Rake and Trail

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Rake and Trail

Post by 8541Hawk » Fri May 31, 2013 5:57 pm

The last transferred thread for the day...... this one was written up after a couple new guys had posted up statement like " I pulled my fork up 1.5" (or 38mm to you guys) and the bike works great" which gave me a :eek2 moment....

Once again here you go:

As I've seen a few posts lately about folks making some radical changes to their bikes geometry I though it might be a good idea to post up the basics of how a bikes front end actually works and what you are changing when you raise or lower the forks in the triples or add (or take away) ride height in the rear.

While I will be using bike related examples, these same principals apply to anything with steerable wheels, from cars to jet aircraft or even a shopping cart.

The main thing to look at is "Trail" which is the distance by which the axle "trails" the point at which the steering axis (not the fork axis) of a motorcycle contacts the ground.

Trail is required to ensure a steerable wheel is stable in following the steered direction. You can visualize trail as a lever arm that tends to center the steered wheel behind the steering direction. More trail means a longer lever arm and greater centering force (or stability).

Motorcycle steering geometry is determined by four measurements:

Rake - The angle of the steering axis (where the triple clamp mounts to the frame), measured from vertical.

Tire Radius - Half of the tires outside diameter.

Offset - The distance from the steering axis to the fork axis

Trail - The distance from where the steering axis contacts the ground to where a vertical line through the axle contacts the ground.

When you change rake, steering offset or tire radius, you change trail and the relative stability of the bike. More rake angle gives more trail. Less rake angle gives less trail. More offset gives less trail, Less offset gives more trail. More tire radius (a bigger tire) gives more trail. Less tire radius gives less trail.

All pretty simple measurements and their relationship is the basis of a motorcycles steering behavior.

So why mess with the factory settings?

Until recently streetbike manufacturers were pretty conservative in choosing steering geometry, consistently compromising in the direction of stability. Which makes perfect sense, steering geometry may be pretty simple but stability is not. It includes many things including strength and stiffness of the structural parts, tire characteristics, weight placement and many other things. So it is best to stay away from steering geometry that might cause instability in a complex system.

With this information setting up a bike for the track or even getting a streetbike to turn easier involved reducing trail or trying to make the steering geometry less conservative.

Normally this is accomplished by sliding the fork tubes up in the triple clamps (this steepened or decreased the rake angle and shortened trail) Increasing the rear ride height steepened the rake even farther and reduced trail even more (or lowering the rear will increase trail)

So feel free to experiment with settings, just remember the less trail you have, the more unstable the bike becomes. Make small changes, as your life does depend on it......

Also manufactures no longer are as conservative with there steering geometry. Many late model bikes are close to the limit of stability as delivered, so much more care is required in making changes to these bikes (The SH is not one of these, but be careful making changes on late model sports bikes).
To err is human, to forgive is divine. Neither, however, is Marine Corps policy

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Re: Rake and Trail

Post by lloydie » Tue May 27, 2014 9:09 am

Interesting reading .
Thanks hawk

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Re: Rake and Trail

Post by simo » Tue May 27, 2014 9:27 am

Aye. Makes a lot of sence Hawk. :thumbup:
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Re: Rake and Trail

Post by fabiostar » Wed Nov 19, 2014 11:09 am

good read sir. :thumbup:
the older i get,the faster i was :lol:

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Re: Rake and Trail

Post by linden46 » Sun May 24, 2015 1:16 pm

Good article thanks

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Re: Rake and Trail

Post by darkember » Sun May 24, 2015 2:23 pm

You are a wealth of knowledge Hawk :thumbup:

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