• 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
One other thing about springs and ride height. Ideally suspension should be set up so that with a normal load the suspension is 30% compressed, as this should minimised topping out and bottoming out. But I think the great majority of B and C Vincents are set much lower than this. With the original springs the figure is more like 70%! Although the AVOs have a shorter extended length than the other choices, with the suspension set to the magic 30%, the seat is too high for me, and I am 5 ft 11 inches tall.
 
Last edited by a moderator:

Bill Thomas

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.
Morning Rob, Page 30, " Whichever damper is chosen, It is worth while fitting the longer eyebolts, They also prevent the fork from moving too far up, Thus avoiding the fork position where the trail is smallest and the fork most prone to "Speed Wobble". "
I think this was the second edition of Know Thy Beast. Cheers Bill.
 

MartynG

Well Known and Active Website User
VOC Member
Morning Rob, Page 30, " Whichever damper is chosen, It is worth while fitting the longer eyebolts, They also prevent the fork from moving too far up, Thus avoiding the fork position where the trail is smallest and the fork most prone to "Speed Wobble". "
I think this was the second edition of Know Thy Beast. Cheers Bill.
It's in the first edition of KTB as well -

Martyn
 

greg brillus

Well Known and Active Website User
VOC Member
I can't imagine the forks being at full compression long enough to cause a speed wobble.......Remember these were the same guys that got it wrong in the first place. These days a manufacture would have called for a major 'Recall" if such a bad handling issue existed on a modern machine.
 

hadronuk

Well Known and Active Website User
VOC Member
It's in the first edition of KTB as well -

Martyn
Aha! Thanks, I was looking in my 4th edition, as my 2nd edition is a bit delicate. Using that as a reference, as was able to find the rewritten section which is on page 39.
Referring to the long eyebolts, it now reads:
"They may also prevent the fork from moving too far up, thus avoiding the fork position where the trail is smallest and the fork, in theory, more prone to "speed wobble"."

Sound like he was less certain and doubt was setting in! :)

I agree with Gregg. Very little time for an oscillation to build up as the forks momentarily bottom, plus when trail is low, the self centreing action is weakest and less likely to drive a resonant oscillation.
 

Bill Thomas

Well Known and Active Website User
VOC Member
The thing is Greg, If you had ever had a " Full Blown Tank Slapper ", You would know that once it's going, It only stops after you have been thrown Off !!.
I will say that my Brother, Does not use the Standard Damper, And he has been riding longer than me, And on the road was faster than me !!.
I also rode for many years, Without any trouble, Most of my Racing was done without an Hydraulic Damper.
For years I had full faith in my bikes, And with Red Arrow tyres on my racer, I could do anything with it, It's just that my luck ran out !!. Cheers Bill.
 

greg brillus

Well Known and Active Website User
VOC Member
My guess is that if the front end hit something hard enough to compress the forks till they bottom out, the instant rebound would send the fork action to the other extreme. I have had various tank slappers on two different bikes, luckily none bad enough to toss me off. Though the one on my RZ 500 Yamaha was enough to strain both my wrists and scare the life out of me. I don't believe there is anyone on this planet that if they have ridden bikes hard long enough, sooner or later something will happen to them, though these bikes are prone to handling issues, some are lucky to escape it. I just prefer to do all I can so it doesn't happen to me .........That's why I went to so much trouble on the racer.
 

Bill Thomas

Well Known and Active Website User
VOC Member
The thing with the Vincent is there is so much weight forward of the steering head, I see the Ducati down tubes are almost in line with the Stering head.
Cheers Bill. Like a bloke on the T.V. said at the end of his prog', " May Your God Go With You ".
 

davidd

Well Known and Active Website User
VOC Member
Back when John Emanuel was becoming aware of this problem others were also aware of the problem, but never got very far. It seemed that most who questioned why the handling was sometimes horrible were focusing on the trail. Many thought that it was the change in trail that caused the problems. Phil Irving would never answer the question, and it was posed to him in MPH. I later learned he was quite fearful of the threat of legal proceedings being instituted due to the Vincent's high number of accidents resulting from speed wobbles.

I don't know what his answer would have been, but I was always of the opinion that it was not the the fact that the trail changed, but what was changing the trail that was the problem. As the posts on this forum have suggested, it was the fault of the axle path, and the axle path was the fault of the lower link position. I also thought the work Tony Foale had done on rake and trail seemed correct. He would say that you need some trail as opposed to no trail...that's all. I suspect that does not account for side car work, but it helped me look past the changing trail issue.

It is interesting that Eddie Stevens changed the language regarding the wobble. I think it is difficult for most to understand how the Girdraulic really works. We all know that tele forks dive naturally under heavy braking, but it seems that no one noticed that stock Girdraulics extend naturally under heavy braking. It is counter intuitive. When Sid would say "The Girdraulics have anti-dive" I would correct him by saying "No, the Girdraulics are pro-extension, just like the teles are pro-dive."

Worrying about the Girdraulics bottoming out during heavy braking would be like worrying about teles topping off during heavy braking.

David
 

greg brillus

Well Known and Active Website User
VOC Member
The really interesting part about all of this that when you look at the fork design, there is very little difference between the earlier tubular pattern Brampton's, Webb, and so on verses the Girdraulics. Only that the Girdraulics have their spindles arranged down the center line of each blade, whereas the former have the lower spindle offset to the rear. The main difference that I see is the travel allowed from the earlier to the later. That is, if the Brampton's had a longer spring thus causing the links to be angled down at the front with the bike laden, I see the same fault arising. I am convinced that if this was actually tried, the Brampton forks could themselves become quite dangerous as well. As we know the axle path is the critical part of all of this, but really what that comes back to, is two (2) completely opposing forces.....one, that when the brakes are applied, the wheel is trying to go backward, and Two, for the forks to compress......the wheelbase needs to get longer. And this my good friends is exactly why the Girdraulic forks "Lock up" I proved that the other day whilst riding that series "D" Rapide. I know you are thinking, yes but if we change the angle of the links, we then lose some of our valuable travel. And I would say, Yes you are correct, and that is exactly what happens to some degree when you install some of David's shorter springs. But when this new stem kit is installed, you regain this travel and more, because it is really only the rear of the lower link that changes. This then causes the spring box cases to be closer together, which means you need different springs to the original, and this is quite experimental to a certain point, depending on the weight of the bike/rider and so on.
 
Last edited by a moderator:

hadronuk

Well Known and Active Website User
VOC Member



I also have an interesting but lengthy journalists summation of the Cranfield and Imperial study. It is in the form of an image of the page. I will try to upload it elsewhere "images"? I could paste it here if no one minds?
 
Last edited by a moderator:

timetraveller

Well Known and Active Website User
VOC Member
Two things; first the springs have arrived and the first pair of 30 lbs/inch springs are now in the post to chrislaun. I have notified Greg but he will probably not get the message until the Australians are up and about. The second point refers to a few words in hadronuk's page copies above. There it states that in order for a speed wobble to occur the bike must be leaning over. NOT SO. When mine developed at Cadwell I was about two thirds of the way down the top straight, flat out at about 100 mph and the bike was definitely bolt upright with me as low on the tank as I could get. Passing over the joint in the tarmac surface started it but the inertia of me and the bike probably kept it going. Later examination of the track surface showed a whole line of black curves, first one way and then the other, where the front tyre had skidded as the tyre could not rotate, with the handlebars at full lock each side. There is a lot of kinetic energy in about 600 lbs of mass at 100 mph and it takes some time to dissipate, even when making black skid marks on a tarmac surface.
 

Bill Thomas

Well Known and Active Website User
VOC Member
When I was a Jag' Mechanic, I used an electric wheel balancer, We had a lot of trouble with new tyres, They were not round !!, It was funny to see them get to a certain speed before they went into a big wobble, No amount of weights could stop the trouble on the road, You could get them better, But not good enough. Like he says, Everything has to be right, Or wrong in our case !!. Cheers Bill.
 

bmetcalf

Well Known and Active Website User
VOC Member
Both my Vin crashes were as I changed lanes on the Interstate (motorway) at about 60 mph, so not much of a lean. When I finally took the Girdraulics apart for maintenance, the steering head balls were in terrible shape, some were square! I don't think the crashes were because the forks were locked up. Now that I think about it, I had them apart almost 20 years ago, maybe I should take a look this winter.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Hello Ron, On the balancer they were out of control !! Nearly took the Machine out the of the floor, They were Big wheels !. But there was no stablity in the machine, On the road, because there was all the steering to soak it up, It was not so bad, We could get the balance right on the machine but on the road we thought it was because the out of roundness, The tyre is smacking the road, Some people said it was tracking or wear in the joints, But the only cure was to change the tyre. Cheers Bill.
 

Latest Forum Posts

Latest Forum Threads

Top