[imslow]

so when iwas at Loudon I had changed gearing because of the enormous torque that the new engine puts out. Running 17/40, used to be 16/42. But I found that I was hitting 5th at near the end of the straight. So does this mean I should go to 17/42? Bigger in the rear?

I always get confused here. What's an easy way to think about this?

I noticed tht Matt camelhairy went from a small rear to a big rear sprocket to get up t. 3 hill faster, but that of course put him screaming on the straight.

As for the MT, I was in 6th (maybe it was 5th but I think it has a 6 speed) before getting to start finish. Should I put on a smaller rear sprocket? Or if I do that will I not be able to get up the hills?

 

[UngaWunga]

If you want to use 5th, go to 17/42.
You could also try the 16/40.
Normally, one down in the front = two up in the rear.

Larger in front = higher top speed
Smaller in rear = higher top speed

There's a ton of gearing calculators out there... google around.

 

[FR]

This site has a great chart for the Yammie 500. It shows the XT/TT, and I'm not sure, but I think the internal ratios for the SR tranny may be different. You could probably find that out from one of the thumper sites, or ask Mark at www.thumperstuff.com
But the top section of the chart will give you a good comparison of final drive ratios. I think Sprocket Specialists has a similar chart.

http://www.members.aol.com/rotaxjohn/websprocket.html

How much low speed acceleration do you anticipate? I like to keep a lower number on the front for better accel off the turns. Of course I only have to setup for one gear, but still, I would think you could go down to a 16, maybe even a 15 front, then find the right rear sprocket to get you into fifth halfway down the straight. Particularly if you can get onto the straight in 2nd...
Your engine has enough torque that once you leave the line you don't need 1st gear anymore.



FR



Edited by - FR on May 03 2007 3:10:40 PM

 

[jbranson]

You don't gear around hp...you gear around rpm. If a certain gear was proper for 40hp...it's still proper for 60hp. Whatever final drive ratio puts you at your max rpm at the fastest part of the track. Fastest part of Loudon is just before you back off for t1...so you'd want to be in top gear about the start finish.

By Max rpm I mean the maximum rpm you WANT to run...not necessarily the factory redline etc.

Exceptions are correcting bad situations...like a hill etc. If not getting up a hill are hurting your lap times more than losing a few mph on the straight...then gear for the hill.

You wouldn't re-gear a bike because of a torque increase unless the modifications that increased torque also lowered the max rpm limit.

If you are trying to move a mountain...you don't use a shorter lever when you get stronger...you use the same size lever and move the mountain easier (meaning quicker lap times).

Gearing is all about fitting as many usable gears into your useable rpm range. Hence the close ratio gearboxes found on race bikes. You don't want to only be able to use 4th and 5th while everyone else is using 2nd, 3thrd, 4th and 5th.

The wider the power band, the fewer total gears in the gearbox you can get away with (more area under the hp graph). That's why GP 50cc bikes with a powerband 2,000 rpm wide had 8 - 12 gears in tranny.

In Frank C's situation...if he had another gear to work with he wouldn't have to compromise his Straightaway gearing....instead of over revving in a low gear up the hill, or bogging in a high gear...he would have a gear inbetween the two.

Use ALL your rpm...and as many gears as you can.
JohnnyB