What oil in the forks will produce a smooth ride ?

Exactly as the various written words L.A.B. showed. IMHO, more importantly than the various written words: Do a bench test of a Norton damper, you will find that the rod goes down quite easily and has resistance coming up and that the faster you try to pull it up the more that resistance increases.

Hopefully, everyone who has their forks apart check to see that the dampers work! I usually pull the sliders from the fork tubes with the dampers in place and test them so I know if I need parts. If they are already apart, I install the damper in the slider, pour in some oil, and test. If everything is fine, I install the fork tube.
 
jsnorton said:
Thanks Marshg246, I will be rebuilding my forks this winter with a seal kit from Andover Norton as they are 50 years old now and have never been rebuilt. I will be checking all the components and replacing as needed. I will use the SAE 30 and give that a try.
Click to expand...
JS,
You should check out the bushing and seal kit from NYC Norton that comes from Cosentino.It is very straightforward and works beautifully.
Mike
 
L.A.B. said:
I think he means axes (plural of axis). See Greg's reply in post #35.
Click to expand...
Les,
So am I correct that axis is the center to center of a fixed distance? I am just trying to understand the terminology.
Thanks,Mike
 
YING said:
So am I correct that axis is the center to center of a fixed distance? I am just trying to understand the terminology.
Click to expand...

I believe the point that's being made is the fork legs can be "out of parallel" when viewed either 'side-on' as in the drawing below or out of parallel when viewed 'head-on' but not necessarily both.

What oil in the forks will produce a smooth ride ?
 
Last edited:
YING said:
Les,
So am I correct that axis is the center to center of a fixed distance? I am just trying to understand the terminology.
Thanks,Mike
Click to expand...
Axis and its plural axes have several dictionary meanings some of which get stretched into other uses. It's pretty common when looking from different angles to say from different axis or more correctly different axes. Since the question I was asked seemed to be a typo of axis or axes, I stuck with the terminology.

The forks need to be parallel with each other. Generally, the side view as in the diagram L.A.B. showed, is the only one that can be corrected. If not parallel when looking from the front but OK from the side, the top and/or bottom yoke is damaged, incorrectly made, or incorrect in some other way.
 
L.A.B. said:
I believe the point that's being made is the fork legs can be "out of parallel" when viewed either 'side-on' as in the drawing below or out of parallel when viewed 'head-on' but not necessarily both.

What oil in the forks will produce a smooth ride ?
Click to expand...
This.
 
YING said:
Howard,
I don’t understand what an axi is.Please help me out on this.
Thanks
Mike
Click to expand...
LAB is correct. We routinely butcher the queens English, not maliciously. The surface plate can only show parallelism in one plane. The plan that is perpendicular to that cannot be measured that way. You'd need a I.D. mic, for instance, to show if the fork tubes were parallel, as viewed from the front.
 
concours said:
LAB is correct. We routinely butcher the queens English, not maliciously. The surface plate can only show parallelism in one plane. The plan that is perpendicular to that cannot be measured that way. You'd need a I.D. mic, for instance, to show if the fork tubes were parallel, as viewed from the front.
Click to expand...
Is it still the Queen's English????
 
With the bike on a center stand, loosen the fork top nuts and lift the wheel checking for friction. Now loosen the axel nut and pinch bolt and repeat - if its better then you have misalignment and binding. Then remove the springs and check that the damper rods are in the center and not binding. This whole operation is best performed when you're assembling the forks and haven't installed the seals. Make all your checks and push in the seals later. The bronze bushings cause a lot of wear and friction but they can be upgraded. Bronze was all they had when they designed these forks.
 
L.A.B. said:
Unfortunately, the information in the Haynes manual has been copied from the forks section of the factory manual which not only contains certain inaccuracies but also some known errors in the description of how the Roadholder damping works.

jsmotorsport.com

Norton Turcite Fork bushings and damping upgrades

For your Norton fork to ride smoothly with out bottoming or topping out you must install the slippery JS Motorsport Turcite bushings along with the aluminum sleeves.
jsmotorsport.com

"COMPRESSION DAMPING:​

If you have damping tubes with holes below the conical section then you need to cover those holes with JS alum sleeves to achieve a hydraulic bump stop and avoid bottoming “clunk”. Then drill only one 1/8″ hole in the middle of the conical section as in the photo below. The value in compression damping is reducing front brake dive and bottoming in racing – but you will feel more bumps when cruising. I recommend a single 1/8″ hole per conical section which gives minimal high speed compression damping and is good for the street. A smaller 3/32″ hole will give you more compression damping but you will feel more bumps. Larger than 1/8″ will give the smoothest ride but no compression damping. If you already have 1/4″ holes in or above the conical section then you won’t have compression damping and that is fine for the street. If you already have two 1/4″ holes and you definitely want compression damping then you will have to fill the holes, smooth them off and drill only one 1/8″ hole in each tube."

www.accessnorton.com

No fork compression dampening

75 MkIII, 15k miles, one owner. I just replaced the springs with Progressive springs and filled with the appropriate amount of 20W fork oil, pumped the fork to remove any trapped air and took it for a test ride. On the test ride I noticed that the forks bottom easily on bumps and seem to have...
www.accessnorton.com www.accessnorton.com

"Compression damping? Stock forks dont have any.."
Quote from the late John Bould inventor and original manufacturer of the Lansdowne dampers.

www.nocnsw.org.au

Norton Owners Club of NSW - Norton Roadholders - The Hole Story by Peter Crespin

Post date: Dec 25, 2011 12:51:16 PM
www.nocnsw.org.au www.nocnsw.org.au
"It does not take a degree in mechanical engineering to work out that there may as well be no taper at all above the lowest edge of the holes. This is because, as before, there is no real restriction of oil movement during fork compression, since it just squirts up inside the damper tube through the holes."

If you believe there is a measurable amount of compression damping in the original system then perhaps you'd care to have a go at describing how that compression damping is achieved.
:)
Click to expand...
So I'm given to believe that both the drawings and the commentary reprinted by Haynes in the Commando book (appears in the Dommie book also) were done by the Norton marketing department and they never actually made the forks as depicted and described. I've had many of these apart for bushing replacement and to replace bent stanchions and never really paid any attention to the where the holes were placed. I just believed the manual and when customers complained of topping out forks we just put in heavier oil.

I don't know about measurable compression damping but the system can only compress as fast as oil can go through the holes that aren't shown in the drawings. Admittedly four 1/4" holes will flow a substantial amount of oil but I don't recall encountering damper tubes with four holes and yes even two will flow relatively a lot of oil but there will be some compression damping even at that, particularly with heavier oil. We used 20wt unless someone complained about topping out as most of out customers rode in an area that rarely exceeded 80F. And they frequently rode at 30+F in the rain. I don't remember complaints from Norton riders about forks bottoming even when braking. BMW riders were another story altogether and we serviced those too. Both marques were often ridden as commuters in all weather crossing Puget Sound on the ferries.

My own MKIII Interstate has progressive springs that I installed and I have never experienced forks bottoming unplesently. I weigh 190lbs. I use 20wt Bel-Ray fork oil. It's entirely possible that the previous owner modified the damper tubes but it doesn't show up in the records I got from him that go back to 1985. I've only had these forks apart to change bushings, and again, never paid attention to holes.

I'm not totally naive about fork bottoming jolts. On my first ever jump riding a Matchless scrambler I closed the throttle after launching, which resulted in the bike landing front wheel first with a loud bang that moved the handlebars a couple of inches. Nothing bad happened to the bike and I was instructed not to shut down in the air. Next lap I ran off into the blackberries and had to be rescued. I don't do that sort of thing with my MKIII (or anything else these days).
 
Thomasa said:
So I'm given to believe that both the drawings and the commentary reprinted by Haynes in the Commando book (appears in the Dommie book also) were done by the Norton marketing department and they never actually made the forks as depicted and described.
Click to expand...

Yes, unfortunately, it is true that the forks do not perform exactly as described. It is well documented and you might have read about it in the Crespin article link by now regarding the Covenant modification etc. and there's also the dynodave article (link, below) plus a few forum discussions:

www.rgmnorton.co.uk

COVENANT FORK DAMPER CONVERSION KIT (IMPROVES DAMPING)

www.rgmnorton.co.uk
Thomasa said:
I've had many of these apart for bushing replacement and to replace bent stanchions and never really paid any attention to the where the holes were placed.
Click to expand...

Pre-Commando and early Commando damper tubes had four holes below the taper (see jsmotorsport.com link in post #40 for more information). Later (so the majority of) Commando damper tubes had two 1/4" holes midway along the taper and what is shown in the Haynes/Factory fork diagrams, however, it seems the change was somewhat ineffective.

Thomasa said:
I don't know about measurable compression damping but the system can only compress as fast as oil can go through the holes that aren't shown in the drawings. Admittedly four 1/4" holes will flow a substantial amount of oil but I don't recall encountering damper tubes with four holes and yes even two will flow relatively a lot of oil but there will be some compression damping even at that, particularly with heavier oil.
Click to expand...

Except for the fact that the damper valve is open on the compression stroke (manual diagram C1) so the oil is mainly moving around the damper piston, not ejected from the damper tubes in sufficient quantity to provide any effective damping unless the holes are significantly reduced in size/area (jsmotorsport.com link again).

Thomasa said:
I don't remember complaints from Norton riders about forks bottoming even when braking. BMW riders were another story altogether and we serviced those too.
Click to expand...

If you are referring to the BMW R5/6/7 series forks then I found the anti-dive springs fitted in place of the 'rubber' compression buffers plus progressive springs, 10W oil and the damper piston modification to eliminate the fork 'clatter' substantially improved the BMW 'pogo stick' forks.

Thomasa said:
My own MKIII Interstate has progressive springs that I installed and I have never experienced forks bottoming unplesently. I weigh 190lbs. I use 20wt Bel-Ray fork oil. It's entirely possible that the previous owner modified the damper tubes but it doesn't show up in the records I got from him that go back to 1985. I've only had these forks apart to change bushings, and again, never paid attention to holes.
Click to expand...

I also fitted (RGM) progressive springs, Except for the odd occasion then the bottoming isn't exactly harsh but could be improved upon as stated previously. The rebound 'clunk' with the standard forks is more noticeable when the bike is on its main stand when the forks are unloaded and is more of an annoyance but not an entirely satisfactory situation for the damper assembly to be the extension stop instead of the extension buffer described in the manual "Extending" diagram notes that simply cannot happen due to the damper assembly reducing the fork extension* movement by approximately 1.5", the stanchion holes never reaching the level of the upper bush so are never "blanked off" hence the Covenant (or RGM extended upper bush) conversion.

*(The fork movement figure given in the factory manual is 6.0", however, this is reduced by the attachment of the damper assembly which reduces travel to approximately 4.1".)
 
Some years ago I made these cross sectional sketches of the fork.
Fully compressed on the left, fully extended in the middle, fully extended with damper cap removed on the right.

What oil in the forks will produce a smooth ride ?


What oil in the forks will produce a smooth ride ?
 
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