Choosing Sprockets

[RichieW13]

If you use 15/45 gearing you get a resultant ratio of 3.00:1. 14/42 gearing will get that same 3.00:1 ratio.

Will both of those setups actually get you the same net results? Is there any difference at all between the two setups?

I only chose those sprocket combinations as examples because the math is easy. I don't think I need to go that fast.

 

[los36]

I don't know the physics involved, sometimes changing C/S sprocket gives you the opportunity to reduce chain torque. Chain torque affects the way the rear suspension works. I think that Team Honda used to run 12/49 instead of 13/52 on their 125's (13/50 was stock).
I don't know for sure, but I think that the closer the sprockets are in size, the less chain torque there will be. I guess in your example, the two options have sprockets pairs that are the same size, relative to eachother.
AMP Research used to sell something that supposedly removed chain torque...it was a couple of extra chain rollers that made the chain run parallel to the swingarm (top and bottom).
I think that I've thrown enough random B/S out there. Maybe someone can come along and refine it into fertilizer! ;)

 

[RichieW13]

Yeah, I remember the old ATK's used to have that chain roller mechanism that enabled the chain to pivot differently. I think it was designed to have less affect on the suspension.

 

[Layton]

All of that "chain torque" stuff is just a bunch of BS for the average rider.

The only real difference is that the 15 tooth front sprocket wouldn't wear as fast and would not put as much stress on the chain due to it's large diameter. Chains don't like little sprockets which is why you seldom see a front sprocket smaller than a 12.

Hope this helps. :)

 

[WoodsRider]

IMO, different sprocket combinations can be used to adjust the wheelbase. A 13/52 and a 12/48 are the same ratio. The 12/48 would use a shorter chain thus a shorter wheelbase for quicker steering. The 13/52 would require a longer chain and longer wheelbase for more high-speed stability.

 

[Jaybird]

"Chain Torque" is sort of a misleading term. Torque is the twisting or turning effort around a shaft tending to cause rotation. So, torque is the force placed on the sprockets, not the chain. Chain pull is the force the chain sees, a linier stretching, if you will.

12t sprockets are a mistake in our sport. They have been tried and failed to perform without accelerated wear of chains.

Look at it like this: If you have a 4:1 ratio using a 13/52 combo and another using a 12/48, then they both accomplish the same amount of work to make the rear tire spin around one revolution. The thing is that the 12 tooth front has to work much harder than the 13 tooth to get the job done.
Imagine 13 guys carrying a telephone pole. One of them drops out and the pole is still being carried, but the load that the 13th man was carrying has now been distributed in equal amounts to the 12 others, making their job harder.
(on the whole scope of things 13/52 you have a total of 65 teeth, or workers...12/48 uses a total of 60 teeth, or workers...5 less workers to accomplish the task)

This extra work required by the 12 teeth does not really make the sprocket wear any faster, actually it places more stress, or chain pull, on the chain. This extra force does two things, it accelerates the wear on the chain, which in turn is what causes the sprockets to wear faster...AND it indeed effects the suspension damping effect, due to a different tension being placed on the working side of the chain. I would imagine graphing out that force difference would be quite a math task, and it probably means squat to the average spode, but it (running smaller CS sprockets) does in fact change things with suspension as well as chain tension.

 

[los36]

I was at the track last weekend and some guy pitted next to me asked me a related question, oddly enough. He was asking about different ratios becuase the 52 tooth rear he wanted to run didn't clear the lower chain guide. I told him to either add a link to his chain and adjust the wheel further back, cut the plastic in the chain guide or run a 12/49.

 

[SuzookKING]

there is an associated weight factor in there as well, smaller sprockets tend to weigh less and less # of links in a chain looses weight as well. I lost 5Lbs off the chain and sprocket swap I did on a 907cc street bike....(and that's reciprocating weight the motor has to spin up ;)) granted on a mx bike it won't equate to that much as I went from a 532 o-ring chain & steel rear sprocket to a 520 non-o-ring racing chain & aluminum sprocket. Yes the wear was more on both ...but the difference was worth it on the pavement in my opinion :yeehaw:

 

[jmics19067]

AND it indeed effects the suspension damping effect, due to a different tension being placed on the working side of the chain.

how much difference could be attributed to the different angle of the chain in relation to the countershaft sprocket and swingarm pivot. would adding 1 tooth on the front sprocket and 3 teeth on the back change this angle?

 

[Jaybird]

The effect of angle would be minimal at best. All it would really do is change where the center of gravity is on the chains sag. I don't think it would effect the suspension reaction one iota. The angle is constantly changing as we ride anywho, no matter the set-up.
Now if you have your chain adjusted too tight, it will make mucho difference, as it will find a point that goes past the recommended sag often. When it does find this point, the energy...or extra chain pull, would be transfered as excess torque and overhung load, which could in turn, damage wheel bearings and races.

 

[SuzookKING]

The only time I have seen the chain angle majorly effect the way a bike accelerates is in drag racing on a rigid rear suspension.

 

[Jaybird]

Wheel hop?

 

[SuzookKING]

to help "plant" the rear tire to the ground, slight changes in CG

 

[jmics19067]

The effect of angle would be minimal at best. All it would really do is change where the center of gravity is on the chains sag. I don't think it would effect the suspension reaction one iota.

I thought the whole idea of an anti chain torgue system was to run the chain parallel to the swingarm. If that angle changes wouldn't the effect on suspension change?

From what I understand, that the pulling of the chain by the front sprocket is trying to pull the wheel down<trying to shorten the distance between the top of the rear sprocket and the swingarm pivot>. With the anti torque system when you have the chain parallel to the swingarm all you are doing is trying to pull the wheel against the adjustment bolts. I guess this would be the point I am trying to make.