Gearing new kart ??

[Cutters]

Hi all, I'm new to karting and need a little help with my new project. I've recently built a new kart for my kids and i'm up to the gearing part.
I'm using a Honda GX270 (9hp)it has a 6:1 reduction box built in with no clutch and has a 1" output shaft. My axle set up is a 30mm diameter and set up to be a single wheel drive. I'm not after top end speed, as my kids are still young. My rear tyre Diameter is 16".
I was planning on going from my engine to a jack shaft to bring the rev's back up (1:6) then i will fit a max torque 12 tooth #35 pitch to jack shaft and run a 76tooth #35 pitch sprocket on the axle. this would leave me with a drive ratio of 6.3:1
So i guess my question is will this work, without over loading my clutch. Im not at all worried about top speed.the kart will mainley be used in a yard with very little hills...

Any help would be appricated..thank you

 

[Doc Sprocket]

Can you not remove the redux case?

 

[Cutters]

Unfortunately not.. I looked into this and the reduction box side cover is the same cover used to hold the main crank shaft bearing.....

 

[Doc Sprocket]

So- haow about sourcing a factory stock "non-reduction" side cover? I'm just thinking that it'll save you some hassle, weight, maintenance, ratiating, mass, etc to eliminate the jackshaft.

 

[Cutters]

This would be an option but could get expensive only because the crank shaft would also have to be replaced as it is too short to leave me much of an out put shaft...apart from the hassle of the gearing up again do you think that a 6.3:1 final ratio to 16" rear wheels will be ok? Or will it excessively load up a centrifugal clutch?...
Thank you

 

[Doc Sprocket]

How young are the kids? Between their age and the clutch, 6:1(ish) may still be too high.

Is it practical for you to sell the engine and then buy one that does not have a built in redux unit? For if you need to gear lower yet, now you're looking at a jackshaft to gear up for the clutch, and then a second jackshaft to gear down again to rduce to the final drive ratio- kinda clunky...

Another option for you may be to just set up a belt system and tensioner clutch arrangement. You can then either a)run the tensioner off a separate "clutch" pedal, b) set it up like a lawn tractor and tie the tensioner into the brake pedal, or c) connect the tensioner to the throttle. Bit of a learning curve for the driver, but not much. Took my kids no time to figure out how to drive the lawn tractor. In this case, the clutch is tied into the brake. Press it to disengage the clutch and stop, release it to go. Just thinking out loud.

 

[Cutters]

Thanks for the quick reply Chris. The only thing with the belt set up is my boys are 6 & 8 and a third pedal could be a little too much for my young bloke. And if incorporated into the brake pedal I'm worried they might try to get out while the kart is stopped but not off therefore it may try to take of or jolt forward and hurt them or any other kids waiting for there turn. If I was able to govern the speed/throttle with a 6.3:1 ratio do you think that the 16" tyres would put too much strain on a centrifugal clutch. Would take off be an issue? By boys are not "big" kids.
Your help is appreciated?

 

[Doc Sprocket]

You don't want to use the throttle to limit the maximum speed like that. The clutch will burn out quickly from all the excessive slipping.

Okay- 3600RPM, 6.3:1, 16" tires. My speed calc says 27.2mph as a theoretical top speed. That's spooky fast for a 6-year-old. Gear down. A lot.

For any "step out of the kart and it takes off" concerns, you could always rig a lanyard to a kill switch like a jetski has, or any number of other solutions...

 

[redsox985]

Maybe even steal one from an old treadmill? Those magnet style ones with springy cords.

 

[Cutters]

Never had anything to do with tread mills...
Do they have a magnetic clutch?

 

[Doc Sprocket]

I believe Redsox was referring to the kill switch and lanyard assembly.

 

[Cutters]

If I did try a belt system what ratio/ diameter pulleys would you recommend ? Would I still need a jack shaft?

 

[Doc Sprocket]

Gear ratios are gear ratios. When gearing with gears or sprockets, you do the math with tooth counts. With pulleys, you do the math with pulley diameters. FWIW, you're generally better off with sprockets as they tend to be smaller overall, as there's only so small you can go with a pulley before belt slip becomes a nightmare.

 

[Cutters]

Thanks for that, I'll have a play and let you know how I went.

 

[B man]

what about hooking the tensioner and throttle to one peddle? that way as you throttle the rpms and tension are moving together? and why not a seat kill switch out of a lawn tractor?

 

[Cutters]

That sounds like a good option also...b-man.
Has anyone done a silimar belt/clutch set up? Are there any pics around or web sites to see?

 

[Cutters]

It's been a while, been busy fabricating and testing. Tried clutch and brake together but spring tension required to keep belt tight enough mention brakes were a little too had for the kids. So what I got now is a separate clutch lever that is spring loaded on and off position works over center. Took the kids no time to pick up, as you said toy story. Happy days now the kids buzz around at up to 25km p/h

 

[r_chez_08]

IMO i would sell that redux box. Im pretty sure they are expensive! Then I would use the money to buy a new side cover and a jackshaft. Sounds like the simplest setup.