FF: Forks Modified Steering Stem

[timetraveller]

I was in touch with John only a week or so ago. He does not know what strength springs he used. He had a large number made and then kept experimenting until it felt about right. He tried a Thornton damper front and rear. He found that the rear one did not work at all when it was at the angle which it would be when on the bike. He showed it to me years ago and it was fine when upright and hopeless when horizontal. I assume that they are not all like that or someone would have complained by now. He eventually had a very special damper made for the front which was longer than standard and which required an outrigger bracket suspended from the lower link in order for it to fit. He believes that this was a waste of time and money and that the Thornton on the front was good enough. He has now lent me his Thornton and it is much softer than the AVO. When new, the AVO is very difficult to move by hand whereas the Thornton, which now has some play in it, is just about the same as my 60 years old Armstrong. John is worried that the handling will be worsened by using a 'stiff' damper but Chris' experience does not back this up. It is at this stage that I admit that I do not understand damping and am prepared to believe real world experience on the bike rather than pumping the things up and down by hand. Both the original Vincent and the Armstrong seem to have the same resistance in the compression and extension phases, which cannot be right by modern standards. This is also true of the Thornton which John has lent me but when that is moved in and out there is a definite 'squidging' sound of fluid moving about and when fully compressed it then pushes itself open as there is something inside creating a gas pressure. I do not know whether this is intentional or due to air leaking into it with use. For the rear John used long springs and had a very special and expensive damper made which, if I have understood correctly, could be adjusted for both the compression and extension strokes as well as a third adjustment for something else. I'm not sure what. He believes that the rear modification was very worth while.
John contacted me a while ago, worried that I was not warning people that with the new steering head the front mudguard could move so far backwards and upwards that it could hit the front cylinder head. This has not proved to be a problem so far with any of the road users who have fitted this modification so where the difference lies I do not know. He is also worried the there is now so much movement at the top of the fork legs that the control cables, brake, clutch, valve lifter etc. might have to be repositioned rearwards. As no one has mentioned this I assume that it turns out not to be a problem for most people. Sorry that there is not more information but you now have all that I have.

 

[greg brillus]

In all three instances I have installed the kit, firstly on the racer, which had a very many mods to the front end other than just the changing of the stem, I have gone to the effort of attaching the shocker with long and short eye bolts, then moved the forks fully up and down as far as they will go using my strength, but not to the point of compressing the bumper rubber inside the shock absorber. By the scribe lines drawn on paper using a texter pen in the hole for the wheel axle on both the racer and my road going Rapide, the rearward travel was only about 15 mm on the racer and perhaps a little more on the road bike. This would only change and not by much, if the steering head angle were different, and /or if the shocker allowed more upward travel. It is possible I guess that John's design is slightly different to yours, but only you would know that. But my understanding was that the ones we have, are based on the lower spindle being removed from its original location in the original stem, and the lower link simply pivoted downward by the desired amount thus causing the link center to also move slightly aft, and because there is no eccentric anymore, the change alters the trail by nothing. It is also clear that both the upper and lower links are now almost exactly parallel to one another. Remember John Renwick was also aware of this issue, but his fix was to drill another 9/16 hole further up the fork leg, and although no one is going to carry out that mod, it clearly would have given him a similar result. Whatever we end up with in the way of springs and shocker units, the main aim is to alter the action and therefore the Axle path so as to duplicate that of the Brampton forks, and you don't see any of them with the links pointed downward. I feel that some may think that the travel available will be some huge full up and downward sweep of the links, but this is simply not possible. In reality the travel is probably more like 45 degrees at best. The fork action needs to be more like telelscopics, where the bike dives in the front under braking and the bike rises and falls on the front as you open and close the throttle. I rode a series "D" Rapide yesterday with what I felt was quite a soft front end, and I deliberately tried accelerating up to speed and hitting the front brakes quite hard to see what the front end did. It basically ceased to function, that is, it was almost locked up, and only very occasionally did I see the upper link move slightly over the bumps in the road. I know all three bikes with the conversion I have carried out so far, do not act in this way at all.

 

[bmetcalf]

With my standard Rapide, the clock moves noticeably up and down while riding. Of course in reality the sprung mass is moving up and down with respect to the road surface, but we know what it looks like to the rider. Greg, were you referring to the upper link movement while braking?

 

[davidd]

Thanks very much gentlemen, you both make a lot of sense. I hope you don't mind if I ask some follow on questions that I hope are useful to the discussion?
Both my pull-down spring mod and this mod are designed to reduce the probability of tankslappers, so allowing softer springs to be used safely.
But as tankslappers are thankfully very rare anyway, how can we know if we have succeeded?
Is there anything in the handling of the revised geometry bike that lends support to this view?

Rob,

In my own case, I tested the shortened springs to a point where I felt the possibility of the Vincent wobble had been eliminated. I found after having guessed at what caused my wobble I could induce the wobble by replicating the conditions: hard braking on a slightly rough road surface. With long springs in place I could release the front brake as the wobble set in and avoid the fall. Doing the same experiment with the short springs simply ended in a fast stop.

I would note as a reference to Martyn's fall that although there are reports of wobbles with no braking involved, the common element is lightness in the front end. I found this conclusion in a University paper that Rob posted in the Comet Suspension thread. It seems that when the front end gets light there is potential for a wobble. In Martyn's case the bump he hit caused the front end to get light. It is not intuitive, but lightness of the front end is the cause of the wobble under braking also. It is the seizure of the front end that causes the front tire to bounce on and off the surface which ends up lightening the front end grip enough to cause the horrible castor reaction that we often see in the front wheels of our shopping carts when the frame of the cart is bent just enough so the front wheel barely touches the floor. I would argue that the lack of front end compliance would be the common element in both accidents.

Back to the stems, I would hope that testing similar to what I did with the short springs could be done. I was lucky to be able to do back to back testing, but general road testing with some experimentation should yield useful experience. Also, as mentioned above, better front end compliance in general will add to the proof that the mod is working. I think of Norman's experience with Dick Sherwin's D with a disc brake where he found he could squeeze the brake and cause the front end to snap to full extension at will. On a bike with modified brakes and a modified stem you would not be able to do the snap to full extension, the bike would merely slow down.

David

 

[BigEd]

Rob,
........ I would note as a reference to Martyn's fall that although there are reports of wobbles with no braking involved, the common element is lightness in the front end. I found this conclusion in a University paper that Rob posted in the Comet Suspension thread. It seems that when the front end gets light there is potential for a wobble. In Martyn's case the bump he hit caused the front end to get light. It is not intuitive, but lightness of the front end is the cause of the wobble under braking also. It is the seizure of the front end that causes the front tire to bounce on and off the surface which ends up lightening the front end grip enough to cause the horrible castor reaction that we often see in the front wheels of our shopping carts when the frame of the cart is bent just enough so the front wheel barely touches the floor. I would argue that the lack of front end compliance would be the common element in both accidents.

Back to the stems, I would hope that testing similar to what I did with the short springs could be done. I was lucky to be able to do back to back testing, but general road testing with some experimentation should yield useful experience. Also, as mentioned above, better front end compliance in general will add to the proof that the mod is working. I think of Norman's experience with Dick Sherwin's D with a disc brake where he found he could squeeze the brake and cause the front end to snap to full extension at will. ...........

David

I have just completed fitting two discs on the front wheel of my Rapide. They seem to work very well.
I haven't noticed the front end snapping to full extension. (I'll take more notice next time out.) I have Rob's degressive spring fitted to the Girdraulics so perhaps they are preventing it.

 

[Nigel Spaxman]

I only have about 3000 miles on my Rapide so I don't have much experience. I read a lot about Vincents over the previous 14 years while I was building this bike and it was because of this reading that I fitted a hydraulic steering damper. I find the handling of my bike is very good and it is the rear suspension that I am going to work on this winter. I want to fit longer springs. The front suspension on my bike is a lot different than any other bike I have ridden. It works well and in many ways it is better than many more modern bikes. One of the advantages is that the suspension continues to work even under heavy braking. The suspension doesn't dive nearly as much as other bikes when you brake. On many bikes you have to avoid braking over large bumps. That is not the case with the Vincent. The problem is that if you happen to begin heavy braking when the suspension is extended that the suspension could lock in the extended position. I have never noticed that happening and apparently Ed hasn't either. It is because of this problem that it is important with the standard set up that the lower links need to be pointed level or slightly up when the bike is at rest. With the suspension in this position at rest it is unlikely the suspension will get into the fully extended position. With the modified steering head more of the suspensions range can be used because there is no problem with the fully extended position. It is a very interesting idea.

I am hoping that by fitting the hydraulic steering damper and having my suspension settled pretty low that I can avoid tank slappers. I have found my bike has a very solid feeling to it that I like.

 

[Chris Launders]

Nigel, surely if you fit longer rear springs this will raise the middle of the bike and steepen the steering which will make the steering more nervous.
I rode one once that a friend wanted me to have a look at, it had had sidecar springs fitted and the eyebolts at their maximum, the thing was lethal, we soon altered that !!!
Chris.

 

[hadronuk]

OK Gentlemen, I give up on a longer damper. After all, it won't be me doing the testing!
TTs description of his Cadwell crash sounds a little similar to the scenario described in the Cranfield and Imperial paper, where a road irregularity of just the right wavelength momentarily forces the forks sideways provoking a self sustaining oscillation. MartynG's accident sounds much more like the scenario where the front wheel leaves the ground and touches down off line. The high trail then forces the wheel to turn violently, again provoking self sustaining oscillation.
Anyway, I can speculate for ever so I will stop.
I greatly regret I was not able to complete my Girdraulic force model. It could have been used to look at spring rates and the wheel and whether the geometry makes this progressive, friction in the links, different geometries, and the balance point of the opposing forces in the forks under braking.
I might have another go when I am retired. Oh, I am retired. Memory not what it was.

 

[Nigel Spaxman]

Chris Probably the steering will be a bit lighter by raising the rear of the bike. The thing is right now the bike is dead steady and the steering is not particularly light. I just want to have some more rear suspension. If I was riding solo all the time it would be fine the way it is, but most of the time I am riding with a passenger. I also want to fully suspend the seat to reduce the shocks to my passengers spine. I am planning on trying some Thornton rear springs along with the Koni damper that I already have.

What do the experts here think? How much can I raise the rear to get more suspension travel without ruining the handling. It looks to me as though some other bikes are riding a bit higher than mine. Mine has sagged original springs.

 

[hadronuk]

I started with some incredibly long Petteford springs and I was not aware they had a negative effect on handling, unless you include falling over when parking on a slope because I could not reach the ground.