Jackshaft Ratios

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?
 
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.
 
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.
 
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?
 
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.
 
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?
Why can’t you just drop one tooth on the engine or add one to the jackshaft side? What is the tooth count on their one to one primary?
 
Have you put mini gear sizes into a spread sheet to see if it might suit your thinking about what your trying to accomplish?
 
Are you still thinking of running the clutch on the crank or on the jackshaft secondary side ? Depending on where the clutch is located, it’s a simple clutch weight and spring change depending on how fast the shaft (crank shaft or jackshaft) is spinning to get your clutch engagement for the correct engine RPM.
 
You will definitely want to run your clutch on the jackshaft side. A heavy 4 disc clutch on the crank can deflect or bend it at high rpm causing chain misalignment and all of the problems that come with that. It WILL cause the discs and sometimes the floaters to wear out very quickly. They are not cheap, think $80-120 per night. We run a belt from the crank to the jackshaft and a chain from jackshaft to rear sprocket. Belt drive sprockets can be bought on McMasters at very reasonable prices. Going to plug the Birky clutch here, super solid reliable. Spend a little now and you will be hundreds of dollars ahead at the end of the season. I learned the hard way, you don't have to. Best of luck my friend.
 
Jackshafted Crf450r with a Birky clutch
 

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look on burris website. I think he has some useful info on there for his 4-cycle jackshaft.that guy about with 1.2 was close to what i was shooting for when i planned on using one my jackshafts.its just easier run without jackshaft on 4 stroke
 
God that’s beautiful lol
Off topic but i was watching some mx podcasts and pro riders was talking about the 450’s that they are too much power and they are skilled riders and its so easyto make a mistake with them motors.that alot of people are getting hurt with them engines.that they are pushing to go to the 350’s instead of 450’s.
 
skilled riders? how skilled?

The only ones who want the 350's are those who can't win with a 450

Why not 150's and make the engine playing field even for everyone?

I thought racing was about seeing who's fastest?
 
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