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Thread: Beam Axles - Front, Rear or both.

  1. #21
    Senior Member
    Join Date
    Mar 2005
    A few weeks ago on the "suspension for spool" thread I said I would post a sketch of a Twin Beam-Wing concept. I think this is a more suitable thread, so here it is (warts and all ).

    (Edit: Made image bigger.)

    Main features:

    * This is an "aero above all" car. Everything else is there just to support the aero. I figure "drive on the ceiling" at less than maximum FSAE speeds (ie. DF=W at <100kph, (maybe <70kph?)). And more is possible!

    * Aero is direct acting on the wheels, and easily adjustable for F/R balance via the flaps. Since wings are close to ground, and with wheels as skirts/end-plates, drag is low (no induced drag). The streamlined fuselage and wheel pods also lower drag for good economy.

    * The aero centres of pressure are unlikely to be exactly above wheel axle lines, so some force acts on chassis through BJs causing some pitch/heave. This can be fixed by either; a) not worrying about it, b) reshaping the wings, or c) interconnecting the wings with two torsionally flexible spars running alongside the chassis and attached flexibly to the beams on their axle lines. Extending this last solution sideways gives a full width "live" undertray.

    * The rest of the car is as simple and robust as possible, for quicker build and more testing. Chassis is mandatory roll-hoops/side-impact-structure plus a minimum of extra tubes. Direct acting springs can be soft because little aero load through them, and wheel cambers unaffected by body roll, pitch, or heave. So dampers also soft.

    * If the beams look a bit bulky, then think of them as the main spars and ribs of the heavily loaded wings. Each beam-wing is attached to chassis at only four hard points, namely the 2x spring-dampers, 1x heavy duty ball-joint (say 12mm or 1/2"), and 1x low friction "peg and slot" for lateral control (using, say, 2 x 12mmID 6201 ball bearings). Jigging can be done with stringline and tape measure.

    * Rear wheels are 10" diameter (x 8" wide), but fronts are smaller, maybe 8" diameter. Less is more! This is subject to finding appropriate tyres, maybe from off-road quad racers with knobs cut off. If necessary, fronts can be narrow 10" diameter. Note semi-circular panels in front wing that turn with the wheels.

    * Steering is bevel-gearbox-&-pitman-arm, for less friction, backlash, weight, and cost than R&P+UJs, and better ackermann (two sinusoids to work with).

    * "Necessary ballast" is Royal Enfield! Shown is the old style engine with the separate gearbox relocated to front of crankcase. Jawa or similar single also possible, either upright as shown, or laydown as discussed "Objectively" elsewhere.

    * Important note: the final drive chain as shown will give some anti-squat under power (good), but also some wedge (bad - less LR load, more RR). So OS out of left turns, US out of right turns. This can be fixed by moving engine sideways so chain is on car centreline (or, less desirable, move BJ so in-line with chain).

    * A Rob Woods/UB style spool-axle is shown, but a live diff can also be fitted (this is the reason for the 4 x axle bearings). Also camber and toe can be made adjustable by using two half axles connected in the middle with CVs (or well greased splines).

    * A De-Dion layout with chassis mounted diff is also possible (my preference). In this case only the 2 outer axle bearings are required, together with CV'd half-shafts. BUT, this requires different kinematic location of the beam-wing to the chassis or else too much pro-squat (I didn't have room to show this on sketch - it's only A4). De-Dion is structurally similar, just some relatively small (but important) kinematic changes, so can be done as bolt-on "option".

    * Finally, a first year or "limited resource" team can do this car without the aero. Use a RE, B&S, or similar engine and you have a simple lightweight car that is quicker to build than the usual wishbones-and-pull/pushrods&rockers-everywhere cars. And, all other things equal, it will have high grip and benign handling because of soft springs with no camber change (not possible with normal independent suspensions).

    Comments and criticisms welcome!


  2. #22
    Very interesting concept. Lots of good ideas.

    Packaging an effective braking system on the front axle may be the most difficult challenge with such small wheels, especially if the intent is to have low scrub radius to minimize fore/aft wheel so that it packages well with the aero.

    If the car has to compromise on tires in order to maximize aero, this does not bode well for the novice FSAE driver who "drives too fast in the slow corners and too slow in the fast corners"

  3. #23
    Just wanna say: that's a beautiful sketch/drawing. How did you do it?
    Rex Chan
    MUR Motorsports (The University of Melbourne)
    2009 - 2012: Engine team and MoTeC Data acquisition+wiring+sensors
    2013 - 2014: Engine team alumni and FSAE-A/FStotal fb page admin/contributer


  4. #24
    Z, what is constraining the rear axle laterally?

  5. #25
    Senior Member
    Join Date
    Jun 2007
    Perth Western Australia
    @ Nihal - A ball joint at the front mount and a "peg and slot" at the back, labeled P&S and detailed in a break out.

    Pencil sketching is becoming a lost art. They don't encourage it nearly enough. I'll bet you Z can do such a sketch 10 times faster than you can do it CAD, and it communicates the concept perfectly.

    @Z - Are you sure your not sitting in on our tech team meetings some how??? Or maybe we are channeling you? Some of our guys do talk funny.
    It turns out surface area trumps the "twin wing" concept by quite a bit, and a full floor is superior. At least according to our relatively inexperienced aero team. Did you see the photo from Aus comp of our floor sitting on the ground without the car? Huge surface area.
    That, and our 1 piece (as in both sides) flexure lower A arms do/are pretty much what you have here, but with the addition of top arms etc providing semi independence and much larger load bases to provide camber and toe stiffness easier.
    As a aero mounting platform it is really good. We can run amazingly low and not touch.


  6. #26
    Senior Member
    Join Date
    Mar 2005

    I have looked at brake packaging with zero offset steering in 8" wheels before and don't see any major problems. Essentially there is a lot of fresh air in standard 13", and even 10", wheels that doesn't have to be there.

    I have suggested the 8"s partly because of the rear weight bias (~40F:60R), but mainly because I don't think any more is necessary. Smaller diameter means less weight, lower CG, less stresses on axle (shorter lever arm), so less weight again, and so on.

    If, at the initial design concept stage, you start thinking "13" wheels", then you end up with a powered DOWN F3 car. If you start thinking "smallest possible wheels", then you end up with a powered UP go-kart. Normal power go-karts are quite fast.

    The only problem I can think of is ready access to tyre information. No TTC data on 8"s (?), so the team has to do its own research...


    It's newsagent bought ball-point pen (black, medium point), white-out, and A4 paper. Smooth lines with a plastic ruler and ellipse/circle stencils, plus some freehand. Some pencil construction lines to begin with, then erased.


    As Pete says it's a P&S same as at the front. This is really only suitable for the relatively dirt-free and short travel conditions of circuit racing. I wouldn't recommend it for production cars, and definitely not for off-road racing.


    Previously I have mostly suggested a full width live undertray. The Twin-Wing concept is to get people thinking about how easy it is to start with beam-axles, then add some (plywood?) skins to turn them into wings. Then add flaps, extend the chord, more flaps+++. Also, with this path the F/R aero balance should be easy to adjust.

    Regarding camber and toe stiffness, I don't see any problem at all. In fact, better than wishbones. A beam takes the forces from the wheel bearings to the chassis via a one-piece shell-like structure (the beam) that can be closely aligned with the axis of the tyre's "force screw". With wishbones, these forces first go up and down the often flimsy upright, then through the small outer BJs' (with lots of local deformations such as the BJ bolt in shear), then through the wishbones, then again through more small BJ's at the chassis. It's a detail design thing, but I reckon a sloppy wishbone layout is much worse than an average beam.

    I'd love to see more detailed pics of your car but at the moment I'm using a borrowed "box of grief" that randomly seizes up when on the web (averaging every ~10mins!). I'm in the process of upgrading (grrrrrrrrrrr ).

    BTW, I believe you run a similar steering layout? It's gives good geometry, eh?


  7. #27
    First of all, Z, I hate you. Your overall skills (including drawing) and understanding is waaaay higher than anyone I know. Onto the topic...As I have mentioned in the other thread ("suspension for spool" or "objectively", do not remebmer) I made a quick CAD during Xmas vacation. There are a couple of things that trouble me with all that approach. First of all, in order to make space for a complete "live" undertray/underwing and the pick up points for the beam axles, you have to raise your CoG a fair amount. You could separate the chassis mounting points (having 2 BJ's per axle instead of one) but this leads to other problems (increased roll stiffness). [EDIT: Separate BJ's could allow lower chassis/component placement for a greater length, as the chassis should be upswept (higher) only at the "peg n' slot" mounting ponts, allowing for a not-so-higher CoG compared to double wishbone.] I have come to a solution using something between UWAM and ADFA concept, namely a single piece lower A-arm/beam and two conventional upper A-Arms, mounting on the beam as close to centerline as possible (for minimum camber compliance). With that configuration, you have constant road camber during pitch/heave/roll, but now I'm worried about single wheel bump... All in all, I do not know if it's worth the risk (and the higher CoG/unsprung) compared to the traditional solution we use for years now....

  8. #28
    Quite a few different ways to skin a cat.

    How about a pair of stiff rear trailing arms to control toe, and a rear truss to tie the tops and bottoms of the rear hub carriers together.
    With the lateral location device of your choice.

    A bit lighter than a monster DeDion beam, and should be potentially a lot stiffer.

    Just another species of twist axle.
    Dare to be different.
    Cheers, Tony

  9. #29
    Tony, what you mean is something like this?


    If I get it right, yes, but removing the rear live axle and interconnecting the hubs by something else. (another tube on the upper point like the one on the bottom points?). IMO this is only applicable in the rear suspension (you have a chassis in between at the front), and I wouldn't do that on the rear either... You have a perfectly stiff tube (the live axle) to interconnect the "hubs", why adding extra weight and complexity? Unless you want to run a chassis mounted diff instead... Your solution interpreted for the front suspension is what ADFA uses! There are almost limitless ways of building what essentially is a beam axle....


    On with my thoughts on Z's approach... Wheel pods for decreased drag are a great idea, but not sure if still applicable after UWAM being unable to use theirs on the FSAE-A 2011.

    Wheels: I completely agree with you on the wheel choice, and basically this was our reasoning in choosing 10"; significant decrease in weight (unsprung), decreased rotational inertia of the wheel assembly, smaller complance due to smaller lever arm meaning even lower weight on both wheel and upright. Screw fancy CF 13" wheels; our wheel assembly with 8" wide tires weighed just shy of 4kg, we are trying to get below that figure for 2012. The main issue with 8" diameter wheels is neither packaging (we can fit 200mm rotors to our 10"s) nor tire data (eitherway they are not available for modern 10" Hoosier R25B tires yet). It is the lack of sticky 8" tires... I cannot imagine any commercially available tiire to be anywere close to grip levels as modern FSAE tires. So 10" for me (6" wide on the front, 8" on the rear if you worry about temperature distribution due to weight distribution).

    Steering: I love the Pitman arm solution. Actually I've been looking into it after seeing in UWAM's car in 2008. By far the easiest way to achieve high Ackerman geometry (impossible with R&P), cut off a kg or two, and spare some 600 euros. I still have my doubts about the bevel gearbox; mainly because of backlash issues. Back in 2007 (my first year with the team) we were forced to use one, and it was completely rubbish, both in terms of backlash and weight. Maybe it was the fact that we used the fisrt off-the-shelf industrial gearbox we found...

    Aero/kinematics: The main reasoning behind using such a suspension is (although IMHO it should not be) the possibility of "live aero". I think that this is also possible with a "conventional" double a-arm layout. The main gain here is increased grip during accel/braking. A double wishbone suspension can be designed for "optimizing" wheel camber during roll (FAP's close to CoG or infinitely stiff ARB's or high amounts of camber gain during bump, etc) and thus decreasing longtitudinal acceleration capability OR for "optimal" camber during longtitudinal accelerations (parallel A-arms or huge anti-dive/squat), and thus decreasing lateral acceleration capability. (NOTE: All of the above "solutions" to the "optimal" wheel camber have their side-effects). By using a beam axle suspension, wheels maintain their "optimal" camber during accel/braking (pitch) AND roll. In single wheel bump, IMO both solutions are equally bad regarding wheel cambers (unless you use parallel wishbones); in the axle solution though, the wheel loads are coupled, making things, ehm, more interesting.

    Overall: Aero is equally feasible either in a wishbone on a beam axle solution, although in the latter the undertray could be lighter (using beams as aero-load bearing structures/part of the undertray), so I'll leave it outside. Pros.: Easier manufacturing (less pick-ups), possibility for lighter chassis, softer suspension setup, constant wheel cambers (increased accel./decel.) Cons.: Reaction of all suspension forces in 2 points per axle (need to be bulky->heavy), higher CoG by a fair amount, "coupled effect" on bump per axis, higher unsprung mass. Need to try a lapsim with the two main parameters (increased lontidual acceleration/higher CoG) and see what effect those have on laptimes.... Any thoughts?

    EDIT: Look what I found on facebook...
    Interesting, huh?! Not much like the one described here, but much resemblance in wheel choice (Z?), although 6" here...

  10. #30

    Here is a hugely massive example of a Lotus97 trailing arm SLA rear suspension.

    This is going to be quite good for control of toe and rear bump steer.

    Now suppose you removed the SLA links and connected the four ball joints on the hub carriers together via a simple X frame.
    That would hold the hub carriers parallel, equidistant, and vertical to the track, while still allowing some independence of wheel movement.

    This has probably been done before somewhere (?) but If it has, I have never sen it.
    Cheers, Tony

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