FF: Forks Modified Steering Stem

[timetraveller]

Shortening the spring cases has been important on the few bikes I have actually worked with this mod. Because the new centre of rotation for the lower link is now about one inch below where it was, the shortened spring boxes don't look out of place. Even on a stripped down racing twin with 33 lb spring the outer spring box was hitting the fork leg when braking hard and with a bump.
I do wonder about the use of a Vincent damper with the long eyebolts. The Vincent damper is longer than the Armstrong, hence the use of the longer eyebolts on the Ds. When I was playing about with the geometry of the new system in combination with the new springs and dampers the combined length of the damper and eyebolts is one of the limiters for the range of travel over which the new geometry works. Without the springs, and by replacing the front wheel spindle with either a rear one or a felt tip marker, one can move the wheel up and down and produce a track on a suitably supported sheet of plywood, or similar, and actually see the movement path of the front wheel. At the moment there is clearly some disagreement as to what the best combination of springs and dampers is for various weights of bike and rider. The total travel available was carefully developed and anyone who alters the range of movement on their own bike is into unknown territory and is responsible for their own safety.
Whilst sitting here in the high temperature and humidity I have had chance to think about some of these matters and will write more fully when back in front of a PC.

 

[Chris Launders]

Hi Greg, I had reason to use mine recently with no damping and it didn't go much further than when using the damper, it was just less controlled.
With the damper I'm using just over 2-3/4" on the spring box and having shortened the spring boxes 1" and have the same amount now unused so you MAY get away without it but it's very very close, too close I think, of course you could take much less off in practice.
I have the bearing type AVO set on minimum.
Chris.

 

[macvette]

Took the bike out for a test ride, it has not been necessary to shorten the spring boxes. There is village on my route which has at it's entrace and exit, series of 4 groups of sleeping policemen in groups of 4 with the spacings starting at 3ft, reducing to 1ft edge to edge. Each one is 2 ins high with small radius edges so you can image the rapid cycle the suspension goes though.
The wheels stay in contact all the way through and I can see the suspension reacting to each bump so I'm happy.

 

[timetraveller]

I think that Chris and Tracy Tilley are also using the minimum setting on their AVO damper.

 

[vibrac]

Hi Greg, I think my friend has the 36lb springs but I'm not sure, and the bearing type AVO, having tried it it's much the same as my twin but possibly fractionally stiffer as the damper has less weight to control. I liked the Armstrong damper when doing the damper testing but as you say they don't make them any more, the original AVO (for the standard set up) gives virtually no control when used with your bearing conversion that's why there's a special AVO to go with it. I had just wondered if there would be enough damping in a standard type AVO for a single with the bearing conversion but he says there isn't.
Chris.

Well IMHO the special AVO damper is too strong for a Comet with bearing mod which is what I also infer from Gregs earlier comments

 

[bmetcalf]

I think that Chris and Tracy Tilley are also using the minimum setting on their AVO damper.

I assume these dampers are sent out at the minimum setting.

 

[macvette]

Shortening the spring cases has been important on the few bikes I have actually worked with this mod. Because the new centre of rotation for the lower link is now about one inch below where it was, the shortened spring boxes don't look out of place. Even on a stripped down racing twin with 33 lb spring the outer spring box was hitting the fork leg when braking hard and with a bump.
I do wonder about the use of a Vincent damper with the long eyebolts. The Vincent damper is longer than the Armstrong, hence the use of the longer eyebolts on the Ds. When I was playing about with the geometry of the new system in combination with the new springs and dampers the combined length of the damper and eyebolts is one of the limiters for the range of travel over which the new geometry works. Without the springs, and by replacing the front wheel spindle with either a rear one or a felt tip marker, one can move the wheel up and down and produce a track on a suitably supported sheet of plywood, or similar, and actually see the movement path of the front wheel. At the moment there is clearly some disagreement as to what the best combination of springs and dampers is for various weights of bike and rider. The total travel available was carefully developed and anyone who alters the range of movement on their own bike is into unknown territory and is responsible for their own safety.
Whilst sitting here in the high temperature and humidity I have had chance to think about some of these matters and will write more fully when back in front of a PC.

I started my execise to check suspension movement because the majority of posts on this subject are interesting but subjective. The seriesD besides having an Amstrong damper also had single softer springs around 55lbs / ins ( I think). The damper provides the limit to suspension travel. In my case the damper even with the long eyebolts, at full extension, was hard against its internal stop and after riding and checking its movement, it was clear that it was operating biased too close to the extended position.
By taking off 22mm from one 45 lb spring, the load on the damper against its extened stop has been reduced and it now operates more equally within it's range.

 

[greg brillus]

Tim all I can suggest is just try another older damper if you have one, the softer the better. I have done another Comet but I could not carry out the bearing mod on account of the large eyes were too oversize to fit the bearings without going to a lot more expense. From memory it had a stock Vincent damper and the lighter springs ........It seamed to be ok. Because of the reduced friction with the use of the bearing mod, and if no other tight spots exist in the forks, then the fork action becomes much more active. The pre-load with the supplied springs for these kits is purely a close guesstimate, but like I have said several times, on pretty much all bikes I have done I have had to shorten both springs by some amount to the point where you could nearly attach the upper case with an extra set of hands pulling down on the upper case. This to me says that you need a stiffer spring with less pre-load...........Does this sound familiar. Mac It sounds like you are getting closer to a good result.

 

[macvette]

" if no other tight spots exist" is important. There are about half a dozen Vincents around here which are only ridden ocasionally unlike mine. All but one aside from mine, have stiff front suspension and even prior to my doing this mod, their riders commented on how my bike responed to humps and bumps. A quick check shows that the spindles are badly adjusted so unless this is adressed, you are wasting your time doing the mod or fiddling with springs and dampers.

 

[Chris Launders]

The problem in looking for a softer damper is in my experience there is no softer damper than the bearing AVO other than the original AVO which was designed for use with the standard forks, and I've done back to back testing for Norman with both the AVO's and at least one other experimental one plus Standard, Armstrong, Thorntons, and Koni dampers. in my opinion they could be roughly grouped as the all giving the same performance under emergency braking or hitting pot holes/speed humps, the least responsive to small irregularities by far was the Koni (and I tried three), the standard and the Armstrong were good but the Thornton and AVO were better, there was almost no difference between them but as has been said before Armstrong and now Thorntons are no longer made.
Chris.