Having a few kneee issues, which bizarrely seem to have cleared up since buying a new bike. Now trying to work out what’s caused it, apart from n+1 syndrome manifesting itself…

Turns out new bike’s cranks are 170mm according to manufacturer. I’ve always used 175 assuming as i’m a big lad that’s what would do the job. 32″ inseam and size 9.5 wide if that helps.

Now I know there’s a lot more to consider here but does 170mm sound too short for those measurements? I always thought so but now I need to put a new set of cranks on the other bike that’s caused the issues i’m wondering whether to try 170mm there too.

Cheers

170 vs 175 is really unlikely to make that much difference. All sorts of other things could be different with the fit which are likely to have a larger effect – saddle height being the most obvious.

Come to that, neither is 170 desperately short for your height. Depending on how you measure it, 32″ inseam isn’t especially tall (I thought that’s what mine was and I keep being told that at 5’9″ I’m below average height). I have 170, 172.5 and 175 cranks on my bikes (as short as 125 on other contraptions) and would struggle to tell the difference.

According to Sheldon Brown crank length probably doesn’t make that much difference. In the grand scheme of things 5mm is a very small change, so I would bee looking for something else in your set up that’s fixed your knee pain.

Ever have a big stack on the side that caused issues?

Bent crank arm/pedal spindle? Seen it a couple times.

I am 6’2″ and have always run 175 cracks on the mountainbike, but my roadbike had 170 cranks, so when they wore out I put on 175, I could defiantly tell the difference, and I don’t particularly like them, will go back to 170 when I can afford to, and will experiment with some shorter cranks on my mountain bike as well.

If you drop your saddle to get your leg correct at the bottom of the stroke then the difference is doubled at the top of the stroke. It could be the leverage around the knee at the top of the stroke that is causing your problems.

Shorter cranks just seem to encourage you to spin a faster cadence rather than mashing the pedals, without any conscious effort, this would reduce stress on the knees.

Some people (ie me) seem to be much more sensitive to things like saddle position, crank lentgh and cleat position than others, and I think among my cycling buddies it is the taller ones who seem to have the most problems, maybe longer limbs exaggerate the problems.

For people who don’t have issue with it, the difference between 170 and 175 is nothing.

However, last year I replaced 5 chainsets in one go, switching from 175 to 170 and my knees have been much happier. If you do have issues, that 5mm is a massive difference.

You can visualise how a crank that’s too long could cause biomechanical issues. However, I can’t see a biomechanical downside to shorter cranks.

Maybe they don’t optimise power delivery or some such but then I only ride for my own enjoyment which works better anyway with happy knees.

I don’t know if shorter cranks are the answer for you or not but from my experience, there’s a very real chance they could be.

Another vote for Sheldon Browns article.

Needing new cranks on a budget, I went for 180’s as they were cheaper than 175 and hey, its only 5mm… Started getting knee pain at the top of the rotation…

Went to 165mm as also cheaper than 175 and I am now very happy… probably less of an issue with geared bikes as all the leverage gains with longer cranks can be negated as with a shorter crank, as other gears are available.

Then there are less pedal strikes too…

CRANK LENGTH – Which one?

http://www.powercranks.com/cld.html?gclid=COe9ufyelrwCFZShtAod3H0Anw

Across the three bikes I currently own I’ve got 170mm (commute bike), 172.5mm (road bike) and 175mm (MTB) and can’t tell any difference between them.

…now I need to put a new set of cranks on the other bike that’s caused the issues i’m wondering whether to try 170mm there too.

Can you try the cranks off the new bike on the old bike first before you spend any money ?
Or are the BB or rings not compatible ?

We sent a mountain bike to a mag for test once with 170mm cranks on it. Apparently it made the bike sluggish and hard to accelerate and was marked down for it.
All I ever notice (if I really do) with 170s is that I may go a gear lower at times if it gets steeper and I’ve not yet tried them on the SS. On my road bike I’ve use 170s for years and I’m 34″ leg.

In the world of triathlon it is now all the rage to use shorter cranks with some long distance pros going as short as 150mm. That’s mainly for the reduced hip angle a shorter crank gives you which is very useful for either getting comfy in an aero position or to get the position even lower.

I think the long crank – long lever – more power argument in now thoroughly debunked.

In the past, I’ve used 175 on one side and 170 on the other without noticing the difference…

BUT… As a long-term sufferer of mild knee pain*, I decided to try short cranks about two years ago. Not one to do things by halves, I went straight to 155s and the knee pain all-but disappeared. 😀

Currently on 165s, because shorter than that is difficult to find if you want quality. Definitely better for me than the usual lengths, but I’d say 160 is about my optimum. I’m 6′ / 1.83m tall.

Incidentally, I also seem to be able to get more power down with the short cranks – I hypothesise that the straighter leg more than compensates for the shorter lever, at least within the range under discussion.

*subsequently diagnosed as osteoarthritis.

5mm makes litte difference. Have 165, 170 and 172.5mm. Might be a little smoother on 165, but it is a track bike so is already smooth.

My suggestion is check teh saddle positions relative to the BB VERY carefully. Do the bikes have the same seat tube angle? If not, a half a degree difference is 0.6mm on the saddle rails. Setting your saddle by copying another bike may not give you the same position.

Yeah I know there’s loads more to it, just always thought 175 would be where I should be. Ordered a set of 170’s and shall try those out. Sounds like I could even go shorter.. 😯 There’s a massive difference in the angles etc (160mm slack full suss compared to 120mm carbon ht). Cheers!

I returned to cycling after a ten year gap and developed pain across the tops of my knees, turned out I had the saddle set too far back. After moving it forward the pain went after about a week.

170 on most of my MTB and road bikes now.

Definitely noticed the difference in terms of reduced knee pain and being better able to spin more smoothly.

It might seem silly To some but making my feet describe a 10mm smaller circle has made a positive difference for me. Could be age, build and/or loss of flexibility but I know what suits me better

5’10” & 32″ inseam.

Seems odd to me when people adjust or change all sorts of parts (bars, stems, seating position, etc) to suit them that they won’t change crank length, the bike should fit you not the other way round.

I can’t feel any difference between 175 and 170 but then there’s lots of meaningful differences in the world that I can’t feel so I don’t think that means anything.

But equally, human bodies are good at healing, are you sure it’s not just normal recovery? I’m laid up with a burst rib and bad back and the evidence suggests that playing lots of Civilization and watching Dr Who is causing a reduction in pain 😉

5mm longer cranks on my road bike meant the difference between clipping my toes on the front wheel and not. Should have thought about that before I bought the ‘bargain’ chainset that was sold out in 170…

I think Mike Burrows thinks shorter cranks may well be the Next Big Thing.

Where are people getting 155mm ones from?

I found the same as ray young. I used to get a pain where the outside tendons of the hamstring attach the top of my fibula, both sides, got quite bad if I rode a lot.

I replaced a Race Face XY layback post with a Thomson inline, largely because I’d just been paid and fancied something tarty. Rode the SDW that weekend and was delighted to be completely pain free.

Thorn at SJS Cycles have lots of very short cranks, but all square taper.

It also makes a difference to muscle recruitment which may help when running off the bike (also a good reason for triathletes to go mid foot). I certainly can tell the difference in terms of which muscles get tired when using 150 or 140 cranks (both of which I do quite a bit of riding with) – shall have to try and work out whether my running muscles are less tired with shorter cranks next time I’m out on those, though I guess it’s complicated by using lots of other muscles I don’t tend to use on a bike.

Shorter cranks allow for getting the power on earlier. Max power between 1 and 4 o’clock

How about the STW wanted ads, or freecycle.

I got an old racing bike frame, BB, stem and forks for (literally) a case of Fosters* from STW.

*Yes, I did judge the man for such poor taste, but he otherwise couldn’t have been more helpful!

on a mountain bike I’d pick a crank length depending the BB height and the chance of pedal strikes rather than anything related to efficiency of pedaling or power.

I know its probably a very individual thing but I too switched from 175 to 170 due to knee problems which over time has solved them for me.

Bit shorter leg than you (30 inch) but just seem to suit me better.

I know in theory they should give less leverage but in practice I have found actually seem to give more power or maybe similar power but without the pain!

Some very sensible advice already posted here

😆

run 175s on the road and mtb, went to Newport and the track bikes are 165? certainly noticed the difference, more so when I got back on the road bike after.

I noticed a difference between 170 and 180 cranks on the BMX – in particular, the first sprint out of the gate. I found it easier to get the power down and accelerate.

I noticed a difference between 170 and 180 cranks on the BMX – in particular, the first sprint out of the gate. I found it easier to get the power down and accelerate.

presumably 180 was easier ?

I have bad knees and 175 on MTB and 172.5 on road and cross. When I started track riding a year ago my knees didn’t like the 165 cranks at first After a few months my knees got used to them.

Now I cant tell the difference between any of them !

I am always quite careful about saddle height as I have issues with my knees and so I’m a bit sensitive to it. When I got a reverb I found that after a while on the bike I would get shooting pains through my knees and have to go home early.
Eventually, after much head scratching, I worked out that the issue was that the reverb is in line and the old post a little laid back. So I slid the seat back a bit on its rails and it solved the problem instantly. Who would of though moving a seat less than an inch back could have such an effect?
(175 cranks BTW)

currently sporting 175 on one side and 172.5 on the other, will see how my knees feel after a few hundred miles. LBS owner said he turned up for some 700+ mile audax with a borked crank got a mismatched replacement and didn’t notice a difference. But as someone said if you’ve got strong knees it’s probably not a big deal.

Yes, the 180 was easier.

Where are people getting 155mm ones from

I used these.