Date: 19-03-24  Time: 05:18 am

Author Topic: 91mm fork air gap question?  (Read 1704 times)

beuleux

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91mm fork air gap question?
« on: 01 January 2022, 05:43:39 pm »
Hi all
How important is the 91mm air gap, I  mean if you put precisely 545ml of oil in then all things being equal it must be right, what can change to make the air gap incorrect? The reason  I'm asking is I put 545ml of oil in bone dry forks spring out fully compressed and the gap was about 105mm
Your thoughts please
Thanks 👍
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Grahamm

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Re: 91mm fork air gap question?
« Reply #1 on: 01 January 2022, 06:24:42 pm »
The air gap in the forks gives the oil space to move into as it compresses when you brake. If you had no gap, the forks couldn't compress at all, because the system would be completely sealed.

The only effect changing the air gap has is that it effectively acts as an extra "spring" when the forks are approaching full compression, ie the air gets squashed by the oil and should stop the springs from bottoming out.

If the air gap is too big, the springs will bottom out under hard braking, meaning they can't absorb any more bumps.

It might be worth draining the forks into a measuring jug and checking you didn't get the amount wrong in the first place by mistake (not that I've ever done that...)

beuleux

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Re: 91mm fork air gap question?
« Reply #2 on: 01 January 2022, 09:08:48 pm »
The air gap in the forks gives the oil space to move into as it compresses when you brake. If you had no gap, the forks couldn't compress at all, because the system would be completely sealed.

The only effect changing the air gap has is that it effectively acts as an extra "spring" when the forks are approaching full compression, ie the air gets squashed by the oil and should stop the springs from bottoming out.

If the air gap is too big, the springs will bottom out under hard braking, meaning they can't absorb any more bumps.

It might be worth draining the forks into a measuring jug and checking you didn't get the amount wrong in the first place by mistake (not that I've ever done that...)
Thanks,  and yes I understand what you're saying, I was wondering if there could be any manufacturing differences between tubes or any wear factor which could account for the difference, I have been known to make the odd catastrophic error so I was very careful to measure very precisely and made sure there was 455ml left in the one litre bottle, I will double check the gap again in the morning when I do the other leg, could it be possible I filled the damper assembly too much when I pumped the rod and it needs to resettle
Thanks 👍
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beuleux

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Re: 91mm fork air gap question?
« Reply #3 on: 02 January 2022, 09:03:18 pm »
Hi all
Just an update, after much faffing about and choice words I have concluded that oil cannot be measured accurately with non scientific kit (I know who'd have thought right 😄), graduation marks on oil bottles mean "about" and surface tension causes an eliptical edge on measuring jugs, air gap distance however is a lot more reliable 👍 job done 😎


This was my first go at usd forks and all in all it went well next time will be easy as I've one or two more tweaks in mind, I always appreciate anyone's input
Thanks👍



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Grahamm

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Re: 91mm fork air gap question?
« Reply #4 on: 03 January 2022, 12:02:26 am »
Glad you got it sorted.

When I replaced my fork springs, I used a metal ruler as a dipstick, holding the fork tube upright, then lowering the ruler into the tube until I saw oil on the bottom.

Once I had that level, I just added a little oil, rechecked, added some more etc until I got the right gap.

robbo

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Re: 91mm fork air gap question?
« Reply #5 on: 03 January 2022, 05:55:38 pm »
The proper syringe fork oil tool is not that expensive. Saves a lot of improvising.
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beuleux

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Re: 91mm fork air gap question?
« Reply #6 on: 03 January 2022, 11:07:14 pm »
Unfortunately my steel rule is too wide to fit in the inner tube, that would be the ideal tool, maybe I'll sacrifice a tape measure for my next attempt 👍
« Last Edit: 03 January 2022, 11:11:53 pm by beuleux »
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robbo

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Re: 91mm fork air gap question?
« Reply #7 on: 04 January 2022, 09:27:18 am »
If you’ve something resembling a knitting needle and a pencil eraser(rubber). Stick the needle through the rubber to the required depth. Then place the rubber on the lip of the fork tube to measure the required depth. Or buy the proper tool :lol
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Trebus

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Re: 91mm fork air gap question?
« Reply #8 on: 04 January 2022, 11:26:14 am »
The steel inserts from a traditional style wiper blade work well. Also very handy as a fishing / hooking device 😀

unfazed

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Re: 91mm fork air gap question?
« Reply #9 on: 05 January 2022, 05:14:21 pm »
Easiest way is buy a small syringe (green rubber seal type) put a piece of hose on the end, measure the distance you require from the flats at the upper end of the syringe and cut the hose at the desired length. Fill the fork leg as normal, rest a flat of the syringe on the top of the fork tube and suck out the oil until it stops.

Grahamm

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Re: 91mm fork air gap question?
« Reply #10 on: 05 January 2022, 10:02:33 pm »
Easiest way is buy a small syringe

Useful tip, I'd never heard that one before :thumbup

beuleux

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Re: 91mm fork air gap question?
« Reply #11 on: 05 January 2022, 10:12:25 pm »
Some fantastic improvised tool suggestions, I should have made this thread before I started 😄


My damper rod holder was a pair of calipers and my damper assembly bolt tool was a sawn off 7mm alen key 👍
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robbo

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Re: 91mm fork air gap question?
« Reply #12 on: 06 January 2022, 09:25:33 am »
Easiest way is buy a small syringe

Useful tip, I'd never heard that one before :thumbup
That’s what makes up the proper tool, which isn’t that expensive and is fully adjustable. I think my one came from Motion Pro.
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robbo

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Re: 91mm fork air gap question?
« Reply #13 on: 06 January 2022, 09:30:21 am »
Dirt bike express have got a cheap copy of my gadget for £10.99.
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