Heavy Wheels Sapping Horsepower
 

A recent video from Brenspeed illustrated the loss of horsepower from heavy 20" wheels compared to 18" really caught my attention.


I think many of us are so taken by the appearance of 20's that we overlook the downside of extra rotational mass and unsprung weight.

I love the look of the 20 inchers on my '06 GT but Lordy are those suckers heavy. I'm considering a switch back to 18" and I'm starting to compare listed weights of popular wheels.

Just guessing here but I suspect the less expensive wheels in any diameter likely weigh more.

10 years ago I switched from 17" to 19" on a BMW Z4 and noticed a definite butt dyno loss in giddyup.

I'd like to hear of any experience y'all have on this issue.

I did not have time to watch the whole video, just the intro up to where he claims a 5% reduction in power.

Did he address the following points?

1. 20" wheels turn slower than 18" for the same RPM.
2. RPM is a factor in calculating HP.
3. Therefore with no changes other than shrinking the wheels, the HP will appear lower to the dyno.

I suspect even if you weighted the 18" wheels to be the same weight as the 20s, you would still see the "HP loss."

The diameter of the tires should be adjusted based on wheel size to reach the same OD. 255/45r18 vs 255/35r20 for example would be correct for 18 and 20" wheels on a mustang to achieve the same 27" OD.

I've been checking weights on wheels. I'm surprised that some retailers don't include weight in wheel specs.

Most all "reasonably" priced wheels such as we see on LMR and American Muscle sites are cast aluminum. A few of the 20"x10" wheels approach 30 lbs. each!!! In comparison LMR specs their 18'x10" Bullitt style at 23 lbs.

Forgestar's website did not spec weight but Vorschlag show a D Force 18x10 S197 wheel at 18.7 lbs. Apparently these wheels are on long back-order and it seems they are not even taking orders currently.

Looks to me like weight saving on wheels is governed more by the cast vs. forged style rather than the diameter and style of the spokes/rim.

This is a classic "do you want to show or do you want to go?" conundrum.

If the specs for my 20x9 Advanti DST Sveltos are accurate, they're about 21 lb each. I love the look of the classic Bullitt chrome, but the ones I looked at were 31 lb each, so I settled on the Advantis.


https://cimg1.ibsrv.net/gimg/themust...38a8decee3.jpg

Reduction in rotational mass will always free up more power. If you don't want to swap wheels try your driveshaft. I'm told, but have no empirical data, that you can knock of as much as 2 tenths in the 1/4 by doing this. That seems like a bit much but rotational mass reduction has a much larger effect lb. for lb. than just regular weight reduction.

07 Boss,
Actually I did replace the steel 2 piece driveshaft monstrosity with a 3.5" aluminum item. Not being a drag racer I haven't run for 1/4mi. times but the lighter shaft made a noticeable difference in "spool up time" and the car definitely revs more freely through all the gears. I anticipate a similar gain with light wheels. I suspect Brenspeed's figure of 5% loss with 20" boat anchors is likely spot on.
I intend to switch back to 18" wheels but not until I can find some light 18x10s that don't cost $600 each. The D Force wheels should do the job at around $300 each (if they ever come out of back-order hell).

Thanks for posting the video, was curious what the actual power loss would be with heavier wheels.

I thought my 275/35 Nitto nt555 g2's on 20x10 Niche wheels were heavy at 62.6lbs. My brembo wheels and pzeros weighed 54.8lbs. I felt like I could feel a difference in power when I put the 20's on. Since then I have installed a DSS aluminum driveshaft that's 20lbs lighter than stock but I didn't feel like I gained anything back. I did put it on the dyno last fall so I at least have a baseline if I put it on again this year (even though it won't be an ideal back to back run) to see if the driveshaft really helped.

:doh::doh:62 lbs is heavy indeed. I have 20" AMRs with Mickeys that probably weigh 62 or more. I'm not a young guy and even though I'm in good shape for 67 years old it is a friggin' STRUGGLE to get the rears with the 275 tires back on the hub. Any tire and wheel combo of that size is gonna be weighty but these things are nuts.
Aside from the horsepower drain of spinning all that extra weight the heavy wheels impede suspension compliance; dampers, springs, sway bars, the whole enchilada works harder.

I'm pretty happy with the size/weight combo I have right now. Out back I have 19x11 SVE S350 - 25 lbs. 305/35 PSS - 30 lbs = 55 lbs total with a pretty wide tire. My front 285/35 tires arent as good, so I'm 54 lbs total up there.

I believe the stock brembo wheels and tires are 58 lbs. 19x9 with 255/40

Heavy without a doubt but I was surprised to see the ones in the video were even heavier with a smaller tire.

That sounds about right. My Brembo wheels with the PZeros and about 30% tread left weighed in at 55.8lbs.

And for all you out there who like those big brake kits, this is something to remember too cause the massive discs I see on some cars is just adding rotational mass also. That is why I have stock replacement rotors plus I run a 15" rim back there so i couldn't put anything bigger on there. I have never needed more brake either. I can lock up the tires at any point, so getting more brake seems like a waste. If did Autox or road racing then I would have to look at them.

That's a great video! It's always good to see bona fide proof of something. I would never have suspected a full 5% power difference.

My car is 2010 Roush and has the Roush 20's on it. I weighed one a while back just because it felt heavy, and it tipped the scale at 61 lbs. I totally love the look of the 20's, but hate the harsh ride and overly-sensitive steering, so this video is really helping me make the move to 18" wheels!

Yes, but the big brake kits (like on my 2010 Roush) typically use giant rotors on front only, so this would have zero effect on dyno pull, but would certainly affect 1/4 mile ET's and ride quality. The rear rotors on my car look very close to stock.

But I totally agree about the big brake kits really only needed for hard racing. I wish mine had standard brakes.