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Thread: Suspension Design

  1. #301
    Stever95,

    Cardboard models are great for everything, likewise lego, it makes understanding movements much simpler and is good for testing various ideas quickly before you run with them. For example, yesterday myself and our chassis guy had a three hour long conversation with a faculty advisor about how both front and rear suspension systems worked, trying to explain with words and drawings was difficult but once a model was made from some lego he immediately understood. By a similar notion it is very good for force visualisation as thin pieces will deform giving you instant results, much faster than building an ANSYS sim, more fun and more reliable too!

    Which reminds me, i intend to update my build thread over the next few days, you might be interested

    Thanks,

    Christian
    Aston University Formula Student - VD/Suspension guy.

  2. #302
    Senior Member
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    SDFs.

    The roll camber (S)uspension (D)esign (F)actor is one of about 18 key ingredients to the recipe that defines the handling characteristics of any vehicle. This particular one affects the tire lateral load transfer distribution (TLLTD) because the change in inclination angle attenuates the lateral force generator via the tire's camber stiffness function. Since race tires are bias belted tires and bias belted tires usually have large camber stiffnesses, the lateral force balance of the vehicle can be greatly affected by this SDF. Since a race prepped vehicle operates at high lateral force levels and race tires are still 'listening' to inclination angle inputs, the balance of the car is affected because the understeer at these g-levels is affected, hence its maximum lateral force capability.

    While we are on the subject, the roll camber SDF is one of 18 or more important measurements made on a K&C machine and/or evaluated using finite element analysis programs. for typical production passenger cars, we can guess the following mean values taken from the normal distributions of a thousand or so such K&C tests. Now before you get all edgy about the comparisons between production cars and FSAE or other Rice cars (using my best accent) , I feel the need to say that sure, the rice cars don't have rubber bushes growing at the ends of suspension attachments, etc. and maybe don't even have power assisted steering, BUT (and that's a BIG BUTT), they run at about twice the lateral force levels, so they actually come out pretty close if they are any good. That being said, some rice cars are so spongy, they are actually WORSE that an engineered production car which has rubber or plastic bushings at the chassis attachments and power steering. FSAE car are no different.

    Here ya go. mateys:

    Kinematics:
    Roll centers (force based) 100 mm front, 150 mm rear.
    Roll steers (steer by roll) .05 deg/deg front, .025 deg/deg rear.
    Roll Cambers (camber by roll) 0.75 deg/deg front, -0.7 or -0.1 rear (bi-modal)

    Elastics:
    Roll stiffness Nm/deg 1200 front, 700 rear.
    Lateral force steer (steer due to lateral force) 0.1 deg/1000N front, 0.03 rear
    Lateral foce camber (camber due to lateral force ) 0.25 deg/1000N front, -0.3 rear
    Aligning moment induced steer (deg/100Nm) 0.8 front, -0.08 rear.
    Aligning moment induced camber (deg/100Nm) 0.04 front, 0.01 rear.
    Lateral stiffness (mm/1000N) 0.45 front, 0.05 rear.

    Signs here adhere to the notion that + is an understeer effect, - is an oversteer effect.

    These factors add a significant amount of understeer to any vehicle's handling characteristics, especially if certain types of tires are used or avoided. Anyone measuring understeer in a road test will be disappointed by the lack of fidelity to computer models of handling if they are omitted. There are some individuals (one has a Post Hole Digger and authored a book) who has completely ignored to this day these SDFS.

    It should also be pointed out that correlation of road test measurements to handling models can be VERY high if K&C and tire measurements are made of any vehicle. And, that any vehicle which has had extensive dis-assembly and re-assembly is likely to suffer the slings and arrows of outrageous carelessness.

    Finally, they are probably not really 'Design' factors, but parameters which are carefully monitored and adjusted to make sure they don't get out of hand or range and are symmetric, side to side. This is how a 'neutral steer' car, based on equal weights and the same tire construction all around , winds up with 3.5 deg/g of linear range understeer and a 30 deg/g understeer reading at its max lateral g performance.

    Just sayin' .....

  3. #303
    If only there was someone out there that has done multiple K&C tests on event winning cars...

    If I get some free time in the next week or so I'll run through my database and post some hard numbers (names will be changed to protect the innocent). Bill's elastic numbers seem low (stiff) compared to what teams have achieved in the past but it's been a while since I looked at any of the data. I've noticed that a lot of FSAE teams have traditionally not done a very good job with bearing selection and hub design.

  4. #304
    Senior Member
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    Kansas City Test Results

    These are production only, street car mean values. Distributions are reasonably Normal (unless otherwise noted). Use of pasta in FSAE car steering systems may show in your results. This could explain the observed, lower than expected steering gain (hence understeer indictment) that these cars exhibit in spite of their rear weight bias.

    Actually there is at least one team with multiple tests. They just hired a new Coach, eh ?

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