Compression/Rebound on shocks
 

The springs on my phaeton have been modified (including some leaves removed) to lower the car to a stance I like. The rear radius rods will hit the chassis rails when the car hits a "good" bump. I have approx 2" clearance between frame rail and radius rod. I am trying to resolve this without changing the stance too much.

While playing with the standard shocks (new ones from Brattons), it appears that the compression stroke is always very soft and the adjustment screw only stiffens or softens the rebound stroke.

Is this normal for the Model A shock?

Thanks in advance for your help.
Peace, Phred

Re: Compression/Rebound on shocks
 

I have only played with original shocks, that is how a properly operating original one works

Re: Compression/Rebound on shocks
 

Could you put some rebound bumpers between the frame and spring?

Re: Compression/Rebound on shocks
 

Originally the HOO-DYE shocks were 60/40. 40% dampening on what they refer to as joust and 60% dampening on rebound. Some rebuilders end up with 50/50 when they rebuild them. I figure if there is enough leak by on the old parts then 50/50 is about as good as it gets. As they wear, they would need further adjustment. The bushing was steel on steel so they did wear. Some rebuilders put a phosphor bronze bushing in them.

Springs wear out too and that is what hold the frame away from the suspended parts. The shock is just a damper.

Re: Compression/Rebound on shocks
 

Re: Compression/Rebound on shocks
 

The design tendency of the shocks are to "hold the car down" on vibration. I.e. as indicated the "rebound" is over-restrained compared to the displacement.

The spring is of a design such that as the spring is displaced, it presents non-proportional increase in resistance. (I.e. it grows stiffer faster than displacement) Thus the ride "stiffens up" under increasing loading/displacement.

This is the same principle used later (and with more fan-fare) by the designer of the VW suspension Ferdinand Porsche. VWs had the 60/40 built into tubular shocks and used "increasing rate" coil springs combined with "rubber donuts" to achieve a similar action.

I still suspect you may be "undersprung" and you're counting on the shock to take momentarily the load of compression. Bottoming out is only one aspect of ride and control.

You can test for "critical damping" which is the condition which Porsche and Ford before them were trying to achieve.

Given the usual number of occupants of the car in place, try to displace the car by jumping on the bumper thereby pushing the car down. If the return involves more than one "wiggle" (i.e. comes above the pre-displacement static point or passes through the static point more than once) you are under-restrained.

If it appears slow to return to the pre-displacement static point and never wiggles, then it is "over-restrained."

The ideal condition is the one where there may be little or no "overshoot" - this would be the critical damping condition. THIS is what you want and will afford the best ride and control.

Graphed below.
https://www.researchgate.net/profile...al-damping.jpg
The beauty and engineering of the Ford/Porsche system comes in having the spring "tuned" to the loading of the car. More load yields a non-proportional displacement.

Joe K

Re: Compression/Rebound on shocks
 

Nice fix Phred.

John