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Thread: Locked rear axle, one rear spring softer than the other.

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    Senior Member robburgoon's Avatar
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    Default Locked rear axle, one rear spring softer than the other.

    Give it some gas, which way does your car pull? Towards the heavier spring or away from it?

    Give it lots of gas, is the answer the same?

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    Chest hair required Olitho's Avatar
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    The answers are "Neither and yes." It is a trick question. Miatas have never pulled anything let alone a direction. :-)
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    "Shoe" Stuntman's Avatar
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    Dirt track car?

    Have to look at the entire car as a whole. Tire diameters, suspension geometry, what happens to the alignment as your car squats to one side more than the rear due to the spring stagger, etc...

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    Senior Member Johnny_Se7en's Avatar
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    Have you been turning only to the left?
    If nothing happens and there is no one around why did it not happen?

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    Senior Member robburgoon's Avatar
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    Quote Originally Posted by Stuntman View Post
    Dirt track car?

    Have to look at the entire car as a whole. Tire diameters, suspension geometry, what happens to the alignment as your car squats to one side more than the rear due to the spring stagger, etc...
    Sure, but let's make it easier on ourselves, let's pretend the alignment stays stable, tire diameters are identical, and so on.

    You can answer the question for dirt or asphalt.

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    Senior Member robburgoon's Avatar
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    Since the rear axle is locked, either both tires spin or both tires turn together.

    I'd expect the corner with the heavier spring to carry more of the weight. True?

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    Ok, I'll bite. I think at light throttle, below the traction limit of lightly loaded tire, it won't matter. Past this point the car will try to rotate away from the heavily loaded tire.

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    "Shoe" Stuntman's Avatar
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    Quote Originally Posted by robburgoon View Post
    Since the rear axle is locked, either both tires spin or both tires turn together.

    I'd expect the corner with the heavier spring to carry more of the weight. True?
    True if going straight. But I think your question has to do with cornering, no?

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    Senior Member robburgoon's Avatar
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    Quote Originally Posted by Stuntman View Post
    True if going straight. But I think your question has to do with cornering, no?
    No. Just going straight.

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    likes to left foot brake. passmenow's Avatar
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    fwiw In drag racing with a spool if the car did not launch straight.
    We added more rear spring pre load to the side we wanted the car to turn towards.

    When I broke a left stub axle in the middle of T8 at wsir the car turned left immediately.
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    Senior Member robburgoon's Avatar
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    Quote Originally Posted by passmenow View Post
    fwiw In drag racing with a spool if the car did not launch straight.
    We added more rear spring pre load to the side we wanted the car to turn towards.

    When I broke a left stub axle in the middle of T8 at wsir the car turned left immediately.
    Preload is a little weirder than a higher spring rate.

    The left stub axle turning left makes sense. Your left rear completely stopped generating forward thrust. However that's a little different since we are assuming a locked axle.

    Edit: day 3 of flu fever. F*ck.

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    Senior Member robburgoon's Avatar
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    Nobody?

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    Senior Member superpilun's Avatar
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    It pulls in the direction that the steering wheel is pointed. That was easy, next question!

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    Senior Member robburgoon's Avatar
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    Got a clue from Puhn's book.

    He gave the example of a stick axle drag car with open rear end. The right rear will spin first in stock form due the torque on the rear axle from the driveshaft lifting the RR and pressing down the LR.

    To fix it, he suggested decreasing cross weight to get more static weight on the RR. That way when you step on the gas it goes equal.

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    Senior Member robburgoon's Avatar
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    It's interesting that when cornering the tire with the most weight on it gives up first and when accelerating the tire with the most weight on it gives up last.

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    Sir flink
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    I'm having a hard time with that. The driveshaft torque is balanced by an equal and opposite torque on the engine/trans mounts, to no net effect.

    Sit in the center of a canoe and press down on one side while lifting the other. Does it tip over? Nope, because the center of mass didn't move. The torque you are applying with your arms is balanced by the opposite torque you are applying with your butt cheeks.

    You *can* change the wheel loading a bit by blipping the engine in neutral (the car rocks). But that's conservation of angular momentum and the effect will be very small when the car is in gear.

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    Senior Member superpilun's Avatar
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    well...maybe with a large enough engine and a light enough chassis, and a high enough rpm, that effect is enough to tip the weight towards one side and for the tire on the other side to spin?

  18. #18
    Senior Member robburgoon's Avatar
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    An old muscle car probably flexes. What happens up front may not match what happens at the back. That might be the reason for the ppf in miatas and torque tube in vettes.

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    Sir flink
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    Sure, the chassis will twist a bit, but that won't significantly move the center of mass. I guess a twisted chassis is equivalent to tweaking the corner weights a bit, so you could calculate the change in wheel force by multiplying the twists's displacement (a couple of mm?) by the overall wheel rates.

    Where the overall wheel rate is, I guess, 1(1/(suspension wheel rate) + 1/(tire "spring rate")). Tire "spring rates" are typically in the 1000-2000 lb/in range, iirc, so probably not significant on that car.

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    Senior Member robburgoon's Avatar
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    Puhn was suggesting adding 50 lbs to RR trunk for a typical sedan at the drag strip, whatever that is.

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