Help determining jackshaft ratios:

You know in this instance having the clutch engage at a lower rpm would be a good idea.
Better too have it engaged and pulling with the torque you have and not burn it up.
Smoking a 4 disc is expensive .

Brian has a point , your overall ratio needs to be similar to what some of the other four-stroke engines run as a base line.
 
Thank you all again.
To answer a few of your questions this is for a road racing application. Previous road racers who used a similar engine recommended a FD ratio between 2.0 and 2.3. The 2.1 ratio is fine. It's easy to keep the engine rpm well above the clutch slipping rpm. The four stroke crank is inboard. The chassis I'm using is a 2 stroke Tonykart. The jackshaft allows me to keep the axle gear outboard and avoid a ton of clearance issues regarding the frame rails and seat strut. Otherwise it's a fun project to futz with. Yes smoking a 4 disc is about $350 to send out. I'm about to do it for the second time. Leaving the clutch on the engine crank is not an option, seat and frame rail issues.
 
Compu-kart said:
My current jackshaft set-up is eating thru the Bully 4 disc clutch. I have been advised that my gearing may be too tall for proper clutch engagement. Heat builds in the clutch and the discs are glazed, cracked and melted after less than 2 minutes of track time. Engine is a mildly built GX390 gas clone putting out approx 25-30hp. Here are the numbers:

Crank gear: 22
Jack gear: 36
Clutch gear: 14
Axle gear: 18

Primary side is belt drive. 428 chain on secondary drive. Looking to target 3500rpm clutch engagement and 6500 max rpm. It's a CIK kart raced on asphalt enduro road racing.

Looking for opinions.
Click to expand...

Axle gear: 18 ????? is 18 teeth correct? Or if you were dirt racin we'd say thooths ... :)
 
The strait is the length of the back stretch at Daytona International Speedway. I'm guessing it's a mile or so. The mph sounds right.
 
Just thinking out loud here.
Why not be 1 to 1 to the jackshaft, then 2 to 1 from clutch to axle?
Or something reasonably close?
In my opinion, clutch could spin faster than crankshaft.
Just would remove a lot of torque on the clutch.
 
Sat down tonight to work out the math. Contemplating the following changes.
Crank gear: 22T
Jack input: 19T
Jack Output: 25T
Axle Gear: 60-69T

22 ÷ 19 = 1.10
(60 to 69 ÷ 25) ÷ 1.10 = 2.1 to 2.5FD ratio.

Thoughts are the jackshaft speed will allow clutch slip at 3500 while engine rpm is 3180. Top speed calculated to be 111mph at 6500rpm.

May need to give up the 428 chain for a 35 chain.
 
Yes 95 shaw, good point. That is plan B. The limiting factor would be the availability of 428 chain shifter kart axle gears in the 28-32T range. I need to research that.
 
The 25/22 and the 14T gears are available. It's the 35T axle gear that I cannot find a vendor for. Biggest I can find is a 30T.
 
Thanks for the advice.
I did the math based on what I know is commercially available. If I change the 14T clutch gear for a 12T gear then I can use the 30T to 25T shifter sprockets to achieve my target of a 2.5 to 2.0 FD ratio. Any thoughts on 428 chain binding on the 12T clutch gear?
 
Compu-kart said:
Sat down tonight to work out the math. Contemplating the following changes.
Crank gear: 22T
Jack input: 19T
Jack Output: 25T
Axle Gear: 60-69T

22 ÷ 19 = 1.10
(60 to 69 ÷ 25) ÷ 1.10 = 2.1 to 2.5FD ratio.

Thoughts are the jackshaft speed will allow clutch slip at 3500 while engine rpm is 3180. Top speed calculated to be 111mph at 6500rpm.

May need to give up the 428 chain for a 35 chain.
Click to expand...

My gear chart spread sheet must be wrong. I get 2.073 for the 60 gear and 2.384 for the 69. I don't remember ever using it before and not getting correct ratio's or at least what I thought were correct.

edit: I just ran the 60 axle gear in the burris calculator and got 2.07. ... so I figure my chart is ok and your numbers are not exactly correct.
 
So your planning on using chain for the primary.
Consider this the chain speed is going to be huge. It's long way around at load with no time to cool. You may fry the chain and or teeth.
 
I use a belt drive for the primary side and a chain from the clutch to the axle sprocket.

Yes Mr Kish. I still have to download your spread sheet after I download the .ods program. I rounded off the values bc it was late and my eyes were crossing. I have no doubt ur spreadsheet is precise and accurate.
 
Math is wrong, 22t driver to the 19t J/S = 0.863 25t clutch gear to 60 axle = 2.4 comes to a 2.07 final ratio.

Yes a 12t driver will rob power from the engine turning that tight of a radius. MX guys years ago that they never run anything smaller than a 14t.
 
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