May 6, 2010

New FS 29er stuff

Warning: Long, potentially boring post. Contains applied trigonometry.

Recently I built Sarah a totally kickass full suspension race bike. It was light, agile, and fast. And it wasn’t what she needed at all, because we both have been racing less and less over the years, and there’s a good chance she’ll be pregnant later this year. Meaning that as a second bike to back up her hardtail, it had some significant shortcomings – she needed something really stable, plush, and comfortable, with _lots_ of standover for easy dismounts in any situation.

So I started over (quite a while ago) and went back to the drawing board, or in my case, graph paper (yes, my computer is ancient).

In essence, what I needed was a way to move both pivots (the main pivot, and the center rocker pivot) in 2 dimensions. In the past, I’ve moved the main pivot around quite a bit, and I’ve moved the upper/rocker pivot some as well, but have been restricted by the seat tube/tire/rocker interference problems that can result from some configurations. Luckily, this has gotten MUCH easier to do in the last 6 months or so, thanks to Shimano’s new direct-mount front derailleurs for mountain bikes.

Solution? Bend the seat tube. Or so I thought. It turns out that the area available for the bend in this situation is *really* small – probably only 2″, which needs to bend 16 degrees. It turns out that’s not very doable with .035″x1.25″ tubing, so I decided to cut the tube at an 8 degree angle and weld it back together. Not the most elegant solution (and one that requires some reinforcement) but mechanically, it’s great.

Here’s a picture of the end result: 435mm chainstays, 13.2″ (no sag) BB height, 100mm travel front and rear, and 28″ standover. This is probably the absolute limit for short chainstays for folks who want to run a front derailleur (and I had to do some *serious* modification of the derailleur itself to make it work here), but for 1×9 or 1×10 dedicated setups, there’s really nothing stopping you from doing as short as 425mm or so, even with quite a bit of travel.

This particular frame is about 6.5# with the shock – it’s got a straightgauge toptube and I used a *really* beefy downtube as well, because it’ll get loaded into a case and put on the plane when we travel – so it needs to be able to handle some pretty rough treatment without denting. For normal sized folks, 6.5-7.5# is probably the range, with the really huge guys of course going up from there. So not a weightweenie FS bike, really. C’est la vie.

I’ll do an exhaustive report on the new FS bike that I’ll be building for myself in the next month sometime that will hopefully explain some of what’s going on here in more detail (with actual construction shots and blow-by-blow), but I think it’s a neat setup and I thought I’d do a quick post on this one just to whet everyone’s appetite.