Real Wheel Compatibility?

[Wheelman-111]

Greetings:

On the way to lunch through the parking garage, I decided to have a close look and really check the Michelin 3.50 S-1's clearance astern. It's always dark when I'm working on Flashes. This is a 2001 SA50 Elite SR AF16E block.

Now that I think about it, I have been noticing a sort of low rumble at higher speeds (>45), like harmonic vibration from meshing gears. I'm still not certain what it is, but here's one possibility:

The triangular point of light inside the upper exhaust mounting lug reflects the actual space between the tire tread and the lug. It's less than 1/8 of an inch. I tried to see if there is rubber skid on the case but the light and angle didn't really permit. The tire doesn't have any obvious scuffing, but a hint of darker shading to the right of the contact patch, and the dusty edge is very sharply cut off on that side, where it sort of fades out on the left.

What I don't know is how much tire growth to expect at the speeds this scooter can achieve. Could this light brake effect explain my lackluster 60 MPH-ish top speed and the rumbly sound both? Do I need to break out the RotoZip again?

[Wheelman-111]

Greetings:

Tonight I answered my own question. Based on the eyeball of the 3.50 Michelin S1 on Flash III, got busy with the sidelined F IV while awaiting a bore kit et. al:

Just starting the notch with the conveniently-curved RotoZip tool, munched some metal at the exhaust mounting lug and carried it down in ham-fisted Wheelman fashion where the tire looks like it might make contact as it stretches under centrifugal force.

The result before polishing with a coarse stone should gain about 1/8" of additional clearance.

I guess I can stop making fun of the Forumistas who confessed they'd shaved their engine cases now...

[Wheelman-111]

Greetings:

Seeing the contact issues on the 1998 crankcase led me to inspect that of Flash III, a 2001 case. It seems the 3.50 Michelin was getting intimate with this crankcase too, but in a different spot. Instead of rubbing at speed directly against the exhaust mounting lug, there was a black patch just inside the lug on a flat surface leading into the lug itself.

Got busy with Roto-Zip:


I ground away superfluous lug material just to make it a little easier to thread the axle. It't probably no coincidence that the ride over "the Hump" on the freeway was not accompanied by stubborn speedometer resistance to reaching past horizontal. GPS comparison to come. F3 has touched 59 a couple of times before already - with "brakes" on! Might be capable of a bit more.

It's a good idea to do a physical check of your particular setup. Evidently crankcase molds vary more than a little. Tire profiles also vary between brands. Finally, the profile of a given tire will change depending on the width of the rim to which it's mounted.