• Welcome to the website of the Vincent H.R.D. Owners Club.

    Should you have any questions relating to the Vincent H.R.D. Owners Club, or Vincent H.R.D. motorcycles in general, please contact Graham Smith, Hon. Editor and Webmaster by calling 07977 001 025 or please CLICK HERE.

    You are unrecognised, and therefore, only have VERY restricted access to the many features of this website.

    If you have previously registered to use this forum, you should log in now. CLICK HERE.

    If you have never registered to use this website before, please CLICK HERE.

FF: Forks Modified Steering Stem


hadronuk

Well Known and Active Website User
VOC Member
I am not sure a longer shocker is really needed for the front, especially with this new stem modification. If the front end hung lower with the front jacked off the ground this would achieve nothing, as the position of the lower link needs to be about level or slightly pointed upward with the rider sitting on the bike. This is a primary limitation with the front fork design.
Why do you say "the lower link needs to be about level or slightly pointed upward with the rider sitting on the bike."? I agree that with the standard geometry there is clearly a watershed point which if you are on the wrong side of when braking can result in the forks jacking up. But I am not at all convinced the angle of the lower link is the sole determinant of this watershed point. Load and spring rate are clearly factors and there are large forces in the top link as well. I thought the new geometry has already been proved to eliminate or at least drastically reduce the tendency to jack up under braking? In which case the lower link position may be less critical and the useable suspension travel could be increased to improve comfort and road holding. If your view is based on experience of new geometry forks, then I happily bow to that experience! Even the PhDs with their super computers at Imperial and Cranfield were only partially successful in theoretically modelling bike dynamic behaviour, so what do I know.
 

hadronuk

Well Known and Active Website User
VOC Member
I am struggling with all this information and had decided to wait till a definitive conclusion has been reached but am I right in thinking you meant "extension" not "compression" in the above statement?
I did mean compression, but I may have misunderstood Bill and I can't find the original quote in KTB! I assumed "the forks move too far up" meant relative to the rest of the bike, rather than the bike rising on the forks.
What's your view on the matter Vibrac? As a racer, you must have far more experience of Girdraulics under extreme conditions than us ordinary road riders.
 

hadronuk

Well Known and Active Website User
VOC Member
Very interesting Rob, But Know thy beast thinks, If the forks move too far up, To where the trail is smallest !!, You could have trouble !.
Cheers Bill.
P.S. I was thinking of putting an inner valve spring under the shroud of the standard front damper, On the Comet, Just to give it a bit more springyness, What do you think. Cheers Bill.
Wouldn't your spring when fully compressed reduce available suspension travel and also bottom out with a bang? As I said, I think my original Vincent damper (with the seals and valve mods done by Don A.) does work pretty well on the front, so if I had the time and energy I would try two mods. A small conical rubber bump stop, because although the hydraulic bump stop is theoretically superior, in practice it acts over a very short distance, hence a big thump. Secondly do the Paul Ennis mod of adding a gas bag, but instead of using fragile bubble wrap, use a piece of closed cell plastic foam, which is what many manufacturers including AVO use.
 

timetraveller

Well Known and Active Website User
VOC Member
First of all many thanks to both hadronuk and chrislaun who have given me lots of feedback and of course Greg, who went even further with his additional modification with the use of needle roller and ball race bearings. As far as I am concerned the only thing left to be determined is the spring strength. I have been trying to keep the tests and measurements I am making between hadronuk and chrislaun and myself private until they are finished but it became clear two days ago that there were limits which had been reached. Two things are involved here. One is trying to make the front wheel movement more like that of a bike equipped with telescopic forks rather than the girdraulic path, see MPH for October if you need a reminder. The John Emmanuel modification does that but only if the front forks are not allowed to drop too far down, Additional to the paths published in MPH I did further tests without a damper, which allowed the front wheel to drop about an further inch below the lowest point on the published curves. The first half an inch of this additional movement is more or less vertical and then the lowest half an inch the wheel is starting to move backwards. Only by about 0.1 inch but backwards. Remember that what is trying to be achieved here is that under braking the wheel will only move upwards and backwards which allows the suspension to still do its work while with the standard set up the wheel moves as far back as it can as the bike rises up over the front end and then the suspension is effectively locked until such time as the brake is released and the wheel can move forwards. Noting the above I had thought that perhaps it would be possible to use that extra half an inch of movement, where the lowest movement is essentially vertical, to give a bit more comfort at the front end. However, what I discovered two days ago is that with the new AVO damper and long eyebolts, and the front end allowed to drop as far as it can then in my test bike before the damper is fully extended the inside rear of the top link is hitting the shroud on the damper and preventing the last one or two millimetres of extension. A damper with a smaller diameter shroud would allow a little more movement but we do not want the front end to drop too far or the wheel will get to the part of its path where it has to go forward, albeit only by about 0.1", as it goes upwards. I can get round the touching of the damper shroud be the upper link by grinding a couple of millimetres off the rear of the upper link. I have seen upper links so treated before but cannot remember where or when.
So my provisional conclusions are that if one want the best handling and comfort one needs the steering head modification, the new AVO front damper, long eyebolts, shortened front spring boxes ( if one want to get the maximum possible travel) and ensure that when the bike is sat on its wheels the front of the lower link is just lower than the rear and once the rider is on the bike the front of the lower link is just above the rear. As far as I am concerned all that remains is to determine what are the best compromise spring strength. Chris is using 36 lbs/inch with 3" of preload. The Lighting racer that I help with is using 30 lbs/inch springs, also with 3" of preload. I have ordered a new batch, ten of each, of both strengths of spring so that people can try them and they should be with me any time now. These will be sent out to both Chris ( the weaker ones) to see if they are strong enough for road use and both types down to Greg who can get different riders to try them out so that we can get feedback from different weight riders on different road surfaces. I would like to think that by Xmas or thereabouts all the result will be in and then those who want better handling and more comfort can go ahead while those who prefer the standard look can cease to have to read about this. Cheers chaps.
 
Last edited by a moderator:

vibrac

Well Known and Active Website User
VOC Member
I did mean compression, but I may have misunderstood Bill and I can't find the original quote in KTB! I assumed "the forks move too far up" meant relative to the rest of the bike, rather than the bike rising on the forks.
What's your view on the matter Vibrac? As a racer, you must have far more experience of Girdraulics under extreme conditions than us ordinary road riders.
No I must admit (I think I have before) that I am still at the half a inch of broom handle up the spring box stage (ie 1965) and I cant contribute much, our racer would have Bramptons on it if it were up to me but Ben gets on well with Girdralics, long eye bolts and 18 inch wheels. He has got an AVO damper on the girdralics but that was long before your efforts for a dedicated one. Its a car unit I squeezed into place. But the point is Cadwell and the like are smooth surfaces . public roads are not -not since they started using puny diesel rollers rather than a few tons of steam rollers. No I eagerly await the results of all these perambulations for testing on the cooking Comet.
 

greg brillus

Well Known and Active Website User
VOC Member
Perhaps its time for some to find out for themselves ........I've said enough on the subject now ........Without sounding like a smart arse, I Achieved exactly what I wanted to with the racer, and it works equally as well on road bikes, Oh and I probably spend just a little more time working on these bikes than most.
 

vibrac

Well Known and Active Website User
VOC Member
That would be a great Christmas present (getting the details :)not you stopping:oops:) can you direct me to the details of the steering head modification itself I am sure its on here somewhere.
 

hadronuk

Well Known and Active Website User
VOC Member
Erm, I am rather embarrassed to be asking this question so far into this discussion, but what are all the intended benefits of this mod and how are they being achieved? I have not asked earlier because from the outset I could see that this mod was likely to improve behaviour under braking, and that alone was good enough reason to use it, at least for racing. But what is the nature of the general (none braking) improvements to handling and what is the theoretical reasoning on how this mod achieves them?
 

greg brillus

Well Known and Active Website User
VOC Member
The absolute primary reason for this mod was to try and eliminate "Tank Slappers", and basically turn the bad handling characteristics of Girdraulic's into the tried and trusted good behavior of the Brampton's. Because the links on the Girdraulic's are longer, more travel should be possible, and you have the benefit of using a good modern spec shock absorber which the Brampton's do not have (without fitting some form of coilover up front) and by using better springs suited to this new set up, the ride should be much softer, and not jarring your arms/hands with every bump on the road. As I said earlier, After we did this mod to Neal's Comet, we both agreed that the ride comfort and handling felt superior to the Brampton's. What more could any Vincent owner want.
 

timetraveller

Well Known and Active Website User
VOC Member
Rob, the way that I see this is that if one imagines the bike moving forwards on the road and hitting a bump then two forces are involved. One is upwards due to the road surface change in level and the other is rearward (think parallelogram of forces). With the standard set up the first one and a half inches of movements are upwards and forwards, by half an inch, depending upon exactly where the suspension was before the impact. Thus the wheel while trying to be forced upwards and backwards by the road surface can only move upwards and forwards. I am not saying that this is the cause of speed wobbles. When I had my major 'off' at Cadwell Park many years ago it was a twitch in the handlebars as the bike went over a repair in the road surface on the long straight that turned into a major tank slapper. I was not braking. No great change in height was involved although clearly there was a small change which I had been able to feel every lap before I crashed. However, the front Vincent damper had leaked but everything else was in good order.
The mod also reduces the trail when the bike is at rest and that will be what people were trying to do by using concentrics with the standard set up. This probably makes the bike more 'flipable/easier to lean into corners etc.) but it does nothing to change the general path of the wheel movement.
I am a great believer in real world experience. Whatever the theory if people find it better then fine. Hopefully, once we have the spring strengths finalised more people will be trying this and if they are unhappy I am sure that we will hear about it. So far all feedback has been wholly positive and about twenty five of these have now been sent to people. How many people have actually used them yet I do not know but once the new springs are finalised I would expect the number able to give feedback to increase rapidly. For those who care about the bike looking standard then it is worth while looking at Greg's photographs and deciding whether the look of the bike is seriously different.
 

hadronuk

Well Known and Active Website User
VOC Member
Thanks very much gentlemen, you both make a lot of sense. I hope you don't mind if I ask some follow on questions that I hope are useful to the discussion?
Both my pull-down spring mod and this mod are designed to reduce the probability of tankslappers, so allowing softer springs to be used safely.
But as tankslappers are thankfully very rare anyway, how can we know if we have succeeded?
Is there anything in the handling of the revised geometry bike that lends support to this view?
I can see that reducing the amount and the variation of trail has improved the responsiveness. Is the steering also lighter at very slow (near walking) speed? That is something I would particularly appreciate, as it is embarrassing how much I weave and wobble!
I fully buy the argument that better & more predictable geometry allows softer springing with the concomitant benefits for grip as well as comfort. Colin Chapman conclusively demonstrated it! But we are still up against the basic physics that the shorter the distance over which a impact is absorber, the higher the average resultant force. Careful tuning of spring-dampers helps enormously, but all things being equal if you reduce suspension travel the bump transmitted to the rider must be greater.
So every mm of travel gained is of benefit, but I am unclear if there is still an intention to test with a longer damper?
As I said earlier, I think I can make a case that the bottom link horizontal position may not be the watershed it is always assumed to be. If I am right, not testing with a longer damper might be a missed opportunity to create a mod that is even more widely appreciated.
Slightly off topic: Norman, can you recall if the Cadwell road repair was across or parallel to the track?
 

davidd

Well Known and Active Website User
VOC Member
Erm, I am rather embarrassed to be asking this question so far into this discussion, but what are all the intended benefits of this mod and how are they being achieved? I have not asked earlier because from the outset I could see that this mod was likely to improve behaviour under braking, and that alone was good enough reason to use it, at least for racing. But what is the nature of the general (none braking) improvements to handling and what is the theoretical reasoning on how this mod achieves them?
It is still a good question. The short springs I came up with were a quick work around for the wobble problem. But, the wobble problem is really a braking problem also: with a large amount of pre load on the springs the braking and the up and down motion of the Girdraulic forks is linked. This linking is not safe. It is something that if known, any good engineer would re design the system. It was most obvious when racing and also with the advent of disc brake conversions. However, I would argue that this problem has caused many street accidents, so many, in fact, that the Vincent has become rather infamous for its propensity to wobble. The Egli and Norvin seem to be obvious attempts at a complicated work around.

If changing the pre load to zero could keep the the forks going up and down during braking, I thought that changing the axle path could do the same. I found on my wood mock-up that moving the eccentric spindle lower did exactly that, severing the up and down motion from the brakes, meaning that the Girdraulic forks no longer seized while the front brakes were applied.

As I saw it, the modified stem puts the Girdraulic back to a proper design. This means that the first work around, the short springs with no pre load, could be changed back to springs with pre load when paired with the modified stem. This would be (and is) a great benefit because you can then use all the travel of the Girdraulic without fear of wobbles in order to get the best ride not just the small portion of travel provided by the short springs. This is the tuning that is happening now. I think of the choice of eccentric or concentric to be part of that tuning also.

I do think that one or the other of these mods, short springs or new stem, helps you avoid the possibility of bad handling that can cause an accident. You can ride without incident on the stock Vincent for long periods of time and maybe forever. However, knowing that the stock system is potentially dangerous can be a stopper for some of us. Wobbles are always a surprise and unexpected. This mod goes a long way to increasing that often untapped stability that should be on reserve in a properly engineered system.

David
 

Chris Launders

Well Known and Active Website User
VOC Member
I can confirm that the steering is lighter at very low speeds (no longer "nervous" when pulling away) and there is far less tendency to drop in at low speeds when taking corners etc, it is more stable at all speeds I've been to so far (just about 3 figures) and can easily be ridden hands off.
It really does feel more "planted" and stable.
Chris.
 

MartynG

Well Known and Active Website User
VOC Member
Thanks very much gentlemen, you both make a lot of sense. I hope you don't mind if I ask some follow on questions that I hope are useful to the discussion?
Both my pull-down spring mod and this mod are designed to reduce the probability of tankslappers, so allowing softer springs to be used safely.
But as tankslappers are thankfully very rare anyway, how can we know if we have succeeded?
Is there anything in the handling of the revised geometry bike that lends support to this view?
I can see that reducing the amount and the variation of trail has improved the responsiveness. Is the steering also lighter at very slow (near walking) speed? That is something I would particularly appreciate, as it is embarrassing how much I weave and wobble!
I fully buy the argument that better & more predictable geometry allows softer springing with the concomitant benefits for grip as well as comfort. Colin Chapman conclusively demonstrated it! But we are still up against the basic physics that the shorter the distance over which a impact is absorber, the higher the average resultant force. Careful tuning of spring-dampers helps enormously, but all things being equal if you reduce suspension travel the bump transmitted to the rider must be greater.
So every mm of travel gained is of benefit, but I am unclear if there is still an intention to test with a longer damper?
As I said earlier, I think I can make a case that the bottom link horizontal position may not be the watershed it is always assumed to be. If I am right, not testing with a longer damper might be a missed opportunity to create a mod that is even more widely appreciated.
Slightly off topic: Norman, can you recall if the Cadwell road repair was across or parallel to the track?
I had a massive Tank Slapper on my Comet when going through a very gentle sweeping bend on a country road at around 60 mph. Result was much damage to bike and self.

There was no braking involved. Post incident examination of accident site revealed road works that had left a circular mound on the road surface, about 3 foot in diameter with its crest about 4 inches above the surrounding road surface. This 'hump' was all but invisssible in the dappled light. Chap who was following me said the bike was tracking just fine then it suddenly became airborne (when I hit the mound) and on landing all hell broke loose. I remember the onset of the head-shake and thats it - I regained consciousness 20 minutes later! This is something I do NOT wish to experience again.
 

davidd

Well Known and Active Website User
VOC Member
Martyn,

I do remember your accident and the photo of the mound. There are some, like you, who have experienced wobbles with no braking involved. I have said this before, that the mods probably wont cure all wobbles, but these mods seem to cure the wobbles due to braking. These stories often occurred when Vincents were crossing railroad tracks and were many times under light braking.

I can only imagine that the circumstances that you faced would have caused any number of bikes to crash. Hitting a large bump while cornering means that the suspension is less in play and the stability of the bike can be easily upset. I was in a corner when a Harley hit my front wheel at Daytona and I woke up 4 hours later.

David
 

timetraveller

Well Known and Active Website User
VOC Member
This is a response to hadronuk #51 above. My memory is that the repair was neither along the line of the road nor at ninety degrees to it but at an angle to a ninety degree line; possible something like 10 or 20 degrees to such a line. However it is a long time ago and I was unconscious for some time.
With regards to the amount of travel and more being better. The problem is this. Consider the top link only. As the forks lower this tilts down at the front until at some stage the inside of the front of this link touches the shroud of the damper. Removing the shroud would prevent this. Now consider the lower link. As the forks lower this tilts down more and more at the front. Now imagine a curve which is the path of the front of that link. Once it is below the horizontal then that path is starting to curve towards the back of the bike. If it was possible for the link to drop a long way down then any upwards impact on the front wheel, a bump in the road, would not only have to try to move the front of the link upward but also forwards. It seems to me that once the bike is loaded then the bottom link should be as near to horizontal as possible with its front just above the horizontal rather than below it. The question now is whether a longer damper, with more travel, could be combined with springs of such a strength that enough of that travel was used up when the bike was loaded to allow the bottom link to get to its horizontal position. That might mean softer springs or less preload. If the links were longer then this would all be easier but then all sorts of problems with trail would occur and a whole new can of worms would open up. So far as I can tell by playing about with a front end set up on a work bench and clamped so that the base of the oil tank is horizontal we are not going to be able to do much better than the standard length AVO, which limits the downward angle of the lower link to about half an inch lower at the front, and some soft springs, which, when the bike is loaded, allow that link to rise at the front so that it is horizontal. I need the results of tests with different riders with different spring strength to know where an optimum is. For example, it might be that a longer damper with softer springs might still allow the lower link to get to the preferred angle and allow the maximum possible movement of the wheel. This would also include the compression of the bump stop in the damper, which can give an extra half inch of compression in the damper. The springs have to be strong enough to prevent anything bottoming out and as the bikes can be subjected to a range of loads the best that we can do is to find a compromise that suits the largest number of people. At the moment my GUESS is that the 30 lbs/inch springs might be correct for Comets and stripped down racers and the 36 lbs/inch spring correct for the rest of us. If it transpires that softer spring will work on the loaded twins then it might be that the longer damper and softer springs is the correct combination. The upcoming tests will tell us.
 

Neal

New Website User
VOC Member
I have been following the comments regarding the modified steering column and have found some of the posts may be from members who may not actually understand what the modification has achieved.
I have been in motorsports all my life and have designed and built many vehicles and have learnt sometimes somethings may not be able to be easily explained and sometimes it may be better to have an open mind and just accept that a certain design or modification has improved or eliminated an underlying problem.

I am the owner of the Comet that has recently had the modification fitted and I also own a Brampton Series B that I have been able to use as a reference. The Series B has always handled better and has felt more supple when riding on rougher country roads and this is what convinced me that the Series C Girdraulic's could and needed to be improved.

With the modified steering column the linkage geometry has been improved and has allowed much more suspension travel to be achieved without the fear of the the front end binding up under brakes or the linkage causing problems under full extension when going over rolling type undulations in the road at reasonably high speed.
When the modification was first done we simply shortened some non original springs that Greg had to achieve the desired ride height and took it for a test ride . The ride improvement was immediately evident and has been incredible in all conditions and I believe it will improve even more when we test with a softer spring that will give an even smoother ride over the 25 to 50 mm bumps on most of our roads.

As I am a newby and don't know who actually designed the modified steering column I would like to take the opportunity to thank all that have been involved with the design, manufacture etc and testing.

Neal
 

greg brillus

Well Known and Active Website User
VOC Member
Thanks for that Neal........Norman must have been reading my mind as that was exactly what I was thinking. Do we have any info from John as to what he came up with regarding Shock absorbers and front springs, given he has had more experience with this set up than anyone, perhaps he has come up with his own improvements to the design that we are unaware of.
 

Chris Launders

Well Known and Active Website User
VOC Member
The previous owner of my twin woke up in hospital after what turned out to be a tank-slapper, all the marks showed it had started at a sudden dip in the road surface on a bend.
 

Top