Wheel weight vs. acceleration test question

[gtguy]

Here's the thing: everyone scoffs at heavy wheels, and everyone seeks light, strong wheels, claiming that a decrease in wheel weight equals free horsepower, etc, etc.

Is this REALLY true? Has anyone done a test, where they've taken the same car to the track on the same day, slapped on different wheels of different weights (with same tires) then let the acceleration timeslips be the judge? No butt dynos, no "boy, I put them on and they feel great," but hard and fast numbers?

It seems to me that such a test, if one exists, would go a long way toward resolving the endless weight questions that crop up on this board. Heck, if someone wants to do it, I'd be more than happy to donate my old-school P1s to the party (Chicago-area test, of course).

Kevin

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[yonrin kudo]

Hi,

Its different from a real-world situation, but take a look at this link that was posted last month.
Wheel & tire rotational intertia

~Cheers

[Patrick Olsen]

Yes, it is REALLY true, but like you, Kevin, I think the effect is blown out of proportion by many people on this board. Every time somebody asks about or posts pictures of a new wheel, the first questions in reply are, "How much does it weigh?" and, "How strong is it?"

My street wheel/tire combo is a set of 17x7" UK WRX wheels with 215/45-17 Toyo T1-S's. The UK wheels are supposed to weigh around 17.5#, and the Toyos weigh 20.1# (according to Toyo's website), so we get roughly 37.5# together. My auto-x/track wheel/tire combo is a set of 15x8" Volk TE-37s with 225/50-15 V700 Victoracers. The Volks are supposed to weigh 9.5# each, and Kumho says the V700s weigh 23.8# each - for a total of 33.3# all together.

Not only is the race setup over 4# lighter at each corner, but I also get a 4.7% gearing advantage by using the shorter tires. Guess what? I can't feel any sort of acceleration difference with the ol' butt dyno. I think someone would have to be really in tune with their car to feel a 4 or 5# difference.

Pat Olsen
'97 Legacy 2.5GT sedan

[gtguy]

Interesting. Thanks, Pat. I figured this question needed to be asked, for precisely the reason you state. It seems to me that even before offset (an important question) everyone wants to know what a wheel weighs.

One of my favorite posters (for you cycling geeks) is of Laurent Fignon, sitting on the ground, with a crankarm/pedal combination attached to his shoe. His Campy titanium bottom bracket sheared off during a race. I've since forgotten the manufacturer that was touting their "ours doesn't break" product, but it really got me to thinking about the whole weight vs. durability equation.

I know that with a bicycle, you can really feel the rim weight, as opposed to the wheel weight. My Ksyriums don't weigh any less than my Rolfs, but the Ksyriums are much faster, because they're stiffer, and there's less weight at the rim that you have to accelerate.

At any rate, I hope this post stimulates some interesting debate, and maybe somebody will head out to the track with their buddies and a bunch of wheels.

Kevin

[rkkwan]

Originally posted by Patrick Olsen
I think someone would have to be really in tune with their car to feel a 4 or 5# difference.

Pat Olsen

Oh, I'm sure there are plenty of people on this board that do. Just yesterday, I think, someone sworn that his front strut tower bar totally eliminates body roll and understeer.

-Ray

[jblaine]

The rotational interta thread says it all if you ask me.

Also, Pat's example (a difference of 4 pounds per corner) isn't what the typical i-club rim snob is reacting to. They're reacting to people taking the stock 16" 16.5lb WRX rims off and putting on 22lb 17" rims. There's 5.5lbs right there. Now wrap the outer perimeter of those rims with a set of lead-filled Kumho Ecstas and you have the i-club special.

EDIT: If you're racing, who cares if you can feel the difference? It's there. You can't evade physics

[gtguy]

Originally posted by rkkwan


Oh, I'm sure there are plenty of people on this board that do. Just yesterday, I think, someone sworn that his front strut tower bar totally eliminates body roll and understeer.

-Ray

It DOESN'T!!?? Now what am I going to do...

Kevin

[Austin]

Originally posted by gtguy
It DOESN'T!!?? Now what am I going to do...

You need to install a set of my special 3" (76mm) sway bars to get rid of your body roll. Paypal me all you money and I'll mail you a set.

[Skyline]

Originally posted by jblaine
EDIT: If you're racing, who cares if you can feel the difference? It's there. You can't evade physics

That's fine. If you're racing, you should really only be using forged mag wheels.

Meanwhile, the rest of us who use our cars as daily drivers would prefer to get whatever wheels we can afford that are both strong enough and give us the best handling we can get on street tires.

<== On "heavy" 18" P1s and proud of it.

[WRXforME]

22lb Speedline 2116's (Chrono,SuperTurismo..) Yeah, baby,yeah...
you know what they say about fat bottom girls

Fire away

[Patrick Olsen]

Originally posted by jblaine
EDIT: If you're racing, who cares if you can feel the difference? It's there. You can't evade physics

I agree whole-heartedly, a car with heavier wheels will be slower. However, the question is how much of a difference does it really make. The point I was getting at is that if I can't even feel a difference, then it's probably something that would barely show at the dragstrip. That's just a WAG, of course.

I'd love to see some actual testing of this - identical tires on a variety of wheels. I'd also love to know how much of difference my shorter racing tires really make as far as acceleration goes.

Pat

[jmott]

you could just put a car on a dyno (the type with rollers)

[Coati]

Here's some testing.
The 17" rims are 25 pounds with Pirelli runflats. The 16" rims are, I think, 17 pounds with Azenis (I think this is skewing the braking results a bit):

Here's the results:

All testing was done in the same location, 78 degrees ambient, same direction, 3 runs each

I made the comparison even more accurate by swapping my father's 17" S-Spoke wheels on to my car instead of driving two different cars.

17's
-----
0-60mph = 7.37, 6.74, 6.85 seconds
60-0mph = 127, 125, 133 feet

16's
-----
0-60mph = 6.57, 6.67, 6.52 seconds
60-0mph = 115, 111, 113 feet

Interestingly, but not surprising, it took more power to launch the 17/run-flats. I was launching the 16's around 3200rpm to get the perfect run while it took about 4000rpm to launch the 17's. It's obvious to me that the rotational mass was the factor to get moving quicker (this is also why the first run on the 17's was significantly slower, it bogged a little at just above 3000rpm).

I ran the 17's first to insure that brake fade would not favor the 16's. Turns out the brakes cooled plenty in the tire change, although they were very hot when I changed after I was finished.

It is VERY easy for me to now see where the magazines could get higher 0-60 times. I've launched this car MANY times and it's well broken in, two factors that magazines do not have right now.

I was extremely happy to see the 111 foot braking number, that's quite impressive.

Again, take these numbers for what they are, a comparison via a G-Tech.


__________________
H Stock for Pro Solo's
D Stock for National Tours in 2002!

Here's the whole thread:
http://www.mini2.com/forum/showthrea...5&pagenumber=1

[HoRo1]

The McRaes and Schumachers of the world would notice a 4lb/wheel difference immediately. But mortals - not in a month of Sundays! A bloody WRX with an average driver and full tank of fuel weighs over 3300lbs. Right, now lets take 16lbs off that and ...wow. I'm a second a lap quicker - in your dreams!

I'm still saving for a set of GC06s though - strong and light...OOPS! Just kidding.

[nhluhr]

Originally posted by HoRo1
The McRaes and Schumachers of the world would notice a 4lb/wheel difference immediately. But mortals - not in a month of Sundays! A bloody WRX with an average driver and full tank of fuel weighs over 3300lbs. Right, now lets take 16lbs off that and ...wow. I'm a second a lap quicker - in your dreams!

I'm still saving for a set of GC06s though - strong and light...OOPS! Just kidding.

the weight savings in relation to the car's weight is insignificant, but the issue isn't accelerating the wheels in a straight line, it's accelerating them rotationally.

[nhluhr]

Originally posted by gtguy
One of my favorite posters (for you cycling geeks) is of Laurent Fignon, sitting on the ground, with a crankarm/pedal combination attached to his shoe. His Campy titanium bottom bracket sheared off during a race. I've since forgotten the manufacturer that was touting their "ours doesn't break" product, but it really got me to thinking about the whole weight vs. durability equation.

I know that with a bicycle, you can really feel the rim weight, as opposed to the wheel weight. My Ksyriums don't weigh any less than my Rolfs, but the Ksyriums are much faster, because they're stiffer, and there's less weight at the rim that you have to accelerate.

Kevin -
Yeah, Laurent was using the old campy ti BB! The big issue was that BB was manufactured before they really understood the difference between alpha and beta titanium alloys. "The Professor" learned a lesson that day!

These days, bicycles have come a long way and breaking a spindle is almost unheard of, but on the topic of wheel weights...
My mtb has a set of the Spinergy SPOX (vectran spokes - ULTRA lite) and they make a HUGE difference over a basic Mavic/Shimano/DT wheel. They also have the benefit of being MUCH stronger than the wire-spoked wheel and the spokes tend to be kind of supple, which makes them superb mtb wheels...

now my road bike has a nice set of Open Pro Ceramics with DB spokes... they feel just fine, but when I tried a pair of road SPOX on it, it was horrible, not because of the weight, but because of the lack of stiffness and the fat profile of the spoke - it's not aerodynamic, which on a bike that averages 20+mph, is bad.

The Ksyriums are truly amazing wheels and the SL's are even more spectacular with even more weight shaved off the rim. Can't beat that aero-profile on the Al spokes either! Happy pedaling!

-Nick (All Star Bike Shop, Raleigh NC)

[Senger]

You might call me crazy but I actually did feel a difference between my normal wheels (17x7.5 Rotas w/Kumho 712s) and my stock wheels/tires when I put them back on for a rallyx. I didn't feel much of a difference once the car got going but it did seem that it was easier to get going with the stock wheels. I found that the car would start rolling with less rpms/clutch slip. Also, on hills I would roll back more than I do on my 17s. It's not a night and day type of thing, but I definitely felt it. After putting my 17s back on things were back to normal so I was able to confirm that there was a difference.

[nhluhr]

Originally posted by yonrin kudo
Hi,

Its different from a real-world situation, but take a look at this link that was posted last month.
Wheel & tire rotational intertia

~Cheers

I went thru and put in 2 wheels on that spreadsheet -

a 20lb 17" wheel with a 225/45R17 tire
a 15.5lb 17" wheel with 215/45R17 tire

The torque difference needed to accelerate the car in a straight line with the lighter wheels is about 1.05 ft-lbs...

the difference needed to accelerate the wheel rotationally is between 2.29 and 4.6 ft-lbs... which is like losing 44-76 lbs from the chassis.

If you add the linear + rotational, you get a difference made by going with lighter wheels of approxkimately 3.3-5.7 ft-lbs of torque freed up.

That's not huge, but it's noticeable by those who are "in tune" with their cars... It's about the difference you might feel from an underdrive pulley or a light flywheel.

A notable additional benefit of lighter wheels/tires is the reduction in unsprung weight, which should make the suspension work better!

[Senger]

Eh...double post

[ElDuderino]

A little off topic but...

gtguy, you said time and time again that your STi suspension is better than the coilovers you used to have. How exactly did you come to this conclusion? Do you have any lap times to support your argument?

The reason I ask is because this thread just screams pot, kettle, black.

[jmott]

get back to me after physics 101

16lbs of weight saved on a wheel which must acceleration in its rotation is not the same as adding a 16lbs dumbell to the back seat.

look at th hard data up above, thats "noticeable"
if you don't FEEL the difference, you might SEE it when you lose by a couple tenths =)

Originally posted by HoRo1
The McRaes and Schumachers of the world would notice a 4lb/wheel difference immediately. But mortals - not in a month of Sundays! A bloody WRX with an average driver and full tank of fuel weighs over 3300lbs. Right, now lets take 16lbs off that and ...wow. I'm a second a lap quicker - in your dreams!

I'm still saving for a set of GC06s though - strong and light...OOPS! Just kidding.

[dj_noose]

And with a lighter wheel mass you'll get better cornering. Less rotational mass= less of a gyro effect. Just like if you take a bicycle wheel and spin it while you hold it... Food for thought

[DoinkMobb]

This is my experience with differing wheel sizes and weights:

I used to have 13" steel wheels with super crap tires before I bought a set of WRX rims with RE-92's. I think the stock WRX rims are 16.5 lbs, not sure what my stock wheels were. Anyway, increasing the wheel size by 3" and adding a few pounds at each corner made a noticeable difference in acceleration. I'll trade some straight line speed for being able to maintain traction when it's raining though.
13" to 16" would be like a WRX owner going from 16" to 19", which of course would be a big difference. I can't see how a slightly heavier 17" wheel could make much of a difference.
Also, how much do you think my speedometer is off with 16" wheels?

[Dori Dori]

I just got my wheels about 3 weeks ago. They weigh 12lbs, are single piece forged, and are 17". I wish I could say that the car felt faster in any way shape or form w/ the lighter wheels, but I cannot. The car does corner better, but I feel it's mostly b/c of the much better tires...I went from stock RE92's to S03's in 225/45/17...nuff said.

FWIW, SCC did a wheel weight/acceleration test a couple of years ago on a civic (same size wheels if I remember correctly) and there was a difference in acceleration. The lighter wheels did do better...but not a huge difference.

Also, I don't think that comparing two different sized wheels is in a weight test is fair, because even if the wheels weigh the same, theoretically, it should take less effort to spin the smaller wheel. Just my $.02

[Rich10]

Originally posted by dj_noose
And with a lighter wheel mass you'll get better cornering. Less rotational mass= less of a gyro effect. Just like if you take a bicycle wheel and spin it while you hold it... Food for thought

This effect is small compared to the effects of the narrower sidewall of a tire on a 17 inch wheel compared to that of a 16 inch wheel (assuming the same overall outside tire diameter). My experience with 17 inch wheels is that while they do take more power to accelerate, they corner better.

Rich