• Welcome to the website of the Vincent H.R.D. Owners Club.

    Should you have any questions relating to the Vincent H.R.D. Owners Club, or Vincent H.R.D. motorcycles in general, please contact Graham Smith, Hon. Editor and Webmaster by calling 07977 001 025 or please CLICK HERE.

    You are unrecognised, and therefore, only have VERY restricted access to the many features of this website.

    If you have previously registered to use this forum, you should log in now. CLICK HERE.

    If you have never registered to use this website before, please CLICK HERE.

FF: Forks Modified Steering Stem


greg brillus

Well Known and Active Website User
VOC Member
The change in the modified steering stem changes all of that, as the forks no longer behave in a manner biased toward the suspension suffering seizures. I have not seen enough good photos of the front end of John Surtees racer to see what was going on, but I have seen the footage of his crash. I feel the behavior of the Girdraulic's can surpass that of Brampton's in part because of the longer links, thus enabling more travel. The efficiency of the brakes will be increased because the front suspension is able to work fully and correctly, without the front wheel trying to stay jammed under the engine. If all the Girdraulic equipped Vincent's world wide were converted in this way, it would be a complete revelation......... In fact I would say, that if the factory had realized and fixed this back then, the other Hybrids like Norvin's and Egli's would not have existed, given their main existence was to improve handling. One of the reason's I wanted to build a twin racer based on a Vincent ONLY, and not a twin engine housed in another frame, was that I was certain that a genuine Vincent COULD be made to handle. Until I rode Neal's Comet the other day, even though my bike has the same set up, except my springs are still too hard.......Neal's Comet felt so different to ride, in all honesty if you were blindfolded, you would swear the bike had tele's up front. But please don't tell either of the Phil's I said that...........:)
 

Bill Thomas

Well Known and Active Website User
VOC Member
Very interesting Rob, But Know thy beast thinks, If the forks move too far up, To where the trail is smallest !!, You could have trouble !.
Cheers Bill.
P.S. I was thinking of putting an inner valve spring under the shroud of the standard front damper, On the Comet, Just to give it a bit more springyness, What do you think. Cheers Bill.
 

Hugo Myatt

Well Known and Active Website User
VOC Member
I am surprised that the AVO front damper seemed so stiff. It may be that it needs to bed-in a little as you say.
Hand testing dampers can be very deceptive!
We dyno tested several dampers. All except the original Vincent damper had a fairly "flat" bump characteristic in that the damping force quickly reached a "plateau", then did not greatly increase.
This "plateau" is at about 25 lbs for an AVO front.
The original Vincent damper, being a simple fixed orifice design, was initially very soft, but reached over 60lbs at higher velocities. (Fixed orifice designs are not used now, because they allow low frequency wallowing but are harsh at higher speeds.)
My Armstrong had a plateau at about 11 lbs.
On the road with well bedded in Girdraulics, I found the Armstrong to be comfortable, but just too soft at any speed over less than smooth roads. The final setting for the AVO was chosen to be just stiff enough to give control, but comfort still felt as good as the Armstrong.

There is a very big caveat to be borne in mind on this subject. There is a lot of friction in Girdraulics! My test measurements showed that even a well bedded-in pair of spring cases provide about 8 lbs of damping when the forks are extended and about 24 lbs when the forks are compressed. That's just the friction in the spring cases, I have not measured the friction from the link bearings.
SO THE TOTAL FRICTION DAMPING IN GIRDRAULICS MAY BE GREATER THAN THE HYDRAULIC!
This makes it harder to assess small differences in damper settings, especially if they are not tested on the same bike.
The Thornton damper we tested was for the rear. This had a plateau at about 40lbs mid range and reached 50lbs at max velocity. This is similar to the settings of the AVO rear.
I would be very surprised if a Thornton front damper tested as soft as the Armstrong on a dyno. As I said, hand testing dampers can be very misleading.
I am completely out of my depth with this but I have always been surprised at the pneumatic pumping action of the springboxes. With a well fitting inner and outer spring box minus the spring it is surprising how much resistance there is to pumping the box and how much air has to be displaced. With the front forks acting rapidly at speed I wonder how much resistance is built up. I have often (idly) wondered whether an air escape hole at the top of the outer would soften the fork action but then my position is one of total ignorance.
 

greg brillus

Well Known and Active Website User
VOC Member
The same applies to the rear spring boxes......If you move them in and out you can feel the air compressing, so yes I guess this is a factor, but I would say it's a bit like a feeling the action of a shock absorber in your hands. It may feel stiff, but on the bike and under the leverage it is operating at.....It is probably insignificant.
 

davidd

Well Known and Active Website User
VOC Member
I drilled holes in my spring boxes, front and rear to avoid the air pumping. I had noticed that it was done of the Factory bikes.

Shroud Hole.JPG

It is a little fuzzy, but you can see the hole drilled in the upper spring box below the mounting bolt. The problem is heightened when you put anti friction seals in. I drilled two holes in each rear spring box. I went to the coil over to lower the weight, the number of parts and the pumping.

David
 

vibrac

Well Known and Active Website User
VOC Member
The same applies to the rear spring boxes......If you move them in and out you can feel the air compressing, so yes I guess this is a factor, but I would say it's a bit like a feeling the action of a shock absorber in your hands. It may feel stiff, but on the bike and under the leverage it is operating at.....It is probably insignificant.
Not an effect I have to worry about I have Banana man springs
springs.jpg
 

Chris Launders

Well Known and Active Website User
VOC Member
I was talking today to my workmate who runs a couple of classic race bikes, his opinion was progressive springs were only there to mask poor damping, with properly set up damping would you should only need single rate springs.
 

timetraveller

Well Known and Active Website User
VOC Member
It is certainly not necessary to use taper roller bearings. John Emmanuel who developed this mod does not believe they are required. However, if there is any reason to change the head bearings then the taper rollers are the way that I would go but depending upon who's design of taper roller installation one uses it might be necessary to machine the steering head to take the taper roller bearings. With regard to pneumatic action within the front spring boxes; I am just in the process of trying to find the correct combination of damper, eyebolt and spring box length to use with the softer springs and the modified steering heads so I have had to play about a lot over the last couple of days with the spring boxes. Not surprisingly I would suggest that not all spring boxes are created equal. The ones off my bike, the ex-Cecil Mills bike, can be pumped in and out as fast as one wants with no tendency to build up pressure. However, it is clear that some other people have found that this can happen so it pays to try it out first on your own set of spring boxes to decide whether you need to drill a hole or not. These tests, which will be written up more fully when they are complete, are concerned with the relative amounts of movement on the spring boxes, the new AVO damper and the wheel spindle. It should be noted that with the modified steering head the space between the upper and lower mounts for the spring boxes is now about one inch less than with the standard set up. To ensure that the bottom of the outer part of the spring box does not bottom out on the fork leg or that the top of the inner spring box does not clash with the inner top of the outer spring box I have cut off one inch of the top of the inner and off the bottom of the outer and then moved the fork legs over the full range allowed with the new damper and long eyebolts. It is clear that one inch is correct and note that as the total distance is now one inch less than original the shortened spring boxes do not stand out as short. More details will be available after a bit more work.
 

davidd

Well Known and Active Website User
VOC Member
I think "advise" would be too strong a word on my part. I don't know of any problems with using a concentric. A handful of bikes have used them for decades. It does move the trail half way in between solo Girdraulic and side car Girdraulic. It also pre loads the springs slightly more than solo. This is not an issue with the short springs, but it increases the pre load on old springs.

I am not sure that the taper roller steering head bearings improve the steering as ball races are still considered top quality. I wanted taper rollers because I would take the racer apart routinely and I came to dislike the extra difficulty of disassembly and assembly. One aspect of the taper roller conversion I did not like was locating the cone on the steering stem. I have looked at many stems and they are not necessarily round as that was not necessary. With the taper roller being held tightly on the steering stem, I would think the new steering stem would be superior to the old.

I should mention that although I do not sell my steering stem with the new geometry, I designed my modified steering stem for eccentrics, so not all new geometry stems are made for concentric bearings. Mine uses the stock spindle with eccentric and can be used with a side car.

David
 

timetraveller

Well Known and Active Website User
VOC Member
Having seen David's reply to highbury731's question I realise that there is a need for some clarification. It is not the use of a 'concentric' bearing for the rear of the lower link which is important. It is the position of the centre of rotation of that link. With the John Emmanuel modification this is moved both forwards and downwards and it is this which changes the geometry and the movement of the front wheel on deflection. When I redesigned this I could have stuck to using the original eccentrics and that would have required modifying the lower part of the steering head and a lot more work on the stops etc. which allow the eccentrics to be rotated to allow a change to or from sidecar use. My feeling at the time was that it was unlikely that anyone with a sidecar would be interested in this modification.. It is true that the modification does alter the trail but that is not what is important. It is the path that the front wheel travels upon deflection, i.e. it now travels upwards and backwards as opposed to upward, forwards and then backwards. A graph in a recent MPH shows a comparison of the two paths. David's point about the advantages of using taper roller head races for those who have a need to assemble and disassemble the steering head is well made. The taper rollers make that a much simpler job.
 

greg brillus

Well Known and Active Website User
VOC Member
Thanks David......We worked out that when Neal is sitting on the bike, that 1/3rd of the shock absorber's travel is used, which I feel is probably about right. In the last photo you can see how Neal improvised to reinstall the original friction disc steering damper, although I have supplied him with most all the components to fit a hydraulic unit. In the top photo the gap you see visible on the shock absorber, this moves down so the shroud is just overlapping the lower body of the unit when Neal is sitting on the bike. He is very happy with the whole set up, and the ride comfort vastly improved.
 

timetraveller

Well Known and Active Website User
VOC Member
Thank you Greg and David. Those photographs are really useful. What type of damper is that? It looks to have more movement that the AVO I am experimenting with at the moment.
 

greg brillus

Well Known and Active Website User
VOC Member
Actually the first photo is not quite correct as the bike is still sitting on its center stand, but note also that before the stem change, with the bike on the same stand, the lower front guard stay was up against the mag cowl. So you can see how much that alone has changed. The shock absorber is an original Koni, and we checked its travel which was about 65 mm, and they have a buffer rubber about 3/4 of an inch thick, which I would say would compress another 1/4 inch or more. The photos showing the position of the lower link with and without rider are pretty much identical to mine. I am leaning towards using the longer lower eye bolts to maximize travel, and even with the spring boxes not connected and the forks fully in the up/compressed position, the links are pointing up a long way. The shorter eye bolts would increase this angle, and I am not sure this is necessary. Cheers............Greg.
 

hadronuk

Well Known and Active Website User
VOC Member
Ah, it's a Koni! I thought it might be because of the short dust cover, but I was fooled by the silver coloured body.
Anyone who likes the Koni damping characteristics should like the AVO front damper, as the Koni 76F we tested has a very similar bump characteristic to the AVO front. Assuming the recently rebuilt Koni we tested is representative.

As Timetraveller has mentioned, he and I have been discussing the possible spec for an extra long front damper, and I am happy to report that AVO emailed me yesterday to say they are willing to build it, in spite of the likely low sales volume.

Re the earlier subject of progressive springs. As the first part of their travel is soft, they usually require a higher preload if the correct ride height is going to be maintained. This is totally contrary to David's short and stiff spring approach, which was specificity designed to minimise preload. He and others proved the concept on the track.

Following on from that and also Bills observation that "Know thy Beast" comments that the forks can be dangerous at full compression because of low trail.
This is a debate that has raged since Girdraulics were first produced! I have read every thing I could find on the subject and I have four main reasons (plus my own theories) to side with the fully extended danger area theory:
  1. It is the view of two of the biggest beasts (Surtees and Irving) of the Vincent world.
  2. The most up to date study of tankslappers by Cranfield and Imperial cited evidence that light riders and/or a lightly loaded front wheel were risk factors.
  3. Some of the descriptions of Girdraulic tankslappers report they occurred after the forks went light, cresting a rise for example. I believe both of the Vincent accidents at Goodwood occurred at such a point on the track.
  4. As I mentioned before, modern racers regard a high preload as dangerous because the sudden unloading of the front wheel as the forks extend under full acceleration has been shown to precipitate a tank slapper.
=========================================================================
Some general points about my contribution to these discussions. Whilst I frequently promote AVO dampers, it doesn't follow I think other makes of damper are poor. I do think the other manufactures made a mistake with their offerings for the rear damper. The key fact is that the Vincent damper was designed for the Girdraulics and I think fitting the same spec damper to the back was an economically necessitated compromise which everyone else unthinkingly copied. This is why I think the AVO may have the edge on other dampers at the back.
With the Girdraulics its a different ball game. The Vincent damper travel is unsurprisingly perfectly matched to the fork geometry, so unlike the rear, there is no benefit to be had in changing its dimensions.
In my back-to-back road tests, the AVO gave slightly better control than the Armstrong and was slightly more comfortable than the Vincent. But I certainly wouldn't suggest anybody should expect a dramatic improvement by changing to an AVO at the front unless the existing damper is knackered.
 

greg brillus

Well Known and Active Website User
VOC Member
Neal told me that he had changed the setting on the front Koni from its softest setting to medium, but reported little change. I would say that because he has had little actual riding time since the change, that it is probably hard to tell. I am very confident that the newer generation of shock absorbers go a long way toward controlling the front and rear of the bikes. I personally am not a fan of the original shocks, and would choose a modern AVO or Thornton any day, though I do like to keep things original where possible. I am not sure a longer shocker is really needed for the front, especially with this new stem modification. If the front end hung lower with the front jacked off the ground this would achieve nothing, as the position of the lower link needs to be about level or slightly pointed upward with the rider sitting on the bike. This is a primary limitation with the front fork design. I think what is needed now is more feedback from others who are going to install these kits, though I know some of the projects will be quite some time before they are on the road. Once we have worked out a good starting point with the front springs on the twins and singles, I know of others that want to carry out this mod to their bikes when we are ready.
 

vibrac

Well Known and Active Website User
VOC Member
Following on from that and also Bills observation that "Know thy Beast" comments that the forks can be dangerous at full compression because of low trail.
I am struggling with all this information and had decided to wait till a definitive conclusion has been reached but am I right in thinking you meant "extension" not "compression" in the above statement?
 

Chris Launders

Well Known and Active Website User
VOC Member
I had a Koni on my twin with the modified steering stem and 36lb springs and it was much too hard when hitting sudden bumps and potholes or "corrugated" roads, transmitting shocks up my arms, ok on undulating roads though.
I now have an AVO fitted on it's softest setting and it's great, just riding bumps and ripples, even speed humps at up to 35 mph and not bottoming out.
 

Top