Thursday, February 08, 2007

Some technical stuff about frame design

All of this talk about spooky handling makes me want to discuss rake and angles. I bet you want to too. Sure, you say.

So what is it and what does it matter? Well think of your standard triangular frame. Ditch the wheels, just look at the frame. A steep head or seat angle simply means a larger number - say 72 instead of 71 degrees. If that's hard to imagine just think 'straight up' would be 90 degrees or perpendicular to the road. So a slacker angle is just another way of saying it's a smaller number. So 71 is slacker than 72 or 73, say. And a slack seat tube produces a 'laid back' or comfy bike. Your typical Euro road frame is slack, like a Look, or a LeMond (yeah, yeah, he's a Yank, but he's in the 'comfy' zone of frame design).

But that's not all. Frame designers have to consider trail, caster, center of gravity and wheelbase, amongst many other things. You may like to refer to my automotive handling guide for some more definitions and explanations (cars are just 2 bikes lashed together side-by-side after all).

Anyway, caster is like what you see on shopping trolley wheels - those wheels are even called casters. They clearly point backwards and if by chance they point forwards when you push on the trolley they swing around to point back again. This effect is called self-steering and the effect is called caster. The amount of caster you have is measured as trail. Ahhhh, now that's a bike term.

So if you draw one line down the center of your steerer tube until it intersects the ground and then drop a vertical line from your front hub to the ground, the two lines hit the ground in different places. The difference is measured as the trail, and more trail the greater the caster effect. In other words the greater the self-steering effect and thus the more stable the bike. The opposite applies, of course, in that as you reduce trail the bike becomes progressively less stable and less likely to self-steer. It becomes twitchier, or more responsive if you like. Now zero trail would be a real handful; and if you go beyond that you'd have the bike from hell.

Now a track bike must have great stability, as you don't want it to twitch in the middle of a fast pack of riders during a full-on sprint; and a road bike will want some stability too, but a criterium bike will want more responsiveness. So changing the trail is how you go about altering your bike's handling. By changing the forks (easiest way) or by changing the steerer tube angle.

You get it? Combinations of head angle and fork rake will alter your responsiveness. But wait, there's more. You are the biggest mass on a bike - by far. And where you put your weight matters a lot. If your position is rearwards then your weight will be rearwards, too. Less weight on the front wheel will lighten your steering and make it feel "twitchy" but possibly increase the understeer as well (so you'll feel as though you are drifting wide in corners). You'll find your front wheel lifting off the deck when you sprint, too.

The reverse is true, too. More weight over the front wheel will give you extra bite, so you'll track truer and feel more stable. You can easily test this out by shifting your weight around as you ride, or by altering your position. Raising your center of gravity - by raising the handlebars and/or saddle will also change the relationship between you and the bike and make for increased instability. Bear in mind that a long stem will also give you a spooky 'floppy' feeling when riding out of the saddle... so that's another complication!

All told, it's the combination of all of these factors that make you feel comfortable with a bike. And the longer and more often you ride the more you'll sub-consciously adapt to it by simply adjusting how you move your weight around on the bike. Trouble is that you may adjust yourself into a bad position that robs you of comfort and power. Which leads into the black art of position.

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