Gear Ratio Calculator

[runslikeapenguin]

i think i might be using the calculator wrong, with a 26" wheel and engine speeds of 5000RPM's im getting speeds of like 700mph.

can someone give me what the stock gearing on most of the engine kits is?

 

[ocscully]

If you are asking about the Chinese 2-stroke/Ht kits the motors have a built in 4:1 reduction so input 10t to the first field and 40t to the second field. The output driver sprocket from the motor is 10t so input 10t to the third field and the most common driven sprocket for the rear wheel is 44t so input that to the forth field. You really don't have any control over the primary 4:1 reduction and i'm not real sure but the output from the motor may be pretty locked in at 10t as well? About the only one you can change easily is the rear driven sprocket.

ocscully

 

[runslikeapenguin]

If you are asking about the Chinese 2-stroke/Ht kits the motors have a built in 4:1 reduction so input 10t to the first field and 40t to the second field. The output driver sprocket from the motor is 10t so input 10t to the third field and the most common driven sprocket for the rear wheel is 44t so input that to the forth field. You really don't have any control over the primary 4:1 reduction and i'm not real sure but the output from the motor may be pretty locked in at 10t as well? About the only one you can change easily is the rear driven sprocket.

ocscully

thanks thats what i was doing wrong, i dident have the initial gear ratio set at 4:1

i was like "i can do 105mph at 6 grand!"

 

[wildemere]

its actually 4.1:1 20t drive & 82t driven.

 

[duivendyk]

The calculation is pretty close,I once measured the actual operational radius of my 26" wheel and would have ended up with 28.1 mph,slightly lower.To go 10 miles faster would require an rpm of 10850 ,which appears rather unlikely.

 

[BrettMavriK]

Okay what am I doing wrong?
I'm getting like 392 mph and 6000 rpm.

I am building a homemade jackshaft, but I'm having gear confusion....

-I know the motor gear is a 10t.
-I think the first sprocket on the jackshaft will be an 18t.
-Then on the other side I was thinking of a 28t
-Then to a Sturmey Archer 3 speed which I don't have yet, so I don't know ratios.

I think I'm inputting it wrong.

 

[ocscully]

Okay what am I doing wrong?
I'm getting like 392 mph and 6000 rpm.

I am building a homemade jackshaft, but I'm having gear confusion....

-I know the motor gear is a 10t.
-I think the first sprocket on the jackshaft will be an 18t.
-Then on the other side I was thinking of a 28t
-Then to a Sturmey Archer 3 speed which I don't have yet, so I don't know ratios.

I think I'm inputting it wrong.

I don't have any idea how much total reduction you need for 20" wheels but for 26" wheels most of the kits offered have a total reduction 0f 18:1 - 22:1
So if we are talking about the Chinese 2-stroke motor you need to factor in the reduction that is built into the motor first so in the first field of the calculator input 20t and in the second field input 82t this should give you 4.1:1 reduction, in the third field input 10t and the forth field input 18t this will give you 1.8:1 which is multiplied with the 4.1:1, in the fifth field you would input 28t and the sixth would get what ever tooth sprocket your 3-speed hub comes with which could be any thing from 16t - 22t Then down toward the bottom you need to change the wheelsize fromm 26 to 20 then when you input a given rpm the software will calculate the mph.

You might want to do some searches for threads on 3-speed hubs here on the forum and read thru them as I believe members here have had less than stellar results using 3-speed hubs in there motorized bicycles.

Hope this helps and good luck with your project.

ocscully

 

[BrettMavriK]

Thanks, man,
it's a little clearer now...

Now I'm plugging in numbers like you have said with he first inputs being 20 and 82 and 10. the Sturmey's freewheel is 19t. So, this leaves me with two values; the left and the right side of the jackshaft. I'm getting the 18-22:1 ratios, but at 6000 rpm, I'm going like 18 miles and hour.

I'd like be in the 30mph's at that rpm....
hmmmmm....

'BrettMavriK

 

[duivendyk]

I am confused as to what exactly you plan to do,you mention a jack shaft presumably to get to the right side for the drive to the 3 speed hub,is that a direct drive from the jack shaft instead of via the crank ?,this means you won't be able to pedal the bike.If you intend to use the crank as intermediary you will need a freewheel crank.In that is the case there are 4 reduction stages internal engine / engine to jackshaft/ jackshaft to crankwheel/ crankwheel to hub input/.the calculator could still handle this if you combined the first 2 reductions (4 and 1.8) into one of 7.2 (you could put in say 72 and 10 ) then you have the last 2 reductions left in the calculator for the last 2 stages.
If you go directly from jackshaft to the hub,your ratio would be :based on your figures:
4x(18/10)x (20/28)=5.14,(assuming a 20t rear hub sprocket)This is far too small, you should have a reduction of around 13-17,which is about 3 times as large.So the input sprocket to the jackshaft has to be made larger and/or the output one smaller.For instance 20 t input and 10t output sprocket on the jackshaft gets you a 16:1 reduction.and about 19mph road speed at 5000 rpm.For a 20" wheel the following formula applies:
Speed= 0.06x rpm/R, R is overall reduction.For instance you would like 25mph at 6000 rpm,solve for R, R=6000x0.06/ 25=14.4 So you need around a 14:1 reduction.Understood?

 

[BrettMavriK]

Here's a number set that looks good, but it's way off the 18-22:1 you have suggested:

20-82-10-13-8-19 ------ 12.66:1 Ratio / 28.2 mph@6000 rpm

This would mean in 2nd gear, which is 100% or 1:1 on the SA three speed hub, it would propel me to 28.2 miles per hour at 6000 rpm. This sounds about right because I still have 3rd gear left to get me to at least 38.5 mph or more, depending on motor mods and such.

The ratio at 12.66 however is way off from your suggesstion of 18-22:1
So, is it all in the wheel diameter that these ratios are so far apart, and I shouldn't be worried to go ahead and buy the sprockets?

'BrettMavriK