Author Topic: USD forks and triple tree swap  (Read 1100 times)

Offline adowd90

  • Posts: 1
USD forks and triple tree swap
« on: Jan 10, 2017, 01:01:51 »
Hey guys,

New to the forum and have a querie or 2.
I have just purchased a 1984 Honda XL350 and am aiming to take it into a street/brat tracker type direction and want to know if anyone has installed USD forks with triple clamps to suit off a modern bike - either dirt or street?

I want to lower the front and rear from stock height without having the forks protruding through the top clamp so will need a shorter set (unsure of stock length).

I have done the search on allballs and there is a few options i can take, though it doesnt help me with the length of steering stem - are there any straight swaps that you know of?

How do you measure the length of a steering stem? As obviously there is tapers and threads that have to sit in certain spots relative to the races etc.

Is it just as simple as from the bottom of stem to the top? Or is it the length of steering head? Etc etc

I've had a good search over the last few days and can't seem to find any answers that define exactly what i need.

Would like to avoid having a steering stem machined up if i can as i would like to be hands on with this build as much as i can.

Also, how is the steering stem seated in the lower clamp? Is it pressed in from the top/bottom? Threaded?

I hope i haven't asked any stupid questions.

Thanks in advance!


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Offline Jewbacca

  • Posts: 1153
  • Therapy is expensive. Riding is cheap. You decide.
Re: USD forks and triple tree swap
« Reply #1 on: Feb 11, 2017, 01:39:13 »
There are thousands of different lengths and bearing sizes, so unless there is a similar bike that uses the same stem as yours, you're going to have to modify.

Steering stem length is measured from the top of the lower triple tree (where the stem goes into the actual clamp) to the top of the threads. Now, that measurement is important, obviously you need the stem to be long enough to go all the way through your steering tube.

But the most important measurement is the distance between the bearings. This will determine if the stem can even be used in your bike.  Model "families" (I.e. CB, KZ, GS, etc) often times have very similarly spaced neck bearings. You need to figure out the distance between the upper and lower bearings, then you can search for front ends that have a close enough match (within 2-3 mm as usually the bearing area is longer than the actual bearing, allowing for some compensation) and this will give you a selection of "straight swap" front ends.

Here's where it gets tricky. You can swap a front end that doesn't match up with your frame neck. It requires having the actual stem replaced by one machined to fit your frame. I did it on my TX500 project to fit a CBR600 front end. Seeing as how I'm a machinist, it was easy. If you don't have the skill, or access to a lathe, you can probably get a new stem made for 50-100$. Stems are usually pressed into the lower triple tree and then welded on the bottom of the tube to the bottom of the triple tree. To get them out you have to machine the weld off (again, lathes help) and then press the tube out of the triple tree. *Pro tip, often times the hole in the bottom triple tree clamp isn't dead nuts square with the fork clamps. If you press in a finished stem, it might not be straight with your forks and then you just wasted a bunch of time and money. Have a stem "roughed out", keep all measurements about .030-.050" oversized, then have the lower hole reamed just slightly, so the stem is still snug but can be adjusted. Press the stem into the lower triple, slide the forks in and slide the top triple down onto the forks and stem as well. This will keep your stem, triple trees and forks all perfectly aligned when you weld the stem into the lower triple. Then you can take it all apart and finish machining the stem with it welded into the lower triple. Voila, you have a modern front end that will adapt to your old frame.
1979 Honda CB750K- Soon to be CB750K-SC Night-Ninja (SOLD)
1974 TX500A "Yamahonda""
1983 Honda CB650SC
Gives me a major cramp in my wedding tackle..." Fellow PNW Rocker Dave Simmons