Wishbone Dimensions

[SK]

Guys!! i m stuck with the calculation of desired wishbone OD & thickness of my fsae car...can anyone of you help me out regarding required calculations and your present dimensions of the wishbones..??

[billywight]

SK!! 3/16" OD tube with a .250" wall is pretty common. I would just skip the calculations and go with that.

[Claude Rouelle]

I would just skip the calculations and go with that.


Do count on impressing Design Judge with that approach!

[Charles Kaneb]

Nice one, Billy!

SK, What's the ID on a 3/16" (.1875") tube with a .250" wall thickness?

[SK]

The main issue is that I am not able to determine the wishbone diameter.I have my load acting on the wishbone fixed and also the material for the wishbone. But while I am trying to calculate the dimensions of the wishbone pipe,I am ending up with one equation and two unknowns,the equation being:
bending moment=shear stress*polar modulus

Here,when I have considered the braking+cornering scenario,I have considered the ultimate stress of the material (is my approach correct?), and I am ending up with both OD and ID terms in the polar modulus expression.This is where I am getting stuck.

As far as my knowledge goes,this is the only equation involving all the necessary variables.I need help in seeing this through.Any other suggestions for solving the above issue are also welcome.

[ed_pratt]

SK,

why are you working out bending moments? are your wishbones in bending or compression/tension?

Ed

[Charles Kaneb]

Originally posted by SK:
The main issue is that I am not able to determine the wishbone diameter.I have my load acting on the wishbone fixed and also the material for the wishbone. But while I am trying to calculate the dimensions of the wishbone pipe,I am ending up with one equation and two unknowns,the equation being:
bending moment=shear stress*polar modulus

Here,when I have considered the braking+cornering scenario,I have considered the ultimate stress of the material (is my approach correct?), and I am ending up with both OD and ID terms in the polar modulus expression.This is where I am getting stuck.

As far as my knowledge goes,this is the only equation involving all the necessary variables.I need help in seeing this through.Any other suggestions for solving the above issue are also welcome.

You will end up with both OD and ID terms; both of them are variables. Tubes with differing ODs and IDs are readily available, so you'll have to choose one that meets your needs.

I recommend Aircraft Spruce for 4130 steel tube. and OnlineMetals for mild steel tube.

[billywight]

The main issue is that I am not able to determine the wishbone diameter.I have my load acting on the wishbone fixed and also the material for the wishbone. But while I am trying to calculate the dimensions of the wishbone pipe,I am ending up with one equation and two unknowns,the equation being:
bending moment=shear stress*polar modulus

Here,when I have considered the braking+cornering scenario,I have considered the ultimate stress of the material (is my approach correct?), and I am ending up with both OD and ID terms in the polar modulus expression.This is where I am getting stuck.

As far as my knowledge goes,this is the only equation involving all the necessary variables.I need help in seeing this through.Any other suggestions for solving the above issue are also welcome.

Why didn't you start your original post with this rather than just asking for an answer? That's the reason you get smart ass replies (like mine)rather than useful information. At least now you've shown that you have done some work to figure it out.

[Charles Kaneb]

Aircraft Spruce Round Tubing (http://www.aircraftspruce.com/catalog/mepages/4130tubing_un1.php)

Two things I'd like to point out:

1) If you're encountering large bending moments, you may want to consider the second section moment of inertia of a square tube versus a round one. A square tube puts a lot of material a long way from the neutral axis.

2) Steel tubing is really light, all things considered. It's not difficult to pick up an 8' stick of 1.25"x.120" tube (Honda Accord LeMons rear ARB - it is possible to have too much rear roll stiffness in a FWD car!). Don't lie awake wondering if you could've gone down one size or one wall thickness.

[SK]

Why didn't you start your original post with this rather than just asking for an answer? That's the reason you get smart ass replies (like mine)rather than useful information. At least now you've shown that you have done some work to figure it out.

Point noted..!! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif