I had knee surgery recently and have gotten it to about 80-90 degrees of bend.

My question is this, if I have about 30 degrees fully extended at the bottom of the stroke, how much bend do I need in the "new" knee at the top of the stroke?? I know there are variables that will effect it some, but in general, what's the number??

Well, I accidently brought my goniometer home from work with me so I figured i would put it to use and go out to the garage and measure this. Measuring myself on the bike ( which does have some margin of error as you can imagine) I got 32 degrees at the bottom of the stroke (6 o'clock) and 112 degrees at the top of the stroke (12 o'clock). I took the average of 3 measurements for each but they were all very close to the average.
I'm sure it would be more accurate to ride the bike on a trainer for a short time and settle into a comfortable position then have another person measure, but this is the best I can do at the moment. Hope it helps.
What type of surgery did you have? We used to put post-op knee replacement patients on an upright stationary bike and have then just pedal the part of the stroke that they could manage, using the non-surgical leg to provide a little force to stretch the opposite leg. You can also set the seat height of the stationary bike higher then normal so that you can get all the way around. You will not be riding very much with that high seat so don't worry about it causing other problems. It's just to get the range of motion going.
Just don't use a recumbent stationary bike, they require too much knee flexion.

About 25 years ago, scientist at Pennsylvania State University studied the biomechanics of 7 elite pursuit riders. With the riders riding at 100 rpm in a 50x13, they found that the mean knee angle at bottom dead center was 37 degrees; the mean knee angle at maximum flexion just before top dead center 111 degrees. (Mean range of motion 74 degrees). The authors of the study seemed surprised to find that the knee is still considerably flexed at bottom dead center.

In a chapter in the late Ed Burke's 1985 book Science of Cycling, Peter Cavanagh and David Sanderson describe these studies on the pedaling mechanics of elite pursuit riders in great detail. That book, I think, was revised and reprinted recently under a different title.

You asked what I had done. In 1987 they did a total right knee replacement, a DuPuy "New Jersey" knee. In 1998, they replaced the two bearing surfaces as they were badly worn and the knee was getting fairly unstable. I was/am a serious hiker and golfer, walk the golf course almost all the time I play, carry my own bag a bunch. About five years ago I started to ride for my general fitness and I now ride about 350-450 miles a month with about 12-15,000 feet of climbing. I have homes in Ohio and Arizona so I ride all year 'round.

In 2001 I had my left knee replaced and got another DuPuy knee, newer version but virtually the same thing when looking at the x-rays.

On June 12th, a tad over twenty years after install, the right knee cap came apart and lodged in the joint leading to surgery on the 15th.The surgeon said it just wore out, the plastic was virtually gone and it separated from the metal kneecap. He said he'd never seen one wear out before. Go figure.

They did both bearing surfaces and replaced the kneecap as well. Called a Total Knee Revision. This was the 7th time they have opened up my right knee, and my left has been apart 3 time. No scope jobs for me!!

They took out the staples yesterday morning I was hitting golf balls at the club yesterday afternoon. It felt very stable. I'm thinking that I'll be back on the bike in another week or ten days. I was just curious how much total bend I needed and I appreciate the answer. I have 132 degrees on motion in my left one right now and had about 125 in the right before they gave me the blade on the 15th.

I am hoping I will have to have the bearing surfaces done one more time, maybe 8-10 years out. I'm 66 and so I figure if my health stays together and I continue to ride as I do, I'll have to have both the right and left bearing surfaces done one more time. At least that's my goal.

Two things would contribute to decreased range of motion: post op swelling (which should subside in a few weeks) and the increased thickness of the new bearing surfaces. You should ask your physical therapist and surgeon how soon you can bicycle.

Try an indoor cycling machine (set at low resistance) at the therapists before getting on your bike.

Here are some things you can adjust in your setup to decrease knee flexion and to prevent strain on the knee:
1. Make sure the seat is not too low, as a low seat would create more flexion at 12 o'clock.
2. Consider short (160mm) crank arms which would decrease the pedaling circle and decrease flexion of the knee at 12 o'clock (a minor amount).
3. Consider a compact crank with shorter overall gearing to put less strain on the knee.
4. Move seat fore or aft to a position that is most comfortable.
5. If you use clipless pedals, set the release tension low to minimize knee twist on exit.

On June 12th, a tad over twenty years after install, the right knee cap came apart and lodged in the joint leading to surgery on the 15th.The surgeon said it just wore out, the plastic was virtually gone and it separated from the metal kneecap. He said he'd never seen one wear out before. Go figure.

I just want to chime in and say that it you are wearing out your artificial knees like that, you're definitely doing something right. So many people seem to just give up after a major surgery like that--it's great to hear from somebody who is determined to wear out, not rust out, even after all that.