value of inertia of student car ......

[manshing316@yahoo.com.hk]

... anyone got an value of inertia of a formula student car in roll, pitch and yaw ??? thanks !

[manshing316@yahoo.com.hk]

... anyone got an value of inertia of a formula student car in roll, pitch and yaw ??? thanks !

[Denny Trimble]

Yes.

[manshing316@yahoo.com.hk]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Denny Trimble:
Yes. </div></BLOCKQUOTE>

can you please tell me the values (just roughly)you've got from your FSAE car ? thanks !

[Denny Trimble]

Well, I don't mean to be rude, but we've spent considerable effort and time building test equipment and measuring the inertia of our cars, as well as modeling as many parts as possible in CAD, to get a good estimate early in the design stage.

I don't think our hard-working team-members would appreciate me sharing that info.

If you're working on a simulation that needs inertia inputs, I recommend you make educated guesses, then check them with physical testing later.

[fade]

2, 4 and 6

[Frank]

what you really want to ask is ..

"what are typical radii of gyration for these cars?"

in the absense of measuring I'd guess

300 mm roll
600 mm yaw
600 mm pitch

Frank

[JonBoy]

After making some random MMOI rigs doing everything wrong (too heavy, too much friction, random things wrong with them, dodgy measurements, random joker making them....) they worked measuring MMOI of moderate (150kg) steel objects of known geometry. When it came to putting a 220-300kg vehicle on them it would give very reasonable values in certain axis, (i think yaw in particular). When i get back to civilisation again ill look up some old notes and confirm it. Otherwise it was all just luck. (unless someone in brisvegas can look through my "Epic Tome" of a thesis and confirm it)

*End of random mutterings now*

[Z]

Back-of-envelope calcs...

Assume car+driver = 300kg, wheelbase = 1.5m, track = 1.2m, weight distribution = 50:50.

Now assume (!!!) car+driver is homogenous rectangular solid L=1.5m, W=1.0m, H=0.5m. So Izz = M(LxL+WxW)/12, Kzz = sqrt((LxL+WxW)/12), etc.


Yaw:
Iyaw = 300(1.5x1.5+1.0x1.0)/12 = 81.25kg.m.m
Kyaw = sqrt((1.5x1.5+1.0x1.0)/12) = 0.52m
k.k/ab = (0.52x0.52)/(0.75x0.75) = 0.48

Pitch:
Ip = 62.5kg.m.m
Kp = 0.46m
k.k/ab = 0.37

Roll:
Ir = 31.25kg.m.m
Kr = 0.32m
k.k/ab = 0.29


For dynamic analysis pitch and roll inertias should be calculated without wheels, so above Ip & Ir are overestimates.

So, Denny, how does that compare with the CAD work?

Z

(Edit note: Manshing, The above estimate of rectangular car size is to the nearest half metre. If you use a tape measure on the major components, and MoI and parallel axes equations, etc., then you should get better results.)

[Denny Trimble]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:

So, Denny, how does that compare with the CAD work?

Z </div></BLOCKQUOTE>

Pretty close on roll, but about 40% low on pitch and yaw, for the entire vehicle with driver. Our car has a longer wheelbase, though. Also, our unsprung weight is significant (and we work hard to reduce it), and at the corners of the "rectangle". The (mass * distance^2) term adds up quickly at the corners.