What oil in the forks will produce a smooth ride ?

marshg246 said:

We are not discussing bump stops!

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Neither was I!

marshg246 said:

Without them you would be bouncing down the road.

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You seem a little agitated - maybe you should.

robs ss said:

The Covenant conversion's only existence was to create rebound damping - something the Roadholders were bereft of.

Explained well here by Peter Crespin (UK)

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

Cheers

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That is incorrect as the Covenant conversion is solely concerned with adding hydraulic bump stops, not to create rebound damping. (Edit as it's compression damping that the forks lack, not rebound)

Suspension fluids vary wildly.
So, the discussion will make an engine oil thread seam tame.

Thank you. I've learnt something new. I am surprised, on my Commando I can keep up with mates on modern retros. All without any real compression damping. What a hero, can't wait to brag!

jsnorton said:

I am currently using 20w engine oil in my 1973 Mk1 forks, but the ride on some bumpy roads is almost unbearable. Now, will using a heavier weight oil produce a smoother ride or is it the opposite, a lighter weight oil that will produce a smoother ride ?
Will adding more oil than the recommended amount stiffen up the suspension ?
I have new seal kits that I plan to install this winter because as far as I know they have never been changed and they do leak a little.
I weigh 200 pounds if that helps in the oil recommendations. I do occasionally hear clunking from the forks which seems to be on the rebound.
Any help or Ideas would be appreciated.
TIA

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Back to the start !

Be aware that 20w engine oil may not have the same viscosity characteristics as 20w fork oil

I started a similar thread titled "Fork Oil Grade" in April 2021 and got good information
- particularly the table in post 14 of my thread by lazyeye6

I decided to try Castrol 15W fork oil and am pleased with the fork action. It suits my old codger
riding style on the slow A and B roads, with traffic, horses, cyclists road works, police, speed
cameras etc around where I live.

I will post another comment in my thread to bring it to the fore in new posts. Hope you find it helpful.

marshg246 said:

We are not discussing bump stops! Yes, the standard forks can clunk when over extended. Are you referring to noise damping? None of the rest of us are. The dampers in the forks allow them to compress with little to no resistance and extend with resistance. That's what any shock absorber does. Without them you would be bouncing down the road.

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As @L.A.B. has pointed out, and I have re-read Peter Crispin's notes, the Covenant conversion does indeed only provide hydraulic bump stops at full compression and extension. My bad!!
I made the mod some years ago to my 500 and my recollection was that it improved my virtually non-existent extension damping.
I have installed the Landsdowne inserts in my 650 and it is much, much better again.

I guess I got caught up in the double whammy of being wrong and matching your bedside manners. (you're not related to a certain DynoDave are you?)
Cheers

I've always thought you need both compression and rebound damping, to avoid gradually compressing further and further on more bumpy surfaces (plenty round here). So, was genuinely surprised there's no compression damping to speak of. Guess the springs are strong enough to avoid that, as I have no complaints about how they ride. Not jolt free, but no jitters.

Mart UK said:

Thank you. I've learnt something new. I am surprised, on my Commando I can keep up with mates on modern retros. All without any real compression damping. What a hero, can't wait to brag!

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Or, fit a set of Lansdowne dampers and they'd be the ones keeping up although you'd lose your bragging rights!

Mart UK said:

I've always thought you need both compression and rebound damping, to avoid gradually compressing further and further on more bumpy surfaces (plenty round here). So, was genuinely surprised there's no compression damping to speak of. Guess the springs are strong enough to avoid that, as I have no complaints about how they ride. Not jolt free, but no jitters.

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Although considered excellent in their day (so the others couldn't have been much good) the damping of the Roadholder forks is extremely basic by modern standards and can be improved upon.

robs ss said:

I guess I got caught up in the double whammy of being wrong and matching your bedside manners. (you're not related to a certain DynoDave are you?)

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"Tin hats everybody"

Mart UK said:

I've always thought you need both compression and rebound damping, to avoid gradually compressing further and further on more bumpy surfaces (plenty round here). So, was genuinely surprised there's no compression damping to speak of. Guess the springs are strong enough to avoid that, as I have no complaints about how they ride. Not jolt free, but no jitters.

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You do need damping both ways IF you want forks that actually function correctly / properly by modern standards.
What we lose sight of sometimes is that these forks were designed 80 years ago (give or take) when hydraulic telescopic fork technology was in its absolute infancy !
What is really surprising is that they work as satisfactorily as they do!
But there can’t be any real doubt that the modern kits by the likes of Maddass, K-Tech, Maxton, Cosentino, etc are on a different planet functionality wise.

I've been very pleased with the Fauth fork mod and ATF! It worked so well that for a while after making the mod about 10 years ago, I would go out and intentionally look for bumps/potholes, etc!

marshg246 said:

The fork tubes and thereby sliders are probably not perfectly parallel. If you don't have gaiters you can verify with a flat plate - I used this: Amazon product ASIN B07T7ZMGL2
Another way to see if there's a problem is with the springs removed, the bike on a jack and the front wheel off the ground, the wheel must move up and down freely. A slightly twisted (forks not parallel) front end will fail this miserably yet still somewhat function on the road.

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Greg,
That is a good tool to have.You can get the forks in proper alignment
Mike

YING said:

Greg,
That is a good tool to have.You can get the forks in proper alignment
Mike

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Agreed. But only one of the two axi can be measured with that.

robs ss said:

As @L.A.B. has pointed out, and I have re-read Peter Crispin's notes, the Covenant conversion does indeed only provide hydraulic bump stops at full compression and extension. My bad!!
I made the mod some years ago to my 500 and my recollection was that it improved my virtually non-existent extension damping.
I have installed the Landsdowne inserts in my 650 and it is much, much better again.

I guess I got caught up in the double whammy of being wrong and matching your bedside manners. (you're not related to a certain DynoDave are you?)
Cheers

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So, next time an OP posts and you give a correct and definitive answer you'll find it OK for me to simply say your wrong with absolutely nothing to back me?

concours said:

Agreed. But only one of the two axi can be measured with that.

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True, but the other axis is fixed by the top and bottom yokes. When in doubt I measure the distance between the inner edges of the bottom yoke and the distance between the fork bottoms. I've only ever found one wrong and that one could not be made parallel the other way (why I measured). Finally figured out that the bottom yoke was bent.

marshg246 said:

I use SAE 30 (not 30w). I'm heavier than you. Extra oil is a bad idea. The heavier you are the further the forks compress for a given bump. The oil controls the rebound and has little to nothing to do with compression. So, if you are hitting bottom a lot, you need heavier springs. If you are hitting the top (clunking on rebound) a lot, you need heavier oil (or you have worn-out dampers).

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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.

Thanks everyone for your help.

jsnorton said:

Thanks everyone for your help.

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Never owned a set of those forks, but I know this.... If the fork oil was not drained and replaced regularly over the last 50 years, it basically turns into light lapping oil and wears everything out of spec. Don't skimp on parts replacement during the rebuild. Good luck.

A quick look at page 90 in the Haynes manual for 68-77 Commandos by Jeff Clew will disabuse one of the notion that there is neither compression nor rebound damping. There is both.

The ride quality can be maintained or possibly greatly improved by paying close attention to how you install the front wheel assembly. Don't tighten any pinch bolts or the axle nut until you have bounced the bike on the front suspension several times. This will ensure that the fork sliders are the correct distance apart to maintain parallel action throughout the travel, provided that the fork tubes are straight and provided that the axle is straight (check both). Don't add or omit any washers for the same reason. If you've had the clamp nuts in the triple tree loose or had a fork tube out, observe the same routine before tightening the pinch bolts. The tubes must be seated properly in the clamps.

Fork stiction can cause a rough ride and the choice of seals and fork oils can reduce this problem.

Heavy oil in the fork will do two things: increase compression damping and rebound damping. Increased rebound damping can cause the fork sliders to ride lower and lower in the travel on a washboard road with increasing spring tension with each bump, resulting in eventually a rigid ride. This is true of rear shocks also. Of course the reverse is true with light oils, both forks and shocks will top out. My forks occasionally top out but this is the price I'm willing to pay for a little plusher ride.

The damping must be matched to the spring rate. Using a heavier oil will increase the damping and have the spring operating under greater tension after the first bump of a series, giving the impression of stiffer springs. Stiffer springs will cause a perceived lack of damping unless higher viscosity oil is used.

One of the easiest ways of improving the ride is to use progressive springs front and rear.

Thomasa said:

A quick look at page 90 in the Haynes manual for 68-77 Commandos by Jeff Clew will disabuse one of the notion that there is neither compression nor rebound damping. There is both.

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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.

"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."


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

"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.