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FF: Forks Godet Modified Steering Stem


davidd

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I thought that it was best to make some comments on Godet's modified steering stem in a separate post because there is a slightly complicated history.

Back in 2014 I was often in contact with Patrick regarding certain aspects of the design of the new Grey Flash. In one email he said that he had given up on trying to get an 1-7/16" original carb because he could not find one, even to borrow. I told him that he needed to find one because it was critical to the project. I explained it was critical because from a competitive point of view it was a mistake to give up the extra 4 mm of choke size on a converted street bike that is competing against bikes that were specifically designed for racing. The big carb that was ordered from Amal by Phil Vincent specifically for the Grey Flash was one of the few advantages that made a Grey Flash faster than a Comet. To that end, I asked Carleton Palmer to provide Patrick with one of the original big carbs, which he did.

As an aside to one of our emails about the carb, Patrick said that Bruno Leroy had a horrible first running of the new Flash at Cadwell. Bruno reported to Patrick that the bike was wobbling and jumping into the air every time he entered a corner. Patrick stated that he had to solve this handling problem, but he did not know exactly what the cause was. Of course, I knew exactly what the cause was and told him that I could solve the problem: he was using a very powerful front brake, which caused the front end to seize under braking. This could only be cured with a modified steering stem.

I contacted Norman to see if he had a spare steering stem and he did not. He had kept one for his own machine, but at that time I think that he was not certain that there would be any more batches. It was only three weeks before the IOM TT and time was of the essence. I knew that Pat Manning had purchased two of these stems from Norman, so I told Norman not to worry, I would send one from the US. In fact, I had Patrick's UPS account number to ship the carb. I packed up one of Pat Manning's stems and sent it to Patrick.

It turned out that the stem calmed down the wild nature of the handling under braking and performed very well at the TT. This was reported on this forum and I was very happy for all concerned with the modified steering stem project. At the time, Greg Brillus had proved the concept on his twin racer, but Patrick, who was experiencing the problem, confirmed that this as a true cure. Because the stem given to Patrick was a loaner, I asked to get it back. Patrick took it off and sent it. However, I had given Patrick a mechanical drawing of my modified steering stem right from the start. My steering stem cures the braking problem like the JE stem, but the geometry is slightly different and the Girdraulic performs differently. Both geometries get rid of the braking problem.

Jumping to 2018 and I was helping David Tompkins with his racing efforts. David was not using a modified steering stem and had a crash that I thought might be caused by the geometry problem. When that happened, I sent him one of my steering stems. David was hesitant to try it and quizzed me hard about it. He wrote Patrick asking about the steering stem Patrick makes. Patrick replied:

"I have manufactured a batch of aluminum revised stems exact to David's drawing." August 8, 2018. This is Patrick's photo:
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Thus, the stem looks like the JE stem, but has my geometry. I was a little surprised by this, but pleased that the geometry had worked so well for Patrick. This stem was the same that was used on the Gallur Grey Flash ridden by Cam Donald. I posted that article on the forum here in the first post:


Because Patrick made my stem geometry look like the JE stem, you can't always judge this book by its cover. If you see what looks like a JE stem It could be a DD stem if it came from Patrick. I have given the mechanical drawing of my stem to many folks. To date, Patrick was the only manufacturer to make it. I made a small batch for my use. I spent a lot of time making the stem work the way I wanted it to work decades ago. However, both stem designs fix the problem and Norman has been very generous with his efforts producing the JE stem.

This was a slightly complicated bit of history, but it is a slightly complicated problem with the Girdraulic.

David
 

timetraveller

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Even on that photograph I cannot see the difference between the DD steering head and the JE one. Some of the angles for weight reduction look very similar so, without asking for commercially sensitive details, it would be interesting to know just how much difference there is. I just cannot see any difference.:confused:
 

davidd

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The difference is not visible at those angles. The geometry changes quite a bit as the spindle is lowered from the bearing platform. If I had a side shot of Godet's stem it would show the bearing platform closer to the spindle hole like this:
26120
The DD stem on the left and the JE stem on the right. If the spindles are lined up and you look up at the bearing platform there is a obvious difference in this distance. The greater the distance between the spindle and the bearing platform the more teloscopic the axle path becomes. The shorter that distance the path becomes more like the Brampton.

Anyway, I agree with Norman that you cannot tell the difference from a glance. This makes it more difficult to identify how the Girdraulic will perform, short of riding it.

Here is another one of my stems made by Jim Young for his sidecar rig:
26122

26123
Jim apparently wanted to use the two spools on the front for something.

There is nothing commercially protected here. I think many Vincent owners have the drawing I made. I think it is a nice stem, I am glad that Patrick loved it and Cam Donald loved it, but I am not in the retail business. Although they act differently, both stems cure the braking issue that plagues the Girdraulic.

David
 

davidd

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Bruce,

Yes. Renwick had a serious accident with his sidecar rig. It prompted him to change the geometry.

David
 

Chris Launders

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I would suspect that in normal use on the road us ordinary folk would be hard pressed to notice any difference between the two, however the difference between them and standard is immense in terms of stability and suspension improvements.
 

timetraveller

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With regards to what the differences are in front wheel movement; readers might recall that I put an image of the path of the front wheel spindle on here some time ago. It is repeated below. One thing to note, and it is my fault, but there has been some confusion as to whether these two graph can be overlaid with regards to the horizontal position, i.e. the trail. No they can't. The same set up was used for both tests but it had been dismantled and then reinstated between the two sets of measurements. It is the shape of the curves which should be noted, not where they start off.
26129
Note that the values are in inches and you can see that if the front lower spindle is up to about 0.2" below the rear lower spindle then there will not be any forwards movement as the forks rise and the bike dips. If the front is, say, one inch below the rear then there will have to be about 0.1" of forwards movement before the wheel spindle starts to move backwards. Contrast that with the standard set up where there is about half an inch of forwards movement over the first 2.5" of vertical movement hence the problem of suspension movement when the front brake is applied. If David has a similar plot then it would be very good to have a comparison.
Note that I was so impressed with John Emmanuel's fork movement that I asked him if I could copy it. This has never been a commercial exercise. John has never received a penny from allowing his design to be copied and I just about break even. If it turned out the David's design was superior then I would have no trouble making copies of that if David gave his permission. This is an exercise in making our bikes safer and more comfortable, without changing their appearance in any significant way, not making money from fellow Vincent owners.
 

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davidd

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So which modified steering stem achieves the best results, if it is in fact quantifiable.
Peter,

I have only promoted solutions where there seemed to be problems. When Simon asked about sidecar use and use of the eccentrics, I think the advice on the JE stem was initially "no" and I said mine was designed to use eccentrics or concentrics. It would do sidecar work. When Greg's FF bearings were used by Chris he was concerned that there was too little friction and his video showed lots of telescopic motion when the throttle and brakes were used. That seemed to lead to the design of different dampers. I mentioned that the problem was caused by the design of the axle path and it was not really the problem of the dampers or friction or lack thereof. It was the inclination of the JE design to have the forks move up and down. I did not see this as a problem, per se, it was a choice of design. John Renwick, for example, liked the telescopic action of the revised Girdraulics. I did not design my stem that way. I opted for the Brampton fork action. It was more vertical and had less inclination to bounce. The wheelbase and trail changed less. However, that is what I intended. As a result I did not have the bottoming problem or the fender hitting the engine at full compression.

Additionally, when Greg was installing the JE stems it seemed that some owners were making an effort to adapt the original manual steering damper. I had designed my stem to use the original manual steering damper. In fact, Jim Young contacted me after making one of my stems and said that he my damper could only accommodate the C damper and needed to be modified if I wanted the D damper with the extra disc included with the D damper fit properly. I revised the drawing to include that feature. I had also designed the damper to use a hydraulic damper like the JE stem.

I think JE did a brilliant job in identifying this problem so early. I did not know about John's efforts, but I tackled the problem with a blank slate like he did and came up with a slightly different solution. It is a little difficult to look at some of the characteristics and call them "problems" as they may not be problems for some riders. I recently gave a seminar on the Girdraulic fork and I passed around my stem and the JE stem. I mentioned all the above differences. When asked which was the best to buy I said the JE stem as I do not sell stems. The JE will take care of the problem.

I am not an engineer, but I have worked on these forks for long enough that I have a reasonably good idea about how they work and why they work the way they do. An engineer with CAD could see all of this in an appallingly short amount of time. Norman has done a great service for the owners and the JE stem makes the Vincent a much safer bike. He has been nothing but helpful, so there is no contest.

Oldhaven did a lot of work to plot the action of the Girdraulics. His work shows what is going on with the forks. The graphs don't have to be perfectly accurate. If you have been following the details and understand that the Girdraulic is a leading link fork, not a girder fork, you can see where the pivot of the leading link is located in the first two graphs. In the JE graph the leading link pivot is located very low (noted as JE_pvt). The stock pivot point (noted as "path_cc") is very high. In the graph on the right, the DD stem shows the leading link pivot in the middle of the almost vertical path. This location can be compared to the same stock pivot location, which is in both graphs, and the much lower pivot location of the JE pivot.
26140

In the views below I added lines that would show the leading link for each ( I have just copied Oldhaven's double graph that is above, twice below for ease of use.)
26141
The leading link of the JE path when compared to the stock path (in the graph directly below it) shows a complete reversal, which is good. The stock leading link goes to extension when the brakes are applied and the JE link does not, it resists extending. However it does not resist compression. When grabbing the brake it will want to bottom out.

The DD stem also resists extension under braking, but it is less telescopic. If you think of an Earles fork BMW front leading link it becomes a question of which graph you chose for how the lower link on the fork is positioned. I set the DD link pivot slightly below parallel to the ground, which was the same as the BMW.

I also selected the position that more closely reflected the attitude of the original Vincent. That is, if you look at the start and stop positions of the axle paths (for the DD it is the bottom right) They are closest to the start and stop points for the stock axle path. The JE axle path raises up the bike's front, which increased the rake, which is something I did not want.

I know that all of these details are pretty mind-numbing, but this was part the process I used in taking the decisions I chose. I was much happier to read Cam Donalds review that the handling was to his liking.

David
 

timetraveller

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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.
 

davidd

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VOC Member
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

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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

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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.



 
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timetraveller

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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

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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.
 

kettlrj

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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

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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
 

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