# Gear Ratio and Jack Shaft Confusion......

#### BrettMavriK

##### Member
K, my brain is starting to smoke.
I've been trying to calculate numbers for my homemade intermediate jackshaft
plans, so I can purchase the sprockets. I know gear reduction is needed, but how much for these motors? 4:1? 3:1? 6:1?

And I know I'm not calculating it correctly. (wish I knew the formula)
I know the motor has a 10 tooth on it, and I'm using a Sturmey Archer 3 Speed Hub on the 20" Stingray rear wheel.... it's just the in between gears on the jack shaft I'm having trouble with.

I was thinking of the first sprocket from the motor on the jackshaft would be an 18t. Then from there, I get lost and don't know if I should be going down in reduction or up in multiplication.

Anyone have a good equation to figure this out???

'BrettMavriK

A

#### andyinchville1

##### Guest
HI,

Pablo and Ghost (of Sick bike parts to the left (sponsor)) make a shift kit for the HT engines...you might try to contact them and ask them about the gearing in their kits (I have a kit of theirs but it is at the bike shop being installed so I can't tell you the sprocket sizes you seek).

Hope this helps you.

Andrew

#### BrettMavriK

##### Member
Thanks

I have spoken to them a little about it. The problem I'm having is I hope to use only 4 sprockets in my setup from the engine sprocket to the wheel sprocket, and they are using 6 in theirs. Plus, I'm using a 20 inch wheel and not a 26.

Here are my points where sprockets are:

Engine - 10T
Left Jack Shaft- ??T
Right Jack Shaft- ??T
Rear Wheel Freewheel 19T (Sturmey Archer 3 Speed)

I'd like to cruise at about 36-38 miles an hour if possible with motor mods.

I was thinking of plugging in the values for the right and left side jack shaft as follows:

Engine - 10T
Left Jack Shaft- 18T
Right Jack Shaft- 42T
Rear Wheel Freewheel 19T (Sturmey Archer 3 Speed)

Am I way off???

The other sprocket which the pedals turn is out of the mix and separate from the drive system, although another sprocket with a freewheel will go on the right side of the jack shaft for it.

I'm definitely missin' something here...
'BrettMavriK

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#### BrettMavriK

##### Member
K, the fog is lifting a bit...

I see the direct drive stock kits are running a 44t or a 36t sprocket
So it goes to show anywhere from a 3.60-4.40:1 ratio is needed.

Allrighty.

Since I'm using only 4 total sprockets from the engine to the rear wheel.
1. Engine 10T
2. Left Jackshaft 18T
3. Right Jackshaft 9T
4. Rear Wheel 19T

This gives a 3.8:1 ratio on a 20 inch rear wheel, but, I also have 3 speeds built into the rear hub. So, anywhere from 3.6-4.4:1 is achieved with gears.......or am I completely way of course???
'BrettMavriK

A

#### andyinchville1

##### Guest
Hmmm....My brain is somewhat dead tonight BUT what are the ratios of the hub?...Is it something like .75 / 1/ 1.33 or something?....I think the under and over drive were roughly 25% under and or 33% or so over if I recall correctly?.

Andrew

#### BrettMavriK

##### Member
Yep.
75%,100%,133%

I am running a direct jackshaft to the hub. Front crank is out of the loop and on a separate chain.

'BrettMavriK

A

#### andyinchville1

##### Guest
Hmmmm again....We must be gluttons for punishment but I'm with ya! ;-O

This would make a persons brain burn!.....FWIW I know a 27T sprocket on a 20" wheel is similar in performance to a 35 T sprocket on a 26" wheel....and I know that several folks in FL are using 27T sprockets for high speed cruising on the flat lands on a 26" wheel....this would be like running a 21 T on a 20" wheel.

Of course all this assumes direct gearing....the jackshaft and internal gears compounds problems that have to be thought out....

OK...looking at it more I believe you want the equivalent of a 32T sprocket on your rear wheel..that way OD makes it act like a 21T sprocket on a 20" wheel which is like a 27T on a 26" wheel which I know is doable on the flats...

Since you have a 19T cog on the hub we need to manipulate the jackshaft gearing so that it makes the 19T on the hub "act" like a 32T sprocket...thus the jack shaft ratios must ultimately reduce the gearing by XX % roughly...

engine driven 10T to a 13T on one end of the jackshaft would reduce gearing by XX %

Soooo I am thinking that one end of the jack shaft needs to be 13T (the engine drives this 13T), and the other end of the jackshaft needs to be a XXX Teeth...to make the rear an "effective" 32 T Rear sprocket.

OK....Engine 10T drives 13T at one end of the jackshaft, yielding a % reduction in gearing, other end of the jackshaft XX T drives the 19T on the rear hub yielding the other reduction needed.

I think that does it....what do you think? We jyst have to figure out the XX parts... BUT at least we know where we need to get to...The effective 32T....

Andrew

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#### andyinchville1

##### Guest
OK....ate some dinner so back to it.....

I think gearing needs to be reduced by 68.4% (that is to make the 19T sprocket you have on the hub "act" like a rear 32T sprocket).

Since you have an internal hub with an Overdrive of 1.33 that would make a 32T act like a 21T which is roughly the same as a 27T on a 26" wheel

The direct gear would make a 32T act like a 42T sprocket on a 26" wheel (close to "normal" stock).

First gear being 25% under drive, the 32T would act like a 52T on a 26" wheel....definitely pretty low gearing!

So the only figuring left to do is determine what the 2 sprockets on the jack shaft should be.....KNOWING we need a 68.4% reduction (to make the 19T THINK it is a 32T rear sprocket).....

We know the engine drives with a 10T sprocket and we want the ratio to NOT be even to reduce wear on components....we can arbitrarily choose 13T....this is about 23% reduction here....thus we need 45% more reduction....(I hope my math is right)

So, we know the rear driven sprocket is 19T.....thus the question becomes : What sprocket driving this 19T sprocket yields a 45% reduction in gear ratio?....The answer being 8T ( 8 divided by 19....actually yields 42%) or 9T ( 9 divided by 19...yields 47%)...

Does this seem reasonable?....My percentage math is only so so BUT I know you would like to make the rear hub THINK it has a 32T sprocket on it....(then all the gears would be usable).

Hope this helps you.

Andrew

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#### andyinchville1

##### Guest
Hi Brett,

I think your last set of sprockets would yield too much gear reduction....I figure it at 102% overall reduction VS. what I calculated as needed being about 68.4% reduction....

My math is subject to errors tho....

There is one way to find out tho....Are you going to build it?

Andrew

PS - I vote "YES" ;-)

#### BrettMavriK

##### Member
Hi Brett,

I think your last set of sprockets would yield too much gear reduction....I figure it at 102% overall reduction VS. what I calculated as needed being about 68.4% reduction....

My math is subject to errors tho....

There is one way to find out tho....Are you going to build it?

Andrew

PS - I vote "YES" ;-)

Here's where I'm at so far.....
'BrettMavriK

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