What’s in this cross member and can it be drilled

[Quickstep192]

The crossmember that highlighted in red is between the wheels in the front just aft of the fuel tank. It’s clearly a stout structural member. I’m wondering if it would be OK to drill and tap two 5mm holes in it.

Any information is appreciated.

[Quickstep192]

Here’s the picture.

[mepstein]

I wouldn’t drill it. An aluminum cross member is half the weight. I’ve had both on a digital scale.

[john walker's workshop]

Just a steel tube. Two 5mm holes won't hurt it.

[mepstein]

Sorry. I was thinking lightening holes. My bad for skimming over the question. Two small holes won’t hurt it.

[911obgyn]

A hole in a tube less than 1/3 the radius of the tube actually makes it stronger.
From some long gone investigating i did years ago.

[PeteKz]

That does not make any mechanical engineering sense. It may make it "stronger for the weight" but does not make it stronger relative to the original piece. However, to the OP's question, I wouldn't worry about drilling a few holes for attaching something.

[Quickstep192]

I used to work with an engineer who talked about how they designed the wings of fighter aircraft so the wings didn’t break off from the stresses of dogfight maneuvers.

He explained that a series of tiny holes were drilled in the wing right where it met the fuselage. Because,,, nothing ever breaks along a perforation!

[Walt Fricke]

It really makes no sense to drill this part (the catalog callls it the "reinforcing crossmember) for lightening, since its replacement - the aluminum part from later models - bolts right in and is quite a bit lighter.

It should be possible for an engineer with FEI experience or some other relevant skill to calculate what size holes and where in the tubular part could be cut without getting below some minimum reserve of strength. Remember that most of the forces on the front wheels flow through the two 12mm bolts which attach this member to the chassis. Braking forces have the chassis trying to move forward against this piece, while the leverage from the tire contact point try to bend it. Cornering forces would seem mainly to be trying to compress one side and stretch the other.

I rather doubt there is much information from experiments doing this. I once asked a 356 race mechanic how they decided how many holes and where to drill/cut them. He said we cut until the part breaks, then go back and use one with one less hole. This isn't a very appealing approach for most of us. Lots of 356 race cars had a lot of Swiss cheese like holes all over the place, but that knowledge is approaching extinction.

When it comes to lightening a 911, pick weight higher up and farther aft.

[ChrisHamilton]

Originally Posted by Walt Fricke

It really makes no sense to drill this part (the catalog callls it the "reinforcing crossmember) for lightening, since its replacement - the aluminum part from later models - bolts right in and is quite a bit lighter.

It should be possible for an engineer with FEI experience or some other relevant skill to calculate what size holes and where in the tubular part could be cut without getting below some minimum reserve of strength. Remember that most of the forces on the front wheels flow through the two 12mm bolts which attach this member to the chassis. Braking forces have the chassis trying to move forward against this piece, while the leverage from the tire contact point try to bend it. Cornering forces would seem mainly to be trying to compress one side and stretch the other.

I rather doubt there is much information from experiments doing this. I once asked a 356 race mechanic how they decided how many holes and where to drill/cut them. He said we cut until the part breaks, then go back and use one with one less hole. This isn't a very appealing approach for most of us. Lots of 356 race cars had a lot of Swiss cheese like holes all over the place, but that knowledge is approaching extinction.

When it comes to lightening a 911, pick weight higher up and farther aft.

Think the OP was asking about drilling two 5mm holes and tapping them, not lightening the crossmember. Appreciate the good info though.

[Walt Fricke]

Ach - I got confused there also. Perhaps because two 5mm holes toward the middle of that thick steel tube would have no appreciable effect on its strength.