Twin-tube versus monotube shock discussion

[underradar92]

Ok' I think I've got it. It kind of makes sense to me since I've been inside a damper-tube (non cartridge) type motorcycle fork, but I haven't been through a cartridge or twin-tube fork. That's why I have some gaps in my understanding. This is a great thread, thanks guys. Just out of curiosity, are AGX's mono or twin tube?

 

[jtmcinder]

The AGX is a twin-tube.

- Jtoby

 

[wongster]

so what brands that are mono tube?

 

[pneumo]

As to that picture from "How Stuff Might Work," it has a huge error with regard to how twin-tubes work. The picture shows the base-valve just sitting off on the end doing nothing. In reality, oil moving from the main chamber under the piston-head to the outer chamber must pass through the valve. I will attach a cut-away of a Koni Sport.

- Jtoby

It's my understanding that the base valve is a spring loaded one-way valve that lets oil into the center chamber but won't let oil out of the center chamber. The piston is also a spring loaded one-way valve that lets oil flow up (relative to the pistons motion) during compression, then pumps oil out the top of the center chamber during extension. So the piston controls compression damping, and the base valve controls damping during extension. The reason for this is so oil is constantly circulated from the outer chamber to the inner chamber. This also keeps the inner chamber filled to the top with cooled, bubble free oil.

I had a chance to play with a stock strut, GR2, and Koni yellow, and found that if I turn it upside down and cycle the shaft in and out the center chamber would fill with air, and damping would decrease dramatically. If I turned it right-side up and cycled the shaft again, the air was pumped out and normal damping returned.

I wonder if it would be possible to build an inverted twin tube shock by reversing the action of the valving?

 

[jtmcinder]

It's my understanding that the base valve is a spring loaded one-way valve that lets oil into the center chamber but won't let oil out of the center chamber. The piston is also a spring loaded one-way valve that lets oil flow up (relative to the pistons motion) during compression, then pumps oil out the top of the center chamber during extension. So the piston controls compression damping, and the base valve controls damping during extension. The reason for this is so oil is constantly circulated from the outer chamber to the inner chamber. This also keeps the inner chamber filled to the top with cooled, bubble free oil.

You have most of it exactly backwards. Look at the pictures I posted of Koni Sports. The foot-valve controls compression and the piston controls rebound.

As to the above question about available monotubes: many of the newer, Asian, true coilovers are monotubes with separators in the base (i.e., Type D in Wret's picture). Note how several tout that they are "short-throw"; that's a big hint that they have wedged the separator at the bottom of tube. The single adjustor that changes both compression and rebound is a bleeder in the piston-head.

- Jtoby