Going from 172.5mm to 170mm cranks

[Soloist604]

To add more fuel to the fire, I've had multiple bike fits in the past (with arguably the best fitter in my city) and they've always said that they can "work with" both sizes when doing their Retul-based fit. Makes me think that all the noise about going from 172.5mm to 170mm is just that...noise.

[Turkle]

I notice an immediate difference when I switch between 172.5 and 170s.

My trainer is the Stages SB20 bike, so it's easy for me to switch crank length without adjusting any other factor, like saddle or bar position.

My fitter wanted me to switch from 172.5 to 170, so I did it first on the trainer. I immediately noticed the difference. Better for the hips, way worse for my knees. So I've stuck with 172.5.

These threads always get so many comments from riders that say they can't tell the difference. I almost can't believe it! The difference is immediately noticeable to me.

[John H.]

I agree with most of what has been written. Crank length is more of a fit solution than performance solution- Unless you were previously on a wildly wrong crank length.

1.) Folks with lower saddle heights tend to be the ones on cranks that are too long. I think most rider with saddle heights <70cm should be on 165 or shorter.
2.) Short cranks tend to help a lot with janky hips and sizable guts. They just create space.

As another poster said- Try may fix one issue but create another. I was one 170 cranks for several years. In the last year I switched back to 172.5. No power increase, but the inner hamstring issue I had on the left side, and the right outer knee issue have sorted themselves out.
That said, I am still curious to try something noticeably short like 165 (my saddle height is ~72.5). More to see what it does for an aero position.

[unterhausen]

I got some 172mm cranks BITD. I thought they bogged me down. Pretty funny. Then more recently, I bought some used cranks from someone and rode them happily for a year before I realized they were 172.5 and realized it doesn't matter. I think my gravel bike has 170mm cranks on it now, but I had 175mm cranks before and I could switch back and forth with my road bike with 170mm cranks and not notice a difference. That includes sprinting downhill (in a 42-11) at the end of the weekly wednesday gravel world championships. The difference in saddle height matters when changing them on the same bike, but just barely.

[JMT3]

Me 6’ with a 35” inseam. Started many years ago on 175mm cranks. Went to 172.5mm cranks over 10 years ago and they felt much better than 175mm cranks. 3 years ago went to 165mm after all the threads on Slowtwitch and love them. The funny part when I pushed 175mm cranks I rode a 54, 42 front and a 11-18 straight block cassette and 60 to 75 RPM cadence was my normal. Now I ride a 53, 39 and a 11-26 11 speed cassette and my RPM average 85 to 90. Also could get by with junior gearing, if that is still a thing since I am rarely in the 11, 12, 13 or 14.

[deluz]

A few moths ago I tried switching from 170 to 165.
It felt better in some respects but worse in others.
When climbing a steep hill it put more stress on my legs and my knees were feeling it. If I had a low enough gear to spin or more power it would have been ok but I was already in my lowest 34/32 gear. Also I think after a lifetime of riding 170 it is just what my body is used to.

[robertbb]

Originally Posted by deluz

A few moths ago I tried switching from 170 to 165.
It felt better in some respects but worse in others.
When climbing a steep hill it put more stress on my legs and my knees were feeling it. If I had a low enough gear to spin or more power it would have been ok but I was already in my lowest 34/32 gear. Also I think after a lifetime of riding 170 it is just what my body is used to.

Yeah, 165's requires lower gearing than 170's.

That was the first thing I noticed when I made the switch.

Interesting that cleat position is rarely if ever discussed in context of crank length (on forums at least). If you're in the "move your cleat back as far as the shoe will allow" camp (which is where I'm firmly positioned) then 165's are well advised, along with proper seat height/fore/aft adjustment. Makes a huge difference.

[eddief]

your knees may ache with longer cranks.

[jacrider]

I'm in the middle of a transition from 172.5's to 165's.

I like the feeling of being able to spin faster. Climbing out of the saddle has taken me a while to get used to, felt 'choppy' at first, but getting better.

Have ridden this bike exclusively for about 750km. Today I rode my road bike which still has 172.5's and I felt like I had to push the pedal to get it over the top of the pedal stroke. Took me 15-20 minutes to forget about the difference and just enjoy my ride.

I wouldn't say I'm totally comfortable yet on this bike. I've messed around with the fit too much based on the shorter crank length and can't quite get a comfortable fit yet. I've had this bike for 5 years and feel confident I'll get it dialed in.

Made the change based on recommendation to help with hip and knee health as we ride into our 50's.

[MXLeader]

A 2.5mm crank length difference is less than the thickness of two dimes.

When I did a search of what this means in terms of knee and hip angles, I ran across the Appleman website where they have a primer on how crank length affects a cyclist's biomechanics. I found it to be an interesting read.

I haven't checked to see if the link was included in previous crank length discussions but figured it might be helpful to the OP and others in this current thread.