Compression vs. Rebound- Not What You Think?

Most people seem to find the effect of compression forces from shock absorbers and what they do fairly easy to understand. How rebound forces from the shocks affect the car seems to be more difficult. This may be a clue that most of us are thinking incorrectly about what the shocks do during a turn.

Time for a thought experiment. I’m not going to help you out with diagrams or pictures. You can find those lots of places. Consider this a challenge. Just THINK about it.

Say you turn the steering wheel left. The car turns left and at nearly the same time weight begins to shift from left to right. The body of the car, because it is supported by suspension springs, begins to roll to the right. The right-front spring compresses, putting more load onto the right-front tire. The left-front spring extends and removes from the left-front tire. The additional vertical load from the springs on the right-front tire is directly proportional to the extra spring compression. The reduced vertical load on the left-front tire is directly proportional to the reduced spring compression. Voila! We now understand weight transfer. Right?

Are you sure?

Caveat: For the purpose of this thought experiment please forget about the energy dissipation effect of shock absorbers. That amount of energy is small, in any case, and has more to do with how the tires are controlled over bumps, and with controlling resonances of the sprung mass, which we are not talking about here.

Let’s go with this idea a little longer even though most of you already know it’s wrong, or at least incomplete, and consider the shocks.

When the right-front spring compresses the shock is also compressed. The shock resists this motion with a compression force. That force pushes down on the tire via the suspension linkages and up on the body. So, the shock adds to the weight transfer and slows the body roll. There’s more downward force on the right tire while the shock is moving, which is prior to the steady-state condition, so it speeds up weight transfer. Once the body stops rolling the shock force becomes zero but by then it has been traded for spring force and we are steady-state cornering, fat, dumb and happy, as they say.

This is the way I thought about shock forces for a long time and it’s not wrong. But, then I started thinking about the left-side shock during this same turn. It’s extending as we enter the turn as the body begins to roll and is therefore resisting that extension with rebound force. So, it pulls up on the tire, reducing the load at the tire patch and at the same time pulls down on the body of the car, again slowing the roll and, again, somehow speeding up the weight transfer. Note: Rebound forces always pull weight/load off the tire just like compression forces always add weight/load to the tire.

Now I have a question for you: Where did the load go that got pulled off the left tire?

Well, the only place it can go is over to the right tire. That’s the definition of weight transfer. It can’t vanish. It has no buddy it can turn to and say, “Hold my beer.” So, how does it get to the right tire?

Think about that for a minute.

See the problem? How does the load that the left shock picks up from the tire patch get over to the right side and push down on that tire? It’s not the force that the right shock puts down to the right tire. (It would still transfer weight to the right even if there were no compression force in the right shock.) Do all the free body diagrams you want… I don’t think you’ll find it. We think we understand how the right shock pushes down on the tire with compression force, but how does the left shock, all by its lonesome, send weight over to the right?

Maybe it will help to understand or at least appreciate the dilemma if we remove the right shock entirely. The left shock is over there, still resisting the roll of the sprung mass of the car by picking load up from the tire patch, but how does it speed up weight transfer and create a load on the right tire? Magic?

One way out of this thought-experiment dilemma is found by gradually stiffening up the shocks until they won’t move at all. Just replace them with solid bars. You can leave the springs in there, but they can’t do anything if the shock is locked up, right?

Now we have a kart. The suspension cannot articulate. The springs are just dead weight.

Is there any weight transfer when we turn our new kart-like car? Of course there is. Turn the wheel left and it still wants to throw you out the right door. Plenty of weight shift from one side to the other. The tires on the outside of the corner see much higher vertical (and horizontal) loads during a turn even though there is no working suspension.

This is called unsprung weight transfer and it happens faaast, i.e. instantaneously, as long as we neglect the elasticity of the tires and the structure. If the tires produce a lateral force there is instant, unsprung weight transfer. Can’t get faster than that. (Please don’t bring quantum physics into this!)

All cars with a working suspension have both unsprung (fast) and sprung (slow) weight transfer. Sprung weight transfer is often called “elastic” weight transfer and has been discussed previously here.

Springs create (or allow) the sprung weight transfer at the expense of unsprung weight transfer and then add a little extra to the total thanks to lateral movement of the CG as the body rolls. The softer the spring, the more and slower the sprung weight transfer. The full weight transfer cannot be complete until the sprung weight transfer is complete, i.e. the body stops rolling. In the end the total weight transfer is the unsprung weight transfer that happened instantly plus the sprung weight transfer that took a finite amount of time.

If the shock is replaced with a solid bar and prevents the spring from moving then you get the fastest possible weight transfer. Effectively, you have converted the sprung weight transfer (that would have taken some time to accomplish) into unsprung weight transfer that happened instantly. You have also slightly decreased the total weight transfer by eliminating the extra bit that comes from allowing the body to roll.

Now you know the answer: both shocks, left and right, by acting to some degree like stiff, solid bars and resisting body roll during the transient event, temporarily convert some of the elastic, sprung weight transfer into unsprung weight transfer. They speed up weight transfer by “borrowing” some elastic weight transfer from half a second in the future and bringing it into the RIGHT NOW. The stiffer the shock the more like a solid bar, the more like a kart, and the more weight transfer gets borrowed from the future. This also makes it all the easier to dynamically overload the tire.

This is why shock forces that are too high, compression OR rebound, can contribute equally to causing that right front tire to go over its lateral grip limit during turn-in to a left corner and start to slide, producing understeer. It’s the total of the shock forces that matters.

Yes, the extra compression force from the right side shock pressing down on the tire helps to counteract the increase in instantaneous weight transfer. But, similarly, by jerking weight off the left front tire with too high rebound force you can start an oversteer sliding event from the inside tire which then cascades to the outside tire. Maybe some very experienced and sensitive drivers can actually feel the difference between compression-produced understeer vs. rebound produced. I doubt that I can.

Moral of the story: Even if you have near zero of shock force in one direction, say very little compression, if the rebound forces are too high you can make the same bad things happen. And vice-versus.