Rear spring rate vs. shock damping issue, and then some
#2631731
03/12/19 09:14 AM
03/12/19 09:14 AM
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Joined: Jul 2007
Posts: 531 Virginia
JimG
OP
mopar
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OP
mopar
Joined: Jul 2007
Posts: 531
Virginia
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I'm trying to sort out some handling issues on my 1971 Satellite 4-door police car clone. I've tried to make a decent-driving car, within the constraints of 15-inch tires. Here's what I've got in a nutshell:
FRONT: 1.03" T-bars Suspension rebuilt with Moog hard parts Firm Feel upper control arms Steering box from Firm Feel Polyurethane LCA bushings Helwig anti sway bar. I think it's 1-1/8, it's been a while since I installed it and the memory fades Koni shocks 225-70-R15 Cooper Cobra tires (being a police car clone, I'm stuck with 15" wheels) Alignment is 1/2 degree negative camber, 5 degrees positive caster, and I do not remember where the toe-in is set.
REAR: 6-leaf Mopar Performance springs, intended for the left side of late-60's 440 & Hemi B-bodies. I can get the part number if needed. Helwig 3-way adjustable anti sway bar. I think it's 3/4, it's been a while since I installed it and the memory fades Koni shocks 235-70-R15 Cooper Cobra tires (being a police car clone, I'm stuck with 15" wheels)
The problem is when driving on a straight road with slight dips in the pavement, the rear end is "bouncy" for lack of a better term. Constant, tiny up-and-down movement. Harsh dips are handled much more effectively and satisfactorily than tiny ones. I've cranked the Konis from one extreme to the other with many stops in between, and the problem cannot be rectified. So yesterday, I pulled the QA-1 double adjustable shocks off another car and put them on the Satellite and will see if they can help the problem.
A FEW OBSERVATIONS AND QUESTIONS:
The rear springs give the impression that they are too stiff. I have removed one and two leaves for trial. That seems to help but the car squats too low in the rear. I'm confused though, as these same springs were factory-installed on 60's cars with bias ply tires. Wouldn't radials want more, not less spring rate? BTW, I have removed the U-bolts and checked that both saddles sit squarely on the spring perches... I understand that if the rear end were in a bind here, similar symptoms would occur. I could just remove two leaves and get the springs re-arched (or buy different springs) but I'm somewhat surprised that these springs exhibit the symptoms I'm seeing.
Compressing the rear suspension by pushing the rear bumper down causes the tires to visibly compress. I've tried 25 to 38 pounds of air in them... it seems to be happier with less inflation. The fact that the tires compress and not the springs also seems to indicate too-stiff rear springs.
I have the QA-1 shocks on now and I'm thinking of trying a low number on the compression damping and a high number on the rebound damping, which would seem to make sense if springs are too stiff.
I'm way past my knowledge and experience in this area, so any advice is appreciated.
Jim
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Re: Rear spring rate vs. shock damping issue, and then some
[Re: JimG]
#2632275
03/13/19 12:52 PM
03/13/19 12:52 PM
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Joined: Jan 2003
Posts: 5,394 Pikes Peak Country
TC@HP2
master
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master
Joined: Jan 2003
Posts: 5,394
Pikes Peak Country
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Lets see what I can throw out.
IIRC, and that's no guarantee these days, the XHD springs are around 160# rate. So not radical high, not really low. I doubt they are too high for your vehicle. Remember that if the springs take 160# of pressure to compress one inch and your tires have 35# or air supporting them, that is a huge difference in resistance, especially with a 70 series tire. I wouldn't worry about this aspect.
I wouldn't say a radial wants more or less spring than a bias ply as it all part of the combo. If anything, the construction of a radial may allow a softer rate to get similar performance because of sidewall compliance, but that's tire theory way beyond my pay grade.
Oscillation or porpoising is typically because of match front to rear spring frequency, not rates. But, you say these are relatively minor oscillations of small road irregularities instead of major dips, so I wouldn't say this is the issue.
I might lean towards an issue with high speed resistance of the shock. Shocks control spring travel with high speed valving, like bumps and ruts, and low speed valving, dips on the road or cornering. These are two separate values built in to the shock. Single adjustable shocks are designed to address overall compression or rebound and do not differentiate between high and low speed pistons speeds within the shock. To generate enough high speed resistance might require cranking the shock up so high that the low speed resistance is now producing a rough ride.
Since you have gone to a double adjustable now, I agree that you might want to try softening up the bump to allow it to soak up the hit and firm up the rebound to push it back into the pavement faster.
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