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I was doing some reading on reducing unsprung mass and came across an interesting post/graph.

A shop in Canada ran a few dyno runs using wheels of different weights to measure the actualized effect of wheel weight on measured power. It's a bit of an ad for HRE wheels (which are the lightest of the set they tested), and the lack of replicate runs irks me a little, but it's an interesting rough guide nonetheless.

https://mbworld.org/forums/c63-amg-...ects-wheel-tire-weights-wheel-horsepower.html

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)
Dyno Run - Effects of wheel/tire weights on wheel horsepower by HRE Wheels, on Flickr
 

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I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
If it is a DynoJet dyno, like it says in the title, then it will make a difference. I converted a couple of DynoJet dynos, which are inertia dynos, so that they would calculate HP at the crank. It does require making a couple of measurements/approximations of the driveline inertial components. We were able to match the engine dynos very closely.

Later,
Eldon
 

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2005 Elise LSS Saffron Yellow
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I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
Exactly this. If your dyno is measuring power output directly (water brake, eddy current, electro dynamic, or even a Prony brake), then there will be no difference in measured output between wheels of different masses, because the dyno is making a steady state measurement of torque, so the rotating masses have already been accelerated to the speed of measurement.

If your dyno is an inertia dyno, you're not actually measuring torque (or power output) directly. You're measuring the time the driveline takes to accelerate a large rotating mass to some maximum speed (storing kinetic energy in the roller). Anything that changes the total angular momentum of the system will change the rate of acceleration, and thus will change the measured power. I'd call it an error in reporting if the difference wasn't entered into the calculations.

In reality, the engine's power output is the same with both wheels and both dynos. The difference is how much kinetic energy you're storing in the system in the inertia dyno.

That said, yes it's an immediate performance gain for a car you want to accelerate and decelerate quickly to have lighter wheels because there's less angular momentum to store in the wheel when accelerating, and less to dissipate when decelerating. The car will handle and ride better too, because unsprung weight is the enemy of good handling and ride quality.

It's yet another reason that Chapman was fanatical about weight.
 

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Premium Member
2005 Elise LSS Saffron Yellow
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809 Posts
I would think it would show that on an inertial dyno but not a load dyno. Not meaningless but different
Exactly this. If your dyno is measuring power output directly (water brake, eddy current, electro dynamic, or even a Prony brake), then there will be no difference in measured output between wheels of different masses, because the dyno is making a steady state measurement of torque, so the rotating masses have already been accelerated to the speed of measurement.

If your dyno is an inertia dyno, you're not actually measuring torque (or power output) directly. You're measuring the time the driveline takes to accelerate a large rotating mass to some maximum speed (storing kinetic energy in the roller). Anything that changes the total angular momentum of the system will change the rate of acceleration, and thus will change the measured power. I'd call it an error in reporting if the difference wasn't entered into the calculations.

In reality, the engine's power output is the same with both wheels and both dynos. The difference is how much kinetic energy you're storing in the system in the inertia dyno.

That said, yes it's an immediate performance gain for a car you want to accelerate and decelerate quickly to have lighter wheels because there's less angular momentum to store in the wheel when accelerating, and less to dissipate when decelerating. The car will handle and ride better too, because unsprung weight is the enemy of good handling and ride quality.

It's yet another reason that Chapman was fanatical about weight.
 
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