Talk Morgan Forums Morgan Sports Cars Classic Morgans (pre-CX) Function of rebound spring

Excuse me for being a little blonde, but there is a part of the front suspension I do not understand: when you hit a bump the stub axle is moving upwards, the force and motion being absorbed by the main spring. Then the spring recoils and moves the stub axle onto and sometimes beyond the neutral point. This "extra" movement is then absorbed by the rebound spring which then recoils again etc., hence the nodding dog syndrome. I am aware of the idea of shortening the rebound spring to reduce this, but isn't dampening the rebound movement the main task of a good shock absorber? In other words, is the rebound spring really needed? I have no intension of removing it (shortening it, yes), but I just wonder. I find that the "upper part" of the system - king pin/main spring/shock - has analogues on a modern car, but have difficulty seing an analogue to the "lower part", i.e. teh rebound spring. Or am I just being blonde?

The lower spring is essential to prevent a sudden shock or banging of the wheel hub assembly travelling downwards on rebounding from excessive shock. If it were not there then the repeated hammering of the hub down upon the lower cross member would lead to failure of the weakest component part.
The rebound spring is already much shorter than the main spring. I see no reason to reduce it's length any further.
"If it aint broke dont fix it." !

Ok, Moggo - your first part makes sense, but as I understand it was originally introduced as a security device to avoid the downwards rebound bang you are describing. Now, the actual length of the rebound spring creates a force counteracting the main spring, a preload. At least I cannot see any reason for this preload, and there are many advocating cutting the rebound spring so it just kisses the underside of the hub/axle. The length itself is not important, and I am of course aware of it being shorter than the main spring. The preload and the oscillating (nodding dog) movement of the axle is the main issue.

Just to add to Graham's comments, without a rebound spring even if the shock absorber were left with the task of limiting rebound, it would tear itself apart in time unless it had substantial internal buffers.

Robbie

Read the attached article in Morganatica. It is written by Peter Ballard who knows more about suspension systems than many of us will ever know.

It is NOT a commercial for the SSL system, but a theoretical and practical examination of the MMC front suspension, and the role of the springs.

https://sites.google.com/site/morganatica/suspension/main-rebound-spring-interation

A viewing of the YouTube video of Morgan Suspension indicates to Me that the job of the rebound spring is more complex than I thought. Unfortunately I can not find this video. Maybe somebody else can find it for Me.

Making sure the balance between both springs and the damper is correct should sort any nodding dog issue. Have a read of the Peter Ballard description. Take your time to absorb it and get the ideas sorted.

This one maybe?

https://youtu.be/1YhaTTb7n8M

That is a better one than the one I was thinking of.

It's not just about bump and rebound. It's also about cornering and roll stiffness.

As you turn a corner the outside wheel loads up against the main spring and the unloading of the inside wheel is controlled by the rebound spring.

If you remove the rebound spring, there is nothing to control that unloading and that side of the car would just continue to lift until it runs out of space as the stub axle impacts the bottom crosshead bracket. This will give you no roll stiffness control and a horribly wallowy feel as you turn in.

If it's just a case of too short a rebound spring you will initially have very poor control of the roll stiffness until the inside jacks up a bit and then the stub axle contacts the short rebound spring to give you back a degree of very belated roll stiffness. Not as bad as no rebound spring but still a very wallowy turn in..

The solution is to use perfectly matched bump and rebound springs with the rebound spring just kissing the stub axle at rest.

As said before, read the Peter Ballard article and take time to assimilate it because it's not immediately obvious. Or at least it wasn't to me.