What oil in the forks will produce a smooth ride ?

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jsnorton

jsnorton

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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
 
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).
 
The weight of the oil will make no difference if the bumps are causing the suspension to bottom out on compression due to short or weak springs. You need to check your static rise height with the normal rider on board and off any stands, aim for 1/6 of full front suspension movement and 1/3 of the rear. If its still uncomfortable then you need to rebuild the forks to get the damping back, the valving is normally worn and low damping will allow too much suspension movement, a thicker oil will restore some damping but better to get the valves working with 20wt oil.
 
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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Is that right? No compression damping?
 
Mart UK said:
Is that right? No compression damping?
Click to expand...
Generally true of all simple shock absorbers. Our dampers have holes at the bottom. When the damper moves down the oil is ejected from those holes with little to no resistance. On the way up there is no release for the oil other than around the bottom of the damper. The bottom of the damper isn't a piston but a more or less square plate with rounded corners in a round tube.

You can see this easily with rear shocks. Remove the springs. The shock will collapse with almost no effort but it somewhat difficult to extend and the faster you try to extend it the harder it is.
 
Compression damping works with the spring force, rebound works against the springs so the rebound damping is always higher so it can actually have an affect. Reducing the holes in the bottom of the damper tubes will increase the compression damping which is underwhelming even with unworn internals.
 
It would be nice if my forks could react a little quicker on compression at slow speeds. Sometimes it feels like the forks don't budge over small holes or bumps in the road. Any solution for that?
 
seattle##gs said:
It would be nice if my forks could react a little quicker on compression at slow speeds. Sometimes it feels like the forks don't budge over small holes or bumps in the road. Any solution for that?
Click to expand...
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.
 
seattle##gs said:
Any solution for that?
Click to expand...
Digressive damping, these are shim stacked fork valves which give high damping at slow fork movement speeds on smooth roads and then as the forks move faster over bumps the shims deflect and the damping reduces. I have CBR600RR shim stack valves in my forks to achieve his type of damping. You can tune the low speed damping with needles that restrict the small hole through the centre and the shape of the damping curve by changing the diameter, thickness and number of shims in the stack.
 
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.
Click to expand...
The Atlas has the metal shrouds over the forks so I can't use the plate glass method you describe.

I can put a jack under the motor and remove the two fork caps which should free up the forks to perform your next test.
 
another factor is top fork bush stiction, which has been covered in other threads, and for which aftermarket low friction bushes are available from NYC Norton and JS Norton.
 
Does the clunking reported by OP happen just on rebound or also on compression?
I had clunking quite often and on compression. Turned out the threaded damper rods had come unthreaded from fork caps. This meant no damping could happen, just spring action. Clunking was end of rods bashing on caps.
Another clue was the rear wheel was not fully clear of floor when on centerstand. Makes sense b/c the forks were extended more with no damper rod limiting the extension, so front end sitting higher than correct, rear sitting lower when on CS.
 
Tornado said:
Does the clunking reported by OP happen just on rebound or also on compression?
I had clunking quite often and on compression. Turned out the threaded damper rods had come unthreaded from fork caps. This meant no damping could happen, just spring action. Clunking was end of rods bashing on caps.
Another clue was the rear wheel was not fully clear of floor when on centerstand. Makes sense b/c the forks were extended more with no damper rod limiting the extension, so front end sitting higher than correct, rear sitting lower when on CS.
Click to expand...
it seems to be just on the rebound.
 
Despite some of the comments above standard Norton forks have no rebound damping, and never had, so "clunking" as the forks top-out is inevitable.
Two common ways to remedy this are:
1. Do the "covenant conversion" modifications (not difficult), or
2. Install Landsdowne inserts from our own @madass140 - more expensive but superior.
Cheers
 
Wot no rebound nor compression damping? What's the fork oil for? Not what I feel on the road. It feels ok on some pretty average road surfaces. It felt pretty soft and vague before I changed my fork oil.
 
Mart UK said:
Wot no rebound nor compression damping? What's the fork oil for? Not what I feel on the road. It feels ok on some pretty average road surfaces. It felt pretty soft and vague before I changed my fork oil.
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I stated there is no rebound damping. Of course there is compression damping:rolleyes:
Cheers
 
robs ss said:
I stated there is no rebound damping. Of course there is compression damping:rolleyes:
Cheers
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What we have here is proof of alternate universes! Whoever thought that the first one we found would be 180 degrees out! Or maybe is the toilet left swirl verses right swirl syndrome causing roadholding to work backwards. :)

You might want to try a bench test of a Norton damper before saying others are wrong. 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.
 
marshg246 said:
What we have here is proof of alternate universes! Whoever thought that the first one we found would be 180 degrees out! Or maybe is the toilet left swirl verses right swirl syndrome causing roadholding to work backwards. :)

You might want to try a bench test of a Norton damper before saying others are wrong. 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.
Click to expand...
The Covenant conversion's only existence was to create rebound damping - something the Roadholders were bereft of.

Explained well here by Peter Crespin (UK)
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
Cheers
 
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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)
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
Cheers
Click to expand...
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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