Scaling/corner weight questions.

[MadBill]

Quite right; at the hit there's way more going on, but the OP says: "As the car leaves the starting line and carries the wheels out, it will make a gradual move to the left", suggesting a longer-term issue.

[Olefud]

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?

It’s a dynamic torque reaction –I think. If just the engine and drivetrain were suspended, the engine would want to rotate one direction and the rear axle the other. This is similar to an airplane that, at slow speed, high power, tends to yaw such that opposite control is required. This is independent of the front wheels. However, once the rear springs are wound up, there would be a structural, torque-resisting link between the axle and the engine through the chassis.

It might be interesting to see what the “bump” steer is at the rear axle when in the wound up condition.

[ijames]

Quite right; at the hit there's way more going on, but the OP says: "As the car leaves the starting line and carries the wheels out, it will make a gradual move to the left", suggesting a longer-term issue.

I'm just guessing, of course, but since the op said "gradual" I thought it possible that at the hit the car could pivot slightly to the left as the wheels came up so it was no longer pointing straight down the track, and then as it moves forward it would gradually go more and more left even though it was now travelling in a straight line. If its path is a continuously curving arc then there is more going on, like different rear tire radii under load or whatever.

[MadBill]

The OP hasn't chimed in since Sunday but hopefully he soon will and will clarify matters...

[Warpspeed]

Yes, we are talking very sudden violent shock loading here.
The whole mass of the car will tilt sideways to eventually equalise the rear spring loads exactly as you suggest, but that will take a short but finite time.

[John Wallace]

Drag Racing Leaf Spring Suspensions

[Olefud]

Yet another diagnostic tool is tire temperature profile. Best if just launch and shutdown – but useful after the run if the probe is a bit under the surface.

[CMcAllister]

Well...the short story is we made some small changes to preloads (corner weights) and rear shocks (allow one side to rebound a little quicker than the other). After the owner picked the car up, he took it home and put new tires on it. He went to 2 different tracks last weekend and the car was fine. I asked and he said he had never rotated the old tires. I don't believe a small circumference difference would cause this pull; the car was straight as a string down track with the old tires.

As I noted in my OP, the rear corner weights change quite a bit when the car is lifted in the center at the front. This takes the influence of the front springs out of the picture and the rear weights at that point are not the same, side to side. The springs and preload are still influencing the rear corners.

I believe that adjustments made in the shop; IC, spring selection, shock settings, corner weights, preload, etc., are just to establish a baseline, based on the combination, and verify that the chassis and components are not bent or failed. You have an expectation of how the car will behave. But once you go to the track and hit the pinion gear with whatever you have to hit it with, things change and there's no way to see how much load is being applied to each contact patch as the suspension reacts and the car moves from the starting line. Adjustments from that point to correct problems and/or improve performance are based on observation, video, data acquisition, time slips, seat of the pants, experience and opinion.

I try to read as much about this subject as I can find, both reference material and conversations between knowledgeable people. Opinions among even experienced people about what's happening, how to define it, how to label it and what's important to pay attention to, vary quite a bit to say the least. Not right or wrong, just different. I have formed some of my own opinions, based on results and what seems to be common sense to me. Adhering to the old expression "If the results don't match the theory, change the theory." has served me well. I'm still fascinated by the physics of what is happening in a race car, once you apply torque to the pinion gear.

Thanks for the feedback.