Practical Ways to Select the Proper Motorcycle Fork Length

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June 14, 2010 | By: Scott Holton

Today’s post will help you guys who are taking on a new bike build. As a tech here at J&P Cycles, I’m often asked how long a fork I should purchase? There’s really no way for me to answer that question because it’s something you must decide for yourself. That being said, here’s an explanation of the easiest way I have found to help people arrive at the best answer. 

  • First, we need to mock up the rear wheel we’re going to use with the frame we have.
  • Next — and most important — we’re going to block the front of our chassis up to give us the ride height and “look” that we want. Once this is established, everything else is downhill.
  • We also need to know the radius of the wheel/tire combination we are going to run. For a 16-inch or a 21–inch, we’ll use a radius of 13 inches. For a 19–inch, we’ll use a radius of 12 inches
  • Take a broomstick or mop handle and drop it through the steering neck. Of course, now you’ll need to know use what wheel radius we are running up front in order to get our first dimension point.
  • Measure the radius of the wheel we’re going to use from the broomstick (straight to the floor), and then mark the broomstick at that point. (this mark represents our axle center).
  • Next, measure from the point of the mark to the top of the frame. This gives us the desired fork length.

A stock FXST has a fork length that is very close to 32 inches. If you need a 34-inch length fork, you would need to order a +2 fork. If you need 38-inch fork, then +6 is what you’ll need. If you need a fork shorter than 32 inches, we would select -2 or -4 (depending on what you measure).

OK, we’ve talked about tube type forks, but Springer’s measure slightly differently. According to Paughco, the builder of the highest-quality, largest-volume custom Springer’s available today, the measurement is also from the top tree (top of the steering neck), but the bottom measurement goes to the center of the rear leg pivot. For a Springer, the stock length is considered to be 27 inches. A stock HD late-model Springer is +3 inches (30 inches) for comparison purposes.

As always, our dedicated tech staff is available to answer any of your questions. Reach them via e-mail, phone or chat (to get started, visit the J&P Cycles Customer Service page today).

Comments: 11 Comments | Categorized Under: Tech Tips

Comments (11)

btw, if it helps, my tire it runs on the front is a 2.25/17

Hello all. I have a strange project and need some help. before i get into the guts of the question i just want to ask that no one give me crap due to the bike choice, im poor and work with what i can afford.
I have a Pagsta mini, looks like a Honda Rebel knockoff. It has the tube style forks and i am wanting to switch those out for springers. I also am pondering angled tripple trees. Ultimately my goal is to lengthen the bike and raise height so ultimately i will also be welding a ridgid end onto it instead of the swaybar rearend and putting on dif size wheels. I measured the forks in two places. 21 inches or so from the cross member at the neck base (not sure of correct term) , 27 or 28 from the triple tree. Soooo to find a classic chopper style springer that would be applicable what should i do?? Please forgive my ignorance but i do think once completed this should be quite a conversation worthy bike.
Thanks for any and all help

I am wanting to change from a glide front end to a springer front end. I am wanting to check the trail to make sure I dont need to do any other changes to the frame rake to get proper tail. i need to know how to measure trail on a springer front end. any help with this would be helpfull. All of the rake trail calculaters i have found tell how to measure trail with a glide, but say nothing on springers.
Thank You for any help,
Lil Ricky

Back in the day, before we ever heard of rake & trail, I’d remove the front end, make a chalk line on the garage floor, cut the gusset side of the neck with a torch, stick an iron pipe through the neck, heat the neck, and bend it up with the pipe until I thought it looked right.

I guess math is cool, but “looks right” is “right” always works better . . .

[...] like to understand how to select the proper length fork tubes, feel free to read our blog post on Practical Ways to Select the Proper Motorcycle Fork Length from senior J&P technician Scott [...]

I’d like to see something on how to plan fork length and trees when changing the rake angle to maintain the same ride height, or is this just something that needs to be figured out using the process described in the article?

Matt, there are a couple Rake and Trail Calculators available on the internet. The one from RB Racing, http://www.rbracing-rsr.com/rakeandtrail.html allows all the possible variables entered to give you what you are looking for.

Id like to see something about changing rake angle with trees and how to figure out fork length to maintain the same ride height, or is this something that just needs to be figured out using the process described in the article?

I’d like to see something about rake and fork length..how to figure what fork length to porder when changing the rake of the trees to maintain the same ride height. Or is this just something that need to be figured out by using the process mentioned in the article??

Hi, I’m very interested in Linux but Im a Super Newbie and I’m having trouble deciding on the right distribution for me (Havent you heard this a million times?) anyway here is my problem, I need a distribution that can switch between reading and writing in English and Japanese (Japanese Language Support) with out restarting the operating system.

Good article but I think it would be more helpful if you would use actual pictures of the bulleted process. Visualization helps out a lot for things like this.

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