Crankarm - 170mm vs 172.5mm

Ok...as some of you may know, I am in the process of shopping for a new bike. I am selling my Cervelo Carbon Soloist (51cm) and in the interim, I am currently back to riding my old Trek 2100 WSD (54cm). Problem is...the Trek is a triple, and I don't like it. I stumbled upon a used 105 double crankset with BB and attached pedals (Shimano 105's SPD-SL) for a really good price. The only thing holding me back is...the crankarm length is 172.5mm...whereas on both my Cervelo and Trek bikes, the crankarm length is 170mm. If I were to go ahead and swap out the 170mm triple for the 172.5mm double...will I notice a huge difference??? I really hate to pass up the opportunity to swap my triple for a double at a really cheap price. Yes, I know I would also need to swap out the front derailleur too...and I'm already searching eBay for a used one. If I could make this swap happen for under $100, I'd be a happy camper. But that crankarm discrepancy has me a bit concerned. Would appreciate any feedback. Thanks!

Linda

I have had a discussion with several people about crank length and the answers are so varied. :( One lbs mechanic insists the difference between a 172.5 and 170 is minimal while my bike friend with tons of racing experience says it is a pretty big difference. :confused:


My 54" Trek came with a 172.5 while the 56" does as well. When I was having knee issues (that probably are more quadriceps) I wanted to try the shorter crank but I never did. One reason I am hesitant is I feel like I am going to loose the torch I get from the longer crank. Right now its not broke so I am trying not to fix it! LOL

So... to sum up... I think it could be different for you, especially going up and not going down in size.

Btw I have a triple and I have no complaints whatsoever!

You will probably notice more of a difference in the change from triple to double (is it compact or standard?) than the crank arm length. I have one bike with 170 (my old bike) and the rest have been 172.5. I really don't notice much of a difference between the two in terms of how they fit.

2.5 mm is less than 0.1 inches.
I've gone from 170mm to 155 mm cranks, when I went from 170 to 165, I could tell no difference.
Unless you have knee issues, I doubt you'll be able to tell any difference at all.
The other changes you are thinking of will make much more of an impact.

I think it depends on your legs. Is there any way you can test ride a bike with 172.5mm cranks to see how it feels?

I can ride 165, but if I'm on 167.5, I have to compromise on seat height to avoid knee issues (and wind up with calf issues instead).

My BF and I just bought new cranks and with the arms laying side by side, it was hard to eyeball the difference between my 170 and his 175. I think different thicknesses of socks might have more of an effect than crankarm length!

actually... socks won't change the effective crankarm length, they'll just raise the center of the pedal stroke. The circle will still be 5 mm longer in diameter...

Thanks for the feedback, ladies! I'm glad to hear that, for the most part, the difference between 170 & 172.5 would be minimal. I'm not at all concerned about going from a triple to a double...as my Cervelo (that I'm selling) is a compact double, so I know what to expect. As I stated in my original post...I would only consider doing this swap, if I could do it for under $100. I don't want to sink a ton of money into a "temporary" bike. Especially since I'm actively looking for a new bike, so I'd rather put my hard-earned dollars towards that.

Perhaps it's my imagination, but I have a bike with 170s and a bike with 172.5s, and I do feel like I notice the difference. I can ride both without pain or issues, but I feel like I have a little more power with the longer cranks, while I spin a little more easily with the shorter cranks.

I've said this before, but when I bought my current bike, the LBS owner tried to switch me from 165s to 170s. I said OK, I would try the 170s. Within one ride, I knew I hated them. I felt like I was struggling to turn the cranks and pushing much harder than I ever did, no matter what gear I was in (the bikes had the same gearing). In theory, the LBS owner said the longer cranks would "unleash the power he knew I had in me," which I suspect was just a crock. I am a spinner; I always have been. I didn't start out trying to be this way, or train any special way, but it's what I feel comfortable with. In fact, I can barely stand on the bike and only do very rarely.
On a group ride a couple of weeks ago someone asked me how I trained to get such a high cadence. I replied that it's just the natural way I ride (and I don't think an average cadence of 80-85) is that high.

It's really personal preference.

My legs are very sensitive to any changes in fit, down to mere millimeters. I had a really hard time finding the right saddle position when I got my first bike. After much agonizing over fit, the LBS noticed that the cranks were 175mm. My bike's specs were for 172.5mm. So, since the bike was built wrong from the factory, they swapped out the cranks. Any knee strain or discomfort I had been having was gone.

I know plenty of people who absolutely do not notice a 2.5mm difference between cranks.

Aside from comfort issues, longer cranks will help you apply more torque to the cranks (one reason some use longer cranks for TTs).

Crank length is, as Crankin pointed out, as much a function of how one rides, their modus operandi, as it is due to leg length or femur size or whatever. I too am a spinner, and typically crank my little legs on my 165s at 90~95rpm and it feels quite normal on the road bike and I rarely stand or mash. I used to have 162.5 arms on a previous bike and I felt a difference going to 165. Small but not imperceptible to me, and if felt right. Maybe if I were a masher, a torquer, I would want 167.5 or 170 like I use on the MTB. Remember, it's 2.5mm further out in all directions- your leg will come up higher, go down lower, and move forward and backwards 2.5mm further out from center.

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Originally Posted by tzvia

Crank length is, as Crankin pointed out, as much a function of how one rides, their modus operandi, as it is due to leg length or femur size or whatever. I too am a spinner, and typically crank my little legs on my 165s at 90~95rpm and it feels quite normal on the road bike and I rarely stand or mash. I used to have 162.5 arms on a previous bike and I felt a difference going to 165. Small but not imperceptible to me, and if felt right. Maybe if I were a masher, a torquer, I would want 167.5 or 170 like I use on the MTB. Remember, it's 2.5mm further out in all directions- your leg will come up higher, go down lower, and move forward and backwards 2.5mm further out from center.

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Hmmm...that's an interesting point you make there. I'm wondering if a change in crank size would also warrant a change in height or fore/aft of my saddle? Any thoughts on this?

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Originally Posted by nscrbug

Hmmm...that's an interesting point you make there. I'm wondering if a change in crank size would also warrant a change in height or fore/aft of my saddle? Any thoughts on this?

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Fore/aft unlikely, height yes. If you set your saddle height based on your knee angle at the bottom of the pedal stroke, it will have to come down by 2.5 mm if you're switching to a crank that's 2.5 mm longer. Conversely, if you set your saddle height based on your knee angle at the top of the pedal stroke (as my knees require), then it will have to come up by 2.5 mm to preserve the same angle.

the 2.5mm doesn't sound much but when the pedal is at the bottom, its 2.5mm lower and at the top its 2.5mm higher for a total difference of 5mm. Some people will notice some will not.

For 165mm versus 172.5mm, that's a big difference most people will notice. If you notice that your seat position height and fore/aft moves by even a few mm makes a difference, then you will probably notice the difference between 172.5mm or 170mm spin on a bicycle with both crank arm length and go with the one you feel comfortable. If your inseam is short, you may prefer 165mm. Torque you have to generate is higher on 165mm but it would be easier if your knee isn't bent all the way. My performance was much better with 165mm instead of standard 172.5mm.