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Thread: Suspension Design

  1. #71
    Originally posted by Drew Price:
    Our first year car it was a miracle that the car kept goiung in a straight line when you let go of the wheel, and the left and right sides were just about symmetric to each other.

    Packaging, load paths, and nothing failing during brake test should most of what beginning teams focus on.

    After that start playing with kinematics.
    That's about it
    Cairo University Racing Team Technical Director 2011-2012
    Tyres and Vehicle dynamics
    Suspension team head 2010

  2. #72
    After reading a good chunk of this thread, I am amazed by one thing. A couple of posters have stated that beam axles are OK - on smooth roads only - and no-one, not even the proponents of beam axles, has challenged this view. The reason for the above fallacy is that most people's experience of a beam axle is the traditional live-rear-axle on RWD cars. For starters, most of these have LEAF SPRINGS, however the real problem is UNSPRUNG MASS. A live-rear-axle is typically a cast iron or fabricated steel housing, wheels, hubs, brake assemblies, solid steel drive axles, crownwheel & pinion, differential assembly, rear universal joint and a good chunk of the tailshaft - ALL UNSPRUNG MASS.

    At the front, a beam axle on a road car is a packaging nightmare - much harder to justify and implement correctly than strut or double wishbone especially on FWD which has become the standard for small and medium size cars.

    For FSAE purposes, a well designed and implemented beam axle system (front, rear or both) could match the best cars out there for handling while offering improvements in weight, cost, construction complexity and reliability.

  3. #73
    If you add up all the absolutely unavoidable unsprung weight of, tire, rim, hub, upright, brake disc, and caliper, it can all add up to a surprising amount.

    And the unsprung fraction of outboard suspension springs and linkages themselves will probably not be all that significant in the final total unsprung weight.

    Another factor at the front is camber gain and scrub.
    When you are going straight ahead, you don't really want either, in fact both can be a positive detriment.

    You only need some slight additional front camber gain when cornering, and you can get that when the front wheels steer by being clever with caster and KPI, even with a beam axle...

    A beam axle does not have to be a massive rigid single beam, it could be a fairly light spidery flat truss, to locate the widely spaced upper and lower ball joints laterally.
    Cheers, Tony

  4. #74
    Agree with all that Tony, and even the spidery truss fron beam will have better camber compliance than a double wishbone.

  5. #75
    Originally posted by js10coastr:
    <BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
    This thread isn't about suspension design anymore....
    I think it's now a cross between "LALALALA I CAN YELL LOUDER THAN YOU!!! LALALA" and

    </div></BLOCKQUOTE>

    How many other people hovered over the image for 10 seconds waiting for the alt-text to come up :/
    Nathan Tarlinton
    UOW FSAE 2010 - 2013

  6. #76
    Senior Member
    Join Date
    Nov 2010
    Location
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    Sorry Chapo, didn't mean any offence. I just never saw the ADFA car go as fast as it needed to to win. I assumed it was a driver training thing rather than a car thing, and I fully appreciate the time constraints you guys have that stops you from doing things above and beyond design/build.
    Jay

    UoW FSAE '07-'09

  7. #77
    Z

    If it makes you feel better, the "current" ADFA design has retained beam axle suspension. Not as a direct result of the previous car, but as a concept that fell out of a requirements analysis. I was originally a bit cynical of the beam axle and how it was implemented, but always have appreciated the rationale behind design of the ADFA car. As I have become more familiar with the nuances of the car, it has become more apparent that the beam concept can be competitive with the podium finishers both on and off the track. After all, we took the beam axles to design finals at FSAE-A and FSUK.

    There was also a lot of test and development capability left in WS11 (the last of the original ADFA beam axle iterations) but unfortunately for the moment the full capability of the platform remains unexplored. However, the potential that was evident in WS11 and the fact that we now have an operational beam axle car to use as an R&D platform is an additional reason for us to consider future beam axle concepts.

    For all we know at this point, a new requirements analysis could have the beam concept iterated and attached to an RHS space frame with 8" wheels and a 250cc engine...

    Loz

  8. #78
    Senior Member
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    Mar 2005
    Location
    Australia
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    1,690
    So..., with the polite, pre-dinner social chit-chat now quietening down, I would like to start the "main course" of discussing the pros and cons of these different suspension types.

    ... the major suspension types are,

    1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
    2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
    3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
    4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
    5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
    6. Strut-wishbone (eg. McPherson at front, strut at rear),
    7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
    8. 5-link (eg. recent fashion, mainly for NVH reasons).

    Of these, any of the first five are more than adequate for a winning FSAE car.
    It is good to see that most posts above regarding beam-axles have been positive. I might add some more later, but most of my views are already on Olly's "Beam Axles - Front, Rear, or Both" thread.

    Instead, here I would like to get FSAEers thinking about the other suspension types on that list. I will cover the others in the "top five" later, but first a very brief, and hopefully also rational (which means critical ) assessment of the suspension types I see as LESS suitable for FSAE.
    ~~~o0o~~~

    Back on page 2 (before the hors d'oeuvres ) Claude suggested "I always have been convinced that you can make a good car with a McPherson...".

    Tony suggested "Good solid toe control at the back would have to be be pretty high on my wish list. A single stiff rear trailing arm, with transverse upper and lower camber control links would give excellent toe and bump steer control while being simple."

    Both also supported double-wishbones or 5-links, at least at the front (corrections welcome).

    And I said "I don't like any of the above options, and have rational critcisms of them which I will give later..."

    So...
    ~~~o0o~~~

    Strut-Type (aka McPherson).
    ====================
    I see this as a cross between a Sliding-Pillar and Wishbone/5-Link suspension.

    Its main advantage, as argued by Ford's Mr McPherson (late 1940s?), is that it replaces the upper-wishbone-and-its-outer-BJ-and-chassis-mounts, plus the damper, with a single beefed up damper assembly, for a small cost saving.

    Its main disadvantage IMO, which it shares with the Sliding-Pillar, is "stiction" in the sliding parts. This is especially so when the strut is loaded in bending, as during racing when there are significant horizontal loads at the wheelprint. I will cover this more under the Sliding-Pillar, but this is a significant and ongoing problem for racecars with these type suspensions. In fact, many race series ban "exotic" low friction bearings on the struts for "cost" reasons (even though they are available over the counter at your local bearing shop...).

    Another disadvantage is that strut front suspensions intermingle the suspension kinematics with those of the steering. Very briefly, you either have a lot of camber-compensation (= short FVSA, = good for FSAE) and large SAI (large KPI, = bad), or else small camber-compensation (= bad, depending...) and small SAI (= good). However, as noted earlier, FSAE cars can do just fine with very little suspension movement, so such kinematic issues can be largely ignored.

    In FSAE there would also have to be some chassis structure reaching out towards the top of the wheel to support the top of the strut. This stucture is already there on production cars, so comes at no extra cost. But on FSAE the whole assembly ends up looking very similar to a double-wishbone, except that it has a few less BJs, but the additional stiction.

    Bottom line, IMO, this option combines the disadvantages of Sliding-Pillar with those of Wishbone, with no real advantages over either.
    ~~~o0o~~~

    Single-Stiff-Trailing-Link plus two Transverse-Camber-Control-Links.
    ================================================== =====
    For advantages, I agree entirely with Tony that "solid toe control at the back" is very important, and can be achieved with this type of suspension. However, the same toe and camber control can also be achieved with any of the Swing-Arm types (3, 4, or 5, on the list). For example, any amount of camber-compensation (ie. FVSA length) can be had by selecting between Lateral, Longitudinal, or Semi-Trailing/Leading arms.

    One disadvantage is that poor choice of Camber-Link geometry can give a motion screw (ISA) that moves to poor locations during suspension movement (ie. the kinematics can easily give bump toe-out, = bad). By comparison, the Swing-Arm ISAs always stay where you put them. But, of course, moving ISAs can be cured by stiffening up the springs. "Any suspension will work, if you don't let it.".

    A bigger disadvantage is that the two Camber-Links require four BJs, plus their mounting brackets, chassis nodes, etc., together with the main Trailing-Link BJ. Any of the Swing-Arm options only require two BJs, or a single "revolute" joint.

    Bottom line, IMO this option is half-way between the Swing-Arm and Wishbone types, so it has some of the Wishbone's complexity (lots of BJs and brackets) which is unnecessary in FSAE.
    ~~~o0o~~~

    Double-Wishone or 5-Link.
    ====================
    Clearly, these types have the most components. The DW has 8 x BJs, plus 2 more on the spring-damper (Push/Pullrods covered below). The 5-link has 10 x BJs, plus the spring-damper. Each BJ also requires a mounting bracket, and the complex load path through these can lead to a lot of compliance. At the chassis each BJ/bracket should be at a node of a spaceframe chassis (often with ~6 tubes converging on it...), or requires local strengthening of a monocoque. All of these attachment points, at chassis and upright, should be accurately positioned. Inevitably, there is a lot of fiddly, time-consuming, and thus expensive work required.

    Claude claims these advantages,
    "Providing some suspension pick up points adjustability, with a 5 links or a double wishbone you can get and/or adapt the heave and roll camber variations, roll centers, pitch centers position and movement to what the tires (and good drivers) "need" ..."

    Firstly, how many teams provide multiple mounting points for each wishbone? Most teams seem to decide on a kinematic arrangement at the beginning of the year, and then build the one car, which they race at the one competition at the end of the year.

    Secondly, most of the important tuning adjustments are also easily done on the simpler suspensions. Again, briefly, tyre-type/pressure/toe/camber/etc., and steer-axis adjustments, can be done on a suspensionless car. Spring-rate and damper adjustments (say for LLTD) can be done on any suspension that allows a bit of up-down movement. Lateral and longitudinal n-line slopes (= roll and pitch center heights) can be done on any of the suspension types, usually with fewer adjustable brakets than with wishbones. Which leaves only camber-compensation that would require some "cutting and butting".

    Lastly, "what the tyres (and good drivers) REALLY need" is a shed-load of aero downforce (check the lap-sims! ). Building unnecessarily complicated suspensions, and dicking around with n-th degree refinements to "roll/pitch centers position and movement" is a very poor use of scarce resources, IMO.
    ~~~o0o~~~

    Push/Pullrod-and-Rocker actuated Spring-Dampers.
    ======================================
    Of all the features on an FSAE car, this is the one I consider the least justifiable.

    I could repeat the criticisms I have given elsewhere for these "fashion accessories", but this post is already too long. However, if anyone wants to provide supporting arguments (eg. "they help lower the unsprung mass"), then I will happily discuss...
    ~~~o0o~~~

    Comments, criticism, or vitriolic bile (preferably of an educational nature), all welcome...

    Z

  9. #79
    Z,

    I certainly understand why you consider push/pull rods and rockers to be "fashion accessories", but I think they can provide some practical advantages.

    -Typical FSAE dampers are quite small and typical FSAE chassis are quite narrow, so I think many teams would have to go to a lot of trouble to get an acceptable place on the chassis to mount the dampers with a direct actuation arrangement.

    -Rockers allow for easy setup changes. Ride height and corner weight adjustments can be made with the push/pull rods instead of spring preload so that damper travel is maintained (damper is not extended or compressed due to changes).

    -Although this is not the case for some, my team found it far more economical to manufacture new rockers with a different motion ratio than purchase new springs to achieve a wheel rate change. The motion ratio changes also affect damping, while a simple spring change does not.

    -I believe that ARB's would be heavier and harder to package with direct actuation (if you use them). Maybe I am wrong.

    -The additional cost of using rods and rockers is minimal when your team can get bearings and rod ends for free.

    ----------

    Beam axles sure do seem like a good idea for FSAE/FS, but perhaps it really is the "monkey see, monkey do" mentality that explains why we don't see more of them. I spent this summer designing a nice, simple 4-link deDion rear suspension and our chassis guy designed a really neat, compact chassis for it. When the fall semester started, we gave a detailed presentation to the team which clearly described the pros and cons of switching to our beam axle design. Some of our team fell asleep or left the room until it was time to vote. The team voted against it. The primary reasons cited included "No one else does it, so it can't be good", "Double wishbones didn't break last year", and "Carroll Smith said double wishbones are the best and he knows more than you" (I am the only "suspension guy" on our team). Some of our teammates were actually quite angry at us for "wasting the summer working on a different design" (even though most of these team members did zero work themselves). Frustrating, but I try to look at it as another way that FSAE is preparing me for the real world.

  10. #80
    Brent, I think you hit it on the head. Same applied for us, it was cheap and quick to drastically change motion ratios and progressive/regressive behavior with just a rocker change.

    I believe Carrol Smith makes a comment in one of his books that if you're really clever, you could have a 'dry' set of rockers and a 'rain' set of rockers, so if the track gets damp or wet you can just swap the rockers to simultaneously raise the ride height and soften the wheel rate without having to corner-weight the car again like you would have to with a spring change.
    _______________________________________

    Northwestern Formula Racing Alum
    Head Engineer, Frame/Suspension 2006-2009

    My '73 Saab 99 Road Race Build

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