Hazards of weakening the frame by drilling holes?

[Scargo]

I notice in street and some R versions that people are drilling holes in the frame to mount thing with. Like 3/8" to 1/2" for seat belt/harness mounts and around 3/8" for nut-serts.
Isn't it weakening the frame significantly to drill large diameter holes in it?

Yes, I know it depends on multiple factors, like where, what plane and whether it's a major or minor structural member. Then, I guess number of holes in a single member can add up. I plan on using every opportunity to add brackets, via welding, rather than drill into the frame anywhere, but I'm building a race car. I realize this is problematic for those with painted frames and no real fabrication/welding capability.
How do you know what kind of overbuilding is done by FFR, that allows for big holes?
Is is SOP and nobody ends up with cracked frames?
Is there any rule of thumb for an FFR kit car?
I see the main frame, including roll cage is made of 1020 and I'm assuming the smaller square tubing is "mild carbon steel tubing" (whatever that is...).

[2FAST4U]

There is no record of any FFR frame cracking, breaking, deforming, or otherwise failing due to holes drilled

[Mechie3]

Here's my take on it:

1: It depends on the ratio of the hole to the size of the member. A 1/2" hold on a 1" member shouldn't cause problems. A 1/2" hole on a tube that has a 1/2" ID (thus leaving just the walls) could potentially cause problems.

2: Depends on loading. Drilling a hole vertically (like the seat rails/seat mounting) shouldn't cause many problems. In example 1, if a tube is loaded in bending vertically with a vertical hole, then the wide walls are still there providing the most support. Polar moment of inertia is 1/3 * base *height^3. The height adds most of the rigidity, not the top and bottom plates that were removed in that one section with a hole. If you're bending horizontally with a vertical hole (ie, bending about the axis of the hole) then you might have issues. You've effectively removed all of the "h" term from the rigidity equation and are left with a much weaker point.

3: Depends on the number of holes in a row and how many other structural members

4: Depends on the strength of the tube vs how much load it must carry and the factor of safety. I would hope there is a decent factor of safety built into the car where a couple holes won't affect anything.

That said, I wasn't a fan of drilling to mount the seats. The tubes do seem a little thicker there, and the loading is mostly out of plane with the axis of the drilled hole, but for something so critical it makes me a little nervous. Some of the tubes are thin, like the center tunnel tubes. When I mounted my shifter I was actually able to start to crush the tubes just by tightening the bolts. The ratio of hole to frame member size and the thickness of the metal was too thin.

[Bob Cowan]

For racing tech inspection, you should do as you plan - weld on tabs. According to Hank Padilla at Hanks Hot Rods, drilling holes in the roll cage is generally not allowed. If you look at a lot of roll cages, you'll see a 1/8" hole drilled in a roll hoop somewhere. That's to check wall thickness.

[C.Plavan]

Dont drill into the round bars. The square bars are OK as long as you are not cheese grating it.

[ssssly]

If you were just drilling the whole and leaving it there it would be one thing. But remember, you are going to almost completely fill that hole with a bolt, that you are going to torque into the surface.

Generally, drilling a hole up to half the width of a square tube will cut the strength by approximately 1/3. Over half the width 2/3. Filling it back up with a bolt, will get you back to 80-90% as long as it's less than 75ish% width of that side of the beam.

However all of this assumes center of flat side. Off center holes exaggerate the loss of strength, and extending a hole to two adjacent sides through a corner, will destroy all structural strength.

Moral of the story, drill in the center and make the holes just big enought for whatever bolt is going through it and you will not see any appreciable decrease in strength.

[Scargo]

2FAST4U You seem to have built a number of FFR kits. What is your approach to mounting things to the frame?
What can you share/suggest? BTW, I cannot find anything on the internet about any failures either.

ssssly: You sound a lot like an engineer (as in, you seem to speak with authority). With my engineering background, and a lot learned by osmosis, I find your claims hard to believe. If a hole were supported internally, as with a sleeve welded in, then you could get some strength by compression. Without anything but washers it is hard to see the tensile going up dramatically. The square tubing will begin to crush eventually and that would create curved surfaces and less contact area, so you only have limited friction or stiction to be gained by a bolt with washers (as I see it). How else does one use a bolt contribute to strength? Reduced flexing?
Show me the money!

[Wayne Presley]

Or weld in anti crush sleeves

[DodgyTim]

A lot depends on the hole location and the direction of the forces applied


As an example a square tube laid horizontally across two supports, then loaded from above~
The top surface is placed in compression
The bottom surface is under tension
Halfway up the sidewall is the neutral axis, where effectively there isn't any stress
A reasonably small hole drilled horizontally across between the neutral axis's won't make much difference to overall strength, and is unlikely to be a point where stress cracks start, if the load is cyclical
A hole drilled vertically goes through both the high compression and high tension zones, so is more likely to reduce strength and cause a stress raiser.

Best case would be to correctly weld on tabs with adequate width so that the load doesn't cause a stress raiser, or to weld in crush tubes as Wayne suggests.

Loose crush tubes and flat washers/plates rely on bolt torquing to distribute the load, and can work well provided the surfaces have a high friction coefficient, and the crush tubes are cut correctly.

[Jeff Kleiner]

Or weld in anti crush sleeves

Wayne beat me to it but that is my suggestion also.

Jeff

[DMC7492]

Wayne has the answer!, weld in sleeves, aircraft use this method to maintain strength. No crushing of the tube and bolt torque can be achieved without deformation.

[billjr212]

any tips from the experts on best technique for welding in a crush sleeve?

This was my initial thought, **but it is based on exactly zero experience**:
1. drill hole all the way through frame exact size of bolt to be used
2. drill 1 side out wide enough to slip in crush sleeve (ideally the back side, from which any force would be applied)
3. sleeve length should be ID of frame + thickness of 1 wall of frame (so it would sit against the inner wall on 1 side and flush with outer on the other side)
4. fully weld sleeve on the side with the larger hole
5. tack weld on side with smaller hole
6. redrill through crush sleeve in the event there is any welding splatter to ensure bolt will slide through
7. grind welds flush with frame as needed.

Thanks!

[Scargo]

There's obviously the holes for riveting on panels (that are commonplace). Any issue with them being too close together, in a straight line, or too close to a corner or joint, as far as weakening or providing a place for crack propagation? I am seeing people pop in 1/4" holes for holding wiring, brake lines and such just about anywhere. This is not an issue regardless of where it is?
I guess this is an area where I would respect the observations of someone who has done this to a race car and after a season it had no cracks or failures. I would lean towards what the flyboys do and is considered best practices, but I don't know what all those are.

[FFRSpec72]

There's obviously the holes for riveting on panels (that are commonplace). Any issue with them being too close together, in a straight line, or too close to a corner or joint, as far as weakening or providing a place for crack propagation? I am seeing people pop in 1/4" holes for holding wiring, brake lines and such just about anywhere. This is not an issue regardless of where it is?
I guess this is an area where I would respect the observations of someone who has done this to a race car and after a season it had no cracks or failures. I would lean towards what the flyboys do and is considered best practices, but I don't know what all those are.

So I have built a FFR challenge car from ground up that has been on the track since 2005 and runs 25+ races on very rough tracks per season, no issues with holes for rivets, wires looms, brackets, etc. I don'[t have any holes in my cage area, that I stay away from. So what I do to my 818R is taken from my challenge car experience and what works and what does not.