Light wheels and performance gains

I was in a discussion yesterday with another forum member, and we were talking about the performance gains of a lightweight wheel/tire setup. I thought i'd share my thoughts on the matter, and open it up to discussion with everyone.

My thoughts:
(ALL ELSE BEING EQUAL) As long as wheels/tires aren't ridiculously heavy, you'll notice no real performance gain from a lightweight setup.

The truth of the matter is, you're far more likely to change *other* factors when making a switch to a lightweight wheel, and those factors will likely effect the performance more than the wheel weight. Things like offset (track width), tire width, sidewall, compound can all make a big difference.


Here's something i posted a while back:
The extra amount of power required to spin the wheel, (or stop it when braking) is really quite minimal compared to moving the car. You'd notice almost 0 improvement in 0-60 time with a lightweight wheel/tire setup (on a glassy smooth road). The big reason why lightweight wheels make a difference in lap time is because they're unsprung. (lighter brakes, for example, will have almost the same effect as lighter wheels/tires in terms of lap time improvements). Lighter unsprung components let the suspension do its job without all the inertia of pieces in contact with the road.

Now wheels are like a "best of everything" improvement because it lowers total weight of the car, unsprung weight of the car, AND rotational weight. And it's true, that wheels are the worst kind of rotating weight due to their high polar moment of inertia (compared to say, a crank pulley or drive shaft)... but rotational weight really isnt helping you anywhere near as much as the unsprung weight savings. But unsprung weight has virtually the same impact as sprung weight on a smooth road--so you're only realizing the full benefits on bumpy roads. Consequently, for something like Rally (bumpy roads, long suspension travel) it probably makes more difference than tarmac racing (smooth roads, stiff springs with little travel).

I'm not saying there's no difference in performance, just that its negligible compared to the performance gains you'd get from nearly anything else for the same money. Unless you're highly competitive and need that extra 1/10th of a second, or severly limited by mods due to classes, i'd look elsewhere for performance on a budget.

I don't really know for sure but I can only think that it has to effect straight line performance (even significantly in some cases) in that it has the same effect as a light weight flywheel, which has significant effects on performance and how the power transfers through the drive train and its effects on torque. Like a fly wheel, when you reduce the rotational mass you increase an engines ability to accelerate. A wheel would have the same effect and each light weight wheel reduces mass at least as much as one flywheel and then times that by four. I suppose once you remove the effects gear reduction, your total effect would be less noticeable though.

One way this can be tested is with controlled criteria and VirtualDyno. Take a car and take several runs on the same stretch of road and average the results. Then change the wheels and do it again and compare the results.

I'm going to have to go with Holy on this. I believe the difference is a bit more significant than you think.
I swap wheel/tires twice a year. The BBS are about 20 lbs per wheel and 23 per tire total 43 lbs. The Work XT7's are about 26 lbs per wheel and 25 lbs per tire total 51 lbs.

I shed 32 lbs of weight twice a year and notice the difference in a lot of aspects including straight line performance. That's 32 lbs total of rotational mass added and subtracted.

In the end it is my feelings or observations and a true test would be warranted. I bet I free up more power with the lighter setup.

The way I see it is like Holy said, you are freeing up power by reducing weight, rotational weight. A lightweight flywheel, carbon driveshaft. lighter rotors, etc. All do the same thing as lighter wheels.

Think of the heavier wheels as having your hand on a drills chuck while using it. It takes more power to overcome that resistance in order to spin the bit. Take your hand off and more power is free to spin the bit.

http://grassrootsmotorsports.com/articles/speed-holes/

http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

http://www.carcraft.com/projectbuild/116_9905_light_vs_heavy_wheels_comparison/

Clearly results will vary. And my thesis isn't that lightweight wheels are tires arent better than heavy wheels and tires...just that they're usually not a significant performance increase per $.

Spamby, yours is bordering on significant, but clearly that's also up for discussion. But its much easier to ADD weight vs the BBS than to subtract it.

It's too early for me to do this math, but as an exercise, how much energy does it take to spin a wheel (you'd have to make some assumptions about the moment of inertia) from rest to 13.5 rev/sec (i think thats 60 mph?) vs a 3300 lb car to 60 mph in, say, 5 seconds. (or at 5.5 m/s^2 contant acceleration)

http://www.thecartech.com/subjects/auto_eng/Road_loads.htm

I bet you'll end up with the wheel taking an hp or two, while the car will take a few hundred. I dunno, i could be wrong.

The first article was about speed around a track and the myth that plus sizing a wheel/tire combo will make your car faster. A nice read but I don’t see how this fits into the equation of rotational mass at the hubs hampering the power output. We all know donking out a car is not going to make it corner any better. I have actually noticed this phenom on some of my sport truck projects.

The second article shows about the same as the first, plus sizing in no way guarantees better/quicker car. This one went a bit further into showing data about loss in acceleration and mpg’s.

The third article provides a bit more data and it was interesting to see the results. I would hope that the reaction time of the driver, temperature, and everything with the car remained consistent. Also, the tire temperature must remain consistent throughout the test. A true scientific test is controlled and repeatable with all variables taken into consideration.

The last was nice to see a more scientific approach.

“Clearly results will vary. And my thesis isn't that lightweight wheels are tires arent better than heavy wheels and tires...just that they're usually not a significant performance increase per $.”

When you put it like that, no it really isn’t. When considering the price of lightweight wheels and the power you regain, the power per dollar figure is not very great. You may gain 10 lbft of torque with a setup but protuning for a few bills might net you 30 or 40 lbft of torque.

“ Spamby, yours is bordering on significant, but clearly that's also up for discussion. But its much easier to ADD weight vs the BBS than to subtract it.”

Agreed wholeheartedly. It is much easier to shed weight from the non BBS GR wheels to something else. Our BBS wheels are kind of a special thing in the idea that they are lightweight from the beginning and the cost per power dollar is significantly reduced when going from those to say something lighter because, as I have seen it, only a few more lbs is said to be had with comparable wheel sizes. So in this aspect the power to dollar ratio is negligible and may be better spent on a turbo upgrade with supporting mods or a new built short block.
I had always figured my power loss was about 10-15 lb ft of torque through this increase in weight. I paid about $150 per lbft of loss. LOL I should work on Wall Street.

Funny thing is that I was under the impression that the Work wheels I had purchased were only a few pounds heavier than the BBS. I pulled the trigger and when I received them I immediately weighed them and was sorely disappointed to see 6 lbs difference per wheel.

A guy with the non BBS factory wheels would have a wheel weight of about 28 lbs per. He could pick up a set of wheels for about 1200-1600 and realize an 8 lb savings per corner. 32 lbs overall. Maybe figure in $100-150 in power per dollar and he is well within the my alltime figured dollar per power.

In the end, my thoughts are that the weight of the wheel combo is a significant factor when it comes to power. Although, measured in small numbers like a few tenths of a second or a few lb ft of torque, in the overall picture it is a significant number.
We tend to get hung up on best bang for the buck and what mods make the best power increase. We tend to want the chicken dinner all the while forgoing the half a dozen or more mods that can increase power and shave lap times down. After all, lap times are published in tenths and hundredths of a second and all it takes is a few tenths or hundredths to beat your competitor or break into a new time range. Ever been at the track and watched a guy rip the interior of his car out in the pits? He was being beaten by mere tenths and the loss of a hundred or two of pounds gave him the win.

Another area that goes overlooked but is often talked about is the drivers perception of feel for the car… or the actual change of the feel for the car. As you have stated, the lightweight wheels may have more of an effect on the suspension and chassis rather than power numbers. This reason may also contribute to faster times in part due to the lighter setup.
Which leads me to this one, the psychological aspect. I wash my car and somehow the car is happier with me and now wants to be faster when I drive it. LOL. But really, a new part on a car somehow makes us believe we are doing something better and thus making the outcome positive. We either perceive small changes as great or manifest positive changes when in fact there are little to none. This isn’t an outry lie or fib but a real thing that happens to us all. Remember when you were a kid and got a new pair or Roo’s? Those Kangaroos made you jump higher and run faster, right? Lol. With all of that babble said, the human factor in place, a new found confidence may have cropped up and we push the vehicle a little harder and thus equate the better performance to the new widget we installed.

I guess i agree with everything you said. lol.

I think for ~2k you could do a turbo, springs, injectors, intercooler, etc... that would probably effect track times more. Lightweight wheels are undeniably better than heavy wheels, its just a question of how much.

All the links i mentioned tried to keep everything the same, especially tire compound. Only (maybe) the last one was the same size tire with different wheel weights. But that's exactly the point i was trying to make. Most of the time when you upgrade, you also change tires, sidewall aspect, etc... that effects the performance more than the wheel weight. And there are plenty of reasons to upgrade, especially class limitations. Just...wheels would the one of the later things id do for *performance* vs other upgrades, unless i was limited to wheels.

I had reread my comments as I was coming down with a migraine type headache when I wrote that... had to see if it was gibberish or not. LOL

Yes. Wheels would be one of the last places for me to look for power performance. Tracking may be another story.
And yes, increasing wheel/tire diameter (overall) will definitely decrease performance more so than weight. This would effectively lower gear ratios, numerically, and cause for sluggish acceleration.

I would still love it for someone to do a little experiment, on a dyno, comparing power numbers from different wheel weights.

Great thread and conversation.

I recently switched from 18x8 Work VS-XX wheels that were about 21 lbs/each to Rota Grid's in 17x8 that are about 17 lbs/each.
(Don't quote me on those weights, I can't seem to find my notes when I weighed them).

Anyway, I seem to feel a difference in accelerating and braking. At wide open throttle, it's hard to tell if any difference is made,
but when driving carefully (to not be a speed racing asshole + get better gas mileage) around town, I REALLLLLY seem to notice
the difference. Moving away from a stop sign seems to require MUCH less throttle input and it seems like I stop sooner and smoother
with the lighter wheels.

Also, it seems that I need to be a little more careful when launching now. I didn't have as much of an issue spinning the tires with
the 18's, but I can absolutely ROAST the tires with the lighter 17's.

Do you guys feel the same things...?

Did the tires stay the same? Tire compound, inflation, summer/winter rubber etc... can make a big difference on fuel economy and especially the roasting tires thing.

Oh yeah, I understand that. For the most part, the tires stayed the same. I'm sure they are
a bit lighter due to the size difference, but the same tire otherwise. On the snow tires, those
are even easier to roast, which is not good because I want them to last a long time...lol
I always run the same tire pressures on the street, with the exception of the snow tires.

I know there are a lot of people out there that throw randomly stats and numbers around
without knowing shit, but I remember reading something a few years ago that was interesting.
I've searched the web, trying to find information to support it, but to not avail. Here's basically
what was said:

For every 1 lb. of UN-SPRUNG weight you remove from the car. The handling (accel/decel/cornering)
characteristics respond as if you removed 4 lbs. of SPRUNG weight from the car.

The idea makes a bit of sense, but as to the actually numbers, I'm pretty skeptical. Especially,
because I've never heard this anywhere else.

Thoughts...?

Batmobile_Engage said:

Great thread and conversation.

I recently switched from 18x8 Work VS-XX wheels that were about 21 lbs/each to Rota Grid's in 17x8 that are about 17 lbs/each.
(Don't quote me on those weights, I can't seem to find my notes when I weighed them).

Anyway, I seem to feel a difference in accelerating and braking. At wide open throttle, it's hard to tell if any difference is made,
but when driving carefully (to not be a speed racing asshole + get better gas mileage) around town, I REALLLLLY seem to notice
the difference. Moving away from a stop sign seems to require MUCH less throttle input and it seems like I stop sooner and smoother
with the lighter wheels.

Also, it seems that I need to be a little more careful when launching now. I didn't have as much of an issue spinning the tires with
the 18's, but I can absolutely ROAST the tires with the lighter 17's.

Do you guys feel the same things...?

Click to expand...

I roast the BBS and 245's during the winter. Stop and go seem crisper. Decel seems slower, meaning it takes more time to decel upon throttle lift. Summer setup is the polar opposite. WOT I can't tell a difference. Bigger heavier wheels on, I notice the spring rates decrease and the ride is a bit more compliant.
Lighter weight wheels seem to be compared to something like lighter driveshafts; A bit crisper, smoother and just somehow feels a little better, all around.

Batmobile_Engage said:

I know there are a lot of people out there that throw randomly stats and numbers around
without knowing shit, but I remember reading something a few years ago that was interesting.
I've searched the web, trying to find information to support it, but to not avail. Here's basically
what was said:

For every 1 lb. of UN-SPRUNG weight you remove from the car. The handling (accel/decel/cornering)
characteristics respond as if you removed 4 lbs. of SPRUNG weight from the car.

The idea makes a bit of sense, but as to the actually numbers, I'm pretty skeptical. Especially,
because I've never heard this anywhere else.

Thoughts...?

Click to expand...

I have heard similar rhetoric but can't really quote anything. In the end, regardless of how much and where, any weight savings is good savings.
I am a bit of an audiophile and I have resisted adding anything to the car due to not wanting to increase weight. And the Subaru stereo is absolute garbage IMO and I cringe everyday I have to hear this Mickey Mouse crap. LOL, sorry for the small rant!

Batmobile_Engage said:

For every 1 lb. of UN-SPRUNG weight you remove from the car. The handling (accel/decel/cornering)
characteristics respond as if you removed 4 lbs. of SPRUNG weight from the car.

Click to expand...

I think that it could be close to the truth....in some circumstances. An example unsprung weight is almost entirely near the bottom of the car. So when you remove unsprung weight you usually RAISE the center of gravity. That might make your road-holding worse. For JUST a roadholding test, i might rather have a pound removed from the roof than from the brakes.

But generally, a pound off the unsprung is probably better than a pound off of the sprung stuff...im not sure I'm ready to lock that ratio at one-to-four though!

Yeah, I don't like that ratio either. Just mentioning what I've heard before.....with a grain of salt.