Fork seals and tying down?

[Reckless]

I've Posted pics before
$1-00 worth of 4x2 (prob free from a local building site), the side of a old nail box (for the cross bit), 4 screws, 15mins of time and a bit of old tyre black. Eliminates all doubts and you can pull the bike hard down, drive like a nutter and its nice and secure without any danger to the fork seals etc. For the above effort I'd rather use one! Debate over!!!

[ricey]

Using a spacer heightens your chances of loosing your bikes more than a faulty tie down or not tying off correctly, I know of more people loosing there bikes from loosing there spacer than tie downs.

We drive on the roughest roads to get to the tracks, thats when they fail ,they simply jump out.

[Reckless]

I see your exactly what you mean but with the T ones I make outa old timber, rest back on a nice angle against the fork tubes, the bolts that mount the guard dig into the top and the knobs on the tyre hold the bottom. With only 12-15 PSI in the tyres I pull them down so the spacer pushes into the tyres a bit, so you have the air pressure as a further gap filler if something loosens off. No chance in my opinion of these coming out.

I completely see your point though, if you just have a piece of timber in there or one of the ones you can buy that don't have the T thing in them. Good point!

[telliman]

I see your exactly what you mean but with the T ones I make outa old timber, rest back on a nice angle against the fork tubes, the bolts that mount the guard dig into the top and the knobs on the tyre hold the bottom. With only 12-15 PSI in the tyres I pull them down so the spacer pushes into the tyres a bit, so you have the air pressure as a further gap filler if something loosens off. No chance in my opinion of these coming out.

I completely see your point though, if you just have a piece of timber in there or one of the ones you can buy that don't have the T thing in them. Good point!

a strop over the back wheel limits any movement at all

[Rogue]

Secured off road bikes with rope before tie downs were invented then have used tie downs for years never had a blown seal or stuffed shock

[CRF119]

no you will never get them to pop the seals out, as all the oil dose is pass through valves to the other side, it is simply there to slow the shock down not act as a spring, thats the springs job. I have flat landed some crazy things there is rubber scrapping on both front and rear of my gards from bottoming and seals are still going well. I do have the cool push button bleeders which i push after every hour or so.

Only way i can see them poping seals is if they are over full.

[Mental Trousers]

I have read numerous times that its just a myth that tieing down your front forks causes leaking seals and I would tend to agree, The oil is never under pressure it simply passes through the valves, If the suspenson can handle the abuse a motocross track throws at it I very much doubt that pulling it down while transporting it will harm it, I guess extended periods of being tied down could cause the springs to compress slightly but thats about it.

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Fork fluid doesn't compress at all (that's 1 of the properties that make it useful in forks). However, the air in the top of the forks does and that gets compressed to something silly like 20 atmospheres (it's different for each bike that you tie down because of air gap length before and after blah blah). And compressing air above a fluid is a great way to create an emulsion (gasses dissolved in the fluid) which sucks as a damper in forks. It's also the air pressure (that 20 or so atmospheres) that forces the fluid through the seals if they're slightly dodgy.

If you haven't noticed it affecting the damping then chances are it's been a bit sludge like since the 1st time you tied it to the trailer, which is probably straight after you changed the fluid so you could get to the track and test it.

[Badjelly]

For the record I don't compress the front suspension on my road bike at all either.

So you never sit on it?

[xwhatsit]

So you never sit on it?

No, it's a rigid frame with girder forks and he just wheelies it everywhere.

[Ocean1]

Fork fluid doesn't compress at all (that's 1 of the properties that make it useful in forks). However, the air in the top of the forks does and that gets compressed to something silly like 20 atmospheres (it's different for each bike that you tie down because of air gap length before and after blah blah). And compressing air above a fluid is a great way to create an emulsion (gasses dissolved in the fluid) which sucks as a damper in forks. It's also the air pressure (that 20 or so atmospheres) that forces the fluid through the seals if they're slightly dodgy.

If you haven't noticed it affecting the damping then chances are it's been a bit sludge like since the 1st time you tied it to the trailer, which is probably straight after you changed the fluid so you could get to the track and test it.

20atm sounds right, lemesee… if the inside dia of the stanchion is 40mm, the static weight on the front suspension is 100kg and we pull it down by another 100kg with the tie-downs we get… 15atm, 226psi. It don’t take a lot of water to make a mess of some oils, but the partial pressure of any actual water in that wee gas cell is likely to be sfa, and one of the characteristics of most oils for that application is a high resistance to emulsification.

The only time I ever really had trouble like that was when I was pissing around with a pressure controlled fork breathing system on a dirt bike (because it was pumping up). The continual cycling of fresh moist air (and probably the occasional serving of fresh moist water) did affect the oils’ damping performance. I had to ditch the breather and change to different seals.

Oh, and once or twice from impromptu drowning lessons.

[geoffm]

Now, I'm a silly fellow with no idea about suspension, and my only experience with forks is the ancient technology of damper rod units. However, unless the oil is well over-full, what pressure are you putting on the fork seals? Fully compressed the oil shouldn't be causing hydro-lock, should it? The damper unit should mechanically bottom out first -- on my forks, there's a wee extra spring on the bottom to stop a painful *smack* of parts hitting together, but certainly the oil shouldn't be hydro-locking.

Airgap at the top compresses, pressurising the inside of the fork. The airgap is a progressive rate spring in effect.

[Robert Taylor]

I have read numerous times that its just a myth that tieing down your front forks causes leaking seals and I would tend to agree, The oil is never under pressure it simply passes through the valves, If the suspenson can handle the abuse a motocross track throws at it I very much doubt that pulling it down while transporting it will harm it, I guess extended periods of being tied down could cause the springs to compress slightly but thats about it.

Fork bushing clearance on practically all ''upside down'' forks is usually very liberal so that friction is minimised. That means in turn that fore and aft play is very significant and the seal lips neccessarily distort to accomodate this very fact. When the forks are in constant movement there is usually no problem but there is a finite service life with such expendable wear components.
When the forks are compressed in one position for a period of time with no movement the inner tubes will thrust hard against one side of the bushings and therefore distort the seal lip in one direction for that same period. Because of this they are a lot more likely to leak in this situation. This can be exacerbated by poor fork tube centralisation and tubing alignment by rough and ready axle installation. Its also not smart to buy cheap seals and find the cheapest guy to instal them who has neither the proper tools or fussy work ethic.