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 Motorcycle Safety
 Technical/Maintenance
 Sprocket diameter size
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 05/29/2013 :  7:21 PM
quote:
Originally posted by gymnast

When I added a sidecar to my 78 Sportster I went up 3 teeth on the countershaft sprocket to pull the heavier load. This also resulted in lower top speed and slightly lower cruising speeds (to minimize engine wear and vibration).

? The countershaft sprocket is the front one and 3 teeth on the front is a huge number, resulting in less torque at the rear tire. Gearing changes don't change cruising speeds. They change the rpms at a given speed so maybe you meant rpms there. More front teeth or fewer rear teeth lower rpms at a given speed and raise top speed if the bike can reach red line in top gear. Many can't.
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gymnast
Moderator
4263 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 05/29/2013 :  7:35 PM
You are correct. I went down three teeth from 21 to 18, on the countershaft, not up. And, as you indicate, the rpm for a given speed increased, The 18 tooth sprocket is noticeable smaller than the 21 tooth sprocket. My error. (Not a good idea to post on 3 hours sleep after spending 12 hours on airplanes and in airports).
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rayg50
Male Moderator
2082 Posts
[Mentor]


NYC, NY
USA

Honda

Shadow Spirit 750DC

Posted - 05/29/2013 :  9:19 PM
quote:
Unless you do the majority of riding on Interstates, and find the rpm's annoyingly high ...
That is the major reason it is such a popular mod for the Shadow. I avoid highways (no fun) whenever possible but lately I've had some need to use them and can see why someone might want to make this mod. It was something that hadn't hit home until I had experienced it on a longer ride.

I do not currently plan to make the switch but wanted to understand it just in case I find myself riding more highway than I would like. Perhaps when the time comes to replace the chain and sprockets I will give it a shot but at least now I have a better understanding of what it is and the trade off involved. There are times when I have benefited from every ounce of the little acceleration this bike has. I don't know if I want to reduce it for an because of the occasional highway ride.

Edited for clarity.


Edited by - rayg50 on 05/29/2013 10:21 PM
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 02/06/2014 :  8:44 AM
quote:
Originally posted by scottrnelson
A lot of people also have a hard time understanding whether their chain will still work, but the amount of movement of the rear axle for a one-tooth change is almost exactly 1/4 of the distance between two pins on the chain. If you have that much adjustment room, you can use the same chain.



Why is that so, Scott, would you explain more,
I've asked myself this question too.
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scottrnelson
Advanced Member
6886 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 02/06/2014 :  9:57 AM
quote:
Originally posted by twowheelsbg
quote:
Originally posted by scottrnelsonA lot of people also have a hard time understanding whether their chain will still work, but the amount of movement of the rear axle for a one-tooth change is almost exactly 1/4 of the distance between two pins on the chain. If you have that much adjustment room, you can use the same chain.
Why is that so, Scott, would you explain more,
I've asked myself this question too.
Let me try to put it in simple terms. The chain comes close to touching half of the teeth on the sprocket. Lets put on a sprocket with four more teeth. Half of those new teeth are on the side where the chain is touching and half are not touching the chain. You agree that only the part of the chain touching the sprocket are affected by the number of teeth, right? Now let's lengthen the chain by two links (you can't do just one) along with this sprocket with four more teeth. Those two new links will fit exactly in the two new teeth on the side of the sprocket that the chain touches, so with the bigger sprocket and longer chain, we end up in exactly the same spot in the axle adjustment and the chain is adjusted properly. Does this part make sense to you?

Now if we just add the longer chain and keep the old sprocket, the axle needs to slide back by the distance between two pins because you'll end up with one new link on the top between the sprockets and one new link on the bottom. Does this make sense too?

So it should follow that if we leave the old chain on there and put on the new sprocket with four more teeth, the axle would have to move forward by that same distance - the distance between two pins in one link in the chain.

If you only add two teeth, it's 1/2 that distance you need to move the axle, and if you add just one tooth, it's 1/4 that distance.

I've swapped sprockets on quite a few bikes that I've owned and have witnessed this firsthand. I went from a 37 to a 41-tooth rear on one Ducati that I owned, I've also gone from 14/45 to 15/48 on my Honda, which is a total of four teeth. I just barely have enough total range for that change.

If this isn't clear enough, try drawing pictures of what we're doing here.
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twowheelsbg
Male Junior Member
50 Posts


Burgas, Burgas
Bulgaria

Suzuki

Posted - 02/06/2014 :  1:48 PM
Thank you, nicest explanations are in practical terms.
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