Suspension dynamics after changing spring length

[MRDAGERUS]

I am interested in this: how the rear suspension dynamic or ride properties will change as a result of replacing the existing 14" spring with 12" new spring of the same spring ratio?

Suspension type: single longitudinal swing arm w/upper and lower lateral links, single 15 deg inclined coil-over spring.
All dimensions/values (before and after) are the same, including the spring rate, except spring free length is reduced from 14" to 12" (new spring, NOT cut). To keep the spring rate the same, diameter of the wire has to be increased and coil count decreased when making a new spring.

Your thoughts would be appreciated.
Thank you.

 

[BrianPetersen]

Presumably you mean "spring rate" (which is a meaningful term) in place of "spring ratio".

Without MUCH more detail concerning the vehicle ...

You are going to be closer to the jounce bumpers and thus have less travel available in bump. The consequence is more frequent bottoming ... worse ride quality, and less grip when it happens. That's a general principle.

Then there's all the "it depends" and "it's complicated" factors.

Depending on the angles and locating points of all of the relevant links, the front-view instant-centers will move. Generally, with "normal" lateral link angles, the instant-centers will move downward relative to the bodyshell. Instant-center below ground level is generally regarded as bad. It will be perceived as a loss of roll stiffness (the antiroll bar will be called upon to make a greater contribution) and/or a change in the way the car behaves in transients. "Anti-roll" achieved through high roll centers happens immediately with lateral acceleration. "Anti-roll" achieved through spring rates and antiroll bars waits until the car actually starts leaning. On the other hand ... lower instant centers mean less "jacking". Whether ANY of this is meaningful or not, depends upon a whole lot of factors that you haven't told us.

It might affect the wheel alignment. It might affect the static camber. It might affect the toe. Those (hopefully) can be adjusted out. It might affect the camber curve (the trajectory that the camber follows in response to bumps, dips, and roll). It might affect the toe curve (the trajectory that the toe follows in response to bumps, dips, and roll). The latter might affect whether you have roll oversteer or roll understeer. Again, it depends on a whole lot of factors that you haven't told us.

Maybe if you told us exactly what vehicle, make model and year, perhaps someone might have actual experience with that particular vehicle.

The type of suspension that you describe is consistent with C2-C3 Corvette, second-generation Corvair, a number of off-road buggies, and of all things the late model Fiat Doblo compact van. There are sure to be others that I don't know about. The Doblo is the only one I know of that is a modern design, and it has the lateral links non-parallel in both top view and front/rear view, presumably to fine-tune the toe and camber control.

 

[SwinnyGG]

Also consider that a 15 degree angled coil-over will no longer be angled 15 degrees at normal ride height if you change the spring length. Depending on other geometry, this will have varying effects on the effective spring rate at the wheel as well as the behavior of the dampers- you're changing the motion ratio at ride height.

 

[GregLocock]

Here's a reasonably modern example

https://themustangnews.com/does-the-mustang-need-irs/

You haven't told us the motion ratio of the spring, so a 2" change in spring length might well give anything up to 75mm change in ride height.

A modern multilink with trailing arm is a fairly linear device for kinematics, so long as the lateral arms are more or less parallel. The big danger is that you'll have used up much of the air gap before the jounce bumper engages and so will have a very harsh ride. If you move the JB up then you may not have enough energy absorption for potholes and you'll ground out on typical road profiles.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[NormPeterson]

except spring free length is reduced from 14" to 12" (new spring, NOT cut). To keep the spring rate the same, diameter of the wire has to be increased and coil count decreased when making a new spring

Something sounds off with that.

Two actions that each increase the spring rate won't compensate for each other. Are you perhaps thinking about maintaining the same final position in bump from some given inertial input instead?

Of course, there's still the pesky matter of displacement input coming up from the pavement that doesn't care so much about inertias.


Norm

 

[BrianPetersen]

Original poster has not chimed in with "the big picture" - such as exactly what vehicle this is, and what it's being used for, and what the objective of this spring replacement is - Why are they proposing to replace the spring.

And Norm picked up on the inconsistency in the original post: Shortening the spring, reducing the number of active coils, and increasing the diameter of the wire act in the direction of increasing the spring rate. Normally, if for whatever reason you are lowering the ride height, increasing the spring rate in compression is what one would do to try to minimise the amount that the vehicle ends up hitting the jounce bumpers.

It's not uncommon in that situation to use dual-rate or progressive springs in the interest of the springs not coming off their mounts in extension going over a crest or a large dip.