Science versus anecdotal.

[Vince]

I need to know the answer to the age old question, does pulling the forks down really had in a trailer situation pressurises the forks to the point where seals can blow. Its happened to me but I keep hearing it's a coincidence as no added pressure is applied to the seals when the fork is fully compressed for hours at a time. Or is bouncing on road bumps an added issue while travelling like this.My anecdotal experience and gut says yes added pressure,and blown seals has happened but I am not an engineer. Any difference with this between conventional and upside down forks.

 

[wdietz186]

Pulling the tie downs way down won't blow oil out of the the seals unless the oil is too high and the vents in the fork caps are plugged. It can accelerate the sacking out of the fork springs tough. If the seals leak when transported they were tired to begin with. Ideally, tying the bike down using just the sliders [around the leg over the caliper/mount] is the way to go as it doesn't compress the fork and you can get the strap really tight. I always tie the rear down for stability along with cinch straps on wheels to the trailer rails. Haven't lost one yet! In my van I use one of those roll in wheel chocks and set the rear up on the pit stand using tie down straps just snug enough to limit side to side movement, works well too!

 

[Dellortoman]

Sounds a bit implausible to me Vince. I've always transported bikes with the tie-downs pulling down on the suspension (front and rear) and never had a problem with fork seals. I reckon it's the safest way to tie bikes down because the compressed suspension keeps the tie-downs tight, even if they stretch a bit.

The seals can't actually blow out because they're retained by circlips. And the seals are designed to cope with pressure, so a sustained pressure inside the fork legs won't hurt them. If the internal pressure forces any air or oil past the seals, then the seals are beyond their best anyway.

If you want to avoid pressurising your forks for any reason, it's dead easy. After you've tied the bike down, briefly undo the plug in the middle of the fork top cap (or depress the plunger in the Schrader valve if you have one). That'll release the internal pressure.

I disagree with wdietz186 that "it can accelerate the sacking out of the fork springs".

Even if you crank it down so hard that the springs are coil-bound, you'll not take the springs beyond their elastic limit.

wdietz186 may be referring to "spring creep" which happens below the elastic limit, but it's very much temperature and time dependent, and we're talking high temperatures and long times. The effect is negligible at normal temperature over short time periods. In the practical world (and in my opinion) you'll never make any permanent change to the free length of a fork spring simply by compressing it for a short time with tie-downs. By "short time" I mean anything that's not measured in years.

 

[Vince]

The reason I asked is I read on another thread about a Fireblade owner who had his bike shipped from Queensland to Melbourne and when it arrived it had fork oil dripping out. I had an old trail bike that blew lots of fork oil out after a 130ks trip on bitchumen and then 20ks of 4 wheel drive track strapped down hard in my trailer. Thats a sample size of 2 failures in maybe multiple hundreds of other trips of mine with zero issues.So what I get is mostly no, but on warn seals maybe. I love some other posts on the other thread, its IMPOSSIBLE.

 

[Paul Marx]

I've noticed that repeated trailering with the use of tie downs damages seals.

Paul

 

[Dellortoman]

I've noticed that repeated trailering with the use of tie downs damages seals.

Do you have a reason to believe it's the trailering rather than the riding in between the trailer trips that's damaging the seals?

 

[sweetas]

No expert on forks or suspension.... but from an Engineers POV - the oil in a Fork is to dampen the movement, up & down.  The oil is only under pressure on one side of a fork valve when the oil is moving/been pushed through the valve.  The pressure equalises on either side of the valving when movement stops.  I understand, some complex forks have 2,3 or 4 sets of valving so they can achieve different release rates for oil during compression and decompression.  But doesn't change the Principle.  The oil pressure in our forks equalises to ATM once movement stops.

 

[Simonr501]

I think the issue is a SUDDEN compression of the forks by a trailer going over a bump or such like.
The forks are already compressed and this not doubt raises pressure quickly, perhaps over hundreds or even thousands of times per trip - trailers are not known for having soft and compliant suspension.

Be interesting to figure out how many seals blow out when the bike is in a van?

 

[CLEMTOG]

its an age old problem
tying the bike down using only un-sprung components is probably the best, in theory, that is the way that I do it with cars and so does everyone else (pretty much) strapping them down to the trailer by the wheels, using strangle straps, I would be happy to do this with bikes,using the fork sliders at the front and the wheel itself at the back, but that means the straps are too low and at a very shallow angle, so I strap down using the handlebar ends and the rear carrier or sub-frame, this means that the bike cannot move about on its springs, which I would prefer it to do, it does work OK but I did once experience a problem (on route Breganze 2017) when the trailer went over a very large pot hole which I did not see, black tarmac hole full of black water, the crash "in" was enough to compress the springs on TOG even further than the tension that I had applied and on rebound was enough to break a strap, maybe that strap had a tiny flaw or rip, I don't know, but for sure it broke, and I stopped and replaced it. What I did not notice (very stupid of me not to check) was that the wheel which had traversed the road defect had also suffered, two studs had snapped and the nuts gone, the other two were also damaged and (maybe) 50 miles later the wheel was clattering loose, was damaged and only one nut remaining, with 600 miles to go to home, I was very lucky to be near the house of a Swiss Laverda friend, whose workshop proved invaluable for repairs which lasted without further trouble for the remainder of the journey, even though they were very much bush engineered temporary fixes, thankfully a spare wheel is carried, so that helped, studs were swapped from the good side and plain bolts used to fill all vacant holes. ( I now carry in addition to a spare wheel, replacement studs and nuts and a few of the bolts just in case.... and always a few spare straps,  so beware
CLEM

 

[Dellortoman]

The oil pressure in our forks equalises to ATM once movement stops.

Oil pressure may equalise across the damper valve when movement stops, but it doesn't equalise with atmospheric pressure outside the fork leg. Think of a fork leg like a bicycle tyre pump, when you push the plunger in with your finger over the air outlet hole. We've all done it. The pressure inside increases, and stays that way until you let go of the pump handle and it boings back out. 

It's exactly the same when you compress a fork leg.  There's no way for the air inside it to escape so the pressure inside the fork increases.  Since the oil and air share the same enclosed space, they're at the same pressure, which is higher than outside the fork.  Pressure remains high until the fork is allowed to extend again.

I think the issue is a SUDDEN compression of the forks by a trailer going over a bump or such like.
The forks are already compressed and this not doubt raises pressure quickly, perhaps over hundreds or even thousands of times per trip - trailers are not known for having soft and compliant suspension.

Sorry Simon, but that doesn't make any sense to me. What you describe is no different to when the bike is travelling along the road under its own steam. You compress the forks when you climb aboard, and then they are compressing further and rebounding all the time as you ride along. That's what they're for.

Anyway, if it's tied down on a trailer or in a van by the handlebars with the forks compressed against the tie down straps, there is no appreciable movement in the suspension.

 

[Simonr501]

What I am saying Cam is perhaps the trailer AMPLIFIES the forces because of uncompliant suspension!
I am also speculating that a van will have better suspension than a trailer?

 

[Paul Marx]

Do you have a reason to believe it's the trailering rather than the riding in between the trailer trips that's damaging the seals?

Yes.
The seals let go a lot more often when I trailer what is an otherwise regularly used bike, used weekly at least.

Paul

 

[hooksey]

Yes.
The seals let go a lot more often when I trailer what is an otherwise regularly used bike, used weekly at least.

Paul

Let some air outta ya tyres. 

 

[earemike]

I?m not adding by saying that I was taught not to tie a bike down too tight in a trailer - just enough to do the job.

I?ve also seen people tie down hard there is little if any give in the suspension. I doubt the suspension was engineered for that purpose so why push your luck?

I?ve also had bikes shipped internationally (& across oz) which seem to have warped disks when they arrive. I?m guessing tightly tied down & a little twisting/movement.

I can?t prove anything but had better luck tying bikes down enough they don?t fall over.

 

[Andy]

Let some air outta ya tyres. 

Actually yes. The trailer, not the bike...

I always adjust the tyre pressure on my trailer depending on load, makes a huge difference

 

[sweetas]

Oil pressure may equalise across the damper valve when movement stops, but it doesn't equalise with atmospheric pressure outside the fork leg. Think of a fork leg like a bicycle tyre pump, when you push the plunger in with your finger over the air outlet hole. We've all done it. The pressure inside increases, and stays that way until you let go of the pump handle and it boings back out. 

It's exactly the same when you compress a fork leg.  There's no way for the air inside it to escape so the pressure inside the fork increases.  Since the oil and air share the same enclosed space, they're at the same pressure, which is higher than outside the fork.  Pressure remains high until the fork is allowed to extend again.

This is what I love about the internet..... People can say what they want, right or wrong, with no consequence. its Utopia!  Wish my professional life had no consequence for been wrong.