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Thread: Best way to make AER forks plush

  1. #21
    KTMTalk Member Van Beida Riffer's Avatar
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by dogdog View Post
    OP here. Thanks for all input.
    I have a 2020 300 xcw, and I have really come to love my revalved Xplors. They were very mushy and not worth a darn for me in anything other than rocks or technical riding before the revalve/addition of mid valves, and adjustable base valves. They are now still plush but I can ride whoops and big drop offs with total confidence.
    If I decide to go with spring conversion, I would like to add the midvalve and do a revalve. Anyone have any DIY links to something like that?

    or maybe I'll just find some Xplors and set them up like my 300's. I don't race, but I sure do like that 350 chassis and motor. Just not the AERs.
    Since it seems I might be biased against air springs, both these options sound ok to me, an idiot. I might favor leaning toward a setup you know that works, sounds like less fussing. For me, it would depend on how cheap I could get XPLOR forks plus whatever mods you have done. I do have the AER WP conversion and, it works, for me. The AER damper is really easy to work on, too. I have no handy references for working on it, though. MXTech might have some vids.
    2017 KTM 250SX - cranky woods bike

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  3. #22
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by ktm520 View Post
    No, it's due to the dynamic compression of a gas. Just the normal laws of a gas PV=NRT

    I have an extensive background in custom tuning mtb suspension as well. I think the bigger issue with the AER spring is the shape of the curve. Imagine a plot of the curve over layed on a coil spring curve. The coil is perfectly straight, with a slope equal to the rate. A simple air spring designed like the AER starts off higher (more preload) and then crosses the coil line shortly after the sag point, and then is quite a bit SOFTER through the mid stroke, and finally crosses back over in the last 25% to be more progressive at the end. The soft mid is the bigger issue, and then coupled with the firmer initial 1/3 makes the setup a compromise. It either has a softer initial rate for compliance in small stuff with a soft mid, or a supportive mid with a overly firm initial rate that rides too high.

    Agree with this and I too have reduced the size of the negative spacer 34cc in my bike. Which helps make the initial stroke softer. With this reduction I do run my pressure 7 psi higher to get enough support. I run 225-250cc oil bath. I would also say I sort of like that the curve is not exactly linear like a spring. Seems to handle a medium sized hit by using slightly more travel but then ramps up and does not bottom. Pretty interesting our experiences are similar.

    This is all directly comparing to a coil of course. I've found a decent setup with my 19 AER by turning down the outside diameter of the negative spacer to closely resemble the '21 spacer, running more bath oil volume (230cc), and reducing hsc significantly. The negative spacer is key to being able to run more psi for adequate mid support while still have a decently compliant initial rate off the top. I was able to run ~8psi more after mod'ing the spacer, everything else staying the same.

    The mtb industry has done a significant amount of work to make an air sprung fork get close to the performance of coil. However, that just recently happened in the last couple years. I've had the best luck, and prefer, Manitou's IRT dual stage air spring. This design is similar to MXtech's A48 cap. I can definitely see how this helps a lot in tuning

    To the OP. I think the best bang for buck would be to do a coil conversion and a simple revalve that changes the base valving and adds a midvalve. That could come from a diy kit like those that have been mentioned, or a reputable tuner that works with the stock cartridge.
    Good advice. Even after my air spring changes that have been very positive, I am running a coil (bought it used) on my bike . The other thing is it is a technical change to reduce the negative air spring spacer. Just taking the air spring apart has risk of seal damage due to heat and/or mechanical damage as it is pulled apart. Now if the air cartridge seals are being replaced/upgraded anyway that reduces the risk. I had my spring kit and I took the risk and everything worked out well for me. I really did it to know 1st hand how close I could get the air spring to a coil. I have 2 friends looking at new bikes with AER. I tell tell they should plan on some dollars ($750 Min) to get them right. They are calling tuners now. Curious what they hear. They are just trail riders and like the stock xplorer forks but those bikes a priced like gold at the moment.


    One last thought. I still think the bleed between negative and positive chambers is not perfectly following the ideal gas law. No proof but the forks are just sluggish at top of stroke. I continue to have interest in what manufactures do with air bleed.

  4. #23
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    Default Re: Best way to make AER forks plush

    I'm currently using Racetech's Goldvalve kit in my fork, and shock. The recommended stacks were maybe in the ballpark, but still pretty firm. Check out this thread. kj's stacks would get you want you want and he's also on a coil spring. Don't overlooking revalving the shock on an XC model, as it will be required also.
    https://ktmtalk.com/showthread.php?5...-Valves-in-AER


    Quote Originally Posted by Van Beida Riffer View Post
    Yep to most of that. The "hammock" curve as I've heard some call it. Also yep to the MX-Tech (yamaha "speedo and tach" air fork, too) fork cap idea... I'd like to understand that some more.

    In case anyone is interested in the math, http://lhtech.com/pdf/automation/pne...pring-Data.pdf, section 9 is a surprisingly good summary of what I was talking about.

    If y'all know where I worked, you'd maybe expect me to be some sort of an expert, but most days, I consider myself a curious idiot.

    I've dyno'ed MTB air springs at different strokes and speeds. I was actually surprised when I dyno'ed a 20mm stroke (near top out) at different speeds. The math happily checks out, but I did not anticipate there being that much of a difference at such short strokes. Now, to be fair, I'm not totally sure this is the difference I was feeling swapping from air to coil springs, but there was a difference and it presented to me like too much high speed compression damping on square edges with the AER spring vs coil.

    I can fix that specific feeling by dropping pressure and running very little damping, but that's not pleasant on all trails, for the compromises that come with that setup. I did love it for hopping around rock gardens, though.

    I do find it interesting that the spring rate on the "flat" or mid part of the AER spring curve (measured at low speeds) is off a "normal" coil setup by about 1.3-1.4x. This also starts to explain why the K-tech spring conversions were spec'ing a 7.0N/mm coil spring with lots of preload and I hope a top out spring (7.0N/mmx1.38=9.66N/mm, or 4.83N/mm for 2 spring forks). I'm not sure if that is still the case. It was like they were trying to match the air spring exactly without considering the high speed behavior, or just "normal" dirt bike coils, either. The WP coil conversion, by contrast, uses "normal" dirt bike rates.

    When I talk about equating this behavior to a damper, I'm making pretty dubious statements... My thinking: If I was dyno'ing a damper that is pressurized somehow, I'd get the gas/IFP spring force component accounted for before I did a speed sweep on the damper. If I did this with an air spring, I'd get a "damping" curve, basically, with more "damping" at higher speeds, probably with lots of hysteresis. I think this matters somehow, as increased damping affects ride quality more than just increases in spring rate. I haven't actually done this, but now I want to. A coils should not have this behavior, or a least not as much of it... There is still the outer air chamber to consider. Enzo fork subtanks now come to mind.

    Yadda yadda yadda, air springs feel different at different shaft speeds and this affects both through the stroke and at top out (like someone else was saying). It makes me wonder if the new air spring changes with the longer dimple seek to make the top out feel more consistent, since, to my mind, the longer dimple might allow just the top out spring to influence that feel, since the negative chamber isn't being compressed so much on top out.

    Sometimes I wonder if this feel is just the air piston seal grabbing... But, then that wouldn't be affected as much by pressure changes.

    I dunno.


    And yeah, flex bars are nice.
    All good stuff man. Thanks for sharing that link. When I do spreadsheet models of air springs I usually just go the isothermal route for simplicity, and it's just a tool, not an absolute.

    I didn't mean to say the speed sensitivity of air springs isn't an issue. That's not something I've ever been able to test, or even put much thought into until I read this thread. I'll have to chew on that more.

    I haven't done a coil conversion . . . yet. It's happening soon.
    Last edited by ktm520; 11-24-2020 at 04:09 PM.

  5. #24
    KTMTalk Member Van Beida Riffer's Avatar
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by ktm520 View Post
    I didn't mean to say the speed sensitivity of air springs isn't an issue.
    Frankly, I really don't know if that behavior is that big of an issue. There's too many variables at play for me to confidently say that. If you figure "it" out, lemme know... As you can likely tell, I like chewing on these ideas, too.

    At the end of the day, I seem to prefer coil(s)... most of the time.
    2017 KTM 250SX - cranky woods bike

  6. #25
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    Default Re: Best way to make AER forks plush

    I do find it interesting that the spring rate on the "flat" or mid part of the AER spring curve (measured at low speeds) is off a "normal" coil setup by about 1.3-1.4x. This also starts to explain why the K-tech spring conversions were spec'ing a 7.0N/mm coil spring with lots of preload and I hope a top out spring (7.0N/mmx1.38=9.66N/mm, or 4.83N/mm for 2 spring forks). I'm not sure if that is still the case. It was like they were trying to match the air spring exactly without considering the high speed behavior, or just "normal" dirt bike coils, either. The WP coil conversion, by contrast, uses "normal" dirt bike rates.

    I wasn't totally following you on this and then look at your link. Good read. Thanks.

  7. #26
    KTMTalk Member Van Beida Riffer's Avatar
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by celler View Post
    I wasn't totally following you on this and then look at your link. Good read. Thanks.
    Glad you enjoyed it. I don't always follow my own thoughts, either.
    2017 KTM 250SX - cranky woods bike

  8. #27
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by islandlife View Post
    did you re-valve the shock as well? Or is there enough adjustability with the stock valving to get a good balance with the 7k tuned forks?
    Shock was firm, I am 150 lbs. 50A. I had Direct Suspension re valve and a WP Bladder kit, it is excellent paired with the 7K forks - very balanced front to back.

  9. #28
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    Default Re: Best way to make AER forks plush

    Quote Originally Posted by lithoman View Post
    Shock was firm, I am 150 lbs. 50A. I had Direct Suspension re valve and a WP Bladder kit, it is excellent paired with the 7K forks - very balanced front to back.

    I'm 175 lbs and ran the stock shock until it was time for an oil change. I did a re valve when I needed an oil change. Rebound on the stock shockis a little fast and if you close it up it will make the compression hard. I tend to runny pretty fast rebound so it was workable for me.

  10. #29
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    Default Re: Best way to make AER forks plush

    6' 185lb 48yr old B GNCC rider, new 2021 250xc, AER still stinks and sad after 10hrs ready to spend $2K on suspension, KTM should be embarrassed and I'm surprised how many say the 2021 is better. I had a 17 and think it was better, but maybe because the aer are better on a 4T. If you have ridden KYBs or Lucky's these are no where even in the same ballpark and the stock KYB on Yamaha without revalve are night and day better, forget after a revalve. I had a 2014 with 4cs and had mxtech do their magic and it was actually better than the KYBs. The problem with the AER is they can handle either slow riding with low air pressure or big hits with high air pressure, but you can't get strong handling with plush ride over braking bumps, rocks, or roots no matter what at faster speeds.
    Jason
    -------
    '21 KTM 250xc tpi
    '18 YZ250x (never selling, but my 15yr old has claimed it now)
    '17 KTM 450sxf FE (sold)
    '14 YZ250F (sold)
    '14 300xcw Six Days (sold)
    '14 CRF 250R (traded in)
    '09 KTM 200XC (sold)
    '03 Kawasaki KX 250 (sold)
    '05 Honda CRF250R (sold)
    '03 KTM 125 SX (sold)
    '00 Yamaha YZ250 (sold)
    '97 Kawasaki KX 125 (sold)

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