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Old 09-02-2004, 06:46 AM
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Flywheel Inertia

In another thread, concern was expressed over the effect of wheel and tire assembly rotating inertia on dragstrip performance. In that thread, it was correctly noted that small improvements will result from reductions in wheel inertia.

I was afraid that those reading that thread would conclude that reductions in flywheel inertia would yield equally small results. This thread should indicate that this might not be the case.

When engineers calculate acceleration performance, they convert, mathematically, rotating inertias to "equivalent weights." They then sum these equivalent weights and add the sum to the static weight of the car. With this little trick, they can then treat the problem as one involving only linear acceleration of a single mass. In other words, it makes their job a little easier.

The value of the equivalent weight depends on the rotating inertia of the item under consideration AND its position in the drivetrain. A wheel and tire assembly and a clutch and flywheel assembly are sitting at opposite ends of the drivetrain. The rotational acceleration of the wheel is much less than that of the engine flywheel. Also, the torque available for accelerating the wheel is much greater...due to the ring and pinion and transmission gearing...than that available to accelerate the engine flywheel. So, while the rotational inertia of the wheel and tire assembly might be about 4 times greater than that of the clutch and flywheel assembly, the equivalent weight, in first gear, might be 1/20th as great! The equivalent weight of a steel wheel and tire assembly might be 25 pounds and the equivalent weight of the clutch and flywheel assembly, in first gear, might be 500 pounds.

Now, when we start adding 500 pounds to the static weight before calculating acceleration, it's obvious that some performance can be gained if we can reduce that number. Since most of that inertia is in the flywheel, a change to aluminum is an obvious benefit. Also reduce the inertia of the clutch and that 500 figure should be pared down to 200 or less. Some clutch and flywheel sources claim an 80% reduction in inertia. Unfortunately, I haven't been able to get any "solid" numbers from them for comparison.

It should be evident that, while it might not be worthwhile to devote much time and/or money to reduce wheel and tire inertias (beyond the obvious switch to aluminum wheels), there are obvious benefits in reducing the inertia of the clutch and flywheel assembly.

If you can get any numbers with which to work, the equivalent weight value is equal to the rotating inertia (pounds mass inches squared) times the square of the ratio of total gearing (axle ratio times transmission ratio) to rear tire radius. (For wheel and tire assemblies, the value of total gearing would be "1.")

In case you're wondering, no, I don't work for any of the clutch or flywheel manufacturers. (Wish I did. This living on Social Security and a pension is no fun.)

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Old 09-02-2004, 07:53 AM
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Absolutely correct. The faster they spin, the more important inertial effects become.
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Old 09-02-2004, 04:08 PM
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Re: Flywheel Inertia

Quote:
Originally posted by BillyShope

It should be evident that, while it might not be worthwhile to devote much time and/or money to reduce wheel and tire inertias (beyond the obvious switch to aluminum wheels), there are obvious benefits in reducing the inertia of the clutch and flywheel assembly.

If you can get any numbers with which to work, the equivalent weight value is equal to the rotating inertia (pounds mass inches squared) times the square of the ratio of total gearing (axle ratio times transmission ratio) to rear tire radius.
Try downloading the Tilton catalog, loads of good info...
afterall, they are the pre-eminant racing clutch manufacturers...

http://www.tiltonracing.com/

SteveL
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Old 09-02-2004, 06:44 PM
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Thanks, Steve, for the lead on Tilton, but, unless I missed it, they still don't give inertia values. Since I don't have an immediate application, I didn't download a catalog, but I figured, if they had inertia values, they'd give them in the information at their site. I could be wrong, though, and would urge anyone with "a need to know" to either download the catalogs or contact Tilton by phone or email.
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