Blown seals?

No, nothing to do with Neka's last trip to Seaworld

Reading a recent article in AMCN about tying a bike down, got me wondering about the accepted wisdom that you don't want to ratchet the front of your bike down too hard when tying it down in the trailer cause it will blow the fork seals. I've read and heard this elswhere over the years too, and there are plenty of products claiming to help you tie down your bike without blowing the seals as well.

Now as far as I understand it, the forks in my bike (like most bikes with conventional forks) have no pressurised components. The damping is done by forcing the oil to go through holes or valves, which can be adjusted etc etc.... The seals will have pressure on them when the forks are moving, the quicker they are moving the more pressure there will be, but when stationary it is just the spring holding them up.

So how on earth is slowly ratcheting down the forks going to put any significant pressure on the seals? Sure the spring will be compressed, and eventually bottom out, which they wouldn't be too happy about, but no pressure. So what's the story?

Good Question "Dingodiggencrim" Mick.

Perfectly rational Mick and I agree totally.

I think the damage was done back in the days of air assisted forks (remember when dirt bike forks used to have a schrader valve to increase/decrease air pressure internally).

These would increase air pressure when tied down and blow. Conventional wisdom is having a hard time moving on from it.

Not an issue these days unless you have a leadwing or similar air assist type job. As you say theres no pressure internally other than spring.

From personal experience, we've always tied the dirt bikes down by compressing the crap outta the forks and we haven't blown seals other than what you would normally expect, in fact I'd suggest the belting a dirt bikes forks cop is a shitload worse than anything you could do to it with a tie down.

I use tie down braces now also but that's just to stop the front from bouncing down and letting the tie downs slap around

Last edited by Glen on Fri Nov 03, 2006 3:06 pm, edited 1 time in total.

Yep, I'm with you guys. Although there is an amount of air in the forks, the measurement to the top of the oil (150 odd mm), which is then increased when the springs go in and the forks fully extended to put the caps on. This is about 250mm of trapped air in each leg.
It is possible that compressing the forks "may" put pressure onto this air space..
My experience is, I've never had a problem with it nor know anyone who has...

Old joke but a goody

A penguin takes his car to a mechanic to get it fixed. The guy says "no worries, leave it here and I'll take a look at it for you".

The penguin goes across the road to buy an ice cream (penguins like ice cream you see). He goes back to the mecahnic in half an hour and say's "so, what's up". The mechanic says "I think you've blown a seal". The penguin says " No no thats just a little ice cream left on my beak"

Boom Tish.......

...The penguin says " No no thats just a little ice cream left on my beak".......

Isn't it - The Eskimo says " No Mate, its just some frostbite in my beard ." ??

Anyway, Mick - I have had a set of forkseals in a dirtbike "blow', because the knob who tied them down ( not me surprisingly), just pulled the living shit out of the tie-downs, almost to the point where the forks where fully compressed, and tied them off at that point !!! I was in another car and didn't see him doing this - of course by the time we got to the property (and after 20km of dirt roads) - the seals had well and truly shat themselves. (This was a 3 hour trip....)

The reason is that the trapped air in the forks does not compress, and when your trailer is bouncing up and down on dirt roads, the bike jumps up and down as well........enough of this will cause the seals to blow - there is already a shitload of *constant* pressure due to the tiedowns (this is the real problem), and any ongoing increases from bumpy roads(however small), just make matters worse.

The reason that seals don't blow in normal riding is that the pressure is not constant for 2-3 hours, like it is on the trailer. If you are travelling on smooth highways only ( eg roadbike to a trackday), there is less chance of a seal blowing due to tie-downs being too tight. (and every chance the trip is less than 1 hour anyway..)

Tie-down supports for dirt-bikes will stop seals blowing, and tying down your roadie from the top of the fork lowers ONLY is the best method. This way, your bike rides on its own suspension in the trailer, and will never ever blow a fork-seal from being trailered. Only for older forks of course, those u/side down bastards are, well, a bastard !!

Perfectly rational Mick and I agree totally.

I think the damage was done back in the days of air assisted forks (remember when dirt bike forks used to have a schrader valve to increase/decrease air pressure internally).

These would increase air pressure when tied down and blow. Conventional wisdom is having a hard time moving on from it.

Not an issue these days unless you have a leadwing or similar air assist type job. As you say theres no pressure internally other than spring.

Spot on!