exwestracer
11-30-2011, 12:45 PM
Home today keeping an eye on a very sick wife, so I figured I'd add another installment...
So far we've looked at lowering the front of the car, and some effects of adding or changing sway bars. This time we'll explore some common terms (and misconceptions) about the rear of the car. Specifically typical solid axle suspensions. One thing that seems to be often misunderstood is the concept of instant center and anti-squat as it relates to a PT type car. Anti-squat is a very important advantage of a solid axle suspension (if designed correctly), but can also get us into serious trouble from a handling standpoint....
Anti-squat is really a simple concept. It is using the rearward rotation of the axle under acceleration to counteract the rearward weight transfer from that same acceleration.
Whenever we "punch it", we are pressed firmly back in the seat from inertia. The same thing happens to the car itself. The vehicles weight (mass) doesn't want to move, so it tries to lever back over the rear suspension; causing the body to drop down over the rear tires, or "squat". This is NOT a good thing. Anytime the body goes down over the tires, that weight is not being used to aid traction.
At the same time, the tires are trying to push the car forward. Again, the car's mass resists acceleration, so some of our engine torque is absorbed in the rear axle pinion climbing up the ring gear (rotating the housing backward)...Isaac Newton's "equal and opposite reaction" at work... The good news is, we can make use of this "lost" energy to help traction. Let's connect the rear axle housing to the chassis with a simple lever...just a steel bar welded to the axle housing and running forward to the frame. When we accelerate, the rearward rotation of the housing will press the bar up against the frame HARD; attempting to lift the car off the ground.
I cribbed this drawing off a Fiero site (!) of all places, but it's a very simple look at the effect I have been discussing.
51888
The rotation arrow on the tire is the direction the tire itself is turning. Keep in mind the axle housing is trying to rotate the opposite way ...note the force arrow UP where the bar meets the chassis "box". Sorry guys, but that's all a ladder bar or torque arm suspension is. Well, not ALL; but that's exactly how a torque arm manages the axle rotation torque. Now, the car has quite a bit of mass, so it's not likely the rear axle will be able to pick it up off the ground (and we wouldn't want it to!), but it's gonna TRY. As our lever is trying to lift the weight of the car, the fulcrum of the lever is getting heavier at the same time. In this case, the fulcrum is the rear axle housing, which... if I remember correctly... is attached directly to the rear tires. So the weight of the car is transferred directly to the tires, increasing traction. As the lever is lifting the car, it resists the natural tendency of the weight to fall to the rear..."anti" squat.
Where the lever attaches to the chassis detemines how much of that lift affects the rear or front of the car.
51896
As usual, it's not really that simple.
Of course, a simple lever may not always be the best way to attach the axle to the chassis, but we can get the same effect using a set of short links; a 3 or 4 link suspension.
51895
This is a version of a fairly common diagram, but it shows all the relevant info. The point at which the link planes converge is called the instant center, or IC, of the rear suspension. This is sort of like the attachment point of our simple lever, but the forces applied through this type of suspension are very different. We still have lift, but not in the same proportion as we had with the simple lever. You may also have noticed that the links allow the IC to move around in the chassis as the suspension travels up or down. In the lower half of this drawing, we see a line running from the bottom of the rear tire to a point above the front axle. This is referred to as the "neutral" line or 100% line. It passes through at the height of the center of gravity as shown. Any IC that falls on this line will exactly counteract the force trying to squat the car over the rear tires, so the result is "neutral" movement of the rear suspension.
All of this has nothing to do with springs or shocks. it's all done through the leverage of the suspension links. There's more to it than just slamming the tire into the road however, especially when the car needs to brake and turn as well.
So far we've looked at lowering the front of the car, and some effects of adding or changing sway bars. This time we'll explore some common terms (and misconceptions) about the rear of the car. Specifically typical solid axle suspensions. One thing that seems to be often misunderstood is the concept of instant center and anti-squat as it relates to a PT type car. Anti-squat is a very important advantage of a solid axle suspension (if designed correctly), but can also get us into serious trouble from a handling standpoint....
Anti-squat is really a simple concept. It is using the rearward rotation of the axle under acceleration to counteract the rearward weight transfer from that same acceleration.
Whenever we "punch it", we are pressed firmly back in the seat from inertia. The same thing happens to the car itself. The vehicles weight (mass) doesn't want to move, so it tries to lever back over the rear suspension; causing the body to drop down over the rear tires, or "squat". This is NOT a good thing. Anytime the body goes down over the tires, that weight is not being used to aid traction.
At the same time, the tires are trying to push the car forward. Again, the car's mass resists acceleration, so some of our engine torque is absorbed in the rear axle pinion climbing up the ring gear (rotating the housing backward)...Isaac Newton's "equal and opposite reaction" at work... The good news is, we can make use of this "lost" energy to help traction. Let's connect the rear axle housing to the chassis with a simple lever...just a steel bar welded to the axle housing and running forward to the frame. When we accelerate, the rearward rotation of the housing will press the bar up against the frame HARD; attempting to lift the car off the ground.
I cribbed this drawing off a Fiero site (!) of all places, but it's a very simple look at the effect I have been discussing.
51888
The rotation arrow on the tire is the direction the tire itself is turning. Keep in mind the axle housing is trying to rotate the opposite way ...note the force arrow UP where the bar meets the chassis "box". Sorry guys, but that's all a ladder bar or torque arm suspension is. Well, not ALL; but that's exactly how a torque arm manages the axle rotation torque. Now, the car has quite a bit of mass, so it's not likely the rear axle will be able to pick it up off the ground (and we wouldn't want it to!), but it's gonna TRY. As our lever is trying to lift the weight of the car, the fulcrum of the lever is getting heavier at the same time. In this case, the fulcrum is the rear axle housing, which... if I remember correctly... is attached directly to the rear tires. So the weight of the car is transferred directly to the tires, increasing traction. As the lever is lifting the car, it resists the natural tendency of the weight to fall to the rear..."anti" squat.
Where the lever attaches to the chassis detemines how much of that lift affects the rear or front of the car.
51896
As usual, it's not really that simple.
Of course, a simple lever may not always be the best way to attach the axle to the chassis, but we can get the same effect using a set of short links; a 3 or 4 link suspension.
51895
This is a version of a fairly common diagram, but it shows all the relevant info. The point at which the link planes converge is called the instant center, or IC, of the rear suspension. This is sort of like the attachment point of our simple lever, but the forces applied through this type of suspension are very different. We still have lift, but not in the same proportion as we had with the simple lever. You may also have noticed that the links allow the IC to move around in the chassis as the suspension travels up or down. In the lower half of this drawing, we see a line running from the bottom of the rear tire to a point above the front axle. This is referred to as the "neutral" line or 100% line. It passes through at the height of the center of gravity as shown. Any IC that falls on this line will exactly counteract the force trying to squat the car over the rear tires, so the result is "neutral" movement of the rear suspension.
All of this has nothing to do with springs or shocks. it's all done through the leverage of the suspension links. There's more to it than just slamming the tire into the road however, especially when the car needs to brake and turn as well.