3rd link offset??

[MadBill]

I don't think locking the chassis down on stands will give you an accurate result. In effect, you have eliminated the rear roll center and any possibility of torque roll, which is what you are trying to offset in the first place. Interesting concept... I wonder if there is a way to lock the drive wheels tightly enough and SAFELY "brake stand" the car on the scales?

Hmmm... Well, first the roll center is defined by the Watts link location, which would still be in place (horizontal on the bottom of the diff) and second, I'd be twisting the DS exactly the way the engine does; it's just that the torque reaction would show up relative to the sprung weight of the axle assembly instead of the corner weights.

Good thought re brake-torquing the car! however Norm's right, either way the brakes would cancel out the twisting force.

Plan 'B': Fit A frames with bases of say 3 ft. length in place of the wheels and set the car, springs intact, on two scales per side, one each at the front and rear of each frame base. This way, axle torque would be resisted by vehicle weight and would show up as differential loading on the scales.

Theoretically though, I believe the results should be linear from 0 ft-lb. on up, so the 'Armstrong' method of inputting torque should suffice. It couldn't hurt to use a T bar, but as the torque reaction at the pinion is pretty well centered in the track, the resultant load on the scales should be equal.

[exwestracer]

Bill,
My point in the earlier post was that the roll torque you are compensating for is coming from the engine block trying to twist in the chassis. Unless you model it with that force applied, you will get A/S; but you won't have the differential spring loading you get on launch.

[Norm Peterson]

. . . I believe the results should be linear from 0 ft-lb. on up

I agree about the linearity of this.

I doubt that Billy Shope's application or any other similar derivation would work if it wasn't linear, and if you're pushing so much torque at it that things are starting to deform under that load far enough to invalidate the assumptions that you're making, you should probably be mounting the axle solidly to the chassis anyway.

It couldn't hurt to use a T bar, but as the torque reaction at the pinion is pretty well centered in the track, the resultant load on the scales should be equal.

The torque part of it will divide properly, or it will at least divide up the same as it does when the torque comes down the driveshaft.

It's the single pulling force that changes things.

If you're pulling downward, it'll add a downward force at the pinion that isn't there when you're driving, and it will also be divided between the rear tires. Depending on the length of your lever, it would be possible that the scale reading on the RR would remain unchanged while the LR would show about double the change that torque alone would cause.

If you're pulling horizontally, you'll be adding a horizontal force at the pinion, which will resolve exactly like the lateral load transfer from unsprung mass does when you're investigating that. If you're pulling horizontally from below the axle, this effect will work against what you're trying to see. Above the axle and it will add.

Think [YourPullingForce] x [PinionHeightAboveGrade] / [RearTrack].




Any other direction would involve some of each of the above.


Norm

[MadBill]

Bill,
My point in the earlier post was that the roll torque you are compensating for is coming from the engine block trying to twist in the chassis. Unless you model it with that force applied, you will get A/S; but you won't have the differential spring loading you get on launch.

Good point. The torque reaction in the chassis would be reflected in a change in front wheel loadings, so scales would be required there to determine the exact percentage of cancellation. However, if all that was required was to observe the relative effect, the change in rear wheel loads alone would show this. (On further thought, wouldn't the resultant asymmetric loading just be the mirror image of the rear's?)

[Norm Peterson]

Now we're getting chassis rigidity and the front vs rear distribution of roll stiffness involved. A perfectly rigid chassis with no front roll stiffness - or a wheelstanding car where all of the engine torque reaction has no choice but to go rearward - will oppose the right to left rear tire load shift that comes from driveshaft torque with exactly the same amount of chassis torque in the opposite direction. In such cases, I don't think that offsetting a 3rd link or TA could gain anything at all.

Ladder bar suspensions, the practice of removing or de-activating front sta-bars, and the large by huge drag race rear "antiroll bars" are efforts intended to approximate the above.


Norm

[MadBill]

I agree about the linearity of this.

I doubt that Billy Shope's application or any other similar derivation would work if it wasn't linear, and if you're pushing so much torque at it that things are starting to deform under that load far enough to invalidate the assumptions that you're making, you should probably be mounting the axle solidly to the chassis anyway.



The torque part of it will divide properly, or it will at least divide up the same as it does when the torque comes down the driveshaft.

It's the single pulling force that changes things.

If you're pulling downward, it'll add a downward force at the pinion that isn't there when you're driving, and it will also be divided between the rear tires. Depending on the length of your lever, it would be possible that the scale reading on the RR would remain unchanged while the LR would show about double the change that torque alone would cause.

If you're pulling horizontally, you'll be adding a horizontal force at the pinion, which will resolve exactly like the lateral load transfer from unsprung mass does when you're investigating that. If you're pulling horizontally from below the axle, this effect will work against what you're trying to see. Above the axle and it will add.

Think [YourPullingForce] x [PinionHeightAboveGrade] / [RearTrack].


Any other direction would involve some of each of the above.


Norm

Since it is the relative change in L/R loading we're looking for, the pinion downward load from say applying 100# downward force on a 24" bar would also be divided evenly or almost so between the contact patches. To play it safe though, you could use two torque wrenches pulled in opposite directions to cancel each other. Plan 'C': Leave the driveshaft in place, slip a 2 x 4 across the frame rails under it near the front end for support and apply the torque to the front yoke. The 'Torquemaster' stands in the car for the test to ensure no further quibbles re accurately reproducing actual conditions.

[exwestracer]

Now we're getting chassis rigidity and the front vs rear distribution of roll stiffness involved. A perfectly rigid chassis with no front roll stiffness - or a wheelstanding car where all of the engine torque reaction has no choice but to go rearward - will oppose the right to left rear tire load shift that comes from driveshaft torque with exactly the same amount of chassis torque in the opposite direction. In such cases, I don't think that offsetting a 3rd link or TA could gain anything at all.

Ladder bar suspensions, the practice of removing or de-activating front sta-bars, and the large by huge drag race rear "antiroll bars" are efforts intended to approximate the above.


Norm

If we stand at the front of the car and watch our typical Chevy transmit torque to the axle, the driveshaft is trying to extend the LR and compress the RR, while the BLOCK is trying to extend the LF and compress the RF. I believe those forces are compounding, not offsetting?

[MadBill]

I think they are a matched set, per Newton's Third Law": "For every action there is an equal and opposite reaction." if we crank up the LR spring say 3 turns, the RF load increases also and the LF and RR decrease.

Slightly off topic, but how much and where does the reaction occur, if for example, the engine is rubber mounted and the transmission is in first gear with say a 3:1 ratio? How about if engine and tranny are hard-mounted? What if we had a 1:1 spur gear set on the tranny output shaft, reversing the driveshaft rotation?

[exwestracer]

The reaction occurs wherever there is a limit to freedom of motion. The rubber will absorb some of the torque as we know, but that is still where the reaction load is fed into the chassis. The reverse rotation thing was tried for awhile in circle track racing (spinning the ENGINE the opposite way, mostly open wheel cars), with some success. In those light cars, the engine (block) torque reaction did quite a bit to counteract chassis roll in the corners. I believe reverse rotation been banned just about everywhere though...

[Norm Peterson]

If we stand at the front of the car and watch our typical Chevy transmit torque to the axle, the driveshaft is trying to extend the LR and compress the RR,

In terms of forces, that's adding load to the LR by stealing it from the RR.

while the BLOCK is trying to extend the LF and compress the RF.

Absolutely.

But since the actual input is really a powertrain torque reaction being applied to the chassis (either as couple forces at the engine mounts or as a pure torque on a motor plate), it will divide front to rear in proportion to the relative front and rear suspension roll stiffnesses.

Strictly speaking, the chassis torsional stiffnesses between the engine mounts and the front suspension, vs that between the engine mounts and the rear suspension, should also be included, though their effect would be minor unless somewhere along the chassis you have a section that's really soft in torsion.

I believe those forces are compounding, not offsetting?

Continuing with the line of thought regarding this torque as a chassis torsion to be distributed according to where the stiffnesses are, not all of the powertrain torque reaction is taken by the front suspension, not even if the engine mounts were to be located ahead of the front suspension. Driving the RF down relative to the LF eventually becomes driving the RR of the chassis and therefore the RR suspension down relative to the LR and re-planting the RR tire to some extent. This also means that some of the "planting" of the LR from driveshaft torque is being "undone" by the upward movement at the LF dragging the LR of the chassis up with it a bit as well. Overall, this is a good thing since two equally loaded tires will have more total grip (and in this situation will naturally want to drive straight instead of having some yaw moment being developed from unequal forward bite).


Norm

[exwestracer]

I believe you are correct about the forces, Norm. I was just looking at it in terms of both reactions promoting chassis roll, rather than one resisting the other.

I still think measuring rear travels on a "real" launch would be the most accurate way to predict how far the top link would need to be offset. Now that I think about it some more, I remember an article in Circle Track (maybe) years ago about a stock car that only used a ladder bar offset to the right, and a single link at axle centerline on the left. They were trying it to get some roll steer effect going, but the idea is basically the same.

[Norm Peterson]

The reaction occurs wherever there is a limit to freedom of motion. The rubber will absorb some of the torque as we know, but that is still where the reaction load is fed into the chassis. The reverse rotation thing was tried for awhile in circle track racing (spinning the ENGINE the opposite way, mostly open wheel cars), with some success. In those light cars, the engine (block) torque reaction did quite a bit to counteract chassis roll in the corners. I believe reverse rotation been banned just about everywhere though...

Got to thinking about this a bit. It appears that in this situation the intent is to equalize front tire loading rather than rear to mitigate a push on corner exit when the front is also going light. Rear tire loading would be made somewhat more unequal and would also be generating a yaw moment that would oppose the push without having to put the axle itself into loose steer. Sound even close?


Norm

[exwestracer]

Got to thinking about this a bit. It appears that in this situation the intent is to equalize front tire loading rather than rear to mitigate a push on corner exit when the front is also going light. Rear tire loading would be made somewhat more unequal and would also be generating a yaw moment that would oppose the push without having to put the axle itself into loose steer. Sound even close?


Norm

That's it in a nutshell, Norm. If the driver didn't have to rely on "hitting the stagger" to get the car off the corner, he was more in control of his own fate... I believe Mr. Yunick got the ball rolling on reverse rotation LONG before anyone else even heard about it.