J&P Community

 I'm having a custom motorcycle built with a 1980 XS650 engine + rigid frame + Big Twin style 49 tooth sprotor:

We ran into some problems now. The rear sprotor is limiting the speed of my motorcycle. I think the sprotor was originally intended for the narrower Vtwin style engine, but I'm using a parallel twin engine which comes out a bit wider because both cyclinders face front as opposed to the side with a traditional American Vtwin style.  I am going for a vintage Harley WLA look so we're using 18" wheels.  I spent all this money on big bore kits & engine upgrades, but I just found out we will be limited by the large big twin style sprotor we were forced to use because of the narrowness if the frame to engine. 

The stock sprocket is 34 tooth, but the sprotor is 49 tooth.

Got these numbers from an expert with the Yamaha XS650 engines:

-A standard Yamaha XS650 17/34 tooth ratio does 4,117 RPM @ 60 MPH with a 18" rear wheel.

-With my 49 tooth rear sprocket and a 17 tooth front sprocket, I will have 5,933 RPM @ 60 MPH.

-With my 49 tooth rear sprocket and a 18 tooth front sprocket, I will have 5,603 RPM @ 60 MPH.

Someone suggested I could have an intermediate shaft with reducer sprockets 17:24.

-Using the stock 17 sprocket would give you 4202 RPM @ 60 MPH.

-A 18 tooth rear sprocket with a 17:24 intermediate shaft would produce 3968 RPM @ 60 MPH.

The problem is I don't know what a intermediate shaft or reducer sprockets are!  Does anyone here have experience with or know what this is & where to get it?  Is it very complicated & expensive thing to do?

Sorry, I apologize for the long winded question.  I spent a lot of money on this project and was probably ill prepared for all the problems that I encountered.  If anyone can think of a easier/simplier solution I'm all ears!

Wow what a question!!! Looks like you have done your research. First of all I use the following website for gear ratio calculating in relation to tire circumference/gear ratio/final drive ratio. They happen to have your stock gearing already preloaded into the website which would be helpful. http://www.gearingcommander.com/ 

An intermediate shaft or jackshaft what some people call is a way to have a gear reduction/increase in a situation just like yours, or a way to offset the sprockets in wide tire situations. Here is a good picture explaining how and jack shaft works.

Here is an example of a design of a jackshat

This will require you to run two separate chains as well as some way to mount the shaft and have a bearing support for the shaft to spin on. This is an extremely custom job which will require some ingenuity and engineering on your part on where to mount the shaft and how to support and align the shaft.

Once the jackshaft is installed the opportunity to modify ratios is endless based on what type of final drive ratio you are looking to achieve. Based on your the numbers you listed above

 17t front and 17t(jack shaft) 24t (jack shaft) and 49t final drive sprocket

(17/17=1) X (49/24=2.04) making the final drive ratio 2.04:1 . Stock on the XS would have been 2.00:1 which is pretty close to getting you the desired rpm. Hope this helped and i did not confuse you.

Thank you, that was the best explanation I had so far!  At least I will understand better in the future.  I totally forgot I posted the question here.  Thank you for the time you took to craft that response.  I didn't really understand it before, but now I do. 

 No problem sorry i didn;t see it sooner.

 Anthony, Where (or who) did you get the graphics from? I'm jealous, I've wanted to be able to add those type graphics to my explinations for quite a while now.