Car and Driver: Wheel weight and acceleration

[trey100]

We've always understood that reducing weight at the wheels improves performance but I don't recall ever seeing instrumented testing. C&D did that as part of their GT350 article. Interesting results. To be honest I never expected the improvements to be so pronounced. I'll try to attach a snapshot of the text assuming you'll find it interesting.

Hopefully it shows up. Enjoy.

[Fundguy1]

Cool. Good info.

[08njSTEP]

Over 10 years ago I saw a company that was making CF wheels. Their top model was CF barrel w/ magnesium center. Picked 1 up and could not believe the weight. As I recall the 20x9.5 was around 15lbs. Unreal. So I then asked how much? $10k a set in 2005. My fear would be curb rash. CF really looses strength when scratched. Cool article, thanks for posting.

[trey100]

More text attached.

I'm trying to understand the numbers. It says the carbon wheel and tire combo weighs in the 40's and the aluminum wheel and tire is in the 60's. I assume that is per tire/wheel. The M3 wheels are only 22-23 lbs and the tires are probably similar so that weight is just around the 45-46 lb area which would be similar to the GT350 R carbon wheel/tire combo. Doesn't seem like the same level of savings could be had on an M3.

Edit: and check out those replacement costs!

[trey100]

Originally Posted by 08njSTEP

Over 10 years ago I saw a company that was making CF wheels. Their top model was CF barrel w/ magnesium center. Picked 1 up and could not believe the weight. As I recall the 20x9.5 was around 15lbs. Unreal. So I then asked how much? $10k a set in 2005. My fear would be curb rash. CF really looses strength when scratched. Cool article, thanks for posting.

The article says they are much stronger but does mention scratches, etc. in terms of costs it doesn't look like they went down any. $15k for all 4.

[tdott]

Originally Posted by trey100

More text attached.

I'm trying to understand the numbers. It says the carbon wheel and tire combo weighs in the 40's and the aluminum wheel and tire is in the 60's. I assume that is per tire/wheel. The M3 wheels are only 22-23 lbs and the tires are probably similar so that weight is just around the 45-46 lb area which would be similar to the GT350 R carbon wheel/tire combo. Doesn't seem like the same level of savings could be had on an M3.

Edit: and check out those replacement costs!

305s Front, 315s rear and still lighter? Go measure the weight of wide enough wheels and tires and you'll see the difference.

[Fundguy1]

Yeah avant-guard cast 19x11 with 305/30-19 are 62.5 pounds. 19x8.5 with 235/35-19 are like 50.

[trey100]

Originally Posted by tdott

305s Front, 315s rear and still lighter? Go measure the weight of wide enough wheels and tires and you'll see the difference.

Haven't looked at 315s but the F80 19s are 19x10 and are 24 lbs. Michelin PSS in 275 are 25 or so. Point being the M seen pretty optimized already.

[sygazelle]

These light wheels improve performance but at an incredibly high price. There may be cheaper ways to gain that performance difference.

Runflat tires are about 26 pounds each and conventional tires are about 22-23 pounds each. Not the kind of weight improvement carbon fiber wheels give, but a good way to reduce sprung weight and improve performance.

I can't weight (incorrect word intended) to wear out my RFT and put proper tires on my car. If I were a true enthusiast I would have done it when the car was new. I'm just too much of a cheapskate.

[DieGrüneHölle]

Originally Posted by trey100

Haven't looked at 315s but the F80 19s are 19x10 and are 24 lbs. Michelin PSS in 275 are 25 or so. Point being the M seen pretty optimized already.

A Carbon Rev 19x10 rear is around 16.5lbs. So you are looking at about 7.5lbs savings per corner(rear) on the F8x(considering you keep the same tires sizes). Compared to the 14lbs savings per rear between the GT350 and GT350R. About half of the weight savings, enough there to see a difference.

I remember an old EuropeanCar magazine Z4M build. They put on some lighter Forgline wheels and gained hp/tq on the dyno(don't remember the exact numers 10+)just from having less rotational mass.

[Vigilante375]

As they have always said: lose 100lbs of weight or 10lbs of rotating mass, shed .1 sec off in the 1/4 mile.

Not a shocker to me with this test. Just goes to show that losing weight in the most looked over area's will make a very significant difference.

[trey100]

Originally Posted by Blipit_

A Carbon Rev 19x10 rear is around 16.5lbs. So you are looking at about 7.5lbs savings per corner(rear) on the F8x(considering you keep the same tires sizes). Compared to the 14lbs savings per rear between the GT350 and GT350R. About half of the weight savings, enough there to see a difference.

I remember an old EuropeanCar magazine Z4M build. They put on some lighter Forgline wheels and gained hp/tq on the dyno(don't remember the exact numers 10+)just from having less rotational mass.

Exactly. I think that's what I was getting at - whether similar savings could be had on our cars. Probably not to the level of a carbon wheel. I switched from the RF on my 335 as well.

Either way, the numbers reinforce the impact on one key area of the vehicle. Maybe it was just me, but I always thought the improvement was nominal. Not the case.

[MKSixer]

Originally Posted by trey100

Exactly. I think that's what I was getting at - whether similar savings could be had on our cars. Probably not to the level of a carbon wheel. I switched from the RF on my 335 as well.

Either way, the numbers reinforce the impact on one key area of the vehicle. Maybe it was just me, but I always thought the improvement was nominal. Not the case.

This article only captures straight-line performance. There are also major gains in directional changes, as well. (Less rotating mass to engage in a velocity change.)

Lighter is always better in performance cars. (As long as it is safe.)

[MKSixer]

Originally Posted by sygazelle

These light wheels improve performance but at an incredibly high price. There may be cheaper ways to gain that performance difference.

Runflat tires are about 26 pounds each and conventional tires are about 22-23 pounds each. Not the kind of weight improvement carbon fiber wheels give, but a good way to reduce sprung weight and improve performance.

I can't weight (incorrect word intended) to wear out my RFT and put proper tires on my car. If I were a true enthusiast I would have done it when the car was new. I'm just too much of a cheapskate.

Agreed. (But not on the cheapskate part.)

One of the first things I did when I purchased my M6 was dump the run-flats. I went with Conti standard tires because they were so light, even in the widths that the M6 uses. My next step was to go with the PZero Corsa System. Even thought they were marginally heavier, about 2-3 lbs per tire, the compound more than offset this difference.

Cheers-mk

[Fundguy1]

I'm in the process of loosing 30 pounds of mass myself. That's 3 pounds of rotational mass lol.

[MKSixer]

Originally Posted by Fundguy1

I'm in the process of loosing 30 pounds of mass myself. That's 3 pounds of rotational mass lol.

Nothing pays dividends like Driver Mods whether they are on the skill improvement side of the equation or the mass adjustment side of the equation.

[08njSTEP]

Insert Danica Patrick in seat. That just sounds inappropriate but can't figure a better way.

[Viffermike]

Originally Posted by 08njSTEP

Insert Danica Patrick in seat. That just sounds inappropriate but can't figure a better way.

No, that's not quite the same thing. Danica would be sprung weight. Lighter wheels are unsprung weight. Reducing unsprung weight provides an exponential benefit compared to sprung weight. It's simple physics: specifically rotational inertia.

A comparison: An engine with a lighter flywheel revs quicker and spools down quicker, but at the cost of momentum and tractability (read: smoothness). Another comparison: engines with a longer stroke produce more torque (which is the converted rotational inertia of the crankshaft), but at the cost of the same two other factors: momentum and tractability. Both are exactly the same things. Ergo:

Permutation A: lighten the hub (i.e., the wheel compared to the tire), and you retain more inertia while gaining rotational acceleration. This is why tires are so often ignored when it comes to lightening: the farther in you can eliminate mass, the less of an effect the reduced weight will have on inertia.
Permutation B: lighten the rim (i.e., the tire compared to the wheel), and you gain some rotational acceleration but at a much greater cost to smoothness and inertia.

The effect works on other wheels, too. Racing bicycle wheels have been made of carbon/fiberglass/etc. for many years with the same exact benefits as goals: decrease mass (particularly near the hubs), and increase the performance possibilities of the rider -- the engine.

It's also been common knowledge among hybrid owners that the lightest possible wheel-tire combo can not only make a significant difference in acceleration and handling, but it can also save substantial fuel -- less rotational mass to have to turn to get the car moving means less fuel burned.

[FSociety]

What about going from 20s down to 19s within the BMW OEM wheels? Still a factor right

[MKSixer]

Originally Posted by Viffermike

No, that's not quite the same thing. Danica would be sprung weight. Lighter wheels are unsprung weight. Reducing unsprung weight provides an exponential benefit compared to sprung weight. It's simple physics: specifically rotational inertia.

A comparison: An engine with a lighter flywheel revs quicker and spools down quicker, but at the cost of momentum and tractability (read: smoothness). Another comparison: engines with a longer stroke produce more torque (which is the converted rotational inertia of the crankshaft), but at the cost of the same two other factors: momentum and tractability. Both are exactly the same things. Ergo:

Permutation A: lighten the hub (i.e., the wheel compared to the tire), and you retain more inertia while gaining rotational acceleration. This is why tires are so often ignored when it comes to lightening: the farther in you can eliminate mass, the less of an effect the reduced weight will have on inertia.
Permutation B: lighten the rim (i.e., the tire compared to the wheel), and you gain some rotational acceleration but at a much greater cost to smoothness and inertia.

The effect works on other wheels, too. Racing bicycle wheels have been made of carbon/fiberglass/etc. for many years with the same exact benefits as goals: decrease mass (particularly near the hubs), and increase the performance possibilities of the rider -- the engine.

It's also been common knowledge among hybrid owners that the lightest possible wheel-tire combo can not only make a significant difference in acceleration and handling, but it can also save substantial fuel -- less rotational mass to have to turn to get the car moving means less fuel burned.

Very nicely explained. Thank-you.

Emboldened: Think Lexus LFA...they have trouble maintaining piston velocity on discontinuous surfaces because of the ultralight flywheel.

Cheers-mk

[Viffermike]

Originally Posted by MKSixer

Very nicely explained. Thank-you.

Emboldened: Think Lexus LFA...they have trouble maintaining piston velocity on discontinuous surfaces because of the ultralight flywheel.

Cheers-mk

Exactly. It's also why racing motorcycles with inline fours can rev so rapidly and produce so much horsepower, yet be relatively easy to ride: a very short stroke (relative to the bore) doesn't produce a ton of torque until high in the rev range. That's why I'll always have more respect for motorcycle road racers who decide to ride a V-twin; it's a much more difficult engine to use smoothly.

[08njSTEP]

Danica as driver mod