Chain ring and crank arm length?

[craig69]

I am currently running a 36T chain ring with 175mm cranks. I am just wondering what others use and would it make much difference changing to a 41T with 170mm cranks.
What about casettes. Im using a the 11 - 32 which seems pretty standard, but for those guys who use road bike casettes, what size do you use and does it make much difference when racing.

P.S. I am 6ft and ride a medium frame.
Cheers big ears.......

[micksta323]

the length of the crank arms determine how much torque (twisting motion)you can put down. chainrings mean acceleration/climbing. it would be harder to turn a 42t ring with 170mm cranks, than with 175mm ones. you would then take off quicker with a 38t/36t ring etc. change rings for fast open, or tight closed up courses. also clearance comes into play with crankarm length ie: hitting the ground when leant over. as for road cassettes, i use them because they have a closer gear range, they change up (accelerate) smoother. i use a 12-27 on my dh bike and a 12-25 on my jump bike. i find that i never go above 2nd or 3rd gear when riding so needing a 32t biggest on a cassette is unnecesary. hope this helps you.

mick.

[benmwatkins]

the shorter your crank arms the less chance you have of them and your pedals hitting rocks/the ground. 170mm cranks are very common on medium sized bikes and i use 165mm cranks for even more clearance and my short ass legs. You would have to go to NASA to measure the difference in feel between 5mm different crank arm lengths, theres not alot of difference unless you went from 180mm down to 165mm, then its really noticeable.

36 tooth chain rings are great for two reasons, one..... if you run that size and a chain guide/bash ring set up to match, you'll have lots of clearance over logs/rocks/ect. so your chain/ring guide wont hit too much stuff, and the best advantage is your chain line.

Think about this, if you use a small ring on the front (like a 36) you will kind-of compensate by using the smaller cogs (gears) on the back. What this means is that when you stand behind your bike and look along your chain from the back cassette to the front chain ring, you'll use mostly the bottom half of your rear gears (cassette) which in turn gives you a better chain line (straighter) for shifting gears and general rding.

Because most (not all) dh bikes have wide bottom brackets usually the chain line is pretty bad (when running 40-44 tooth front rings) using mostly the top half of your casette which means the chain comes off your cassette and then goes outwards (looking from the rear again) and you get alot of chain scrape/drag and pretty average shifting.

Seeing as you are six foot 165mm may be too short, but 170mm would be better than 175/180mm.
yeah also micksta made a good point above. i use road cassettes on my 4X and DH bikes and you get a super smooth gear change when shifting cause theres not a massive difference in cog size. what that kinda means is when accellerating hard and shifting down to pick up speed you wont suddenly feel like you need to pedal twice as fast and its a gradual change through the gears. quite often on my xc bike i feel like my gears are too far apart cause im used to my dh cassette (if that makes sense-not that riding xc makes sense!)

[craig69]

What brand and type of road casette do you guys use. I race road as well and use durace, but i have the old ultegra group set with a 9 speed 12 - 25t casette on an old bike. Can it change straight over onto the DH bike with out modifications.

Ive got SRAM x7 rear deraileur and shifter. Would they be compatible with the shimano casette.

[RaID]

What brand and type of road casette do you guys use. I race road as well and use durace, but i have the old ultegra group set with a 9 speed 12 - 25t casette on an old bike. Can it change straight over onto the DH bike with out modifications.

Ive got SRAM x7 rear deraileur and shifter. Would they be compatible with the shimano casette.

they will be compatible

im currenttly running a SRAM cassette 12-26 but probably the next cassette will be an ultegra

[craig69]

Cool thanks for that. :)