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Discussion Starter #1
I realize this has probably been discussed before, but I can't find it.. . . .
What effect does switching to a 16t front sprocket have on the indicated mph--better, same, or worse?
thx in advance
Barry
 

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I changed my rear sprocket from the stock 47 to a 43 yesterday. Sometimes a large reduction can cause some problems but the four tooth reduction worked out very well in my opinion. Let me preface that by saying I LIKE LOW RPM’s and I have no interest in doing stunts. Let’s face it, the Wee is a very capable bike but I wouldn’t call it fast. My other bike is a BMW 1100RT which putts along at 3500 RPM on the highway and I like that. Nearly every bike I’ve ever owned has gone through a gear ratio alteration, sometimes several. I know most people change their front sprocket but over the years and dozens of sprocket changes, I prefer the rear. There are several reasons for this and if anyone is interested in my humble opinion I can explain further.
Dino
 

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Discussion Starter #5
front vs rear sprocket

Yes, I would be interested what the advantages would be for a rear sprocket vs a front sprocket change. . .
 

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To start with, several years ago I crunched a lot of numbers to determine what the affects of changing front versus rear sprockets is. My background as an ME peaked my interest so I wanted to understand the details of the mechanics involved. Reason: The basic principles of drive systems did not explain the seat of the pants differences I felt on my bikes.
This may be a little long winded but here are my seat of the pants observations and technical reasons. When going to a smaller rear sprocket, the tooth engagement on the front sprocket increases due to the chain to sprocket engagement angle. This is a good thing because the more teeth or percentage of a tooth that is engaged increases sprocket and chain life as well as the ability to deliver torque/ horsepower. Next, on every bike there is a percentage of HP/torque that is sacrificial due to the chain and rear sprocket. When you change the rear sprocket all of the same output from the front sprocket is still available prior to the parasitic affect the chain and rear sprocket impart. Put another way, a larger front sprocket reduces the HP/ torque available to the chain, rear sprocket and rear wheel. When you change the front sprocket, there is additional strain placed on the all the transmission components due to the longer lever arm of a larger diameter gear. In some cases (like the 47 to 43 sprocket) the chain will require shortening. This is also a good thing. The reduced chain length and sprocket diameter is reduced rotating mass that the engine does not have to turn. That means less sacrificial load on the motor. When doing minor rear sprocket changes, quit often there is no need to take a link out. This has a minor benefit which is to move your rear wheel backward increasing the wheelbase improving ride quality and stability at higher speeds. There is also the increase in available ratio changes when the rear sprocket is replaced. On average you can expect roughly 100 or so RPM change per tooth. And, for the Weestrom you can buy as small as 40 teeth which I would not recommend. So, after dozens of sprocket changes on dozens of bikes, I find the affects when changing the rear sprocket are much less noticeable in acceleration at both slow and fast speeds. Basically there will not be a huge change in the way the bike performs. What you will see is a reduction in RPM with slight changes in performance.
Dino
 

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Discussion Starter #7
rear / front sprocket change

so if a rear sprocket change results in approximate 100 rpm change per tooth, what would be the rpm change for a front sprocket change ie. from a stock 15t to a 16t (or 17t for that matter) ?
 

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Not quite, It is a ratio of the two sprockets which results in a change of the final drive. The 100 RPM per tooth is for the rear sprocket.
15:47=3.133
16:47=2.938
15:43=2.866
As you can see, the 15:43 is a larger step
 

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so if a rear sprocket change results in approximate 100 rpm change per tooth, what would be the rpm change for a front sprocket change ie. from a stock 15t to a 16t (or 17t for that matter) ?
From a 15 to a 16 was a drop of about 500 rpm at 70mph (actual by GPS).
 
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