FF: Forks Modified Steering Stem

greg brillus

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Hi there to all interested in this subject, just wanted say that I have just installed this kit, along with the change from the large lower link eye bushes to sealed bearings, to a friends Comet. In fact it is the bike belonging to Neal Lowe who lives not far from me here on the Gold Coast, and has a write up about his two bikes in the latest MPH. So he has had a taste of the difference between the Girdraulics on his Comet verses the Brampton's on his "B" Rapide. Well the conversion process went very well, only hold up was getting the link eye's honed out to suit the 32 mm OD bearings, but that was not too difficult, and I soon had all 4 bearings pressed into the link and installed on the new stem, which we had fitted the day before. I ordered one of the new Kawasaki type steering dampers to be installed in the next few days. I had a small batch of attachment brackets made as per Norman Walker's design from 6 mm 6061 alloy, and these bolt straight on with two 6 mm Allan headed cap screws. Ok the final part was what springs to use, and I had already had some 40 Lb springs made at two different lengths, but these were too long, and even the short ones when installed in the front of my Rapide, was topping out and I could not press the suspension down at all...!!!! In the end on my bike I used a pair of David's springs of the red version which are 75 Lb's each. These work quite ok, and still probably a bit heavy still, but not unacceptable. Ok .....Back to the 40 Lb springs.......We decided to use these and chop them down in increments and see what happens, first we cut 25 mm off and tried this, but it felt too heavy, so we chopped another 25 mm again, and this felt quite good. The right side spring box felt very harsh, so I removed it and thoroughly washed it out to remove any grit and old grease, reassembled, and refitted the front wheel. We hooked up the front brake cables, adjusted the brakes and tightened the front axle. The front end felt very good, and dived with the front brake held firm. It was time to go for a ride......It fired up first kick, and I took off up my drive way, and off down the hill and around the block. The bike felt absolutely fabulous, the font end rising on acceleration as I changed up through the gears, followed the bumps and divets in the road with a nice soft ride, no harshness at all, back off the throttle and touch the front brake, and the front dives nicely....Not too much, but just as it should. I returned up my street where Neal was waiting for the news......I handed him my helmet and sent him off for a test ride. When he returned he could not believe the transformation, and he agreed as I did, that it actually felt better than the Brampton's on his "B" Rapide. All in all I felt very happy with the spring choice, though I will try some of the ones Norman has made up in the front of my Rapide, as mine still feels a bit stiff. The springs really take some experimentation to get right dependent on the rider and the bike itself. But by todays efforts, I can honestly say that Neal's Comet feels better than any other I have ridden, and I think from Neal's reaction that he would totally agree. Now he has plans to upgrade the rear suspension as well, and as we know these things can be a work in progress. But given this is the first Comet I have carried out this modification to, I can say with absolute confidence that it is most definitely worth the while. Big Thanks to all who contributed to this outcome, especially Norman and David. Cheers for now...............Greg.
 

greg brillus

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Tim I'm sorry that my efforts to put photos on here are not good, but I can email some to David, and he will kindly post them on here. I am not trying to promote this set up based on the fact that I spend a lot of time working on these bikes, it has come from a keen interest in trying to improve the suspension full stop. Obviously from the changes I made building the racer, it was easy to do the same though a much less modified version to my road going Rapide. Perhaps the Comet felt so good on account of it's relative light weight verses a twins extra 30 Kg's, I know that Comets generally feel more agile with better brakes and so on, so perhaps this combination coupled with the changes to the geometry with the new stem just gives the bike a totally "Better feel" to the front end. I know that when talking to Neal, he said that by comparison to the Brampton's, he did not feel the Comet's suspension felt good at all, remembering this guy has spent his whole working life around race cars and he understands suspension very well. But I could tell by the time I had ridden the bike to the end of my street, that the change was quite amazing...... All of a sudden the front end was working exactly how it should.......And not a steering damper in sight.........:).
 

davidd

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Greg,

That is great news! I thought it would take some effort once the mods were in place to select the best spring rate. It was quite encouraging with Chris' reports that lighter springs were working well, because I always thought that an overall lighter rate could improve the ride quality. I had always used the stock Vincent spring rate at full compression as a benchmark for the new springs even though I realized that it might be too high. Unfortunately, that may have skewed the range to be too high. I think the data you are generating will be a great aid to pegging the correct range of springs. Starting with the Comet is good as it establishes the lower limit of the range.

It was always a goal to get the handling to be at least as good as a B. It turns out that the handling of the Girdraulic can be better due in great part to the damping and the relative stiffness of the Girdraulic compared to the Brampton. I do remember making the change on my racer. I went out at Miller with the hope of watching the front end and noting its movement as I had done so often in the past. I returned to the pits and realized I had not glanced at the front end even once. I tried many times to focus on the movement which was so obvious to me before and I was never successful. I could only conclude that the movement I was used to looking at had gone away and the the mods had made a "calming" effect on the movement. I think that this was a result of the brake no longer causing up and down movement of the fork, which had been so apparent.

Just on handling in general, I would mention to those who are interested in mods that the use of 18" wheels on a Vincent really "wakes up the handling" as Carleton Palmer said when he changed to 18" rims. I have mentioned it before, but it has had such a profound effect on the handling of our racing singles that I did not want to be thought of as hiding the results. I see so many racers using 19" wheels I am not certain that anyone else has tried this out.

David
 

vibrac

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Ben has always used 18" wheels, I have them on my cooking Comet but only because I needed good trial tyres for the trials outfit
Perhaps I am wrong about 20" giving best steering :oops:
bencorner.jpg
 

bmetcalf

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An 18" front would really throw off speedo accuracy, are there alternate ratio gears that would fit or can a chronometric be adjusted?
 

davidd

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Ben has always used 18" wheels, I have them on my cooking Comet but only because I needed good trial tyres for the trials outfit
Perhaps I am wrong about 20" giving best steering :oops:
View attachment 15307

Tim,

That is good to know! I am sure that a good rider like Ben has used that mod to his advantage many times. I think it is hard to compare the street bikes to the race bikes, but I noticed on the street bikes that when you do something aggressive, like quickly changing the line in a corner to avoid someone or something, the stock bike will respond with a "Yes, that is theoretically possible." The racer would shout "Do it faster next time!"

I would note that John Renwick was often using 18" wheels front and rear on his racers.

Bruce, I would have to look, but were the D's off that much?

David
 

bmetcalf

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I think my Series C with a 19" front was approx. 10 mph fast at a true 70 mph. My 21" is spot on, based on a bicycle speedo I jury-rigged.

If the bicycle unit was right, a 19" reads 77 and an 18" reads 81 at a true 70, assuming similar tire aspect ratios, using circumference ratios.

The upside for me is bragging rights to fast riding speeds without the risk. :cool:

Bruce, I would have to look, but were the D's off that much?

David
 

timetraveller

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Thanks Greg for the write up. Regarding the springs; these are now ordered and should be with me next week. I have ordered 10 off of the 30 lbs/inch and 10 off of the 36 lbs/inch. Chris Launders has been using the 36 lb one on his road bike for several months now and they are clearly not too strong. On the Lighting replica we raced at Brands and Goodwood earlier this year I fitted 30 lb springs and on that bike they were about right. We measure the total movement of the spring boxes by fastening a cable tie around the inner spring box at the level where the bottom of the outer spring box is and then see how far down it is pushed after using the bike over bumps at speed. Chris is getting about 2.5" of movement. He has shortened his spring boxes, inner and outer, by one inch to allow for more movement. Note that with the new set up the distance from the top to bottom fixing for the spring boxes is now one inch less than the standard set up so the shorter boxes do not show up as different. I have tried my own spring boxes and the inner will move fully up into the outer and is stopped at the top. So on these particular spring boxes the flared lower section is not doing anything and therefore in my case it would be possible just to cut off the top of the inner and the bottom of the outer.
Feedback on this project is the best way to ensure that anyone who wants to can benefit from it. So far the best feedback I have had is from Chris on his road going twin and Greg with his light weight racer. The extra information on the Comet is very useful. Once I have the new batch of springs my intention is to send two of the 30 lb ones to Chris to try on his road twin. My suspicion is that they will be too weak but we will see. I will also send out two 36s and one 30 to Greg and others who are prepared to road test them on twins. It is possible that one 36 and one 30 will be about right, or even two 30s. My guess is that on Comets two 30 will be about right but the springs might need to be shortened. At the moment the new springs are made to be 16.5" long which will give a 3" preload. This is to ensure that the ride height and the angle of the lower link is correct. That is that the front of the lower link is just pointing upwards when the bike is fully loaded with the rider on board. On the Comet, once you have the lighter springs I would suggest trying them out full length and then if they prove to be too stiff, cutting of half an inch progressively until the preload gives the correct angle at the front.
This now leaves the damper at the front. I lent Chris one of the new AVOs and at its lightest setting it seems to be superb. However, during this last week John Emmanuel has been in touch about the damping and other matters and it transpires that I, at least, do not understand damping. The new AVOs, even on their easiest setting are still only just moveable by hand. That is as delivered, so whether they free up a bit with use I don't know. Yet John, using a Thornton damper on the front found that about right. Now John has sent me his Thornton damper and it is very light on compression, about the same as my old Armstrong or Vincent dampers. John believes that a light damping is required. Chris finds the relatively firm damping of the AVO gives superb control. Clearly there is more to damping than I know about.
There is one final thing about dampers. Rob Staley (Hadronuk) who has done most of the development work with AVOs for their new Vincent dampers says that there is 3" of movement on their front dampers. This total movement is important as it affects whether short or long eyebolts should be used with these dampers. So far all Chris' test have been done with long eyebolts.
What I am trying to achieve here is that following John Emmanuel's development which improves the handling I am trying to find what spring strengths and lengths will give the maximum comfort and movement at the front end. Laney Thornton did this years ago for the standard set up but the modification to improve the handling deserves to be developed to give more comfort as well. Watch this space.
 
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greg brillus

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Yes on both my Rapide and the Comet yesterday we used longer eye bolts, so the front end hangs lower on full extension without the spring box's in place. Using the original Koni shock unit on the Comet, it has around 60 to 65 mm of travel, and once the spring box's were in place and the bike on level ground with the rider seated it was showing about 1/3 rd of it's travel used, which I feel is about correct. Before attaching the spring box's we lifted the front end up to full compression and we concluded that it would take one hell of a bump for the suspension to move that far, but whether the spring boxes or the damper bottom out in travel first I don't know. But with Neal sitting on the bike the lower link was pointed upward at the front by about 5 degrees at the most. I will also try and show some pictures of the rear set up on my Rapide showing the Thornton coilover, and the re fitting of my original spring boxes in place using telescopic tubes inside to replace the springs. This is purely a visual thing, but it does improve the look of the rear I feel, even though some might think it not necessary.
 
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