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compact cranks and crank length

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compact cranks and crank length

Old 03-01-07, 09:46 PM
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Rutnick
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compact cranks and crank length

I probably already know the answer but I'll post it anyway. I used to use 175mm crankarms until last year. I'm not really sure why I changed to 172.5 other than I got one heck of a deal on some Dura Ace cranks last year. Well, since then I've been using 172.5s on all my road bikes but continue to use 175mm on my MTB bikes.

I just came across some compact cranks I would like to try but they are 175mm in length. Other than having to adjust my saddle height slightly, any possible problems I might encounter? I mean it is 2.5mm.

I'll probably just run the compact when I do mountain climbs (which will be about 3-4 times this year) and RAIN.

Mountain climbs:
3 State 3 Mountain
Brasstown during Tour De Georgia
Some Mountain climbs in W Virgina/Virgina
1 Possible 1 week ride that will take me to the smoky mountains

The rest of the time I will use my standard 53/39 gearing. Any downside other than I might like the cranks enough to keep them on the bike always?
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Old 03-01-07, 10:25 PM
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not really. Just remember that you've got the extra 2.5mm at the top and bottom of the pedal circle, so once you drop the saddle 2.5mm, this totals 5mm at the top of the stroke. So your knees come 5mm closer to your chest through the top of the stroke
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Old 03-02-07, 07:33 AM
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Originally Posted by Rutnick
Any downside other than I might like the cranks enough to keep them on the bike always?
That's what happened to me, even though with the Shimano Compact crank, it's easy to switch between crank sets.

I wouldn't worry about the seat. You'll probably not notice 2.5 mm, and if you do, then over time you won't.

Don't forget that each time you switch cranks, you'll have to adjust the height of the FD, and maybe adjust chain length. A check of shifting and an adjustment of the RD could be in order, too.
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Old 03-02-07, 11:05 AM
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Originally Posted by 531Aussie
not really. Just remember that you've got the extra 2.5mm at the top and bottom of the pedal circle, so once you drop the saddle 2.5mm, this totals 5mm at the top of the stroke. So your knees come 5mm closer to your chest through the top of the stroke
Can you folks tell the 2.5mm difference in crank lengths ? I always wondered about that.
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Old 03-02-07, 11:58 AM
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Originally Posted by snhrider
Can you folks tell the 2.5mm difference in crank lengths ? I always wondered about that.
last night i went out to see the difference in cranks between my comfort bike and hybrid bike. the comfort bike has a 170 and the hybrid has a 175. when i ride one on one day and the other the next, i really don't notice a difference. however, i've always wondered why i'm faster on my comfort bike despite the mtb wheelset and slower gearing. maybe its because of the crank length?
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Old 03-02-07, 12:09 PM
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Originally Posted by snhrider
Can you folks tell the 2.5mm difference in crank lengths ? I always wondered about that.
When I used to race bmx I ran a 180 crank and I couldnt ride anything but it, I would notice a difference between that and 175's and 182's even.
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Old 03-02-07, 12:48 PM
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looks like a good excuse to have two chains!




Originally Posted by NoRacer
That's what happened to me, even though with the Shimano Compact crank, it's easy to switch between crank sets.

I wouldn't worry about the seat. You'll probably not notice 2.5 mm, and if you do, then over time you won't.

Don't forget that each time you switch cranks, you'll have to adjust the height of the FD, and maybe adjust chain length. A check of shifting and an adjustment of the RD could be in order, too.
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Old 03-02-07, 07:17 PM
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Originally Posted by snhrider
Can you folks tell the 2.5mm difference in crank lengths ? I always wondered about that.
if the saddle height has been adjusted, I'd immediately notice that my knees are coming up 5mm higher or lower at the top of the stroke. If someone changed my cranks from 172.5 to 175 without telling me, and the saddle height was left alone, it might take me a few more minutes to realize I was pedaling a 3% bigger circle, and that I was reaching an 2.5mm at the bottom of the stroke.
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Old 03-02-07, 07:40 PM
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Takes a little less effort on the pedals, but only by a little bit.

I have 172.5 on my beater, and 175s on my touring/road bike (nothing flashy yet...) and while I'm slower on the beater, I think I would be anyway even with the same crank length. The nicer bike also has a triple, the beater is heavy and has a regular road double. Hard to compare... (oh, and the beater is six in the back, the other bike is nine.)
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