Have a question about weight transfer

[OustedFairlady]

A friend and I have been engaged in a little friendly debate about suspension squat.

 

The long and short of it is that I believe that the majority of suspension squat is caused by weight transfer, while he believes that the majority of it is caused by the engine's torque pulling the rear end into the car.

 

Which of us is correct/more correct?

[johnc]

A friend and I have been engaged in a little friendly debate about suspension squat.

 

The long and short of it is that I believe that the majority of suspension squat is caused by weight transfer, while he believes that the majority of it is caused by the engine's torque pulling the rear end into the car.

 

Which of us is correct/more correct?

 

You are 100% correct. Your friend is 100% wrong.

[JMortensen]

Not so fast there John. Try putting an automatic 280ZX in drive, put your left foot on the brake and step on the gas. Then put it in reverse and do the same (foot still on brake). The ass end will change altitude by about a foot, and none of it is weight transfer. The simple answer is that it depends on the geometry of the suspension.

[johnc]

My answer was in response to the OP's question which included this statement:

 

while he believes that the majority of it is caused by the engine's torque pulling the rear end into the car.

 

I stand by my original reply.

[kamikaZeS30]

Isn't the weight transfer a result of torque, in a sense, though? Inertia.

[johnc]

Weight transfer is the change in load as a result of an acceleration. Torque is a moment rotating an object around its axis. Two different things.

[AkumaNoZeta]

This conversation started with a discussion of live axles too. That may, but more than likely won't, change the context of the conversation. I guess OustedFairlady was right when he said I was referring to wheel hop, but I still hold on to the idea that fast drag cars don't squat and may even lift up a little in the rear.

[RTz]

... but I still hold on to the idea that fast drag cars don't squat and may even lift up a little in the rear.

 

 

That is largely the result of torque reaction. Live axle+geometry. Something an IRS car isn't able to make efficient use of.

[AkumaNoZeta]

Ok, how about this scenario? Say someone takes a Foxbody Mustang and lowers it dramatically without changing the the chassis mount locations of the control arms, then it would develop pro-squat. Right?

[OustedFairlady]

I have to agree with JMortensen, as far as the auto car goes. The rear tires try to spin, and failing that, they transfer their torque to the surrounding components and induce a slight squat in the car. However, my argument was that in the bigger scheme of things, squat wasn't a detriment, but rather a boon to those in search of traction (EDIT: Let me say that I'm speaking in regards to drag racing, specifically). The type of squat I was referring to was induced by weight transfer, where the weight of the car goes to the rear, presses the rear of the car into the ground, and allows for traction by loading the rear tires/suspension.

 

I also asserted that any perceived "torque squat" was controllable by spring rates as well as damping rates, and was, therefore, irrelevant in a performance car.

 

Am I right or wrong?

Edited March 17, 2011 by OustedFairlady

[JMortensen]

Your description that torque is transferred to the components inducing a slight squat is wrong, as is the idea that the car leaning back is a cause of the weight transfer. A gokart transfers weight even though it has no suspension at all. The weight transfer is a function of the center of gravity, the wheelbase, and the amount of acceleration g force. The car leaning back is the result of weight transfer, not the cause of it. What the suspension does depends on the geometry. I used the 280ZX as an example because it has semi-trailing arm rear suspension so it reacts to torque pretty dramatically. Other kinds of suspension (like 240/260/280Z for example) do not. On live axle cars it depends on the control arm geometry as Ron said.

 

As far as traction goes, in theory you don't want anti-squat. Anti-anything (squat, roll, dive) works by trying to freeze up the suspension so that it won't move. This makes the suspension effectively act stiffer and stickier and makes it react to bumps in the pavement, etc more harshly, so that hurts traction. Pro squat probably isn't that great either, because the more power you have, the more likely you are to make the suspension bottom and the weight transfers regardless of the position of the suspension. Using the torque from the car to compress the suspension isn't a good idea because when it bottoms it can't move, and basically you get the same thing as you had with the anti-squat, but worse. In an ideal world, I think you want the springs to do the springing and as little anti or pro geometry as you can get so that the suspension can move freely to keep the tires on the ground. I think in practical terms a lot of really high hp cars have a little bit of anti-squat built in because they transfer so much weight when the launch. I'm not a drag racer though, so that is all road racing based info. How true it is for drag racers I don't know.

[johnc]

BTW... squat is not a bad thing in drag racing. Depending on the car it can soften the initial launch loads on the rear tires. My 1966 GTO would get quicker 60' times by letting the back end squat about 3". Any less and the tires would break loose and any more would just waste time. As a driver I had to know the delay in actual forward movement that squat imposed and stage the car accordingly.

[OustedFairlady]

Your description that torque is transferred to the components inducing a slight squat is wrong, as is the idea that the car leaning back is a cause of the weight transfer. A gokart transfers weight even though it has no suspension at all. The weight transfer is a function of the center of gravity, the wheelbase, and the amount of acceleration g force. The car leaning back is the result of weight transfer, not the cause of it. What the suspension does depends on the geometry. I used the 280ZX as an example because it has semi-trailing arm rear suspension so it reacts to torque pretty dramatically. Other kinds of suspension (like 240/260/280Z for example) do not. On live axle cars it depends on the control arm geometry as Ron said.

 

As far as traction goes, in theory you don't want anti-squat. Anti-anything (squat, roll, dive) works by trying to freeze up the suspension so that it won't move. This makes the suspension effectively act stiffer and stickier and makes it react to bumps in the pavement, etc more harshly, so that hurts traction. Pro squat probably isn't that great either, because the more power you have, the more likely you are to make the suspension bottom and the weight transfers regardless of the position of the suspension. Using the torque from the car to compress the suspension isn't a good idea because when it bottoms it can't move, and basically you get the same thing as you had with the anti-squat, but worse. In an ideal world, I think you want the springs to do the springing and as little anti or pro geometry as you can get so that the suspension can move freely to keep the tires on the ground. I think in practical terms a lot of really high hp cars have a little bit of anti-squat built in because they transfer so much weight when the launch. I'm not a drag racer though, so that is all road racing based info. How true it is for drag racers I don't know.

 

Perhaps I misrepresented myself...I do know that the weight transfer is what causes squat; when the car launches, the center of gravity is pulled rearward, increasing load on the rear suspension, thereby causing squat. Not that the squat is somehow actually causing the transfer of weight. Inertia, i.e., "an object that is in motion tends to stay in motion, and vice versa" is the underlying cause of weight transfer and thus squat. My original statement:

 

The type of squat I was referring to was induced by weight transfer, where the weight of the car goes to the rear, presses the rear of the car into the ground, and allows for traction by loading the rear tires/suspension.

 

Our debate was regarding what causes squat, and although I asserted that sure, torque can cause minor squat (such as your 280ZX example), the vast majority is attributed to weight transfer, especially in drag racing. Of course, I admit I may be wrong, but I'm not perfect and I certainly don't know everything!

 

In any case, I think the point is well taken. Squat is generally caused by the transfer of weight to the rear of a car during launches or acceleration.

[260DET]

Not so fast there John. Try putting an automatic 280ZX in drive, put your left foot on the brake and step on the gas. Then put it in reverse and do the same (foot still on brake). The ass end will change altitude by about a foot, and none of it is weight transfer. The simple answer is that it depends on the geometry of the suspension.

 

It's not weight transfer in that example Jon, its torque reaction. Which does not mean John is wrong, two different things, both of which can influence squat.

Edited March 21, 2011 by johnc

[johnc]

Torque reaction does have an effect on squat, never said it didn't. My "100% right/wrong" answer above was in response to the OP's original question.

 

 

EDIT: Ya know, I don't think Jon's example above is a good one. What you're seeing in the automatic trans brake torque example is the suspension reacting to the applied brakes through the rear suspension. If the brakes are not applied, the vast majority of the reaction will be due to weight transfer, although there might be some suspension reaction to the inerita/grip of the rear wheels.

[JMortensen]

EDIT: Ya know, I don't think Jon's example above is a good one. What you're seeing in the automatic trans brake torque example is the suspension reacting to the applied brakes through the rear suspension. If the brakes are not applied, the vast majority of the reaction will be due to weight transfer, although there might be some suspension reaction to the inerita/grip of the rear wheels.

Mountain bikers have this stuff nailed.

 

It's not the brakes that do the work in my example, it's the engine torque. If the brakes are not applied and you put a lot of torque through the driveline from a stop, it will compress the suspension above and beyond what a different suspension system that doesn't have semi-trailing arms would do. If you "launched" in reverse, the rear end would lift more than a typical front drive car would do going forwards. The "brake" when you're driving regularly is the inertia of the car. The faster you're going, the less noticeable the issue is going to be, but the geometry of the suspension will jack it around regardless of whether the brakes are on or off. In mountain biking circles this would be akin to pedal bob, only there isn't really a bob on a car, so it is just referred to as squat. If the impulses through the crank were slow enough, it would bob, and you could see that separately from the squat caused by weight transfer.

 

It is true that the brakes alone have an effect on the rear suspension when the car is moving, this effect is commonly described as "brake jack" in mountain biking circles. It also reduces traction by making the suspension not want to move, and also in the case of the 944 actually compresses the rear suspension under braking, so it reduces nose dive to some degree as well. When you step on the brakes hard, the whole car just lowers down to the ground. It would in theory be possible to do a "floating brake caliper" to fix if someone were really interested in it, for most situations like street driving or even road racing I don't think that's necessary, but I'd speculate that it would probably make a discernable difference in a rally car.

 

 

http://2btherapy.com/index.php/bikes/brake-therapy-floating-brake-kit

 

 

Edited March 18, 2011 by JMortensen

[kamikaZeS30]

Weight transfer is the change in load as a result of an acceleration. Torque is a moment rotating an object around its axis. Two different things.

 

Yes, I know WHAT torque, weight-transfer and inertia are. But, I'd like to see someone drag-launch a car without a power-train.

 

But, torque works against inertia during acceleration, or am I wrong? I.e. the reason the car moves (accelerates) is due to the rotation of its drive-train: crankshaft, flywheel, transmission, drive-shaft, differential, half-shafts, stub axles, hub, and finally, the wheels and tires. It also seems that a car, regardless of rear-suspension geometry, will react to (in varying degrees) application of work (in this case torque).

 

In simple terms what I am saying is that torque when it becomes acceleration (forward moment) causes the weight-transfer. I guess I don't understand why there is the prevailing sentiment that these are all isolated occurrences?

Edited March 18, 2011 by kamikaZeS30

[johnc]

But, I'd like to see someone drag-launch a car without a power-train.

 

[johnc]

I guess I don't understand why there is the prevailing sentiment that these are all isolated occurrences?

 

Because its easier for people to understand and figure out what's going on. Isolate parts of the system, analyze them in detail, and then you have a better understanding of the system as a whole. In this thread we're trying to understand the effects of weight transfer and torque from the powertrain on the suspension. Without separating the two how do you propose we measure the effects of each on the whole?

[JMortensen]

Yes, I know WHAT torque, weight-transfer and inertia are. But, I'd like to see someone drag-launch a car without a power-train.

 

But, torque works against inertia during acceleration, or am I wrong? I.e. the reason the car moves (accelerates) is due to the rotation of its drive-train: crankshaft, flywheel, transmission, drive-shaft, differential, half-shafts, stub axles, hub, and finally, the wheels and tires. It also seems that a car, regardless of rear-suspension geometry, will react to (in varying degrees) application of work (in this case torque).

 

In simple terms what I am saying is that torque when it becomes acceleration (forward moment) causes the weight-transfer. I guess I don't understand why there is the prevailing sentiment that these are all isolated occurrences?

Nobody is saying that the occurrences are isolated, but their effects through the suspension can be mitigated. Anti-squat is the tendency for the suspension to EXTEND under power (not weight transfer, you still might have squat because the amount of weight transfer is greater than the amount of anti-squat). Pro squat is the tendency for the suspension to COMPRESS under power (not weight transfer). If you accept that anti-squat and pro-squat exist, then it should be pretty easy to figure that there is a middle point where you have neither pro nor anti-squat, and that's what a Z has. It will still squat, but it squats due to weight transfer. The 280ZX has pro-squat, the Z32 if I recall has anti-squat.