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.

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.