Profiles' 145mm crank arms

it's is not that big a difference though really is it?

Go from 175 to 165 and within a few days you would not notice it. It is only 10mm more than than.

How is 145mm only 10mm less than 165?

How is 145mm only 10mm less than 165?

It isn't, but it is only 10mm more than going from 175 to 165.

The problem people seem to have is that they don't gear down accordingly. Shorter crank arms enable faster rotation as smaller circle for leg to move.
Power is the same as force is less (shorter lever) but speed is more (higher RPM).

it's 20mm 10mm on the top 10mm on the bottom

Trust me, I geared down sufficiently, I was running 34 X 21.
When you hit around 155mm and below in crank length pretty much any normal riding becomes difficult as there isn't enough torque.

For a specific purpose, such as polo/pedal strike related issues, I fully recommend it.
However the OP stated that he was considering it as a way of getting rid of knee pain, in this case I think he'll find that he's wasted a lot of money buying 145 cranks, not getting on with them and having virtually no chance of re-selling them.

That doesn't add up and is not what others have experienced. Maybe they just weren't for you?

Are you saying that a slight incline on a gearing of 34 x 21 with 155s was killing your knees?

It can be better for knee problems/other problems because the range of movement is lowered due to the smaller circle.

Please bear in mind I have a missing ACL in one leg (a knackered knee) and a fully working knee on my right leg. SO I can compare both a working knee and a none working knee.

With reference to your second sentence, I am saying exactly that.
The required force required for a given gearing combined with the reduced movement in the joints meant that my knee issue in my left leg worsened and I started getting pain in my good right knee as well.

For reference, I use 165mm cranks on my fixed wheel bikes, 172.5s on my road bike and 170 or 175s on my mountain bikes.

That doesn't add up and is not what others have experienced. Maybe they just weren't for you?
With reference to the latter part of this statement. You are quite right that maybe the shorter cranks just weren't for me but may work for others.

However, as the OP has a similar issue with knee pain I thought it pertinent to join in this discussion. I am in no way saying that he shouldn't try it, but 145mm is a hell of a drop from the crank length that most people would use and my personal experience would recommend against it.

With reference to the latter part of this statement. You are quite right that maybe the shorter cranks just weren't for me but may work for others.

However, as the OP has a similar issue with knee pain I thought it pertinent to join in this discussion. I am in no way saying that he shouldn't try it, but 145mm is a hell of a drop from the crank length that most people would use and my personal experience would recommend against it.

fair points and you speak from experience, although you may have different symptoms. The problem is it can turn out quite costly to try I suppose (purchase of cranks arms and sprocket/chainring).

In my experience I prefer 165 over longer so who's to say I wouldn't like 155s. As I have just installed some 165mm BMX cranks this morning I guess I had better stay with those for a while :-)

Agreed that it can end up costly trying them all out. But it's the kind of thing which doesn't need to be done often. Surely a cyclefit type scheme would be able to give some pretty solid opinions?

I myself wasted the money on lot of different length cranks over the years and worked it all out through trial and error.

It's also worth noting that shorter cranks change the rotating position of your legs in relation to your seat tube so you may have to play around with different saddle positions, or even a new frames, to get it spot on.

Realistically it all comes down to intended use of the bike. For distances it really makes sense to get everything set up perfectly. For 10-15 miles across London you can probably ignore all advice and do whatever you like.

As an aside, I know a mountain biker (Sunday XC racer & tourer) who uses different length cranks on each side of his bike to compensate for knee pain. I believe the crank for the weaker knee is 5mm longer.

i'm bookmarking this thread for april 1st.

The problem people seem to have is that they don't gear down accordingly.

The ghost of Sheldon Brown remains the most useful guide:
http://sheldonbrown.com/gain.html

I rode 72"/140mm and 90"/175mm for comparative purposes, i.e. precisely the same gain ratio. If you're using a normal urban gear of 65" on 165mm cranks, you need to go down to 57" on 145mm cranks, otherwise you've effectively increased your gear and will find you have to press harder on the pedals at any given speed, which is unlikely to be kind to the knees. Note that even this reduction in gearing will still leave you pedal force unchanged, although the reduced range of movement may be beneficial in some circumstances. Conversely, the increased pedalling frequency could just as easily be a detrimental influence on other joint conditions. In other words, consult your doctor before trying to fix joint problems with changes to your cycle equipment.

Thanks again for all the relevant opinions and info, I think I'm going to give it a try, as I have everything I need, except the crank, so the outlay shouldn't be too bad.
The problems with my knee aren't going to go away even if I was to stop cycling all together, so worth a try.

Thanks for underlining the word relevant. Here are some relevant questions: where does your knee hurt? Subpatellar? Medially? Laterally? When? Do stairs hurt it? Going up or down stairs? Is it only after a long ride?

My brother does unicycling, and KrisHolm, a very good brand in this field sells double insert cranks 150/125mm so you can change crank length in minutes. My brother also does freestyle (like flatland bmx on a unicycle), and he's running 90mm cranks on his 20" build.
Basically, it's hard to get going and to slow down but top speed is improved.
it's useful on a unicycle because it gives you some sort of gearing without the need for demultiplication. I'd only consider shorter cranks for my massive pedal overlap and ground clearance.