[Turninconcepts.com]

Just as a side note - rear stroke on the ohlins sportlines (by hand pressure) is only 4.5 inches. No idea how far into the bump stop that took it though.

[DuncanG]

Originally Posted by stretch

Your modified AST's still have a stroke problem even with longer springs? I would have expected all bottoming out issues to have stemmed from the bundled 7" springs up front, which would be expected.

Well it was much improved with the long springs but it was clear the bumpstops were being excercised rather hard (much too hard). With (from memory) 3.5" bump travel they were much inferior to my Ohlins with only 2.5" bump. Bump handling is not all down to the spring (contrary to some old Koni tuning guides) but the damper plays a huge part in it.

Originally Posted by stretch

Anyway, is that an awful lot of hysteresis for a damper? I'm having trouble thinking of what would cause that. Even the pressure due to gas (starting point at 0mm/second) is reversed for the two lines, what could cause that?

The divergence of the lines at 0mm/s is the most basic illustration of hysteresis. The fact that the lines dont reconverge at higher speeds is indicative of rather large hysteresis. If you imagine the plot unfolded on the y-axis you'd see a more familiar (to an engineer) hysteresis loop. See Penske, Ohlins, Roehrig etc for some basics on hysteresis and cavitation.

[DuncanG]

Originally Posted by Turninconcepts.com

Either way, we're going to find out if the larger HS orifice helps the HS damping. I'm hoping it does or I'm going to have to figure something out that I don't think AST is building.

You might like to ask AST if these contain a base-valve? I'm starting to think that the hs-bump collapse I felt might be related to cavitation as a consequence of no base-valve. A base valve would impact on cost and damper stroke.

It'll be interesting to see how your experiment works out. Do you have any gear for monitoring wheel position? Even a camcorder mounted on the side of the car pointing at a wheel can give you some useful feedback.

(edit - changed 'foot-valve' to the more common 'base-valve')

[stretch]

Originally Posted by DuncanG

You might like to ask AST if these contain a foot-valve? I'm starting to think that the hs-bump collapse I felt might be related to cavitation as a consequence of no foot valve. A foot valve would impact on cost and damper stroke.

It'll be interesting to see how your experiment works out. Do you have any gear for monitoring wheel position? Even a camcorder mounted on the side of the car pointing at a wheel can give you some useful feedback.

Ah, I read through Penske's manual again and their own example shows hysteresis about as severe as above. Their own literature says this is affected by, "The type of piston, the shims used and the relative position of high and low speed adjusters."

The Penske manual, the only one I'm familiar with regarding damper construction, deals with needle (bleeder) valves and shim stacks (with variable preload). So, what's a "foot valve" and why would it help hysteresis? The only mention of it that I can find defines it as the bottom valve in a twin-tube damper, but we're discussing monotubes here.

Edit: I found this on hysteresis reduction through pressure balancing: http://www.roehrigengineering.com/ca...0Balancing.pdf

[Turninconcepts.com]

Originally Posted by DuncanG

You might like to ask AST if these contain a foot-valve? I'm starting to think that the hs-bump collapse I felt might be related to cavitation as a consequence of no foot valve. A foot valve would impact on cost and damper stroke.

It'll be interesting to see how your experiment works out. Do you have any gear for monitoring wheel position? Even a camcorder mounted on the side of the car pointing at a wheel can give you some useful feedback.

Our gear for that one is pretty low tech. It's Tony and Rob following, and watching how things move. Although, we do have a suction mount and a camcorder so we could do that too.

[DuncanG]

Yes but Penske don't say whether thats a typical or a bad example - maybe its not so bad, dunno

The base-valve is not about reducing hysteresis but cavitation. The Roehrig article on pressure balancing gives some insight on that. Yes all twin-tubes have a base-valve and its making me wonder if thats a reason why in the budget market twin-tubes can be better.

The simplest monotube arrangement consists of the rebound chamber, piston, compression chamber, floating piston then gas. Resistance to compression is provided by suction on the rebound side of the piston as obviously you cant create pressure on the compression side since thats coupled straight to the floating piston and gas. Too much compression -> too low a pressure in the rebound chamber and the fluid cavitates (in effect boils) so limiting compression. There's a good video of that on the Roehrig site.

The next step up in quality provides a base-valve that is fixed in between the main piston and the floating piston. It provides some resistance to compression (but not extension, by means of a valve). Now in the compression stroke there is increased pressure in the compression chamber with less suction needed in the rebound chamber so cavitation can be reduced. Ohlins shocks for the S2000 and some QA1 socks are examples of this type. The base valve also means that a lower gas pressure can be used leading to a lower 'rod-force' on the shocks.

The next step up again takes us to high-end shocks with the gas and floating piston in an external tank with adjustable valves.



I don't have the definitive answers - just trying to shed some light on possible pitfalls and trying to work out why I found the ASTs so disappointing.

[stretch]

Well, Dennis Grant has said 125psi is sufficient for eliminating cavitation in Bilsteins, which use a very similar design to AST (no head/foot valve) as far as I can tell. AST seems to be running much more gas pressure than that. A 22mm strut shaft would have a surface area of 0.59 square inches. At 100lbs of extension force from the strut, that'd imply a gas pressure of 170 pounds per square inch.

[DuncanG]

Hmm, I've used Bilsteins and they were fine - no hs-bump collapse. So I guess that theory is busted.

<edit><edit>(edit...)
Or is it?
From another thread -

Originally Posted by Stretch

Bilstein claims they run the highest gas pressures in the industry to prevent cavitation and foaming: 360psi. Koni's (for the WRX application) are purely hydrolic and have no pressurized gas, but even pressurized twin-tubes see a much lower 50psi. (Recall that for dampers with bump/rebound valves in different places, such as Konis, the probability of cavitation and thus the need for high pressure is greatly reduced.) The elimination of 360psi is quite significant, as the thick shaft of the strut displaces gas as the damper compresses. That means we have all that pressure working to push that shaft outwards.

So, I was curious what a 360psi change in all four struts would do to the STI's ride height. I don't know what shaft diameters Bilstein uses, but Whiteline (per the FAQ linked above) claims to use a 22mm shaft in their dampers. A 22mm diameter is really beefy so this is an extreme example, but that means an 11mm or 0.433 inch radius. Pie*0.433^2 = 0.58 square inches. At 360 pounds per square inch, that amounts to a spring rate of 209 pounds of upwards force just from the shock.

So it would seem Bilstein are running, from factory, about twice the gas pressure of AST.

The fact that Dennis Grant doesn't have cavitation with 125psi at only 3inch/sec for his auto-X tuning doesn't mean much in the real world where I expect my shocks to work at 30+inch/sec.

[edit]
However the ASTs have a much bigger piston than the 40mm Bilsteins so that reduces the need for such a large pressure. The hi-speed bump softness I felt in the ASTs was probably just down to soft 'comfort' valving.
</edit></edit>

[LOLSTi]

Originally Posted by Turninconcepts.com

Our gear for that one is pretty low tech. It's Tony and Rob following, and watching how things move. Although, we do have a suction mount and a camcorder so we could do that too.

I'm high tech, damnit!

[TraSTi]

^^lol

[stretch]

TiC, see if you can source these tophats: http://www.vorshlag.com/images/catal...ne1_e46_01.jpg

So it would seem Bilstein are running, from factory, about twice the gas pressure of AST.

The fact that Dennis Grant doesn't have cavitation with 125psi at only 3inch/sec for his auto-X tuning doesn't mean much in the real world where I expect my shocks to work at 30+inch/sec.

</edit></edit>

Dennis Grant says 80psi is the minimum for the 3inch/sec sweep, but 125psi "never cavitated". Bilstein advertises the 360psi figure on their site, but I wonder if they actually use that pressure in all their products. Lower pressures are better so long as cavitation isn't a problem, as far as I know.

Anyway, your dyno graph would have me believe the AST's have a little more than 100lbs of extension force, equating to ~200psi internally. That's in line with what I thought most monotubes ran at. (Edit: I see your spreadsheet uses 50kg, or ~110lbs and ~186psi)

[Turninconcepts.com]

Originally Posted by stretch

TiC, see if you can source these tophats: http://www.vorshlag.com/images/catal...ne1_e46_01.jpg

Ummmmm.... funny that you mention those...

[DuncanG]

Originally Posted by stretch

Anyway, your dyno graph would have me believe the AST's have a little more than 100lbs of extension force, equating to ~200psi internally. That's in line with what I thought most monotubes ran at.

How do you deduce gas pressure from that dyno graph?

[stretch]

Originally Posted by DuncanG

How do you deduce gas pressure from that dyno graph?

By looking at the persisting force at zero piston movement.

[DuncanG]

You mean the pink trace at 120N (27lb) or the blue on at -120N (-27lb)?

No - thats the hysteresis, not the gas-force, as that has already been corrected for in the dyno-plot spreadsheet.