Author Topic: How can I repair my Plus 80 forks?  (Read 5567 times)

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Offline KiwiJohn

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How can I repair my Plus 80 forks?
« on: 20 Sep 2006 at 02:49 »


This is a long story.  My Plus 80 forks are damaged at the top where one conical piece is quite distorted.  A previous owner has made an asymetrical top plate to compensate and at first I thought that was the extent of the damage.

However, putting the bike on the road has shown very little if any inherent straight line stability and almost no trail.

The two main fork tubes do appear to be parallel but the real issue may be that the central steering pivot tube is not parallel to the main fork tubes.  This may be because this central tube is bent or perhaps it has been reseated in the tripple bracket or maybe the triple bracket moved on the fork Tubes.  This misalignment may be totally accident damage or it might be another consequence of the 1965 owner trying to 'chopper' the bike!

Having read the posts regarding brazing the frame repairs I am very wary about trying to reset the bracket on the fork tubes.  The last thing I want is for one of those tubes to fracture miles from home and maybe in traffic.

I feel an unavoidable step is to remove the central tube and have it trued up.  If it was then reseated in the bracket parallel to the main tubes I would have the trail back.

Does anyone have any ideas on how to remove that tube?

Thanks.

Offline Doug

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Re: How can I repair my Plus 80 forks?
« Reply #1 on: 20 Sep 2006 at 03:29 »
Considering the other damaged components of the front fork, would it not be simpler to get a set of Mark 5 forks and convert them to Plus specification? I think this would entail the following:

Moving the brake reaction arm lugs one inch further up the fork tubes. Removing the brazed on headlamp mounting ears and replacing with the Plus detachable headlamp mounting ears (the pressings are the same, except the Plus are drilled for bolts.) Placing the two-way dampening shuttle and springs from your Plus in the Mark forks.

Anyone else know of any other differences between the Plus and Marks front forks (besides paint)?

This should be easier than trying to remove and reposition the steering stem, and too you would still have the damage to the upper portion of your Plus forks to repair (or live with.)

As for the headlamp ears, these are fairly easy to remove. I removed the ones on my Mark 3, as it was easier to straighten them out off the forks and braze them back on when done. Or, you could just leave the Mark brackets in place. Or you could just use the Mark forks in their entirety. The brake reaction links are at a slightly wrong angle, but beside aesthetics it works fine (it has been done before.)

-Doug
« Last Edit: 20 Sep 2006 at 11:52 by Doug »

Offline KiwiJohn

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Re: How can I repair my Plus 80 forks?
« Reply #2 on: 20 Sep 2006 at 07:42 »
Yes Doug, another set of forks may be an answer or when they arrive from the other side of the world I find they are no straighter than those I already have.    Still I may eventually resort to doing that.

Offline eddie

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Re: How can I repair my Plus 80 forks?
« Reply #3 on: 05 Nov 2006 at 12:30 »
John,
    I would suggest that you should first check whether each leg of your forks is straight (at this stage dont worry if the two legs are twisted).  Then check whether the steering head stem is straight (an easy way to do this is:- obtain a piece of steel tube with a bore the same as the internal diameter of the botom steering head bearing, get the end machined true - this can now be slid over the stem and with it pressed against either the bottom bearing or the bearing seat, the top should be concentric with the stem). If these checks prove the legs and stem to be straight, then the damage is in the bottom yoke. The next task is to get the tops of the legs and the stem in the correct alignment using a new/good top bridge plate as a gauge. Small amounts of adjustment may be possible with the forks cold, but any major tweaking will require the yoke to be heated between the legs and the stem to prevent fracturing - only heat to a just perseptible dull red and dont allow any brazed joints to become overheated. When you have finished, the bridge plate should fit without having to spring the legs and the two legs should have no twist in them (check by laying the forks on a flat surface).
       With this done, your machine should handle buch better.
                                Good luck with the repairs,
                                                      Eddie.

Offline KiwiJohn

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Re: How can I repair my Plus 80 forks?
« Reply #4 on: 05 Nov 2006 at 17:11 »
Thanks Eddie, I never expected to be able to heat the yoke enough to bend it without overheating the brazed joints.  I have a new top bridge plate which clearly shows something is awry!

 

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