[fuelrod]

Hey guys:
I'm lowering the front end of my Trumpet build, and need some help.
The bike is a bit of a frankenbuild, so your expertise is needed.

Frame is a '58. Front end is from a 71-72 OIF. I measure the rake for that frame and the 71-72 front end at ~24 degrees.

With that setup, and a 21" rim, my tire is way too close to the frame, and I've got about 2" of suspension travel before dining at the shit buffet. Not ok. My solution is to use preunit trees to gain another 3 degrees (3 degrees, right?) of rake, and make up some new fork tubes (I need them already) to graft the 71-72 forks (34.5 mm) to the PU trees (33 mm).

With all the measuring + math I'm doing, I'm still unsure how much the fork tubes should be engaged into the fork lowers. I want to shorten them up so I actually have suspension travel, but I'm not sure how much I can shorten them.

 

[hillbillyhell]

http://www.bikeexif.com/triumph-thunderbird-motorcycle

There's a '57 I just finished with early OIF forks. No shortened tubes, no preunit trees, you're makign my head hurt getting that complicated with it haha. It's about an inch shortened, but no need to redo tubes for that little amount.

 

[fuelrod]

Clay,
First of all, that bike is stunning. So, so, so tidy. I hope mine comes out half as nice.

Second, I'm thrilled that someone else has done this, and I'm not the only asshole putting "late model" forks on a PU.

Finally, yes- this is a mind fuck. I swear I'm not trying to make it this complicated. I've drawn so many triangles at this point, pythagorus must be giving me the finger from his grave.

I have literally a half inch of clearance between the back of the front tire and the frame. I have 2 inches of suspension travel. The only differences I can think of between your build and mine (aside from the fact that yours is done, and mine is a pile of parts stained with blood, beer and anger) is ride height. My frame rails are sitting about 5" from the ground. Is that a lot lower than your 57?

 

[hillbillyhell]

Heh, thanks. This one was stained with plenty of anger, emotional deal.

I didn't measure the finished ride height, but I'll tell ya what the deal was. I knew since preunits are so little and have so little frame rake that what you're running into was going to be a problem. So I set it up nose high. (I also wanted a little of that stock rake long fork "chopper" vibe, it was important to this bike)

If you were to set mine up on the table with the bottom rails level and the rear wheel touching, it sits 3" high. Sitting level on the table it absolutley looked like it was not going to work, same as you've got. You can't build the early no rake stuff level and still have a front suspension. Just doesn't work. When you add the front end it picks everything up and it all works somehow. If you look at the frame and engine in the pictures you can see what I'm talking about. It doesn't maybe look as nose high as it is because I built the exhaust to be level at ride height, not level with the frame.

If you're sitting at 5" level frame with the rear wheel on the ground, you might be too high for your taste by the time you get enough front end under it. Not sure what you'd do there except change the rear frame. Wow, I hope some of this makes sense, having trouble explaining.

 

[fuelrod]

Clay that makes perfect sense. I've been battling for a level frame, but allowing the front to sit higher will definitely get me more room for the front end. It's going to be a fine balancing act I think- It's looking like very minor adjustments to ride height on the front will make a big difference to suspension travel and clearance between tire + frame.

I do have an old set of PU trees... I could always bore them out to 34.5mm too I 'spose.... Seems a little sacreligious, but it may get me there more easily than a set of custom fork tubes.