Bleedin heck! not john Lansdowne again!!!!

Regarding the old cheshnut. fork sticking.

This is the problem; STICKY SLIDERS.After a few simple tests , your concerns about fork sticshion,could be the nature of spring compression :!:

I placed a standard commando spring down the stanchion and applied a known weight; ie 21.5 kg the spring reacted by compressing 28mm, But i could compress it another 10mm ,total 38mm..by pushing onto the 21.5 kg , and it stayed there :shock: Pulling back the said weight and allowing it to settle again , well it was back to the 28mm. The answer is simple...as the spring deforms it twists to impart a force to the tube wall. this action is a frictional force at right angles to the centre line, Appling more weight increased the outward force's . having seen many springs with there sides worn.conferms this. AND WEAR AND FRICTION GO TOGETHER This is the effects by static loading and not under road usage...but the effect is there! Interesting ?

Interesting hypothesis John. Through what angle did the weight rotate from where you compressed it to where you released it (and where it stayed)? That is, how much twist did you impart to the spring?

Hi, The weight was in a vertical downward direction,acting directly upon the spring with no side thrust, I did not twist the spring only the weight produced a "Cork screw "effect.

I am now sure "This buckling "is the common cause that produce's a sticking effect. Most complicated problem's have simple cause's, Also can Cheap stanchions with poor machined inner surface's add to the problem. Plus no one can argue that springs can rub the sides ..i have loads with lots of wear. and wear means friction..well it did when i went to school.........many moons ago :!:

Having just had a phone call from my local spring manufactuer, is words " This is a unsuitable application" Extream buckling and internal binding will result" Springs fitted to external surfaces are better suited , firstly the larger dia resists this effect, and second the increased dia change doe's not matter with external...he's only been in the job 40 years...so what doe's he know!!!?........So whats the answer? ....none i am affraid...grin and bare it lads.

John

Some years ago the industry started using air pressure to modulate the resistance on the compression of forks and put some small (what appeared to be tire valve stems) valves to add air or take out air as needed. I suspect it didn't work too well since I don't see them any more (maybe the MX lads still use them). But since air would not offer any friction it seems like a logical choice, except that the seals needed to keep the air in probably needed to be so tight that they would create more stiction than a spring. OK back to the drawing board on this. BTW the super expensive Ohlins have gone to exotic plating on the stanchions to fight the stiction.

Scooter

John, very good insight on fork spring phenomena. External is
neat solution I'll wait for you or others experiments and wear.
Flat or oval coil spring might help but for less coil binding travel.

Alas I've been aware of this a few years and my off the cuff
finding is that the fork sticking occurs prior to significant
side wall drag and past that the external and internal
forces simply over whelm any spring drag.
But this could add a sense of ratcheting to travel when
powering enough to unload front a good bit so it tends to follow
smaller road texture. Powered turns will create this condition.

Personally I think most striction occurs in the top bush
slack fit on stanchion for a tipping rocking long lever
that puts a side bind load on the bush interface near
its ends.
Likely some stanchion flex adds its intermittent drag too.

I see effects of this when approaching low and hi side
steering speeds in tights,
forks blur to focus on in fro/aft vibration but not in side-side.
When it hits resonance of unloading tire scrub-howl-squeal
surface inter action,
time to back off or enter next phases of handling trills.

It occurs to me to try tighter bush fit yet but with dry friction
coatings and maybe micro polish.

hobot, can't slide a unicycle very much.

I've seen what you describe back in my bicycle wrenching days. I performed many mountain bike for rebuilds and there was always evidence of the springs marking the internal walls of the stanchion. Mind you, all the higher end mountain bike stuff utilize alloy stanchions but the marks were always there.

As for the air dampend forks, again mountain bikes use air dampened systems all the time. They were more of a light weight racing option and I noticed the air seals on those rear and front shocks would leak a lot sooner than the oil dampened units. Both required maintenance but the air sprung ones requied servicing more. You also have to use a special high pressure pump since some of the air sprung shocks would take about 190+ psi.

Hi John,
Absolutely correct - spring will corkscrew when compressed, shorter harder springs will not show it so well but longer springs will. It is a check that we have to carry out 20mm Gambo, General purpose machine gun, MSI's 30mm and some other small arms, most of the springs are stranded and then wound into the recognisable spring. It is usually the strands on the out side that are easily identified as worn or sometimes broken due to this effect. The springs that have a telescopic compressible carrier holding it together through the centre show the corkscrew effect really well.

Interestingly the use of air springs in moutain bike forks is more the norm than coil springs now, they are also more complicated than most new motor bike forks! One thing that I have seen that may reduce the rubbing is the use of a thrust bearing on one end of the spring, as the spring compresses this alows it to rotate at one end, this approach seems to work reasonably well on moutain bike shocks with titanium springs which have few coils

Cheesy said:

Interestingly the use of air springs in moutain bike forks is more the norm than coil springs now, they are also more complicated than most new motor bike forks! One thing that I have seen that may reduce the rubbing is the use of a thrust bearing on one end of the spring, as the spring compresses this alows it to rotate at one end, this approach seems to work reasonably well on moutain bike shocks with titanium springs which have few coils

Click to expand...

Early mountain bike forks actually used elastomers instead of springs. Back on those days one of my clients was MountainSpeed who made metal spring replacements for shocks such as the RockShox Judy. The springs were actually made by Eibach, which are very well known spring makers.