Flattened A arm tubing

[Pennyman]

So it looks like a lot of schools flatten the ends of the A arm tubing when using spherical bearings on the outer side. I assume the reason for this is to reduce the depth of the bearing cup for clearance reasons. A couple pictures of what I'm talking about are found below:

http://lh6.ggpht.com/SJSUFormulaSAE/SGxoLCOpNRI/AAAAAAAAAFw/QWTNp1-sGEw/s576/100_2845.JPG

http://lh3.ggpht.com/SJSUFormulaSAE/SGxoMGKal2I/AAAAAAAAAGI/MHhcb0F3BW4/s400/100_2849.JPG

http://lh4.ggpht.com/SJSUFormulaSAE/SGxp99zMyWI/AAAAAAAAAIs/AG4JPnDEKEM/s576/100_2902.JPG

Now, I know that a tube will fail near the middle due to buckling, but I'm wondering about how the change in shape effects the strength of the tubing. What are people's experiences with this design?

[Pennyman]

So it looks like a lot of schools flatten the ends of the A arm tubing when using spherical bearings on the outer side. I assume the reason for this is to reduce the depth of the bearing cup for clearance reasons. A couple pictures of what I'm talking about are found below:

http://lh6.ggpht.com/SJSUFormulaSAE/SGxoLCOpNRI/AAAAAAAAAFw/QWTNp1-sGEw/s576/100_2845.JPG

http://lh3.ggpht.com/SJSUFormulaSAE/SGxoMGKal2I/AAAAAAAAAGI/MHhcb0F3BW4/s400/100_2849.JPG

http://lh4.ggpht.com/SJSUFormulaSAE/SGxp99zMyWI/AAAAAAAAAIs/AG4JPnDEKEM/s576/100_2902.JPG

Now, I know that a tube will fail near the middle due to buckling, but I'm wondering about how the change in shape effects the strength of the tubing. What are people's experiences with this design?

[VFR750R]

this is a case where i would suggest physical testing. Weld a tube to a bearing cup and then load it with weights until failure or until you're satisfied it will have suficient safety factor for your design.

although this is a often used design method, many teams just use rod ends, which when loaded like this are plenty strong, and offer some adjustment.

don't copy those uprights.

[Steve O]

We flatten our a-arms next to where we weld on the bearing housing. It is so the a-arm will meet up with the housing.We heat treat our a-arms after they are welded and when stress testing them, as designed the a-arms snap in the middle of the a-arm under high stress... I don't remember numbers but it far exceeded normal operating forces and was representative of a crash into the wheel. In a crash condition our a-arms break to save the more expensive components like the wheel, brake components, and frame. I will say that our flattened portion of our a-arm is much smaller than theres; however I don't think it will make too much of a difference.

Remember when looking at those pictures that the arms have been weakened primarily in the up/down,Y, direction. The forces your a-arms sees are not in Y, your forces are only front to back (z). The Y force is absorbed primarily by your push/pull rod and shock/spring.

The only real mistake that the team there made was using Risse Shocks... I can say this only because last year our team bought 4 new ones, 2 of which didn't work out of the box, sent them back and then one of those 2 new ones didn't work, sent it back, and that one then blew out at the competition!

Steve

[Ppada]

I think with the flattened edges you can use smaller spherical... or larger diameter tube...
That's the main reason i can think of.

[RacingManiac]

Much like Steve O mentioned, we also flatten our a-arms for same reason. We have fixtures made for the operation such that each one will end up the same. We basically heat up the ends with torch and crimp them in a vise with the fixture installed. Then we mill out the half-moon/fish mouth portion to mate with the bearing housing.

We also stress-relief the a-arms at an outside heat-treater after to relief welding stresses. All welding/stress relief operation were done in a jig of course...

[TMG24]

Make sure when you flatten your ends you don't kink or dent the tube. Design judges look for that.

[Pennyman]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by TMG24:
Make sure when you flatten your ends you don't kink or dent the tube. Design judges look for that. </div></BLOCKQUOTE>

Taken care of. Made a flattening tool using a couple scrap chunks of Al. Every one came out nice. I'll post pics a little later.

We also tested an A arm with the flattened tubing. We ended up bending the test fixture before any measurable strain was recorded.

[jaccccc]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by TMG24:
Make sure when you flatten your ends you don't kink or dent the tube. Design judges look for that. </div></BLOCKQUOTE>

Taken care of. Made a flattening tool using a couple scrap chunks of Al. Every one came out nice. I'll post pics a little later.

We also tested an A arm with the flattened tubing. We ended up bending the test fixture before any measurable strain was recorded.

Did you ever get a chance to take pics of your fixture? I'm researching crush tubes, and am interested about the manufacturing of them to ensure consistent tapered angles and such.