Jackshaft Ratios

twostroketoowoke

Site Supporter
We recently acquired a 4 cycle jackshaft and my two cycle experience is almost useless and has my head spinning. I keep seeing 1-1 primary ratios so guys can run shorter chains but we want to use it so we can get high final ratios without dragging a big rear gear. I don’t like the idea of a clutch being crank mounted because heat on chain primaries is atrocious. But let’s say we do a 1.75 primary and we put that clutch on the secondary for an open modified. That clutch needs to hit at 4500-5000 engine rpms, but the primary makes the jackshaft speed engagement for the clutch be in the high 2000’s. Is that even achievable? What are you guys doing to get small gears with big short track ratios?
 

95 shaw

Site Supporter
I know that clutch engagement rpm is achievable. The issue would be in being able to handle the torque as that increases with the jackshaft ratio.

Need a 3 disk at minimum. 4 better.
Might be why birkys are popular on jackshaft setups.
 

twostroketoowoke

Site Supporter
My thoughts too, that plate pressure plate is gonna take a beating on a 4 cycle jackshaft. We run a 2.71 primary on two cycles so dry clutches engages about the same as a 4 cycle clone speed on the crank.
 

twostroketoowoke

Site Supporter
The only mathematical thing that makes sense to me is running somewhere between a 15-21 on the crank with the clutch, 24-25 on the primary rear and then massaging the secondary that basically won’t change to get a small rear gear and then the smaller the tracks are we then go down on clutch driver.
 

95 shaw

Site Supporter
I would think you could use typical clone gearing for track for final baseline, then jackshaft gearing to make jackshaft rpm be similar to clone engine rpm.

So, thinking 7000 rpm from a 9000 rpm engine.

9000/7000= 1.28 or 15-19

I think I've seen references to 15-21 (1.4) jackshaft setups.
 
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We recently acquired a 4 cycle jackshaft and my two cycle experience is almost useless and has my head spinning. I keep seeing 1-1 primary ratios so guys can run shorter chains but we want to use it so we can get high final ratios without dragging a big rear gear. I don’t like the idea of a clutch being crank mounted because heat on chain primaries is atrocious. But let’s say we do a 1.75 primary and we put that clutch on the secondary for an open modified. That clutch needs to hit at 4500-5000 engine rpms, but the primary makes the jackshaft speed engagement for the clutch be in the high 2000’s. Is that even achievable? What are you guys doing to get small gears with big short track ratios?
So my question is are ypu required to run a Jackshaft, is there an advantage?
 

twostroketoowoke

Site Supporter
So my question is are ypu required to run a Jackshaft, is there an advantage?
No requirement but on bullrings where we want 8600-9K revs it’s hard to achieve when you’re limited to a 11-68 or so because anything bigger will drag the ground or catch a rut. There’s tiny tracks that we want a ratio that could be upwards of a 11-76 equivalent but it’s not possible on dirt without wrecking a rear gear.
 
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