I’ve recently developed painful knees when cycling that hangs around for days after.
I think it’s got worse since I’ve got a new full suss mountain bike and doesn’t really go now.
I’ve changed from 175mm cranks on the old bike to 170mm cranks on the new one. I’m not sure if that’s the cause of the pain though.
What should I be looking at changing with my set up? Crank length? Saddle position fore and aft? Saddle height? Although it is about the same height as my old bike.
Should I fork out for a bike fit? Will it help?
Any suggestions/experiences would be welcomed.

It could be just getting used to the bike. Ive spent the summer on the road due an injury. Managed to get rid of knee pain that I devloped when changed from just mtb to just road. Now, back on the mtb, my knees ache again.

Set up is important, try and relax the legs more going down hill and make sure you move to absorb everything. Also, warming up, stretching helps

Your legs are moving 10mm more more to so you saddle will feel low when your knees are at the top of a stroke. Maybe that needs getting used to.

Knee pain is a bit vague, behind knee caps, external ligaments, sharp pain, dull ache ? You are right that a new bike could be a factor assuming you where riding regularly before.

External ligaments sounds bloody painful.

Check the saddle fore and aft. drop a line from the tip of the saddle and measure the setback at the bottom bracket. Start at 5cm behind the BB. A small change in seat tube angle can move you a cm further back without realising. This, for the same saddle height, will put more strain on your knees due to over-extension.

Move your saddle forward half a cm at a time would be my suggestion if you were happy with the height previously

Good job iv seen this at 43yrs old my right knee needs TKR and my left is going to have its 2nd Op this year,I found being clipped in caused alot of knee pain so i went to flat pedals to which gives my knee more movement and made it pain free.i also found that if the seat was to slip down say 1/4-1/2 inch this also gave the knees pain and cramping calf's....i did like to ride aggressive biggish drops,odd jumps and rough DH bits and jump tracks which has now stopped due to pain....iv read that 29ers are good for riders with knee pain as its less effort than other sized wheels to ride.im in the process of building a 29er to test this theory..another one is pushing a to harder gear when climbing hills or riding a SS bike...1 x 10 might be good but could be a problem for us with knee problems...im 30T with 11-36 and its just about right balance plus take your time climbing as long as you get there who cares.
Hope you find what your looking for…May the force be with you.

Thanks all, the pain is right behind the knee cap.
I ride with flat pedals.
The seat angle on the new bike is slacker than my old so I will try moving the saddle forward.

Try some nurofen and stop mashing the big gears!
And the saddle angle& seat post height etc.

There are lots of guides available online regards where the knee hurts and what you should try changing to alleviate.
I’m my case I needed to move the seat back and it cured the pain quite quickly.

Although it is about the same height as my old bike

About simply may not be good enough, if you are sensitive to position like a lot of us are then even 5mm out can be the difference between a 100 miles without fuss and crippled after 10.

If you still have you old bike then get the ruler out and start measuring everything, set-back, height etc. and start by trying to replicate that on the new bike.

But you need to pay attention to other changes that may have happened too, lower or wider bars, shorter/longer top tube, even a different saddle can all mean that your body is positioned differently and consequently means knock-on effects elsewhere so you may need to fiddle around and experiment a bit too.

Plenty of guides online too but often you get a particular fitters POV pushed across and they don’t always agree about what is ‘right’ so take it all in by all means but work out what will work for you, not just blindly follow a formula as it won’t always work for everyone.

Thanks, I’ve tried to replicate reach with the new bike and tried to keep the feel the same.
I have changed the cranks from 175mm to 170mm but from what ive read before that shouldnt cause knee problems.
I will measure the setback from seat to BB on the old bike and try to replicate that.