18 inch rear wheel

[John]

I know that we have discussed trying to find a different gear ratio for the middle or final drives, and we've talked about using a Standard's 17 inch rear wheel on a Special, but what about an 18 incher from a 750-850 Special? There shouldn't be any problem with the extra height, should there? The Special and the Standard use the same swing arm, and the Standard can easily accept the 17 incher so...
That should help to slow the Specials engine speed down some when travelling at or above the freeway speed limit. It will also help with top end speed. I know the rotor will need to be changed, as the750-850's are a smaller diameter.

[TopCatGr58]

Okay John, try to follow this "crazy train" of thought!

Overall diameter of Dunlop 140/90-16 is 25.72", times 3.1416(Pi) gives 80.8 divided by 12" gives 6.73ft. One mile of 5280ft divided by 6.73ft/1-wheel rpm= 785 rpms=60MPH.

I checked several tire companies, maximum overall diameter found was Dunlop EliteII of 120/90-18 at 26.88"....following same process = 750 wheel rpms, a savings of only 4.5%.

4000 crankshaft rpm, reduced at clutch to 2414, increased thru 5th gear to 2735, reduced thru final drive to 829 wheel rpm, roughly about 64MPH, which is about what I read on my Special!

Now, follow the 750 wheel rpms back thru the reductions and increases of final drive, 5th gear, clutch gears, you get 3619rpms, a modest almost 400 crankshaft rpm reduction.

AND, the size rear tires you can find in 18" are either skinnier than the OEM to be taller, or much fatter, ie. 130/70-18 which again brings the overall diameter back to the equivalent of the 16" tire!!

IMHO, I don't think it's worth it !

[John]

Follow it? I'm going off the rails...

Just reading that made my head hurt. Yours must be killing you.
I know Yama put the 18 incher on the 750 to try to give it a little longer legs. The one that I had was quite quick, but it topped out to soon.
Thanks for doing the head work for me. I saw one in ebay a little while ago, and went hmmm...