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In the four strokes, they have quite a lot of damping ("stiff") compared to years past on ktm as well as other brands and bikes and ironically this has been well received by many riders who all used to say the ktms were too "stiff"
Plush and soft are two different concepts.
Stiff and harsh... also not universally associated.
The two stroke aer mid valve design is a poor boy version of a cone valve. If you buy a cone valve for a two stroke version fork... you made a mistake imho
the shortcomings with any aer system is stiction and rate of spring rate progression...although most of that vs a spring fork is a myth. Same progression rate since spring forks DO have an air spring which is used to help control bottoming (air gap adjusted via oil volume)
Air pressure build up on the tac and aer designs is almost self correcting due to the balance chamber also experiencing pressure change.
Ironically people never talked about air pressure build up in spring forks.. but it happens exactly the same and is a larger percent change to influence. 14.7 (atmosphere) psi up to 17 psi is 15 percent or so
145 to 147 is less than 1 percent
Both a spring fork and an air fork will climb at the same psi rate to temperature change...it's basic gas law 101. Nitrogen doesn't fix this - common myth
Moisture free air and nitrogen will perform nearly identically in psi vs temperature.
The Kyb psf1 and psf2 in production version are garbage imho and shouldn't be considered when discussing air forks. The sff air fork is quite good too.
People forget that the sff spring fork on the 250 never got rave reviews at first either, and the Damping side between air and spring sff showa is the same
They test ride every bike, they work on everything. Two years before 4CS forks were in the United States, Decker had them on his KTM450 working on settings.
If you take apart KYB and Showa forks or shock, I'll bet 50$ you couldn't tell the difference if I mixed the parts up. The piston looks almost identical, but Showa is actually built slightly better. They are work on the same principle, so If you understand how a shock works... you can make anything work.
Regarding the AER fork, or any fork, you would be shocked how sag can affect the feel of the fork. Most people also keep opening compression trying to get rid of the spike (which AER doesn't have, the mid-stroke is smooth) and the fork gets worse! Too much air pressure they get springy, too little and they dive/feel terrible. I'm talking within 3psi.
Also, people don't realize you still need to bleed the outer chambers, it does make them feel rigid when they warm up and 3psi builds up. Open your bleeds!
This fork feels absolutely amazing once it's right, better than my Yamaha SSS. That's my humble opinion, but it took a little bit.
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The air within your forks at "0" will heat and up and gain 3-4 psi if measured post post.
This happens on damping side too
All spring forks do this it just wasn't talked about.
You will gain 3-4 lbs too which some people like making the front feel more planted. Personally I prefer lower weight
Air only changes pressure due to temperature rise.
The rate of rise in temperature is effected by volume of air(gas) and temperaure rise defined by
Ideal gas law : PV =nrT
the temperature rise is from friction of sliding seals and hydraulic conversion of energy due to damping. The stiffer your damping the more heat u will produce. This is why shocks get so hot. On pro level bike it's around 300f in a long moto.
The air side of the aer fork doesn't produce damping, thus it's temperature rise is ONLY due to friction, this is a good thing.
On sSS forks the damping heat and friction heat are both heating the air within the fork. 2-3 psi on a fork starting at atmosphere (14) is a lot. 2-3 psi out of 145 is not nearly as much as a percentage.
Proof... after a moto on sss forks immediately go bleed the bleeders. You will have pressure every time.
I have used the KYB and AER fork on the same bike. To me , feel and bottoming resistance in stock valving and sprung for my weight, very similar.
I was going to do a air vs nitrogen test on the AER, you are saying you won't see an advantage in build up pressures?
You won't see oil oxidation as quickly I suppose
If you had scuba air with zero moisture it would be the same or if you live in low humidity areas its close.
Pure Nitrogen being inert vs air that has oxygen has advantages in oil and internal parts long term since oxygen is reactive but for weekend warriors this isn't much an issue.
I haven't touched on why air feels different than coils... as the science behind it is actually really cool and there is a variable spring rate based on fork velocity with air that doesn't occur with a spring. But this is more complicated and harder to explain for people who just want their fork to work... and don't care how
Nitrogen was originally used in the Air Force, because its dry. Accumulators at altitude were freezing, and Nitrogen solved the problem. People that think Nitrogen filled tires, forks, or shocks actually changes anything, have drank the Kool-Aid and been sold a bill of goods.
You can't see a difference on the shock dyno either, if you wanted to know. Ambient air is mostly Nitrogen anyway..
When someone says "why", well, because who carries around a tank charged to 2500psi with ambient air? Answer, nobody. Nitrogen is simple and cheap. A fully charged tank goes a long way, not needing an air compressor.
Then i went to milestone and came up short on a few big jumps and almost died. Blew right through the stroke. So i added air to 150, then to 155. Some compression, etc. Landings were better but i couldnt turn anymore. Wrecked 5 times in corners with knifing and pushing. Sucks so bad.
So i understand what both sides are saying. These forks can be dangerous and can be awesome depending on what kind of riding you do and how hard you push them.
Sent them to FC for a performance package this week. I think they will be great, but if not then i have to go kyb springs. Been hurt too much and im getting brittle. I cannot live with these forks as is. Too dangerous.
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Adding air also made it real bouncy so i added rebound and compression. Next thing i knew is i couldnt turn anymore.
ill see if FC can make it better. Im confident it will be a great fork. It sure was before coming up short on big jumps. If i didnt suck so bad maybe id he fine stock.
For the shock, MXA setting are better.
Long story short - if the fork compresses very quickly - it doesnt have time to transfer the heat generated into the air out into the fork and atmosphere -this makes it "stiffer" or a stiffer spring rate
Then if the fork compresses slowly - it has more time to transfer heat - and it acts softer/lessor spring rate.
Hence why the air fork doesn't feel "consistent"
This is a pressure vs volume chart of a two "idealized" air springs - neither which are achieved in moto - but in moto you have the range between those two as possible.
Compared to a coil spring - the coil spring would be the same rate all the time at any speed.
Granted - a large percentage of "progression" and bottoming control is via air spring even on coil spring forks...but in that instance the faster your about to bottom - the stiffer your fork is - so it's a plus
Combined with the fact that the SFF SPRING showa fork is considered harsh - there is no real reason the "tac air" version which shares the same valving side - would be any better.
to this day i like the SFF TAC fork - and the WP air - and I'd bet good money those forks installed on a yz chassis would open most peoples mind's as to how much role the bike/chassis/and shock play to perceived fork harshness.
Often times - the shock function is causing more issue than the forks are.
Both pneumatic and coil spring forks have a total spring rate (a curve) which is the sum of the main spring and the pressure acting on the tube diameter due to trapped air volume.
In the pneumatic spring fork, the pressure change due to temperature is a change to the main spring rate (the primary pneumatic spring). You can think of that as a small percentage change to the spring responsible for most of the fork's total spring force. This primary pneumatic spring force is responsible for nearly 100% of the total spring rate when the fork is near the top of it's travel.
On the coil-spring fork, temperature makes a large percentage swing in the trapped air volume, however this is a small portion of the total spring rate in the fork. Given most OEM fork settings on a coil spring fork, the trapped air volume contributes little to the total spring rate near the top of the fork's travel and a significant amount toward the bottom of the stroke.
That gets real complex when you start adding in the different balance chambers, oil-heights, diameters of various pistons, etc. but the bottom line is a different set of curves will result for each type of fork in the real world. Even if the rate curves are ideal (better) for the pneumatic fork, most people aren't willing to fuss with setting pressures before they ride. Most people don't bleed their coil-spring forks or even check the pressure in their tires.
For the guy who is willing to take the time to learn how it all works, it's entirely possible to find a setting that feels as good, maybe better than a coil-spring while saving two or three pounds. Hopefully the suspension companies will continue to R&D various air-fork designs to revisit this stuff someday.
Another thing I'm wondering about is the inertia of the coil spring. Is it correct just to add half of the mass of the spring to the unsprung weight or is the whole thing more complicated? How are the dynamics of sucha fast moving coil spring?
Yes, the shock has a huge influence on how the forks feel. I bet a lot of guys that can't get their forks to work right, have a bike that is too high or too stiff in the back.
People swear they can feel 1 step in spring rate change very easily noticed - say from a combined rate change of .92 to .96 kg/mm
That's around 5 percentish. So if the air spring varies 5 percent or more - logically the rider should notice it like he notices spring rate changes
To answer cw
I don't agree in total spring rate change. 1 percent is 1 percent on the air fork - period. You rise 1 percent in pressure - your about a 1 percent increase in spring...very close on these systems.
On the spring fork - total spring rate isn't the issue with pressure - it's preload on the fork.
On a TAC fork - or WP aer - both the balance chamber AND the pressure chamber will be at the same temp or there abouts - on the TAC - it's totally seperate so the smaller tac chamber will potentially experience a larger pressure rise than the main chamber - although I haven't seen much of this in testing.
On the WP aer - it exchanges air between the two chambers all the time - making any pressure rise in the main - an equal pressure rise in the balance - something very smart on WP's part to ensure the main and balance rise together at the same rate.
On a coil spring fork, any pressure rise MOST effects the TOP of the stroke - similar to spring preload adjustments
As an aside to that- a customer here locally had a new spring bike 18 - said it was super soft in his opinion. The stock stacks werent all the soft once I tore it down - but I went considerably stiffer on the base valve and a step forward on mid off his feedback
He said it was still "way to soft"
I had an idea to his issue - added 5mm preload each fork - a large change - and he said it was ungodly stiff
Went to 2.5mm preload each side - still "ungodly" stiff
Back to oem valving with 2.5mm was his sweet spot. The reality is the valving was in the ballpark - his complaint was mostly to do with pitching under braking....but he described that as "too soft" at "all times". Most customers would find themselves very happy on any bike with the right spring rates (if you are way out of weight range), sag correct (and tested to find their happy place), fork height changes, and oil height/spring preload changes on the fork with stock valving
Some bikes - certainly just shitty - but last year and this year - most the bikes are quite rid able - and decently balanced where both ends work together.
I think 90 percent of suspension complaints are chassis feel complaints - I hear it all the time. I want my "bike A" to feel like my buddies stock yamaha. Well - you shoulda bought the yamaha and went riding - rather than buy "bike a" and went bitching and moaning the next 8 months about your suspension feel
And yamaha owners - I wish my yamaha turned like "bike A"
Well - you shoulda bought that other bike instead of trying to make the yamaha turn like something else.
No knock on the current yz - it's a very neutral handling bike IMHO.
If anyone read the chad reed racerx team story - just a factory triple clamp change rocked his world in feel.
Imagine what difference a whole chassis can make to feel then...
I don't profess to have any clue as to what changes in the clamp and chassis design would do this...but I have tested yamaha forks and internals that I totally love the settings on on other bikes - and totally HATED the feel on those bikes.
Not sure either - I use the lower spring perch groove on nearly ALL of the new showa 49 builds I do. Seems to help a ton for some the "complaints" you hear.
I have the forks on my 2018 Alta Redshift MXR. I'm 190lbs and Vet Intermediate.
I used to run 10.5 bar with the compression 12 clicks out, but was having trouble with front end traction (it was harsh initially). I then decided to drop down to 10.3 bar with the same 12 clicks out on compression and that helped, but it lost the bottoming resistance. So i decided to go back up to 10.5 bar but go 18 clicks out on compression and it feels a bit better.
I know the valving on the Alta is a little different than the KTM's, but are you KTM / Husky guys experiencing the same behavior? Higher pressure with less compression damping feels better?
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