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Thread: why should rear roll centre higher than the front?

  1. #1
    hello everyone,
    I'm from a young Chinese FSAE team. I have not enough experience to solve some problems. Anyone could help me?
    (I'm not good at English, hope you can understand me and point out my errors, thank you! )

    Question: some experienced designers give me advice about ensuring the rear suspension roll center higher than the front one, why is that?

    My guess: the higher roll centre, the faster lateral weight transfer, this would make a car sharp in tight corner.

    I always doubt myself about design. :-P
    -------------------------------
    If you like to train driving in LFS, contact me 297116008@qq.com , I would like to have big fun & improvement with you

  2. #2
    There is no hard fast rule that your rear roll center should be higher than the front, just as much as there is no rule of what color your car should be.

    The distribution of your lateral jacking force ratios is entirely up to you based on what YOU want the car to do.

  3. #3
    Originally posted by NR-Cruelty:
    My guess: the higher roll centre, the faster lateral weight transfer
    You are correct about the higher roll center causing a quicker load transfer. So, why would you want the rear to transfer load quicker than the front? Well, you're approaching a corner, and you turn the wheel, the front tires get a slip angle and produce a lateral force. The rear tires haven't yet, until they get some slip angle. Therefore, the rear is lagging behind the front. Having the rear roll center higher helps the rear to "catch up" with the front.
    Adam
    Any views or opinions expressed by me may in no way reflect those of Kettering University, it's students and administrators, or our sponsors.

  4. #4
    acedeuce802,
    Thank you very much.
    But why we need the rear catch up with the front earlier?would a car be easy to spin if the rear catch up with the front in lag?
    -------------------------------
    If you like to train driving in LFS, contact me 297116008@qq.com , I would like to have big fun & improvement with you

  5. #5
    Yes, if you over-do it, it will promote a oversteering attitude during turn in; it's up to you to decide if this is good or bad ant to what extend. I personally prefer a slightly oversteering vehicle during transients but with a slight understeering attitude on steady state cornering.

  6. #6
    Thank you.
    Is the faster lateral weight transfer means faster slip angle forming?
    What are the factors which relative to the rate of forming rear lateral force?
    -------------------------------
    If you like to train driving in LFS, contact me 297116008@qq.com , I would like to have big fun & improvement with you

  7. #7
    Originally posted by NR-Cruelty:
    Is the faster lateral weight transfer means faster slip angle forming?
    Yes, the slip angle will form faster. Keep in mind that you are still going to be seeing the same slip angles, thus generating the same lateral force. A quicker load transfer does produce a faster more agile car but at the expense of a harder car to drive. The driver must have a faster reaction time to keep up with the quicker load transfers. There is a balance to be found between how agile the car is, and how difficult to drive it is.

    Edit: I'm very new to this too, and I'm using knowledge just from past research, and OptimumG, so please correct me if I'm wrong!
    Adam
    Any views or opinions expressed by me may in no way reflect those of Kettering University, it's students and administrators, or our sponsors.

  8. #8
    Load transfer means you build rear slip angle faster? Don't think I believe that, particularly given that it should be compromising both cornering stiffness and ultimate traction.

    Who cares what a book or seminar says about X, Y, or Z. Show me your sim work. Prove it.

  9. #9
    There are two completely separate issues here.

    Steady state understeer/oversteer determined by roll stifness and relative lateral load transfer at each end.

    Transient response, or how fast load transfers at each end.

    The two are quite independent and are tuned to suit the particular vehicle's mass distribution.

    The most common solution usually ends up with the roll centre higher at the rear, and a higher roll stiffness at the front. But not always...
    Cheers, Tony

  10. #10
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    NR-C,

    Your English is good.

    The belief that "higher-rear-roll-centre = good" is, IMO, purely a historical artefact. The development of cars throughout most of the 1900s had most of them with rear beam-axles (high RC), or rear swing-axles (high RC), and independent front suspensions (low RC). This was simply for reasons of convenience of packaging, low cost, robustness, etc., and had little to do with good handling.

    Unfortunately, the commonness of this higher-rear-RC has lead many people to think that it must be important for good performance. It is not. Fact is, there are many cars with "lower-rear-RCs" that perform very well.

    For a beginning team I suggest you have both RCs about same height, and both about 0-5 cm above ground. Concentrate on a simple, robust design that can be built quickly, so that you can start testing early. Design in adjustability for toe angles (accuracy +/- 0.1deg), camber (+/- 1 deg), and different spring rates (+/- 10%). Don't bother with adjustable kinematics, or ARBs.

    Z

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