+ Reply to Thread
Page 3 of 4 FirstFirst 1 2 3 4 LastLast
Results 21 to 30 of 37

Thread: A Arm Load Calculations

  1. #21

    Rod end budget

    Harry (and all),

    Worth to ask....

    What is the cheapest?

    - 20 rod ends of 6 mm or 10 rod ends of 6 mm and 10 rod end of 5 mm?

    - 1 rod end of 6 x 1.0 left and one of 6 x 1.0 right OR one 6 x 1. 25 right and 6 x 1.0 right. In the second case you increase the adjustment resolution of, for example, a toe link by 8 as in the first case 1 turn would make 2 mm of length change and in the second case 0.25 mm length change (1.25 mm IN and 1.0 mm OUT or the other way around)? Considering that a small toe adjustment has a huge influence on the car behavior (especially rear)....

    - Instead of 2 rod ends of 6 mm, 2 spherical joints of 6 mm and make the adjustments (push or pull rod, toe link) with shims?
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  2. #22
    Thanks everyone for the helpful discussion! After some more work, its clear the cone strikes (or people standing on or lifting the car) from A-arms is the worst case load.

    In the 3 FSAE cars I have been around in my time here, we have only had one “super student” issue, and that was a rack of tubing falling over in the trailer and landing directly onto an upper wishbone. Took us one night to remake it, and we were back out testing the next day. We have always been very good about teaching new people how to properly lift the car, and which parts of it they can stand/sit on safely. Not that it will never happen, but it happens so rarely that I would rather just remake the links or have spares ready to go than need to make them stronger and heavier.

    The cone strike/cone drag case is important though, and is guaranteed to happen several times a year. After doing simple worst case bending situations, lifting up ¼ or ½ the weight of the car, this is what determined the sizes of our lower wishbones, front and rear. These loads were more significant than any induced by accelerating or hitting bumps.

    Our front uppers are pretty heavily loaded by our pullrods, and our rear uppers (we are running pushrods in rear) are the lightest loaded of them all. These numbers come from our analysis from different load cases and suspension geometry as Claude has described. Its good to know that if we have him for design we will do well at least on this topic – I agree with and have already done everything he described.

    One question I still am not sure of an answer to is how much deflection/compliance is significant. It seems the wishbones will likely never yield, but they will definitely have some potential to deflect and introduce some compliance. Part of me says “well I know we can’t measure toe much more accurately than 1/16 of an inch, so maybe half that much compliance would be no big deal”. But another part of me thinks “well if we have some compliance in the a-arms, and some compliance in the spindle, and some in the inboard sphericals, and some in the outboard sphericals…” then we get this tolerance stack-up affect that could make things much worse. I am aware of software that exists to analyze compliance in all these joints and tell me what it will do to my alignment, but how much is acceptable?


    Quote Originally Posted by Z View Post
    Max,

    While I am here...



    So how much do your pushrods-&-rockers weigh?

    And the extra chassis structure needed for the rocker-mounts?

    Do you really need all that stuff?

    Z
    Z: It is hard to just give weights for these two different cases, as the front half of our chassis and our rear structure are designed around having rockers, and would look a lot different if we were going to run direct acting. A lot of other changes would have to be made in order to package and get the desired motion ratios out of the Penske dampers that we are running this year. I know there are other longer dampers or different ways to package them, but we selected these dampers because we already have them and they've been reliable for us in the past (unlike our Ohlins), among other reasons. We actually did run direct acting front dampers last year, but have moved back to pullrods in the front for this year. I can give you two numbers that I have calculated: the tradeoff between CG height and weight. Based on the load sensitivity of the tires from TTC data (and validated by our own data), I’ve found that if you want to go 0.1 seconds faster around the skidpad you can either cut 40 lbs from the car or lower the CG by 1.59 inches (or get a better driver, or spend more time tuning, etc…). Our pullrods allow us to put the rocker and damper ~3-4 inches off the ground, which improved our CG significantly over a pushrod setup in the front, and still a little better than a direct acting setup. The argument that direct acting would be simpler and maybe lighter is valid, but I hope I have adequately defended our case. Either way, there are plenty of good teams with rockers, and plenty of good teams with direct acting shocks.

  3. #23
    "....I know we can’t measure toe much more accurately than 1/16 of an inch..."

    That is why I do NOT believe in the fishing string method to measure toe.

    I have seen some amateur racing guys using a laser beam attached to a fixture itself attached, for example, to the front rim and projecting a red point on a "flag" attached to the rear of the chassis or to the rear rim. And vice versa; rear beam projecting to the front.
    See attached pictures.

    Another idea; if you really want to use the fishing string to measure the toe, I would not only advise you to look at the difference of (fishing string to front of the rim) - (fishing string to front of the rear of the rim) which gives you the toe in mm....
    ....but also [(fishing string to front of the rim) +(fishing string to front of the rear of the rim)]/2 which would be in fact a measurement of the distance from the fishing string to the rim is the rim was flat.
    Then compare that measurement of the LF with the RF (and LR with RR); if the 2 measurements are not the same that means the car is not symmetrical and the two 1/2 tracks are not equal. Up to you to find out why: chassis, upright, wishbone, rim....

    In a perfect world the 2 fishing strings should be parallel and the middle of the distance between the 2 fishing strings should correspond to the inline axis of the car.
    It would be good the tubes that hold the fishing string (tubes that should be perpendicular to the inline axis of the chassis) are attached to brackets that fall (with pins for example) in holes that are in the chassis and that theses holes were part of the chassis design and manufacturing.

    These are the kind of tons of tip and tricks that are shared in the OptimumG Data Driven Seminars
    Attached Images
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  4. #24
    Senior Member
    Join Date
    Apr 2008
    Location
    Colorado Springs, CO
    Posts
    460
    Quote Originally Posted by maxhouck View Post
    I am aware of software that exists to analyze compliance in all these joints and tell me what it will do to my alignment, but how much is acceptable?
    This is the part with the real engineering! I don't think anyone can conclusively tell you how much is acceptable, you have to take what you (and your team) knows about your tires, your drivers, your kinematics, etc., and figure out the relationship between compliance and performance, the "cost" relationship between performance factors (e.g. mass and yaw inertia vs. camber loss), and where you think the balance point should be. There are countless tools you can use to help, like decision matrices, lap time simulations, etc., but the hard part is wrapping your head around all the primary and secondary impacts of the different options, without forgetting about them (similar to the cone strike/drag discussion).
    Mountain Lion Motorsports

  5. #25
    Well, Mikey and Maxhouck, how much will one degree or one 1/10 of one degree on let's say the LR tire toe or camber will change your lateral and longitudinal acceleration, your yaw moment, your balance, your control and stability on entry and your control and stability at the limit?
    Make at least that steady state analysis.....
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  6. #26
    Senior Member
    Join Date
    Apr 2008
    Location
    Colorado Springs, CO
    Posts
    460
    Quote Originally Posted by Claude Rouelle View Post
    Well, Mikey and Maxhouck, how much will one degree or one 1/10 of one degree on let's say the LR tire toe or camber will change your lateral and longitudinal acceleration, your yaw moment, your balance, your control and stability on entry and your control and stability at the limit?
    Make at least that steady state analysis.....
    Sounds like a very good place to start
    Mountain Lion Motorsports

  7. #27

    C Vs C&C

    By experience the 2 priorities I would focus on are 1) Rear toe compliance Vs Fx and 2) Steering compliance. Always good to measure and/or simulate it with steering rack locked AND steering wheel locked. That way you know what additional compliance there is the steering column

    C is the enemy of C&C. Compliance is the enemy of the driver Control and Confidence
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  8. #28
    Senior Member
    Join Date
    Mar 2008
    Location
    Buffalo, NY USA
    Posts
    340
    No need to reinvent anything, there is an excellent small book, "A Chassis Alignment Procedure" -- recommended:
    https://www.barnesandnoble.com/w/a-c...ten/1115530973

  9. #29
    Indispensable but not sufficient
    Claude Rouelle
    OptimumG president
    Vehicle Dynamics & Race Car Engineering
    Training / Consulting / Simulation Software
    FS & FSAE design judge USA / Canada / UK / Germany / Spain / Italy / China / Brazil / Australia
    [url]www.optimumg.com[/u

  10. #30
    Junior Member
    Join Date
    Jan 2011
    Location
    Findlay, Ohio
    Posts
    12

    Oh dear compliance!

    Quote Originally Posted by Claude Rouelle View Post
    By experience the 2 priorities I would focus on are 1) Rear toe compliance Vs Fx and 2) Steering compliance. Always good to measure and/or simulate it with steering rack locked AND steering wheel locked. That way you know what additional compliance there is the steering column

    C is the enemy of C&C. Compliance is the enemy of the driver Control and Confidence
    Every real object known used in engineering has compliance. Tires, springs, bridges and buildings have compliance. Hell, even the international space station has compliance in its structure.
    I think it's incorrect to make a blanket statement saying 'compliance is the enemy'. I would argue that lack of understanding of compliance and how it affects handling is the real enemy. Too many formula teams (even when I was on the team) would see compliance as an evil being that we need to conquer without understanding what it actually is.

    Here's some good resources on how compliance affects vehicle dynamics - http://papers.sae.org/2002-01-1218/, http://papers.sae.org/760713/, http://papers.sae.org/760711/ along with Fundamentals of Vehicle Dynamics by Thomas Gillespie and RCVD by Milliken have plenty of information on how to integrate compliance into your model.

    You don't need a multi-body vehicle dynamic simulation to incorporate compliance into your simulation. If you are modeling tires, springs etc. then you're modeling compliance.

    Back to the question of 'how much compliance is acceptable'? I would rephrase that question to 'what is compliance doing to my vehicle handling'?
    Then you approach it in a more practical way of integrating it into vehicle design rather than chasing shadows in an attempt to 'eliminate it'.
    Last edited by sidkash14; 11-02-2017 at 09:16 AM.
    Sid Attravanam
    ----------------------------------
    Susension/Vehicle Dynamics, UTA FSAE Alum (2009-2012)
    Vehicle Dynamics, Cooper Tire & Rubber Company

+ Reply to Thread
Page 3 of 4 FirstFirst 1 2 3 4 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts