There are three dimensions on road bikes that when changed can make significant changes in the way a bike will steer. They are 1) head angle 2) fork offset and the result of these numbers is the third, trail.

Quickly, trail is the amount the wheel follows the bike (in your brain) and a a big number means the bike has a strong tendency to go straight. a small trail number and you change lanes when you turn your head to spit.

Trail is a measurement taken on the ground. If you project a line through the center of the head tube all the way to the ground and make a mark. Then using a framing square or a plumb bob, you locate a point on the ground directly below the axle. The difference when viewed from the side of the bike is "trail".

Randy mentioned something in the recent PX thread about different forks having the same steering geometry. The top row is a demonstration of that. Only the radius of the bend in the fork was changed but it has a large visual effect. No change in steering geometry.

The second row is what changes when head angle and fork rake (offset) are changed.

We can look at this a bit and discuss it more later if you like. There are other small factors, wheel diameter/fork length that effect the steering geometry. This is a small part of overall handling which weight distribution and leverage make big effects on handling. The image on the bottom right is a bike with a 74 head tube and a bent fork with a short offset number. It's counter intuitive that it would have a greater tendency to go straight. That is what I was talking about at the beginning, "the amount the wheel follows you" can really be felt in this example when ridden. You may have to click on the image to see it clearly. My monitor looks like you are peering through a screen door so it's difficult to see anything without a blur!

I copied this to frame builders, I hope that is OK!


steeringgeo by frankthewelder, on Flickr