Damper rebound

[Austinspace75]

Hi,

I'm trying to understand whether some opinions I'm hearing about damper rebound are reasonable and useful, or are actually just a misconception.

The two things that seem at odds with each other are:

1. The opinion some people hold that with a damper unit freed from the car/strut/spring, when they push down on the rod enough to push it into the damper and then release it to find that the rod DOESN'T push itself back out of the casing either quickly or at all, they then describe that damper as having 'no rebound'.

2. Or is it that, regardless of what you may find the damper does when exercised in an isolated state, rebound is something else entirely. Once fully assembled on the car, a damper's rebound is the amount of RESISTANCE the damper will offer to a re-opening spring, not the amount of ASSISTANCE.

Can anyone shed any light on the truth please? Is there a relationship between 1 and 2?

And also, why is it that some dampers do indeed push their rods out of the casing all by themselves after being manually closed?

Thanks,
Tom.

[phoenix]

Answering your last question first, the only type of damper that will push the piston/rod out after compressing it (on or off the car) is a gas pressurised damper. With a damper that isn't gas pressuried, the rod/piston will stay wherever you leave it after compressing it.

Both types (pressurised and non-pressurised) offer resistance to the piston/rod moving in both directons; on closing (that is the bump damping) and on opening (the rebound damping).

When fitted to the car, rebound is the energy in the spring which was absorbed during compression and is then released. The energy released is then damped (controlled) by the rebound damping provided by the damper. The energy released by the spring in rebound will always be far greater than the energy in a gas pressurised damper which you note pushes the piston out at a steady by quite slow rate.

So, 2) is correct and there is no relationship between 1) and 2)

[Austinspace75]

Thankyou for the clear description! Much appreciated.

[RPD Motorsport]

Sounds like your common sence and phonex's help have been spot on.

Anyone who expects a non-pressurised damper to shoot back out un-assisted and comments to say it's got no rebound is clearly showing their wires are crossed! As you have gathered high rebound will slow the dampers return stroke.

[elitemotorsport]

As i posted in another thread there are many many misconseptions about dampers in all paddocks.

I designed them for a living and it used to be a source of great frustration having to explain what really goes on inside one to almost everyone i spoke to about it. I still have to give lessons on dampers to damper manufacturers sometimes.

Being a bit of a snob i would say that if the damper rod doesn't force itself out of the body it has no place on a race car. The twin tube unpressurised dampers are awful really in proper engineering terms, but, i do appreciate that financial restraints have to be taken into account. It's very hard to setup a car properly with a cheap damper as there are so many limitations and compromises with them.

There's an article on track engineering dampers on my site (click here) which might help you out. But in general remember that dampers are velocity sensitive devices not displacement sensitive. I.e they only apply a resistance in proportion to how fast the damper rod is moving, not how far it's pressed into the body.

D.