FF: Forks Godet Modified Steering Stem

davidd

Well Known and Active Forum User
VOC Member
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:
26116

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

Well Known and Active Forum User
VOC Member
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

Well Known and Active Forum User
VOC Member
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

Well Known and Active Forum User
VOC Member
Bruce,

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

David
 

Chris Launders

Well Known and Active Forum User
VOC Member
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

Well Known and Active Forum User
VOC Member
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.
 

Attachments

  • 1551093142538.png
    1551093142538.png
    309.4 KB · Views: 17

davidd

Well Known and Active Forum User
VOC Member
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
 
Top