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I'm 1.74 meter tall. I've two MTB's, one of them being 26" frame and the other one is 27,5". They both have crank sets with 175mm crank arm length. As a chondromalacia sufferer I wonder if replacing my crank sets with those have 170mm crank arms would be better for my knees. What is the point of having shorter crank arms for a rider?

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Shorter cranks mean a smaller rotation circumference your feet/pedals have to travel to do 1 revolution.

That also means your knees and hips move through a smaller angle, your knee won't come up quite as far.

Downside is that you have less leverage on the pedal, and therefore you'll be putting less power into the crank, so the physical reaction is to pedal faster. This is because your knees and shins and feet are moving a shorter distance and not building up as much momentum.

The upshot of that is with everything else being equal, a shorter crank arm will result in a slightly higher cadence, so you will "spin" more.

For the average rider, changing crank length between 170, 172.5 and 175mm is no great deal. Those three lengths are extremely common, and picking up a replacement set used on ebay or similar is easy.

What's hard is looking at results from any of the "crank arm calculators" - My height of 195 cm and long-legs suggests a crank arm length of 195-200mm and they are exceedingly rare.

In your case, find a donor bike with shorter cranks that suit, swap them to one of your bikes, and try it out.

In your specific case of chondromalacia (knee pain) I'd also look at shoes, pedals, placement of foot on pedal, and make sure your pedal axles are not bent.

If you use cleats/clips then also consider how they affect your entire leg and the pedal stroke. Even something as simple as a miss-aligned saddle can affect your pedal stroke.

I've also had relief using knee straps, thick reinforced and formed pads that velcro around the knee above and below.

The last resort is always see someone with medical AND riding skills, in person. A sports-physiologist may be able to advise better than some randoms via the internet. This is also something that may be helped by a proper bike-fit using any of the systems out there like Retul / Retu:l etc. Downside is these visits have significant costs.

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    I also come out needing unreasonably long cranks if I try to calculate. Pedal strike would be a real issue then. But in general, a rider is more likely to have serious issues with too-long cranks (leading to too much knee movement) than too short. So it's not a problem for us tall people but might be for the OP 175mm seems like it could be a bit long for their height (depending on how much of it is in the legs); I know riders a little taller than that who switched to 170 or 172.5mm cranks for their knees.
    – Chris H
    Commented Mar 3 at 9:16
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    170mm sounds more suitable for my case. My chondromalacia is due to patellar tilt. Probably too tight IT band which causes the patella to a lateral tilt. Resting, stretching and dry needles help. Also walking and keeping the knee fluid get enough blood stimulation is crucial not to make things worse. Never sit too long.
    – Ender
    Commented Mar 3 at 9:38

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