Drilling frame rails?

[Iceni John]

I plan on drilling some 3/8" holes through the lower flanges on my Crown's frame rails, for the hangers for the generator/tanks/batteries.   I just read on a fire truck website that holes should never be drilled into flanges of frame rails, only into their sides  -  the article did not explain why.   My frame rails are about 5/16" thick, and for most of their length are nested (i.e. are about 5/8" combined thickness).   Crown used 90,000 PSI tensile strength steel for their frames and body skeleton;  as far as I know it is not heat-treated per se (unlike some trucks that have specific warnings to not drill or weld on their frames), because there are plenty of welds from the factory on all sides of the frame rails.   The spring hangers are bolted through the sides and the bottoms of the frame rails, so this would suggest it should be OK for me to drill some more holes in the lower flanges.   I would thoroughly debur all holes, and they would not be closer than an inch or so from the vertical sides.

Does anyone here have any ideas on this?   I would hate to find I have weakened some critical part of my bus's understructure  -  however, it is so massively over-built that it probably has plenty of reserve strength (if Isambard Kingdom Brunel had made buses they would probably have been Crowns!).

Thanks, John

[kyle4501]

The flanges of the frame rail is where the strength comes from.

I would be very cautious in where you drill. There are many places where it wouldn't matter, but there are also places where it does. Any hole removes material which increases the stress the remaining metal has to carry. Force divided by cross sectional area = stress.
(Sometimes, you can reduce a stress concentration by removing metal, but that is a different topic.)

If I had to drill thru the flange, I would do it in a low stress area. If you don't know how to determine where those areas are, don't drill. Instead, use existing holes & bolt a bracket to the frame rail that will allow you to put the hole where you need it. (The factory engineers know where the stresses are & how the holes & welds effect the strength of the part.)


Or, you can take your chances & drill away while hoping it is overbuilt enough to handle the extra load you're adding.

[bobofthenorth]

If you are just making hangers why not drill the sides of the frame rails?  That way the bolts will be in shear which is a better design anyway. 

[NJT 5573]

Holes in the side, never in the bottom. Make the holes larger than the bolts. Clamp it togather, (no tight holes). Grade 8 bolts, SAE washers, grade 8 too. Fine thread, no lock washers. Torque wrench if you can.

[Chopper Scott]

Never drill in the rails especially the lower rail. Later

[Iceni John]

Thanks so much for the good advice.   I researched this further, and it seems that holes should only be drilled through the center of the rail's side web, where the stresses should be neutral whether the frame is experiencing positive or negative vertical loads.   I've now come up with a simpler less-expensive and more practical system to hang my tanks:  I will use 1"-wide 1/8"-thick steel strips that are already drilled every inch with suitable holes, and I will give them a quarter-turn twist to allow them to be bolted to the square tubing that will support the loads.   Both 110 gallon fresh water tanks, the 115 gallon grey tank, the poo tank, the house batteries, the propane cylinders and my generator slide mount can all be mounted this way.   Everything will be cross-braced to withstand forces from any direction.   If anything leaks it will just drip onto the ground, and I can keep all my bellybin space for Other Stuff.   This basic design is adaptable, strong, cheap and easily made.

Once again the collective wisdom and experience of this forum's members has saved me from many possible problems.   I learn something valuable each time I read this website.   Thanks again.

[boogiethecat]

And after three busses, I've finally learned that the easy way to put a big (1/2 to 3/4") hole in  the side of a frame rail is NOT with a drill bit!  Instead use a step bit from harbor freight and you'll be amazed at how much easier it is!!!