wolverine

Being an engineer, I've been doing some acceleration testing on my 545 6sp. I've been running a 2-way test on the same strip of road, first with my BBS 19" RS-GT wheels and Goodyear F1 GS-D3 tires, then with my OEM 18" wheels and Michelin Pilot A/S tires. First, the rolling diameters of the BBS setup is 25.496 in, and the OEM setup is 25.579 - a .3% difference. I've been running from 2000-5000 rpm (35-86mph) in 3rd gear, and timing with a stopwatch. I've tracked temperature, tire pressures, barometric pressures, wind and car weight to make sure I'm comparing apples to apples. My results suprised me. The times were slightly different depending on test direction.

19" BBS setup times:

8.58 seconds SD of .04 one way
9.05 sd of .05 other way

18" OEM setup times

8.85 seconds SD of .06 one way
9.36 seconds SD of .05 other way

The BBS tire/wheel setup is 10 lbs less than the OEM setup, for a total of 40 lbs. (Actually, the runflats are 4 lbs heavier than the Michelins).

.3 seconds is one heck of a difference in times, but the car definitely feels more sluggish with the OEM setup. Does this make sense with what anyone else has experienced? Am I forgetting something?

Jimbo

Driver?

wolverine

I was pretty careful to eliminate as much driver input as I could. 3rd gear, at 1500 rpm floor it. Click stopwatch on at 2000. Keep pedal floored. Click stopwatch off when you hit 5000 rpm. Brake, repeat. Other than watching for the occasional car or truck, not much for the driver to do.

robg

The Goodyears are stickier than the Michelins, and will put the power down bettwer. But, starting in 3rd, should this make this much of a difference?

wolverine

Don't think so - traction was not a problem with either tire.

300TTto545

It would be interesting if you could get the same results blinded (have some one else change the tires and don't look).

How many runs did you make?

Are Michelin Pilot A/S runflats and that is the weight diff?

Hormazd

Wolverine:

I think your exercise has answered its own questions. It looks to me like we have two issues here:
1) Wheel and tire weight.
2) Wheel Diameter.

The weight should make a very small difference, probaly not noticeable at all. 40lbs is nothing. Like having a suitcase in the car.

Now the diameter... the extra inch is giving you an extra 3 inches in circumferance. I would think 3 inches on each revolution of the wheel would make a big difference. This would not only get you going faster, but might also provide inaccurate speedometer readings. Does this make sense? I am not the engineer!

Hormazd

realtyman

I'm not entirely sure what his circumference differential is but it will affect the overall final drive ratio. More circumference should make it slower but more top end (if not electronically limited).
I do know that "unsprung" weight makes a huge difference in handeling but I'm not sure about accelleration.

Aaron

I think the weight of the wheel applies to how much centifugal force you must apply to get it going. If you look strictly at tire/wheels size, the amout of rotation at the axis will equal the distance each wheel rolls. So, if you the wheel has a circumference of 56 inches, and the axis moves at an angular disposition to create 56 inches, then you will get one wheel turn. If a different wheel has a 44 inch circumference with the same angular disposition at the axis, then you will get approximately 1.27 turns, but your DISTANCE will be the same 56 inches and rate = Distance/time, so the rate or speed will be the same, so size alone should not affect speed, but the force required to rotate a heavier wheel does matter, I believe, in acceleration because now you are trying to create a change in velocity over time and that requires you to rotate a heavier mass just as fast as say a lighter mass.

I am rambling, but think about a big wheel. If my wheel is 20" made of plastic with a rubber covering and yours is 18" made of cement with a rubber covering, during take off or acceleration, you have more weight to pull than I do(even though your wheel is smaller), and therefore you must work harder to get to the finish line, but the distance is the same, regardless of wheel size and weight.

Also, I have been taking some robust engineering classes, and I don't believe your measurement system can be that accurate. The fact that you are driving and timing alone make me think this is not that repeatable. Your thump speed, hand eye delay, etc., creates their own tolerances within this measurement.

Sorry for the rambling post...

price9195

Hormazd, the 40 lbs rotating mass is not like carrying a 40 pound suitcase. There was an excellent article recently in one of the mags (I'll try to find a link) discussing the dangers of oversized wheels in relation to braking. The angular momentum changes in the rotating wheels are tremendous with slight changes in weight. Understand this article was in relation to braking forces, but the analogy to acceleration is obvious. As realtyman says the unsprung weight plays a major role in handling but also in acceleration and braking.