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Roadholder Fork Question #730827
04/04/18 12:29 pm
04/04/18 12:29 pm
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ogrilp400 Offline OP
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G'day All,
When Roadholder forks, in this case short ones on a Featherbed, are at full extension, what is acting as the limit stop?

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Re: Roadholder Fork Question [Re: ogrilp400] #730837
04/04/18 1:44 pm
04/04/18 1:44 pm
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The damper assemblies.

Re: Roadholder Fork Question [Re: ogrilp400] #731010
04/05/18 7:14 pm
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I've used short Roadholders on my Tritons for ~40 years. Until about 10 years ago, all I'd done was to replace the oil, bushes, seals and on one occasion, springs.
They'd always seemed ok to me, though I'd never had the opportunity to try an "as new" set for comparison.
After several years of my local MOT man criticising the poor damping, I thought I'd better look into it.

Number one issue I think comes from moisture rusting the steel damper rod, which then grinds away the orifice in the damper cap. Once this has opened by a few thou, rebound damping is pretty much lost.

In looking for replacements, I came across several sites describing the design faults in short Roadholders. Here is one that is comprehensive, though could have been easier to read:

https://www.inoanorton.com/Articles/Roadholder.html

I have equipped myself to follow those suggestions, but first I bought new damper tubes and rods from Andy Molnar.
I got the steel rods, though they can be got in alloy, I'd prefer stainless but not available.
The new rods were ~1" longer than my originals, which meant that the damper valve no longer hit the damper cap at full extension.
It does though allow the damper rod to hit bottom of the damper tube in theory, though in practice with good springs I have not experienced.
Even so, I would prefer this to trying to pull the damper tube apart.

What I found with the new parts was that the forks behaved so well (merely the damper tube working as intended), that I haven't felt the need to go further, though I do see the logic in doing so.

I have tried several other experiments over the years, including air assistance for adjustable pre-load, moderately successful, but at the end of the day, I'm happy with the forks at the moment.

Re: Roadholder Fork Question [Re: ogrilp400] #731012
04/05/18 7:20 pm
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Another informative site:

http://atlanticgreen.com/forks.htm

Re: Roadholder Fork Question [Re: ] #731019
04/05/18 8:06 pm
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Originally Posted by Charles DB
On Commando roadholders it is the slider top bush coming into contact with the stanchion bottom bush that limits extension.


No, it isn't,

The bushes are still a good 1.5 inches apart when the Commando dampers top out.

http://atlanticgreen.com/forks.htm

The Commando fork diagram below shows that with the damper piston approaching full extension, the bushes are still quite some distance apart and "Oil trapped between bushes forced back through big hole then small hole into stanchion as big hole becomes blanked off" simply doesn't happen.

[Linked Image]

Last edited by L.A.B.; 04/05/18 8:35 pm.
Re: Roadholder Fork Question [Re: ogrilp400] #731025
04/05/18 8:39 pm
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"I would be surprised if the unsprung weight of the front end (wheel, hub, brake assembly, tyre, tube, mudguard, fork sliders and damper tubes) was dependent on only the damper rod to prevent the front end assembly separating on the short roadholders."

I agree, it is surprising, but true! In both Commando and short Roadholders, the limit of extension is when the damper valve hits the top end of the damper tube. I've not known it happen but conceivably the damper tube could be ripped apart. This wouldn't result in the forks coming off though!

If you read the references, the faulty reality will become clear.

Re: Roadholder Fork Question [Re: ogrilp400] #731028
04/05/18 8:50 pm
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"The standard arrangement has the top and bottom bush coming together"

If it had been designed properly, that would be true, but it doesn't happen because it wasn't.

That's the point of the longer bush that you refer to, and it applies to Roadholders with damper tubes (ie after long Roadholders).

The 2 bushes will not come together, because the limit of travel f the damper mechanism does not allow it, by a large margin.

Re: Roadholder Fork Question [Re: ogrilp400] #731030
04/05/18 9:05 pm
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That's interesting Charles, did you have a special long bush made, or use a bit of another bush? I enquire because I've been considering for some time, but haven't got round to it yet.

Have you done the changes to the holes in the bottom of the damper tube as well? Though I guess you're talking Commando, in which case I believe that situation is slightly better than with short Roadholders anyway.

Re: Roadholder Fork Question [Re: koan58] #731032
04/05/18 9:10 pm
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Originally Posted by Charles DB
The standard arrangement has the top and bottom bush coming together.


No, it can't happen because the dampers top out first with the bushes still approximately 1.5 inches apart (see diagram above).


Originally Posted by Charles DB
There is a design fault (on Commandos) in that the top bush is not long enough to blank off the oil stanchion bleed holes which would create an hydraulic bumpstop.



Correct because the standard bushes never come close enough together to even begin to blank off the holes.



Originally Posted by Charles DB
A modification is to fit an extended top bush which then creates the required bump stop and so slows down the extension and prevents the violent coming together of the two bushes.


The extended upper bush (or Covenant modification) adds the required hydraulic bump stop (and I've used both) but the fact that you can fit an extended bush means the originals can't come together. It's plain enough from the diagram posted above that it can't happen.



Originally Posted by Charles DB
There was an excellent site www.members.shaw.ca/randell/bump_stop which apparently is no longer on line and which, from memory, contradicts the atlantic green site, no disrespect.


I don't remember any contradiction.





Re: Roadholder Fork Question [Re: koan58] #731036
04/05/18 9:24 pm
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Originally Posted by koan58
That's interesting Charles, did you have a special long bush made, or use a bit of another bush? I enquire because I've been considering for some time, but haven't got round to it yet.


RGM Motors sell extended bushes.
https://www.rgmnorton.co.uk/buy/ext...ping-out-2-9-74mm-o-all-lgth-pr_2298.htm

Or there's the Covenant kit.
https://www.rgmnorton.co.uk/buy/covenant-fork-damper-conversion-kit-improves-damping_2294.htm
(But don't bother doing the blanking dowel part of the modification as it's completely ineffective)


If you really want to transform your Roadholders then I recommend you fit the Lansdowne damper kit (now being manufactured by Don Pender)

http://www.tritonmotorcycleparts.com/



Last edited by L.A.B.; 04/05/18 10:16 pm.
Re: Roadholder Fork Question [Re: ] #731090
04/06/18 8:32 am
04/06/18 8:32 am
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Originally Posted by Charles DB
There is also a noticeable reduction in total fork travel (to no detriment in use I might add) which contradicts the damper limit theory?



No, it doesn't.

If you actually read the INOA and dynodave pages and look at the diagram I posted above then you would understand why there's no contradiction (because the damper unit reduces overall 'fork' stanchion and slider travel from just over 6" to around 4.6" leaving the upper and lower bushes approximately 1.5 inches apart on full extension, so any upper bush extension would have to be in excess of 1.5 inches before it had any effect on the fork stroke.

I've quoted the relevant sections and as far as I remember there was no contradiction to this in the shaw article.

(my underlines).
http://atlanticgreen.com/forks.htm
"The fork damper rod tops out in the damper, 0.6" before the fork is extended enough to start to block off the big hole in the fork tube. This topping out action is also 1.25" short of reaching the smaller bump stop hole. The damper topping out limits the fork to 1.55" less travel than the bushes would otherwise allow."

https://www.inoanorton.com/Articles/Roadholder.html

"Sadly, unlike the Matchless version, neither arrangement works properly. A quick glance at the illustration shows that as the slider moves down the stanchion, the oil trapped in the space between the top and bottom bushes is squeezed out through the large upper holes and the smaller lower holes. This provides some damping to supplement the effect of the separate damper assembly. In theory, as the slider nears the end of its travel, first the large holes and then the small holes are blanked off as they disappear inside the top fork bush. This leaves a small cushion of oil trapped between the two bushes to stop them hitting each other and putting excess strain on the materials (and your eardrums). As far as it goes, this design is perfectly sound. Unfortunately, it literally doesn't go far enough. 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. If those of you with Roadholders don't believe me, remove the fork top nut from one side of your forks, (having first taken off your front wheel and mudguard) and fully extend the fork leg. You will notice that the damper rod disappears an inch or so below the end of the stanchion. In other words, when the top nut is in place, it and the attached damper rod stop the forks from extending as far as they otherwise would. For those doubting Thomases who are still unconvinced, have a look at the underside of the damper tube top and you'll see the tell-tale marks in the alloy where it has been struck by the damper valve........................

To provide a proper bump stop on full extension, it is only necessary to arrange for the holes already in the stanchion near the bottom bush, to be blanked off in sequence. At first sight, the easiest way might seem to be to make up a longer damper rod to allow the forks to extend fully. However, even if the fork springs were packed up to provide the extra extension required such an arrangement would not be desirable because of the limited overlap, or engagement, of slider over stanchion. To maintain adequate stiffness of the front forks, there should be several inches of stanchion inside the slider even on full extension. The best way of providing a bump stop in this design is simply to make longer top bushes - about one and a half inches longer in fact. The exact length is adjusted so that even when completely extended, with top and bottom bushes in contact, the damper valve stays just clear of the underside of the damper tube top. With such bushes fitted and the fork topped up with oil, the last fraction of travel is properly cushioned as the oil holes are blanked off in turn until the hydraulic lock occurs."




Last edited by L.A.B.; 04/06/18 10:23 am.
Re: Roadholder Fork Question [Re: ogrilp400] #731101
04/06/18 10:58 am
04/06/18 10:58 am
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I read that website before it disappeared and cannot remember it saying that mod reduced travel, there must be a reduction if you over extend the bushes, but if you extend the bushes so the bushes meet just before the damper valve clashes the damper top then it will be minimal eg 1 or 2 mm. Your other option is to extend the damper rods but then you have the issue of once the standard bushes are close together with the clearances to the stanchion the stanchions can bind in the bushes.

Re: Roadholder Fork Question [Re: ] #731102
04/06/18 11:01 am
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Originally Posted by Charles DB
I am simply reporting my findings after carrying out the modifications as described in the web site I referred to, but sadly no longer available, followed by road testing - approx. 15,000 miles to date.


Referring to a possible contradiction (and one I don't remember) in an article that's no longer available holds little credence. Perhaps somebody has a copy?

Originally Posted by Charles DB
Roadholder (and the Commando service notes) has demonstrated to be a mine of misinformation on many occasions and simply because the editor saw fit to publish an article from a club member does not guarantee its accuracy. Cut and paste all the articles you like but sorry, that is how it has proven to be.


And what's to say the information in the Shaw document wasn't equally flawed if you can't provide a copy?

The measurements which can be verified and the necessary information to support the fact that your thinking is off is all there in the two articles and the fork diagram.
Edit: I can also state from my own measurements of Commando Roadholder components that the information is accurate therefore I'm not simply quoting any apparent "misinformation".



Last edited by L.A.B.; 04/06/18 11:54 am.
Re: Roadholder Fork Question [Re: ogrilp400] #731115
04/06/18 2:34 pm
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LAB,
"Or there's the Covenant kit.
https://www.rgmnorton.co.uk/buy/covenant-fork-damper-conversion-kit-improves-damping_2294.htm
(But don't bother doing the blanking dowel part of the modification as it's completely ineffective)"

You've obviously tried it, it seems to make sense to me, why do you think it does't work?

My interest is short roadholders with the holes below the apex of the damper body. Are you talking about later ones with the holes on the taper flanks?

Re: Roadholder Fork Question [Re: koan58] #731120
04/06/18 3:13 pm
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Originally Posted by koan58

"Or there's the Covenant kit.
https://www.rgmnorton.co.uk/buy/covenant-fork-damper-conversion-kit-improves-damping_2294.htm
(But don't bother doing the blanking dowel part of the modification as it's completely ineffective)"

You've obviously tried it, it seems to make sense to me, why do you think it does't work?

My interest is short roadholders with the holes below the apex of the damper body. Are you talking about later ones with the holes on the taper flanks?



Yes, the RGM Covenant kit consists of two sleeves that blank off the holes in the slider as the forks reach full extension which the standard upper bushes don't do, also two alloy 'dowels' intended to blank off the later Commando damper tube holes above the apex of the taper. As they're not intended for Short Roadholder damper tubes I won't go into detail.
https://www.rgmnorton.co.uk/csi/1245184/f/pdf/050007_covenant_fork_damper_improvement_kit.pdf
[Linked Image]


To blank off the earlier (short Roadholder and early Commando) four damper tube holes below the taper requires a different pair of sleeves.





Re: Roadholder Fork Question [Re: ogrilp400] #731131
04/06/18 3:43 pm
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And the way I see it, that would be worthwhile for the earlier arrangement?
Thanks LAB

Re: Roadholder Fork Question [Re: ogrilp400] #731132
04/06/18 3:49 pm
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I hope Ogri has found this useful, as usual a simple question resulted in maybe more than expected! He has forks like my own, so hopefully it has been of value.

Re: Roadholder Fork Question [Re: koan58] #731135
04/06/18 4:06 pm
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Originally Posted by koan58
And the way I see it, that would be worthwhile for the earlier arrangement?


Yes, it should then provide a (somewhat basic) hydraulic compression 'bump' stop.
Two holes need to be drilled in each damper tube above the taper to allow oil into and out of the damper tubes similar to late Commando tubes.

[Linked Image]



Re: Roadholder Fork Question [Re: ogrilp400] #731167
04/06/18 10:03 pm
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The real question here, as a diversion, is why Nortons/AMC didn't ever vastly improve the rather basic damping in any of these Roadholder forks ? The prewar tele roadholders were little more than undamped pogo sticks, and progress was slow after the war too .The benefits of 2 way damping were known from manx efforts, but this never carried across onto the road bikes. "Built in the light of experience" was a slogan rather than a truism. ?

The damping setup in the AMC Teledraulic forks from AMC was better too, from the WW2 start
But this didn't carry across into the Roadholders either, when the teledraulics were abandoned.
Resting on their laurels ??

Re: Roadholder Fork Question [Re: ogrilp400] #731199
04/07/18 9:20 am
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Perhaps I omitted the words "effective" in my comments..
Both in respect of 2 way damping, and incorporating some Matchless content in them.

This older discussion by Mr Crespin doesn't paint a very rosy picture of the oem forks - as we are all familiar with...
https://www.inoanorton.com/Articles/Roadholder.html

Re: Roadholder Fork Question [Re: ogrilp400] #731201
04/07/18 9:22 am
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There may be compression damping in theory but the holes are too large for effective damping but going smaller risks too much damping over pot holes which is a problem common to all fixed orifice damping. To get the rebound increased I used alloy rods and alloy damper caps with a PTFE bush to separate the same metals so no wear like in an AMAL carb, now been changed to Showa 20mm digressive dampers from a CBR600RR for the best of both worlds, high damping at low fork speed defections and lower damping at high speed deflections. The other issue is the valve having to change position from compression to rebound, it uses the oil flow to achieve this and within this movement there is no damping, so on a flat road when you need damping rates to be at their highest there is no damping. The Landsdowne dampers split the damping between the legs and this gets round the lack of damping as the valve moves and gives adjustable damping but its still fixed orifice so has to be a compromise when you adjust but is still a definite improvement over the original set up even with upgrades.

Last edited by kommando; 04/07/18 9:38 am. Reason: added section on valve movement
Re: Roadholder Fork Question [Re: ] #731223
04/07/18 2:54 pm
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Originally Posted by Charles DB
.. Rebound damping was achieved through oil forced out between the aluminium damper cap and the damper rod..


Wrong again .
It is the clearance between piston and damper tube that controls rebound damping .


Re: Roadholder Fork Question [Re: ogrilp400] #731226
04/07/18 3:10 pm
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I have the consentino kit and 2 diy kits I did myself, if you do diy its the same showa parts and the only issue is the springs as the spring rate alters if you shorten them to suit the available space. There are other springs which work and are available new and second hand. On the diy I added in a compression bump stop which the consenting lacks, did not add a top out as you can adjust the rebound enough not to need it.

Re: Roadholder Fork Question [Re: ogrilp400] #731281
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"The valve closes as the slider extends and effectively seals the damper tube forcing oil up and through the gap between damper rod and aluminium cap, which is what gives the damping effect."
Precisely, except its arse about face. The flap valve is open during compression of the forks, it closes on rebound to damp that reaction. If the clearance between the rod and cap is much bigger than it should be, or the damper within its cylinder, then the damping action will not be what it should be.
As I described earlier, when I put new damper units in my forks, it was a transformation.
These damper bodies should be viewed as 10,000 ml service jobs, I reckon!

Re: Roadholder Fork Question [Re: ] #731323
04/08/18 10:28 am
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Originally Posted by Charles DB


P.S Or do you mean the cupped valve at the bottom of the damper rod? If you do then you are wrong again (sic). The valve closes as the slider extends and effectively seals the damper tube forcing oil up and through the gap between damper rod and aluminium cap, which is what gives the damping effect.


Charles , I said : "wrong again " , because in this thread you have shown that your understanding of the Roadholder fork is rather limited ( to put it politely ..) , so I will try to explain :

The aluminium damper cap is a sliding fit over the damper rod , with minimum clearance .
The piston has a clearance in the damper tube of around 0.40 mm , and over a larger circumference than the rod .
When the piston rises ( = rebound ) oil is trapped between the closed piston ( valve ) and the damper cap .
A small amount of oil can escape upwards via the damper cap , but by far the most oil will escape downwards , because of the relatively large clearence between the piston and the tube wall .

Unless your damper cap is completely worn out , of course .






Last edited by ludwig; 04/08/18 11:30 am.
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