Test:
Davenport Motorsports (www.davenportmotorsports.com) 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://www.flickr.com/photos/hrewheels/6757639011/http://www.flickr.com/photos/hrewheels/6757639011/ by http://www.flickr.com/people/hrewheels/, on Flickr

 

Nice analysis! I'm glad to see someone has finally done this test! Well done!

 

Interesting test and results. Thanks for posting.

Nice to verify what we often discuss in theory and to add some empirical evidence and quantify the results for this particular wheel/tire combination.

 

So hopefully this will shut people up about roatating mass! I've been saying it all along

This shows about 1hp gain to 1lb savings

Now redyno with the lightest tires you can find

 

Very nice write up! Thanks for sharing Lon!

 

Great test!

Throw some lightweight rotors on and re-dyno.

 

For the non believers
https://mbworld.org/forums/c63-amg-w...lly-users.html

 

bringing an old thread back to life !!

Rotational mass in an interesting topic not talked about much here. Dyno shows that having less weight on the rear tires shows up as extra power since it has to spend less energy spinning the lighter wheel.

Now what happens when you also lighten up the front wheels? This obv will only show up on a dyno for an awd car that is also putting power to the front wheels. In real world driving it should have just as much of an impact as the rears and the power gains here should be theoretically doubled correct? The feel of acceleration should be more than just 8hp that was shown for rear wheels only.

I recently changed my rotors to girodisc 2 piece rotors that measured almost 13 pounds lighter EACH in the front and 6 pounds lighter EACH in the rear. At the same time also switched to a lighter wheel (forgestar cf10) and the difference is far from subtle the butt dyno is telling me the car is flat out quicker and i can tell by how much faster it dumps each gear out. Also the braking advantage is obvious with lighter rotaional mass but i felt an equal gain in steering response/agility ! All that weight taken off the front tires combined with a stickier tire compound has made the car feel much more agile and i feel like i can carry more speed thru turns as the directional change is alot "crisper"

 

Indeed - I had a similar experience on my P-car after a rotor & wheel upgrade a few years back. One other thing that was immediately obvious is that the car tracked the road surface better with the lighter rotor/wheel combo. There are a couple of bumpy corners at a local track that I occasionally drive at, and while with the OEM rotor/wheel combo the car had a tendency to 'skip' over the bumps, it hooked up much better with the lighter setup. It easily shaved off a couple of seconds from my lap time.

 

What are the top 5 lightweight wheels for C 63?

 

Yep, I posted my dyno sheet years ago showing the difference between my stock CLK wheels and my 15 pound per wheel cf Dymags, the Dymags showed an increase of nearly 20whp.

Not only will light wheels net you some nice gains but light weight rotors will as well as will a CF drive shaft and lightweight pulley. Any time you can remove rotating mass from the driveline you are freeing up lost hp.

 

Interesting thread. Thanks for bumping it up.

So far I was okay with the looks of my OEM 18"s. This gives me a valid reason to "upgrade" from the my stock OEM wheels to lightweight HREs.

 

This raises a couple interesting questions about wheel size. Does an 18" wheel & tire combo weigh the same as a 20" of the same brands? If yes, the move to bigger wheel diameters may not be such a good idea.

Rotational inertia is partly related to weight. The other big factor is WHERE that weight is. Weight in close to the hub doesn't add nearly as much inertia as weight out close to the tread. A bigger wheel puts that heavy tub out closer to the tread diameter. Does an 18" wheel & tire combo have the same rotational inertia as a 20" of the same brands?

Think about the wheel sizes in formula one and on top fuel dragsters. Are these examples of intentionally NOT going to bigger wheel diameters?

Insights appreciated . . .

 

Too bad us car guys don't get same we use on our bikes. Love running our forged magnesium rims. We see big gains on dyno too. Weight is huge consideration. Now if HRE starts making forged Mag. Never mind I can imagine cost.. We drop $4k for just a pair..

 

Not sure on dyno charts, since I live on one, but hre you guys should have compared same tires! We can see over 5% just from Pirelli to Dunlops race tired same size due to weight & compound! I'd like to see HRE vs Stock same size same exact tires (and psi as well) I still think you'd be impressed but really hate dyno comparisons with different variables

 

Correct on both counts. All other things being equal, an 18" wheel will weigh less than a 20" wheel. Air - even when pressurized to 40-ish psi - still weighs less than metal.

Yes, although the wheel mass and thus rotational inertia is only about half the story. The other half is that the high-profile tires act as shock absorbers, which are absent in F1 (there is some flexing in the suspension bits and bushings, but that's pretty much it), and also that the cornering (lateral g) loads are so high that the sidewall needs to deform, again acting as a shock absorber of sorts.

There was a discussion at http://backfires.caranddriver.com/fo...-profile-tires. Keeping the brake sizes small as someone suggested is obviously not the issue, as the current size brakes are more than adequate to repeatedly stop the car without experiencing brake fade past the point where the wheels lock up.

Of course, F1 regulations specify the allowed wheel sizes so running lower-profile wheels is not an option, but I assume you meant why the regulations themselves haven't changed.

 

i second this question

 

Sorry for bringing this old topic,

But I was wondering if there is 2 set of wheels , 18 and 20, they both have same tire brand, and they weight exactly the same with tires.

Which one will have a better acceleration ? The 18 or the 20 ?

I was thinking that the 20 will be faster since the wheel will move bigger distance comparing to the 18 ?!

 

I may be wrong, but I don't believe there is any change in the circumference in the tires, at least not a significant amount. True the 18" wheel is smaller, but the tire has more side wall, than the one on a 20" wheel. My understanding was tire/wheel manufactorers tried to keep the end circumference as close to stock as possible in order to keep the vehicle's odometer accurate for the DIY's out there who can't reprogram the computer to adjust for changes...

I'd be interested to hear if this is actually correct or not.