Air Assist on Front Forks, VL800 - Page 4 - Suzuki Volusia Forums : Intruder Volusia and Boulevard Forum
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post #31 of 54 (permalink) Old 10-13-2013, 04:03 PM
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Originally Posted by dfinitlydisturbd View Post
Don't forget we're dealing with inside dimensions not outside. Total travel of the fork from extended to fully collapsed is 5" (number obtained by measuring both states and subtracting)...
Well, yes, we are talking about the space inside the fork, but 5" of the outside diameter of tube has disappeared inside the slider. The available space inside the slider has been reduced by the steel tube sliding into it, so you need to use pi x 4cm x 4cm/4 x 12.7cm = 160cc for the volume change. Of course, the initial volume has now been reduced because you put the springs back, so the volume ratio goes up.

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post #32 of 54 (permalink) Old 10-13-2013, 06:58 PM Thread Starter
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Actually to avoid having to sort out for multiple variations in diameter I used only the inside diameter of the fork tube...treating the entire assembly as a single cylinder that has contracted by five inches in length. As pressure readings continue to hover between the results established by the mathematics I suspect that the 5" by 3/16" column created by the tube wall thickness has a minimal impact on the overall. Note: your math is in error in the above it is
(Pi X (R X R)) X H for the volume of a cylinder
not
(Pi X (D X D)) X H .

Last edited by dfinitlydisturbd; 10-13-2013 at 07:06 PM.
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post #33 of 54 (permalink) Old 10-13-2013, 09:44 PM
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The Kawasaki Vulcan 750's had air fittings on the forks, and ran a stock seal without a bladder or anything like that. They said not to run more than 7 pounds of air pressure in them for fear it might blow out the seals.
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post #34 of 54 (permalink) Old 10-13-2013, 10:38 PM
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Originally Posted by dfinitlydisturbd View Post
Actually to avoid having to sort out for multiple variations in diameter I used only the inside diameter of the fork tube...treating the entire assembly as a single cylinder that has contracted by five inches in length. As pressure readings continue to hover between the results established by the mathematics I suspect that the 5" by 3/16" column created by the tube wall thickness has a minimal impact on the overall. Note: your math is in error in the above it is
(Pi X (R X R)) X H for the volume of a cylinder
not
(Pi X (D X D)) X H .
If the wall thickness is 3/16" (4.8mm) then the contribution of the tube wall is ~36cc, which seems very significant if the volume change was 52cc, and slightly less significant if the change was ~160cc.

My maths is correct. The area of a circle can be expressed as pi x r^2 or (pi x d^2)/4 You missed the "/4".

However, since the original purpose of this analysis was to determine the maximum pressure in the forks so that comments like MrFreeze's one above could be considered, maybe it would be easier to test the pressure difference starting with a gauge at zero on the extended fork and look at it again when compressed? Those pressures you used in post No.27 were they actuals, or calculated ones?

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post #35 of 54 (permalink) Old 10-13-2013, 10:45 PM Thread Starter
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yeah, air assist is nothing new for sure.

Would love to see a diagram of them...would love to see a decent batch of images of the internals of the various aftermarket air forks out there as well. Have found several that rely soley on seals and for the most part appear to be little more than stock forks with heavier seals and retainers.

Going over the basics of the VL800 fork it's nearly identical to the construction of both air as well as hydraulic pistons The primary difference being the hollow fork tube and the method of securing the seal, which so far going over everything is the weakest point in the design. Though is easily remedied through the use of a heavy clip to replace the spring type unit and/or swapping out the seal (which is up to the task as is) for a heavier unit.
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post #36 of 54 (permalink) Old 10-13-2013, 10:57 PM Thread Starter
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If the wall thickness is 3/16" (4.8mm) then the contribution of the tube wall is ~36cc, which seems very significant if the volume change was 52cc, and slightly less significant if the change was ~160cc.

My maths is correct. The area of a circle can be expressed as pi x r^2 or (pi x d^2)/4 You missed the "/4".

However, since the original purpose of this analysis was to determine the maximum pressure in the forks so that comments like MrFreeze's one above could be considered, maybe it would be easier to test the pressure difference starting with a gauge at zero on the extended fork and look at it again when compressed? Those pressures you used in post No.27 were they actuals, or calculated ones?

BRG
All pressures are mathematically achieved, and confirmed by compressing the forks completely at pressure. Ie. At 80 P/SI the fork is compressed completely and the pressure read from the gauge 120-ish.

I've not seen your method before, but it seems to work. And yeah I missed the divided by four. I still haven't gotten around to preparing a key of all values obtained, and used to get my numbers, but rest assured as soon as I have the time to spend I will post them. (I killed an hour preparing the post I lost. Since I'll essentially be starting at zero again I expect to need at least a half hour to get everything prepared to my satisfaction)

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post #37 of 54 (permalink) Old 10-13-2013, 11:27 PM
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Aaaah they're actuals. Well using those figures, your airspace volume ratio would be about 1.4:1. I assume that's without the springs in.

Thinking about the seal issue, fork seals are usually of the lip type (like a bearing oil seal), so when you increase the pressure inside, the lip presses on the shaft (fork leg) harder. It may not blow out, but what it will do is wear much faster. So, initially it may hold, but then it will probably start to leak a bit, and because there's so little volume inside the fork leg (not like a car tyre), the pressure will leak out really fast, any you'll have to top it up a lot. I've done a bit of work with air suspension, and I found that you really need something that's designed to do it from scratch. Otherwise the seals and the internal dimensions are not right.

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post #38 of 54 (permalink) Old 10-28-2013, 05:33 PM Thread Starter
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Well it's been almost a month of running the forks pressurized. Have been running with the forks pressurized at 60 PSI for about half of that with sawed off springs which give about an inch of travel with no air. Still rides great, no squeaks, creaks or leaks. Accidentally pressurized to 150 (they increase in pressure VERY quickly only takes a couple of seconds of being sidetracked to go from ok to oh ****), thankfully nothing gave way. Still haven't upgraded the seal retaining clips (to many other pressing matters at the moment).
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post #39 of 54 (permalink) Old 11-10-2013, 03:31 PM Thread Starter
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Still holding, still no issues. Cept, that it's pretty crummy weather to be ridin' in lately. Oh well, at least what snow that does manage to fall between the rain has been goin away as quick as it comes.
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post #40 of 54 (permalink) Old 11-20-2013, 03:46 PM Thread Starter
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Still nothing to report. Still haven't replaced the stock clips, still no leaks.
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