For the longest time, I could barely ride 10 miles without my knees aching so badly that I would have to skip a day or two. A friend with whom I work, a certified physical therapist, told me to try riding in lower gears. For two weeks I ignored her advise, and stayed in the tall gears, but still suffering pain. So this weekend I tried to move into the lower gears and about half way through my ride, I shifted to the tall ones. Guess what? No pain. In fact, my legs got a better workout. I have been suffering this pain for so long that I think this may be a fluke or something. I will add some mileage this coming weekend. Anyone have similar experience?
Well if you thought of it like a bearing on a crank shaft or something, the more force you put on the bearing laterally, the more wear and tear you give it. Slowing your cadence puts more pressure on your knee joints for a longer period of time through a down stroke, thus creating more wear and tear on the knee joint bearings, the cartilage. Best bet is to get a cadence reader on your bike computer and keep an eye on it, more so than your speed.
Lower gears will help you a lot in several ways. It's been shown that high-spinners (bikers focusing on riding in low geras) usually have lower heart-rate and therefore sweat less and get tired more slowly than the riders who tend to use high gears more. I, unfortunately, am one of those who is terrible at spinning in low geras so I mostly use high gears and from time to time I experience quite a bit of knee pain. Lately, however, I started focusing on using lower gears.
I have a cateye astrale 8 that reads my cadence and I try my best to keep my cadence between 90-110 and have experienced very little knee pain. I can also verify what was said above in that those who spin instead of mash end up with lower heartrates, sweat less, and don't get tired as quickly. My friend tends to mash gears at a 65-70 cadence and he can't keep a consistent speed very well.
Surly Cross-Check, '85 Giant road bike (unrecogizable fixed-gear conversion
Work to have a higher cadeance, and you will go faster, your knees will feel great, and you will achieve the aerobic benefits of cycling to a much greater degree. If you're trying to lose weight, that will happen faster as well.