I'm not at all confident about buying the right gears. I have a T72 on the back rim and I believe a T12 up front right now. I plan on buying a new clutch (so the "T" may change) and a final drive if I need to. This is for my 7yr. old and I can't make a mistake. I understand there are four total gears, can you give me the correct "T" numbers for 5/1 and 6/1. I can stare at the gear ratio chart for an hour and still think I'm going to screw up. I'm new to this. Thank you.
Stay with a 12 tooth on the clutch and You'll keep the 6;1 you currently have with the 72 rear.. with the setup you say you have, that will work well. Just buy a good quality clutch. I'm running a Hilliard extreme duty on one for my 8 year old with a 2hp Briggs and it will haul my 250lb butt around without overheating...
I guess I don't understand what the jackshaft accomplishes. Other than being able to line up gears, what is the benefit when I have 6:1 already? Are the two jackshaft gears the same size, if so what size? I don't mind spending the money for either a stock js (I think I would have to move the brake drum/final drive unit to the other side) or a new setup that's all snazzy that the kid would love (drum remains on left, fitment issues may arise with clutch cover). I know I missing something that's probably glaringly obvious, but I don't know what it is.
Welcome Endo, Google or search here "jack shaft calculator" and plug in your numbers of teeth etc. and it will figure out your final drive ratio.
That way you can either get two of the same number of teeth for the jack shaft gears (to keep the same ratio) or some extra gears and really confuse yourself, like I did, lol.
Also, take the calculators that give you a "top speed MPH" for a loose guide of MPH as there is more than wheel sizes and gear ratios involved with top speed...
That way you can either get two of the same number of teeth for the jack shaft gears (to keep the same ratio) or some extra gears and really confuse yourself, like I did, lol.
Also, take the calculators that give you a "top speed MPH" for a loose guide of MPH as there is more than wheel sizes and gear ratios involved with top speed...
Last edited:
If the "T" numbers were 12 (clutch), 12 12 (jackshaft), and 72 final drive, my ratio remains 6:1. By changing the gear on the js running the final drive, it would fractionally change? Is that how people get odd ratios like 4.10:1? My question remains, what benefit does the js offer other than fine tuning both the ratio and the alignment. I want to spend the money wisely. I should mention that there is no js on now. Thanks.
And not one word on tire dimension here. That is the largest factor in the computation, and it is not computed into ratio on any calculator beyond the nebulous and distracting "top speed" number.
Generally, a chain clutch is not suitable for any drive tire diameter in excess of 15 inches. Do not fall into that "top speed" box, as it can be very misleading.
What kind of bike is this? Chances are, someone here will know exactly what it takes to make it work well for your goals.
Generally, a chain clutch is not suitable for any drive tire diameter in excess of 15 inches. Do not fall into that "top speed" box, as it can be very misleading.
What kind of bike is this? Chances are, someone here will know exactly what it takes to make it work well for your goals.
For example I'm running 6.8 to 1 on a 18" tire (17" real dia.) 6.5 hp on the flats.180lbs ..smooth takeoff, no wheelies clutch ok.coz I'm running only flat pavement i can get away with that ratio..off road i would need 9 to 1 to 10 to 1....burning the clutch depends on where you ride,ratio, tire size and weight of rider...to help you decide tell us those and we won't have to guess..
Last edited:
Yep...Gearing is important. But as "automatic" as the clutch is it still depends on rider input with the throttle to get the clutch fully engaged as soon as possible minimizing slippage. My son recently had his first ride on a mini bike. We were doing some slow speed yard riding. After watching him for a minute or 2 I stopped him and told him don't baby that thing, get on the throttle and go. Be more aggressive with your riding.
Bottom line is that clutches slip and heat up. The idea is to get full engagement as quick as possible.
Bottom line is that clutches slip and heat up. The idea is to get full engagement as quick as possible.
Sorry for the confusion. I'll lay out my plan, this might be long.
The bike is a 1970 Bonanza Scout MB300 (Scooterboy id'ed an identical one a few years ago). It has an H25 w/o a governor. 12T up front and 72T final drive. I'm putting a Honda (real one) GX160, 5.5 hp on in addition to new throttle (one that I can adjust to fit a child 55-75 lbs). I picked up 5" shoes from OldMiniBikes and they're on ready to go. The rims are 6", the present tires are 4.10/3.50-6 double Indian head. They will work, but an upgrade for lawn/field riding is needed.
My overall goal is to have a dependable engine that I can self-govern through throttle control (hopefully a predator plate will do the trick). I'll be able to blast the frame, forks, and rims ( glass bead) before painting. I swapped out the wheel bearings front and back a couple of days ago. I have fenders on the way but the front shocks probably mean that one can't be attached on the fork sides. The back will have tabs welded on and installed as normal.
The whole idea of the jackshaft was to be able to alter the ratio to compensate for my son's lack of experience (7 yr old). The more I thought it seemed I didn't need one if the throttle plate will work. The chain has a long run at present and I saw the js setup and thought it would help. I hope that makes sense now. I want to be able to speed it up as he progresses. Thanks.
The bike is a 1970 Bonanza Scout MB300 (Scooterboy id'ed an identical one a few years ago). It has an H25 w/o a governor. 12T up front and 72T final drive. I'm putting a Honda (real one) GX160, 5.5 hp on in addition to new throttle (one that I can adjust to fit a child 55-75 lbs). I picked up 5" shoes from OldMiniBikes and they're on ready to go. The rims are 6", the present tires are 4.10/3.50-6 double Indian head. They will work, but an upgrade for lawn/field riding is needed.
My overall goal is to have a dependable engine that I can self-govern through throttle control (hopefully a predator plate will do the trick). I'll be able to blast the frame, forks, and rims ( glass bead) before painting. I swapped out the wheel bearings front and back a couple of days ago. I have fenders on the way but the front shocks probably mean that one can't be attached on the fork sides. The back will have tabs welded on and installed as normal.
The whole idea of the jackshaft was to be able to alter the ratio to compensate for my son's lack of experience (7 yr old). The more I thought it seemed I didn't need one if the throttle plate will work. The chain has a long run at present and I saw the js setup and thought it would help. I hope that makes sense now. I want to be able to speed it up as he progresses. Thanks.
So now I'm back to square one. Do I use a jackshaft, gear it so it's slower yet make the engine work overtime. If this is a matter of clutch burn I guess I don't mind going through a few until the boy can handle the bike. Do I save the money and go w/o the js and stock up on clutches? If this is the trade off so be it. I have seen clutches that engage at different rpm's, what clutch would best suit this particular situation. BTW, thanks for all the input from you guys.
I would not use a jack shaft. Jack shafts are used primarily to align drive chains where mounting is offset, or where chain run is excessive. You are dealing with neither of those issues on this bike.
Restore it as you are planning, and use the standard entry level clutch. Your seven year old isn't so heavy that the clutch will burn up quickly.
Those clutches start to engage at around 2000 RPM. I would not let concerns about burning out a clutch drive my decisions on power train. Instead, build the bike as you intend, no jack shaft, and concentrate on that, with an initial means of limiting RPM, or even sitting behind him as he learns to operate the throttle and brakes. (grin) That last- riding double and putting around, is hard on clutches. Make sure you oil the bushing well, and often, keeping the oil off of the shoes- only on the bushing.
I'll bet he is able to handle the bike pretty quickly.
Restore it as you are planning, and use the standard entry level clutch. Your seven year old isn't so heavy that the clutch will burn up quickly.
Those clutches start to engage at around 2000 RPM. I would not let concerns about burning out a clutch drive my decisions on power train. Instead, build the bike as you intend, no jack shaft, and concentrate on that, with an initial means of limiting RPM, or even sitting behind him as he learns to operate the throttle and brakes. (grin) That last- riding double and putting around, is hard on clutches. Make sure you oil the bushing well, and often, keeping the oil off of the shoes- only on the bushing.
I'll bet he is able to handle the bike pretty quickly.
I can't help with the gear thing because it confuses me but I can tell you the motovox I built with a 6 1/2 hp 212cc predator engine had a 75 tooth rear sprocket and 12 tooth front & the mini bike was dangerous.
I went down on the rear sprocket to a 58 tooth and it is a lot safer..
My cat mini bike that I am building now has a 68 tooth rear sprocket and I can not find anything smaller so I went with a 13 tooth front sprocket.
I don't know how the cat will Handel but the motovox was dangerous and I would not give it to my son unless you don't like him.
I don't think anyone can help much without all the information.
What kind of bike?
Tire size?
Childs weight?
I went down on the rear sprocket to a 58 tooth and it is a lot safer..
My cat mini bike that I am building now has a 68 tooth rear sprocket and I can not find anything smaller so I went with a 13 tooth front sprocket.
I don't know how the cat will Handel but the motovox was dangerous and I would not give it to my son unless you don't like him.
I don't think anyone can help much without all the information.
What kind of bike?
Tire size?
Childs weight?