Jackshaft Understanding.( Help Needed)

M

MidwestOutlaw

Member
We've been going back and forth about trying a jackshaft and not quite sure how all that works. They recommend running a 15 on the clutch driver to properly cross reference everything but what are the advantages and disadvantages to running possibly a smaller front driver or even a larger one? Anybody can read the paper and use it as it says but what are some tips and tricks to using one?
 
What engine?

Normal would be to run a cog belt to the jackshaft and a chain from the jackshaft output to the axle gear.

Beyond that it's about ratio's you need and if your running normal kart gears or mini gears.

we ran mini gears and a 4cycle type disk clutch without ever having a problem on our K30

Biggest thing is to make sure your belt from the engine to the jackshaft is wide enough to handle what ever hp your using.
 
paulkish said:
What engine?

Normal would be to run a cog belt to the jackshaft and a chain from the jackshaft output to the axle gear.

Beyond that it's about ratio's you need and if your running normal kart gears or mini gears.

we ran mini gears and a 4cycle type disk clutch without ever having a problem on our K30

Biggest thing is to make sure your belt from the engine to the jackshaft is wide enough to handle what ever hp your using.
Click to expand...
All will be chain driven clutch to j/s input then j/s output to the axle.
 
I would not disagree .
Unless it's class specific rules .
deviating from recomennded setups , leads to poor inconsistent results .
where if you want advice from mfg it's going to be iffy .
changing to bigger or smaller will affect chain life some . also likely to affect clutch action and life .
 
I don't think a lot of people understand jackshafts. They're literally for one thing, tuned RPM. No matter what engine you're running, you have an RPM range that you want to run that engine.

Jackshafts are to get the correct ratio from the engine to the tires. Common 2 stroke ratio are 2:1, 2.5:1, so that at upwards of 10k rpms that you need to make power in some two strokes gets the axle ratios within reason. Clutches can't be tuned for 10000 rpm. But they can be tuned for 3-5k. Then gearing and everything is more relatable to what is already being used. So to get a clutch to engage at say 8k rpm if that is when it comes on the pipe means it needs to be tuned for the ratio of the jackshaft. Use 2:1 as an example would put you at 4k rpm. This goes for 4 strokes as as well, correct ratio for tuned racing rpm.

So if your engine needs to operate outside of some of those rpm parameters or you just need a convenient way to transfer power to a clutch like with the 450's just to adapt to kart style clutches without the trans you'll need a jackshaft. A lot of this generalized, because there are lots of combinations that don't account for each engine. So nothing too specific as far as math goes. But, ultimately these are the major reason you'd use a jackshaft.
 
Use the minimum amount of the stiffest springs available with the heaviest weights at purple plate flathead rookie engagement rpm.

anything other then that ain't as good
 
Quite a limited amount of info for the question .
Benefits of a jack shaft are . Imo .
Reduced load on crank shaft .
Alter or change output location .
Allow high rpm with respect to gearing .
Reduce or alter clutch engagement and abuse .
 
flattop1 said:
Quite a limited amount of info for the question .
Benefits of a jack shaft are . Imo .
Reduced load on crank shaft .
Alter or change output location .
Allow high rpm with respect to gearing .
Reduce or alter clutch engagement and abuse .
Click to expand...
i think this is exactly why our junior sprints run them. i can see no other reason
 
The other reason to run a jackshaft is that you can only reduce the front pinion so far in size, and on a small track, having too large a rear gear can cause issues, so a jackshaft allows for a smaller rear gear to be run. Common ratio for a jackshaft is 19-21:57 (primary side) // x to 65'ish @ wheel (secondary side)
EDIT: fixed primary tooth count...thanks Chuck!
 
Last edited:
Ted Hamilton said:
The other reason to run a jackshaft is that you can only reduce the front pinion so far in size, and on a small track, having too large a rear gear can cause issues, so a jackshaft allows for a smaller rear gear to be run. Common ratio for a jackshaft is 19-21:57 (primary side) // x to 65'ish @ wheel (secondary side)
EDIT: fixed primary tooth count...thanks Chuck!
Click to expand...
Ted, you’re in the 4 cycle section. That mumbo jumbo don’t mean anything here... typically you’re only gonna see a big block or high horsepower industrial 4 cycle on a jackshaft.
 
MidwestOutlaw said:
We've been going back and forth about trying a jackshaft and not quite sure how all that works. They recommend running a 15 on the clutch driver to properly cross reference everything but what are the advantages and disadvantages to running possibly a smaller front driver or even a larger one? Anybody can read the paper and use it as it says but what are some tips and tricks to using one?
Click to expand...
Are you running a big block? For big block you want to use a 14 or 15. Typically a 15. Anything smaller and you'll be running a bushing driver instead of a bearing. the disadvantages there should be self evident. 15 is right in the middle of the road for just about any ratio you need to hit. The only reliable way to run ratios needed for tiny bull rings both big block and small block. I think with bigger clutch drivers you would have more slip? I'm not a clutch expert. Sprocket clearance in the rear is big also. it doesn't take much to break a rear sprocket, hub, and damage a chain after bouncing it off the track a few times. It's also easier and quicker to change the jackshaft secondary at the track than the engine driver.
 
Last edited:
Seems to me that the jackshaft on a Jr sprint shortens the amount chain needed which helps keep it from coming off.
 
Jimbo said:
Seems to me that the jackshaft on a Jr sprint shortens the amount chain needed which helps keep it from coming off.
Click to expand...
only chain issue we have ever had was when we about turned one over and it broke. We do have an extremely tall axle gear guard. Ive cut off and moved our motor mount forward we have the longest chain i've seen around our parts.
 
Say your running no jackshaft on a 2 cycle, clutch on end of crank with chain to rear sprocket (which I think was somewhat common on stock-ish 100cc'ers in the past)...to get any kind of ratio on a small oval, to get into decent high end rpm range, musta needed maybe 8-9 tooth on the clutch and high 60's on the back (35 chain)....I could see the chain and gears and clutch getting very hot and taking on a lot of abuse....enter the jackshaft into the mix, and most if not all those issues gone....
 
You must log in or register to reply here.
Back
Top