Jackshaft considerations for reeds

b0son

New member
I’ve got some TM K11B’s collecting dust since my local track put in a clutched karts only rule :(

Putting a clutch on them was not an option without putting in a JICA crank half. Then I stumbled upon the jackshaft… all problems potentially solved.

Though I have some questions on the best way to go about it.

I normally run 10t front and 80-85t rear. My first thought was put 10t either side of the jackshaft, but this would result in the clutch running at up to 20,000rpm! So I was wondering, should I gear it down to reduce the clutch rpm?

A lot of people run Yamaha KT100s here (I'm in Australia), so a clutch to suit one of these would probably be best for replacement parts … most are centrifugal, though there is also the Tomar (have never used such a clutch so no idea if they are suitable for 15000rpm+).

Anyone have an idea on the best approach for implementing a jackshaft on a high-rpm motor?
 

alvin l nunley

Site Supporter
Anyone have an idea on the best approach for implementing a jackshaft on a high-rpm motor?
The most common reason for the jack shaft is to make it possible to run engines like the KT100 on short tracks and keep the engine in its power range RPM’s. With a typical KT pipe, (not a Veeve) most clutch karts use an RPM range from about 10,000 to 15,000. Maybe a little higher or lower.
A gear range on the primary would be about 2.5-1 (14/35 for example) thru 3.0-1. The secondary gear set would then be adjusted to fit the track you will be racing on. Over all ratios, engine to axle, would/could be from maybe 8-1 thru 10-1. Even lower than 10-1 is not unheard of.
Most jack shaft drives have the clutch on the secondary side of the jack shaft. This means they are absorbing in the range of 2.5-3.0 times the torque compared to a clutch mounted on the engine. It takes a strong clutch to hold that torque, but it does it at a much lower RPM.
Ga day mate.
Comments, compliments, criticisms and questions always welcome.
 

ttownwideglider

New member
For the kts we use the jackshaft as an overdrive setup. We set the ratio at about 2.7-3.0 to 1. So the engine is turning about 10500rpm and the jackshaft i and clutch are turning about 3,874 rpms. We lock the clutches around 10,000-11,000 rpms where ever the powerband hits for the engine. Once the clutch is locked in it doesnt matter how many rpms you turn on the top end becuse the discs are no longer slipping. The overdrive helps greatly with the amount of wear the discs are subjected too becuse the clutch is turning so much slower than the engine when it locks up. A lot less expence in clutch rebuilds. The belt drive for the jackshaft will require a crankshaft pully on the kts it keyways on a tapered shaft.Dont know how your engine crank is set up but some guys make special drivers for their engines. The rear pully on the jackshaft is where the large overdrive pully is located. This also eleminates the weight of the clutch on the crank so the engine seems to accelaretes faster.Plus this make gear ratios extremly flexable, you can change the # of teeth on the clutch driver, the# of teeth on pullys for the overdvie ratio. You can go to a very low ratio 13 to 1 with a much smaller axel sprocket for short tracks. Hope this helps you. Some of the other guys on here my be able to help you better with your type of engine. Jack Burroghs and Jon Brogden or John Hilton can probably help. See ya Mitchell
 

b0son

New member
is it common to run different chain types? ie. 219 off the engine and bigger for clutch->axle? I was thinking of running something bigger off the jackshaft for longevity, my 428s on my shifter are bulletproof, but my reeds wear through 219s pretty quick. what determines which size you would use? power? what would you recommend for >30hp?

i'm also not planning to spin the clutch up, I want it to be as close to direct drive as possible, with clutch engagement as low down the rev range as I can get away with. what sort of clutch, especially if I gear the clutch to run much lower rpms?
 

arc100

Member
I’ve got some TM K11B’s collecting dust since my local track put in a clutched karts only rule :(

Putting a clutch on them was not an option without putting in a JICA crank half. Then I stumbled upon the jackshaft… all problems potentially solved.

Though I have some questions on the best way to go about it.


I normally run 10t front and 80-85t rear. My first thought was put 10t either side of the jackshaft, but this would result in the clutch running at up to 20,000rpm! So I was wondering, should I gear it down to reduce the clutch rpm?

A lot of people run Yamaha KT100s here (I'm in Australia), so a clutch to suit one of these would probably be best for replacement parts … most are centrifugal, though there is also the Tomar (have never used such a clutch so no idea if they are suitable for 15000rpm+).

Anyone have an idea on the best approach for implementing a jackshaft on a high-rpm motor?

Nice to see another aussie on here. I live in Australia and run a Buller 4 disc j/s. The only j/s in the country. If you need to know anything call me anytime. Mob: 0402 487 310

USA guys run imperial everything except for Margay so you will need to know what you can and cant fit to metric karts and what crank driver hubs are available tapers that fit and stuff like that.
 

arc100

Member
You have to run the #35 chain which you cant get in a competition chain in Australia either. Well at least I've never seen it. I get all my stuff from USA. You will need all the consumables as spares because you wont be able to use anyone elses stuff because it just doesnt excist here. It cost a few bucks to set it all up and you WILL have to mess around a lot to get it to fit but once you're up and running you will NEVER EVER go back to a DD (direct drive) set-up.
 

arc100

Member
Also another good option is a Birky clutch on the j/s. Not sure how well they go with a standard 100S but anything reed should be fine. The Birky's are bullet proof. However the Buller and Birky will both handle 60Hp+.

Check out www.buller.net
 

b0son

New member
arc100, thanks for the offer, I will probably get in touch after the new year when I've had a chance to have a good look at all my options. I suppose my most immediate question right now is... how do you start it? I have an external starter for my old KT100s with strike clutch, and thought of putting the starter nut on the jackshaft, somehow ... is this appropriate? my engine has a splined crank so no way to attach anything direct to engine.
 

ttownwideglider

New member
On the kts the starter nut is threaded on the shaft where the clutch is mounted and also holds the clutch on. I turned my cylinder and head around and run the clutch or pto side of the shaft on the inside of the kart on with the engine mounted on the right side looking at the kart from the rear.That put the belt drive on the insidenext to the seat and the chain and clutch outboard on the jackshaft, away from the driver. I did this for safty and to clear the seat struts. I had to fab a starter nut for the ignition side which is now outboard, buy welding a starter nut to the rotor nut that holds the rotor on the shaft which is also threaded. Then I drilled a hole in the ignition cover for the starter nut to come out of. If you ignition side of the shaft is threaded you might be able to do the same thing. Dont know if you can reverse the cylinder and head on reed type engines.
 

arc100

Member
As wideglider mentioned mounting the driver off the ignition side is the best way for you to go about fitting the driver. However I didn't reverse cylinder, I just used an inboard ignition rotor extension nut and bearing support that are commonly available in the US. These require a bit of a light skim on the rotor so that they run almost dead true on centre when you torque up the extension nut. The spline PTO thing is a real pain for j/s set-ups.

And for starting, I would just machine a normal splined sprocket down and then drill out a nut and weld the 2 together. Maybe someone on here sells these already.
 

arc100

Member
Can also go with the easiest option of an axle clutch. But not sure you can get them for metric axles. SMC made them for 40mm but not anymore. That way you can still run your 219 chain and sprocket stuff but you wont get the extreme longevity of a #35 chain and sprocket set.
 
Understand on the primary side we run a belt drive. I might have missed that you knew that. Chain on the primary does not hold up. We also run dual ignition crank halves as they are stronger than the PTO halves and hold up much better. As I am sure you cant get clutch disk Down under as well as your 35# chain so that will be something else you will need to get from us. If you are running dual reeds you can run one inboard and one out. Have the 2 drivers in the middle and you can tie the two engines together on a Birky clutch and you can run 428# on the secondary side so it will hold up.jmo
 

b0son

New member
belt drive isn't really an option, the drive side crank is too short (sprocket is held on with circlip, not a nut so not even a thread there). its barely long enough for a chain sprocket (since these engines were never meant to carry a clutch).
 

arc100

Member
belt drive isn't really an option, the drive side crank is too short (sprocket is held on with circlip, not a nut so not even a thread there). its barely long enough for a chain sprocket (since these engines were never meant to carry a clutch).

You have to run a belt drive to the j/s, whether you run ignition side or build crank to suit you still need the belt. Thats what makes the j/s system work so well.
 

b0son

New member
why is a chain-based jackshaft a no go? are they simply unpopular, or problematic? the Burris setup looked pretty nice and was the direction I was leaning :( Cant build a crank to suit as the only crank that would work would be a JICA crank which isn't available anymore, and the expense simply wouldn't be worthwhile
 
With the chain spinning that fast they get hot and don't hold up. You can run one but you will be putting a new chain on every race. The belt is way more forgiving. The Burris is set up for 4 strokes which don't spin as fast is how they get away with the chain. I cant believe you cant find a couple of ignition halves to switch out the PTO, just much more stronger and reliable.
 

arc100

Member
With the chain spinning that fast they get hot and don't hold up. You can run one but you will be putting a new chain on every race. The belt is way more forgiving. The Burris is set up for 4 strokes which don't spin as fast is how they get away with the chain. I cant believe you cant find a couple of ignition halves to switch out the PTO, just much more stronger and reliable.

Brian, we aint got **** here in aus mate. You yankee guys have all the fun stuff. And cheap spares.

Its way easier if he just runs an ignition side driver with the extension nut.
 

b0son

New member
Haha... its a formula A, I'm no stranger to throwing chains at it every other week. If I had more sprocket options, I'd be running a 428 like on my shifter, those things are bulletproof.

Other than lack of parts, the other main issue with crank modifications is that I have three of these motors (at 20,000rpm, you don't get long before it wants to go boom) so the cost of setting all three up would be prohibitive.

Anyone got a pic of an inboard/bearing support setup?
 

b0son

New member
thanks.

I checked the engine and it is 80x116mm, so I think an IAME plate would be closes, though still needing a re-drill. the extension shaft would probably be a good solution, I would just need to check the dimensions to make sure it would fit next to the seat.

is the extension shaft long enough for a clutch? looks like it might be, in which case, it may negate the need for a jackshaft altogether if I could just bolt the clutch straight onto the ignition side.
 
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