Help determining jackshaft ratios:

C

Compu-kart

Member
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.
 
It could be as simple as not having enough weight (or too much spring on the clutch). Based on what you have stated above, you're clutch is mounted on the jackshaft, correct ? If so, at 3500 engine RPM, the clutch engagement RPM is ~ 2138 rpm. You may not have enough weight on the clutch to make it lock up.
 
Apologies for not making the question clear. The clutch is set to engage at 3500 rpm (clutch speed). Need to adjust the jackshaft ratio to where the clutch engages at 3.5K clutch rpm and the max engine rpm (after clutch engagement) is 6.5K max.

- - - Updated - - -

Yes, clutch is on the jackshaft.
 
Do you feel as though the primary and secondary ratios are what you need for the track you're racing ? If so, I would call Buller or Brian Carlson and ask their thoughts regarding the clutch set-up. I was thinking you could change to softer springs and add weight so the clutch would engage at the correct engine RPM (max torque) and be at a lower clutch speed (which would reduce heat in the clutch if properly engaging). I am interested in what you find out as we're thinking of taking our Sudam to a local sprint track; I was anticipating a spring and weight change to our clutch.
 
Thank you for the lead Mr Cooper. Mr Buller recommended a one to one point three primary side ratio. This would drop the jack gear from 36T to 19T.
I took it all apart and am trying to source a supplier for the belt drive gears needed.
 
Yes, it was engaging and disengaging multiple times per lap. The lock up was set at 3500. Heat built up quickly. Not good. Thanks!
 
Compu-kart said:
Thank you for the lead Mr Cooper. Mr Buller recommended a one to one point three primary side ratio. This would drop the jack gear from 36T to 19T.
I took it all apart and am trying to source a supplier for the belt drive gears needed.
Click to expand...

Might be I'm incorrect seems the jack gear should be a 29 or 30.
 
My mistake. The ratio should be one to one point one five (1.15). He recommends a 19T, I will double check. This is my first experience using a jackshaft. What charts or formulas can I use the determine the final drive ratio?
 
Gear ratio from engine to jackshaft times gear ratio jackshaft to axle equals gear ratio from engine to axle.
 
Compu-kart wrote:

"The clutch is set to engage at 3500 rpm (clutch speed)."

The sentence you wrote is not correct. The method you use to determine clutch engagement or your concept of what's needed from a clutch is flawed.

Your clutch is set to slip at 3500 rpm (clutch speed).

Your solution if there is a possible solution for your application is to use heavier weights and springs along with maybe changing how your jackshaft is geared.

I am the person who caused jackshafts with a 4cycle clutch to become wide used over axle clutches in UAS applications. I did not come up with it but developed my own theories on it working with Donald Raiti, who's dad built Goodhope Speedway in Ohio, remembered it being done years earlier. The original application was for Yami dirt oval racing. Enough said on that. PER MY THEORY>> You should use the smallest driver possible on your clutch because it moves stress placed on the clutch and gearing towards the axle. The bigger clutch gear you use the more stress you put on your clutch. Without looking at my gear chart you probably need to adjust all your gearing to be able to get better reliability from your clutch. It was ME who first explained to Kermit Buller how we used a MCD 4cycle clutch on our jackshaft. I also explained how we used the clutch to Mr Hortsman and he sent me a new clutch for sending him a picture of our setup. Until then he had never heard of using a 4cycle clutch on a jackshaft for higher hp applications. The big debate while again developing the concept between Donald and I was about clutch gear size. Donald was convinced a 19 was the way to go on the 1/5 mile Goodhope Speedway and I with my stress theory used a 15 driver going to a 13 for 1/8 mile tracks. Donald had failures, we did not.

And unless your putting a new clutch on every time you spend 2 minutes on the track your NOT doing what is necessary to fix your on track clutch problem off the track.


By the way because our MCD had not come in yet we used an everyday Noram 4cycle clutch with adjusted springs and weights successfully. Your problem is not about the clutch, it's about your ability to maintain, gear and set your clutch properly for your application.


If you pm me your email i'll send you my jackshaft spreadsheet. No I won't send it out to anyone who wants it, just you.
 
Thank you Paulkish. You are an authority on jackshaft application. Much respect. I haven't been karting too many years and I thought this would be a fun project, so far so good! I'm a bit confused on a few aspects of your explanation. I agree on the smallest clutch driver. I'm using a 14T, all good there?
Yes, poor choice of wording on my part. The clutch is set to begin slipping at 3500 rpm. It was calibrated by a gent who was recommended to me by several independent sources. I trust his work without question.
The thing is I'm sure jackshaft/clutch theory is an intriguing topic, when and if I retire I will can dedicate myself to understanding its subtleties. I understand I need to re-gear my set up. Such is the reason for this post, to obtain opinions on the best method to do that. So far there have been a few that got me thinking in new directions. I look forward to receiving your jackshaft spreadsheet and using it to get this puppy racing right. Yes I agree with you, I am acutely aware there is currently a problem.
Ha, no I don't think it's a great idea to put on a new clutch after 2 minutes of track time. Hopefully that bad habit can be done away with in short order!

Thanks flattop and 95 shaw. Very useful and practical chart.
 
I sent it to you feel free to share it on here. The green blocks are for the four gears and automatically update for additional teeth. If you print it you will get hard copy to take to the track.

ps... you may not know Al but he stole how I did parts of my spread sheet from me to use with his 9 sheets. grrrrrrrrr I still have the email exchange with him and a few years later he DENIED stealing it from me. again grrrrrrrr ... :)



... bob, Holier then thou and upstanding Al did do that to me ! grrrrrrr That and his getting Jean banned from Pete's site for his normal arrogant way is why he and I did not get along well.
 
When racing your axle and the back of the kart flexes. Do you want to put the strain of the flexing on a small compact driver or a tall driver, verses putting it to the axle gear.

... OH Forgot to tell you. I also used mini gears of less then 59 teeth on the axle. There available now to buy but I had a jr dragster hub machined down so I could put 4 bolt mini bike gears on it. ... I think they were mini bike gears but anyway you can buy mini gears now for karts. The small gears will solve your gearing problem and you can run then outboard over the motor mount rails if you need to. I used them both ways. You don't have to have both the output from the jackshaft and the clutch on the same side of the jackshaft if you use mini gears. Plus you will be able to use less chain and reduce rotating mass and increase acceleration.
 
I'm with Paul on the primary gear sizes.
Seems way too small. For the load.
Wondering out loud if a vbelt or dual vbelt might be easier to get and still transfer the load.
 
I understand that this is a road course and things are different than what i do. But I figure the ratio at a 2.10, is that enough gear to keep from dogging the clutch coming off the corners ?
 
Figuring your overall gear ratio is just math. The issue comes when you put your clutch on the jackshaft instead of just leaving it on the engine.
 
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