Sick Bike Parts New 21T Jackshaft Sprocket

[FurryOnTheInside]

I have an update for all related to the 21 tooth sprocket. I did install one on Friday and it was easier than I expected. It was as simple as adding 2 links to the chain, removing one of the 1/8 inch shims between the engine and rear mount and adding 3 of the thin shims. Can't report back on any performance aspects though as it has been raining like crazy in Washington Sate.

Thanks Ghost0. Can you let us know what sprockets you're using on the RHS atm and what if any changes you make to allow best use of the new 21T LHS sprocket, also your engine's compression ratio (or cranking pressure would be my preference actually) and how you find starting the engine with the new gearing. Thanks.

 

[Ghost0]

Well I installed it on my "dirt bike" so currently it is geared with a 9 tooth and standard 36/44 chainrings on the RHS. I would suspect I will want to change the 36 to a 40 or 44 to get the final drive back to where I like it.

Like I said I have not done any riding so I can't really answer any of your questions about starting the engine. As far as compression I don't know that either. It is a standard "slant" head and I have never much difficulty turning the engine over.

 

[FurryOnTheInside]

Well I installed it on my "dirt bike" so currently it is geared with a 9 tooth and standard 36/44 chainrings on the RHS. I would suspect I will want to change the 36 to a 40 or 44 to get the final drive back to where I like it.

Like I said I have not done any riding so I can't really answer any of your questions about starting the engine. As far as compression I don't know that either. It is a standard "slant" head and I have never much difficulty turning the engine over.

That's okay I wasn't expecting an answer yet! Just that I know with the increased (engine : pedal) ratio that the engine would require more force through the pedal to turn over.. only 23.5% extra I think.. but I really don't trust my maths lol.
A 44T final drive chainring should be equivalent to the 36T if you trust my maths lol. So with both crank sprockets being equal size there could be some issues with chain retention devices clearing the outer chainring perhaps needing an extra large gap between the chianrings?

 

[Guest-918393]

I dont understand any of the math behind calculating the speed when it comes to a jackshaft setup honestly, but i just installed one and im not satisfied with the top end, its a stock jackshaft kit/gears. Any tips on which gear to change? I live in a flat area so i mostly need top end since wide open roads

 

[5-7HEAVEN]

A single sprocket change might be the answer.

Please identify all your sprockets, chain rings and cassette gearing.

There are online calculators to assist


We can try to help.

 

[5-7HEAVEN]

Let me take a wild guess.

Since your goal is high speed, let’s try to make your last sprocket pull like a 32-tooth single rear sprocket.

Your engine has built in 4.2:1 gearing.
A 32t sprocket with a 10t engine sprocket = 3.2:1.
4.2 x 3.2 = 13.44:1.

A 21t jack shaft to the 10t engine sprocket = 2.1.....

A 45t chain ring connects to a 10t jack sprocket = 4.5:1.....

Using a 34t-10t cassette and a 30t chainring sprocket, the 10t cassette gear INCREASES gear ratio = 10/30 = .333.....

Sooo, 4.2 x 2.1 x 4.5 x .333 = 13.23:1.....


That’s very close.

FWIW, your First gear would be 44.98:1.
That’s equivalent to a stump pulling 107-tooth sprocket.

That’s the best of both worlds.

 

[Ghost0]

If he is running the stock set up then he would be using the 17 instead of the 21 which would be a slightly taller ratio. What most customers fail to take into consideration is that speed is still dependent on horse power. All the gears in the world won't make a little 4 cylinder car go 200 mph. We usually ask the customer to provide top speed data in each gear. Then we know if there is enough power to pull a taller gear. For example on my bike it runs its fastest top speed in 5th gear not in the highest gear 6th. That means that no matter what gearing I have my engine cannot pull a taller gear than the ratio I have in 5th. The only way to go faster is more horse power. Now if they they show more top speed in every gear then the engine would appear to have enough power to pull a taller gear. If that is the case then swapping the 36 tooth chainring for a 40 would be my recommendation.

 

[5-7HEAVEN]

If he is running the stock set up then he would be using the 17 instead of the 21 which would be a slightly taller ratio. What most customers fail to take into consideration is that speed is still dependent on horse power. All the gears in the world won't make a little 4 cylinder car go 200 mph. We usually ask the customer to provide top speed data in each gear. Then we know if there is enough power to pull a taller gear. For example on my bike it runs its fastest top speed in 5th gear not in the highest gear 6th. That means that no matter what gearing I have my engine cannot pull a taller gear than the ratio I have in 5th. The only way to go faster is more horse power. Now if they they show more top speed in every gear then the engine would appear to have enough power to pull a taller gear. If that is the case then swapping the 36 tooth chainring for a 40 would be my recommendation.

Changing from 2.1 to a 1.7 would have a 19% rise in gear ratio.
So, of course you'd have to change the chainring to accommodate the gearing loss.

You also recommend change from 36t to 40t.

What would final drive be, say, using a 34-10 cassette?

 

[5-7HEAVEN]

If your engine cannot pull a taller gear than the ratio you have in 5th gear.....

then lower your overall gearing.

Say you have 13.23:1 gearing, using (21/10), (45/10) and (10/30).

Changing (45/10) to (45/9) lowers gearing in top gear to 14.7:1, equivalent to a 35t sprocket.

Still not pulling hard with a 35t?

Then put the (45/10) back in, and use 7th gear, which is equivalent to a 38t gear.

Of course, these are just ballpark figures.

If the riders enjoy pedaling with or without assisting their engines, then other factors need consideration.

 

[petey350]

What engine are you using?