Quote:
Originally Posted by bluex
Why is that?
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I'll try to explain with enough proof to make sense but without getting too far into the engineering.
A control arm by intended use, notably a lower control arm, needs to be resistant to bending in the vehicle's primary direction of travel and direction of jounce. As a way to judge a shapes bending resistance, engineers use the second moment of inertia. The natural shape of 'A' arms makes them very strong in the direction of travel, therefore I'm only really commenting on the direction of jounce.
I'll be very general here, but it makes the point-
For the flat plate arms, I think they are 3/4" thick. Let's assume they are 2" wide at the bushing. Their second moment of inertia (or area) is 0.0703in4.
As a comparison we could compare that to a 1.5"OD x .120" wall DOM tube. Many use 1/4" wall but that's overkill. The second moment of inertia (or area) for this tube is 0.249in4. Essentially ~3.5 times stiffer.
Second moment of area rewards height in the dimension parallel to the force. This is why a wood 2x4 holds more weight and is stiffer when its standing up vs laying down. Same with an I-Beam. Speaking of I-beams, we can also compare areas to see how efficient the flat plate arms are at getting their strength..
Comparing the areas(aka weights, assume same 'length' & density), the tube is .520in2 vs the flat plate of 1.5in2, nearly ~3 times less.
The argument could be made that the arms get wider than 2" towards the center and are stronger than I'm giving them credit for where it counts. Let's assume the 3/4" flat plate arms were wide enough to give them the same second moment of area as the tube, how wide would they be? 7". And keep in mind a tubular control arm usually has 2 tubes in it and some plate. That's comparable to 14" plus of flat plate and the weight is next to nothing.
I won't get into the performance benefits of lighter components vs heavier ones, but I figured I would touch on the strength/safety side of things. I'm sure the flat plate arms are suitable for mock ups and rolling around the shop, but I'd never put my families life on them.