FT: Frame (Twin) Rear Frame Member alignment and repairs to achieve alignment

oexing

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I had to do same job on two rear frames a few weeks ago, Argentinian history as you can guess. My approach was to disengage the upper tubes from the doweled lower bigger tubes . You cannot possibly rectify the lot with all joints bolted up - and no good way to use brute force under a press. This would lead to curved funny shapes as you´d get bends where there were no before. Mostly snake like shapes then. So I got me the big welding torch after undoing the scews and levering the upper tubes out of dowel fits. So you put some big tube or monkey wrench on the rear end and heat at the suitable place to red and bend to your liking. You´d want to get axle of taper roller bearings parallel, resp. horizontal when viewed from behind, plus the wheel axle parallel as well. I made me two lengths of plastic disks to fit in the Timken bearing seats with central bore for pushing some straight stainless rod through to be placed on v-blocks or parallels on a table. So then you check the wheel axle slots for same heights left and right. Otherwise you´d have a rear wheel not vertical !! The shock absorber bores are not critical here, they may be allright after having all else in positions.
Once you got the lower big tubes correct with swing arm bearing bores horizontal and parallel on your table - plus rear forked ends parallel/same height too, you then look at the curved tubes on front and the upper tubes that have to engage the dowels again. For arriving there you have to think about where to go with the torch, either bending the curved tubes back when too short to reach the dowel. Or maybe just bending down or up when the loose ends are too high or low. Depends on what defect you got.
Anyway, with the torch you can feel the redhot tube to go where you want it, some lever force is certainly required but you get good control about what place has to give for the job to be done. A few hours will go for sure, but this depends on your standards of perfection then.
In case the forgings at the rear are f***d up a lot you could consider to have some welding done for perfect seating of nuts and spacers of the wheel axle components. Big washers are desirable, not original but an improvement, so you don´t squash faces again later.

Vic
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Bob

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Non-VOC Member
Craig, the only real tool you need per-say, is a descent press.........this is so you can "Hold firm" the forward end of the frame and then use brute strength to twist the other using lengths of bar, tubes whatever it takes........I have straightened many bent Girdraulic fork blades, and sometimes it takes 3 people to do this, especially if they are twisted.........Because the rear frame is triangulated, it is very strong, the tubes are the only weakness, so they will shift if you can exert enough force, whilst keeping the other end firm. Good luck with it all...........Cheers......... Greg.
Craig, I hope you can find some 'Twisted' people to sort this out, mind you that should be no problem after the current lockdowns!
 

Bill Thomas

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VOC Member
I spoke to Bob Culver some years ago, And asked Him if He had a Jig,
He said He didn't but would have a go and He would bend it cold,
So I thought I may as well have a go myself,
You have to be careful, The brazed end bits can break, But I was lucky,
I wonder if heat may upset the brazing, And that's why He does them cold.
 

oexing

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Temperature control for hot bending is no real problem, you see the red colour and keep some distance from brazed forgings. I would not want to bend cold, takes a lot of force and you rarely get kinks out of tubes at the right place. It is really a safe operation with a torch , unlike with a little help and hope from a press when you would be challenged how to place that unit under it to get the desired results. It is a very threedimensional shape so easier to hold it in a decent vice and apply heat for a one-man bending job. I did not clamp on castings for bending, only gripping the tubes and used some patience for having red heat for bending.

Vic
 

Bill Thomas

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I do remember Stan at MLG Motor cycles, Where I worked in the mid 6o s on BMW s,
Used heat on a proper jig, Chased the bends up and down for hours !.

The Vincent rear frame is a hard one to do, Because the tubes are meant to have a huge offset,
It's like they built the Bike and and thought S--t, The wheels don't line up :D .
Lets give it a bend here !.
 
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oexing

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Rear frames offset, really ? Looked pretty symmetrical to me. At least one can cope with spokes tensions but the sprockets have to be aligned for sure.

Vic
 

craig

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VOC Member
I am trying to best to fully understand the instructions for crash damage in Paul Richardson's book page 40.
I have edited this page 40 text for my clarity.
My"twist" appears to me to be 7.0mm/ft

Crash Damage
Slight lateral bending can sometimes be corrected by heating the tubes adjacent to the lugs and re-setting each fork end individually.
Mandrils inserted through the pivot bearing and the rear-axle slots should be parallel, and the maximum permissible twist is 3/32 in. (2.38 mm.) per foot of mandril.

The inner fork-end faces should be parallel, 8.3/8 in. (21.27 mm.) apart;
the right-hand face (purple) is 9/16 in. 04.28 mm.) outside the face of the pivot-bearing lug,
the left-hand side face is 1.1/4 in. (31.75 mm.) outside the left-hand face of the pivot bearing lug.

The apex lug (yellow) is central to the fork ends, and the ears of the seat-stay lugs should also be parallel.

MO22cropRFM3.jpg


Is there a how to article on replacing the fork ends on an RFM?
Can you unbraze a fork end by heating it and pulling it off?


RFMRearLugs2.jpg


Are there dimensional drawings of the RFM that can more accurately describe the dimensions?
Top view?
Side view?
Pictures of disassembled RFM?
Depth of bore, engagement joint of brazed joints?
Tubing dimensions? Suppliers of tubing?
 
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Monkeypants

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Non-VOC Member
Rear frames offset, really ? Looked pretty symmetrical to me. At least one can cope with spokes tensions but the sprockets have to be aligned for sure.

Vic
They are offset. I became aware of that fact when searching around for Egli frame drawings.
There is a very nicely drawn Egli swing arm drawing out there that is symmetrical. It will not work with a Vincent engine. For a standard type Egli which uses standard dimension head brackets, the swing arm needs to be built to the same offset as a standard Vincent RFM . A friend warned me about the symmetrical drawing, which was a good thing as I had it and was considering building from it before learning of the offset. He had spent a great deal of time building a beautiful swing arm to that drawing, only to chuck it in the scrap bin after discovering it would not work with the Vincent engine.

In the end I did something quite different (monoshock and 180 rear tire) and that required an even greater offset in both the head brackets and swing arm. None of the collection of Egli frame drawings that are readily available would suit my plans.
It was simple enough to make a full sized drawing based on what was needed.
Since then I've noticed that most motorcycles have some sort of offset in the swing arm.
It's like Bill says, the designers must design the swing arm last and just make the offset whatever it has to be in order to line everything up.
Come to think of it, that's what I did as well!

Glen
 
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