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Triple tree fabrication

Posted By: Notthepainter

Triple tree fabrication - 05/23/19 11:20 pm


I talked to a guy at work who's retiring end of June. He's in charge of a water jet machine and he doesn't mind making a few parts for me. I've been thinking of adapting some cbr600 f3 forks on my 1978 t140. So I think I'll ask him to make some yokes and a disc adapters (I'll have a stem turned somewhere else). I have no experience making such things. That's a good opportunity to learn.

- is Al 6061 appropriate for the yokes? For the disc adapter?

- Anybody would know which tolerance I should use at the lower yoke / stem interface?

- Anybody think it's a bad idea?

Tank you!
Posted By: DMadigan

Re: Triple tree fabrication - 05/24/19 6:49 pm

Water jet is not very high tolerance, especially when cutting thicker pieces. The jet tends to wander at the bottom of the cut and unless cut slowly the bottom cut trails the top. As a way of roughing them out it works but you have to machine the tube and bolt holes anyway so you do not save any time water jetting first.
You will not be able to hollow the underside by water jet so more machine work.
Why not use the CBR yokes and make a stem to fit your frame?
The disc adapters will have to be machined.
6061-T6 is a very common alloy. "Appropriate" depends upon your design and the loads.
Machinery Handbook gives interference for press fits.
Posted By: Notthepainter

Re: Triple tree fabrication - 05/26/19 12:07 am

Hi DMadigan,

Thanks for your answer!

Yes other people have told me the same since I posted. Water jet won't be accurate enough, especially for the interference fit for the steering stem.

It's ok though if I cannot hollow the underside.

There are drawbacks to reusing the cbr600 yokes. They're ugly, the steering stops won't work as well, the Honda steering stem is much larger than the triumph one which means making an adapter... And it says "Honda" on it! But maybe I'll change my mind once I see an estimate from the machine shop.

I'll do some research and post some updates.
Posted By: kommando

Re: Triple tree fabrication - 05/28/19 11:59 am

There must be a US version of this guy

As well as the yokes listed he will do custom yokes ie CBR600 with a hole for a Triumph stem not a Honda stem.
Posted By: t ingermanson

Re: Triple tree fabrication - 05/28/19 4:02 pm

this looks like maybe the US version of the top yokes guy:

more expensive than your buddy at work, but results will be much better and safer too.

watch your ride height, rake, offset, and trail numbers. i don't want to sound like a know-it-all or worry-wart, but those numbers are vital to keeping the bike working well and keeping you safe. no joke. there are a lot of dangerous bikes out there with these conversions. the cbr fork will probably be much shorter than your triumph fork from '78. it will take some doing to keep the bike handling right.
Posted By: kommando

Re: Triple tree fabrication - 05/28/19 4:38 pm

Or just look up some yokes off a database.

and check what geometry it comes up with, I upgraded a Norton front end and ended up with Gen1 R6 yokes holding F4i forks with 20mm increased trail. I bought some special bearings that took the R6 stem and a perfect fit.
Posted By: old mule

Re: Triple tree fabrication - 06/03/19 2:11 am

I used Honda VF500F forks on my 78 Bonneville, I wanted somewhat less trail for quicker steering, Honda yokes wrked. And I wanted a slightly steeper fork rake, 18 inch wheel did that. Bonus was speedo drive from RD250 front wheel, reliable Japanese speedo. I used single disc.
That for and Ohlins shockers transformed the bike, embarassed a couple first year Ducati Monster owners soon as I got it on the street.

Of course I have reduced the Triumph's value greatly, but I don't care. No restorer would want a post 1970 Triumph anyway, we used to say.
Posted By: Notthepainter

Re: Triple tree fabrication - 07/14/19 1:10 am

So as expected my project is moving very slowly. I have other priorities (kids, work, house).

I understand the waterjet is not going to cut it (haha). The good news is that I found another colleague who has a cnc milling. So the new plan is to machine triple trees, top in Al 6061, bottom in steel 4041.

I'm wondering what to do with respect to the top yoke. I'm hesitating between two solutions:

1) Except from the t140, all bikes I ever owned (german, japanese) were such that above the top steering bearing, one (or two) fine thread lock nut sets the steering freeplay, the top yoke is above and in contact with that lock nut, and finally a last nut secures everything, by pushing the yoke against the bearing lock nut.

2) As you probably know, the t140 is different, a screw in the top yoke simply makes the top yoke compress some specially shaped bearing lock nut, locking the lock nut onto the steering stem.

Solution #2 would probably be cheaper, since I may be able to press out a steering stem from an existing t140 triple tree. But intuitively, solution #1 makes more sense, mechanically speaking.

Does anybody have an opinion?
Posted By: Allan Gill

Re: Triple tree fabrication - 07/14/19 12:56 pm

The pre 69 bsa twins had a castlated nut which pushed down on the top bearing cup, in effect it was doing the same job as on the Japanese bikes. A top yoke but then threaded onto that and the top yoke clamped against that nut.
Posted By: DMadigan

Re: Triple tree fabrication - 07/14/19 2:56 pm

I presume you meant 4140 steel? Why steel? Are you trying to put more weight on the front wheel? Most all lower triple clamps are aluminum.
The T140 uses a sleeve nut that slides through the top triple clamp and pushes on the dust cover over the bearing to set the clearance. A pinch bolt keeps it from turning. This way the spacing between triple clamps is controlled by the headlamp mounts, not the bearing spacing.
Another common way to set the bearing clearance is to use two thin (1/4") nuts with slots around the periphery and pin spanners to lock them together. The triple clamp is locked to the post above the threads either by a bolt through the top or pinch bolt.
Posted By: Notthepainter

Re: Triple tree fabrication - 07/15/19 1:35 am

Hi, DMadigan,

Yes, you're explaining much better than I do the two concepts I am contemplating. Thank you! So the question is, does the t140 system have any inconvenient? It seems less stiff.

Regarding the material, yes, I mean 4140 steel. Before I decided on fabricating the yokes, I looked at existing last 90's 41 mm ones: sv650, cbr600 F3, kawasaki zx6r. All lowers were steel, all uppers were aluminum. I know some people will say al 6061 is fine, that's also what cognito moto uses, they may be right, but 6061 is never described as fatigue resistant. Al 7075 may be, but it's expensive! I'm designing that thing blind, I have no idea of the loads a yoke sees, all I can do is use my caliper and get some inspiration from what's already there. I prefer a triple tree that is heavy and may bend, than something that will fail into pieces. So steel it will be.

And thanks for your answer!
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