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I've recently developed pain at the front of my knees when cycling. I suspect this is caused by, or at least made worse by, my bike having cranks that are too long for my leg length (172.5mm cranks for 77cm inseam).

Assuming crank length is a factor, a long term solution would obviously be to get shorter cranks. However, I can't afford that right now.

Are there any bike fit adjustments that I could make to open the angle of/reduce the strain on my knees at the top of the pedal stroke? I've already experimented with various seat height and setback changes without much success. I'm prepared to temporarily compromise other aspects of my bike fit, if it would allow my knees to recover.

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    Are you getting (nearly) full extension at the bottom of the stroke? – Daniel R Hicks Sep 22 at 22:46
  • Yes. I've experimented with a range of saddle heights, but currently have the saddle fairly high, and so my legs are near but not completely straight at the bottom of the stroke – NotRelevant Sep 23 at 0:46
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    You can get bolt on crank shorteners that may be use for you to try. They are now overly expensive, and easy to install and remove when done with. The do alter the Q-Factor, which may affect knee pain. You may be better visiting a bike shop for a professional bike fit, ideally someone with an interest in rehabilitation rather than just a focus on performance. – mattnz Sep 23 at 1:48
  • Are you using clipless pedals and if so, what are the angles and float set like? I had rather severe knee pain at one point that was completely resolved with cleat angle adjustments. – Deleted User Sep 24 at 16:03
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    Just a general comment: knee pain can have many causes. If you are able, I would find with an experienced bike fitter first, or a sports-oriented physical therapist. I worked with both for knee pain. The first helped me change my saddle height and suggested some stretching exercises. The latter helped more with stretching and conditioning. I would suspect the cranks alone aren't the cause, but I'm obviously not you and I don't know what research you've done. even so, I'd cautioning against assuming it must be the cranks. – Weiwen Ng Sep 24 at 19:51
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If you are using clipless pedals and cleats one simple thing to try is moving the cleat as far back to the center of the foot as the adjustments allow.

This worked for me and I got the idea from some articles on bike fit and cleat placement at cyclingnews.com. This was from at least 10 years ago so those articles may be long gone. This is a recent article from velonews that exposes a similar philosophy.

Personally, my experience has been that switching to 165mm or 170mm cranks from 175mm makes a big difference in knee pain and comfort on the bike in general. If you do go to 165mm cranks it's important to make sure you have appropriate gearing. Knee pain can be just as much related to force through the joint as the range of motion. Keeping your cadence high helps your knee health a lot.

Riding at a very easy pace with high cadence has been the most effective tool for recovering from meniscus damage as I get older.

  • Thanks this is the kind of advice I was hoping for - I can see how moving my cleats back would affect knee angle at the top of the pedal stroke. I will try tomorrow. – NotRelevant Sep 24 at 21:25
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Product options

You can purchase a bolt-on plate for either side that will give more pedal positions.

enter image description here

Any competent machine shop should be able to make you something like this, given the cranks to work from and test-fit.

Downside, this will increase your Q-factor by at least 10 mm, which could be uncomfortable. Adds weight and surface area,and leaves end of crank arm poking well below your pedal.

On the positive side, it doesn't change your bike and can be removed in the future. Can be fitted to hollow crank arms that can't be drilled.

Link: https://tadpolerider2.wordpress.com/2015/05/11/short-cranksetscrankarms/


Another similar option is a swing-arm pedal. Its very similar to the plate above but includes a hinge/swivel.

enter image description here

Perhaps best-explained by an animation:

enter image description here

From https://highpath.co.uk/pulse-swing-cranks/

Downsides, they add double the Q factor change of the plain bolt-on plate. Significant cost, and there's now another moving part in your transmisison.

Upsides, you can shrink your pedal circle, and you can also lower your pedalling circle if that's what you need.

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    A quick definition of "Q Factor" (If such a thing exists) would be greatly appreciated! – Matt Holland Sep 23 at 22:27
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    @MattHolland God point - done. bicycles.stackexchange.com/a/64382/19705 – Criggie Sep 23 at 22:51
  • Actually, speaking of Q-factor, this would move the OP's pedals outward and change his/her knee alignment while pedaling ... which could affect his/her knees more. unless, of course, his/her pedal position was too far in to begin with. – Weiwen Ng Sep 24 at 19:52
  • @Criggie - both your answers are interesting, and the swing cranks are neat from an engineering perspective, but I was hoping for adjustment advice rather that stuff to buy ideally. Should I update my question to clarify this? – NotRelevant Sep 24 at 21:24
  • @NotRelevant Could do - but also upvote the answers that help and to also accept the one that helps the most. Leaving the other answers may help someone else another day. – Criggie Sep 25 at 4:12
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Depending on your cranks, it may be possible to shorten them. This is much easier than lengthening them.

This requires either a well-equipped workshop, or access to one. The basic method will be

  1. mount your cranks to a junk/spare BB axle that can be held in a vise.
  2. Position this in a pillar drill so that the drill axis is parallel to the BB axle.
  3. Mark exactly the length you require. Get someone else to measure your mark to confirm.
  4. Drill pilot hole, with cutting fluid.
  5. Test parallelity with BB.
  6. Drill correct size hole, again with cutting fluid.
  7. Tap using the correct 9/16" left and right hand thread taps.
  8. Deburr, refit, test ride.
  9. (Optionally) you could trim off the old pedal mounting hole, and round off the end for weight and aero savings, but its also less to catch on the ground or on roots/grass.

You'll also need to use cranks of a construction with enough meat to still work after shortening. Some cranks are hollow, and some are simply too slim to have enough metal left. Ideally they should be flat on the outside, or have enough metal to grind flat.

from https://highpath.co.uk/crank-shortening/

Carbon fibre cranks are not suitable for shortening by this method. Instead, buy some junk spares off your local ebay and use them for the experiment. I see steel cranks may be unsuitable too, due to age and lack of metal once drilled.

Further links:

https://highpath.co.uk/crank-shortening/
http://faqload.com/faqs/bicycle-components/drivetrain/crank-arm-shortening

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    Wouldn't shortening the cranks make knee pain / issues worse? My understanding was pain in knees generally means your saddle height is too short, I'm not sure how shortening the cranks compensates for this, wouldn't raising the seat compensate for shorter cranks and it's far easier and does not require a workshop full of tools. – Dan K Sep 23 at 5:16
  • @DanK fair points - the biomechanics of the human body have a lot of variables, so OP's perfect saddle height may not match the optimal height based on body metrics. OP wants to try shorter cranks, and here are some solutions. Personally I want to try riding with longer, but anything over 175mm is completely impossible to find here without going expensive+new. – Criggie Sep 23 at 5:20
  • @DanK Even if you have the correct saddle height, a longer crank will create a tighter angle (and more pressure on the knee) at the top of the stroke. – Andy P Sep 23 at 14:05
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    My understanding is that knee pain at the front of the knee can be caused by the leg being overly bent at the top of the portal strike, and this increasing the forces on the knee. Higher saddle would alleviate this, but obviously eventually hits a limit. Shorter cranks should allow for reduced leg bend at the top of the stroke whilst still being able to reach the bottom of the stroke not completely straight legged. Assuming I've understood correctly. – NotRelevant Sep 23 at 14:06

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