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Thread: RMIT front suspension design - wishbone angle - rollcentre location

  1. #1
    I searched a while for any info regarding RMITs front suspension design and did not find what I was looking for.

    I was wondering if anyone from RMIT and also other teams have some opinions on RMITs angle of the front wishbones. Is there any old topic on this?

    I am interested in discussing:

    - why such an angle on the top wishbone? to place the rollcentre almost under the ground plane, to give high camber gain?

    - if the location of the inner wishbone mounts makes sense in structural terms for loadpath in the carbon chassis

    - if the location of the rollcentre very low was designed so that a lot of the roll stiffness is given by the springs and dampers rather than the suspension links.

    I think I have answered all the questions myself but I d like to hear some opinions since most cars seem to run less steep upper wishbones. I know a certain car that ran RCs below ground level on the rear with some quite bad results (admittedly linked to use of Fox shocks with no time to refine damping and motion ratio on the rockers)
    ...we got emergency on planet earth

  2. #2
    I searched a while for any info regarding RMITs front suspension design and did not find what I was looking for.

    I was wondering if anyone from RMIT and also other teams have some opinions on RMITs angle of the front wishbones. Is there any old topic on this?

    I am interested in discussing:

    - why such an angle on the top wishbone? to place the rollcentre almost under the ground plane, to give high camber gain?

    - if the location of the inner wishbone mounts makes sense in structural terms for loadpath in the carbon chassis

    - if the location of the rollcentre very low was designed so that a lot of the roll stiffness is given by the springs and dampers rather than the suspension links.

    I think I have answered all the questions myself but I d like to hear some opinions since most cars seem to run less steep upper wishbones. I know a certain car that ran RCs below ground level on the rear with some quite bad results (admittedly linked to use of Fox shocks with no time to refine damping and motion ratio on the rockers)
    ...we got emergency on planet earth

  3. #3
    I'd guess it would be to obtain a very low front view virtual swing arm length. This would mean very little camber change in roll, to the detriment of camber change in bump.

    Given the nature of the courses, biasing your suspension towards roll as opposed to pitch would make sense to me.

    Matt Gignac
    McGill Racing Team

  4. #4
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I am interested in discussing:

    - why such an angle on the top wishbone? to place the rollcentre almost under the ground plane, to give high camber gain?

    - if the location of the inner wishbone mounts makes sense in structural terms for loadpath in the carbon chassis

    - if the location of the rollcentre very low was designed so that a lot of the roll stiffness is given by the springs and dampers rather than the suspension links. </div></BLOCKQUOTE>

    The kinematic roll centre isn't necessarily below the ground because of the high angle on the top wishbone. The ICs are actually well above the ground.
    Initial reasons for going to this design included the following:
    a) Packaging
    b) Load Paths feeding into the carbon monocoque
    c) Kinematic roll centre constraint (relative to the chassis....)
    d) Tyre orientation

    (d) is the interesting one here. The question is will your tyres be able to provide longitudinal grip when camber is not optimal? Also, given the original design philosophy from the honorable G. Pearson, we placed more emphasis on lateral acceleration than longitudinal acceleration and tried our best to make gains there.

    As long as your compromises can be justified, I sure there's any number of paths you can take successfully.

    Pat Drum
    RMIT Racing Fan

  5. #5
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Matt Gignac:
    This would mean very little camber change in roll, to the detriment of camber change in bump.
    </div></BLOCKQUOTE>

    It might even gain negative camber in roll on the outside tire.

    The roll center could actually be quite high but given the angle of the lower arms it was probably bought down in design.
    John "Jack" Vinella
    University of Washington Alumni 06' 07' 08' 09'

  6. #6
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> It might even gain negative camber in roll on the outside tire. </div></BLOCKQUOTE>

    I think by the time you find tyres that retain some sort of longitudinal grip at large camber angles, you might also find that you've lost out on lateral grip when the outside tyre rolls over in the corners.
    Even though in your kinematic software you are achieving no change in camber with chassis roll, sometimes the centre of the contact patch of a deformed tyre in a corner means all your negative camber has been effectively negated.
    Just another compromise...
    It's Sunday night and by the end of the weekend I start speaking in lots of double negatives sorry. I'll try better next time.

  7. #7
    the question is probably better worded:

    "theoretically speaking (neglecting packaging), why is RMIT's Front-View-Swing-Axle so short?"

  8. #8
    Quote Jack "The roll center could actually be quite high but given the angle of the lower arms it was probably bought down in design".

    Jack, that statement shows something of a misunderstanding about Design.
    The Design Judges don't know and don't pretend to know everything. So we listen to the teams defense of their design, and let me tell you, RMIT have suffered the inquisition about their VSAL and load paths many times.
    Obviously, their defense was sound as shown by their placing in the Design sector of several various FSAE and FS competitions around the world.
    Personally, I don't like their solution very much, but that is my opinion and they have defended their design to me on quite a few occasions. They then have backed up their arguments with an exemplary performance on track.
    Credit is due where it is deserved.

    Pat
    The trick is ... There is no trick!

  9. #9
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> The roll center could actually be quite high but given the angle of the lower arms it was probably bought down in design.
    </div></BLOCKQUOTE>

    It's Monday morning now and the brain is in work mode again. I now understand what I think you mean by this statement, and I have a feeling that Pat (Clarke) may have misinterpreted. I think you mean that we brought the roll centre height down because of packaging reasons?
    We actually started with our preferred roll centre height first (as I think is a good idea...), and found our compromise for SAL, packaging, load paths and THEN designed our chassis around that.

    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> the question is probably better worded:

    "theoretically speaking (neglecting packaging), why is RMIT's Front-View-Swing-Axle so short?" </div></BLOCKQUOTE>
    Is that a question Frank?

    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Personally, I don't like their solution very much, but that is my opinion and they have defended their design to me on quite a few occasions. </div></BLOCKQUOTE>
    I think you'll come round Pat... I'll have better constructed arguments ready next time I see you.

    Pat Drum
    RMIT Racing Fan

  10. #10
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by PatClarke:
    Jack, that statement shows something of a misunderstanding about Design.

    Pat </div></BLOCKQUOTE>

    What a meant by "design" was the "design process" not the "design event"
    John "Jack" Vinella
    University of Washington Alumni 06' 07' 08' 09'

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