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FF: Forks Modified Steering Stem



greg brillus

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VOC Member
#1
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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VOC Member
#3
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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VOC Member
#4
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

Well Known and Active Website User
VOC Member
#5
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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VOC Member
#6
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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VOC Member
#7
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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VOC Member
#8
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

Well Known and Active Website User
VOC Member
#9
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

Well Known and Active Website User
VOC Member
#10
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.
 

davidd

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VOC Member
#11
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.
From memory the Thornton damper I tested had 2.85" of movement to the rubber snubber. The snubber could only yeild about .25" once it was contacted, so that would be the total travel. The damper was the intended limiter of the movement of the Girdraulic as far as I could devine.

Thornton originally instructed owners to have 30% sag, but later changed that to 50%. I was always puzzled by this because there is no physical way to adjust sag on a Vincent, which was why I opted to make shorter springs that could be adjusted up to set the sag. It has become easier to think of sag in terms of the angle of the bottom link when loaded. Of course, the new steering stem positions the back of the lower link, that is the end with the large eyes, lower, making the back of the lower link resist rotating over the front spindle during braking. I would think that the new soft springs should err on the side of being shorter rather than longer for the same reason. Spacers can always be added. I chose 14" for the distance between spring platforms in the spring boxes, but I found it varied on different bikes. I think I measured 15 inches on a D once.

Thornton also recommended that the short eyes be used on the lower link to hold the damper due to the damper's longer reach than the Armstrong. Greg was aware of this, but it does not seem to be an issue with the new stem. It will be interesting to see if the longer eyes prove to be more useful with the new stem or if they both might be used for some fine tuning of the ride height or damper throws.

Thanks to Greg, Norman and Chris for doing all the juggling!

David
 

greg brillus

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VOC Member
#12
The changes that occur when the stem kit is installed are actually more than meets the eye. At first it seems that the short eye bolts would be fine, and in reality, it is really only altering the shaft position of the shock absorber.......But that is not actually the whole story, when the longer eye bolts are used the forks hang lower, meaning that more of the lower spring box is exposed. This means that when you install the springs the preload is different......That is, if the short eye bolts were used the preload would be more. I know it sounds a bit confusing, but it makes sense when you assemble it all. I wanted to set up my bike and now Neal's Comet is the same......By trying to keep as much useable travel as possible, whilst keeping the lower link in the "Safe Zone." Also, the attachment point for the upper spring boxes is now further forward than standard, and this seems to be about 3/16th's of an inch, this too will alter the spring rate because the upper point is now closer to the fork leg. It is interesting to note that the springs I had made were 40 Lb's per inch, and we effectively cut 50 mm off each one, this gave the springs an overall length of about 15 mm longer than David's springs. With this set up, the ride comfort was excellent.
 

macvette

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VOC Member
#13
An 18" front would really throw off speedo accuracy, are there alternate ratio gears that would fit or can a chronometric be adjusted?
My D speedo was refurbished when I rebuilt the bike. This involved measuring the distance covered by one revolution of the front wheel under load and noting the direction of rotation and no of turns of the speedo drive on the front wheel for the guy working on the speedo. He then calibrated the chronometric using this info. My understanding ( limited), is that this is done by adding or subtracting weight in the form of washers to a rotating part of the speedo to speed it up or slow it down as necessary.
He also recalibrated a speedo I found for my 1938 Empire Star ( it was originally supplied without since they were not compulsory at that time). This would imply that the speedos can be recalibrated to cover 19" (series D) to 21 " (Empy).
Open series D speedos are different from series C in that they require a right angle drive gear box on the back because of being mounted in the head light.
Road testing against my friends 2yr old BMW 1200 RT at a steady 70 and 80 mph shows agreement within 1 to 2 mph so it's good enough for me. Don't ask what BMW speedo error is , I don't know
 

timetraveller

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VOC Member
#14
An update here re. the total travel on the new front AVO damper. Following a message from Rob Staley (Hadronuk) I have gone back and used a sash cramp on the damper to exert a serious amount of pressure. It moved a further 1/4" without too much effort so a total of 3" is correct if the bump stop is fully compressed. Sorry to have gone off on a tangent with that chaps.

The previous post that this refers to has been corrected. Moderator
 
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hadronuk

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VOC Member
#16
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.
I am surprised that the AVO front damper seemed so stiff. It may be that it needs to bed-in a little as you say.
Hand testing dampers can be very deceptive!
We dyno tested several dampers. All except the original Vincent damper had a fairly "flat" bump characteristic in that the damping force quickly reached a "plateau", then did not greatly increase.
This "plateau" is at about 25 lbs for an AVO front.
The original Vincent damper, being a simple fixed orifice design, was initially very soft, but reached over 60lbs at higher velocities. (Fixed orifice designs are not used now, because they allow low frequency wallowing but are harsh at higher speeds.)
My Armstrong had a plateau at about 11 lbs.
On the road with well bedded in Girdraulics, I found the Armstrong to be comfortable, but just too soft at any speed over less than smooth roads. The final setting for the AVO was chosen to be just stiff enough to give control, but comfort still felt as good as the Armstrong.

There is a very big caveat to be borne in mind on this subject. There is a lot of friction in Girdraulics! My test measurements showed that even a well bedded-in pair of spring cases provide about 8 lbs of damping when the forks are extended and about 24 lbs when the forks are compressed. That's just the friction in the spring cases, I have not measured the friction from the link bearings.
SO THE TOTAL FRICTION DAMPING IN GIRDRAULICS MAY BE GREATER THAN THE HYDRAULIC!
This makes it harder to assess small differences in damper settings, especially if they are not tested on the same bike.
The Thornton damper we tested was for the rear. This had a plateau at about 40lbs mid range and reached 50lbs at max velocity. This is similar to the settings of the AVO rear.
I would be very surprised if a Thornton front damper tested as soft as the Armstrong on a dyno. As I said, hand testing dampers can be very misleading.
 

greg brillus

Well Known and Active Website User
VOC Member
#17
I must admit that of the three stem kits I have fitted to three different bikes, all have had the lower link eye bushes converted to sealed bearings, and I'm sure this must reduce the friction, though I don't imagine that friction is anywhere near as high as it was with the link in a better position. I know when we did the conversion to the Comet the other day, the right spring box was very sticky in it's sliding motion, and the front end felt very ordinary, but once we sorted that out, it felt much better. We were wondering why it seams not possible to install progressive, or duel rate springs to the front of these bikes. Does anyone know why, and has anyone ever tried it......? I would be prepared to give it a go. I have more people interested in the stem kits now.
 

MartynG

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VOC Member
#18
Folks,

In the dim recesses of my memory I can recall hearing about low friction FF15AS Spring box outers. As I recall they were lined with some form of hi-tech, low friction coating. But thats it.

Am I dreaming - possible considering the meds I am currently on - or is/was there such a thing? In the search for a low friction front end they may be of benefit

Martyn
 

chrislaun

Well Known and Active Website User
VOC Member
#19
Hi Greg, I installed progressive springs in my Atlas and Norvin, they made a big difference, I think the maths involved working out what we would need are quite complex and at present just sorting out what poundage is required for the modified stems is enough.
 

hadronuk

Well Known and Active Website User
VOC Member
#20
I had to replace my spring cases as one could not be straightened sufficiently. On the test rig, the badly bent one was only producing an extra 4 - 6lbs of friction, but that was enough to give a very poor ride over smoother roads, as the forks remained stuck until a larger bump was encountered. The replacement cases from VOC Spares have a plastic lining in the larger diameter part of the outer case. These new cases in a test rig measured exactly the same as my less bent but very old and internally pitted case pair. Ride over smooth roads was much better, rough roads felt much the same. So I conclude that friction in the spring cases has a big impact on damping and comfort over smooth roads, but perhaps much less influence over rough roads. Secondly the plastic lining may be good for reducing wear, but not friction. So provided your old cases are straight, clean and well greased, there may be no functional benefit from replacing them. (Plus the dimensions of the new cases were wrong, resulting in too high a ride height. I did report this, it may be fixed now.)
There is one other possible spring case problem. Some replacement springs were right on the top tolerance for diameter. This means that as they compress, expand and buckle they can jam in the cases. My suggestion that the original spring specification was wrong received a very frosty response from one very senior and knowledgable club member, but nervertheless, I believe a new batch of springs was made.

It is probably extremely arrogant of me to say this, but I think John Surtees may have made a big mistake in fitting progressive springs to his racer, and it may have been a factor in his tank slapper and crash at Goodwood in 1998 (?).
I have always suspected but have been unable to prove that Girdraulics are very soft, perhaps dangerously soft, in the extended area of operation, but become too firm when compressed. So the last thing they need is progressive springs to increase this. Both John Surtees and Phil Irving commented that Girdraulic handling problems are most likely when the forks are extended. Conventional modern racers have in the main stopped using progressive springs in telescopic forks and now mostly use pull-down springs, which are degressive, the opposite of progressive. My standard geometry Girdraulics are certainly comfortable with soft degressive springs and I have had no handling problems.
 


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