Bicycle Mechanics - I have a clunk in my Trek 820

I have a Trek Mountain Bike 820 that I am going to retire this winter and get a new Trek Madone Road Bike. My 820 is close to 15 years old and this summer has started to clunk only while pedaling. Not on every revolution all the time but does seem to be in the same spot all the time. Another problem is you can't hear the clunk but you can feel it in your feet. Even more evident now that I have clip less pedals. But I am not sure I want to totally retire this old bike. It has been great and I may want to use it on the trails in the winter plus my wife loves this one over her 820 because of the fit.

Is some of the cranks on this bike the same as many other bikes. I have a couple old bikes that ride nice just that they are beat up, wheels bent and derailers probable worth less also. I was thinking if generic I could steal the crank and back gears maybe buy a new chain to keep this bike running and maybe this winter I would have time to do the work myself. Hate to part with it as the frame has barely a scratch on it.

Hi, well based on the age of your bike it is probably the crank bearings since they were cup and ball type not the newer cartridge bearings, grab the crank arms and see if there is any play up and down or side to side.. But this is also the era when Shimano had recalled cranks so inspect these as well for cracks of the drive side arm. Now also if in the same pedal spot I would look at the chain for a stiff linknwould create a skipping sensation while pedaling, or excessive play in the pedal it self. If you use a different crank swap the crank BB with it as they will probable have a different off set

Likely the BB as explained above or perhaps in one of the pedals. If you feel it in both feet then it's likely the BB bearings. If more in one foot perhaps a pedal.

Not likely the chain if it happens at the same spot in the pedal stroke. A tight spot in the chain would not appear in the same spot of the pedal stroke because the chain doesn't pass around fully for each crank rotation. If you check it for play in the cranks or a roughness of the bearings you'll likely find the BB or a pedal that is toast. If you shift down to the small front ring you can typically lift the chain off the ring and let it sit on the BB shell. That'll let you turn and test the cranks without the chain being part of the equation.

While you are at it unmount both wheels and see how smoothly the axles and the freewheel are turning.

All in all there certainly is no need at all to retire the bike. If you're keen on learning to do your own work this is a golden opportunity to learn to do the basics of strip, clean and relube jobs as well as learning to more effectively and properly diagnose your own bicycle woes. With a bit of work and a little money spent on specialty bicycle tools you'll easily be able to keep your 820 running well and smoothly for another 15 years and even more.