What was wrong with air assist on forks ?

[MidlifeCrisisGuy]

Back in the 80s forks had Shraeder valves and you could add air pressure to the fork chamber to increase the spring rate.

 

Now we have air bleeders to ensure that the fork pressure is zero before we start riding.

 

What was wrong with adding air pressure to a spring fork ?  Why don't we do it anymore ?

 

Note: I'm not talking about air sprung forks.  I'm talking about adding a few PSI to spring sprung forks instead of using a heavier spring.  It sure would make setting the static and rider sag a lot easier.

[LiamRM212]

This reminds me of the time I emptied the air out of my dads '82 kdx175 forks. He rode it and was like WTF did you do?

[KTM_Pat]

80's were crazy with all the different suspension experiments and schemes. One could argue that some types were superior to todays types but were unservicable by the average rider. Sorry can't answer ur qeustion.

[rjg]

I am not sure how you could add air pressure to the modern dual chamber fork design without a big design change....maybe I am overlooking something simple.

[MidlifeCrisisGuy]

Put the Shraeder valve where the air bleeders are.  Pretty simple.

 

Modern forks would work better than the old forks for adding air to because the damping oil and components are now separate from the spring chamber.  The pressurized air won't be absorbed by the damping oil.

Edited August 21, 2014 by MidlifeCrisisGuy

[YZEtc]

I believe it's because air pressure (the air spring effect as the fork compresses) is very progressive, with resistance going up sharply as the fork goes through it's travel.

It was decided that the fork works better if the sharply-rising air spring was minimized by starting with zero air pressure and using the fork springs and better damping methods to control the fork's movement.

[KTMjake22]

Ride an mx all day, forks get harder as air builds up, making it more tiring because it is more harsh. The forks will be more plush just changing springs and oil.

[Suspenders]

By adding 5ml of fluid to your forks you make a very small but noticeable increase in bottoming resistance.

You can adjust in very small increments very easily to get optimum performance and all you have to do is bleed the forks to get back to baseline.

Where is the advantage using air unless it is a pneumatic fork.

[knudsen]

Both Suzuki fork types I have spec 0 PSI, 79 RM400 and 85 SP250. Well, I have 82 SP250 forks too, but they are the same fork. So, they were bleeders.

 

 

I believe it's because air pressure (the air spring effect as the fork compresses) is very progressive

 

That is correct. The more air volume, the more plush the beginning of the stroke. The more air pressure, the stiffer the beginning of the stroke.

[CrobarGreg]

Back in the 80s forks had Shraeder valves and you could add air pressure to the fork chamber to increase the spring rate.

Now we have air bleeders to ensure that the fork pressure is zero before we start riding.

What was wrong with adding air pressure to a spring fork ? Why don't we do it anymore ?

Note: I'm not talking about air sprung forks. I'm talking about adding a few PSI to spring sprung forks instead of using a heavier spring. It sure would make setting the static and rider sag a lot easier.

I have. 88 cr250 with the air valves. They were put on there not only to bleed air but as an emergency fix at the race track when needed. You can add 5lbs or so. It's basically the same as upping your oil level. Gives you more bottoming resistance. You also can blow out your forks seals easier. It's not a good idea to use them to air up. I use them to bleed.

[knudsen]

I think some of them funny bikes with bald knobbies that the pirates ride on-road have a rear shock that works like an automotive air shock, so if they get a real big GF or goat on back they can pump 'em up and level the ride.

[Yz295]

It is in fact the same problem that occurs with the modem PSF as with the old air assisted fork

Air heats up with use, expands and makes the fork stiffer

That's it in a nut shell and at the time having the forks stiffer at the end of the race wasn't acceptable to riders, do it died a natural death, now they just accept it and ride slower after 10min or so to accommodate for the stiffer fork

The biggest difference between the death of air assist and popularity of PSF is marketing, PSF are marketed as a must have for weight saving ( and this should not be discounted as the savings are real and a benifit) but the old problem exists just as it did in the old day

To drive home the point Showa do not sell a customer version of PSF, kyb is currently the only supplier to oem of PSF, kyb is majority owned by yamaha, yamaha do not spec PSF on their bikes they use SSS spring forks, SSS forks are not available to other oem manufactures

Air assist and PSF are a good fork when sorted but for anything else than club racing ( where races are typically 10 min or less) they must be filled with nitrogen to combat the ramping affect with air temp rising

We return all our customer forks back filled with nitrogen as our minimum standard

Hope that answers your question

Yz295

[harrperf]

We return all our customer forks back filled with nitrogen as our minimum standard

 

 

 

This doesn't do much to combat pressure change to temp.

If a user uses "dry" air - or scuba air, the difference between it and straight nitrogen vs temp is almost neglagible.

It's when humidity/water comes into the equation their is a big difference.

Showa is now selling their version of an air fork - and it's brilliant vs the kyb version.  The worst aspect to overcome on air forks with pressure rise is mostly the initial part of the travel or preload increase.

The showa fork uses an "air" negative spring that will proportionally climb in pressure vs temp just like the main spring does vs temp - essentially self balancing out. Quite smart.

[Yz295]

Haven't seen the Showa (customer fork ) here as yet I have had a look inside a set of factory forks so I know what your talking about

With regard to nitrogen I had to go back into my notes from testing

@ 32c ( around 90f) 39% rel humidity I put air in one leg nitrogen in the other after 20 min Moto ( practice) 4.2psi more pressure in the air leg tried it again with compressed air ( scuba) 1.9psi more in the scuba leg than the nitrogen filled leg

That's quite a large and noticeable difference when your racing and tired and it's a repeatable test, in both cases the nitrogen increased pressure as well but the air increased at a higher rate

Yz295

[mog]

When you say only yamaha has sss , the crf 450 and kxf 450 used exactly the same fork for a good few years

[Freestyler]

I use nitrogen all the time in normal twin chamber forks. Because of the different kinds of tracks in Belgium it is the perfect tool for tuning sag in the forks.

First I remove 4mm of preload of the springs  and then I make a bridge to one shrader valve so the pressure is always the same in the two legs.

I use zero pressure for hardpack enduro tracks and 0,5 bar for sand like Lommel.

[Yz295]

Don't know that the kyb fitted to crf and Kxf are the same as fitted to yz

Yz295

[Yz295]

The KYbs used on the honda and kawi are nearly identical to those use on the mighty yz.

Speed sensitive suspension is fitted to everything, the mear act of forcing oil through a hole at different velocity will give different dampening at those velocities with everything else being equal

SSS is a marketing term that was applied to the yz kyb and not those on the honda or kawi for very good reason

As with all things fast, the devil is in the detail

in this case the valving and spec that make the kyb fitted to the yz far superior, were not shared with ( the customers) honda or kawi

Yz295

[Yz295]

I use nitrogen all the time in normal twin chamber forks. Because of the different kinds of tracks in Belgium it is the perfect tool for tuning sag in the forks.

First I remove 4mm of preload of the springs and then I make a bridge to one shrader valve so the pressure is always the same in the two legs.

I use zero pressure for hardpack enduro tracks and 0,5 bar for sand like Lommel.

I'll try this tomorrow I hadn't thought of doing this cheers Craig

Yz295

[highmarker]

the op's q about air assist.  It's viable and can be handy where loads change and adj is needed. Like a big bike with a 6 gal tank loaded with or without gear at different times.

 

Many times with the volume and psi of air needed to do the job the air will top out the fork, no free sag. Riding you get a clunk each time it tops out and the front end may not sit in to the travel needed for good turning.   A newer better design air fork uses a reverse spring effect in addition to the main compression spring effect to prevent this top out. Any heating of the air or increase in pressure also makes the reverse spring effect stiffer so there is some compensation.  Even a coil spring fork suffers from air warming up stiffening, so it's not a toatl solution either.

Edited August 28, 2014 by highmarker