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  • Offline Greenman   gb

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    Effect of changing the front fork air gap?
    on: Nov 27, 2021, 04.47 pm
    Nov 27, 2021, 04.47 pm
    Iím about to change the oil in the front forks of my 2018 Roadie.  I have the Triumph recommended weight of oil (some very useful stuff on here about comparing different brands and specs of fork oil) but Iím thinking of maybe reducing the air gap.  Iím 80kg suited and booted and the static sag measures 61mm, which I think is near enough? I do find the forks a bit soft and I get lots of dive on heavy braking so I thought maybe put a bit more oil in.

    I believe the standard air gap is 103mm and I thought to maybe reduce this by 10mm.  I appreciate I could just do it and experiment but if I can get it good enough first time it would save me the hassle of stripping out the forks 2, maybe 3 times.  Does anyone have experience of changing the air gap on a Roadie?
    Just doing as much as I can for as long as I can.

  • Offline K1W1   au

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    Re: Effect of changing the front fork air gap?
    Reply #1 on: Nov 29, 2021, 03.51 am
    Nov 29, 2021, 03.51 am
    I changed the springs for Wilbers when I had to replace a fork seal.
    I suspect that would be a better option than changing the oil level. It made a noticeable improvement to my bike.

  • Offline awjdthumper   gb

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    Re: Effect of changing the front fork air gap?
    Reply #2 on: Nov 29, 2021, 07.48 am
    Nov 29, 2021, 07.48 am
    I would have thought the air gap is responsible for at most about 50 mm of fork travel compared with over 200 mm in total, mostly provided by the springs. It sounds that, if you are suffering from a lot of dive on braking, the springs and their spring rates need to be addressed first with adjusting the air gap only likely to provide a bit of fine tuning :084:
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    Re: Effect of changing the front fork air gap?
    Reply #3 on: Nov 30, 2021, 06.56 pm
    Nov 30, 2021, 06.56 pm
    *Originally Posted by Greenman [+]
    I do find the forks a bit soft and I get lots of dive on heavy braking so I thought maybe put a bit more oil in.
    Does anyone have experience of changing the air gap on a Roadie?
    Well, I don't have much experience in changing fork oil air gap (for adjusting fork performance) -- but that won't prevent me from commenting...
    Keep in mind that suspension springs do one thing whilst damping does another.
    In days gone by, manufacturers actually added air valves to the fork caps so air-pressure could be added to the forks for extra spring support. Turns out that air really didn't provide a very nice spring assist -- difficult to get the just-right amount and didn't really react like a good coil spring.
    (there have even been competition models with vented forks for more 'pure' suspension performance)
    But, more oil / less air should provide spring assist.
    There are a terrible lot of lies going about the world, and the worst of it is that half of them are true.Ē
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  • Offline Greenman   gb

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    Re: Effect of changing the front fork air gap?
    Reply #4 on: Nov 30, 2021, 07.12 pm
    Nov 30, 2021, 07.12 pm
    Fair points, and thanks. I did think about changing the springs and have done on previous bikes and it worked for me. Generally I find the suspension ok for the sort of riding I mostly do though I do get the feeling that if/when I up the pace Iíll be less happy, but thatís another day.  For now Iíve reduced the air gap by 7mm, not a lot and I probably wonít notice it but I was changing the fork oil anyway and I had the time.

    At 9500 miles the fork oil was probably original and I did wonder why it was green in the RH leg and red in the LH? 
    Just doing as much as I can for as long as I can.

  • Offline Newhorizons   au

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    Re: Effect of changing the front fork air gap?
    Reply #5 on: Dec 01, 2021, 03.04 am
    Dec 01, 2021, 03.04 am
    My 2 cents worth. But on a 15 XC.

    In order;
    1.Put a plastic spacer under the retainer to preload the spring.
    That helped the dive a bit. Added 30ml of additional Oil becasue I also raised the forks 10mm into the triples (have new dog bones also so same 10mm)

    2.Recently drained the forks, cleaned the seals with a seal saver plastic thingy and refilled with 5wt oil.
    I refilled with the same quantity of Oil drained. Interesting that one side was like 50ml less??? I refilled to higher capacity.

    However, this change did create a much tighter front end and handling was noticeably better.
    During ISO I had changed the foot pegs also so not sure if that is what stopped the peg scraping or not, but I regularly scraped if there was a dip in mid corner. Recently did 2000klm in the high Country twisties on the gas and not one scrape.

    I would suggest maybe a bit more preload, it would have been nice if there was an external nut or screw but the spring pre load and the new oil did it for me.

    About the same weight suited up and I had maybe 15 kg of clothes and spares for 5 days with 2 extra turns on the rear shock. Desired result.

    I will go back and check air gaps now but I have the desired outcome, so checking is more about curiosity and logging in my notes. But, always a but, right. Need to set up for a big dirt adventure next year and just hope that playing with the compression and rebound will give me a softer ride if required because the original set up was scarey dive on the front. Good luck.


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    Re: Effect of changing the front fork air gap?
    Reply #6 on: Dec 01, 2021, 01.58 pm
    Dec 01, 2021, 01.58 pm
    *Originally Posted by AvgBear [+]
    In days gone by, manufacturers actually added air valves to the fork caps so air-pressure could be added to the forks for extra spring support. Turns out that air really didn't provide a very nice spring assist -- difficult to get the just-right amount and didn't really react like a good coil spring.
    The other (1987) bike I regularly ride is equipped with 270 mmm travel Marzocchi front forks with Schrader valves fitted to the caps. I think the idea was to be able to stiffen up the forks by pressurising the air possibly when the bike was used on the road rather than off-road where the softer suspension came into its own. I've never tried doing this because I find the suspension almost perfect on the normal roads and I never suffer from brake dive because the front brake is relatively weak compared with modern standards.
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  • Offline Greenman   gb

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    Re: Effect of changing the front fork air gap?
    Reply #7 on: Dec 01, 2021, 03.29 pm
    Dec 01, 2021, 03.29 pm
    I fitted Schrader valves to the fork tops of a Z650 I had in the 80s; I was a sucker for every handling upgrade fad.  AFAIR they didnít do much, although at least my fork seals didnít blow - never thought about that at the time.

    On a different note yesterdayís No.1 stupid thing was to think adding more oil meant a bigger air gap  :138:  Luckily I came to my senses before fitting the fork leg.
    Just doing as much as I can for as long as I can.

  • Offline Newhorizons   au

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    Re: Effect of changing the front fork air gap?
    Reply #8 on: Dec 01, 2021, 11.32 pm
    Dec 01, 2021, 11.32 pm
    Yeah, I had a Suzuki PE dirt bike with the Schrader set up. It had a really low pressure guage as we were only talking around
     + or - 10 psi. I have heard of fork seals weeping due to the additional pressure or maybe that was old oil foaming?

    Always felt that the spring and compression valving was still the most important components and that the air pressure was about stopping the forks 'bottoming out' on the big hits.
    Eg that very small air gap left on full comp, right at the finish was a kinda soft stop.

    The bigger issue with forks is being a bit too stiff into the motion if it has too much oil / small air gap. Did that on my old Bandit. Filled new oil and didnt properly evacuate the valve stacks. Ideally you want as much travel being used during normal operation and if correctly valved then bottoming should not be a problem.
    I know guys who got their big Dirt Bikes suspension done (DR650, KLR etc) were stunned by just how plush they felt, the increased wheel on ground and traction ... but they didnt 'hit' hard. But NOT cheap and needs your kitted weight and then setting the sag. The sag is then the base level for your suspension range.

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    Re: Effect of changing the front fork air gap?
    Reply #9 on: Dec 02, 2021, 04.32 am
    Dec 02, 2021, 04.32 am
    *Originally Posted by Newhorizons [+]
    Yeah, I had a Suzuki PE dirt bike with the Schrader set up. It had a really low pressure guage as we were only talking around
     + or - 10 psi. I have heard of fork seals weeping due to the additional pressure or maybe that was old oil foaming?

    Back in the mid '70s I had a Trials bike that we drilled tiny horizontal holes on the fork tube caps. It was the Trials theory at the time and made the suspension very subtle but when I took it trail riding and hit big bumps or went over jumps and the forks compressed quickly there would be tiny jets of fork oil shot out of the top of the forks.

     


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