Topic: pros and cons of "air" forks

[jaguar]

i have a set of forks off a dohc 750 or 900.  they have air fittings on then and i assume that they are air charged.  they are a leading front axle set so i dont want to use then for my bike but was thinking about the air charged part.  what are the pros and cons to them?  can i make then work on my 78F front end?  should it bother?  i plan to use progressive springs and or racetech valves.
thanks for any help and knowlage

[tbone]

You'll need to use the triple trees too. Those forks are a larger diameter.

[Soos]

You'll need to use the triple trees too. Those forks are a larger diameter.

I dunno about that.
The cb750DOHC that i snagged the dual disk setup had the same 35mm tubes my 650 had stock.
And it is the same diameter my '74 750 had on it.

Are the DOHC air assisted really a larger diameter on the DOHC's VS. the non air-charged ones?


And the benifit of the air charged is the ability to control the ride height?
Or possibly the rebound by adding pressure to the legs?

Anyone know for sure?


l8r

[ofreen]

I had an '82 CB900F for a few years.  According to the manual, you could run up to 14 PSI in the fork, if I remember correctly.  The more pressure I ran, the less compliant the fork became.  The usual explanation for this is the the increased air pressure squeezes the seal against the tube leading to more stiction.  The fix was installing Progressive Suspension springs.  I then ran 0 PSI, and the fork worked pretty good.

[Geeto67]

adding air into the fork works like a preload adjustment. at 14psi it is like running full preload (ie almost a rigid suspension) and you will have very little travel. basically you are putting the fluid under pressure and since it doesn't compress it will in-turn put added resistance on the spring as it moves through the fluid (thus also compressing the gas in there as well - it is not a traditional preload in that you are compressing the at rest height of the spring - really you are just adding more resistance to its movement).

IIRC the DOHC cb750s that don't have air forks are 35mm, the ones with air forks (1981 and 82) may be 37mm (the cb900Fs for those years are def 37mm). I think cb450sc nighthawks have 35mm forks and air caps but the inside diameter of the fork tube might be different.

There were air cap kits sold for SOHC bikes back in the 70's and 80's. I think I have a set minus the nipples kicking around. The advantage of the honda setup over the kits is the balance tube that equalizes air pressure between the forks, where as with the aftermarket kits, most of them you added air individually and had to equalize it yourself with a pressure gauge. To be honest you can make your own air caps, just drill the stock sohc 4 caps and install a pair of schrader valves - that is how we used to convert old dirtbike shocks to air suspension.

I personally would not run more than 10 psi in a SOHC cb750 fork. for me 6 to 8 psi always seemed to work well when I had air caps. However it is a lot of stress on the seal and you will probably end up doing fork seals every 3-4 years. A good heavier weight oil and some decent fork springs or cartridge emulators will really make a bigger difference than a set of air caps will - still it is nice having no mess tunability.

[ofreen]

at 14psi it is like running full preload (ie almost a rigid suspension) and you will have very little travel. basically you are putting the fluid under pressure and since it doesn't compress it will in-turn put added resistance on the spring as it moves through the fluid (thus also compressing the gas in there as well - it is not a traditional preload in that you are compressing the at rest height of the spring - really you are just adding more resistance to its movement).

Actually, that isn't quite true.  Air is compressable and the air in a fork acts as a spring.  When I ended up using the full 14 psi in that fork, it is because over time and miles the stock springs were getting sacked.  Even at 14 psi, I was using up more than half the fork travel just sitting on the bike.  It was far from rigid.  When I installed the Progressive springs, I found that 0 psi worked fine and the added benefit was less stiction and a more compliant fork.

[Geeto67]

at 14psi it is like running full preload (ie almost a rigid suspension) and you will have very little travel. basically you are putting the fluid under pressure and since it doesn't compress it will in-turn put added resistance on the spring as it moves through the fluid (thus also compressing the gas in there as well - it is not a traditional preload in that you are compressing the at rest height of the spring - really you are just adding more resistance to its movement).

Actually, that isn't quite true.  Air is compressable and the air in a fork acts as a spring.  When I ended up using the full 14 psi in that fork, it is because over time and miles the stock springs were getting sacked.  Even at 14 psi, I was using up more than half the fork travel just sitting on the bike.  It was far from rigid.  When I installed the Progressive springs, I found that 0 psi worked fine and the added benefit was less stiction and a more compliant fork.

this is kind of what I was trying to say but wasn't doing a very good job of it. The air is compressible and is a sping, but the more pressure you put in the less compressible it becomes. at 14psi the recommended max pressure there is still a lot of give but you are using most of it up putting weight on the bike. MY 900F forks are very stiff with 14 psi (I run them at 0psi because I run 15w fork oil and like a cushy ride) but only after I have my full weight on the bike.

stiction in a motorcycle fork is a binding of the fork caused by the tubes being offcenter from the sliders (usually caused by lateral loads and distortion). I don't think it is the right term for what you are talking about here. Stiction in physics is a threshold force necessary to overcome the static cohesion of two objects pressing against each other.

Air in the fork actsl like a spring but what does it press against? The fluid in the fork, the walls of the fork itself, and the spring. As the chamber gets smaller, the air becomes more and more compressed and works against the compression of the actual spring in the fork. It also presses against the fluid and makes it more difficult for the spring to pass through it since the fluid cannot compress but it's natural tendency is for the leve to rise as more coils displace it. 

[ofreen]

The air is compressible and is a sping, but the more pressure you put in the less compressible it becomes.

Another way to say it is that an air spring is progressive.  The more you compress it, the more force it takes to compress it further.

stiction in a motorcycle fork is a binding of the fork caused by the tubes being offcenter from the sliders (usually caused by lateral loads and distortion). I don't think it is the right term for what you are talking about here. Stiction in physics is a threshold force necessary to overcome the static cohesion of two objects pressing against each other.

There is stiction between the seal and the fork tube.  Press the seal harder against the tube, and stiction goes up.

[tbone]

You'll need to use the triple trees too. Those forks are a larger diameter.

I dunno about that.
The cb750DOHC that i snagged the dual disk setup had the same 35mm tubes my 650 had stock.
And it is the same diameter my '74 750 had on it.

Are the DOHC air assisted really a larger diameter on the DOHC's VS. the non air-charged ones?


And the benifit of the air charged is the ability to control the ride height?
Or possibly the rebound by adding pressure to the legs?

Anyone know for sure?


l8r

Got a set of an 82 750 in the shop and they're 37mm.
Also got a set off a 77 750SS and they're 35mm.

[rbmgf7]

where were these available? i have a set installed on my 78 750F from the PO. it has the air valve on the fork caps but it also takes oil.

[jaguar]

you guys are such a wealth of info.   thanks for the help

i have a set of factory honda seals for my 78F front end.  those seals will be ok as long as i stay under 14psi?  i kinda like the idea of making the air caps for the front end.   even if i just run 0psi its should be nice to have the ability to run more.   is there anything else i should know or do to make the set up work better?

[jaguar]

i just checked what size the air forks were that i have.  they are a 35mm.   but the caps have larger threads and do not fit into my 78F fork.  also they are about 2in longer.   i pulled them apart to see if i can use my 78 lowers with the air upers but they look much different.  any idea what forks i have?  im almost sure that they came off a dohc.  deff have leading axle lowers and air caps with the cross over.

[Geeto67]

you could have a 1979-1980 DOHC fork (35mm) with aftermarket air caps. The honda factory setup has only one fill and a balance tube, aftermarket will have two valves for you to fill it with air. It could also be a cb450SC fork which is 35mm and has air caps (and looks similar to the cb750 forks).

and with SOHC cb750 forks you want to keep the pressure under 10 psi, not 14 like the cb900fs. They could take the added pressure because the forks are larger. Go to 14 psi on a SOHC fork and you are liable to breech the seal or have it fold over.