FF: Forks Modified Steering Stem

[vibrac]

the length of the head mounting from the side of the bracket to the pivot point on the damper body is variable as is the position of the pivot on the body of the damper and in type two the position of the clamp on the spring box is also alterable to some extent I have had one on a twin but ever since I have had Bramptons I have not bothered

 

[timetraveller]

First my apologies to all those who are still waiting. Having had two sets of engineers try, and fail, to machine out the eyes at the rear of the lower damper I have now found a local company who has a line borer. In fact, not one but two companies. Both companies cannot start the work for three weeks but yesterday I was with one company who has done work for me before; machining a 2.5 foot diameter gear for a telescopes. They showed me exactly how they could set it up to ensure that both the new holes would be in line and the correct distance from the front bearing holes. I have seven of these lower links waiting to be machined and, except for one new one, none of them has the eyes parallel and in line with each other. I have been loathe to place these into the postal system as at £400 each that was a lot of money to risk with people who do not provide adequate insurance for the parts they transport. This is what has been causing the problems.
I have already started to send out kits for those who will use Oilite bushes and therefore need the AVO dampers with the standard settings. I expect the new AVO dampers, with the stiffer settings to be used with ball races in the lower link, to be delivered later this week and once I have them I can start and send out the kits to those who will be machining their own lower links. I thank you all for your patience.

 

[timetraveller]

Right; this is going to take a while so please stay with me. I got a phone call from the chaps who have the line boring equipment this afternoon. Although they told me they could not do the job for three weeks they had nevertheless got on and done some measuring and it is not good news. I had given them seven lower links to be machined, one of which is brand new and belongs to Stu Spalding. It is unpainted and clearly never been fitted to anything. They had set up several of the links with a spindle through the two front Oilite bushes. This is an as new original Vincent spindle supplied from my bike. The spindle was placed in vee blocks on a surface plate and the rear two eyes allowed to rest on the surface plate. They then clocked the inside of the rear eyes off the surface plate. At first I thought that the potentially different thicknesses of the forged eyes could cause a problem but I soon realised that by measuring the inside of these eyes the thickness of the outer material makes no difference. You might have to think about this for a while, I certainly did. With the old lower links the differences in height were all over the place but generally tens to several tens of thous. I then got them to do the same with the brand new one. There is 22 thou difference between one side and the other. Twenty two thou gentlemen. To take the eyes from 1.25 inches to two thou under 32 mm one needs to take a cut of four thou, that is eight thou off the diameter. To imagine where this misalignment is please imagine taking hold of a lower link by the two rear eyes so that the front is away from your body. Then imagine pushing down on one side and up on the other. That is the amount of twist in that direction. We then set up the new lower link in such a way that the distance from the front axis to the inner rear of the eyes was measured. On the new link the holes were parallel with the front spindle. We did not do the same with the older links. And there gentlemen we have the problem. The two previous people who claimed they could machine these eyes and then told me that they couldn't never explained why. The new chaps have been kind enough to not only explain but also to allow me to play with their surface plates, vee blocks etc and to see exactly what and where the problem is.

So we are now at a stage where we have to make a joint decision. It is not just old lower links which have distorted with time but even a brand new one is so out of line, twenty two thou, that it is impossible to machine four thou out and make the eyes in line. There is absolutely no problem machining each side separately so that the eyes are the correct size to take the ball races and are exactly in line with the front spindle. However, they cannot be made so that the two eyes are essentially at the same height. It would be possible to bend the lower links to that they are more in line but the chances of that bending staying constant in the future is considered to be just about zero by the machinists and it is not a route they would wish to go down. The only positive thing to be got out of this is that all these links, apart from the new one, have been in use for many years without the riders feeling any problems. I can see only two choices here and that is why this is so long because it affects all of those of you who are going to go for the bearing mod, or even continue to use Oilite bushes. The first would be to make new links, heat treat them to remove any residual stresses and then give them to someone who would be competent to machine them accurately. This might mean two machining operation. One a course cut to relieve some of the original stresses, de-stressing again and then a final cutting to size. This would be prohibitively expensive for most of us as new ones already cost about £400 and they, or at least this one sample, are not accurate. The second option is to accept that we have less than perfect machines and to just open up the eyes independently and just accept the fact that there will be some errors which are probably not detectable to the rider. For my self and my own lower links I will go for the second option. Please let me know either via this Forum or by PM what you wish to do.

 

[greg brillus]

I just straighten the links that are bent and hone the eyes out as a pair, the problem can't be that bad if the bikes have been on the roads all these years. If I'd shown you all the original lower link off the racer you would have told me to throw it in the bin..............The arms were bent in an "S" so I had to heat and bend them a little at a time to get them as true as I could. I had a lower link here the other day that looked like one of the arms had been re- welded back on..........

 

[Oldhaven]

My experience with the bent headstock on my B tells me that we probably have more problems with bent parts in the front end than we care to know about. I think I have heard from davidd that he has never found a headstock that was not at least somewhat bent. As long as the bike doesn't try to ride off toward Sawyer's we probably would never notice. I do wonder if the eyes being out of line might have more consequences on friction/stiction and wear with bushings rather than bearings.

 

[Steve G]

I just straighten the links that are bent and hone the eyes out as a pair, the problem can't be that bad if the bikes have been on the roads all these years. If I'd shown you all the original lower link off the racer you would have told me to throw it in the bin..............The arms were bent in an "S" so I had to heat and bend them a little at a time to get them as true as I could. I had a lower link here the other day that looked like one of the arms had been re- welded back on..........

Believe that may be MY lower link to which Greg is referring. . . Greg, should I be worried??

 

[timetraveller]

The thing is Greg that I did question the guys about bending the lower link back into shape and then machining it. I use a 25 tonne press to push the steering stems in to the lower motion blocks with the new steering heads so there would be no problem doing the bending. The problem is that, in their opinion, any straightening would likely be only temporary and the link would very likely try to revert to its previous shape, or some other. My own guess is that as with the new design there is no single spindle through both sides of the bearing position (originally eccentric position) so that the assembly of all the parts would probably pull the lower link straight enough. Anyone who has seen the new steering heads will have seen how very strong and rigid the mounting of the concentrics into the lower motion block is. I think that the lower link will distort to fit rather than anything else move but it is not the kind of elegant engineering that I like to send out.

 

[Martyn Goodwin]

It is not just old lower links which have distorted with time but even a brand new one is so out of line, twenty two thou, that it is impossible to machine four thou out and make the eyes in line. .

So even brand new links from the Spares Co are 'not correct' from an engineering perspective?? It is not a cheap part!

I wonder if anyone has brought this to the attention of the Spares Co. and what, if anything, they can do about it.

 

[greg brillus]

Well ideally the link eyes and front spindle should all line up. A bit like saying that once you start up your engine after a complete rebuild that your perfectly aligned crank assembly will change and the flywheels big/end will shift and the Sun may not come up tomorrow as well. I have a complete set of Girdraulic forks here that are so bent that I cannot fit the lower axle back through both holes. Now That is something to worry about........Whereas if your link eyes are out by 10 or 20 thou, I would not get too concerned, It would only be a worry if you cannot assemble the forks without using excessive force. As far as the engineers claiming the straightened links would likely return to their "Bent Condition" well I'm not so sure, the links are quite strong and it does take quite some effort to get them near straight again. I've had some experts tell me how weak the rear frame members look............I say to them, have you ever tried to straighten one. Something made up almost entirely of triangles is very strong. Of all the lower links I have done so far with the bearing mod, they all seem to assemble up ok, and the fork spindles return back into position without any heaving or excess strain. It is important for the holes of the eyes to be square and parallel so the 4 bearings don't bind up. One of the trickiest parts of trying to straighten the link is trying to hold on to it.

 

[greg brillus]

Believe that may be MY lower link to which Greg is referring. . . Greg, should I be worried??

Steve, it did have some very odd looking markings on it, but it would be near impossible to know how they got there. It did take some effort to get straight, but they always do, and it did not fall apart even with all the bending and tweaking I did to it. I would not worry to much............they are a very strong component.