I am about to order my sprockets for Cafe ala Carte which for those of you that haven't been around for a while is slow project of building a 60's style cafe bike powered by a Savage engine.

Now throw out any thoughts of actual ratios as I am running an 18" rear wheel with a tall 110/90 tire (that alone gives me a 7.5% taller gearing with stock drive ratios).

Concentrate on the sprocket question at hand.  For the ratios I want I can buy either a set of 16/17 front 49 rear or a 15/16 front 46 rear.
Both will give me fundamentally the same ratios.

What I am looking at is which set to go with.  Here are my thoughts.  For the look of the bike I would like the smaller set better.  However, I have always pushed for a larger front sprocket rather than smaller to avoid undue stress on the counter shaft bearing.  That however was on GSX-Rs and may or may not be an issue here.  We can talk HP of the Gixxers but here we have large torque and I don't know the robustness of the bearing on the Savage engine.  Also, going with the larger front gets me more room going over my swing arm with the chain. Note this is not a stock Savage frame so don't think about your stock clearances.

Thoughts??

Thanks
Smokin Blue

actually size of the front does affect the load.  It is a question of does it affect it enough to cause a problem?  I am doubting it would here.  It is a matter of torque arm length.  For a given torque value the force (tension in the chain) would be multiplied by the torque arm (radius of the front sprocket)  To get an equal torque a smaller front sprocket will have a larger tension force on the chain.  Then through balancing the forces (theory of statics in engineering) the tension force has to have an equal counter force provided by the counter shaft bearing.  So the bearing with the smaller sprocket sees a larger force applied to it.  Again I have seen it cause problems but I think the range that I am talking about here would be a mute point.

Right now my head is to go with the smaller set to keep the back from looking too big for the bike.

T-Mack, are you saying that if I could find 2 sets of sprockets that delivered the exact same gearing that the forces on the bushing on the front sprocket would be greater with the smaller front sprocket?Wouldnt the chain loads be identical except for the losses created by the tighter wrapping of the chain using the smaller set?

justin_o_guy2 wrote on 01/01/09 at 21:59:03:


Bushings??  You mean in the chain? The tranny output bearing would have same load (apprx 30 some HP)

It's all about surface area and spreading the load.   A higher load will wear things out faster.    

EXAMPLE: Let say 1/2 the front sprocket is grabing the chain.  (maybe more but half is easier to calc).  
-  On a 15 tooth that's 7.5 teeth to distribute the 30 some HP or about 4 HP per tooth.
-  On a 17 tooth, that's 8.5 teeth or only about 3.5 HP per tooth.

NOTE: remember the stock belt is fat.... lots of surface area to spread the load.......

The thing to remember on a chain drive is that only about two or maybe three teeth on each sprocket actually carry the whole load. Because of chain and sprocket wear the rest carry little or no load.

Another thing about the big single is that the load on the chain will be somewhat pulsatile, or 'jerky.' The pulsing load may have an effect on the output shaft bearing. That's one reason for the rubber bushings in the pulley/hub assembly. The belt drive also helps a little to absorb that jerkiness. I don't know about the S40/Savage, but many vehicles have a cushion drive assembly (usually a two-part gear with springs in it, or springs in the clutch disk on a car) to help absorb engine vibrations.

Assuming you are transmitting the same power, bigger sprocket sets will cause the chain to travel faster, but with less tension. The product of the speed and the tension will be constant if the power is constant.

I know where I am headed...thanks everyone for the discussion!