Fork Hyper Extension

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SvenSven

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After much study, replacements, upgrades, and fine tuning I seem to have the Commando in fairly decent semi-modern, reliable position.... for now anyway. Norton Owners know what I mean.

So what now? Well, now we are out of the fog ages (1st step after the preceding dark ages) the forks tend to hyperextend once in a while when going over road bumps at moderately high speeds, say 50pmh+. Brits tell me they are only too familiar with this based on common rough roads. Honestly, those of us in the US are fairing just as badly as the infrastructure wears and we don't repair things, and ... anyway! So here is my quick summary:

1973 Commando 850
Both forks rebuilt including new stantions and Andover bushings, seals, etc.
Landsdowne internal fork mods with adjustable compression and rebound
150ml of fresh clean red SS-8 Honda Suspension Fluid (10W oil)
Hagon 2810 rear shocks with adjustable rebound controls

Please share with me any ideas, and more importantly potential solutions you may have.
Cheers
SvenSven
-->FJ
 
dennisgb said:
Read this...it's long, but it tells the story
Click to expand...

As Lansdowne dampers are fitted, it won't be the right story.
 
Let me describe this better. The experience is the reverse of a "bump" when forks bottoms out. Hyper extension is when the fork's bottom sections (sliders) fly away from the stanchions and hit their maximum extension travel. This is likely the upper bronze bush hitting the threaded fork collar. When this happens you get a metal on metal thud or clunk. Again its the fork stopping itself from being pulled apart and the stanchion separating from the slider.

I've had this happen on my Honda 1995 CBR1100xx Super Blackbird until I rebuilt the awful stock non adjustable forks with RaceTech parts, changed shim stacks, oil viscosity, different spring, gold valves, pre-load washers and effort galore. Now they are perfect for my riding style.

Thanks.
 
SvenSven said:
This is likely the upper bronze bush hitting the threaded fork collar. When this happens you get a metal on metal thud or clunk. Again its the fork stopping itself from being pulled apart and the stanchion separating from the slider.
Click to expand...

It doesn't happen, or at least shouldn't be happening, either with the standard damper assemblies or the Lansdowne dampers, as the fork extension stop is provided by the damper assemblies which top the forks out approximately 1.5 inches short of the point where the upper and lower bushes come into contact.

Did you try increasing the Lansdowne rebound setting?
 
Best me and bunch of others can figure is Norton built Commando Roadholder to take advantage of a bunch of springs from the only maker that'd still do business with Norton and told Norton would cost a bit more to change. Hyper extentions implies something broke like your elbow folding up backward in a crash. Full top out extension clunk is damper rod valve striking underside of damper cap which don't have robust threads to stand much of it beside pilot annoyance. If lengthening the damper rod to eliminate the clunk with stock spring length it leaves spring loose to bounce inside at some point of extension, so a top bush extension is put on to both take up the spring gap by limiting travel and also covers the stanchion holes to give soft-silent hydraulic top out stop. KIts sold for top out bother w/o changing anything else about Roadholders.
 
Simple answer and wait for it! It is not quite as dumb as it may appear - no. Only because it is finally handling just right now, almost like a modern bike. If no other options are available, I will then of course make a few compromises by changing Landsdowne settings to see if a middle ground can be found. I hate the middle ground though, especially after so much work...

Oh joy! Crazy thing is, with numerous other modern bikes in my stable, this frigging thing is consuming an enormous lot of my time and attention - albeit with mediocre rewards. BUT most of you all, are already well familiar with this Norton ownership sillyness!
 
hobot said:
Best me and bunch of others can figure is Norton built Commando Roadholder to take advantage of a bunch of springs from the only maker that'd still do business with Norton and told Norton would cost a bit more to change. Hyper extentions implies something broke like your elbow folding up backward in a crash. Full top out extension clunk is damper rod valve striking underside of damper cap which don't have robust threads to stand much of it beside pilot annoyance. If lengthening the damper rod to eliminate the clunk with stock spring length it leaves spring loose to bounce inside at some point of extension, so a top bush extension is put on to both take up the spring gap by limiting travel and also covers the stanchion holes to give soft-silent hydraulic top out stop. KIts sold for top out bother w/o changing anything else about Roadholders.
Click to expand...

But none of that is relevant to the Lansdowne damper kit.
 
john robert bould said:
Try screwing in the rebound adjuster 1/2 turn ?
Click to expand...

Do you think adding an extra 20cc or so of oil to each fork leg might help?
 
Ugh sorry didn't know this was a Lansdown kit only thread instead of DIY fork improvement but hey I ain't and don't argue the Lansdown is best off the shelf way to avoid slip shod inventiveness hassle and discoveries.

Do note extra fluid in non Lansdowns does not help top out much at all and risks hydro lock up as air spring pocket pumps its self down. 175 ml is max I could get away with or jerked off the path even creeping with feet down.
 
hobot said:
Ugh sorry didn't know this was a Lansdown kit only thread
Click to expand...

It isn't, but it wasn't relevant so only serves to confuse the issue.
 
I missed fogged over the Lansdowne kit part and don't mean to confuse or distract but would like to know how much fill Lansdowne take before air pocket spring pumps down to nothing to hydro lock.
 
Might it be that the damping rates are set to far part. So when the forks hit a bump the compression is controlled by the spring and damping is set low so the oil flows freely but the rebound damping is set to high so the oil can't return fast enough. Most of the time this will go unnoticed but over a rapid enough serries of bumps the forks have no time to reset to correct lenght and they extend like hydraulic jack being pumped up until road smooths an they have time to recover.

This was a problem with fully adjustable forks when they where first put on road machines an owners moved them to far from factory settings. My friend had a GSXR750wp bought from a man who said it was close to unrideable at times. We simply set it back to standard setting and it was brilliant (my friend even got it very cheap as it looked like a major and expensive problem but turned out to be few minutes to sort an cost nothing)

Contact Landsdowne an see what they say it may even be a fault with the dampers.
 
L.A.B. said:
dennisgb said:
Read this...it's long, but it tells the story
Click to expand...

As Lansdowne dampers are fitted, it won't be the right story.
Click to expand...

I think, although I might be wrong, that the uncovering of the oil bleed hole still can happen with the Landsdowne dampers installed.

Maybe John will see this and chime in.
 
dennisgb said:
L.A.B. said:
dennisgb said:
Read this...it's long, but it tells the story
Click to expand...

As Lansdowne dampers are fitted, it won't be the right story.
Click to expand...

I think, although I might be wrong, that the uncovering of the oil bleed hole still can happen with the Landsdowne dampers installed.

Maybe John will see this and chime in.
Click to expand...

I'm not sure what you mean by: uncovering an oil bleed hole?
Your link:
Unlike the Matchless, Triumph and countless, other designs, the large and small oil passages are not actually blanked off at all; this is because the slider never moves far enough down the stanchion for the top bush to cover them. What happens is that the delicate damper valve assembly strikes the underside of the damper tube top at a point where the stanchion oil-holes are still about an inch below the top bush. In practice, therefore, instead of a nice oil cushion providing a proper bump stop, the damper valve has the job of limiting fork extension when it suddenly hits solid metal - a job it should never really be expected to do.
Click to expand...

As the forks extend, oil holes in the stanchion should become increasingly blanked-off as the forks reach maximum extension if the system worked as it was designed to do, however, the forks never extend far enough for this to occur unless the upper bushes are lengthened (RGM upper bush conversion) additional sleeves (the Covenant conversion) or longer damper rods (Fauth mod) are used, or the oil holes in the stanchion are repositioned, and the Lansdowne kit doesn't offer any increased extension travel over the original damper assemblies.
 
I meant that the oil hole(s) are still not covered (open) even if a Landsdowne damper kit is installed...am I wrong?

L.A.B. your posts seem to be conflicting...why is the oil hole issue not in play with the Landsdowne kit if the dampers are the same length and none of the other mods you site have not been made? I'm totally confused now :(
 
dennisgb said:
I meant that the oil hole(s) are still not covered (open) even if a Landsdowne damper kit is installed...am I wrong?
Click to expand...

No not wrong, "still not covered" is a better description. :)

dennisgb said:
L.A.B. your posts seem to be conflicting...why is the oil hole issue not in play with the Landsdowne kit if the dampers are the same length and none of the other mods you site have not been made? I'm totally confused now :(
Click to expand...

It's not an issue with the Lansdownes, or at least it shouldn't be, because the later versions supposedly have built-in hydraulic extension buffers.
 
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