FF: Forks Modified Steering Stem

hadronuk

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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

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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

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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

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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.
 
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vibrac

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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

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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

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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

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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

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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

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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.
 
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