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Crankarm length and chain compatibility

Old 09-13-13, 06:10 AM
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asmac
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Crankarm length and chain compatibility

I have to replace my crankset and have a couple of questions.

1. My existing crankarms are 170mm but the one I'm looking at is available in 165 and 175 only. What difference does it make? Does it affect effective gearing or easier/harder on the knees? What are the criteria for choosing?

2. The new models (XT, SLX) are 10 speed but the rest of my drivetrain is 9 speed. Will 10 speed cranksets work with 9 speed chains?
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Old 09-13-13, 06:22 AM
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mudfreek
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the length doesn't matter think of what u will be riding over lots of rocks and roots you want shorter crank arms for more clearance
for smoother rides u want longer u will get more power per stroke
the length of a crank arm will determine the amount of torque applied to the pedal stroke/force and as far as chain and crank compatibility u shouldn't have any issues running a 10 spd chain ring with a 9 spd chain
the chain is wider 10 ring will work for 9 chain not the other way
hope this helps

Last edited by mudfreek; 09-13-13 at 06:25 AM.
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Old 09-13-13, 06:36 AM
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(1) 170mm is a fairly common crank arm length, but if you're looking at older / out of production cranksets, then maybe the 170s aren't available, only the slightly less common (?) sizes. As for impact, well, that depends on how tall you are, how good your knees are, whether you are looking at a road or mountain bike, etc. (Since you mention XT, I assume this is a mountain bike.) Mountain bike crank arms tend to be on the long side on the theory that the rider can stand and get more leverage to go over obstacles. On the other hand, if you're on a road bike you may prefer slightly shorter crank arms in order to be able to spin. Me, I like 170's for all my bikes and won't use anything longer--but I'm 42 and have had lousy knees for 25 years, so YMMV.

(2) I recall reading a number of years ago Sheldon Brown's view that rating cranksets as 8, 9 or 10 speeds is marketing BS; rather, derailleurs, cassettes, and chains are what make the drivetrain, not the crankset. I've never had any problems mixing 8 speed cranksets with 9 speed drivetrains, but I don't have any experience with 10-speed (or later) stuff. I believe the 11-speed stuff does have narrower chainring spacing than of old, so St. Sheldon may no longer be right on this.

Mike
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Old 09-13-13, 07:18 AM
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Bill Kapaun
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Originally Posted by mudfreek View Post
the length doesn't matter.........
Maybe it doesn't matter to you, but 175mm means enough knee pain that I only ride when absolutely necessary to get groceries etc.
165mm means I have been able to do 70 miles in a day on my hybrid.
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Old 09-13-13, 07:29 AM
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As you have noted from the above posts, crank length is a highly subjective matter, some riders are sensitive to it and others indifferent. I've ridden both 170 and 175 mm cranks and never noticed any significant difference but I have 170 mm cranks on all my bikes now. Bill finds the difference quite significant.

If you MUST buy this crank, use your height as a guide. If you are over say 5'10" go for the 175, otherwise buy the 165. Actually, I'd buy a crank in the length you are used to.

The 10-speed crank will probably work with a 9-speed chain but you may get some rubbing on the big chainring in the smaller cogs and middle chainring. A 10-speed chain will work with 9-speed cassettes so that is a safe option.

Note to mudfreek: Please capitalize and punctuate your postings. What you wrote would be a lot more useful if it didn't take three readings to decipher it.
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Old 09-13-13, 07:57 AM
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Originally Posted by HillRider View Post
.......If you MUST buy this crank, use your height as a guide. If you are over say 5'10" go for the 175, otherwise buy the 165. Actually, I'd buy a crank in the length you are used to...................

Note to mudfreek: Please capitalize and punctuate your postings. What you wrote would be a lot more useful if it didn't take three readings to decipher it.
I'll chip in some more-
I have a "bad" knee with limited range of motion.
With 175mm, my knee runs out of "bend" and I'm basically lifting my R leg up over the top by pushing down on the left.
@ 62 RPM, It actually throws my leg off the pedal @ the 12 o' clock position.
170mm I can spin 80 RPM, but have varying degrees of knee pain from day to day.
165mm and I can spin 85 RPM. NO PAIN!!!!
160mm and my cadence actually drops to 82 RPM. My chubby legs just don't want to spin that small of a circle and it FEELS too short.

BTW, I'm 65 and am 5-11, although I used to be almost 6-1.
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Old 09-13-13, 08:25 AM
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Originally Posted by dubes View Post
(2) I recall reading a number of years ago Sheldon Brown's view that rating cranksets as 8, 9 or 10 speeds is marketing BS; rather, derailleurs, cassettes, and chains are what make the drivetrain, not the crankset.
The difference between cranksets of different speeds isn't the chainring spacing but the design of the ramps and pins which are tailored to the various chain widths. If you've ever tried to run a wider chain on a crankset designed for a narrower chain and had chainring/chain rubbing near the extremes of the cassette then you've noticed the more aggressive chainring design.

The problem with just swapping chains to correct this problem is that with index shifters you really want your front derailler cage width matched to your chain too. I won't bother any more with trying to make mismatched parts work. I've tried in the past and been fairly unsuccessful, with index systems at least.
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Old 09-13-13, 08:26 AM
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As far as crank arm lengths go, I can ride 175mm cranks for shorter distances but prefer (and have on all my road bikes now) 172.5mm cranks. I get a bit of knee pain with the longer cranks but nothing with the slightly shorter ones. I have good knees too but apparently sensitive ones!
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Old 09-13-13, 10:07 AM
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OK. Thanks, all. I'm persuaded that I should spend the extra $50 for a 9 speed XT instead of the 10 speed SLX. I've been using 170mm cranks but will go with the 165mm version. (170 is on backorder anyway so that makes it easier). I flirt with knee pain - it comes and goes and seems to depend on saddle height and cleat positioning, 1/2 an inch either way) and I want to do whatever I can to discourage it.

BTW, I see that asking "will it work" was not reasonable. The answer I take away is that it "will work" but not optimally or perhaps not as well as it might work otherwise.

Last edited by asmac; 09-13-13 at 10:14 AM.
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