I was wondering if anyone knows why you can't run a computer from your rear wheel.

You can and many do.

Traditionally the problem is that you need a wire from the sensor to the head unit, which is a mess on some bikes. The front wheel is closer, so less wire mess.

I don't know if this is still true, but the range of the wireless ones used to be too short for rock solid communication between the sensor and the computer.

Yes, that was my experience.

Here is my understanding....

Wireless ones need to have the sensor close to (and pointing at) the computer, and having the sensor on the back wheel would require you to put the computer behind the seat or something like that. But it would "work" though.

For wired ones, they just simply don't send you a wire long enough for the back wheel because most want it on the front.

I have a cheap Sigma computer, and I wanted to put it on the back wheel. My bike shop was able to order me a replacement sensor with a longer wire for $9. I think it was called a "rear wheel sensor kit." It works fine. I had to use a bunch of scotch tape to get the wire to run from the back wheel all the way to the handlebars. Mounting on the front wheel is MUCH easier.

That's the way I see it anyway. I could, of course, be totally wrong about everything I just said though.

Thanks all, I would like to use one on my rear wheel, because the the one I have to use on the front wheel doesn't have all the functions I want.

My Cateye strada cadence (wired) lets me run from the back wheel with no problem. Same wire that runs down to my pedal for cadence. Works well. Cheap too.

I use a wireless Cateye V2c. It has a single sensor on the chainstay, with a magnet on one pedal post and one on a rear spoke. I get cadence without a second sensor, which is what sold me on this model. I have no connectivity issues whatsoever, and have a really big bike (68ST, 60TT). The current versions of the wireless computers use the 2.4Ghz frequency, same as most WiFi, and have a greater range than the older computers. There is no longer a need to run wires, nor limit where you put the sensor(s).

My combination HRM/computer says you should place the sensor as close to the computer as possible to minimize interference. Prior to that computer, I had a Cateye w/cadence and that was designed to get the speed off the rear wheel since you had to run a wire back to the pedals for cadence anyway.

I do wonder though if using the rear wheel vs the front does affect the accuracy of the wheel circumference setting. I imagine the rear tire deflects more at the same pressure due to the additional weight. I tend to inflate the rear wheel to a little higher pressure but I'm not sure if that compensates, over compensates, or makes no difference at all.