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Old 08-03-2014, 08:00 AM   #61
randyo
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I do not get it

does not people understand how forks work ?

when you compress the fork, you are compressing the spring, not the fluid

all fork seals to is keep the fluid inside the fork, the spring action is dampened by fluid moving from one side of a valve to the other, fork fluid is never under pressure of going past the seals, it is only under pressure of going from one side of the valve to the other
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Old 08-03-2014, 08:19 AM   #62
shelion
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For giggles, I pulled out the rider's manual for my bike (2010 BMW 650GS) and flipped it open to the "Securing Motorcycle for Transport" section. The relevant sentence is this one: Push motorcycle onto transport surface, and do not place on side stand or center stand.

It goes on tell where to place the straps front and rear, and to tension them all evenly.

So, BMW at least, tells owners not to trailer using the center stand. I would imagine that Honda, Kawi, and Suzuki manuals would say similar about the models that have center stands.
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Old 08-03-2014, 08:27 AM   #63
markk53
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Originally Posted by davevv View Post
I have to disagree, conditionally. I've been tying bikes down since I started riding in '61 and have had it happen to me, but only once. The bike was my new '74 Yamaha RD350 with very few miles on it. I was loading the bike without any help and only had rope to tie it down with instead of ratchet straps. With the bike on the side stand, I tied off the left side of the handlebars with no pressure. Then with a loop on one end of a rope, I ran the other end over the right side of the handlebars, through the tie down point and back up through the loop. Pull down on the loose end of the rope and it compresses the forks and stands the bike up at the same time. Then a couple of half hitches and it's secure. I'd done it plenty of times before on other bikes with no problem. Only this time, I also got a squirt of oil out past the seal on the right fork leg. It had never shown any signs of leakage before, but it leaked from then on until I changed it.

I said conditionally, because I realize it's possible the seal was defective to start with, but the bike was practically brand new and had never leaked up to that point. So for what it's worth, that's my story. Did yanking those forks down to the bottom cause the leak? I don't know, but that's when it showed up.

You think on that very same day a hard landing from a wheelie wouldn't have done the same? You can yank the bike down harder than a hit from aa 300 lb bike dropping 3 feet?

I'm thinking you're right, it was defective or had some crud in it that cut the seal.
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Old 08-03-2014, 08:29 AM   #64
windblown101
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Quote:
Originally Posted by randyo View Post
I do not get it

does not people understand how forks work ?

when you compress the fork, you are compressing the spring, not the fluid

all fork seals to is keep the fluid inside the fork, the spring action is dampened by fluid moving from one side of a valve to the other, fork fluid is never under pressure of going past the seals, it is only under pressure of going from one side of the valve to the other
Fork seal issues aside for the moment I'm intrigued by this post. I'm no suspension expert so I'm willing to be convinced that my preconceptions are wrong.

If there is no pressure build up in the forks as they compress why does changing the air gap have an affect on fork action, especially towards the end of the stroke? For that matter why have an air gap at all if it's just oil moving past the valving that affects fork action?

Why have an air bleed in dirt bike forks to remove built up pressure?

As the forks compress the internal volume of the sealed chamber reduces. Where does the air or oil go during compression and how does it get back into the sealed fork chamber when the fork extends?
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Old 08-03-2014, 08:29 AM   #65
markk53
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Originally Posted by ttpete View Post
I think that some seal problems come from cinching the forks down without cleaning off the hardened bug smash and shoving it through the seals.

I'm thinking you're right.
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Old 08-03-2014, 09:32 AM   #66
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Quote:
Originally Posted by randyo View Post
I do not get it

does not people understand how forks work ?

when you compress the fork, you are compressing the spring, not the fluid

all fork seals to is keep the fluid inside the fork, the spring action is dampened by fluid moving from one side of a valve to the other, fork fluid is never under pressure of going past the seals, it is only under pressure of going from one side of the valve to the other
You don't think that there's pressure involved in that action? When your folk is moving rapidly, and the fluid is providing a damping force against that action, there's pressure. that's where the damping force comes from.

Granted there's not as much pressure as if it was a gas piston instead of springs, and also I'd agree that if you compress the fork and hold it in that position, whatever pressure there is on the fluid will equalize.
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Old 08-03-2014, 12:26 PM   #67
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Old 08-03-2014, 04:33 PM   #68
markk53
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Quote:
Originally Posted by windblown101 View Post
Fork seal issues aside for the moment I'm intrigued by this post. I'm no suspension expert so I'm willing to be convinced that my preconceptions are wrong.

If there is no pressure build up in the forks as they compress why does changing the air gap have an affect on fork action, especially towards the end of the stroke? For that matter why have an air gap at all if it's just oil moving past the valving that affects fork action?

Why have an air bleed in dirt bike forks to remove built up pressure?

As the forks compress the internal volume of the sealed chamber reduces. Where does the air or oil go during compression and how does it get back into the sealed fork chamber when the fork extends?
Doesn't really matter. The pressure under compression is still insufficient to blow a seal out.

Face it, mechanically a seal would have to blow essentially inside out. Not simply leak. A leak is caused by damage to the seal lip and that damage will not happen simply by compressing the forks. If it would, the seals would be damaged every time the forks bottom out when riding.

I just had my KLX tied down in my truck, only about 1/2 travel used. Had my Zephyr in it a few weeks back, maybe 2/3 compressed. Neither would generate the pressure of bottoming the forks on either bike and both have been bottomed out one time or another without blowing the seal.

Back in the 80s Honda along with many others used to use air pressure assist in the forks - up to 10 psi. No blown out seals.

Only way you end up with leaking seals is if the sealing edge gets nicked or damaged somehow.
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Old 08-03-2014, 05:11 PM   #69
windblown101
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Originally Posted by markk53 View Post
Doesn't really matter. The pressure under compression is still insufficient to blow a seal out.
My own experience strapping down bikes would agree with this. All together between dirt bikes and track bikes I've racked up to a few thousand miles towing bikes, all with them strapped down against the suspension and never had a seal leak as a result.

However, my question was in response to the blanket statement that there is no pressure increase when the suspension is held compressed.
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Old 08-05-2014, 09:28 AM   #70
Andyvh1959
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The sidestand and centerstand are designed as static loaded devices. They are not intended for the varied and potentially high dynamic loads of bouncing down the road, even on a trailer with suspension. Those dynamic loads are primarily why our bikes have suspensions. The HD dealer is completely wrong in saying tying a bike down onto it's suspension is potentially damaging to the frame and chassis. In fact it is just the opposite, tying it down onto the side or centerstand directly transmits shock loads during transport into the frame. Tying a bike down onto the suspension, allows the suspension to carry some of the shock load. Properly tied down, means the suspension is not fully compressed.

Also, the suspension load allows for load shifting and maintaining tension on the tie down supports. If a sidestand or centerstand fails during transport, the tie downs go slack, not a good thing.

IF the fork seals are effective enough to maintain an air seal, then compressing the forks does very slightly increase pressure in the forks. Simple matter of fluid contained in a sealed vessel, reducing the free volume increases the pressure. But I have never had a fork seal problem because of tying a bike down against the suspension for transport.
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