42T rear sprocket?


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i'm Bill form the Netherlands and new to the forum so hi all!:)

i'm busy with my caferacer project based on a "79 heritage special, i can make thread of my project if you guys are interested.

the main problem i have now is the setup of my rear sprocket. i'm currently running a 17 inch suzuki wheel and the smallest sprocket I can get my hands on is 42 tooth.

I feel this is way to big right? Can you guys explain to me what this will do with the performance of the bike? Like top speed and such? This is my first XS, so still learning.

Thank you
Welcome to the site.

Yes, 42 is much too large. You should try to find sprockets around 32,33 or 34.

I have a 16 inch rear wheel, and I find 17 front, 33 rear to work really well for me.
Yes, the 650 uses a rather small rear sprocket compared to many other bikes. Some wheel swaps simply won't work because you can't fit such a small diameter sprocket on the wheel. I don't know the diameter of a stock rear sprocket off hand but I can measure one for you. Then you'd know if it would even fit on the wheel.
ok cool thanks man.

what is bike's top speed on that setup? because i also want to take the bike on the highway, which has a speed limit of 80mph over here in the netherlands so i need it to do atleast 100mph.

With a 42T sprocket, you'll get about 85mph @7K. Redline is 7500 so it's probably not a good idea to be cruising down the highway all day at 7, lol. You'll shake the fillings out of your teeth, probably blow the motor up first though, lol. You may need to re-think your rear wheel choice.
Hi BillyJane,
I'm running 17T/38T on my XS650 with Velorex sidecar.
After riding the rig I reckon I shoulda gone 17T/36T instead.
Rides just about perfect in city traffic though.
42T gives you the effect of having a 3 speed transmission with two underdrives.
You'll beat every other XS650 pulling away from stop signs but they'll have a perhaps 50kph higher top speed.
Stock solo gearing is 17T/34T but most long distance riders drop a tooth or two off the rear to run 17T/33T or 17T/32T.
But what to do?
Get an XS650 wheel instead.
Get a custom sprocket made to fit your Suzi wheel.
Or, if you have more money than God, swap in a pair of lower ratio primary gears, (some guy in Australia makes them) that allows running the larger rear sprocket.
Here is an Excel spreadsheet that gives RPM vs MPH for all combinations of wheel size, sprockets and gears.


This spreadsheet uses rolling distance of the rear wheel for one complete revolution. I used 76.5" for a 17" wheel, but you will have to do a rolling distance check with the wheel and tire mounted on the bike for more accurate numbers.

Using the Heiden Tuning high ratio primary gear set and MikesXS 5th gear with an 18 tooth front sprocket and a 42 tooth rear sprocket you will get:

6761 RPM @ 100 MPH
5747 RPM @ 85 MPH
4057 RPM @ 60 MPH
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Hi BJ,
what with your power unit being a "Mystery Engine" having had all manner of unknown modifications done to it; perhaps the late builder has installed a high ratio (although us nitpickers claim it's a low ratio) primary gear set with more than 27 teeth on the engine gear and less than 72 teeth on the clutch?
Check by turning the engine one full turn and see how far the clutch turns.
27/72 stock tooth count turns the clutch 135º.
If the clutch turns more than that your 42T sprocket could be OK.
There are several so called high ratio primary gear sets available. I have included the Heiden Tuning set in my spread sheet. That is a 80/35 = 2.29 ratio and costs 575 Euros, or $782. I believe that the Heiden Tuning set is a stock item.