FF: Forks Godet Modified Steering Stem

[timetraveller]

Thanks for all that David. If someone with your design of steering head could produce a critique of it along side one of the JE system it would be interesting but probably beyond the bounds of realistic expectation.
As an aside to the general discussion here, I have recommended to people who have questioned about the use of the JE design with a sidecar, that the best way forwards would be to use a short top link. From time to time two different sizes of these are available giving different amounts of trail.
I think that we should all be grateful that people like JE and DD are prepared to make their considerable skills freely available to the rest of us. Greg Brillus and Chris Launders also provided significant input/feedback for my work. For those who do not know DD has just sent me drawings of his design but at the moment I already have plenty of the JE designs in stock.

 

[Cyborg]

It's what makes this forum so great!

 

[davidd]

Carleton Palmer has bought one of my stems ( I only made a few before my machine shop closed). He has the JE stem and Carleton very good at those type of comparisons, but it is a matter of when. Right now I am quick to recommend the JE stem, but I am also happy to have made my own for my usage.

Carleton Palmer loves the handling of his D Shadow. He says it is the equal of any street bike he has ridden (brakes excepted). I thought this thread was important to show that there were two ways to achieve good handling and that folks should ask some detailed questions if the steering stem had been changed. This way there would be no surprises.

I have shared the design freely, but you never know what changes a manufacturer might make. Patrick was very clear to me that he used my geometry "exactly". But, I am not sure how many bikes got the modified stem.

David

 

[greg brillus]

After we rebuilt the racer last year and took it to Broadford for the National titles, Phil was able to give me the following comments.............. Although the engine performance was an issue for various reasons, the other changes were, we went from 19 inch rims to 18's and ran a 110 on the front and a 130 on the rear. I also used better brake linings from advice given by David, and I went to a bit more trouble to remove some play that was evident where the swing arm mounts at the back of the engine, this was done by increasing the surface area of the hollow axle nuts and so on. Phil's comments on the bike was that the brakes definitely resisted fading much better, and that the handling and corner speeds were far better, and that he was able to out corner other competitors riding Featherbed framed machines, never a hint of head shake or bad behavior. He felt that the handling was equally comparable to many fast modern's he has ridden, and Phil is a quick rider with a lot of on and off road riding experience. That was always confirmation enough for me..........

 

[Oldhaven]

Excuse the video quality, since I had to hold my phone and the screen was in bad light, but if this works, here are videos or links showing the different actions of Bramptons, JE Mod, and DD Mod, as shown in the model I created. This may help with seeing the different actions that slight variations in pivot locations can make.

 

[timetraveller]

Thanks, that is very instructive. It looks as though DDs system has more forwards movement than the JE one at the lowest part of the travel but whether it ever gets into that position I don't know. Certainly the JE one will move forwards unless one is careful to keep the lower link just about horizontal at the lowest part of the travel. See my earlier graph.

 

[Oldhaven]

If you pause both JE and DD at about 4 seconds and use the slider at the bottom to fine tune, you can see the recommended normal horizontal or slightly above riding attitude of the lower link, (brown lines at the top of the fork line). At that point both geometries are traveling vertically and it is all vertical or back from there. Both cannot avoid some slight forward travel coming up from full extension if the axle is allowed to drop that far, but it is still a lot less than the original and may not be a significant problem with braking. This points out the importance of choosing correct springs and damper lengths. As another matter of interest, the moving dots at the road level show the approximate change in trail as they indicate the steering axis intersection with the road and a vertical line from the axle to simulate the tire contact point. (That moves around in road use of course.) On the Brampton video, it is possible to see approximate change in wheelbase as the front and rear articulate, though that relationship is hard to quantify exactly on the road.

At the top of the arcs both geometries begin to be a bit telescopic, JE a bit more than DD. If you go back to the loooong thread on modified steering head there is a video (around page 19 ?) of what I think the Goodwood bike geometry is, and it is almost entirely telescopic and linear, showing what is possible by moving the pivot points.

The real takeaway seems to be that from what I can see the original axle path moves forward for most of its arc, and only goes close to vertical just before full compression. Both new geometries solve that nicely.

 

[timetraveller]

Thanks. Tremendous stuff!

 

[kettlrj]

When I was in my early 20's (50 years ago) I had a couple of tank slappers on my first Rapide and so I made a line drawing of the Girdraulic forks which plotted the wheel movement. It seemed to me at that time that the path of the wheel was not as it should be, as has since been shown to be the case. I concluded that although the idea of the eccentrics was a good way of being able to offer a fork geometry that was suitable for both solo and sidecar use, it was in fact a compromise solution that did not really suit either situation, hence the need for the short top link to make the fork fully acceptable for sidecar use. I wrote to Phil Irving at the time expressing my doubts about the design of the fork and received a rather gruff reply that there was nothing wrong with the geometry and that many bikes were used both on the road and being raced with no problems. Unfortunately the letter was lost years ago. At the time I did not have the facilities to consider making a different stem and then years of working away in different parts of the world put the idea out of my mind. I eventually fitted my Shadow racer with a front damper unit that was converted to take a spring, which meant that the main springs could be removed. This removed a lot of friction from the forks and made the action very free. My current Rapide is fitted with Bramptons but if I had a bike fitted with Girdraulics I would change the stem to a DD/JE type and also fit a spring over the damper and remove the spring boxes. I would think this set-up would give excellent results.
Regards Richard.

 

[davidd]

Richard,

Phil Irving was under some considerable pressure due to legal liability concerns. At one point it looked like the government might investigate the rash of crashes associated with Vincents. As a result he "doubled down" on the Girdraulics being perfectly fine. In fact, he explained his problem to the late Sid Biberman and asked him to write and submit an article to MPH that stated that the Girdraulics were fine if properly maintained. Sid did so and the article was published and became part of the mythology. Apparently, the investigation was not a priority for the Government and the potential legal issues subsided.

The Girdraulics had been type tested as required for vehicular road use. Ted Davis and another rider put 10,000 miles on a set to prove the forks. They took turns riding the bike and apparently had a local course laid out so they could run off a precise number of miles to document the testing. I suspect the course simply failed to present a situation where the forks would display the bad behavior. I doubt it was intentional, just pragmatic.

George Brown was stunned at how poorly the Girdraulic forks worked for him on Gunga Din. He had crashed at Eppynt with the Bramptons on Gunga and received some serious injuries when a young girl ended up on the track. When he was healed sufficiently to get back on Gunga, it had the prototype Girdraulics installed. George had a horrible time controlling the bike and complained immediately after the race to Cliff Brown and Cliff promised to reinstall the Bramptons. Neither one told Phil Irving or Phil Vincent. It seemed that they were both concerned about anyone questioning George having lost his nerve.

I suspect what happened instead was that someone found out that the handling of the Girdraulics would calm down considerably simply by shortening the front springs. I found that almost all the photos of the racers showed short springs that allowed the lower link to be parallel to the ground or lower. As I found out with the short springs I made, this would have been a good compromise. Ron's diagrams show that if you limit the Girdraulic to the area near full compression the axle path is very good. I think that was the "insiders" solution. However, we know that George never used Girdraulics on any special that he rode after he left the Factory.

I would also agree with you on the eccentrics, but I do not believe they have any impact on the poor handling problem. The eccentrics are fine, except on the one point you make: they did not reduce the trail enough to make the Girdraulic steering as light as it could be by getting the trail closer to zero. Thus, the need to introduce the short top link for sidecar use. The Girdraulic would shorten the trail for sidecar use, just not enough for easy steering.

That leads to a potential problem today. Sidecars rigs will experience the same handling problems and reduction in braking that the solo bikes do with the stock steering stem. However, if a sidecar rig is switched to a short top link, there is a possibility that the the new trail of the modified steering stem when added to the old reduced trail of a short top link, could add up to less than zero, or negative trail. This would not be desirable. So, there is an issue with mixing new and old parts with sidecars.

This is a problem because I don't think that the Factory made any short top links. Thus, there is not "standard" length for a short top link. Different owners made different batches that varied in length. When you consider that we have become use to using "concentrics", which reduce the trail and that few owners use or think about using sidecars, it would not be surprising that someone could assemble some parts that may have some unknown consequences. Whe Jim Young made his stem based on my drawings, Ron checked the trail of both the short top link with the new stem and made suggestions accordingly. This may be a problem of having too many choices!

David