FF: Forks Modified Steering Stem

davidd

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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.

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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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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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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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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davidd

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Some photos from Greg:

Photo 1 Front end unladen
P1010239.jpg


Photo 2 Rear Suspension
P1010240.jpg


Photo 3 Close up of Rear Suspension
P1010242.jpg


Photo 4 Front Suspension Laden
P1010245.jpg


David
 

hadronuk

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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

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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.
 

Martyn Goodwin

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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
 

Chris Launders

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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

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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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