FF: Forks Modified Steering Stem

[hadronuk]

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]

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]

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.

 

[Martyn Goodwin]

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]

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]

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]

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

 

[timetraveller]

The man to thank is John Emmanuel of West Yorkshire.

 

[greg brillus]

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]

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.