FF: Forks Modified Steering Stem

[Nigel Spaxman]

One of the things that works really well on the Girdraulics I have which are more or less standard is that you can brake hard even when going over big bumps
the suspension still works. On most other bikes braking over big bumps can cause bottoming of the forks. With the modified steering stem is this characteristic preserved or will the fork bottom more easily since with the change the fork acts more like a telescopic one.

 

[davidd]

One of the things that works really well on the Girdraulics I have which are more or less standard is that you can brake hard even when going over big bumps the suspension still works. On most other bikes braking over big bumps can cause bottoming of the forks. With the modified steering stem is this characteristic preserved or will the fork bottom more easily since with the change the fork acts more like a telescopic one.

Nigel,

The reason you like the Girdraulic so much is that it is inclined to extend with hard front braking. You are correct that is is disinclined to bottom out under braking, but it also is disinclined to absorb much of the bump.

I designed my modified steering stem not to dive as much as the JE steering stem. Like you, I am a fan of maintaining the attitude of the bike under braking. Having said that, there is nothing wrong with the fork diving under braking, as long as it does not bottom out. Greg Brillus and Martyn Goodwin are quite pleased with the JE stem, so I think they could shed some light on the issue. They have a good idea what spring rates will work on a Comet and a twin with no bottoming.

Alternatively, you can try some short springs and adjust your ride height as Martyn described above. That will enhance the forks ability to absorb bumps under hard braking and will provide the same handling improvements. It is not as "foolproof" as the JE stem, which will provide safe handling whether or not the springing is correct. However, the short springs are fine as long as the lower link is parallel to the ground with you sitting on the bike. When using the short springs the fork is still inclined to extending, but it doesn't have the spring power to do it. As a result, the fork will happily go up and down over bumps under hard braking.

David

 

[greg brillus]

On the series "D" I worked on before Christmas, I found the front end to be quite soft in it's action when pushing up and down on the bars in the workshop, pretty normal I would think for that model of Vincent, and out on the road it handled very nicely giving a good comfortable ride over the bumps and undulations of the road. When I hit the front brakes quite hard, the front end definitely stiffened up and would only rise or change in movement over the heavier bumps in the road surface. The change to the new stem changes this quite dramatically especially with the lower link bushes replaced with sealed bearings. The race bike has the largest four leading shoe front brake I can legally run, and I have hit it probably as hard as I can into near full stop corners and the front end has never bottomed out, and it has never looked like shaking it's head either. It is the ability for the wheelbase to shorten when in use that is the difference from the stock set up to the new stem design. There is no loss in ride height or travel........with longer rear suspension, I had to extend my side stand by 35 mm to allow the bike to sit at the same angle it was before the suspension upgrades.

 

[Chris Launders]

Gentlemen can you put your thinking caps on about this please, just to satisfy a puzzle I've had, when I fitted needle rollers to the bottom links instead of bushes I expected to have to increase the damping to compensate for the reduced friction but I've had to increase the spring rate as well quite a bit to stop the forks bottoming out.

Now an idea has lodged itself in my head that the friction of the link bushes increases as the angle increases, now this may seem strange but I think it works like this;

To simplify things, say the fork action is vertical and the links are around horizontal, the action is purely rotational, but as the angle increases a proportion of the forces acting upwards pulls against the bottom of the eccentric increasing the pressure, the extreme of this is if in theory the link could achieve the vertical ALL the forces exerted upwards would be pressing against the bush/eccentric and there would be no rotation, so we have gone from a position of low pressure/ high movement to high pressure/low movement.

To me the increase in pressure acting on the bush increases the friction so we had a system which had built in progressive damping, of course with rollers almost all of this disappears, which is why I've had to increase the spring rate.

I could of course be completely wrong !!!

Chris.

 

[vibrac]

You may be correct but it all depends on the friction generated as a function of the angel my guess is it is nowhere near linear I bet its the steepest curve at the extremes and flat in the middle. a bit like locking at TDC and BDC only measurement would validate

 

[davidd]

Chris,

I am not sure I am following you, so don't take my lack of understanding as criticism. If I think about fork design in the largest sense possible I would guess that engineers would try and minimize the variables that they are working with. Thus, variables like rigidity, which if low enough can introduce poor steering response, and friction, which, if high enough, could introduce poor suspension compliance would be minimized by the designer. I suspect that the Girdraulic was built with the idea that it was quite rigid and as compliant as possible. It would have been no worse that the bushed Bramptons in terms of compliance, while being much more rigid. That is my guess.

I think they would have used materials of the lowest friction possible for the bushings. They did so for the RFM, using a taper roller, and it performs quite well with the same damper. I have the sense that they shied away from roller bearings in the fork because of the number and cost of such an endeavor, rather than the desire to introduce some method of progressive friction damping by using bushes.

If I follow the angularity idea, I can see it happening at the extremes, but the fork works within a tiny range. I am not sure that there can be a large variation here. If I visualize the front brake lever, I want it working at 90° where it pulls the best. If it is rotated to point straight down, the force in the bushing becomes huge and the lever does not move when the brake is applied. Of course, the more it is angled from this position, the more the pressure in the bushing is reduced. But, it operates in a small range and not near the extremes. I also think that replacing the bushing in the brake with a needle roller would yield a beneficial reduction in friction that might need to be addressed with a stiffer brake spring to make the application of the brake more progressive. But, most folks would happily trade a bushing for a bearing in the brake with the idea that the external brake spring is a more reliable way to modulate the pull rather than adjusting the friction in the bushing.

My racer was very close to ball bearing performance in the front end. I have mentioned that I could bounce it up and down like a basket ball. Most Vincents will not do this and it tends to prove that there is wild variation in the bushed forks. I had no bottoming with the short 75 Lbs. spring in both spring boxes. I could have tuned it a little better, but it worked so well that I moved on to other issues.

David

 

[greg brillus]

Chris I think I understand what you are saying, it is a scenario a bit like the original set up whereby the large eyes in the lower link become warn oval and the large bushes spin in the eyes. The rotational loads become so high that the amount of press fit of the bushes in the link is overcome. The shock absorber will not stop the front end from bottoming, the answer is to use springs of a higher spring rate, like I have done.......I have shorter springs resulting in less initial pre-load, but the spring rate is about 45 lb's averaged between the pair of springs. Like I said earlier, you would think that using a lesser spring rate would increase the softness/comfort of the ride, but I found on the twins that even with very weak springs, the comfort changed little and the bike wanted to bottom quite easily. my bike now rides very nicely and does not bottom, the comfort level is very good, but much better on the lighter Comets which on average are about 30 kilo's less than their bigger brothers.

 

[Chris Launders]

Hi Greg, As I've indicated before it was great with the bushes, 36lb springs and AVO damper, but when I fitted needle rollers to the large eyes it was bottoming easily, I've now worked my way, one stage at a time to approximately the same as you, and back to the same level of comfort and travel as before.
I have a 16.5" 36lb spring and a 15" 56lb spring fitted (average 46lb) giving about the same preload and I've refitted the Koni damper. So far I haven't had it bottom out but I haven't tried the most severe speed humps yet, that's the next stage, if that's ok I'll refit the AVO and see what happens.
The idea with the bush friction was to try and explain to myself why changing from bushes to rollers made so much difference.
Chris.

 

[greg brillus]

Anything that creates friction will have some affect, simply removing the spring box's themselves makes a big difference as well, though that is not such an easy modification, but by going to a single coil over shocker on the racer it did away with more friction (and weight), that with full needle rollers on all the usual bush to spindle locations, reduced the friction to an absolute minimum. The front end of the racer is very fluid in it's action. Look at the friction dampers on a set of Brampton's, it is just adding friction to the fork action, and adjustable as well.........well until the grease minimizes that............

 

[Oldhaven]

I still wonder how the forks would perform with the ball bearing change only. Chris did one change at a time, but did the stem first and there was noticeable change by installing the bearings. Something happened and the only way to know what it did is to try changing one thing only. I have this intuition that the top link angle and the friction in the lower link interact in ways we have not considered. Any friction reduction should affect the way the standard fork geometry links interact and may allow the top link, acting at a different angle, with different force vectors, to play more of a role in pulling the lower link over horizontal and make the forks less stiff or pro extension under braking. I can't see anyone doing just the bearing change, so we probably won't know anytime soon. I do know, going back to hadronuk's friction calculations, that there are dramatic changes in total friction by eliminating the bushings.