Help With AER Forks

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6/22/2022 3:41 PM

Hey guys need some advice here.

I have a '22 KTM 250SXF. I'm a slow MX racer - typically mid pack in the 60+ Vet Novice class in the SoCal OTMX club. Usual track for me is Glen Helen. I'm about 225lbs in shorts/T-shirt, so I'm guessing about 250 in full gear.

I have the rear set up pretty well - 5.4 spring set to 105mm sag with about 7mm of preload on the spring.

For the forks, I have the JBI DIY kit installed, which consists of a sub valve and a new leaf-spring type mid valve - similar to SSS KYB/A-Kit forks.

My issue is I'm not getting anywhere near full travel, and the forks feel quite harsh when landing from a jump, and very busy in braking bumps going into corners. I don't have a lot of confidence in the corners - it feels like I have a lack of front tire traction.

I checked my travel using the o-rings on my forks, and was several inches from bottoming. Granted at my skill level and age I'm not airing-out jumps - I'll usually land about 1/2 to 3/4 of the way over a table top. But even that feels quite harsh on landing. I was initially running about 145-148 psi in the fork. I lowered it down to 138 last sunday, but even at that I'm still at 3" from full travel. This already seems like too low of a pressure given my weight.

MXA says to keep lowering air pressure until I have about 1 to 1 1/2" of travel left, but I'm hesitant to keep going lower - won't that cause my forks to ride too low in the stroke, which in itself will cause harshness? I t seems I'd have to go down to about 120 or so psi to get another 1 1/2" - 2" of travel - and that just seems way too low. They say to do this before messing with the clickers though. (I'm at 12 out on compression currently)

The person that installed my fork kit did fill my outer fork legs with 280cc of oil in each leg. Manual states 230cc is standard, (+10/-50). Could this be part of the issue? Should I try draining 50cc out and see what happens? I'm also confused about whether to drain oil from both legs, or just the dampening side - I see both ways mentioned.

Thanks for your help

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6/22/2022 5:02 PM

First and foremost you need to contact JBI and discuss your issues with them since you installed their DIY valving kit. Second, 280cc is way too much oil for those forks, IMO. Why did your installer decide on that volume?

Not having any knowledge of the JBI settings, at 225lbs, I personally would recommend you run much higher psi around 168-170 +/- and also set your oil volume at the lower range around 180cc in both legs.

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6/22/2022 5:15 PM

slipdog wrote:

First and foremost you need to contact JBI and discuss your issues with them since you installed their DIY valving kit. ...more

I don’t understand either why he went with 280cc of oil in each fork when the manual calls for 180-240, which 230 being the oem setting.
That confirms what I was thinking - that there is just too much oil in the forks. The JBI website does state you can run a little less air than usual. For my weight I would think 168-170 would be typical like you mentioned. Maybe with the JBI kit I could be down in the 150s somewhere- IF I lower the oil volume like you suggest. And you did say both legs, so thanks for clearing that up for me.
I think I might just drain all the oil from the forks and just add the 180cc myself so I know for sure the exact volume I’m starting with.
Appreciate your help Slipdog!

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6/22/2022 9:56 PM

May be a stupid question, but have you bled your outer chamber on the fork with air? It sounds to me like it’s pneumatically locked up.

We are the same weight and I run 157-160 in my stock 22 AER’s.

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6/22/2022 10:09 PM

183Matt wrote:

May be a stupid question, but have you bled your outer chamber on the fork with air? It sounds to me like it’s pneumatically ...more

Matt - yes many times. Sometimes I do hear air escaping on the left (air) side. Is this normal or indicative of a problem? I know you do this to release any trapped air, but how much is normal? And I’ve only ever heard the hiss of air on the left fork - never the right

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6/23/2022 7:36 AM

My buddy had same bike and is similar size and riding style to you. I basically agree with what Slip and others have said. You need more air and less oil, and then probably open the clickers up to get the forks moving. My buddy was running 156 psi with 210cc of oil and he was really happy.

Also, not sure how many hours you have but these forks noticeably free up at 5, 10, and then 15 hours. I've also found that being very careful to get a good bleed on the damping cartridge helps the plushness.

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Powerband in every gear !

6/23/2022 7:58 AM

That harsh feeling is one of the issues with the air forks. On certain jumps (coming up short on a table would be a perfect example) the ramp up of the forks is too aggressive. They call these "Slap down" landings and it's hard to make this go away.

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6/23/2022 9:54 AM

FGR01 wrote:

My buddy had same bike and is similar size and riding style to you. I basically agree with what Slip and others have said. You ...more

Thanks FGR. I’m at about 8 hours on them.

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6/23/2022 9:56 AM

Moto520 wrote:

That harsh feeling is one of the issues with the air forks. On certain jumps (coming up short on a table would be a perfect ...more

Thanks Moto. I’m gonna try the suggestions here to see how much improvement I can get, but if I’m still not happy I might look at an MXT Raven or a Coppersmith Hybrid kit

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6/23/2022 11:23 AM

Moto520 wrote:

That harsh feeling is one of the issues with the air forks. On certain jumps (coming up short on a table would be a perfect ...more

swatdoc wrote:

Thanks Moto. I’m gonna try the suggestions here to see how much improvement I can get, but if I’m still not happy I might look ...more

i went through the same thing. I used the air forks with an open mind and they worked great for certain conditions. I could really feel it in my hands when the forks slammed down onto flat ground after the rear wheel hit. It was enough to make me switch to Cone Valves. It's too bad because there are some things that are awesome about how the air fork feels. i think in the case of these forks.....the media outlets hit the nail on the head with how they feel.

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6/23/2022 11:38 AM
Edited Date/Time: 6/23/2022 11:39 AM

Moto520 wrote:

That harsh feeling is one of the issues with the air forks. On certain jumps (coming up short on a table would be a perfect ...more

swatdoc wrote:

Thanks Moto. I’m gonna try the suggestions here to see how much improvement I can get, but if I’m still not happy I might look ...more

Moto520 wrote:

i went through the same thing. I used the air forks with an open mind and they worked great for certain conditions. I could ...more

That’s one of the places I get a lot of harshness for sure.
What do you think about the air coming out of my left bleeder? It was a good 2-3 second hiss. Bad seal head? If so I might do the SKF glide kit, or just take the $200 I’d spend for that and put it towards a Raven kit

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6/23/2022 11:49 AM

The JBI kit is phenomenal when you get it set up. Best of luck. Contact JB first and foremost instead of asking for help here. He knows the kit better than anyone and will gladly help you

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6/23/2022 11:59 AM

motox11 wrote:

The JBI kit is phenomenal when you get it set up. Best of luck. Contact JB first and foremost instead of asking for help here. ...more

I agree he’s a great guy and very knowledgeable. I was just under the impression that my issue had nothing to do with his parts

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6/23/2022 5:36 PM

A lot going on here, but harsh feeling on slap down landings is often attributed to poor shock setting..... at least, that's what a showa factory trained tuner with multiple ama titles told me cool when he setup my shock, that harsh feeling in the forks disappeared. I had them air neken clamps and i sold them straight after, lol.

Also, over the years I've learned that going too far from stock, it's easy to get lost... I like JB and I've bought stuff from him over the years. That said, that stock 2022 midvalve is a great design and I was able to achieve what I think you are looking for by just a couple of small adjustments on the mid valve and trampoline springs.

Either way, just do what makes you happy, and if that means a new set of cones, then go for it.

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6/23/2022 5:43 PM

280cc is a lot of oil, I would correct that first, I have been running the jbi kit, it works very well, give him a call he will point you in the right direction

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6/24/2022 8:47 PM

I feel you can get the 21+ AER forks to work really well, just needs some tuning. I regularly ride my daughter-in-laws 22 MC250F with Gold Valves, and even though it's really low on air pressure for me, it works great. My friend's similarly equipped 22 250SXF has an excellent feel too. Another friend's 21.5 450 FE is set up for a heavier rider than me, but his forks feel pretty good too. Not quite the ultra precise feel my 350 now has, but very, very usable. Nice slap-down feel too on all the newer AER stuff.
My 17 FC350 (your old bike, swatdoc) could never seem to make the forks work right. Revalved the stock setup, went to Gold Valves, even tried Ohlins TTX-22 inserts. Until I ponied up for Blackjack inserts it always had harsh slap down along with less than stellar small bump absorption.

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6/25/2022 1:19 AM

Thanks for the input Matt. I definitely need to play with these forks some more. I think I've got some good direction on how to go from the responses so far. I think the plan is gonna be to drain about 70cc of oil out of each fork - down to 210 from 280 - this will put me right at 20cc below standard amount. Then start backing out my compression clicker quite a bit.
How's the 350 running?

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6/25/2022 1:20 AM

Bruce372 wrote:

A lot going on here, but harsh feeling on slap down landings is often attributed to poor shock setting..... at least, that's ...more

Ya know Bruce that's a good point. I might also try backing out my compression a tad on my shock - that could help with those slap down landings. Thanks

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6/25/2022 4:15 PM

Bruce372 wrote:

A lot going on here, but harsh feeling on slap down landings is often attributed to poor shock setting..... at least, that's ...more

swatdoc wrote:

Ya know Bruce that's a good point. I might also try backing out my compression a tad on my shock - that could help with those ...more

Its the opposite, if the shock is too soft comp damping, when you land the back end is low and the front wheel hits rhe ground and the forks have the wrong angle off attack, so they start to flex vs compress if that makes sense?

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6/26/2022 8:18 PM

swatdoc wrote:

Thanks for the input Matt. I definitely need to play with these forks some more. I think I've got some good direction on how ...more

As others have mentioned, contact JBI for suggestions on oil levels. May as well, you have their product in there..
Just a thought, since it's really easy to add oil to the forks, maybe drop the level way down to 180cc, and just add 10cc's at a time at the track and record what the findings are.

The 350 is a monster. I put the stock clutch back in, swapped the GET ECU for the stock one, tuned by Twisted Development, and I run it on pump gas. Certainly not as fast as it was before, but I still fear no 450 once the revs come up. If I don't holeshot, it's not the bike's fault. I was really looking forward to racing Mammoth on it (that's the specific reason I bought the 350), but caught covid a few days beforehand and killed that vacation.

I did recently tighten up the sag from 102 to 97.5, the result was tighter steering, but it also helped settle both the front and rear end, which was a bit of an unexpected surprise. Once you get your forks feeling good, that might be another avenue to check.

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