FF: Forks Cracked FF3 Lower Link?

[Gary Gittleson]

Yes, I didn't see how it could have been forged, unless two pieces were forged to form two halves of a clam shell, but that would require welding to put them together. There's no sign of that. Someone needs to take a wrecked lower link and cut it apart so we can all find out, no?

 

[Elanman39]

Well, I've done a bit of digging around on the internet and a few quick sums and the answers are surprising.. According to several different manufacturers websites hand held pistol grip grease guns can generate up to 17,500psi (!). Some claim 20,000psi which I find amazing. I measured my cracked FF3 link and the bore of the front pivot is not particularly central to the forging. As a result the wall thickness at the extreme front of the link is just 1/8" in the central region. Assuming the link is mild steel as it is described and taking mid-range values for yield strength and ultimate tensile strength for mild steel of 50,000 and 64,000 psi respectively the burst pressure for a seamless mild steel tube with 1/8" wall thickness and 1" overall diameter which replicates the central section of the front of my link comes in at 16,000 psi using Barlow's burst pressure equations. So given the age of the parts, the probably poor quality of the steel and the significant offset of the machining it is well within the realms of possibility that I could have cracked the link with my little hand held grease gun. If I didn't it's quite possible someone else has at some stage in the past....
The good news is I have repaired my link as discussed, grinding the crack out to a wide vee and then using mild steel filler rod and an argon arc to weld it back up, taklng the opportunity to increase the thickness of the material locally by bringing the crown of the weld to a peak and then grinding and polishing it back to a nice stress raiser free ridge along the leading edge where it was thinnest. The really good news is there was no measureable distortion and with nice shiny new bushes fitted and jig-reamed the link is perfectly square.
I'll put it all back together tomorrow evening....
In the meantime, please go carefully with those grease guns!
Nick

 

[davidd]

Vic,

That particular link was discovered on David Tompkins's racer. My advice was to replace it, but none was available. He had a few days before leaving for a race. It was stripped and as I remember there were cracks in the paint and that arm was bent. He was worried about the amount of force needed to bend it back. I told him that the lower link is pretty soft and it could be bent easily.

He set it up on the milling machine shown above and gave it a tweak or two and it was done. He intended to take it apart after the race as he wanted to install a new steering stem and taper rollers. He experienced breakage of the lower steering head bearing. I had made two stems that, you will be happy to hear, are metric. This allowed me to fit a metric taper roller and a shim into the stock steering head. I see the old steering stem on the miller, so I am fairly sure of the order.

I am not sure I remember the fate of the lower link. I remember when he swapped to my steering stem he installed bearings in the lower link he ended up using.

I think the lower link is just not quite as strong as one would think. I suspect it is just cast steel. The 1/8" thickness is the same wall thickness used in the steering head and they bend easily also. The steering head is also cast steel. I have not seen a catastrophic failure of the bottom link, which is a good sign. I bought a new lower link for the road racer I am building now. I always buy a new FF1, also.

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David

 

[Peter Holmes]

I have a wrecked FF3 that folded to one side in a rather bad T Bone accident, I should have discarded it, but that is against my nature, although it way beyond safe repair, I would be more than happy to part with it, as long as there was some sort of guarantee no attempt would be made to make it serviceable (well in someones eyes anyway) but it would seem the questions have been somewhat answered now.

With regard to the headstock casting, I took one to the ex-factory engineers, Furness and Searle for repair to see if they considered that possible, the bottom bearing housing was oval, and the bearing cup was accordingly loose, I seem to remember them telling me that it would easy enough to repair as it was a malleable casting, I think they simply put a bearing cup in the recess, and then beat the metal back to make a tight fit on the cup, whilst ensuring it went back also in alignment, I know it sounds awfully crude, but back in the day you just gave them jobs to do on Vincents, and trusted the results, it certainly seemed ok at the time.

 

[vibrac]

Years ago (many,too many!) I did that exact repair job on a head lug top bearing location it is indeed a malleable casting

 

[Simon Dinsdale]

Typ

Years ago (many,too many!) I did that exact repair job on a head lug top bearing location it is indeed a malleable casting

Type 1,2 & 3 (FT1, FT1/1, FT1/2) headstocks were all a malleable cast iron. The later FT1/4 and series D headstocks were a steel forging.

 

[delboy]

Hi folks,
I found at least two quotes from PCV.
[From Vincent V-Twins] "....the bottom link fashioned from forged steel".
[And from Vincent-HRD story] "Forged one-piece fork links could operate against the legs...."
I can also assure you that a forging drawing for the bottom link exists as it was reviewed during the drawings project.

I would think the link is most likely finish forged from a casting [like the legs]. The part line of the dies running horizontally with a line across just where the split is in the photo.
Now, WHY the link has split? I can only guess a fault in the original casting, exacerbated by the bashing of the dies?.
I have never seen one like this in over 40 years of rebuilds. Plenty of bendy ones though!

Peter,
Jack Furness did a similar job on my Comet's bottom steering bearing housing. He was a VERY knowledgeable man though.
It has stayed true and sound all these years.
Regards,
delboy.

 

[Elanman39]

I have now found out that the grease nipple fitted in the centre of the forward part of the FF3 lower link on my bike is not a standard feature, it has been added at some point by a well meaning previous owner. I think this is the root cause of the crack: injecting grease under high pressure into such a large diameter bore creates huge stresses as the calcs above showed and it would not normally be considered good Engineering practice according to a design manual I've been looking at. If such an arrangement is used it's recommended that the maximum pressure is effectively limited by cutting grooves in the bearing faces to allow grease to escape and show to the gun operator that it's time to stop. There were no grooves in the bushes on my link; there are now, on the leading edge where the loads are lowest.
I would doubt that a similar failure will be seen on a link without this additional grease nipple fitted.
Thanks for all your thoughts and suggestions, I hope I haven't frightened anyone. Nick

 

[lee_812d]

There is an article on the forum showing the locations to add these extra grease nipples - the one you have, another on the top link, on each eye where the eccentrics are, and the steering head spindle.

 

[Elanman39]

It might be worth adding a warning about the danger of over-pressurising to the article? How can I find it? Is there a search facility I haven't found yet?