Real World: Wheel Weight Criticalness

[Smoke20]

Has anybody increased their rim size and weight...and felt a subsequent loss in braking or acceleration?? Mathematically, I know rotational mass is a factor...but in the real world, does it matter??

I am considering 22" rims (up from 20's) and really wondering how much this should weigh in my decision.

[CoolVanilla]

We definitively showed that going from 18" to 20" on an RT at near stock power numbers made zero difference both on the dyno and in ETAs at the track. Braking... we never did a critical analysis, but my experience has said no difference.

20s to 22s... never tried it so I can't directly speak on it. Maybe we should at the next round of testing?

[Jaak]

22's weigh a lot more than 20's, but I wonder if that's made up by the tire weight that's lost?

[Hemi31]

We definitively showed that going from 18" to 20" on an RT at near stock power numbers made zero difference both on the dyno and in ETAs at the track. Braking... we never did a critical analysis, but my experience has said no difference.

20s to 22s... never tried it so I can't directly speak on it. Maybe we should at the next round of testing?

Not to argue jason,but the guys out here running the Foose ditch them for the stock wheels at the track and usually pick up quite a bit.Can't comment about other wheels,but the Foose in 20" are ridiculously heavy,especially in staggered.

[CoolVanilla]

Not to argue jason,but the guys out here running the Foose ditch them for the stock wheels at the track and usually pick up quite a bit.Can't comment about other wheels,but the Foose in 20" are ridiculously heavy,especially in staggered.

Fair enough. Here are the results I referenced: GIFO3 - Headers, 18" vs 20" & GSM Mod RESULTS

The 18"s were the stock RT rims & tires, and the 20"s were Diablo Wishbones laced with Toyo Proxes STs.

[Hemi31]

I remember those results.And I can only comment on the Foose.I had no problem with the 20" American Racing's on my R/T.

[Smoke20]

Here is the rule of thumb that I found that applies to straight line acceleration and braking...when considering wheel weight. Take the difference in weight between wheel/tire #1 and wheel/tire #2....multiply this by 4 (to account for all 4 wheels/tires)....this gives you the actual total weight difference. To account for the effect of rotational mass/inertia...take that figure and multiply by 2. This number is the equivalent weight added to the car (in theory).

So if I go from a 20" wheel/tire to a 22" wheel/tire that weighs 25 LBS more, it would be like adding 200 LBS to my car when I have the original 20" wheels on it.

I saw another rule of thumb that stated each 100 LBS costs you .10 in the quarter mile. Therefore, such a change would cost me .20 in the quarter....seems far fetched, these rules of thumb.

[Beast 2]

To really answer this you might want to find out the weight of what you have now vs. what you want to change to. It's possible to get larger custom wheels that weigh less compared to smaller stock wheels.

I have driven a truck that went from a stock set up of about 70 lbs per 17" wheel/tire and that same truck at 95 lbs per 22" wheel/tire (SRT10 immitations) and it was a HUGE difference. Both accel and braking were less responsive with the additional weight. That guy went out and bought new 22" wheels and tires (Centerlines) that are about 70 lbs and it drives like stock again.

[Kim Hufty]

it's not just the total weight but the distance the weight is from the center of the wheel. 22" wheels tend to have more weight farther from the center than a stock 18" wheel. Even if by chance your light weight 22"s weight the same as the 18"s it still takes more energy to move 1 LB that is 11" from the center of the wheel than 1 LB that is only 9" from the center.
This is a general rule and does not mean every 20" or 22" wheel will degrade performance.

[MeSteve]

Wheel weight is not the main factor, it would be rotational moment. I cannot explain the theory or formula, just know that that is the main performance factor that you are looking at. Basically as I understand it, it has to do with the weight, rotation speed, and placement of said weight. It was a major thing when I worked around jet engines. I know they turn quite a bit faster, but the physics are the same.

Anyways, just a fancy way of saying what Kim said.

[Super T]

it's not just the total weight but the distance the weight is from the center of the wheel. 22" wheels tend to have more weight farther from the center than a stock 18" wheel. Even if by chance your light weight 22"s weight the same as the 18"s it still takes more energy to move 1 LB that is 11" from the center of the wheel than 1 LB that is only 9" from the center.
This is a general rule and does not mean every 20" or 22" wheel will degrade performance.

Wheel weight is not the main factor, it would be rotational moment. I cannot explain the theory or formula, just know that that is the main performance factor that you are looking at. Basically as I understand it, it has to do with the weight, rotation speed, and placement of said weight. It was a major thing when I worked around jet engines. I know they turn quite a bit faster, but the physics are the same.

Anyways, just a fancy way of saying what Kim said.

It gets to be a slightly complicated formula, but not too bad... the formula is

J = [(pi) * radius ^ 4 ] / 2

J is the polar moment of inertia in units of length ^ 4


Since the radius is taken to the fourth power, you can see that a little difference in diameter can hurt a lot. Also, think about your standard torque formula... you car puts out xxx lb-ft of torque, which after gearing is equal to yyy lb-ft at the rear axle. Torque, as we know, is

T = F * d (force times distance)

The force is what pushes your car forward. The torque is constant. THerefore, as d gets bigger (tire radius), the net force on the ground goes down...


So, not only do you have a harder time getting them to start and stop due to inertia, but you also have less power input to the ground.

I know this degree would come in handy some day

[Smoke20]

...I smell an Engineer (BSME?)...and this is an even fancier way of saying it.

[MeSteve]

I knew someone would know the specifics. Just out of curiosity, was I very far off?

[hawkxp]

Dont forget, your overcoming both static and dynamic inertia...

On a dyno, you never overcome (or shouldnt) static friction, on the road you may, but dont...

If yout too light, and the static friction (tire to road friction) is too low you will spin the wheels... hence making the tired warm and sticky...

Too heavy, too sticky, to much friction and it slows you down..

All that said, when your doing a "single gear" acceleration from 2000RPM (at the engine, and I have no idea what RPM the tires are at at 2000RPM) to 5000 rpm for a dyno pull, the static friction is never overcome, and the dynamic inertia is no where near as hard to move from 1000 to 2500 (again no idea what the rpm) as it is from 0 to 1000

Meaning, I would not expect wheel size/weight to make ANY difference in a dyno run, and most of the difference in the 64' mark of a 1/4 run

[Super T]

...I smell an Engineer (BSME?)...and this is an even fancier way of saying it.

Yeah, sorry. Bradley University '05

I knew someone would know the specifics. Just out of curiosity, was I very far off?

No, you pretty much had it.

Dont forget, your overcoming both static and dynamic inertia...

On a dyno, you never overcome (or shouldnt) static friction, on the road you may, but dont...

If yout too light, and the static friction (tire to road friction) is too low you will spin the wheels... hence making the tired warm and sticky...

Too heavy, too sticky, to much friction and it slows you down..

All that said, when your doing a "single gear" acceleration from 2000RPM (at the engine, and I have no idea what RPM the tires are at at 2000RPM) to 5000 rpm for a dyno pull, the static friction is never overcome, and the dynamic inertia is no where near as hard to move from 1000 to 2500 (again no idea what the rpm) as it is from 0 to 1000

Meaning, I would not expect wheel size/weight to make ANY difference in a dyno run, and most of the difference in the 64' mark of a 1/4 run

Static friction and rolling resistance are two different things. Rolling resistance (what you describe as too much friction slowing you down) is due to the energy you lose in the constant sidewall deflection and scrubbing on the microscopic level... the tire is constantly changing shape so it has an ever-so-small flat spot on the bottom which causes the sidewalls to bend and also causes the tread surface to "pucker up" were you to have it under a microscope.

Static friction (no slipping) is greater than kinetic friction (slipping). Once you bust 'em loose you continue to waste power until they catch again. There is no such thing as too much static friction. Now, if they were actually sticky, that would cause losses due to having to peel it off of the pavement.

You also get hit by bearing friction... like all friction, it's greater when the thing's not spinning than it is once you get it moving.

Wind resistance is a function of the square of the velocity... so go twice as fast, get 4x the resistance. This is part of the reason gas mileage suffers at higher speeds in cars as boxy as ours.

Finally, the inertia of the wheel is a function of its geometry. It doesn't lessen with speed. It's polar momentum does, bur raising it 1,000 rpm is raising it 1,000 rpm whether it's the first 1,000 or the last 1,000.

Bottom line, if you spin your tires, traction is limiting you, not weight or inertia.


**phew** This is what happens when I get stuck at work for 32 hours straight!

(<---- looks like a fun ride)