Way back in December 2012 (page 9) I wrote a rather long post describing the various types of Swing-Arm suspensions. There I said that Semi-Leading&Trailing Swing-Arms are my second favourite type of suspension for FSAE. They don't have quite as good overall properties as Beam-Axles, but they are perhaps the structurally simplest solution, so in that way are well suited to FSAE.
So here is a sketch of how I would do Semi-Leading&Trailing Swing-Arms.
Some comments (much of this also covered in the above link).
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1. KINEMATICS - These Swing-Arms have a short "Front-View-Swing-Arm" that gives "100% Camber Compensation". This means there is no wheel camber change during cornering, regardless of any amount of body roll. The penalty of short FVSAs (not seen on Beam-Axles!) is that the wheels will have camber change when the body pitches due to accelerating or braking forces, or heaves due to vertical forces from sprung aero or big bumps. FSAE is mostly about lateral cornering forces, with lesser amounts of longitudinal or vertical forces, so this type of front-view kinematics (for independent suspensions) is preferable (ie. better than very long FVSAs). See also Geoff's and other posts on previous pages.
The side-view kinematics have the front wheels' longitudinal n-lines rising slightly up-to-rear, so giving a small amount of anti-dive. At the rear the longitudinal n-lines are such that there is some anti-lift under braking for outboard brakes, but with inboard brakes there is a little pro-lift, and with inboard drive there is a little pro-squat. (Hint: Draw side-view n-lines in the plane of the wheel, and see that they intersect a little above ground near the front wheel. Note that the n-lines for a Swing-Arm are ALL the straight lines in 3-D space that intersect the SA's pivot-axis (= its ISA).)
This side-view behaviour is generally OK for FSAE type racing. The pro-squat is not suited to drag-racing (where large anti-squat is better for good launch), but in FSAE such issues are adequately fixed with Anti-Axle-Bounce springs (see below).
At the front wheels bump-steer is mainly determined by steering tie-rod position (its centreline should pass through, or close to, the SA pivot-axis). Note that a little bump-steer at the front is not too bad, because the driver can correct for it. At the rear the Swing-Arm pivot-axes (ie. their ISAs) should be close to horizontal to minimise bump-steer.
Sloping the rear pivot-axes up-to-front gives bump-toe-out, and so roll-oversteer. This is generally bad for passenger cars (car spins out of corner), but may be good for an FSAE car that understeers through slaloms and hairpins. Sloping the pivot-axes down-to-front gives bump-toe-in and roll-understeer. I suggest it best to keep the pivot-axes horizontal, and then fine tune with static-toe adjustments (see below).
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2. STRUCTURE - The chassis structure required for these S-L/T Swing-Arms is about as simple as possible. I see this as their biggest advantage. All the SA-to-chassis attachments (BJs or bushes) are at the perimeter of the floor. These attachments are at the centreline of the front bulkhead (where strong structure is required for the pedals and IA), at either side of the front and main roll hoops (which are required by the Rules), and under the drivetrain/diff (which must also be strong, least the engine fall out).
A flat floor can join all these points, and this is well suited to carrying the mainly horizontal loads from the SAs. Having all these attachments in the flat floor plane also simplifies jigging and manufacture of the chassis. All this major structure is at the lowest possible part of the chassis, so giving a lowest possible CG. The vertical loads from the wheelprints, which are carried mainly by the spring/dampers, can be fed from the Swing-Arms to the chassis by any convenient path (more below).
In the sketch the structure of the Swing-Arms themselves is a hollow sheet-steel fabrication. IMO this is easy technology to learn, can be done with simple tools, and gives the best strength and stiffness-to-mass properties. Thicker aluminium sheet could also be used, or carbonfibre if you really like the smell, sticky fingers, and extra time and cost.
Importantly, ALL FOUR SWING-ARMS ARE IDENTICAL. This means only one jig is required and fewer spares needed (eg. only have one spare, and it can replace any corner). There are important "Production" advantages here. Namely, once you figure out how to make one good one (ie. which welds to do first, where to rest your elbow while welding, etc.), you can then produce high quality items at high speed.
The two rear "uprights" are also identical, as are the front "upright/king-pins", and all axles and bearings are very similar (for lower spares count), but some parts of the front steering are different (too many funny angles!
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3. SPRINGING - Given that suspension is NOT a very important performance factor in FSAE (rigidly sprung cars have won), a very simple spring-at-each-corner is shown, because it will do. So a conventional spring-damper is drawn at each corner, which feeds its loads into any convenient point on the chassis. These dampers have a Motion Ratio of about 0.6+ (damper/wheelprint movement). This is more than enough for lightweight FSAE cars, and the supposedly magical MR=1 is NOT necessary. In fact, I have shown the spring-dampers neatly tucked away because there is more to be gained from good aero flows than silly MRs!
As noted above, the main disadvantage of these types of Swing-Arms is excessive wheel camber change during heave or pitch motions of the car. The easiest solution is to fit Anti-Axle-Bounce springs (these covered extensively in other posts). These "lateral Z-bars" are essentially the same as the "third-springs" used on many modern aero racecars.
I would possibly implement AAB springs as lateral centre-pivot-leaf-springs (see "Z-Bar" sketch somewhere), mainly for neat packaging and low CG. Perhaps go to an Archery store for inspiration (ie. a lightweight and stiff centre section with the pivot, and lightweight flexible fibreglass leafs at the ends). Zero-droop+rising-rate-in-bounce is also good, and easily done with this type of spring. Other options also possible...
If you use AAB springs as above, then these should carry most of the weight of the car (ie. they control pitch and heave). This way the corner springs really only have to carry the roll forces, and since body roll doesn't affect wheel camber angle, the corner springs can be quite soft. This, in turn, means that the chassis is subject to only low torsional loads, and thus high torsional stiffness is less important. Furthermore, the soft corner springs give the whole car a softish Twist-mode, which is a good thing if the track has any undulations.
Of course, longitudinal Z-bars can also be fitted, giving a completely soft Twist-mode, and thus very predictable and easily adjusted handling balance. (Might have to do another sketch of these one day to show various practical implementations...)
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4. REAR CAMBER & TOE ADJUSTMENT - This is shown at bottom-left of the sketch. A one-piece Swing-Arm-From-Chassis-Out-To-Hub-Bearings would be simpler, stiffer, lighter, etc., but then camber and toe might be harder to adjust, and different SAs would be required for each corner.
The joint shown is essentially a three bolt flange, but with capability to vary the spacing between the three corners of the equilateral triangular flanges. The two forward bolts use shims (eg. washers) to vary the top and bottom spacing and thus adjust camber-angles. The rearmost bolt uses a threaded adjuster in the SA to give finer adjustment for toe-angles.
The "upright" has spherical surfaces machined into it, and these are clamped by the bolts and cup-like washers. This is necessary to accomodate the misalignment when making adjustments. Note that the three bolts are always parallel and with constant alignment to the SA, but the upright moves "out of square" wrt the bolts.
The bolt sizes, etc., shown in the sketched joint would have similar strength and stiffness to a one-piece SA+Hub, although it is slightly heavier. However, this joint should be considerably stronger and stiffer than many FSAE wishbone+uprights because the load paths are more direct, and the "balls" are clamped tight and thus don't have any of the usual freeplay.
For the record, the sketched joint only positively constrains 5 DoFs between SA and upright. The sixth DoF, a rotation about the rearmost bolt, is only constrained by friction. It is quite easy to positively constrain all 6 DoFs (yes, this has been covered before
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5. CENTRE-PLANE STEER-AXIS - The "upright" at the front wheels might seem rather novel (err, unless you play around with tractors
The main differences in the sketch to normal FSAE cars is the use of Tapered-Roller-Bearings for both king-pin and axle, and the very compact overall packaging. These bearings (30ID x 55OD) are more than adequate for the loads, and IMO much better than the 68xx Deep-Groove-Ball-Bearings commonly used for FSAE axles. FWIW, for a given mass TRBs carry much greater loads than DGBBs, and can have better installation stiffness. The main disadvantages of TRBs is slightly higher friction (Mu=0.002 vs Mu=0.001 for BBs), and consequently lower MaxRPM.
One of the main advantages of this design is the low steering friction under very high loads. Here I am thinking of serious aero downforce and the resulting 3+G cornering loads ... minimum! As such, the "upright" (ie. the "king-pin" shaft + outer-housing of the axle bearings) should be made of reasonably good quality steel. A 3" square bar of 4140 would do for a start, hollowed out with "speed holes" as far as you dare.
A similar but different design for the front king-pin/steer-axis can be found on the Citroen 2CV, and is a good alternative.
Close to centre-plane Steer-Axes are quite common in FSAE these days, which is good because they work better than the massive Offset + SAI (= KPI) that used to be common. However, if tyre distortion due to low pressures or excessive negative camber start to give funny steering feel during mid-corner braking, then adding a small amount of Offset (= scrub radius) can help. This is most easily done with wheel spacers (between wheel and hub), or using a wheel with different "offset".
The sketch was done primarily to show that centre-plane steering is feasible in a 6" wide, 10" diameter wheel, with the brake-disc outboard of the Steer-Axis, and relatively large steer-angles are possible (30 degree outer, 45 degree inner).
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As always, comments and criticism welcome.
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