Scaling/corner weight questions.

[CMcAllister]

We are looking at a drag car and what I see has caused me to have some questions. It's a bracket race, back halved, Chevy door car. 3000#, low 10-high 9 second deal. Stock front suspension with coil overs, coil-over ladder bar rear.

The owner has said it will pick the front wheels up and carry them to the 60'. Nothing too violent. As the car leaves the starting line and carries the wheels out, it will make a gradual move to the left. Enough so that it needs to be corrected and will, on occasion, get out near the stripe or centerline. A bigger horsepower engine was put in this year that made the problem more pronounced.

Visual inspection shows spring seat heights about normal and even. Ladder bar adjusters about even. Car sets level. Nothing jumps out.

Scale numbers W/O driver are LR 800, RR 740 with the RF being about 10# heavier than the LF. The driver is pushing 275#. I would expect this car to go right instead of left. I jacked the front end up in the center of the X member to get the front tires off the ground. Rear weights go to LR 795, RR 830. I don't recall seeing rear weights move that much when doing this in the past.

So...does anyone use the jack under the center of the front end for double checking or diagnosing rear weights? Is it useful or am I out in left field? As far as the car, it is an older car, but the shocks/springs are a year or so old. What should I pursue first? Chassis twisted? Shock bound? I'd like some opinions, please. I'm almost afraid to put the weights where I would normally and send it to the track.

[John Wallace]

What's the front weights?

Tire rollout on the rear?

Axle square to chassis?

Sounds like something is off, though.

[CMcAllister]

Front weights are LF-740 RF-750.
Tires are ok.
Rear is in the car square doing a quick string line check.

The car has been doing this to some degree for a while. Shocks, springs and tires have been replaced since the issue was noticed. More cubic inches and power have made the problem more pronounced this year. Also, leaving with the transbrake makes it worse than footbraking.

[CMcAllister]

Driver also reports the car drives perfectly fine with all 4 tires on the ground, regardless the speed or how hard he jumps on the brakes..

[Olefud]

When you jack up the front do you use a socket or some pivot so the car can rotate around the jack point? You might try spring spacers, lowering or between the coils as the case may be, to jack the weight while the front is on the jack.

Also, the lateral center of gravity position should be determined with driver aboard. If it’s to the left, centerline thrust from the tires and an offset CoG reacting can develop a left tending torque couple.

[Warpspeed]

With any sprung live axle car, tail shaft torque is going to plant the left rear wheel and try to raise the right rear wheel.

That, and especially with both fronts clear off the ground, means you cannot really do much effective diagonal wheel weight jacking with the springs.

What is probably steering the car will be the effective large difference in rear tire loading on either side at launch, as well as the different tire slip rates that causes.
I would be experimenting first with setting slightly different tire pressures either side at the back, at least to begin with because it's easy to do....

Another better thing to try would be fitting the biggest stiffest rear antiroll bar you can find (to directly resist tailshaft torque at the axle housing), or some much stiffer rear springs.
Either will tend to even up the loading on the rear tires at launch.

[Olefud]

With any sprung live axle car, tail shaft torque is going to plant the left rear wheel and try to raise the right rear wheel.

That, and especially with both fronts clear off the ground, means you cannot really do much effective diagonal wheel weight jacking with the springs.

What is probably steering the car will be the effective large difference in rear tire loading on either side at launch, as well as the different tire slip rates that causes.
I would be experimenting first with setting slightly different tire pressures either side at the back, at least to begin with because it's easy to do....

Another better thing to try would be fitting the biggest stiffest rear antiroll bar you can find (to directly resist tailshaft torque at the axle housing), or some much stiffer rear springs.
Either will tend to even up the loading on the rear tires at launch.

Agreed that wedge is not going to do anything with the front wheels levitated. However, with the left rear more loaded the expectation would be for the car to drift right rather than left.

The antiroll bar won’t alter the weight transfer. Normally, it can change the relative amount of front/rear weight transfer, but with the fronts in the air all the weight transfer will be at the rear.

Without a torque tube, probably the only static way to balance the yaw problem is to adjust the relative left/right tire traction, but this usually means decreasing optimum traction on the strong side.

The center of gravity concern mentioned above involves the same dynamic forces that lift the front wheels. It can be a substantial player and, with a driver of some substance, is reasonably suspect.

[Warpspeed]

An infinitely stiff rear antiroll bar, would in theory constrain both rear wheels to rise and fall together, no matter how much tailshaft torque was being transmitted, even with relatively soft springing.

I agree it would not be as effective a solution to the uneven tire loading problem as a torque tube, or rigid mounting the whole rear axle assembly.
But it could be a simple practical step in the right direction, especially if it already has a rear bar.

[Olefud]

An infinitely stiff rear antiroll bar, would in theory constrain both rear wheels to rise and fall together, no matter how much tailshaft torque was being transmitted, even with relatively soft springing.

I agree it would not be as effective a solution to the uneven tire loading problem as a torque tube, or rigid mounting the whole rear axle assembly.
But it could be a simple practical step in the right direction, especially if it already has a rear bar.

Yeah, I see your point. It would connect the axle through the frame and motor mounts to the engine -a torque tube of sorts.

[ijames]

Several friends run the rear sway bar from http://hrpartsandstuff.com/ and love it. I bought one for myself but put in on a friend's 8 sec 87 Regal when I got tired of watching him drive 3/8 of a mile to get down the 1/4 . Then he complained about how boring the car was now because all it did was go straight . Disclaimer, I have no financial stake but Paul Ferry, owner of hrpartsandstuff, is a good friend of mine. You didn't say what body this is for. He has bars for G bodies and A bodies, and probably more that I don't know about. Drill two holes for the top end of the links and the rest bolts right on. Doesn't attach to the rear lower control arms so they don't get twisted up. Simple to set up, and just plain works.

[CMcAllister]

Thanks for the feedback. I do like anti-roll bars on 4 links now, after thinking they were a band-aid when they first began to be popular years ago. This is a basic ladder bar/coil over car. Everyone else made some valid points. I was mainly looking for opinions on why a car that is well over 100# heavy on the left rear would turn left at the hit.

I basically decided to treat the symptom. Making a bit of a move to the left usually requires some weight to moved to the left rear. We did this and also made some shock adjustments to try to keep the left rear applied a bit more as the car moves the first 60' or so. Nothing too radical. I am waiting to see how it did this weekend. Beyond that, something has to be bound. I suspect a tight shock, probably a LR. I will get the car back and pursue this a little more. He just didn't leave it with us long enough for anything but a quick look last week.

[MadBill]

I'm no drag racer but as I see it, once both front wheels are in the air there can be no torque reaction affecting lateral loading other than the very momentary one due to drivetrain inertia. Assuming the use of a spool or significant breakaway torque positrac/LSD and with fairly even tire loading, the biggest variable affecting the direction of travel is the relative rolling radii of the tires. With identical pressures, the heavier loaded left tire will 'roll smaller' creating effective tire stagger that will steer the car left like a coffee cup rolling across the ground. Try raising the left/lowering the right a half psi at a time and see if that doesn't straighten it out...

[Warpspeed]

There will be BIll, because the rear axle is decoupled by the springs from the main mass of the car.
The softer the rear springs, the more tailshaft torque plants the left wheel, and removes weight off the right wheel.
This would be a transient effect at launch as the clutch/converter hits hard.

On the other hand, something like a slingshot dragster that has the rear axle housing mounted totally rigid to the frame, that effect cannot happen, and rear tire loadings will be equal no matter how horrendous or sudden the transmitted torque.

I agree with you that unequal rolling radius is probably a big factor here, its probably the most likely explanation for the pulling effect. Best fix would still be something that equalises rear tire loading as much as possible at launch.

[MadBill]

Once the front wheels are off the ground, there is no fulcrum against which the driveline torque can act to affect rear tire loading. The right side spring is compressed by the torque and vice-versa, but the vertical loading will be determined solely by the vehicle weight and lateral CG location.
Visualize the car supported by a knife edge at the center of the front crossmember. Let's say the rear tire loads are 900# each. Now shorten the right spring by six inches. The car 'falls down' six inches on the right, but the loads remain essentially* the same. (*The CG will shift very slightly to the right)

[ijames]

In a static sense I think you are right, Bill, but during the time the chassis is twisting, the rear is planting, and the front wheels are starting to rise but still contacting the ground, the car can definitely make a turn. Even if it doesn't keep turning more once the front wheels are up, it is already headed off to one side or the other. It's a transient thing, ya know?