Gear Reduction Results

[Motra]

There always seems to be questions here about gearing so here's the results of my experiments. I have a 4HP GX120 in a DB that I wanted to gear down so I built this jackshaft (it's still in progress, the shaft sticking out the side won't stay there). I went from 5.83:1 down to 7.46:1............and it isn't quite enough. The acceleration still isn't what I'd like. I think I'll add a couple teeth to the jackshaft driven sprocket and see if that helps more.

 

[oldfatguy]

I have a set up like that on my bike the final drive ratio is about 9 to 1. I am using a 6 hp tecumseh the low gear ratio gives it great acceleration and hill climbing power but the top end isn't that great. This works out good for how I use the bike. You can probably gear your bike to around 10 to 1 which will give it a lot of low end power. This will work out well if your riding area is small [like a back yard] where you don't have the room or need for high speed. A torque converter will give you both the low speed power and the gear change for high speed. If you just run on a short track the clutch /jack shaft will work fine. If you gear it too low the bike will just want to wheelie all the time and dump you off the back. I know, mine has dumped me a few times, all it takes is to hit a small bump while accelerating and the bike will stand straight up. all though it is fun to ride down the street and be able to crack the throttle and have it rip the front tire off the ground.

 

[spewya]

i have a 12 tooth clutch and a 12 tooth sproket on the jack shaft right be hind that and fom that it is spinning a 14 tooth sproket .. im just clueless on what size rear wheel sproket to use to have the combination of low end and top end

 

[oldfatguy]

i have a 12 tooth clutch and a 12 tooth sproket on the jack shaft right be hind that and fom that it is spinning a 14 tooth sproket .. im just clueless on what size rear wheel sproket to use to have the combination of low end and top end

running a 12 tooth to a 12tooth is basically a high speed set up because you are not getting any gear reduction from the jack shaft which is defeating the purpose of having it. You should run the 12 tooth clutch to a larger gear on the jack shaft [around a 20 tooth] this will improve you low to mid range power but you will lose some top speed. The larger the rear sprocket the lower the ratio will be [ most minis were 6 to 1, meaning the engine turned 6 times for 1 at the rear wheel] A jack shaft will allow you to lower the ratio to about 8to1 up to about 10to1. the lower the gear ratio the more low end power you will have but you are trading top end speed to get it.

 

[spewya]

wel these are the gears i have so thats what im useing .. i just need to buy a rear sproket and i need some advise on what size i should buy

 

[oldfatguy]

a smaller sprocket [60 tooth] will give you a higher top end then the larger [72 tooth].
Your jack shaft the way it is set up isn't doing anything for you. You are getting no gear reduction running a 12 tooth to a 12 tooth, and you are losing some reduction with the 14 tooth. This set up is almost the same as running from the clutch to the rear sprocket because you are getting no benefit from having the jack shaft. The size of your rear tire if its small could limit how large of a sprocket you could use. The first thing that you need to decide is how you want to use the bike depending on where you ride it to decide what gear ratio is best for you to use.

 

[gbabins1]

Use this calculator to get the gearing you want. The higher the first number ( ex. 10:1) the more bottom end / less top end.
I just made one for one of my DB's works great.

Burris Racing Jackshaft Calculator

 

[Motra]

wel these are the gears i have so thats what im useing .. i just need to buy a rear sproket and i need some advise on what size i should buy

You'd be better off going from the 12T clutch to the 14T on the jackshaft, then from your 12T sprocket to the rear wheel. The overall gearing depends on the engine you're running too. If it's in the 3HP to 4HP range you'll probably want a 72T, but if it's a 6.5HP or more a 60T would probably work fine. It depends on the type of riding you do too, just like the others have said, lower gear ratio = more low end and less top end and higher gear ratio = more top end and less low end.

 

[spewya]

im useing a 6.5 engine and the tire is 13" so would a 60 tooth rear be good for dirt trail riding with some hills.. i will acasinoaly ride on flat surfaces

and why would i want to switch those two jack shaft gears??

 

[spewya]

when i use that jack shaft calc its not givine me a ratio with 2 numbers its just saing a number with a decimal like its not saing 10:1 its saing something like 4.86

with a ratio like x.xx and what ever x comes out to be is it still the same concept
higher the number= the more low end power and
the lower the number= the faster it will go

 

[oldfatguy]

you can check your ratio right on the bike, just mark the clutch and rear tire [a piece of tape ]

Then turn the clutch and count how many times it makes a full turn to make the rear tire go around once. a 4.86 ratio means that the engine or clutch turns 4.86 times to make the wheel turn once. Most mini bikes are about 5 to1 or 6 to 1 with just a clutch and rear sprocket. A jack shaft lets you get a lower gear ratio then you can get just a clutch and sprocket .For off road trail riding something around 8 to 1 or more will work good and you will give you power and acceration.

 

[Motra]

I've pretty much ignored this bike for a couple months but I finally got back to it. I removed the excess shaft and added a brace, then I took it for a long test ride yesterday. Now I'm happy with the gearing. It pulled me around just fine and topped out at 22 MPH, which will be fine for the kids to ride and that's what this bike is for.