setting up a jackshaft/2cycle

spincycle

spincycle

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trying to find out if there is a formula/mathematics 2cycle racers use to help them figure out what rpm engagement to set up the clutch on the secondary side on a jackshaft...using 57/21 pulleys on the primary side, disc clutch and 35 chain/gears on the secondary side, looking to run engine at 14000 - 14500 max rpms...thx!
 
You decide where you want your clutch to engage for proper/needed operation and then set the gear sizes accordingly.

It then becomes a matter of parts availability and what will work per your jackshafts physical configuration.

Way back when jackshafts all started we used a MDC clutch and brought the clutch in at what a rookie MDC clutch would have been set to.
 
question is, if the motor is at max rpm 14,500 how may rpms is the jackshaft/clutch spinning at if there is a 57/21 (2.713) ratio on the primary side? me thinks that it would be 14,500 divided by 2.713 or 5345 rpms, am I thinking this correctly?
 
paulkish said:
You decide where you want your clutch to engage for proper/needed operation and then set the gear sizes accordingly.

It then becomes a matter of parts availability and what will work per your jackshafts physical configuration.

Way back when jackshafts all started we used a MDC clutch and brought the clutch in at what a rookie MDC clutch would have been set to.
Click to expand...
 
If you want to use clutch similar to what you used on open 4 cycle, need to have initial ratio under 2 to 1.

57/28 may let you use same clutch setup as your animal.

Just a thought.
 
Last edited:
So if your peak torque is 10600 (based on pipe, flex and motor), take the recommended clutch engagement and divide by 2.71 for the needed engagement. So for 10600 / 2.71, your clutch engagement would need to be ~ 3900 rpm. Keep in mind, the tach will still read 10600, but your clutch should come in around 3900 ish. Keep your phone calculator app handy and divide your engagement rpm by 2.71
 
Well hou may already know but it may help someone else.✌️
 

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spincycle said:
trying to find out if there is a formula/mathematics 2cycle racers use to help them figure out what rpm engagement to set up the clutch on the secondary side on a jackshaft...using 57/21 pulleys on the primary side, disc clutch and 35 chain/gears on the secondary side, looking to run engine at 14000 - 14500 max rpms...thx!
Click to expand...
Here's a gear ratio chart you might find helpful. You have to order "Nine Sheets" from me (it's free) if you want a working model, but this has your primaries so you can find your secondaries and the ratios will be right.
Even with the best advice, you may find the stall RPM of the engine isn't exactly what you want. If it was me, I wouldn't worry what the clutch is slipping at, I would worry about the engine RPM when you test it for stall. Who cares what the clutch is slipping it, other than you don't want to have it spinning too fast, you wouldn't want it to overheat.
gear ratios 2.jpg
 
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