Need help with gears ratio

This is a very hard question to answer as there are so many variables. Here is a link to a great diy calculator. Download the exe file and input all info. It will allow you to find what your speed will be at any given rpm http://jimsitton.net/ratiocalc/

Oh, and welcome to the forum. show some pics!

badny8 said:

Thanks a lot WightBoy,
I download it but I am not sure I am doing right.
I live in Costa Rica and we got a lot of hills, so mainly I need power versus speed, I dont really care about my top speed, what I am concern is that the little engine will help my up the hills since the bike is quite heavy (around 80 pounds).
Do you guys know what would be an standard setting for this so take it from there?

Click to expand...

There are no standard settings.

You aren't telling us your rear wheel diameter and motor redline so there
is no way to figure it out.

Give your sprockets like this.

?? teeth on engine
?? teeth on jack shaft

?? teeth on jack shaft
?? teeth on rear wheel

Rear wheel diameter
Engine rpm


Input all the numbers above into the calculator and you will get the speed

Leave the 3rd set of boxes with 1 in them.

Help for Gear Ratio Calculator

Enter tooth count for up to 3 gear/chain sets. If you have less
than 3 gear/chain sets leave the unused fields at the default
value of 1:1.

If you're using a belt drive enter the diameter of the dirve and
driven pulleys in place ot tooth count. Any ratio will work and the
diameters can be entered in any chosen unit of measure as long
as the drive and driven pulley diameters are entered using the
same unit of measure.

Measure your wheel diameter for the most accurate speed. Using
the diameter printed on the tire can introduce errors.

For friction drives use the diameter of the friction drive wheel, not
the size of the driven wheel, as input to the wheel diameter field.

Click to expand...

badny8 said:

Thanks a lot WightBoy,
I download it but I am not sure I am doing right.
I live in Costa Rica and we got a lot of hills, so mainly I need power versus speed, I dont really care about my top speed, what I am concern is that the little engine will help my up the hills since the bike is quite heavy (around 80 pounds).
Do you guys know what would be an standard setting for this so take it from there?

Click to expand...

Is this a single-speed bike? I'm presuming that it is, so if you want to climb steep hills, your top speed on the road MIGHT be 20mph, with engine screaming. With a 15" car rim, let's say your tire diameter is 26". This is to make things simple, since many bicycles have 26" tire size. Lemme make calculations and come back.

Update: Use 29:1 to climb hills @ 7500rpm. Top speed is 20mph. Use 24:1 to climb hills with top speed @ 25mph @ 7500rpm. Find the 5;1 pocket bike transmission. Without it, proper gearing is impossible. Use 8mm T8F chain and sprocket from transmission to chainring. This to maintain strength and keep the chainring sprocket size smaller. Place an 11-tooth sprocket @ the trans and use a 63t chainring sprocket. That's 28.64:1, so the outside chanring should be 24-tooth. The rear hub sprocket will also be 24t. For 24:1 gearing, use all the same gears except the rear wheel hub. Change that one to 20-tooth. That gives 23.86:1.

If it was a regular 26" bicycal wheel, these calculations would work well. Unsure how the sheer weight of the car tire will affect hillclimbing. I know it will put a great strain on the pocket bike's clutch, which is the weakest part of the engine. Good luck.

You're welcome. Your bike will look baad with the huge rear tire.