I spend a lot of time explaining this (sometimes fruitlessly) over the phone, so I'm going to try to do a post about it that I can refer people to. Some of you who are super bike nerdy or amateur framebuilders may find this interesting, others may not.
I want to explain some of what I do to accomodate fitting everything (meaning: tire, chainstays/seatstays, drivetrain, front derailleur, seat tube) together in the rear end of a bike frame. More specifically, I want to talk about how to fit everything together when dealing with a situation that calls for short chainstays, big tires, and gears (meaning a front derailleur). This is something that comes up fairly often, since I build a lot of custom 29ers for smaller folks who really need the rear wheel squeezed up as close as possible. And because I build bikes for bigger folks who are convinced that short chainstays are going to make them into Hans Rey (sorry, dudes, it's not going to happen). But I digress. Taking the desire for short stays as a given, here's what has to happen.
-The tire has to have sufficient clearance to the seat tube. This is consideration number one, because if we don't leave room at the seat tube, all the chainstay/tire clearance in the world won't do any good. For a typical short-stay 29er, 435mm or so is the limit of chainstay length before a typical 28.6mm, 73 degree seat tube will get too close - at least, too close for comfort. There are, of course, ways around this - you can steepen the seat angle (sort of helpful, but limited) and run a setback post, or you can offset the seat tube forward and slack back the angle (think of a downhill bike where the seat tube joins the downtube instead of the BB shell). This is a shot of Jason's flux-covered 29er - as you can see, there's only about 1cm of space to the seat tube (and this is not a new tire) - just enough for the front derailleur cable plus a few mm of clearance. Tight!
In most cases, if anything, I'll offset the seat tube forward 5mm or so (so it's still attached directly to the BB shell). With a 68 or 73mm shell, there's no point in getting any crazier with seat tube offset, because chainstay/chainring/tire clearance is going to put the kibosh on shortening the stays any more anyway. Also, with 5mm of offset, a standard front derailleur will work ok - with any more, you've got to go to an E-type, and they suck balls, as we all know.
Which brings us to the second consideration:
-The tire/chainstay/chainring clearance has to work out. Here's a shot of Jason's 29er (that's a ~60mm width tire, a Panaracer Rampage, and this frame uses 435mm stays). As you can see from the shot, with a 32t chainring, there's just a few mm of clearance to the stay. The tire has 5mm or so on each side (what I consider the minimum). So all is well. But push that stay out any farther, and we'll be running into chainring/chainstay clearance problems. Likewise push the tire forward farther, and we'll have it rubbing the stays. Notice (annoyingly enough) how the widest point of the tire and the chainring almost perfectly line up with each other. Doh!
In this case, I've dimped the stays a bit for extra clearance, but without building a yoke, this is as much tire clearance on a short stay 73mm shell bike as I can easily do.
-What's that, you say? Why do I keep mentioning the shell width? Well, simple - 68/73 is not the only size available. I can also build around 83mm or 100mm shells. This pushes everything outboard quite a bit - 5mm on each side in the case of the 83mm shell, and more like 14mm for the 100mm. Crank selection kinda sucks (most options are downhill/freeride stuff) but if you want serious short stays and tire clearance, this is the way to go. You can stuff a 60mm wide 29er tire into a 425mm chainstay frame (as long as you offset the seat tube) with no problem if you use an 83mm shell. If you look back a few weeks, you'll see an 83mm shell (not that you can really tell from the pics) used on Marcus' 36er, where we *really* wanted to keep the stays as short as possible (check out how close the tire is to the BB shell - with the dropouts all the way at the back, too):
-In the case of singlespeeds, some of this goes out the window, since we A) don't need a front derailleur, and B) can mess around with the chainline to an extent.
If, say, we decided to run a singlespeed setup, and had a cassette-type rear hub, we could stick the chainring on the outboard position on the crank and get a free 5mm of space for the chainstay - meaning another 5mm of space on each side for the tire, or 10mm shorter stays with the same tire clearance (approximately). This of course assumes that we've moved the seat tube out of the way using one of the techniques mentioned earlier. So bottom line - singlespeeds can have shorter chainstays than geared bikes, all other things being equal.
-Of course, if you're willing to live with less space around the tire, or want to run a smaller rear tire, things can get even shorter. Generally speaking, it's safe to say that you can subtract 1mm of chainstay length for every mm of tire clearance you subtract (hence, 25mm tires on a road bike with 405mm chainstays, versus 60mm tires on a 29er with 440mm stays). Some people (myself included) don't like big rear tires. But then again, I'm not a huge fan of super-short chainstays either on my own bikes.
Whew! I hope that made sense...bottom line here is that there are a lot of options to get the wheels where you want them, but there are going to be tradeoffs as well in terms of drivetrain function and parts availability. No free lunch, as usual.