Flywheel - Standard Profile?

[Shep]

Hello,

Can someone help clarify the flywheel rule for me? Exactly what is meant by "standard profile"?

If you look up "profile" in the dictionary, you get definitions such as:

• A side view of an object or structure, especially of the human head.
• A representation of an object or structure seen from the side.
• an outline of an object, as a molding, formed on a vertical plane passed through the object at right angles to one of its principal horizontal dimensions.

In terms of a flywheel, is "profile" viewed normal to the crank axis, or co-planar to the clutch friction surface? Or is it any outline viewed from any major axis, e.g. parallel to the crank axis?

I think we'll all agree that a significant amount of material needs to be removed from a stock flywheel to get it down below 16lbs, and that the resulting topography of the flywheel will be highly modified in the process.

If I attempt to remove most of the weight from the outer portion of the flywheel, but leave a sliver of material of stock height at the original outer edge, such that if viewed directly from the side (normal to the crank axis) it looks like the stock piece, is the flywheel legal? (assuming a weight >= 15.5 lb).

I don't have a cross-section of a stock flywheel handy -- perhaps the basic shape of the flywheel limits my options right off the bat...

Thanks in advance,
Erik

l. Flywheel

Weight with ring gear: 15.5 lbs minimum for the original and uprated engine.
The flywheel may be machined provided the machining to reduce weight to the above minimum weight retains the standard profile. Flywheel locating dowels are permitted.

P.S. And yes, I am trading what my time to machine a flywheel is worth against a bolt-on $435 Ivey flywheel...

[TimW]

I think the intent of the rule is that the cross section of the flywheel has the same shape as the original pre-machined flywheel so weight is removed uniformly across the diameter of the flywheel (as appossed to putting all the mass nearer the center machining only the outer edges and making it paper thin toward the outside, which whould be an advantage...until it fails). I think the rule should probably state a particular minimum rotational inertia of the flywheel instead of a weight, but that would be harder to measure by tech for compliance. Not that 'standard profile' is very measureable either...

Isn't that car done yet Erik?

[Brian]

I will look at answers to this with interest, since I never considered this question. When I lightened my flywheel, I took the mass mostly from the back of the outer rim, so quite far out in the diameter. The flywheel as stock was a casting, obviously, and was factory machined with the casting quite off center. That put it remarkably out of balance, and great large holes were drilled in it to bring it back into balance. I thought about it, and couldn't see a great way to lighten it and retain a semblance of balance, so I chucked it very carefully and machined it totally - removed all of the off-center mass from the casting, removed all of the drilled bits (which were in the outer rim that I thought I should remove anyway, and finished up with a fully machined flywheel that weighed what it was supposed to and was in excellent balance without any holes drilled. I also modified it to suit a 7.25" clutch.

Next time I bought one. It was a PITA, the cast iron was dirty, I had to re-chuck it about four times, I had to drill and tap the new holes for the clutch, I had to check it for balance (at least I didn't have to start drilling to balance it, it was as close to right as I can measure), all this took about a day and a half...

Brian

[John Merriman]

Erik, the typical flywheel modification/weight reduction is achieved by milling off the three "steps" at the back of the perimeter of the flywheel and also by making the cut required for the F3 clutch installation. Thus, the weight is taken from the area farthest from the center. The wording of the rule and the use of the term "standard profile" has always been curious to me but I am quite sure that there was never any intent to mandate taking the weight off uniformly.