Beam Axles - Front, Rear or both.

[thewoundedsoldier]

Pretty cool rig, but I don't approve of welding in shorts!

haha last year I got terrible sunburn like 9 times from welding in a tshirt. always too lazy to walk over and get the sleeves...

I would be very interested to see your results plotted between 0 and 2.5 degrees camber in both directions. I'd expect it to be non-linear and unpredictable, meaning a glitchy car.

I'm going to spend a little while setting up something similar for our solid beam axle rear end TR test.

Very impressive!

[exFSAE]

Originally posted by oz_olly:
I just spent the last week manufacturing a suspension compliance test rig to try and put some numbers to the compliance of our beam axle design. Photos of the rig are available at:

http: // s928. photobucket. com/albums/ad127/oz_olly/Suspension%20Compliance%20Test%20Rig/

(just remove the spaces)

The first picture shows the rig set up with the car to measure camber stiffness and the other pictures (taken during fabrication) show the set up for toe compliance. Notice the leaning tower of G clamps for good measure. I did some quick tests this afternoon just to make sure everything worked and it all seemed to go quite well. The angle results were only accurate to +-0.05 deg as I was using an electronic camber gauge. When I do the full set of tests I will use either linear pots or LVDTs to get better resolution.

My initial results are 285Nm/deg in camber with 1.6g*80kg*0.2413m giving a moment of 303Nm. So we lose 1.07 deg camber at 1.6g with 80kg static corner weight. That's much more compliance than I would like and I have a feeling the toe compliance is going to be worse but now that we have a way to measure it we can quantify improvements.

Cheers

Kinda missing load transfer in there. When you're mid corner, let's assume the inside and outside tires are roughly operating at similar mu's. Might get to oh say 130kg on the outside... 130*9.8*1.6*0.241 = 490N-m. 1.7deg of camber compliance... yikes.

[oz_olly]

Bugger you're right exFSAE. I just did those numbers really quickly. Thanks for finding my mistake when I was writing it I had that feeling that I'd forgotten something important. I'll rework the actual numbers taking into account weight transfer and post my results. The design that I currently have definitely has too much compliance so now that I have measured it I can set realistic design targets for the next iteration. It would be interesting to see some numbers from wishbone suspension FSAE cars. I guess for them the upright plays a big role provided the control arms are loaded in as close to tension compression as possible.

As for Woundesoldiers hypothesis of the compliance being highly non-linear, this is not the case. The moment vs displacement curve is quite linear. There is a bit of non-linearity in the low moment area as any slop in bearings etc gets taken up but then it is pretty straight. I'll post some graphs up when I take more measurements with dial guages to measure displacement.

[MalcolmG]

Originally posted by oz_olly:
It would be interesting to see some numbers from wishbone suspension FSAE cars. I guess for them the upright plays a big role provided the control arms are loaded in as close to tension compression as possible.

...well, I did some camber compliance testing on our unsprung assembly (not including wheel) last year, and although I don't have the raw data on hand to check I did my calculations correctly (or what mass assumptions I based the numbers on), I came up with 0.27 degrees for the heavy end of the car at 1g lateral acceleration. I'll see if I can find my moment vs deflection numbers when I get home for a more useful comparison.

[TheoD]

Brown University had solid axle cars from 97 to 2004. We started with a front solid axle controlled with a peg and slot, that used torsion bars for springs, and rear axle-stressed engine also with a peg and slot and regular shocks.
Over the years we developed the designs to use:
Solid front axle with "stressed" steering rack, controled with Mumford linkage and leading arms
DeDion rear controled with Mumford linkage and trailing arms.

We always used 10" rims during these years (F500 class) and original Mini transmition hardware for diff/ half axles etc

We placed 6th in 2000
and 10th in 2002

After that the newer team members wanted to follow the fashion of Independant suspension and have not performed at the same level...

The 3 main reasons were:
1) Simplicity of manufacturing cause of jigging etc
2) If the independant suspension is so stiffly built that in effect it acts as a solid axle, then what is the advantage? Forgoe the cost and difficulty of it all together...
3) Roll center at or below the pavement!

Desclaimer: This was 10 years ago and I was not the Chassis and suspension designer so I am very rusty on details.

[rjwoods77]

TheoD,

I remember seeing one picture of one of your cars many years ago. Where can one find pics of these cars.

[shark.ashwa]

Originally posted by rjwoods77:
TheoD,

I remember seeing one picture of one of your cars many years ago. Where can one find pics of these cars.

Rob,

Found it!
http://www.brown.edu/Departmen.../The_Past%21.html#12

but then there are only one or two!

Sharath

[oz_olly]

The more I think about beam axles, the more I think a carbon tube live rear axle could be a really good choice for a Formula SAE car. A single (large OD ~2.5in) off the shelf carbon tube with bearing surfaces (sleeves), sprocket and brake disc carriers and hubs all bonded on. The kinematics could be controlled using your favourite linkage system. Major compromises would be no differential, no camber and toe adjustment but I think the system could be made to be of similar unsprung mass to a good a-arm suspension and it would have quite low camber and toe compliance. The system I am thinking of would look similar to the Buffalo car from a few years ago.

Can anyone else dig up more pictures of that Brown car? When Z was talking about the brown go kart, was he really talking about the Brown go kart

Cheers

Edit: Turns out I should read the other posts. Looks like Rob and I see almost eye to eye on this one. Also Z is right, when you take your design in another direction to the general trend you don't have much to copy but you are forced to think about it right from first principles and I think you potentially come out the other side with a deeper/better understanding.

[mech5496]

Oz, thats what I meant with "polished up" beam concept... (http://fsae.com/eve/forums/a/tpc/f/1...51#32420641151) This could go on to CF 4-link bars, pushrods and (why not?) a monocoque, making an 140kg car likely...

[rjwoods77]

Olly,

I am curious as to how you differ on what I described.