To learn and share about dampers / shock absorbers

[DaveW]

The line of semi-active damper that we are designing now at work for off-road, heavy vehicle have built in blow-off as somekind of pressure control. Mainly to help the damper survive in high pressure event that might otherwise exceed the pressure rating of the seals. The general strategy when we are tuning the blow-off is that it should envelope the performance range of the semi-active mechanism such that normal function of the system can be retained, but it effectively clips off the force higher speed range to control the pressure.

Having seen a damper explode spectacularly on a dyno, I think it is a good idea to build in a pressure limiting device. However, it should not be quite so necessary in service use, because tyres, etc. will help to limit internal damper pressures (but don't quote me on that).

The "blow-off" control I have discussed is intended to limit sprung mass peak velocities (& subsequent displacements) when the vehicle encounters large road inputs (e.g. kerbs), & it would (usually) be set to a much lower pressure than that required to protect the damper. If it has sufficient flow capability the same control would, of course, serve both functions.

[RacingManiac]

I think our need arises out of the fact that the electronically controlled solenoid valve has much greater range, but at the extremity it has some unstable tendency that the control may not be able to correct for. And due to the application which is a fairly heavy vehicle designed for off road use that we basically need some kind of mechanically based back-up to ensure the peak pressure is not reached.

In most case the blow-off is tuned such that the main solenoid valve is the source of the control of the damper.

[DaveW]

Good stuff, RM, & good luck with your project. Don't forget to build in a wheel speed simulation mode so that people like me can help to assess the performance of the vehicle on a rig (with non-rotating wheels). Such tests can be very useful to identify & solve vehicle problems, but only if the dampers are working in a representative way. Oh, & don't forget that the speed sensors might emit stray pulses even though the wheels are not rotating....

[Belatti]

One thing I have observed in the shock dyno for a group of particular dampers (8300 series 2way Penskes) is that when you change clicks in bump the rebound curve slightly changes and viceversa, too.

This doesnt happen with Ohlins...

[747heavy]

Not knowing the 8300 in great detail, is difficult to say. I had a look at some drawings, and a least the rebound adjuster, if there is not problem with the check valve, should not influence the bump side.

Having said that, it would be interesting to see, how it looks on the dyno printout.
There is a chance, that it is a "misread" from your dyno. It will depend, on how your dyno software measures friction and gas force, and how he takes them off from the graph, to make sure that your diagram is centered around the zero line.

If you have very "assymetric" friction in bump/rebound direction (due to your settings) at very low speeds, some dyno software will average the value (assuming that the pure friction is/should be symetrical). This will lead to an slightly drift/offset when adjusting either bump or rebound.
Seeing that both adjusters ae bleed adjusters, which will have an effect, at low speeds, that could be an explainantion.
You would need to know, how your dyno measures gas force and friction, and also at which speed, he performes the measurement. At some dynos, you can adjust the speed and method how the measurements are taken.

But there is a chance, that it is a "fault" in the damper, but from the drawings I have seen, it should not be that way.
May others here have more expirience with that particular damper.

[marcush.]

One thing I have observed in the shock dyno for a group of particular dampers (8300 series 2way Penskes) is that when you change clicks in bump the rebound curve slightly changes and viceversa, too.

This doesnt happen with Ohlins...

that is not the complete truth ....it is highly depending on which series ohlins you are talking of and the build of the damper ,there are a lot of options that may or not show influence of rebound clicks on bump behaviour.Í speak not of TTX here .The older 2 ways dampers have huge influence of rebound on bump in certain areas of the adjustment range .this is one reason why you should avoid adjustments close to the end of the adjustment range and reshim .

[Belatti]

Oh! sorry for the misunderstood marcush!
I was comparing an 8300 series (2way) with a TTX (4way) wich, by the way has a complete diferent construction and regulation philosophy.

[marcush.]

the TTX is totally independend R/B by design and being not a monótube design a lot of problems with cavitation are simply not present on this damper.You will also not need significant gas pressure to avoid hysteresis as well.

[Belatti]

Lately, I have been looking at some dual tube street shocks modified to have 4 internal posibilities of adjustment (not external & not pressurization allowed) to race in a local FIAT 600 series (standard suspension geometry and street tyres).









My friend who drives the lil´FIAT told me that both dampers (red & blue) where identical (theoretically) according to they guy who made his dampers. He gave my friend a peak force vs. velocity curve writen with a pen, measured with an old pneumatic dyno as "the info" about how his dampers where adjusted

Of course, when my friend saw these graphs he wanted to kill himself (and his damper service supplier). He started the DIY path now, and hopefully I will help and try to learn with him.

Next path will be to dismantle both dampers in pieces to see why there is such a force and hysteresis difference given that they supposedly have the same pack of shims, pistons and oil.

[DaveW]

I suspect that I am about to display my crass ignorance or, at least, reveal the sheltered life I enjoy working with dampers that behave (more-or-less) as their designers intended.

I believe the trajectories were obtained from twin tube dampers with nothing physical separating the fluid & the air reservoir, so air & fluid will tend to mix when the damper is exercised. When air is "suspended" in the fluid, then it has to be compressed into solution before the damper can develop a reliable velocity-dependent force. This is what appears to have happened in the compression half cycles in both cases. On that basis, I would conclude that the second damper (blue) had rather more suspended air than the first (red). I believe that the rebound half cycles indicate that the both dampers also had an intrinsic stiffness caused by the reservoir air volumes being rather smaller than they might have been, the first (red) being rather worse off than the second (blue). Perhaps the two features are connected. BTW I believe that this type of damper must be fitted vertically (with the correct end up), & may take some time (& exercise) to settle after it has been disturbed.

[WilO]

If anyone is to display his crass ignorance here, I think it shall be me....
Forgive the basic nature of this post, but I'm interested in learning about the inherent characteristics that different types of dampers are likely to display. Some have been alluded to, regarding cavitation and hysteresis effects. I was having a discussion with an engineer and some other folks, and was surprised at the lack of consistency of knowledge on the matter. Appreciating fully that 'all generalizations are false', I'd appreciate any thoughts, perhaps we could progress to valve designs as well. Thanks in advance.

Wil

[DaveW]

Wil,

You might like to try this... as a starting point, but it does contain some nonsense (I think), so you may also like to have a look at John Dixon's book... (referenced on page 1 of this thread, for newcomers).

[Belatti]

Dave, thats exactly what I suspect that is happening with those dampers and, of course, they are twin tube with nothing physical separating the fluid & the air reservoir.

As using pressure is forbiden by the rules, the solution Im gonna try here is to use some kind of rubber, maybe like this one:
http://en.wikipedia.org/wiki/Ethylene-vinyl_acetate
to fill the air volume that you need to have but letting the oil compress it at the same time...

Maybe next week I can plot the results...

[WilO]

Wil,

You might like to try this... as a starting point, but it does contain some nonsense (I think), so you may also like to have a look at John Dixon's book... (referenced on page 1 of this thread, for newcomers).

Thanks for the link and recommendation Dave. I just received the latest edition of Dixons book yesterday, began reading it last night. I'm looking for information that gives characteristics of different architectures (Monotube designs tend to cavitate more than dual tube or other types).

Thanks again for the help!

Regards

Wil

[DaveW]

Wil,

Serious cavitation can occur when a component of the damper force is developed by a reduction in fluid pressure (i.e. when fluid is sucked through a metering valve). This can occur in many, if not all, damper architectures, depending on the hydraulic layout of the damper & how the flow is metered.

You might consider that flow should always be "pushed" through a metering valve (I do, certainly), because cavitation cannot occur, and damper charge pressure can be low (thus minimizing seal friction). However, extreme velocities can cause the metering valve to "choke" which may then cause the damper to self-destruct unless a safety "blow-off" path is included. By way of contrast (from a damper designer's viewpoint), a design that "pulls" fluid through a metering valve is intrinsically safe because cavitation will limit internal pressures automatically.

In my view, self-destruction is more likely to occur on a dyno than it is on a vehicle &, in any case, adjustable "blow-off" can be a very useful control.