used koni shocks--how to determine if they are good?

[gary basil]

i have a set of shocks on the shelf that look good. how do i determine if they are? thanks, gary

[66S]

I use my "seat of the pants" shock Dyno. Hold one end down with your feet or a screwdriver through the lower grommet, extend and compress the shock if you can,try to judge the resistance to movement and compare them, very unscientific. if the shock moves very easily it probably lost its oil past bad seals or is just worn out. if you can compress the shock but can't extend it a seal has broken up and is clogging the orifices, a pretty common problem. If there is resistance that goes away as the shock is extended it has lost some of its oil. some Konis are crimped shut on top and some have threaded caps on top that can be removed with a spanner wrench for rebuilding the shock.

[chris_seven]

The force needed to move the damper will vary significantly with velocity and the variation should be virtually linear until the force limit of test system is reached.

A typical road car damper would have a ratio of around 1.5/2.0:1 between bump and rebound.

This means it needs more force - at a given- velocity to cause extension to occur (Rebound) than for compression to occur (bump)

Clearly at zero velocity there is zero force in either case so estimating the difference between bump and rebound at low velocities is quite difficult.

The adjustment range of a typical Koni would probably allow rebound to be varied between 1.25:1 and 2.75:1 with respect to the bump setting.

The Koni is unusual in that the Classic Road car Adjustable Konis only adjusted Rebound and Bump was always a fixed value. I believe that Koni's view was that for road cars bump was more dependant on unsprung weight than other variables.

If you don't know where in its range the damper is adjusted comparions of Rebound and Bump behaviour are even more difficult to make on a subjective basis.

The only real way is to use a 'shock' dyno and get some real data. Obviously if there is oil leaking past the seals then measurements are largely irrelevant.

The number that Bilstein use are a little odd and we commonly see numbers such as 225/120 referred to and these are often misquoted.

225 would be rebound and 120 would be bump.

These numbers indicates the forces needed in daN (decaNewtons) at 0.52 metres/second.

The factory used to test all of its dampers on an old 'carding' machine which used a bell crank to create the motion and a fixed speed synchronous motor for the drive.

In themselves these numbers are not very helpful and don't really describe the full behaviour of a damper.

Many damper testers are still bell crank driven but use variable speed motors and more easily generate a curve of Force vs Velocity over a wide speed range but again this data is good for matching and comparison and hence quality control.

Clealry a damper tester with a bell crank inputs a sinusoidal waveform into the damper and this means that the 'lissajous loop' produced during a damper test would be trapezoidal in nature and this makes it diffcult to calculate the area conatined in the loop (the true damping beaviour) and the corners, which is where valve really operate are rounded.

A 'State of the Art' Damper Tester should be Servo-Hydraulic and the damper should be tested with a Traingular Waveform. This produces a rectangular lissajous loop and allows much better evaluation of a damper.

Sorry to drift off the topic - old hobby horse.

[Neil '67 911S]

Thanks Chris for a very interesting explanation.

http://en.wikipedia.org/wiki/Lissajous_curve

[66S]

VERY scientific reply in stark contrast to my technique. I'll go back in my cave now

[Haasman]

You could always send your shocks to Koni for testing- Koni is located in Hebron, Kentucky, ten minutes from the Greater Cincinnati International Airport.

BTW, as a general comment, Koni wants to borrow some cars in return for free shocks. See: http://www.koni-na.com/testfits.cfm Not for me thanks but might be a great deal for someone.

[gary basil]

geez, and i thought the internal revenue code was complicated. sending them to kY seems like a good idea