Cantilever Bell Crank question???

atomicjoe23

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OK. . .this is something of a generic series of questions so I'm only looking for a generic answers (assuming generic answers can be given). . .

. . .concerning the bell cranks of a cantilever suspension. . .

Would it be correct to say that the bell cranks are normally designed to create a rising rate for the shock/coil sping?

If so, when (if ever) would it be appropriate/desirable to have a falling rate?

Finally, what makes the bell crank create the rising rate vs. a falling rate?

Thanks!
 

Tech Tim

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I don't know if you would call it a rising/falling rate. To me a rising or falling rate has more to do with shock mounting in relation to it's travel. Or maybe better put: does the shock get easier or harder to compress as it goes through it's travel.

In a cantilever system the bellcrank can be used to give you more travel, less travel or the same amount of travel; it's a leverage ratio. If the bellcrank arm on the link side is 8"s long and the BC arm on the shock arm is 4" long, then we have a 2 to 1 ratio and the shock moves half the distance of the actual axle travel. This could be beneficial in picking up more travel for a shorter shock. In a 1.5 to 1 ratio you could get 15"s of wheel travel from a 10" travel shock, but at the cost of shock control. The shock is working harder to control movement as the wheel is traveling further than the shock shaft, so less area to control it in.

If they are the same length, the shock moves the same amount as the axle, which, other than packaging at the link or a-arm and less unsprung weight, you get no advantages.

If the BC arm measurements are reversed you would need a shock that is double the length of your desired travel (in a 2 to 1 ratio scenario) which would give you no other benefit than being better able to control the dampening as you have twice the distance of shock travel.

Edit: That is not taking into consideration the length of the bellcrank arm to the length of the shock or it's mounting position. A shorter bellcrank arm has a sharper arc and will move the shock position which could cause the shock to change it's rate.

There are a few good threads on here that discuss cantilever and bellcrank set-ups.
 
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