FF: Forks Modified Steering Stem

davidd

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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

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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.
 

davidd

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Neal's Comet with the modified steering stem as mentioned by Greg:

Photo 5 unladen
P1010254.jpg


Photo 6 laden
P1010256.jpg


Photo 7
P1010257 (1).jpg


Photo 8
P1010259.jpg


David
 

greg brillus

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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

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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

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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

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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.
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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

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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

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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

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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.
 
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