easyupbug 

Many years ago I used to race running and cycling and know chondromalacia patellae which in me has now morphed a more dominate osteoarthritis. I would think you might find as I have that the shorter crank lengths and higher RPM are a benefit in reducing the loading on the joint. When you say your knees don't like a changed crank length, was it a minor 5mm you tried or a massive change? Crashes complicate everything so good luck there. As for hips/back it is also complicated, so good luck there as well. I got to where I was seeing a chiropractors monthly for my hip/back pain while extensively traveling internationally and in the US and one finally advised to try a knee pillow and I have not been to a chiropractor in a year and a half. Incidentally, I always enjoy seeing you posts.

OldsCOOL

I’ll be staying with 170. That’s what all my roadbikes and MTB’s have and what I’ve used since 1973. No knee probs which I am thankful for.

Carbonfiberboy 

By the formulae, I should be on 160s, but my singles are 170 and our tandem captain's cranks are 175. They still work just fine. We hike and backpack, I ski Alpine and XC, and we go the gym twice a week. I think cross training keeps becoming a better and better idea as we age.

Trakhak

Not as big a deal as knee surgery, if it were to come to that.

After a year or so of increasingly worrisome discomfort in my left knee, I just last week replaced the 172.5-mm cranks on the bike I use on my smart trainer with 165-mm Sora cranks. So far so good. Maybe a bit of an improvement. I rode 165-mm cracks on various of my track bikes for decades, so at least they don't feel weird to me.

MIght be worth your considering buying a cheap knockoff Hollowtech II crank set in 165 or even 160 to experiment with. I bought a set of cranks, plus a set of chainrings and a Hollowtech II bottom bracket bearing set, for around $60 or so.

Might end up buying Hollowtech II cranks for all my bikes, since I love how absurdly easy they are to work on.

Trav1s

I'm 5'6" with a 29.5" inseam (measured by the book method) and was riding a 170mm crank on my primary bike without much problem. Picked up a vintage MTB with 175mm crank - any ride over 10 mies left me sore in the hips and knees. Meanwhile, I upgraded the primary bike to 165mm crank and found riding to be much easier and more efficient. While my cadence was higher, it was more natural and With the helps of the LBS, the vintage MTB was converted to a 165mm crank and all was well. While 170mm works, 165mm is more comfortable and natural for me...

Tomm Willians

I switched from 172.5 to 165 as when using aero bars I felt like I was nearly kneeing myself in the chest. 165’s made it much more comfortable.

terrymorse 

Did you also move the saddle forward, for a larger hip angle?

Tomm Willians

Unfortunately I have no further adjustment available in that matter.

Calsun

It is common to see riders of all ages with the saddle too low and this puts a lot more stress on the knees. It may be partly from ignorance and partly from adjusting the seat height before they have warmed up enough to all the muscles to stretch. With a new bike or a new saddle I ride for at least 15 minutes before making saddle position adjustments.

Many mountain bike riders have switched to shorter cranks and believe them to be an improvement although their riding environment on dirt trails is quite different from a road rider on pavement.

rydabent

IMO as you get older, you are not as strong, and should keep the longer crank arms for the torque they produce.

PeteHski

Or just ride in a lower gear.

Trav1s

I’ve always wondered about the physics, especially load on the joints. This curiosity is informed by my automotive knowledge, especially with engine construction - the debate of shorter vs longer stroke, rod ratios, and piston height/side loads. I won’t bore anyone with details but all impact bearing life in different ways. If you are not familiar with the ideas, think of a Ford 300/jeep 4.0 inline 6 (long stroke) v/s Japanese 4 Cyl and 6 Cyl engines (shorter stroke) - lug it or rev it - your call. Pair with the right set of gears small engines can do large amounts of work…

Since the gene pool has gifted me with adequate at best knees, I’m trying to be as kind as possible and make cartilage (nature’s bearings) last as long as possible. Shorter crank reduces loads and angles in the knee as well as dwell at the top and bottom of the stroke. The move to shorter cranks has reduced knee and hip pain which allows me to ride longer. YMMV

Charles Lathe

I mostly ran 172.5 cranks on my bicycles, but when I built my touring bicycle fifteen years ago I used 175 Shimano XT cranks because that’s what I could get. When riding on trails, the pedals sometimes touched bumps or rocks so a few years ago I switched to 165s. I like them very much.
I am through with my 60s and have shrunk from 6’ to 5’11”. I have long used the Greg LeMond saddle height formula which measures from the top of the seat to the bottom bracket center without regard to crank length so I haven’t changed saddle height.

Mtracer

I have just ordered a 165 crankset for my main road bike. I currently ride 172.5. My reason is some slight knee pain that has developed. The pain is very similar to when I had 175 mm cranks on a road bike (that's the way it came). In that case, the pain was pretty much only when riding out of the saddle. And noticeable at the top of the stroke.

While I'm 6' 2", and thus why a standard bike came with 175 mm cranks, my height is in my back, not my legs. My inseam is a bit under 33" (floor to book in crouch method). The shorter cranks will reduce the range of the angle my knees go through. I'm hoping it will reduce loads on my knees. My pain appears to be the patellar tendon.

As for the crank arm leverage question. Certainly a longer crank has more leverage. But keep in mind that the difference of 175 and 165 mm cranks is about 6% or equal to about half a gear change. And of course, given gears, this is only an issue when in your lowest gear. However, I think this is offset by having a greater range of the power stroke. IN other words, sure you have less peak torque for the same applied force, but you can apply force over a greater range of the pedal stroke, so you don't need to produce the same peak torque to get the same average torque.

I actually look at the pedaling dynamics data I had going from 175 to 172.5 mm cranks, and the power stroke got slightly larger (more degrees of the pedal rotation) and also moved up a little. And by that I mean the bottom of the power stroke stayed the same, but the top started earlier with the shorter cranks. Now, I didn't analyze a bunch of data, so I'm not beating this drum too loudly.

PeteHski

You can think of it in terms of how fast your feet are moving around the pedal circle. A longer crank has more leverage but you have to move the end of that lever (pedal) through a larger distance per revolution and hence your foot speed is higher for a given cadence. It is literally the same effect as using a lower gear, except you have to bend your knees and hip joints more in the process.

As you rightly state, the longer lever is only a potential advantage when in your lowest gear if you are struggling in that gear. You could compensate for the gearing, but it's usually not worth it unless making a big change to the crank length.

terrymorse 

There's something else that the longer lever crank arm produces all the time: a greater range of motion (ROM).

I don't know if that is or isn't a benefit to the cyclist, but strength training studies suggest that exercises with a full ROM produce slightly better outcomes (strength, speed, power, muscle size).

Something to consider, at least.

Hermes

Age 74, 6' with a 34 in inseam. 165 cranks on track bike and 172.5 on TT and road bike. I plan on changing my TT bike to 165. I changed a few years ago from 172.5. 165 cranks are faster as tested on the track against the 172.5.

PeteHski

It is usually the greater range of motion that causes problems for some cyclists with longer cranks. As far as generating power, pretty much every study I’ve seen concludes that power is not significantly affected by crank length, even over a wider range than is typically available.

terrymorse 

I wasn't thinking about injury issues or power advantages, but the training effect from exercising with a greater range of motion.

Since studies suggest that full range of motion exercises produce superior results, maybe training with longer cranks make you stronger.

Maybe.

On a personal note, I tried shorter cranks briefly and did not like them one bit. They felt very awkward when climbing out of the saddle.

PeteHski

I guess it depends what your riding objectives are. I doubt that there are any studies showing that longer cranks make you stronger. But there are several studies showing that crank length doesn't significantly affect your power output. Pro riders tend to favour shorter cranks too, because it helps them with their aero fit without affecting power.

Then there's the question of how close to your full range of leg motion do you really want to go when you are potentially riding for many hours with significant fatigue? On a 5 hour ride you might be doing around 25,000 reps!

noglider 

I'm sure there is some trigonometry that can help explain why small changes in crank length makes for a very different experience. I've ridden 170 most of my life. I remember the first time I rode a bike with 175 mm cranks. It felt wrong, and I kept lowering and raising the seat, and I couldn't get a good position. Then I realized what was happening. I previously didn't believe it was perceptible, but yes, it is. I can sometimes perceive a 2.5 mm change, but not reliably.

@rydabent, what have you read that supports your view that longer is better for old folks? Most of us here are saying the opposite. It's easy to understand that more leverage reduces the amount of force required, but we can do that by shifting down or just slowing down. Age reduces a body's range of motion. Long cranks make me uncomfortable.

An interesting experiment: I put 155 mm cranks on my fixed gear bike. It's a street bike for errands, not a track racing bike. It does feel a little odd, but it's not at all uncomfortable. I've had them there for years, and I have no plans to change them.

terrymorse 

I don’t accept that reduced range of motion is a necessary result of aging. I’m about as flexible as I have ever been, maybe even more so.

PeteHski

Whether you accept it or not, it appears to be pretty common.

Also there is a growing opinion amongst bike fitters that stock crank lengths are already too long for many riders under 6ft tall and especially for riders shorter than average. Fortunately I’m tall enough and long-legged enough not to have any range of motion issues with stock 172.5 or 175mm cranks. But if I was building a bike from scratch I would fit 170 or even 165 cranks. I don’t see any downside and they can only be better for endurance.

datlas 

Interesting. I am curious if you raised your saddle by 2 cm to compensate for the effective saddle height change. I am thinking some of the improvement may be from a lower effective saddle height.

datlas 

Also, height loss with aging usually is in the spine, so leg length should be preserved (maybe a tiny loss at knee joint).