing the true effects.

http://farm8.staticflickr.com/7027/6...815e24be_b.jpg
Dyno Run - Effects of wheel/tire weights on wheel horsepower by HRE Wheels, on Flickr

Logical conclusion. It's nice to see the effort put into recording and verifying the data.

These are a little misleading. The real difference is less than shown here.

Dyno runs are generally done in a high gear, and the run from low rpm to redline takes just a few seconds, equivalent to going from (let's say) 40mph to 200mph in a high gear in that time, when in reality, it would take even pretty high-powered cars at least a minute, possibly longer. Thus, the dyno isn't presenting as much resistance to increasing the rotation rate as the mass of the car and wind resistance do in the real world. What does that have to do with wheel weight? Well, there is undeniably a difference between wheels, but the reason for that is the need to put more of the engine's energy output into the rotational kinetic energy of the wheel/tire combo. For a given wheel, the energy required to go from 40 to 200mph is what it is, but on the dyno, it has to be done in just a few seconds, requiring a higher percentage of the engine's power output to do it (remember, energy = power x time, so if you have less time, you need more power). On the street, the engine has more time to spin up the heavier wheel, so the power loss is less.

In practical terms, the difference between heavy and light wheels will be felt more in lower gears than higher gears, but will always be considerably less than what those dyno charts show.

Under braking, as long as the brakes have enough power to activate the ABS (which they do on pretty much every car out there), it's effectively a non-issue. You may have to use a slightly greater pedal pressure, and brake fade will occur a fraction sooner, but other than that, it's not a huge difference.

Think of it like this: how much kinetic is there in a 3,000 lb car, every molecule of which is traveling at 100mph, or 200lbs' worth of wheels and tires, only the very outermost part of which is rotating at 100 mph? (the translational kinetic energy of the wheels is included in the 3,000lb) If the wheel is 5lbs lighter than stock, that's a 20lb difference on a 3,000lb (or more) car.


Where it *does* make a big difference is in the ride and handling. An ideal suspension has as much of the mass of the vehicle sprung as possible. The wheel needs to move quickly and precisely to remain fully in contact with the road, and the lower the mass the springs have to move, the better. This is the reason for the slight difference in braking performance using lighter wheels - the suspension manages the weight transfer slightly more efficiently.