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Old 09-09-2019, 07:12 AM   #1
Phred
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Default 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.
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Old 09-09-2019, 07:15 AM   #2
Kurt in NJ
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Default Re: Compression/Rebound on shocks

I have only played with original shocks, that is how a properly operating original one works
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Old 09-09-2019, 08:08 AM   #3
Ranchero50
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Default Re: Compression/Rebound on shocks

Could you put some rebound bumpers between the frame and spring?
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Old 09-09-2019, 09:10 AM   #4
rotorwrench
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Default 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.
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Old 09-12-2019, 06:11 PM   #5
Phred
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Default Re: Compression/Rebound on shocks

UPDATE:Given the limited suspension travel (by design or consequence) on my car and the nature of the Model A shocks, I was able to lessen the "bottoming out" condition by switching the rear shocks side to side. When installed as recommended and the adjustment made to max resistance the shocks probably had a dampening ratio of 20% compression to 80% rebound. Switching the shocks to opposite sides made an inverse dampening ratio of 80% compression/20% rebound.

For the last few days the car has not bottomed out. The ride may be just a bit stiffer/harsher, but not too bad. I may be able to back out a bit of the compression resistance on the shocks to improve the ride and still not have it bottom out.

This has been a quick/easy solution. We'll see how it works over time.

Thought this might be of interest.
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Old 09-12-2019, 08:45 PM   #6
Joe K
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Default 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.

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.

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Old 09-13-2019, 04:37 PM   #7
john in illinois
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Default Re: Compression/Rebound on shocks

Nice fix Phred.

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