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Thread: front design validation

  1. #1
    i have tentatively designed a front suspension and steering setup..
    front track - 1250
    front KPI - 6deg
    front caster - 6deg
    front scrub - 19 mm
    front camber - 2deg
    front RC height - 38.16mm
    FVSA - 2910 mm
    lower control arm is parallel to ground, front view length - 367.92mm

    this the the front view pic -

    the setup was modelled in optimum k with a pushrod setup and kinamatic analysis was done.

    front view pic - https://www.facebook.com/photo...67848&type=3&theater

    top view pic - https://www.facebook.com/photo...67848&type=3&theater

    the steering geometry was designed as such - https://www.facebook.com/photo...67848&type=3&theater

    the given steering geometry gave 29.274 mm as the rack travel for 45 deg of steer angle at the inner wheel.
    due to lack of tyre data things had to be assumed and then analysed for results...

    following are the simulation graphs -

    roll 2 deg, 120 deg angle at steering wheel (45 deg steer angle at the inner wheel) pic - https://www.facebook.com/photo...67848&type=3&theater

    the bump-steer comes around a maximum of .3 degrees in 3 in jounce...

    roll 2 deg 45deg steer angle at inner wheel - and - bumpsteer analysis

    if anyone could plaese help me out with validation of the results...
    thank u...

  2. #2
    i have tentatively designed a front suspension and steering setup..
    front track - 1250
    front KPI - 6deg
    front caster - 6deg
    front scrub - 19 mm
    front camber - 2deg
    front RC height - 38.16mm
    FVSA - 2910 mm
    lower control arm is parallel to ground, front view length - 367.92mm

    this the the front view pic -

    the setup was modelled in optimum k with a pushrod setup and kinamatic analysis was done.

    front view pic - https://www.facebook.com/photo...67848&type=3&theater

    top view pic - https://www.facebook.com/photo...67848&type=3&theater

    the steering geometry was designed as such - https://www.facebook.com/photo...67848&type=3&theater

    the given steering geometry gave 29.274 mm as the rack travel for 45 deg of steer angle at the inner wheel.
    due to lack of tyre data things had to be assumed and then analysed for results...

    following are the simulation graphs -

    roll 2 deg, 120 deg angle at steering wheel (45 deg steer angle at the inner wheel) pic - https://www.facebook.com/photo...67848&type=3&theater

    the bump-steer comes around a maximum of .3 degrees in 3 in jounce...

    roll 2 deg 45deg steer angle at inner wheel - and - bumpsteer analysis

    if anyone could plaese help me out with validation of the results...
    thank u...

  3. #3
    the first analysis excel sheet was for bump steer
    this is the analysis for 2 deg roll with 120 deg steering wheel angle corrosponding to 45 deg steer angle at the inner wheel -

    https://docs.google.com/spread...kNkNGeVZIR1FZU3hxSkE

  4. #4
    I'm hardly an expert but it looks like you are on the right track overall.

    If this is a first year car I would do some side view stuff (anti dive etc), then stick with that and go build it. Then learn from it, and build a better one next year.
    "He who dies with the most toys wins"

  5. #5
    even i hope im on the right one... well, did u check the excel files for the analysis??

  6. #6
    Senior Member
    Join Date
    Mar 2005
    Location
    Australia
    Posts
    1,690
    Ezio,

    Why are you raising the CG as high as possible (ie. spring-dampers very high up)?

    Why are you stressing the steering components so highly (ie. very short steer-arms)?

    Z

  7. #7
    it is our first year car , and in absence of the Tyre data, i'm designing for 100% ackerman.. so if i increase the steer arm length, the bump steer increases as the steer arm moves further into the wheel and in the front view the relative lengths of the LCA front view can the tie rod changes increasing the bump-steer..

    currently it is about 0.3 degrees for around 3 inches of bump

    what do u suggest, should i be continuing with 100% ackerman?

    we're working on the packaging of a pull-rod setup so that the spring and dampers come down....

    any suggestions or changes that are evident???

  8. #8
    from the current setup of the bellcranks, have you got a plot of wheel travel vs. piston travel in the damper? It looks like this geometry is very degressive, which you may not want it to be...

    Also, look at what Z said, if you induce a lot of stress in the steering linkage, you will likely also get a lot of compliance. So you have to find a tradeoff between bumpsteer and ackermann. Maybe you can also find a way to make the bumpsteer actually help your cornering...

    In addition to this, the angle of the front lower a-arm seems to be a bit narrow, which leads to high stresses and high compliance again.

    Just my 0.02$... Overall you are on the right way, jst remember, engineering is all about finding the best compromise for your application, especially in suspension design.
    Lutz Dobrowohl
    2008-2011
    Raceyard Kiel

    Now: Scruitineer, Design Judge, application engineer @Altair engineering

    Whatever you do, do it hard!

  9. #9
    Senior Member
    Join Date
    Mar 2005
    Location
    Australia
    Posts
    1,690
    Ezio,

    First thing you should do is get an overall view of what sort of car you need to do well in FSAE. Have you read the "Reasoning..." thread? If so, what are the results of your "top level" (ie. Level 4 & 3) design processes?

    What I am getting at is that it seems that you are diving in and simply copying some suspension designs that you have seen on other FSAE cars. Unfortunately, those are bad designs. You can be much more successful in FSAE if you greatly simplify your suspension. Double-wishbones are not necessary. Push-or-pull-rods-and-rockers are a complete waste of time and money. Etc., etc.

    To be more specific, tyre data is not necessary to design a good suspension. Whatever suspension you do design should have adjustable toe, camber, and spring rates. But really that is about all you need. The "fine tuning" of those variables AFTER the suspension is built (not lengthy initial design) is enough to satisfy whatever your tyres want.

    Likewise, ackermann has very little to do with tyre data. It is mainly related to the kinematics of a car going around a tight corner. Put simply, you want as much "ackermann" (or dynamic toe-out) as you can possibly get. It is a simple matter, and preferable for structural reasons, to do this with longer steer-arms than you currently have.

    FSAE has negligible bumps, so bump steer is not a big problem. Only if you do a spectacularly bad design will you have problems. Also, the suspension should have some final adjustability to cater for manufacturing tolerances, which you can use to fix bump steer.

    But back to "top-level" design, why did you choose double-wishbones? What is your justification for using this unnecessarily complicated type of suspension?

    Z

  10. #10
    thank you so much Z for ur suggestions,

    i know the steering arm is too short, but i can compromise on bump-steer and ackerman, but increasing the steer arm length increses the rack travel as-well. the titan data sheets for fixed center rack doesn't have that high a rack travel (ie rack speed).

    also i would like to ask, what would be the maximum steer angle (for the inner wheel if i'm running ackerman) that would be enough to negotiate the tightest corners in the track?
    with the current wheelbase and track what i come up is around 40 deg, but it sounds way too high! (is it reasonable enough?)the lock to lock steering wheel angle what i assumed is -120 deg to 120 deg (is it a descent assumption?). with this value, the rack travel what i am getting is 29.274mm for 45 deg inner wheel angle.. now total rack travel would come out to be 29.274X3 = 87.822mm for 1 pinion revolution.

    now if i increase the steer arm length, this value would rise... do i go for a custom made rack then? suggestions would be very helpul...

    thank u so much guys...
    thank u again 'Z' and 'LUNIZ'


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