How Old are Your Suspension Cones?

[Frogeye61]

Interesting. I wonder if there is a difference in the way that the bounce takes place if it is indeed the case that the amount stays the same. Did you also compare the amount of bounce from old rear vs old front rubbers? Would be interesting since they'd have had completely different weights resting on them.

I have changed out the front pair with a used rear pair (from the same car) before and the difference was remarkable. I assume these would have all been the same age of rubber, so in theory the oxidised layer would have provided the same protection everywhere. The difference in driving comfort only lasted a few months though.

In a different case I did install new rubbers at the rear of a car and apart from an improvement in ride height, which continues till this day, the ride does feel more comfortable. You think it is imagination?

I can't tell ultimately. I'm not that technical but when you've spent money on something, it's easy to convince yourself it was money well spent

Rubber is a remarkable thing. I'm told that most suspension bridges made in Victorian times use rubber as an insulator and it is still working well today. The only thing that will change it's consistency is a lot of heat as it changes the way it works somehow

Well, there you go!
Yes, rubber is a remarkable thing. It is an inexpensive material, flexible enough to be used as a spring, and also containing a high degree of internal damping. (We do find a similar mechanism with leaf springs, good and rusty, but not rusted solid. Problem is they wear out quickly due to abrasion.)

All materials are flexible - to some degree. All materials contain internal damping - to some degree. This is notable by seeing mostly various metals like steel & bronze, plastic and rubber being used as springs. Even a piece of stone is minutely flexible and though the damping coefficient would be hard to determine, rest assured it's there, and quite high. (Otherwise we could build a perpetual machine using a piece of stone bouncing inside a vacuum.)

The elasticity coefficient is the amount of force necessary to deflect (compress or stretch) a material of a certain cross-sectional area to a certain distance. Damping is a force in the opposite direction to motion which is proportional to the speed of the motion.

Regarding old rubber, it becomes harder. What this does is it increases the elasticity coefficient, but that doesn't necessarily do anything with the damping coefficient which directly affects bouncing height.

The first thing we notice is that the word "harder" implies - more difficult to deflect the surface. This implies more force for the same amount of deflection. This is the definition of higher elasticity coefficient. The energy to deform the rubber is given back when the rubber returns to its original shape unless absorbed by the damping. What it means is that it could bounce just as high as before, just that the total deformation of the rubber is less.

If the damping coefficient remains unchanged as the rubber ages, old rubber will bounce higher than new rubber because since the deflection is less, and the rate (speed) of deformation is the same, there would be less actual damping. If the old and new rubbers bounce the same it implies that both the elastic coefficient and damping coefficient have changed. (There is most likely, under laboratory conditions a slight change in bouncing height.)

Bottom line. after all this rambling,
1. Old hard rubber gives less "total deformation of the rubber" This is called higher spring rate and is often used in racing cars. It tends to give a harsher ride to a car.

2. Bounce test, unfortunately proves nothing except the spring rate and damping both changed after aging.

[abri]

Thanks Frogeye, that is consistent with my experience I'd say. Old, sagged rubber springs giving a lower ride height and harsher ride on the one hand vs new rubber springs giving a higher and softer ride on the other.

The lower ride height of old rubbers suggests the rubber is already somewhat compressed. Thus the possible "distance" it can give while damping a force (like a bump in the road) is by definition less. The elasticity coeficient must thus be higher, i.e a much bigger force needed to get the rubber to "give" the same amount (as compared to new rubber). This causes harshness.

The less the rubber has been compressed, the more it will "give" - therefore you'll feel a difference if you swap out your old rear rubbers and put them in front, because they're not as compressed as the old front rubbers.

[Frogeye61]

Exactly