FF: Forks Modified Steering Stem

greg brillus

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The change in the modified steering stem changes all of that, as the forks no longer behave in a manner biased toward the suspension suffering seizures. I have not seen enough good photos of the front end of John Surtees racer to see what was going on, but I have seen the footage of his crash. I feel the behavior of the Girdraulic's can surpass that of Brampton's in part because of the longer links, thus enabling more travel. The efficiency of the brakes will be increased because the front suspension is able to work fully and correctly, without the front wheel trying to stay jammed under the engine. If all the Girdraulic equipped Vincent's world wide were converted in this way, it would be a complete revelation......... In fact I would say, that if the factory had realized and fixed this back then, the other Hybrids like Norvin's and Egli's would not have existed, given their main existence was to improve handling. One of the reason's I wanted to build a twin racer based on a Vincent ONLY, and not a twin engine housed in another frame, was that I was certain that a genuine Vincent COULD be made to handle. Until I rode Neal's Comet the other day, even though my bike has the same set up, except my springs are still too hard.......Neal's Comet felt so different to ride, in all honesty if you were blindfolded, you would swear the bike had tele's up front. But please don't tell either of the Phil's I said that...........:)
 

Bill Thomas

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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.
 

Hugo Myatt

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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.

I am completely out of my depth with this but I have always been surprised at the pneumatic pumping action of the springboxes. With a well fitting inner and outer spring box minus the spring it is surprising how much resistance there is to pumping the box and how much air has to be displaced. With the front forks acting rapidly at speed I wonder how much resistance is built up. I have often (idly) wondered whether an air escape hole at the top of the outer would soften the fork action but then my position is one of total ignorance.
 

greg brillus

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The same applies to the rear spring boxes......If you move them in and out you can feel the air compressing, so yes I guess this is a factor, but I would say it's a bit like a feeling the action of a shock absorber in your hands. It may feel stiff, but on the bike and under the leverage it is operating at.....It is probably insignificant.
 

davidd

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I drilled holes in my spring boxes, front and rear to avoid the air pumping. I had noticed that it was done of the Factory bikes.

Shroud Hole.JPG


It is a little fuzzy, but you can see the hole drilled in the upper spring box below the mounting bolt. The problem is heightened when you put anti friction seals in. I drilled two holes in each rear spring box. I went to the coil over to lower the weight, the number of parts and the pumping.

David
 

vibrac

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The same applies to the rear spring boxes......If you move them in and out you can feel the air compressing, so yes I guess this is a factor, but I would say it's a bit like a feeling the action of a shock absorber in your hands. It may feel stiff, but on the bike and under the leverage it is operating at.....It is probably insignificant.
Not an effect I have to worry about I have Banana man springs
springs.jpg
 

Chris Launders

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I was talking today to my workmate who runs a couple of classic race bikes, his opinion was progressive springs were only there to mask poor damping, with properly set up damping would you should only need single rate springs.
 

timetraveller

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It is certainly not necessary to use taper roller bearings. John Emmanuel who developed this mod does not believe they are required. However, if there is any reason to change the head bearings then the taper rollers are the way that I would go but depending upon who's design of taper roller installation one uses it might be necessary to machine the steering head to take the taper roller bearings. With regard to pneumatic action within the front spring boxes; I am just in the process of trying to find the correct combination of damper, eyebolt and spring box length to use with the softer springs and the modified steering heads so I have had to play about a lot over the last couple of days with the spring boxes. Not surprisingly I would suggest that not all spring boxes are created equal. The ones off my bike, the ex-Cecil Mills bike, can be pumped in and out as fast as one wants with no tendency to build up pressure. However, it is clear that some other people have found that this can happen so it pays to try it out first on your own set of spring boxes to decide whether you need to drill a hole or not. These tests, which will be written up more fully when they are complete, are concerned with the relative amounts of movement on the spring boxes, the new AVO damper and the wheel spindle. It should be noted that with the modified steering head the space between the upper and lower mounts for the spring boxes is now about one inch less than with the standard set up. To ensure that the bottom of the outer part of the spring box does not bottom out on the fork leg or that the top of the inner spring box does not clash with the inner top of the outer spring box I have cut off one inch of the top of the inner and off the bottom of the outer and then moved the fork legs over the full range allowed with the new damper and long eyebolts. It is clear that one inch is correct and note that as the total distance is now one inch less than original the shortened spring boxes do not stand out as short. More details will be available after a bit more work.
 
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