I ride a bicycle. I think this makes the crotch wear out quickly.
I’ve just patched the knee and the crotch of a pair of jeans, using another pair as material.
The knee was a flat piece of cloth against a flat piece of cloth. It was easy to get a good fit. The crotch is a saddle shape. Denim is not very stretchy. There is going to be some distortion somewhere.
For the crotch, I cut a piece about 9 inches long and 3 inches wide. The idea is to fold over about half an inch all round. I sewed one end on just below the zip and then proceeded towards the posterior. I ended up with a bit of bunching here and there.
If I was to do this again I would draw a line one inch either side of the central seam of the jeans. I would mark one inch intervals on this line, and do the same on the patch. I would try to keep the marks on the patch lined up with the lines on the jeans.
This way the bunching up would be kept more or less equal throughout.
Hope this helps.
Publunch