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Thread: Beam Axles - Front, Rear or both.

  1. #141
    OOOOOOOOOOOOOKKKKK I see now, I was looking at the preload mechanism as having a cusp, isolating the bearing in the z direction.

    So what happens then if you do constrain it or use extremely short bump rubbers? Mid-beam might refer to the point directly in front of the P&S (reduce movement from 10mm to 5mm, or even less, reducing the angle of attack change to ~0.5 degrees. Should be allowed right? The wheels can still move up and down 1 in (albeit in roll or twist).

    -Zach

  2. #142
    Yeah, well that was my concern. I'm guessing it should be fine as beam axles have been used in the past under this rule with no concern. I was thinking that they were not the same as these, but from what I can tel WS10 had a front beam restrained with a BJ and P&S, albeit with a different steering mechanism, but that shouldn't change anyything.
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  3. #143
    This definitely need a clarification on HOW the suspension movement is measured, as I have mentioned before in this thread. In all competitions I have attended since 2007, one of the scrutineers just pushed down the chassis (usually by jumping on the jacking point) and measured the difference in ride height (chassis to ground). In that case, the aforementioned solution would not work as the suspension would seem to have an operating range of 5 or 10mm... Note here that in 2010 we were forced to change to much softer springs for the scrutineering in order to pass the 1" rule...

  4. #144
    I would generally prefer to have the jacking bar attached to the beam itself, keeping as much of the chassis as forward as possible, the most rearward point being the P&S. In this case there will be as much travel as there is tyre compliance and I'm pretty sure that tyre compliance doesn't count as part of the 1".

    I'm ok with changing to softer springs for scrutineering to demonstrate that the travel is there, so long as they are ok with the fact that it most likely won't be running with those springs on the car.
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  5. #145
    An additional thought that's stopping me from sleeping as I roll it around inside my head, is how, with a full undertray, you allow for twist in the suspension. Would you just let it be taken up in the flexibility of the undertray?

    Separate side and middle sections? Stiffer side sections and a flexible inner section? That is assuming you're going to get less DF down the middle because of packaging for chassis, nosecone, engine, jacking bar in the diffuser section, etc. Or perhaps separate front and rear sections with a flexible membrane between the two (very tricky and probably a poor solution as it will change in shape as it is sucked down, you'd need to be very clever to make it work properly).
    Dunk
    --------------------------------------------------------
    Brunel Racing
    2010-11 - Drivetrain Development Engineer
    2011-12 - Consultant and Long Distance Dogsbody
    2012-13 - Chassis, Bodywork & Aerodynamics manager

    2014-present - Engineer at Jaguar Land Rover

  6. #146
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    Dunk, Zach, Harry,

    A few issues to cover, so I'll do it by subject.

    Peg and Slot.
    ==========
    This acts as a lateral n-line between beam and body. As Dunk said, it is meant only to constrain y-axis movement, while allowing all other DoFs.

    So similar to a Panhard bar, except that always horizontal wrt beam (because the "contact normal" is always perpendicular to the slot edges). The end of a Panhard bar moves through an arc so there is always some lateral (y-axis) movement of the beam wrt body, which then results in steer changes. Bump steer not good, but given the minimal bumps in FSAE, a Panhard bar may be tolerable.

    Many other lateral control methods are possible, but the P&S is reasonably simple, compact, and symmetric (everything is on the car centreline). It is suitable for the short travel, clean conditions of FSAE, but not for off-road racing(!), or even production cars. The two ball bearings (which rotate in opposite directions during movement) and the preloaded vertical plates (one per ball bearing, and preferably with hardened rolling face) give low friction, rattle free control. A "plain" round peg in a vertical slot has higher friction, and eventually wears and rattles.
    ~~~o0o~~~

    Heave and Pitch Control.
    ==================
    The "mid-beam bump rubbers" I suggested would mount between the body and the "P&S-bracket" on the beam. So they would restrict travel of the peg in the slot. Details to suit... ... although compliance with Scrutineers' whims is important.

    One approach may be to use "preloaded coil springs" to control the vertical movement of this middle part of the beam. The beam has +/-5mm of free travel before contacting these springs (in either up or down direction). Once contacted the spring requires its preload to be overcome, say 60kg(?), then it moves at its spring rate, say 20kg/cm. So 5mm travel for any load up to 60kg, then 25mm travel for 100kg (scrutineer jumping on body). Plus the load due to the (soft) corner springs.

    I do NOT think heave or pitch will be a problem aero-wise, but if it is, then simple fixes (Plan B!) are possible.
    ~~~o0o~~~

    Aero Effects of Beam-Wing Pitch Changes.
    =================================
    A very important point here is that wings flying very close to ground have very different characteristics to wings up in the air. Throw away the aeroplane wing profiles!

    Briefly, the trailing-edge height and slope determines (roughly) the mass flow under the wing. The minimum ground clearance (say 20-100 mm, depending on testing?) then determines the air velocity, and hence maximum suction. An aeroplane wing has max suction at its nose, but the beam-wing, as drawn, has max suction at the narrowest ground gap (ie. near mid-chord of the main element, near the axle line).

    So, roughly speaking, if the TE is normally at Z = 300mm, but then drops 30mm due to downward body heave, then downforce drops by ~10%. BUT! this is compensated somewhat by the body putting extra downward load onto the tyres via the springs.

    Furthermore, the flaps can be connected via a simple linkage to the body so that they change AoA according to position of the body wrt beam. So whenever the body heaves upward, suggesting lower spring loads, the flaps increase AoA. So during cornering the body roll will cause more downforce on the "inner" sides of the wings, counteracting LLT. But maybe leave this for second+ year...
    ~~~o0o~~~

    Front Wing Obstructs Rear Wing.
    ==========================
    Keep in mind that the front wing has double flaps (hence double-slots), and the main element is essentially horizontal. The two slots and the underwing gap will feed a lot of air to the rear wing. Just keep the front wing (mainly its flaps) well away from stall.

    To repeat a point in the previous section, an aeroplane wing has a small region of maximum negative Cp (Coefficient of pressure) at its nose. At high lift this Cp approaches -10. A ground effect wing can have a similar suction (ie. low Cp) over a much larger area of its undersurface. (Hint: hence the largish, approximately horizontal main element.) Bottom line is that the front wing doesn't need to run near stall to do its job.

    Note also that military fighter/bombers always carry their stores (ie. bombs. missiles, fuel tanks, etc.) on the high-pressure underwing side. This equates to the upper surface of a racecar wing, because upside-down. Putting obstructions on this high-pressure side of a wing has little effect on the wing's lifting performance (some arguments say that lift is improved, "vortex theory of lift", etc.)
    ~~~o0o~~~

    One Piece Undertray.
    ================
    Here is one approach.

    First remove the front wing from the "Twin Beam-Wing" car (keep the front beam!). Next extend the rear wing's leading edge, at the car centreline, to the nose of the car. Keep the outer ends of these LEs at the front of the rear wheel-pods. The rear wing is now a large triangle in planform with its apex at the car's nose, like a "DeltaWing".

    The front part of this DW undertray hangs from the centre of the front beam, via a single short ball-ended link, to allow for freedom of movement between front and rear beams. The rear of the DW is attached to the rear beam via two ball-joints, one next to each rear wheel, allowing the beam to pitch independently of the DW. The DW is thus suspended at only three points (2R, 1F), so can be made rigid without constraining the suspension in any way.

    Next add two small "trim wings" to the front beam, similar to the sketch but smaller, and perhaps a bit higher, so just above the DW undertray. These are used to adjust aero balance by adding a bit more front downforce, because most of the plan area of the DW is towards the rear.

    The under surface of the DW can be smoothly curved, or it can have "tunnels", or something completely different. All options, if done right, will work well. I might post on the "completely different" option on the "WINGS" thread, when time allows...
    ~~~o0o~~~

    How Much Downforce?
    ==================
    Bottom line is that I reckon it should be very easy to get Cp = ~ -4 over about 1m^2 of the under surface of the two Beam-Wings (say 0.4m^2 front, 0.6m^2 rear), with negligible drag.

    At 15m/s (54kph, 33mph) this gives Force = 0.6 x 15 x 15 x 1 x -4 = -540N = ~54kg. Add to this other (lesser) negative pressures on the rest of the under surface, plus small positive pressures on the upper surface, and you are off to a good start. Because Cp = -10 is entirely feasible!

    Z

  7. #147
    Originally posted by Z:



    Z
    Z, not sure if you noticed, but the 2012 UWA car is in fact your "Z-Bar Concept" drawing shown at top left. The differences are : "Centre-pivot Leaf Springs" replaced by the aero undertray acting as twin rocker-beams and also restraining the beam axles' rotation in the Y and Z axes. An ARB acts at the "centre-pivot" locations (as you mentioned in the "Suspension Design" thread). The "Monoshocks" are replaced with an innovative "W" spring which provides lat' and long' location in addition to springing. Dampers are located at each wheel.

    This is a very clever and innovative car with a very low suspension component count and one major suspension component doubling as an aero undertray. This design and future iterations are capable of great things.

  8. #148
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    Originally posted by Gruntguru:
    Z, not sure if you noticed, ...
    ...
    This is a very clever and innovative car with a very low suspension component count and one major suspension component doubling as an aero undertray. This design and future iterations are capable of great things.
    GG,

    Indeed it is!

    Yes, I recognised it straight away. I would really like to see UWA finish this car and perhaps get it over to some Northern hemisphere comps next year. Any chance? Certainly, I hope future UWA teams continue with the concept.

    To restress what you said above, it is a VERY SIMPLE CONCEPT, with a minimal part count. So much so that I doubt anyone would have believed it could work if it was only suggested as an "idea" on these pages...

    Proof that the concept works, to some degree at least, is one of Rex's (?) photos on the Facebook page that shows the car parked with one front wheel on top of a couple of red and blue boxes, about 10-15 cm high. All other wheels are still firmly on the ground!
    ~~~o0o~~~

    To any interested teams currently running conventional suspensions.

    Some of the UWA concept and the Z-Bar sketch above might be combined to retrofit a fairly simple soft twist-mode suspension to a conventional wishbone car. This is something you might do to a previous year's car as a research project.

    First, remove all normal springs (dampers can stay). Then, looking at bottom right of sketch, at front and rear of car fit lateral Z-bars to control heave and pitch only. These might be lateral steel or glass/carbonfibre leafsprings as shown, or else any conventional "third spring" arrangement. These do the job of UWA's "W" springs (silly me, thinking they looked like "E's"!), although the W's also provide axle location (clever!). Next fit an unsprung undertray, somewhat like UWA's.

    The chassis is now only supported like a bicycle (with single spring at front and rear), so wants to fall over whenever going around corners. So fit a SINGLE lateral U-bar at about mid-chassis with its outer ends connected to the undertray tunnels, thus preventing body-roll (search Rex's photos). During cornering the body leans on this single ARB, which in turn pushes down on the outside tunnel, and lifts the inside tunnel.

    These roll forces (up on inner, and down on outer tunnel) are passed on to their respective wheels by the tunnels acting as "balance beams". So LLTD (= ERMD in above sketch) is determined entirely by the geometry of the linkage (specifically, the ARB/tunnel attachment points), and not by any spring rates, or by any bumps or twist in the road.

    Note that in the above sketch (bottom-right) there are two longitudinal torsion bars, acting as Z-bars, that control both heave and roll. This layout is well suited to production cars because of the easy packaging of the torsion bars, which are conveniently the right size to carry most of the car's weight (ie. heave loads), as well as the roll loads. The UWA single lateral U-bar is a simpler solution, although it does require the undertray, or some sort of side balance-beams to work.

    Z

  9. #149

    Panhard Bar Lateral Location Question

    I am new to the fsae forum and have found a wealth of information here.

    To clarify, I am not involved in fsae as a participant, judge or consultant.

    I am a Tool and Model designer in industry by profession who spends his free time working on suspension analysis and tuning for lower level oval track race teams in the northeastern region of the United States.

    I have been perplexed for quite some time on one particular point of the classical or the static analysis of a live beam axle suspension in common use in the arena I am working in and was hoping someone on this forum may enlighten me.

    The problem:

    A race car with beam axle suspensions both front and rear.

    The main sticking point is the analysis of the live rear axle laterally restrained by a very short (approx. 18 in.) panhard bar offset to the right of the vehicle centerline as viewed from the rear. The panhard bar attaches to the axle just to the right of the axles centerline and to the sprung body just inside the right rear wheel as viewed from the rear. The overall height of the bar is adjustable relative to the axle centerline or ground as you prefer. The bars angularity is cockpit adjustable by the driver while the car is in motion. If we assume the bar to be set level at axle centerline height the range of adjustability is 10 degrees up from axle to chassis to 10 degrees down from axle centerline to chassis as viewed from the rear. The adjustment takes place at the chassis mount of the panhard bar via a vertical 'lead screw' and captive block assembly.

    The remaining rear axle degrees of restraint are two parallel trailing links mounted solidly to the rear axle below axle centerline height (approx. 6 in.). The final restraint needed is for axle housing rotation about the y-axis which controlled by a torque arm from the live axle center section (gear housing) to a linear bearing 'sled' with large heim joint being used to attach the torque arm to the chassis i.e. one degree of restraint of the arm rotation about the lateral axle centerline.

    The constraints in this form of racing:

    There is no data acquisition.
    There is no tire data.
    The track surface may be pavement or dirt.
    There is so little testing time it might as well be considered negligible.
    There is no practice time to speak of.

    You are left with driver feedback, observation, and possible video and thought to analyze possible setup changes.

    My approach to date has been a simplistic 'classical' roll center based model to evaluate front and rear wheel pair loads to have a look at what limit behavior might be at assumed steady state lateral and longitudinal acceleration levels i.e. make spring changes or suspension link geometry changes (IC position changes) assume a 'g' level and calculate (spreadsheet) the front and rear tire pair loads.

    The problem I have had with the 'roll center', 'shear point', control point type of analysis, name your favorite author here, IS the location of that point with the short offset panhard bar with angularity described above.

    All of the classic texts I have (RCVD, Dixon, Olley etc.) say that with a basically planer linkage as I have described is that the roll center (for lack of a better term) is located where the rear axles axis of rotation as defined by the axles locational linkage pierces the lateral rear axle wheel pairs vertical plane. For the linkage I describe the rear 'control point' is always taken where the panhard bar crosses the vehicle centerline plane.

    I see no kinematic nor force based reason for this to be the case and as Dixon points out the rear axles axis of rotation is a piece of engineering fiction useful in locating the notional roll center or force coupling point.

    Well, the panhard bar does not cross the vehicle centerline in this case, what to do?

    Consult Mark Ortiz.

    In Mr. Ortiz's view the roll center can be located in this situation by one of two methods. The first he describes as the simple method of finding the intersection of the panhard bars centerline with either the vehicles centerline or longitudinal CG center plane if the car is not symmetrical and to neglect the internal jacking force and use that point as the roll center. The second he describes as the more rigorous method and uses the panhard bars mid-point to fix the roll center height but also says to include the internal jacking force caused by axle to chassis bar angularity in your calculations and to make the mid-point of the panhard bar the point for this jacking forces vertical point of application.

    I have a great deal of respect for Mr. Ortiz's openness in answering any and all questions and agree with much of what he has written about asymmetric race cars but again I can find no kinematic or statics (force based) reason that makes the panhard bar mid-point any more 'special' then any other point.

    When looking at this problem from a 2D statics point of view you come to realize that the panhard bar is simply a two force link attaching the beam axle to the body and if you free body diagram the body then the panhard bar force line of action 'on the body' is simply the bars angle and you could decompose that force anywhere along that line of action. (Yes, a Z n-line) So no point is 'special' from this point of view.

    Two questions to the forum if I may:

    1. In the situation as described where would you take the roll center height for a classical treatment and more importantly why?

    2. What would be a better approach to getting a high level view of the effects of the short panhard bars position and angularity with respect to wheel pair loading with an assumed steady state lateral loading?

    As one last point of observation I have worked with six different drivers two of which have been multi-time champions in this form of racing and only one of the six can describe the effects he feels when he has adjusted the bar past the point of a positive effect.

    Thank you,

    Ralph

  10. #150
    Ralph,

    This is an interesting question, as I haven’t dealt with asymmetric cars before. The following is based solely on my own reasoning, so corrections are welcome.

    Normally for most suspension types the roll centre is found by making a few simplifying assumptions: the linkage is reduced to a purely planar linkage, and the tyres are pin jointed to the ground at their contact patches. The only way the chassis can move when these assumptions hold true is by rotating about the roll centre. When considered in plane; a beam axle located by a Panhard bar does not restrict the chassis to rotate about a single point (Panhard bar can rotate relative to the beam, and the chassis can rotate relative to the bar), so the concept of a geometric roll centre becomes grey.

    There is also a different force based definition of the roll centre as a point where lateral forces can be applied to the chassis without causing roll. After drawing up a quick FBD I think that the point found from the intersection of a line through the Panhard bar with a vertical line dropped from the CG satisfies this definition if you assume the car is sprung such that the chassis will not roll if a vertical load is applied at the CG. Note though that this definition accounts only for a roll centre height, and does not consider a lateral position for the centre.

    This would be easier to explain with a picture but bear with me. We want to apply a lateral force to the chassis, this force will be reacted by the horizontal component of the Panhard bar force. There will also be a vertical component to the bar force if the bar is not horizontal because the bar force must be aligned with the centreline of the bar.
    This component will either increase or decrease the load carried by the suspension springs; the change in the spring loads can be represented as a single vertical force passing through the CG because we have assumed that the applied force does not cause roll, and that vertical loads passing through the CG do not cause roll.

    Now we have 3 forces introduced by applying the lateral force. These must sum to zero and produce no roll moment for equilibrium. The bar force and the spring change force can be slid along their lines of action to the point where they intersect. We can see now that the lateral force must be applied at the height of this intersection in order for the 3 forces to cancel without introducing a roll moment. Therefore this intersection defines the roll centre height.

    If for some reason the position where vertical loads could be applied without introducing roll was not aligned with the CG; then the roll centre height would be determined by the intersection of the bar axis and the line where vertical loads can be applied without causing roll.

    The roll centre height depends only on the location of the chassis Panhard bar point, the angle of the bar, and the location of the line where vertical loads can be applied without introducing roll. The length of the bar will only influence how this changes with suspension movement, shorter bar = more change in bar angle for the same movement.
    Last edited by nowhere fast; 04-09-2014 at 11:06 PM.
    Nathan

    UNSW FSAE 07-09

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