JS Motorsport Technical upgrade paperwork on Forks

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olympus

olympus

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Hi all
After a bit of advise on the Below technical papers from JS Motorsport on mods to forks
This is a good read with easy achievable mods to improve the forks
Other than fitting RGM extended top bushes I have carried out most of them which includes the plugging of the 1/4 holes in the damper tube conical sections & drilling them with a single 1/16 hole
I'm also using 20 weight fork oil
I have to say is wasn't expecting the compression damping to be rigid as it is...
So my question has anyone else carried out the single 1/16 drilling to the previously blazed in plugs.. and should I be using a lower weight oil??
jsmotorsport.com

Norton Motorcycle Suspension

Norton Motorcycle Suspension Upgrades. Replacement suspension components, Improve the ride quality and the performance on your Commando or Twin.
jsmotorsport.com
 
It reads:

" Generally speaking you do not need compression damping on the street because too much compression damping will give a harsh ride (the stock setup has no compression damping). "

and

"A single 1/16″ hole per tube in the conical section will give you compression damping (smaller than 1/16″ will cause cavitation). A single 1/8″ hole per conical section gives minimal high speed compression damping and is good for the street. "

To check compression damping remove the top nuts and springs only. Bottom the forks and grab the damper rods. Pull them up and then ram them down by hand. It should only take 1 second on the compression stroke. If you don't like the feel of compression damping - do what I did and stay with only one 1/8" hole for minimal compression damping or leave the holes at full 1/4" with no compression damping.
 
jseng1 said:
It reads:

" Generally speaking you do not need compression damping on the street because too much compression damping will give a harsh ride (the stock setup has no compression damping). "

and

"A single 1/16″ hole per tube in the conical section will give you compression damping (smaller than 1/16″ will cause cavitation). A single 1/8″ hole per conical section gives minimal high speed compression damping and is good for the street. "

To check compression damping remove the top nuts and springs only. Bottom the forks and grab the damper rods. Pull them up and then ram them down by hand. It should only take 1 second on the compression stroke. If you don't like the feel of compression damping - do what I did and stay with only one 1/8" hole for minimal compression damping or leave the holes at full 1/4" with no compression damping.
Click to expand...

I have carried out the brass sleeve modification to the damper tube top hats, in addition fitted new damper rods.
The rods are now a beautiful sliding fit.
Can i ask when the rod is lifted (done outside of the fork leg & with the damper tube in oil) how long should it take to lift the rod from the lowest to max posistion.. and more importantly should I be seeing oil escape from around the rod to top hat bush...I don't have any
So should all the oil be passing back through the internal valve??
I have replaced the valve internals and slightly opened the valve to achieve a nice sliding fit within the damper tube without any slop.[/QUOTE]
 
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Rebound damping should always be more than the compression damping so you are looking for more than 1 sec to pull the rod up, I assumed the rebound rate was set by the leakage out of the top.
 
kommando said:
.. I assumed the rebound rate was set by the leakage out of the top.
Click to expand...
Not really.
It is set by the clearance of the piston in the damper tube.
0.45 mm IIRC.
Wear of the damper cap will of course reduce rebound damping.
 
ludwig said:
Not really.
It is set by the clearance of the piston in the damper tube.
0.45 mm IIRC.
Wear of the damper cap will of course reduce rebound damping.
Click to expand...
It will indeed but the fit of the damper rod in the now brass bushed top hat is allowing no oil to pass.
So my question was should the oil only pass internally within th damper valve
When I purchased the valve I also speced the "B" variant so it only needed the merest adjustment to get a lovely sliding fit
 
To answer questions above.

It doesn't matter if a little oil passes through the top collar.

"Place the damping cup on a socket. Tap carefully with a hammer and a tapered carpenter plumb bob to swell the open end to .678″ (.680″ max and .685″ is too stiff)."

Or you can make a valve with .678 to .680" max diameter.

"With used a sleeved cap and the cup flaired to .678″ you can pull the damper rod up by hand (immersed in container with 20 wt oil) in about 3 to 4 seconds and this is what you want."

The rebound damping is controlled by both the top collar and the valve cup. Its easiest to just sleeve the top collar and then adjust the Outside diameter of the valve cup correctly.

More answers to your questions are in the details here:
jsmotorsport.com

Norton Motorcycle Suspension

Norton Motorcycle Suspension Upgrades. Replacement suspension components, Improve the ride quality and the performance on your Commando or Twin.
jsmotorsport.com
 
Still struggling with the rebound damping. Currently compression damping using a single 1/8" hole in the conical section Is giving me with 20 weight oil a push down time just over 1 second so well happy with this.
On the other hand the rebound damping has turned into a full hydraulic situation components assembled and operated on the bench in a container with correct oil, after 2/3 pumps of the damper rod the assembly hydraulics.... solid
Strippd back down I have to ask....
The way the valve is designed is on the compression stroke the valve is lifted off the squared washer and against the peg, oil then flows between the valve and damper rod into the damper chamber.
On the rebound stroke the trapped oil pushes the valve back against the washer and in affect seal it.

I have read many times hear about having th damper cap in good condition & with no wear, and to that degree I have carried out the JS Motorsport mod to brass bush the caps, and these are now a lovely sliding fit.
Studying the fork drawings in the shop manual it clearly shows oil being expelled from the top cap....

So if it's so critically important to have good fitting/unworn top caps (which I understand to give some rebound damping) ...how does the oil get out of the upper chamber if the valve is seated against the washer????? Because at the moment the trapped oil is locking the unit solid
 
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The oil has 2 potential paths, out the top or past the valve, if its locking once the chamber above the valve is filled then you have too little clearance for the oil to pass out either of the 2 paths hence the lockup. So either reduce the OD of the valve or the id of the top damper bush. For compression you have a 1/8" hole giving 1 second, for rebound giving 2/3 sec you roughly need a clearance with 1/3 to 1/2 of the x sectional area of a 1/8" hole.
 
olympus said:
It will indeed but the fit of the damper rod in the now brass bushed top hat is allowing no oil to pass.
So my question was should the oil only pass internally within th damper valve
When I purchased the valve I also speced the "B" variant so it only needed the merest adjustment to get a lovely sliding fit
Click to expand...
I think it must pass through the bush but rate will be small. Could you pulling the rod force it through perhaps not? I've seen an original drawing and the bore diameter differs by less than 0.001" from model to model. ie Commando, Domi, ES2. etc.
 
I have a spare pair of top caps without bushings.. I'll give these a try, I'd like to leave the valves if possible as these are assisting the compression damping
 
Measure the cup OD, the damp tube ID, the damp rod OD and the top cap ID. If you don't measure then you are just guessing and you may never get it right. It sounds like your cups are too enlarged. Show us what you have and compare them to the specs at:

jsmotorsport.com

Norton Motorcycle Suspension

Norton Motorcycle Suspension Upgrades. Replacement suspension components, Improve the ride quality and the performance on your Commando or Twin.
jsmotorsport.com

"New caps measure .380" ID so its better to drill them to 13/32" and loctite (red) in a sleeve of .378" ID .015" wall K&S brass tubing (available at hobby shops). The damper rod shaft should measure .375". Clearance must be tight or you won't have enough rebound damping. "


"To improve rebound damping. Place the damping cup on a socket. Tap carefully with a hammer and a tapered carpenter plumb bob to swell the open end to .678" (.680" or more can be too stiff). The inside of the damper tubes should measure approx .690" max."


"With a sleeved cap and the cup flaired to .678" you can pull the damper rod up by hand (immersed in container with 20 wt oil) in about 3 to 4 seconds and this is what you want."

Match these measurements before going further.
 
I fitted a new cap onto the damper tube yesterday and repeated the test..... once again the assembly locked up solid..
So i fitted an "A" variant damper valve with the brass sleeved top hat.... at last I now have a rebound movement of 4 seconds :)
Stripping the assembly back down I measured the valves
"B" variant is 0.682
"A" variant is 0.677
This minuscule difference has made all the difference, and I now have controlled rebound damping
I will perform the same test on the opposite leg to hopefully achieve the same compression & rebound figures
 
olympus said:
I fitted a new cap onto the damper tube yesterday and repeated the test..... once again the assembly locked up solid..
So i fitted an "A" variant damper valve with the brass sleeved top hat.... at last I now have a rebound movement of 4 seconds :)
Stripping the assembly back down I measured the valves
"B" variant is 0.682
"A" variant is 0.677
This minuscule difference has made all the difference, and I now have controlled rebound damping
I will perform the same test on the opposite leg to hopefully achieve the same compression & rebound figures

Miniscule difference: presumably because that apparently small difference is in relation to chamber they run in, and hence the clearance/ cross sectional area of the GAP, which is what is critical here??
Click to expand...
 
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