Automotive, Holes Subframe etc., Purpose (not for Fastener Access)?

[YoshiMoshi]

Hi,

In automotive industry, I've seen subframes, lower control arms, upper control arms, cross members etc that have holes in them.

Like above. That do not provide access to a stud, nut, that was put in from the other side. Now I understand some of these holes are there for that exact purpose (as can be seen in the picture). I get that. But this isn't the case for a good amount of them. This got me thinking, what is their purpose and function if not to provide access to a fastener? Seems like a good way to introduce water, and crud from the street to the internals of these metal structures. Seems like a good idea to just cover them up, install a grease fitting, a relief valve, and inject grease into the hollow structure to prevent rusting from the inside out. But I highly doubt these holes have no purpose? I've watched videos of people on YouTube replacing this lower control arm (the thing highlighted in the picture) without using the holes for access to fasteners.

I know I can just remove the old component and install the new one and go about my life, but I really don't understand the point of these holes and would like to learn their purpose.

 

[Baluncore]

I really don't understand the point of these holes and would like to learn their purpose.

Holes provide drainage and an exit route for particles that enter through bolt access holes, or through gaps or cracks in welds. Hermetically sealed components are heavy and expensive.

Holes provide accurate jig positioning points. Jigs are needed during the mass production of precision steering components.

Holes increase flexibility by allowing some distortion of the flat metal surface. In plastic designed components, something must give. It is better to flex and deform the sheet material near a smooth hole, than to fracture a weld.

 

[YoshiMoshi]

Hey thanks! I really appreciate it.

It sounds like welding sheet metal over these holes so I could create a sealed unit would not be a good idea then because it would interfere with the ability of the lower control arm to flex? Or even just simply putting a plastic cover or a rubber boot over the hole would also create issues?

I was just thinking of a non sealed metal structure that will likely rust out, might as well as seal it and fill it up with grease to prevent rusting.

Some of the bolt holes on my subframe have plastic covers over them that you have to pry out, probably to keep the threaded studs from rusting to badly. While the ones that aren't meant for this are just left open.

 

[Baluncore]

I was just thinking of a non sealed metal structure that will likely rust out, might as well as seal it and fill it up with grease to prevent rusting.

The main problem is that it cannot be economically sealed.

When the component was manufactured, it was dipped in a series of baths that prevent rusting and coat the surfaces with paint, inside and out. The holes allow the liquid treatment to flow in and drain out. Because the inside is not exposed to environmental scratches, it is more durable than the outside of the component.

 

[Ranger Mike]

Holes in components mean a lighter part weight.

Automobiles before the 1960 had a stamped and welded iron chassis. The body was set on top of this chassis with rubber pads providing isolation. Bolts were used to secure the chassis. Todays pick up trucks still use this type chassis.
Bare chassis weighs 250 pounds and the convertible model had a heavy X frame section to prevent and or reduce body flexing since you did not have a sedan body connecting laterally to the chassis.
In the early 1960s the manufacturers went to UNI body design that used heavy sheet metal to substitute for the separate chassis. One main reason was weight. These cars were a lot lighter as the mass was reduced. Light weight means more fuel economy as the car required a smaller engine , with less horsepower to move the car. Less power means less fuel required. Holes in components mean a lighter part weight. its all about miles per gallon. Plus it was cheaper and faster to make.