Any follow up article on how wheel/tire weight affects performance?

Jun 28, 2018 | Posted in Drivetrain | From the May 2003 issue | Never miss an article

This article comes from all the way back in 2003, so we had to march back to our archives, pick out the magazine, and dig out the scanner.

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Stampie said:Any follow up article on how wheel/tire weight affects performance?

Really dirty math using the number about the flywheels: 20lb or 54% (65% less rotating inertia) smaller flywheel made 115 lb difference on a 3100 lb car in fourth gear (wheels go road speed not engine speed). Essentially the weight benefit was between 5-6x the weight.

In 4th the wheels go engine speed (usually 4th is the 1:1 trans gear) divided by the diff ratio, but then again there are 4 wheels so if the diff ratio is about 4 it washes out. So I think you are safe in assuming that wheel and tire packages have 5-6x the weight benefit. Same for brake disks. Be careful not to multiply by 4 wheels twice however, so you would look at the difference in weight at ONE CORNER and multiply it by 5 or 6 to find the real weight benefit.

Does this above estimate check out in the real world? hmmm, if 100 lbs = .1 in the 1/4, then a 10lbs reduction in wheel/tire combo is .05, plus the 40 actual pounds lost for almost a full tenth. Seems reasonable.

EDIT: The linked test shows .4 improvement (9.0 to 8.6) in miata 0-60 times with a 12lbs per corner advantage. Which seems to be a LOT more than the 5-6x estimate.

The flywheel is such a crazy example because it goes at high rpm. Wheels don't see the kind of RPM that engines/flywheels do (I think 2000 rpm is a rough estimate for end of 1/4 wheel speeds).

Ha! When I saw the first graph I thought "Whoever wrote this uses the same graph layout that David Vizard does..."

If you work backward from the loony 20% driveline loss people talk about with chassis dynos, then getting a lightweight driveshaft that reduces that to 17% would be like adding 30hp to a 1000hp car. But only 3 hp to a 100hp Miata. Because the energy it takes to accelerate X mass to Y speed in Z time scales with engine power! Right???

**In reply to Vigo :**

To a degree, it DOES scale, because the 100hp Miata accelerates slowly enough that the rotational inertia makes little difference. Once you start really movin' (well, acceleratin'), all sorts of things get to be relevant. To use your driveshaft analogy, let's say that a Miata needs to accelerate its driveshaft from 0 to 4000rpm in 17 seconds, in a quarter mile. A car with ten times the power may need to accelerate its driveshaft from 0 to 8000rpm in 9 seconds. Twice the speed in half the time means everything has to get moving at a 4x greater rate of acceleration.

One interesting thing is that, on a decently fast car, hitting the accelerator with your thigh muscles instead of your ankle is worth a significant amount of time. This doesn't matter all that much on a 17 second Miata, but on a 6 second Pro Stock car, it's important.

Man, I wish that they would have explained math in school in THESE sorts of terms... I would have paid much closer attention. Not that I did poorly at it (usually quite the opposite), but I found it rather boring.

Funny thing is, my Supra was basically built around the concept of "lightweight all the things you can afford to, while retaining a reasonably nice place to road trip in," and it worked out really well. Had I correctly picked out the piston compression I wanted, this thing would basically be a sumo-sized Miata, as far as engine response and handling behavior are concerned.

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