OK, as promised here's my calculations, someone please confirm or deny...

Primary reduction gear = 1.81 based on 67/37 tooth gears
Final drive ratio = 2.9565 based on 68/23 tooth pulleys
5th gear ratio = .884 based on posted tranny specs
Stock tire = 140/80/15 w/5.51 inch width, 4.4 inch tire sidewall height, 23.819 inch tire height, 74.829 inch tire circumference, 846.272 revolutions/mile

Putting it all together it looks like this:

846.727 revolutions/mile*2.9565 final drive ratio*.884 5th gear ratio*1.81 primary reduction ratio = 4005.489 rpms @ 60 MPH (because we used wheel rotations/mile, 1 mile/min = 60 mph to convert to proper units)

Did I miss something here?  (I realize the actual tire size may be different than stated due to manufacturing vs. theorectical specs)

I don't get the same numbers as you on some points.

We're good up to the tire circumference= 75.83 but then it goes off... from there is should be 75.83"/12 = 6.32' ft/rev. 5680'/6.32' = 898.73 rev/mi

Then 898.73 x 2.956 x .884 x 1.81 = 4250 engine revolutions per mile. If you deside to do that mile in a minute, then it becomes 4250 rpm


Just for fun, lets figure mine on my '99 chopper:

It is also a 5-speed so... the primary is the same, and I have stock belt @ 68/23 = 2.956; but I'm running a huge 140/90 rear tire that is approx 25." dia. so... it rolls over 6.54'/rev... 5680/6.54 = 868.5 rotations/mi.

868.5 x 2.956 x .884 x 1.81 = 4102 rpm @ 60 mph

So... I'm cranking 150 less rpm @ 60 mph.

My '87 is running the lower primary 68/36 = 1.889, but higher top gear than the '99, ('87 top gear = .875) and the same rear tire size as yours. So it's math would be:

898.73 x 2.95 x 1.889 x .875 = 4382rpm @ 60mph

Believe it or not, I can actually feel the difference between those 280rpms at 60 mph. But man 5th gear is gutless  for passing on the '99. Third is kind of cool though, because it will wind up past 70mph on entrance ramps! Then drop into fourth and blend in with traffic.

I'm pretty happy with the belt drive ratio with the 5-speed and the big 140/90-15. It works with my 250+ pounds and the engine doesn't seem to labor badly even cruising at 70mph. At 55-60mph it is just putting along. I see no reason for a chain with my big rear tire.

justin_o_guy wrote on 12/31/69 at 16:00:04:


I suppose you are asking about the Dunlop 140/90-15
I mis-identified? HMMMMM???? I ask, becuse I also have a Cheng-Shin (sp?) 140/90-15 that is a bit wider, and about a half inch shorter.

And why is it I get the feeling your question is asked in the spirit of a spelling flame? You'll just have to pardon my Dyslexia.

The math remains the same. The Dunlop on the back of my chopper is pretty close to 25" dia.

You made me look though

Corrected calculations:

Barry's Savage...

23.82" dia. rear tire = 74.8" / rotation... 74.8"/12= 6.23' /rot. 5280/6.23= 847 rot/mi

847 x 2.96 x .884 x 1.81 = 4011revs @ 60 mph = 4011rpm
(slight difference in my calculations from not carrying decimals out as far.)


My'99:

25" x 3.14 = 78.5"/12 = 6.54' / rotation 5280'/6.54' = 807 rot/mi

807 x 2.96 x .884 x 1.81 = 3822revs @ 60 mph = 3822rpm
--------------------------------------------------------------------------
My '87:

23.82" dia. rear tire = 74.8" / rotation... 74.8"/12= 6.23' /rot. 5280/6.23= 847 rot/mi

847 x 2.96 x .875 x 1.89 = 4146 rev/mi @ 60mph = 4146rpm

So really, the difference between what my '99 and '87 are turning is a bit more than 300 rpm @ 60mph. I can feel the difference, and like it. If I were considerably lighter the temptation of sprockets and higher ratios would be irresistable to me, but as it is...

justin_o_guy wrote on 12/31/69 at 16:00:04:


Sorry about misunderstanding...

From what I can tell, the Dunlop 404 @ 140/90-15 fits within the swingarm, belt and fender clearances of my 1" lowered rear chopper -just barely- and only because a couple of the fender brace / sissy bar bracket bolts have been removed to keep them from rubbing the sidewall.

Now that I've put the digital calipers on the Cheng-Chin (sp?) and realize that it is a couple of mm wider and about 1/2" shorter, I wouldn't recommend trying to put one of those on the Savage. I'll probably take that new one I got cheap off eBay and trade it at the independant cycle shop towards another Dunlop 404. I think it (the 404) is the biggest thing in both width and diameter that could safely be stuffed under a stock Savage.

Quote:


John, carefull with those 87 numbers.  There have been a few changes.  Top gear ratios are different AND the front and rear pulley sizes are different.  There's a post around here somewhere that highlights those differences...

barry68v10 wrote on 12/31/69 at 16:00:04:


Don't think so... the belt is the same one, and both the pullys are too: Much to everyone's regret... otherwise there would be at least a couple of gearing options with the belt; which there aren't.

Good try though... You made me look