Thread: drilling holes in the chassis

Let me just preface this post by letting you all know I'm not an engineer, I've just been messing about with Inventors "Stress Analysis" tool.

I drew a length of box section, cut some holes in it, and added several thousand Newtons of force, you can see the stress concentrating in points around the top hole:



Cheers,

Originally Posted by uninformed

can an engineer here tell me why a 12mm dia hole in the middle of the 75mm wide chassis bottom flange would be an issue? Isnt there already some there.

The maximum stress on the chassis is on the top and bottom - all the sides do is hold these apart.

A 12mm hole in the top or bottom of a 75mm wide box section reduces the strength at that point by over 15% - whether there is that much strength to spare at that point is a fairly involved calculation. But there is worse - the hole will cause a stress concentration that is likely to start cracking, and a larger hole with less curve may be better - but that loses more strength. There are holes already, put there by the manufacturer - but they not only calculated that it was safe to put the holes where they did, but tested the chassis design to confirm their calcuolations.

John

I agree wholeheartedly with John above. If you consider the chassis rails to act like beams the top and bottom surfaces are considered flanges like those found on I beams and the sides are the webs. As John stated the sides hold the flanges apart and in a position that best resists the calculated compressive and tensile forces. In reality the webs (sides) do contribute to the strength but AFAIK are generally not included in the calcs. So if you drill wholes anywhere (and I would also suggest you don't) the only safe place is in the centre of the sides, right through the neutral axis where there are virtually no compression or tension forces.

The same applies to welding directly across the bottom or top rail, the weld usually doesn't crack but the chassis will right next to the weld, Regards Frank.

while some basic ideas cross over from truck to light vehcile there are also plenty of diffences, ie: material grade, heat treating and chassis shape design.

Welding across the chassis top and bottom cords/flanges may be a no no, but LR have done it in more than one place on my 110 defender.

It certainly is not BEST practice to drill a hole, but will a 12mm hole for drainage lead to chassis failure in real world useage?

The point has been brought up about stress risers, I agree it will induce some, but isnt a radius the best you can ask for. I have seen cracked chassis repairs done and the first thing to do is drill a hole at the end of the crack......

Originally Posted by TimNZ

Let me just preface this post by letting you all know I'm not an engineer, I've just been messing about with Inventors "Stress Analysis" tool.

I drew a length of box section, cut some holes in it, and added several thousand Newtons of force, you can see the stress concentrating in points around the top hole:



Cheers,

good stuff Tim, is this a free programe or website?

The programe is Autodesk Inventor, unfortunatly not free, but I have access to the software at work.

Cheers,

Tim

Originally Posted by uninformed

........
Welding across the chassis top and bottom cords/flanges may be a no no, but LR have done it in more than one place on my 110 defender.

As pointed out above, Landrover were able to calculate the effects of this, and what is more, test the results before committing to production.

It certainly is not BEST practice to drill a hole, but will a 12mm hole for drainage lead to chassis failure in real world useage?

I don't know, and neither do you. And further, you are unlikely to get any engineer to put his signature on an approval to say that he doesn't think it will.

The point has been brought up about stress risers, I agree it will induce some, but isnt a radius the best you can ask for. I have seen cracked chassis repairs done and the first thing to do is drill a hole at the end of the crack......

Drilling a hole at the end of a crack will reduce the stress at the tip of the crack, and reduce the probability of it propagating further - but this is only effective where the crack has propagated into a relatively low stress area, and is normally accompanied by action to strengthen the structure and fix whatever started the crack. And certainly the stress concentration from a 12mm hole will be a lot less than that resulting from a crack, but as pointed out above, it will not only concentrate the stress, but will reduce the strength of the chassis significantly.

John