October 29, 2010

Reductio ad Absurdum – or, my new singlespeed

This is actually more of an introduction than anything, my ride today got canceled due to (another) forest fire in Boulder Canyon, so I have very limited dirt time on the new singlespeed. A real “review” post on that bike will show up at some point, but not today.

Today, I want to talk about chainstays, why I made the stays on this bike so short, and what the pluses and minuses (in my mind) are. You can look at my godawful pictures of my spraypainted bike (powdercoating? Pfah. I’ll do that when we’re buried in snow!)

First, for the sake of argument, we’re going to say that we have 2 identical bikes – ie, same angles, same tubes, same parts. There’s no sense in comparing a 5″ full suspension bike with long stays to a 24″ rigid BMX.

That said, what happens when we make chainstays longer or shorter? Well, a bunch of things.

-We increase rear wheel trail. Just like the front wheel, the rear wheel wants to go in a straight line/follow you because it falls behind the steering axis (imaginary line drawn down the center of your head tube to the ground). Front wheel trail for a mountain bike averages around 80mm, with lots of variation, and higher numbers mean more “stability” (meaning, tendency to stay on a straight line). Rear wheel trail is MUCH higher – on a generic medium sized mountain bike it hovers somewhere around 1000mm, so an order of magnitude higher.

-You can easily notice the change in handling if you vary front wheel trail by 5mm or so and likewise you can tell when rear wheel trail changes (the effect is not as great, though) – as the rear wheel trail drops, the wheel deflects farther (we’re talking about an angular change here) from your intended path if you hit a rock or a root at a funny angle. So if the rock knocked your rear wheel 3″ to the right, the rear wheel angle relative to the front has to change more for shorter bikes, and less for longer ones.

-To illustrate a bit better, think of a bike with 20 foot long chainstays. When you hit that rock in the trail, the rear wheel still has to shift 3″ to the side, but the bike’s angle hardly changes. You might not even notice (of course, you wouldn’t make any switchbacks, but that’s beside the point…) On the other hand, if we have very short chainstays and wheelbase (say, a normal bike) the bike’s angle changes quite a bit – and hence feels less “stable” – you’re getting knocked off course, because when the angle of the bike changes unexpectedly, you have to actively compensate – or else ride off the trail.

-Remember that you can also make the FRONT of the bike longer (ie, longer effective toptube, slacker head tube angle, more fork rake, or any combination of the above – a longer front center measurement, for the geeks out there) and gain rear wheel trail as well. So for our thought experiment (identical bikes) the longer chainstay makes for more stability. In the real world, there’s a lot more to it than that and the chainstay length shouldn’t be considered in isolation from the rest of the frame geometry.

-Obviously there is more to it than “stability”. Many of us like to get up on our rear wheels and wheelie/manual/hop. Short chainstays make this easier, as the riders weight is closer to (or even behind) the pivot point, which is of course the rear hub. Here the length of the front end of the bike matters less (though the position of the rider matters quite a bit). Holding other variables constant, shorter chainstay frames are always easier to wheelie/manual/etc. In many cases, that’s irrelevant, since most terrain is ridden with both wheels on the ground. For folks who like to jump every little hip on the trail, though, short can be a lot more fun.

-The bottom line is that, in isolation, chainstay length doesn’t mean that much. The rider preference and terrain dictate a lot, and shorter isn’t always better. As much fun as it is to manual/hop/bop/jive, it’s not so fun when the trail spits you off your bike on lap 8 at 4 in the morning because you can’t hang on and stay on course anymore. So as with all things, there are tradeoffs involved, and what works for one person might be terrible for another.

In my case, I wanted to encourage myself to be active on the bike and enjoy really feeling/working with the trail, rather than just blasting down at warp speed as I’ve gotten in the habit of doing on the suspension bike. This bike (106.9cm wheelbase, 9cm of front wheel trail and 97.9cm of rear wheel trail, for those who are curious) is intended to do just that – both the rear and front ends are very (by my standards) short. For comparison, my previous singlespeed was 108.5cm wheelbase, with the difference about evenly split front/rear. My (just retired to become my brother’s) full suspension bike is 108.9cm, with most of the extra length in the rear.

In other words, it’s not a race bike, it’s a play around bike for when I want to try to wheelie drop every log step or jump all the little jumps on a Ned trail. If things are super fash/gnarly, I’ll probably break out the full squish (yes, I built myself 2 new bikes this year… bad Walt – more on it when I get it built up) but for everyday XC riding, I think this bike will be a lot of fun.

Ride report of some kind in the next few days, along with initial thoughts about the Black Cat swingers, once I’ve had a chance to give them a fair thrashing.