calling all engineers
10-23-05, 01:14 PM
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calling all engineers
here goes yet another compact crank vs regular question. a 39 x 29 gear has about the same gain ratio as a 34 x 26- are they really equivalent or is there some advantage or disadvantage to having a lesser difference between front and rear # of teeth - 8 vs 10?
10-23-05, 01:38 PM
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Originally Posted by oujeep1
here goes yet another compact crank vs regular question. a 39 x 29 gear has about the same gain ratio as a 34 x 26- are they really equivalent or is there some advantage or disadvantage to having a lesser difference between front and rear # of teeth - 8 vs 10?
"Similarly, derailers come in long cage and short cage versions. The short cage ("road") ones only work with narrow range ("road") cassettes, because they don't have enough capacity to take up chain slack for use with a wide range cassette. Short cage derailers are also limited in terms of the largest rear sprocket that they can clear without having the jockey pulley rub on the sprocket."
10-23-05, 01:50 PM
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If you take the time you can use a Campy short cage with their 13-29 cassette and 53/39 up front even though they recommend using a medium cage for this.
As I have heard it is better mechanically to run the largest ring in front that you can. Don't really know the why or if this is even correct.
The advantage of the compact in this case is more to do with the number of overlap gears between the 53 and 39 on the cassette versus the numbers with a 50/34...
Note: With Shimano the largest road cassette is a 27 though you could go to MtB derailleur and cassett.
As I have heard it is better mechanically to run the largest ring in front that you can. Don't really know the why or if this is even correct.
The advantage of the compact in this case is more to do with the number of overlap gears between the 53 and 39 on the cassette versus the numbers with a 50/34...
Note: With Shimano the largest road cassette is a 27 though you could go to MtB derailleur and cassett.
10-23-05, 02:09 PM
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Originally Posted by CPcyclist
As I have heard it is better mechanically to run the largest ring in front that you can. Don't really know the why or if this is even correct.
Point 1: If you consider the cassette as disposable, but the chain rings as not then you will get less wear on the chain rings and more wear on the cogs when using the big ring with nearly identical gearing.
Point 2: While there is no difference between the ratio of the pedal force applied and propulsive force at the road surface when using the same gear ratio, the required tension in the chain is lower if you use the big chain ring.
10-23-05, 03:41 PM
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Originally Posted by mollusk
While there is no difference between the ratio of the pedal force applied and propulsive force at the road surface when using the same gear ratio, the required tension in the chain is lower if you use the big chain ring.
10-23-05, 09:31 PM
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Originally Posted by mollusk
I'll hazard a guess on this one.
Point 2: While there is no difference between the ratio of the pedal force applied and propulsive force at the road surface when using the same gear ratio, the required tension in the chain is lower if you use the big chain ring.
Point 2: While there is no difference between the ratio of the pedal force applied and propulsive force at the road surface when using the same gear ratio, the required tension in the chain is lower if you use the big chain ring.
That's the only real reason to do it. But even then the difference is small and it's all within the margins of the chain's design. But there is a little less force transmitted through the whole drivetrain. The torque at the rear hub will be the same either way.
You might also say that larger diamater gears make the chain bend less severely. This is true around the gear teeth and through the drailleur, assuming the same length chain. So if you have a choice of gear combinations on the same bike, using the big ring keeps the chain bending to a minimum, while also lowering the tension in it. That ought to be good for something after a couple thousand miles.
