bob1esq

OK, I sort of got this torque arm geometry figured out. It is unbelievably complex. I get that the center of gravity revolves around the instant center. I also get that the higher the instant center, the less forward to rear roll and thus more traction at the rear axle during forward acceleration. I also get that the opposite is true. If you have a lower instant center you will have more roll and thus less force being applied during forward acceleration.

What I do not get is why or how this causes more (or less) force/traction during acceleration. I get that more force is being applied to the rear axle. But I do not get why or how.

Is it because force is being transferred through the LCA from the chassis to the axle? If so, how, and how is this related to roll and the center of gravity? It just seems that the opposite would be true. If you have more roll, it would logically seem you would have more force being transferred and thus more force at the rear axle. I get that if you have more roll that more of the force being transferred to the rear is being absorbed by the springs, and thus less is getting to the axle, but I do not get is why when there is less roll there is more force at the control arms. Where does this force come from and how does it get there? I am so confused

bob1esq

I GOT IT! Think of it like that ride called the "Hammer". As the head approaches the top it goes really slow because gravity is pulling down on it. But, as the head approaches the bottom of its circumference, it goes really fast. More angular speed = more inertia.

bob1esq

Well what is happening when the Instant Center (IC) is above and slightly to the inside of the center of gravity is that the center of gravity swings around and below the pivot (the IC). There are 2 forces working on the CG, one is the force of gravity and the other is the force of acceleration, the car is accelerating forward, so there is a counter-force on the CG pulling the cg toward the rear of the car. because the CG is swinging downward (at first) when the car accelerates, it swings very quickly, because gravity is helping it along. But if the pivot, the IC, were below and to the side of the CG, then the CG would have to swing upward. As the CG would have gravity pulling it down as it tries to roll upward, the roll would be slower, less inertia. WOW, do I got it.

bob1esq

of course, of course, of course, More is not always better. Sometimes too much force too quickly is not a good thing. Once the car stops rolling (around the IC), no more force is being transferred, that is the force on the rear wheels decreases. For racing like drifting, it may be more beneficial to point the control arms down toward the front of the car. This would make the IC below the CG. The CG would roll up and then down over the IC pivot. Hmm. So, there would be very slow momentum or inertia at first, but then it would start to pick up a little and it would be over a longer period of time. So, the tires would bread away easily because little force would be on them, but as you get going the force would slowly increase. Who says you need to be an engineer to figure this stuff out. Oh, and BTW, even mechanical engineers I have spoke to have not been able to figure this out. I guess racing is a great motivator to figuring things out.

bob1esq

So, based on my theory, if you were to change the CG to the rear of the car, i.e. by putting bricks in the trunk, you would want your IC pivot to be below the CG. This would really cause the car to roll back really fast, so fast you may even pop a wheelie.

bob1esq

Just a little clarification. When the pivot is above the CG, it seems that even though the rear of the chassis lifts upon acceleration, the CG is effectively pulling against the pivot pushing it through the control arms into the axle. So, although there is less force on the springs, more force is on the control arms, creating more traction at the rear wheels. The springs are not absorbing the force. By the same token, it would seem that when the car first starts to roll because there is a downward movement of the chassis, the springs in the rear and somewhat in the front would be involved. It would be a very quick yet small downward movement of the chassis. So the rear springs would take the force initially and then the force would be transferred to the control arms once the chassis in the rear starts to roll upward.