Fork Length, Rake and Trail

Harley Davidson Front End
Harley Davidson Front End

Motorcycle Fork Geometry and Measurement

One of the most crucial components when building a custom chopper, or customizing a motorcycle, is the front forks. Whether you choose tube forks, springer forks or girder forks the right front end will significantly affect ride comfort and handling.

Certain geometry and measurements should be understood before purchasing the front forks for your motorcycle or chopper. The first to be discussed will be fork length, followed by rake and then trail.

Fork Length

Measuring Fork Length Diagram
Measuring Fork Length Diagram

Measuring Fork Length

Fork length is measured from the base of the frame neck to the center point of the axle mounting hole. For any off-the-shelf motorcycle this standard measurement for the front forks is considered stock length. When buying forks other than original replacement parts, their lengths are compared to the stock length. Forks that measure longer than stock are listed as Over-Length (OL). Those forks that measure shorter are listed as Under-Length (UL). If the forks are the same length as original the fork length is listed as stock. It is important to note that when the rake angle is changed, fork length may need to change in order to compensate for the change in rake angle.

Frame Rake

Frame Rake Measurement
Frame Rake Measurement

What is Frame Rake (Rake Angle)?

Frame rake is the angle formed by the center line of the neck tube and a vertical line. As a rule of thumb, the greater (higher) the rake angle, the longer the front forks must be. The rake angle can have a profound effect on handling characteristics and the aesthetics of a chopper.

Chopper With A Smaller Rake Angle
Chopper With A Smaller Rake Angle
Chopper With A Larger Rake Angle
Chopper With A Larger Rake Angle
Affect Of Angled Triple Trees
Affect Of Angled Triple Trees

Changing Rake Angle

Rake angles can be changed in two ways. Changing the neck angle on the frame, or adding angled triple trees.

Changing the neck angle involves cutting the neck from the frame tubes and lengthening or shortening the frame tubes to get the desired angle, then welding the neck tube back into place. This is not a job for a novice. Any change in frame geometry can result in serious handling changes. Consult a local fabricator or motorcycle shop. Custom frames can be purchased with a variety of rakes.

The other option is to mount the front forks with angled triple trees. Angled triple trees use mounting holes that have been angled in order to add 2-5 degrees of rake to the front forks without changing frame geometry. These are very useful for adjusting trail when minor changes to fork length, and wheel sizes have been made.

A quick math example, along with the illustration to the right (Affect Of Angled Triple Trees), should demonstrate the overall affect of using triple trees.

  • If the Rake Angle is 35 degrees and the triple trees are angled at 3 degrees, the Overall Rake Angle is 38 degrees.

Trail Measurement Diagram

Measuring Trail
Measuring Trail

Measuring Trail

One of the handling characteristics of longer front forks and increased rake angle occurs at lower speeds. The effect is called "flop," and it refers to the front wheel wanting to fall to one side or the other when turning. It makes the handle bars hard to control, and if flop is great enough, it can actually make the chopper dangerous to maneuver at low speeds. One of the ways to reduce flop is to adjust the trail.

Trail is a horizontal measurement, that describes the distance between a line drawn vertically from the center of the front axle to the ground and where the neck center line intersects the ground. It sounds complicated to explain, but is more easily illustrated to the right. Though no exact measurement could be reached from my interviews and research, it seems the general consensus is that 3 to 5 inches is the desired range for proper trail. Any trail measurement outside that range will result in undesirable handling characteristics.

Trail Adjustment

Trail can be adjusted in a few ways. Before trying any of these options check with a local mechanic or motorcycle shop.

  • Changing¬†the front wheel diameter or tire profile is one way. This will only result in minor changes in the trail however.
  • Changing the fork length is another option.
  • Adding angled triple trees will change the rake angle and will significantly impact the trail measurement.
  • Change the geometry of the frame. This should be done as a last resort, by somebody experienced in frame modification.

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10 comments

Ghost32 7 years ago

I really like the clarity of your diagrams and explanatory text. Not that I've come close to building my own chopper; the ride I had for the most years was a (gasp!) rice burner, a '95 Suzuki 1400 Intruder. And the only customizing I did on that besides a few add-ons and a custom sissy bar was swapping the 27 inch handlebars out for a 36 inch set--got tired of dropping the thing while I was parking it half asleep.

Our last H-D was a little Sporty we sold a couple of years back. Pam (wife) got so busted up she couldn't straddle the back seat any more, and that kind of took the fun out of it for me. Am I whipped or what?

Which reminds me, I'm overdue to stop in and say hey to my bud at Cochise County Choppers in Bisbee...thanks for reminding me.


puppascott profile image

puppascott 7 years ago from Michigan (As far as you know...) Author

When I was writing my book, trail was one of the hardest "measurements" to get an explanation for. None of the experts I talked to would commit to exact numbers or ranges. It was almost as pointless as trying to nail greased Jello to granite.

There is an old saying that goes something like this.

"Any HD rider that tells you he's never owned a foreign bike is a liar." I wouldn't worry about the Suzuki. My first bike was Honda CB900 Custom.

I do appreciate your compliment and comments.

Scott


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ThunderOnWheels profile image

ThunderOnWheels 6 years ago

Excellent hub. I'm a trained motorcycle technician and the books we used in school described the rake and trail just like that.


Gordi 6 years ago

Does anyone know of a site that you can enter your measurements and it will give you the actual fork length you need? Mine are:

Frame Neck 40 Degrees

Neck Height 39 inches

Front Axel Height 13.5

I see lots of sites that will show you the tube length but I need to know if I need 6", 8", 10" over forks.

Thanks


Gordi 6 years ago

...and 0 degree trees


couponalbum profile image

couponalbum 6 years ago from Sunnyvale, CA

I really had a passionate love for Harley Davidson bikes. Very informative article liked the clarity of diagrams. Joined your fanclub and would like to invite you to join mine.


Bikearama 5 years ago

I wrote up a nice explanation of rake and trail including some very simple ways to alter motorcycle rake and trail. Which I think adds to the above information and highlights the relationship between rake, trail and offset which the above doesn't.


Bikearama 5 years ago

I wrote up a nice explanation of rake and trail including some very simple ways to alter motorcycle rake and trail. Which I think adds to the above information and highlights the relationship between rake, trail and offset which the above doesn't. It's at: http://bikearama.com/theory/motorcycle-rake-trail-...


mikeky47 5 years ago

my frame has + 4 in in the down tubes & 34 degrees rake, 21 in front wheel 18 in rear.how much over should my front end be?

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    Reference: Chopper Fundamentals 101 © 2007

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