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SLA Suspension Model

SLA Suspension Model

SLA Suspension Model

I've been fiddling with my own model of SLA suspension for a while, and had been stuck trying to do something that I'm, now that I've done it, unsure that has any practical value...

The 1/4 vehcle model is made in Mathcad, and so far is for non-compliant analysis. It calculates most suspension parameters I could think of, but I would be grateful if someone with a bit of time on their hands would take trouble to double-check me. Also, I'm not versed with software like ADAMS which would help me check my kinematic analysis (I've taken trouble to check velocity domain, but had a troble checking component accelerations in non constant-speed situations- seeing how I had to derive the whole set of equations twice, checking it would be most welcome).

But, my first intention for this post was not to ask for help- in case if anybody is interested or thinks he could make use of it, I will post the Mathcad file (I can also post PDF print of the model for those who don't have Mathcad). I've also applied the model in a small Flash file, which I hoped would be much more pratical and accessible*, because it would require only internet browser to run- but I seem to have messed up something and at one point it started spitting out gibberish instead of useful data. Instead of trying to fix it, I've decided to make a new program, but it might take some time before I'm finished*.

* if I can get a nod of approval for this model from this learned community, I intend to make the said flash file publicly available, and free of charge- as a way of thanking the 'community' (I hope noone will take offence if I mention first and foremost Greg, whom I've met on another forum, as most prominent and from whom I've learned so much)

As a bit of aside, I'd like to brag about this model a bit- unlike most models it does not do jounce and steering separately, but also provides 3D maps (I'm enclosing a shot from presentation of this model showing the data imported to Excel and resultant 3D map of camber angle). It could also be used to calculate compliant geometry, if used in iterations (but Flash is most unsuitale for calculations of such extent). I'd also welcome opinions if such model could be employed to make full vehicle model (actually, I've toyed with the idea but am a bit stuck in the concept phase). Thanks in advance.

RE: SLA Suspension Model

Looks very nice. The best free software I know of for kinematics is wishbone.bas, one of the Locost websites has it. I have checked that against ADAMS and it seems OK.

Best way forward if you want it checked is to post a hardpoint list.



Greg Locock

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RE: SLA Suspension Model

Thanks for prompt ad encouraging response, Greg. OK, for checking geometry, I've used this model (data is from test file I did in Lotus, but pls. ignore spring mounting points*):

                             STATIC VALUES
         X         Y         Z
      (mm)      (mm)      (mm)
   1900.00     63.00    181.92    POINT:1          Lower wishbone front pivot
   2150.00     63.00    181.92    POINT:2          Lower wishbone rear pivot
   2000.00    688.00    181.92    POINT:3          Lower wishbone outer ball joint
   1900.00    243.00    376.92    POINT:5          Upper wishbone front pivot
   2150.00    243.00    376.92    POINT:6          Upper wishbone rear pivot
   2005.00    665.61    421.88    POINT:7          Upper wishbone outer ball joint
   4146.50    600.00    203.60    POINT:8          Damper wishbone end
   4180.00    475.00    593.60    POINT:9          Damper body end
   2030.00    665.61    421.88    POINT:11         Outer track rod ball joint
   2030.00    243.00    376.92    POINT:12         Inner track rod ball joint
   4180.00    486.00    690.00    POINT:16         Upper spring pivot point
   4145.00    645.00    210.00    POINT:17         Lower spring pivot point
   2000.00    688.83    286.92    POINT:18         Wheel spindle point
   2000.00    737.83    287.00    POINT:19         Wheel centre point

Wheel travel is 77mm in bump and 77mm in rebound in 11mm increments, and steering travel is 12mm in 2mm increments. (As far as geometry goes, I don't recall encountering difference between my model and Lotus Suspension Analysis bigger than half a percent, but I checked only 'main' characteristics). Some of the results are shown in enclosed picture (less frequent characteristics are shown only in design position)- jounce, scrub, camber, steering/toe angle, roll centre height, mechanical trail, scrub radius, kingpin inclination, caster angle (plus brake force lever, spindle length, caster offset, vertical force lever, lateral force lever). Unfortunately, I've made this model forgetting I should be able to check svsa geometry calculations.

For given load at contact patch, model will also calculate ball-joint loads and loads at inboard bushings. It can also solve kinematics of pushrod/pullrod mechanism, calculate 'linearity' of springing, contact patch to damper motion ratio, &c. I've lost quite a while deriving the equations describing the model, so one can obtain velocities and accelerations of each individual component (should be useful when making dynamic model incorporating influence of unsprung masses, &c).

P.S. maybe it would be more convinient if anybody is willing to give me hard points of model they've already modelled so they can check it against existing data

RE: SLA Suspension Model

Sorry for the delay in replying. What do you mean odd suspension, Greg? (I'm guessing because the track rod is going *through* the upper wishbone)

Anyways, the reason for this delay is that I decided to make a full comparison with new LSA and added a new way of data input (most programs use hardpoint coordinates, and I guess it would be most convinient for you guys- so it woud make sense to add that) and suddenly I started getting nonsense results... After a bit of frustration and hand-wringing, today I found out that I merely forgot to move two equations after the redefinition of hardpoints. D'oh.

I expect to have this comparison done shortly and then sort out the flash program.

RE: SLA Suspension Model

I'm back with comparison amost complete and would be very grateful if someone was to take a look. I'm enclosing the Excel table with head-to-head comparison with Lotus Suspension Analyzer 5.0c. The comparison was done on their template suspension, but so far only in bump/rebound.

My results are fairly good when comparing 'real' points- error is marginal, meaning below 0.00001, but what may be of relevance is that exception is the track rod outboard end with still relatively small discrepancy, but 10x greater than the rest. What I do not like are derived characteristics- e.g. camber, KPI and scrub radius are almost acceptable (discrepancy mostly around or below 0.1%), and the rest is not so. The main problem is toe angle, where I get big discrepancies (caster angle and mechanical trail are not that catastrophic).

I'd also be much obliged for assesment of roll center height and fvsa length comparison- even though error might be relatively acceptable, percentage wise- I'd say average is 0.5%, the absolute error may be too big. IMHO 900mm may be 1.6% of fvsa length, but it's still not much less than a meter- and that is big.

Thanks in advance.

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