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A lot of posts recently have been made about adjusting this and that along the range of forks. These changes shouldn’t be made capriciously and I hope after this some of you will understand why, I also hope some of you will attempt these adjustments once you understand them.
Lets stick with the basics; Rake, Trail and Offset.
Below is a simple line drawing of a frame, forks and wheel.
First let’s define rake. Rake is the angle from a straight line intersecting the fork over to the fork:
Generally the larger the rake angle the more stable the front end. That is the less likely it wants to turn. Race bikes have shallow rakes around 24 degrees. Cruisers might be around 27 degrees a difference of 3 degrees. Not much, so you can see each degree makes an impact.
Next is trail. Trail is the distance between the imaginary line extending the steering head to where it would touch the ground and the imaginary line extending from the axel straight down to the ground:
Which brings us right away to Offset:
The reason these are connected is because the more forward the steering stem is in relation to the forks (or the smaller the offset) you can see that the trail will be increased or moved the other way (larger offset) even eliminated or made negative. Trail relates to stability of the wheel. If your bike has decent trail you can let your hands off the bars and the bike will travel straight. Two good examples of extreme trail are bicycles and shopping carts. Bicycles have curved forks that push the front wheel out to decrease the trail; most bicycles have very shallow rakes and zero offset. Look at bicycles with front suspensions and you see that the axel is forward of the fork because the fork needs to be straight. While ok at low speeds sneeze while humming along on a bike and yer in the weeds. In shopping carts the rake is zero and the axel is well away from the “steering” head in the wrong direction. So when you push a shopping cart the wheel (even if facing the wrong way) will swing around to the axis of motion. This negative trail and zero rake is what gives them their “zero turn radius” and makes them the poster children of instability. An interesting anomaly is a bike like the Bimota Tesi. The Tesi can actually be set up with zero rake and long trail. This is because the rake, trail and offset is contained in a 4 inch area in the axis of the wheel. A very cool thing about hub centered steering.
Now you can change some of the geometry after the motorcycle has left the factory. This can make things better or tragically worse. In this crude drawing you can see the stock rake in red and the “tilted” rake in blue. The angle of the steering stem to frame has not changed but you can see that the angle to the imaginary lines drawn to the ground have.
Using AutoCAD I defined some basic geometry:
59 inch wheel base.
25 degree rake.
1 inch offset
which gives us 2.89 inches of trail.
Raising the rear 1 inch
(such as adding a different rear shock or changing dog bones)
Results in:
24 degrees of rake
2.72 inches of trail.
However!! *cue dramatic music*
Dropping the forks (actually dropping the frame) so the fork tops now extend 1 inch above the tree should be the same as raising the rear right!?? wrong
This changes our geometry to:
23 degrees rake.
2.11 inches of trail.
Big changes!
Upshot, make these changes carefully. Make changes intelligently. But do make changes. Raising the rear is you best bang for the buck. It decreases your "rake" but changes the trail to a lesser degree.
I would like to illustrate this geometry I need to label the AutoCAD drawing and convert it to a JPG or something..
Other installments in the "more than you wanted to know" series:
A lot of information about Springs
A lot of information about Vacuum
A lot of information about Hydraulics
Lets stick with the basics; Rake, Trail and Offset.
Below is a simple line drawing of a frame, forks and wheel.
First let’s define rake. Rake is the angle from a straight line intersecting the fork over to the fork:
Generally the larger the rake angle the more stable the front end. That is the less likely it wants to turn. Race bikes have shallow rakes around 24 degrees. Cruisers might be around 27 degrees a difference of 3 degrees. Not much, so you can see each degree makes an impact.
Next is trail. Trail is the distance between the imaginary line extending the steering head to where it would touch the ground and the imaginary line extending from the axel straight down to the ground:
Which brings us right away to Offset:
The reason these are connected is because the more forward the steering stem is in relation to the forks (or the smaller the offset) you can see that the trail will be increased or moved the other way (larger offset) even eliminated or made negative. Trail relates to stability of the wheel. If your bike has decent trail you can let your hands off the bars and the bike will travel straight. Two good examples of extreme trail are bicycles and shopping carts. Bicycles have curved forks that push the front wheel out to decrease the trail; most bicycles have very shallow rakes and zero offset. Look at bicycles with front suspensions and you see that the axel is forward of the fork because the fork needs to be straight. While ok at low speeds sneeze while humming along on a bike and yer in the weeds. In shopping carts the rake is zero and the axel is well away from the “steering” head in the wrong direction. So when you push a shopping cart the wheel (even if facing the wrong way) will swing around to the axis of motion. This negative trail and zero rake is what gives them their “zero turn radius” and makes them the poster children of instability. An interesting anomaly is a bike like the Bimota Tesi. The Tesi can actually be set up with zero rake and long trail. This is because the rake, trail and offset is contained in a 4 inch area in the axis of the wheel. A very cool thing about hub centered steering.
Now you can change some of the geometry after the motorcycle has left the factory. This can make things better or tragically worse. In this crude drawing you can see the stock rake in red and the “tilted” rake in blue. The angle of the steering stem to frame has not changed but you can see that the angle to the imaginary lines drawn to the ground have.
Using AutoCAD I defined some basic geometry:
59 inch wheel base.
25 degree rake.
1 inch offset
which gives us 2.89 inches of trail.
Raising the rear 1 inch
(such as adding a different rear shock or changing dog bones)
Results in:
24 degrees of rake
2.72 inches of trail.
However!! *cue dramatic music*
Dropping the forks (actually dropping the frame) so the fork tops now extend 1 inch above the tree should be the same as raising the rear right!?? wrong
This changes our geometry to:
23 degrees rake.
2.11 inches of trail.
Big changes!
Upshot, make these changes carefully. Make changes intelligently. But do make changes. Raising the rear is you best bang for the buck. It decreases your "rake" but changes the trail to a lesser degree.
I would like to illustrate this geometry I need to label the AutoCAD drawing and convert it to a JPG or something..
Other installments in the "more than you wanted to know" series:
A lot of information about Springs
A lot of information about Vacuum
A lot of information about Hydraulics
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