Knocking sound from forks

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daveparry

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Hi all,
I'm just checking my interstate over for MOT, (not used it for a couple of years or so since I got involved with BMW's!) and when I bounce the front end up & down I can hear a metallic knocking noise, sounds like something to do with the damper rods, any ideas anyone? I have a feeling i've heard this before and it's never affected the MOT test in the past, maybe it's normal?
I'm not talking about the heavier clunk of the forks 'topping out' when thay become fully extended but a rather lighter sound,

Dave.
 
daveparry said:
I'm just checking my interstate over for MOT, (not used it for a couple of years or so since I got involved with BMW's!) and when I bounce the front end up & down I can hear a metallic knocking noise, sounds like something to do with the damper rods, any ideas anyone? I have a feeling i've heard this before and it's never affected the MOT test in the past, maybe it's normal?
Click to expand...

Usually normal, especially if you hear it when you bounce the forks statically, as the damper rods and tubes act as the fork extension stops. If you hear the same noise under hard acceleration then it may be worth trying a heavier fork oil or the damper caps and rods may need replacing as they do wear out which affects the rebound damping.
 
Thanks for that LAB, I had an idea i'd heard it before but having not used it for a while couldn't quite remember!

Dave.
 
If it were me, I would change the oil in the forks anyway. Get rid of that old oil and start fresh.
 
Yes Steve, that would be a good place to start, can't remember what oil I put in there last time, probably one of the purpose made fork oils. Maybe i'll try some 20/50, the original spec would have been sae20 grade but I reckon 20/50 should work ok,

Dave.
 
daveparry said:
Yes Steve, that would be a good place to start, can't remember what oil I put in there last time, probably one of the purpose made fork oils. Maybe i'll try some 20/50, the original spec would have been sae20 grade but I reckon 20/50 should work ok,

Dave.
Click to expand...

I have been using Bel Ray Fork oil for longer than I care to say. 20w should be fine. I would be reluctant to use muligrade oil in my forks, the 50w component will make for a pretty harsh rebound damping.

http://www.belray.com/bel-ray-high-performance-fork-oil
 
steveyacht said:
I would be reluctant to use muligrade oil in my forks, the 50w component will make for a pretty harsh rebound damping.
Click to expand...



The "50" in 20w/50 (it's only the 20 part which is "W"= winter) denotes that the oil is the equivalent viscosity to 20w when cold, and SAE50 oil when hot, so a multigrade oil's viscosity remains more stable than monograde but it still thins out as it heats up-not that fork oil gets as hot as engine oil, or at least it shouldn't under normal riding.
http://www.realclassic.co.uk/techfiles/oil030319.html
Knocking sound from forks


VI and how multigrade works

To produce a multigrade mineral oil, polymers are added to a light oil, say SAE 20. At low temperatures the polymers are coiled up and do not restrict the flow, but as the temperature rises they unwind so that at 100 oC (212 oF) the viscosity is equivalent to SAE 50. There is more than one way of achieving this result. The best quality base oils have a higher VI, which is the rate at which the oils' viscosity falls as temperature rises within a given temperature range. This means that less polymer is required to achieve the same result. As it's the oil that does the lubricating, not the polymer, this is obviously preferable and as the polymer also degrades in use the less present to start with the better.
Click to expand...
 
daveparry said:
Yes Steve, that would be a good place to start, can't remember what oil I put in there last time, probably one of the purpose made fork oils. Maybe i'll try some 20/50, the original spec would have been sae20 grade but I reckon 20/50 should work ok,

Dave.
Click to expand...

Duncan Fitchett was using 50 gear oil in is stock Manx dampers.untill along came the Lansdowne Kit, now he use's 10 , some times 5 SAE!
 
If my memory serves me right, theres a thread here some where that states filling/blocking the small holes off in bottom of the dampners solves this problem?
Foxy
 
Foxy said:
If my memory serves me right, theres a thread here some where that states filling/blocking the small holes off in bottom of the dampners solves this problem?
Click to expand...

The holes can be repositioned (by blocking the original holes and re-drilling them higher up the tube) as the oil must be able to escape from the damper tube on the compression stroke, however it is a modification to help prevent the forks bottoming out, not topping out.

http://atlanticgreen.com/forks.htm
 
I have that knocking noise when pressing the forks up & down, so I presume it is the dampers.
Can't get on with that thin fork oil, so at the moment I am using 10-40 which seems fine.
Find 20-50 alright as well.
 
negating loose neck bearings, wheel bearing, brakes etc, up AND down can be something other than the dampers.
The stock dampers make the knock on extension when restricting rebound only.
I presume the engine is stopped and the environment quiet and the main tube locknuts are home against the seals.
Up and down could just be the bottom bushing sliding axially along the main tube.
I doubt you are pushing manually down enough to bring the compression springs to solid.
So is it only on the up or both up and down?
 
Flo said:
I have that knocking noise when pressing the forks up & down, so I presume it is the dampers.
Can't get on with that thin fork oil, so at the moment I am using 10-40 which seems fine.
Find 20-50 alright as well.
Click to expand...
There is a possibility the Knocking can be a Loose bottom bush.as one member had that noise, he glued the bush onto the stanchion and it went...allso if the stanchions are slightly twisted the sticking bush/slider condition will make forks sound "knocky" Try allso pumping the forks with the rear brake applied, not the front brake..sometimes the knock can be from a "Loose" brake. clacking back and forth.
I will not respond to the re-drilling of holes...its a waste of time.
 
It is only a metallic sound on the down stroke. Always done it. Fork bushes are tight. I think it resonates through the mudgaurd.
 
at the seasons end, or now if you wish, remove the fork fluid, raise the bike, remove the wheel, remove the dampener retaining screws, unscrew the fork top nuts and withdraw the spring and dampener as an assembly up and out from the top of the main tubes.
Now move each slider up and down and see if you continue to have the noise.
If it is determined that the lower bushing is the culprit the axial looseness of the lower steel bushing can be permanently cured by shortening the length to leave only a very short chamfer remaining on each bushing end and filling the gap with a custom cylindrical spacer. .003"-.005" clearance will quieten at any viscosity.
Some have used cyanoacrylate but I feel a metal spacer is of more permanence.
Or just accept the idiosyncrasies of a 35 year old suspension that was ancient technology then.
The valve noise will hide the fork clatter anyway.
 
Hi all,

Many thanks for all the replies on this one, it certainly seems to have created some interest!

I drained the forks and re-filled with some 10w-40 (just because that's what I happened to have available) and the noise has disapeared completely.
The last oil I put in there several years ago would have been the Castrol Fork Oil, maybe that is too thin for these older type of forks?

Dave.
 
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