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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Godet Modified Steering Stem
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<blockquote data-quote="davidd" data-source="post: 104955" data-attributes="member: 1177"><p>Peter,</p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>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. </p><p></p><p>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. </p><p></p><p>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.</p><p>[ATTACH=full]26140[/ATTACH]</p><p></p><p>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.)</p><p>[ATTACH=full]26141[/ATTACH]</p><p>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. </p><p></p><p>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. </p><p></p><p>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.</p><p></p><p>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.</p><p></p><p>David</p></blockquote><p></p>
[QUOTE="davidd, post: 104955, member: 1177"] 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. [ATTACH type="full" alt="26140"]26140[/ATTACH] 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.) [ATTACH type="full" alt="26141"]26141[/ATTACH] 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 [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Godet Modified Steering Stem
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