FF: Forks Modified Steering Stem

oexing

Well Known and Active Forum User
VOC Member
The Bramptons don´t have the geometric linkage problem at full extension like the girdraulics, one reason for the new steering head mod. So tank slappers are not expected with Bramptons. Nevertheless I´ll fit hydraulic steering dampers on them, at € 30.- no factor.

Vic
P1060214.JPG
 

MarBl

Well Known and Active Forum User
VOC Member
Thank you for the extensive answer.
As far as I am concerned the modified steering stem seems the way to go. Surely I wont race the bike nor try to reach the ton anytime soon. But you never know when you are confronted with an emergency and in that case I'd like to have the best setup available without altering the apperance of the bike in an unfavorable manner.
May I ask where to get the springs with the unique spring constants? And should I go for an AVO Damper? The Thorntons seem to be rated the best but maybe are no longer available?
On base of my first inspection of the bike there will be quite some work to do anyway, so its best to do it the right way, if its disassembled already.
Many thanks!
 

davidd

Well Known and Active Forum User
VOC Member
There has been so much discussion of this issue I don't blame anyone for having questions. I originally had this discussion with Rip Tragle on this Forum. The new FF2 will prevent a wobble, but it is a specific Vincent wobble that is really un unforced error. You cannot prevent "all" wobbles because a wobble on a motorcycle is part of the two-wheel design. Wobbles can be minimized, but not eradicated.

The wobble that could happen with the Girdraulic occurs because the axle path was designed incorrectly. It angles forward and up. The new stems change the path to either straight up and down (with a slight rearward tilt) in the case of the JE stem to rearward or "telescopic.

I did a full-size mock-up of the Brampton and the Geometry was very close to straight up and down. I did not have working Brampton at the time, but I remember deciding that the difference in Brampton geometry was why the forks were seen as a good replacement for Girdraulics. As a result, I chose a Girdraulic axle path that was closer to the Brampton, mostly because I thought that Vincent and Irving would have done so if they had realized that the Brampton geometry had been abandoned.

So, the new stem won't cure all wobbles, but it will provide handling that is very reliable and predictable.

David
 

greg brillus

Well Known and Active Forum User
VOC Member
Oh dear here we go again..........Ok the big difference between the Brampton's and Girdraulics is actually nothing........yes nothing..........The later factory design was to improve on the strength and total movement of the forks......... Yes they improved the strength and yes they improved the travel........now this later point is where they went wrong........They went wrong in using longer stiff springs, this single factor changed the link geometry to a condition where the forks basically "Over center"........to prove this point and that Brampton's can suffer the wobbles, look at some video's shown on this forum where a pre war twin is loaded to the max with pannier racks and luggage, then a rider of lighter weight takes off and the front end starts to weave left and right. I bet if you could see the links, they would be pointing down at the font.........This is the exact same condition that the Girdraulic's have from their outset. Take a nice open series "D" for a spin, lovely soft compliant suspension........Ok hit a few bumps in the road surface, not overly fast say 60 kph........now apply the front brakes and look down at the upper link and watch what happens to it..........All that lovely soft suspension suddenly stops. All you've got now is the side walls in your front tire....... So lowering the front using short springs works, but the stem mod returns the full travel and you can use special springs made specifically for it.........The shock absorber is not an issue, so long as it feels fine and has not leaked out t's oil........Yes do lubricate all the spindle bushes and spring box pivots......the friction in these front ends is really very high, and this adds to the problem overall. When I ride customers bikes now with a stock front end, I am very cautious, and reminded of how potentially dangerous they can be........Well I rode one the other day that had little or no brakes, so that was a good thing............. ;)
 

oexing

Well Known and Active Forum User
VOC Member
Girdraulics not being my topic, but common girder forks need not be very stiff from friction in their bushes when all is well as designed. Maintenance and line- reamed bushes provided with all correct geometries you actually need these friction discs for reducing liveliness . Friction in typical bushes is not a factor, minimal compared to what you want from hydraulic dampers on teles or rear shockers. So I would not go for ball or needle bearings in girders as much as I like them elsewhere. Instead Permaglide or IGUS bushes , no grease duties with these.

Vic
 

Martyn Goodwin

Well Known and Active Forum User
Non-VOC Member
I understand. I have an Aprilia RS250 Race Replica I ride on the road and the occasional track day. It is sublime. Those that push it to its limits fit a steering damper. I tried one and it made normal riding a pain, so I removed it. Different strokes for different folks and those in between make their choices. I will be fitting one of Norman's steering dampers on my Shadow rebuild because I have a) been thrown over the handlebars of a Series C, b) watched someone being thrown over the handlebars of a C and C) don't want to do it again.
I'm with you on this Al. around 5 years back out on my Comet with original configuration girdraulics, series 'D' friction damper, Ikon front shock, just touring a country road and had a massive tank slapper. I woke up in the ambulance some 30 minutes after the event!

My Comet is now fitted with the modified steering head, thornton front shock (unfortunately Thornton are no longer in business) and a hydraulic steering damper (Kawasaki). At all times I ride with the steering damper on its stiffest setting.

With that setup I still experience the occasional mild head shake, but that's all it is - no tank slappers!

The key to this is how the two types of steering damper works.

The original friction dampers provide maximum damping when the friction pads are moving a very slow speed and as the speed of the friction pads increases (like with head shake) the damping effect becomes progressively less.

Hydraulic dampers are exactly the opposite. At low damper movement the damping effect is low but as the speed of the damper rod movement increases the damping becomes progressively heavier - exactly whats needed to control head shake and tank slappers.

Attached is my steering damper installation.
 

Attachments

  • Fitting an Effective Steering Damper.pdf
    2.2 MB · Views: 49
Last edited:

greg brillus

Well Known and Active Forum User
VOC Member
The steering damper is a band aid to the problem...........personally from about 6 or so years of riding bikes with the steering mod, including a race bike........ I would feel safe running without one all together. But I still run one and would not advise anyone otherwise.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
Martyn, I completely under stand why you would want to run your Hydraulic as stiff as that,
But I think you are making it act like a friction damper ?
Just my thoughts !.
It depends on the gearing of yours, But on mine I run 2 clicks from soft on my Twin and my Comet.
I find it does not have to "feel" like it's doing anything, It just calm's things down.
As does Taper rollers at the bottom.
Good Luck, Bill.
 
Top