FF: Forks Cracked FF3 Lower Link?

[oexing]

Castings or forgings can be challenging while machining them. You need some good planning for setting them up in a jig for getting bores right in the best place. I have found on Brampton forgings/castings some places with very low wall thicknesses. So before sending them to Robbins I did some TIG welding for beefing up these parts with nickel-chrome buffer rods. You will be prepared for some days of rectifying warped components from welding and all sorts of poor alignments.
So for commenting the defect I´d think there is very low wall thickness that showed up after grease pressed in.
I will be interested in reading here how the bearing recesses will be rectified after welding for restoring alignments of bearings. Or otherwise have undersize bushes fitted and line-reamed then for correct fits ??
These days I have finished a girder repair of the 1935 Guzzi which got new IGUS "plastic" bushes , replacing SKF Permaglide graphite/teflon bushes. Those had 30 000 km use , with no wear on stainless spindles, no lube certainly all the time. Spindles are kept for the new bushes of course. Great thing is, you can line-ream the IGUS material, cheap to get as well. The Permaglide are coated bushes, so no reaming is possible with these.

Vic

 

[Elanman39]

The very straight nature of the crack certainly makes me think it is a forging issue Gary; the equivalent of a cold shut in a casting where two advancing surfaces meet and fail to fully bond. I came across a similar issue on a forged steel rear upright on a car I was involved in the development of a few years back. In that case the forging included a hole, I'm not sure if the lower link forging would have had a hole included or if the holes was just bored out afterwards. If the latter then my theory bites the dust...

Good spot Bill, funnily enough I noticed the same thing when I was reviewing the thread a few minutes ago. I have just been and checked and we have aberration number two: the lower damper mounts are not a matched pair, they are about 1/8" different in height. I don't see that this would have caused the crack but it can't have been helping the fork articulation too much. One also has a rather ugly sharp stress raising corner while the other has a much more healthy looking radius. Any ideas where I'll find another with the radius? So much for the high quality build, looks like I need to spend a bit more time checking everything closely.....

Thanks for the comments Vic, The current plan is to pop the old bushes out, vee the crack and weld it back up after warming the forging through and then check the truth of the bush housing bores to see how much distortion we have. If under a couple of thou I'll rebore the link straight and use oversized bushes, if worse then will probably do as you suggest and correct the misalignment by fitting new bushes with small bores and then boring the bushes in situ. The Igus bushes sound interesting.

 

[Bill Thomas]

Well done Nick, It's just that I have broken a damper in the past,
So any sideways stress would worry me,
Mine snapped at the change at the top where the threaded bit screwed on,
It was 1965 , ! On an old Twin outfit.

 

[timetraveller]

It makes me wonder who did the high quality rebuild, and no, I am not asking. First someone made one of the eyebolts wrong and then someone used a mismatched pair when fitting the damper. That must have been obvious during the fitting process.

 

[brian gains]

What's curious to me is how straight the split is. It almost looks like a seam between two mating surfaces. Could the forging have been done that way from a flat, rolled plate? In any case, I'd V it with a dremel and weld it up.

no intention of diverting thread but I had similar query come up where I thought a part, HD selector drum, had been machined from billet but was educated that the item had been manufactured as per Gary's reasoning. Similar split and not full length of drum.
Regardless, interesting alert and I'll be checking my Comet FF3 first thing.

 

[davidd]

The lower link is not particularly strong. They tend to crack very easily. I have straightened quite a few. If they are cracked I replace them with a new one.

As described in the literature, the link has been designed for high strength, but it appears to be less robust than I thought initially.

Due to the poor design of the steering stem, the bottom link takes much more abuse than it was designed for. Under braking, the big eye is exposed to a constant hammering action, which is why the eccentric holes need reclamation after extensive use. If you mount a new steering stem I think a chunk of aluminum would do well as a replacement because the suspension loads are resolved in the springs rather than being resolved by hammering the lower link.

David

 

[vibrac]

I think Chris has it, a high pressure grease gun done it
check the size of the one that goes with the tool kit
a bit like the oil pressure on a Vincent think 1948

 

[Gary Gittleson]

I have doubts about the grease gun theory. A few years ago, I had to repair a leak in the hydraulics of my New Holland 3910 tractor (made in the UK!). One of the "hard" lines rusted through. Rather than find a replacement, I welded up the hole. The material was very thin, probably much thinner than the lower link.

This tractor's hydraulic system runs at 2500 psi. Would a grease gun exceed that? I've had no further problem with leaks.

Gary

 

[timetraveller]

I too have some doubts about the hydraulic theory. Even if enough pressure could be generated to burst the tube why would it spread almost the whole width? Also when one sees a new one from the SC they are as cast all-over apart from the machined surfaces. No evidence of the front tube being made of two parts. I think we are missing something here.

 

[oexing]

Davidd, did you strip the paint of your casting to inspect these "cracks" ? Seems a strange pattern of cracks in the paint to come from a cracked component. I´d have suspected poor paint quality , like from American plastic thermo paint ??
Also, I do not share the hydraulic theory of blowing up the casting by brute force. There has to be just minimal wall thickness for cracking the steel. Not easy to get size of wall along the crack, maybe drill some small holes along the crack and do a guess from that.
I´d be surprised if this item was a forged part, cannot imagine how they would do this shape. So I believe this is a casting like most other parts on classics.

Vic