Cassette sizes

[Jim_bo]

Does anyone make a 10 speed cassette with a 38 tooth cog? I want to go to 170mm cranks, but I really don't want to lose any low gearing for very steep stuff.

[AlexRandall]

1. No, most 10sp derailleurs wont handle a 38T easily anyhow, but unless you add a 38 individual cog and drop the 11 then I don't know any way of getting a 38

2. Shortening your crankarms doesn't change your gearing, just the way the force is applied over distance. The change is minimal to not noticeable depending on whether you notice minor alterations.

[Jim_bo]

1. No, most 10sp derailleurs wont handle a 38T easily anyhow, but unless you add a 38 individual cog and drop the 11 then I don't know any way of getting a 38

2. Shortening your crankarms doesn't change your gearing, just the way the force is applied over distance. The change is minimal to not noticeable depending on whether you notice minor alterations.

Really?? The physics book I used said a shorter lever arm requires more force. Is this not right?

[mitzikatzi]

Really?? The physics book I used said a shorter lever arm requires more force. Is this not right?

I doubt you will notice 5 mm of difference in crank arm length, "force wise"

Gearing stays the same. For every revolution of the front chain ring the rear wheel turns the same number of times. A 36 front chain ring a 18 rear sprocket ie 2 to 1. The crankarm length has nothing to do with the gearing.

[mitzikatzi]

1. No, most 10sp derailleurs wont handle a 38T easily anyhow, but unless you add a 38 individual cog and drop the 11 then I don't know any way of getting a 38

2. Shortening your crankarms doesn't change your gearing, just the way the force is applied over distance. The change is minimal to not noticeable depending on whether you notice minor alterations.

this

[customfab]

Really?? The physics book I used said a shorter lever arm requires more force. Is this not right?

It's correct but it really only applies to single speed applications. Consult that same book for a formula that tells you the difference in force between a 170 and 175 mm crank arm.

[Jim_bo]

It's correct but it really only applies to single speed applications.

I'm sorry... That's just not right. If there isn't a 38 tooth 10 speed cassette, then my question is answered.

[-Todd-]

There is a thread detailing the use of the 38 tooth replacement... Search.

[mitzikatzi]

If your that worried about the amount of force you apply at the pedals changing from 175mm cranks to 170mm cranks.

then you might need to consider
Both 42/21 and 32/16 are 2 to 1 ratio so the same gearing. My thinking is that the amount of force you need to apply at the pedals will change.

maybe it is just my imagination but I think it is easier to climb on 32/16 than 42/21.

[AlexRandall]

The biggest change you're likely to see going from 175's to 170 is the 5mm you need to raise your seat by.

[reamer41]

If your that worried about the amount of force you apply at the pedals changing from 175mm cranks to 170mm cranks.

then you might need to consider
Both 42/21 and 32/16 are 2 to 1 ratio so the same gearing. My thinking is that the amount of force you need to apply at the pedals will change.

maybe it is just my imagination but I think it is easier to climb on 32/16 than 42/21.

It's just your imagination.

[J.B. Weld]

maybe it is just my imagination but I think it is easier to climb on 32/16 than 42/21.

Actually it should be the opposite, more teeth = slightly more efficient drivetrain. By my calculations you should see a .0004397% improvement in efficiency with the 42t, subtract the penalty incurred by the 98 gram weight gain and you're still up .0002973%!