How much does wheel weight affect performance?

[nikitopo]

Originally Posted by Twinz

For the FRS, you are comparing two different-diameter wheel/tire combos. You are changing not just the weight, but the length of the "lever".

The length of the "lever" is almost the same (rim+tire). The distribution of the weight is changing mainly. I am just trying to show you that in reality (not in a lab or by using a thought experiment) different unexpected results are happening.

Racecomp Engineering gave you another example on the same dyno. Not different dynos with different calibrations and let's not start the silly discussion about noise. A good dyno on a 200hp car can be reliable up to 1/2 hp.

[Twinz]

Originally Posted by nikitopo

The length of the "lever" is almost the same (rim+tire). The distribution of the weight is changing mainly. I am just trying to show you that in reality (not in a lab or by using a thought experiment) different unexpected results are happening.

Racecomp Engineering gave you another example on the same dyno. Not different dynos with different calibrations and let's not start the silly discussion about noise. A good dyno on a 200hp car can be reliable up to 1/2 hp.

The difference in gearing is more significant than the difference in weight. When the crank puts out effort to turn the wheel the 2700lb car is where the VAST MAJORITY of the work is coming from.

There are other factors at play too. The fact that the car gets identical 0-60 times is telling too. If I were to ignore the gearing and friction difference between the two tire sizes and compounds, I could point to the time as proof that there is no difference between the two, but there are differences. One has a gearing advantage and the other has a friction advantage.

As for dyno accuracy, my 350Z was on 4 different dynos and no two runs (not even back-to-back runs on the same dynos) were ever within 1/2hp of each other.

If I were to put your car on jack stands, blindfold you and ask you to turn two different weight wheels by hand, you might be able to tell me if one wheel weighed 5lbs more than the other. This is the simplified way most people visualize this question, and it holds true.

However if I do the same thing but put the car on the ground, you would have no idea if one wheel was heavier than the other. The effort required to move the freaking 2700lb car now dwarfs the effort required to move the wheel itself.

A lighter lever is better, yes. And where the weight is located on the lever also matters too, but when the lever is just 0.01% of the weight you are moving, making the lever 0.014% is not that big of a change in the work load.



I think the gyro effect of a spinning wheel and it's resistance to lateral changes (like when you turn the car) are far more noticeable from behind the wheel.

*edit* just checked the article for the track test and they cover the gearing issue vs top speed as well.

[reeves]

Originally Posted by Slammillionaire

Hey guys, I'm going to be in the market for some new wheels in the somewhat near future and I'm wondering how much wheel weight really affects performance. I've been looking between 17's and 18's and I honestly much prefer how the 18's look. I won't be tracking the car at all, but do enjoy some spirited driving through the country side. Some of the sets I've been looking at that are aesthetically pleasing are around 26lbs. Will I be gaining THAT much from going with a set of RPF-1's or something similar instead?

The difference is negligible in feel and performance, and since you're not tracking the car, just get a good quality rim that looks good to you. Don't skimp on the tires either, they could affect performance more than the rims.

[Ernest72]

Go lighter if you can. A friend has 16s for winters and they are much lighter than his 18s for summer and the difference is very noticeable.

[Racecomp Engineering]

Originally Posted by Twinz

This test has two samples with the same outer diameter and the results of those two pulls fall within the "noise" of dyno pulls. My 350Z spent a good amount of time on different dynos and no two back-to-back runs ever matched....even when nothing was changed on the car between pulls.

315/35/20 and 275/40/20 are both 28.7 inches in diameter, so all 3 wheel/tire combos were the same in that regard. I agree that dyno precision is a concern for a test like this, but there is a pretty consistent trend on the back to back to back runs. I do wish they did the test on a car with less power.

But anyway...reducing rotational mass has more of an effect on acceleration than total vehicle mass and that has been both measured and calculated numerous times by a variety of sources. There are some crap experiments out there, but here's a decent one:
http://www.focusst.org/forum/attachm...wheel-test.pdf

The performance differences in that test are much higher than I'd expect honestly, though not unreasonable.

The physics behind reducing rotational mass and unsprung weight also make sense.

Whether a couple of lbs is noticeable to everyone on the way to the grocery store is a different question. My winter wheel/tire set-up is about 5 lbs lighter per corner with a very slight increase in diameter. It is noticeable for me. Everyone's butt dyno is a little different. Overall diameter (gearing), tire compound, wheel strength, and driver skill may have more of a difference when looking at laptimes/feel/driverconfidence.

- Andrew

[Lynxis]

Originally Posted by Racecomp Engineering

315/35/20 and 275/40/20 are both 28.7 inches in diameter, so all 3 wheel/tire combos were the same in that regard. I agree that dyno precision is a concern for a test like this, but there is a pretty consistent trend on the back to back to back runs. I do wish they did the test on a car with less power.

But anyway...reducing rotational mass has more of an effect on acceleration than total vehicle mass and that has been both measured and calculated numerous times by a variety of sources. There are some crap experiments out there, but here's a decent one:
http://www.focusst.org/forum/attachm...wheel-test.pdf

The performance differences in that test are much higher than I'd expect honestly, though not unreasonable.

The physics behind reducing rotational mass and unsprung weight also make sense.

Whether a couple of lbs is noticeable to everyone on the way to the grocery store is a different question. My winter wheel/tire set-up is about 5 lbs lighter per corner with a very slight increase in diameter. It is noticeable for me. Everyone's butt dyno is a little different. Overall diameter (gearing), tire compound, wheel strength, and driver skill may have more of a difference when looking at laptimes/feel/driverconfidence.

- Andrew

Rolling resistance of the tires are also a factor, it's how low rolling resistance tires help the car achieve better fuel economy after all. In the end, there is no doubting that lighter wheels affect acceleration. I recall there was a guy racing an 86 on 16 inch wheels when the rest of the field was on 17 inch wheels and other drivers simply couldn't keep up with him on the power centric courses. I recall the rules were changed to define allowable wheel sizes in his series when it was apparent there was an unfair advantage.

[Twinz]

The few tests I saw from Tire Rack and Grass Roots Motorsports resulted in differences that were unlikley to be felt unless the driver was some sort of car-whisper freak of nature. I can't find the actual tests on line now (they were a while back) but when I search, I find discussions of the test where people discussed the results.
http://classicmotorsports.com/forum/...e/39037/page1/

Maybe the differences in weight were too small and or the speeds too slow. IDK.

The Ford test is by far the biggest difference I have seen in any test....the kind of results that should easily be felt by an average driver.

When I switch from my stock wheels to my 14.8lb Enkei's, I notice a slight difference in the steering but acceleration is a wash. (By the butt-dyno at least) It's 215's vs 225's so the gearing is different too.

I view wheel weight (and weight in general) as a due-diligence thing. I don't want to have wheels that are heavier than they need to be, but I'm not spending $3k on wheels made from unobtainium either.

This was in street class. Now that I'm in a spec class, the lightest allowable wheels are still cheap.

[Spuds]

My very rough estimate for an otherwise stock manual FRS/BRZ was that a 6lb loss in wheel weight per corner was equivalent to a total gain of 2.5hp in a second gear pull. Also, a 1lb loss in tire weight is equivalent to gaining 1.5hp in second gear. These are smaller as you go up in gear, and larger in 1st. Additionally, the faster you accelerate (if you have more power to begin with for example), the more difference it makes

Im sure there is a tool online you can use to get a rough estimate as well.

Im working (slowly) on putting together a better spreadsheet tool to mathematically answer the question of how does rotating part X affect acceleration. Why? Because its interesting. Of course accuracy will depend on accuracy of inputs and some assumptions I made regarding geometry (different wheels have different weight distributions). But I'll see if I can get it done in the next few weeks and post it for you.

[churchx]

Remember that not only acceleration is affected, but also braking. And suspension work/thus grip, how well/fully engine power is transferred to road. Suspension has easier work to swing lighter wheel to follow road defects.
While resulting lap time gain is slight, it's there. Now multiply it by several rounds. If there is any competitiveness in one, then few secs gained by session is not exactly no-difference at all.

[venturaII]

I went from OEM wheel/tire to 12.5lb wheel and tire weight is still within 1 pound of factory, depending on brand. It's not like it's a different car or anything, but at the same time you'd have to be pretty numb to not notice the improvement EVERYWHERE. I plan on further reducing unsprung weight with the Wilwood 4 pot Dynapro front kit when the time comes for me to replace the factory front rotors. It's less an issue of how much weight is removed, and more about where it's removed.

[Skeneypoo]

+10 lbs on the wheels affects performance more than you eating 20 half pound burgers.

[new2subaru]

Originally Posted by venturaII

I went from OEM wheel/tire to 12.5lb wheel and tire weight is still within 1 pound of factory, depending on brand. It's not like it's a different car or anything, but at the same time you'd have to be pretty numb to not notice the improvement EVERYWHERE. I plan on further reducing unsprung weight with the Wilwood 4 pot Dynapro front kit when the time comes for me to replace the factory front rotors. It's less an issue of how much weight is removed, and more about where it's removed.

Wheel and tire sizes?

[ZDan]

Originally Posted by churchx

Remember that not only acceleration is affected, but also braking.

More braking torque will be required to decelerate a wheel/tire with more rotational mass at the same rate, but more braking torque is available simply by mashing the pedal harder. So braking shouldn't be affected, at least on a relatively smooth surface.

Car and Driver tested a golf with 15, 16, 17, 18, and 19" wheels/tires a few years back. Acceleration is affected, braking not so much:
https://www.caranddriver.com/feature...d-tires-tested

Note: 18" and 19" are not directly comparable for grip as they had a lower treadwear-rating higher speed-rated tires (despite being same make/model) vs. the 15 16 17. 15/16/17 results below:

tire sizes: 195/65-15, 205/55-16, 225/45-17
wheel/tire weight: 40, 46, 48 lb.
0-60: 7.6, 7.7, 7.8 sec.
0-100: 22.3, 22.6, 22.7 sec.
1/4-mile: 15.9@88, 16.0@87, 16.0@87mph
60-0: 130 ft, 133 ft, 128 ft
skidpad: 0.83, 0.85, 0.85 g

[churchx]

ZDan: yes, to get same torque one can just push pedal a bit more. But i was speaking that just like car needs less power to accelerate, car brakes will heat up less due less energy to bleed off for same speed drop.