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Disaster recovery
The Black Flash - 23/10/12 at 09:42 PM

Hi all.

For reasons I'm not going to go into, persons, who will for now remain nameless, have drilled two BIG holes in each of my lower chassis rails. Big as in, the full width of the box section. On the outside the holes are 21-22mm, on the inside, 11mm. This pic shows one of the holes, looking from outside: (I've had to cut back some of the aluminium bodywork - a pain but at this point, the least of my worries). What this is for, believe it or not, is just to take an M8 bolt.



Pretty bloody terrible I'm sure you'll agree. Obviously this is major damage to the chassis.
Now I have discussed this with Dax who made the chassis, and we have a plan to rectify it, which seems sensible to me. However given the knowledge on here, I figured that it wouldn't hurt to get a few more opinions, or pointers for things to watch out for.

Current plan is to turn up a bushing to fit into each hole; 21mm at one end, 11 at the other. I did one tonight:


Then fit this in the hole, and weld around each end.


OK. That all seems sensible to me. I'm going to be doing some test pieces first, which I will be doing some destructive testing on as I want to be sure. But can anyone else think of anything else I should be doing, checking, watching out for before I do?

Thanks in advance....

Andy

[Edited on 23/10/12 by The Black Flash]

PSpirine - 23/10/12 at 09:55 PM

That bush looks exactly how I'd rectify it. Just make sure that when drilling you didn't thin out the bottom or top surface of the tube - otherwise I'd be inclined to weld a plate over those faces.

It's not the end of the world, particularly if it's not a "lightweight" chassis - there's certainly no reason it should be any less robust than any other bolt mounting through a rail.

britishtrident - 23/10/12 at 09:59 PM

Personally I wouldn't choose weld such a heavy section component to a thin thickness member.

I would reduce the thickness on the larger end of the bush and or chamfer it this should make for a better chance of a good weld .

mcerd1 - 23/10/12 at 10:18 PM

I'd be happy with that as long as you can get a good weld at both sides


where abouts is that on the chassis ?

britishtrident - 23/10/12 at 10:32 PM



This way the difference in material thickess of the material sections being welded isn't so great and there is less of a stress raiser at the weld.

designer - 23/10/12 at 11:26 PM

I would not do that, too much stress, the proper way is to plate it.

cliftyhanger - 24/10/12 at 04:14 AM

I agree re welding the very heavy new bush to the chassis rail, it needs to be closer in terms of thickness.
I would also plate top and bottom, just to be belt and braces. The extra 100grams will not hurt you, but would be reassuring.

Fred W B - 24/10/12 at 05:58 AM

I would prefer to do it properly. Remove the body panel and section in a new piece of tube from node to node.

Cheers

Fred W B

JoelP - 24/10/12 at 06:42 AM

My more slapdash approach would be to either fit the bush if you need to mount a bolt in it, or just weld a plate over it. Its not ideal from a technical point of view but a locost chassis is hardly a highly engineered thing anyway. The difference in material thickness might matter if it was a wishbone but not here.

welderman - 24/10/12 at 06:57 AM

What Fred said

but if its a total PITA then i would plate it

mcerd1 - 24/10/12 at 07:07 AM

quote:
Originally posted by JoelPIts not ideal from a technical point of view but a locost chassis is hardly a highly engineered thing anyway
but this is a dax rush.....

907 - 24/10/12 at 07:33 AM

OP, pic 3, looks just ripe for a TIG weld.

I would however first clean the rust from the inside and the paint off the outside.

I'd probably rig up a back purge then the resulting weld should be a flush butt on
the outside and a clean penetration fillet inside.


I have several M8 threads in my chassis consisting of a !3mm through hole with
12mm bar (threaded M8) TIG welded in both sides.


Cheers,
Paul G

The Black Flash - 24/10/12 at 07:44 AM

quote:
Originally posted by mcerd1
I'd be happy with that as long as you can get a good weld at both sides


where abouts is that on the chassis ?


That one is about 6 inches forwards of the front of the rear arch. The other is directly below the scuttle.

The Black Flash - 24/10/12 at 07:45 AM

quote:
Originally posted by britishtrident


This way the difference in material thickess of the material sections being welded isn't so great and there is less of a stress raiser at the weld.


Now that is exactly the kind of thing I was after, thanks!

The Black Flash - 24/10/12 at 07:49 AM

quote:
Originally posted by Fred W B
I would prefer to do it properly. Remove the body panel and section in a new piece of tube from node to node.

Cheers

Fred W B


Asked Dax about that, and the advice was that that was not necessary. It's certainly something to avoid if at all possible as I'd have to strip the car back down and remake the sides and floors...

The Black Flash - 24/10/12 at 07:54 AM

quote:
Originally posted by designer
I would not do that, too much stress, the proper way is to plate it.


Interested in this. If I plate it, which I have considered, I'll still need a 12mm bushing welding in. So I'll immediately have a 12mm hole in my plate. Or I could do as I've shown above, and put a plate over the whole lot with just the 8mm hole in afterwards.

Edit - just thinking aloud here.
Or I could make up the plate for the outside, and weld the bushing to the inside of it on the bench, so the plate would only have an 8mm hole. Then weld the assembly to the chassis. Would that be better than just a bushing, anyone know?



Thanks to all who've replied BTW, this is really useful.
Oh, and it wasn't me what made the hole as some seem to think. I do have my moments, but nothing this bad, yet.

[Edited on 24/10/12 by The Black Flash]

daviep - 24/10/12 at 08:11 AM

I'd be happy with your original solution or BT's refinemnet, it's maybe not 100% perfect design practice but I'm sure there will be lots of bigger compromises made elsewhere.

Cheers
Davie

p.s it took less than 5 mins to work the why and the who

chillis - 24/10/12 at 09:17 AM

The integrity of the box section has been lost, the only safe way to repair it is either to plate each side or replace that part of the box section, neither of which look like easy jobs.

mcerd1 - 24/10/12 at 09:53 AM

quote:
Originally posted by The Black Flash
quote:
Originally posted by mcerd1
I'd be happy with that as long as you can get a good weld at both sides

where abouts is that on the chassis ?


That one is about 6 inches forwards of the front of the rear arch. The other is directly below the scuttle.

ok I think I've worked out the cause of these holes too....
I've got the std. Dax parts for this, so mine will be mounted to the underside of the floor with big spreader plates instead


taking into consideration some of the other comments, plating the top and bottom flanges would be the belt and braces way to sort it (probably a plate the same thickness as the tube and ~3 times the tube width long [~75mm] would sound about right to me - I'm no expert on chassis, but I do structural steelwork design for a living so I'm assuming the principal is the same....)
but a nicly fitted and welded bushing like BT's one sould be all you need on the sides as long - as the welds are good (might be worth making a test piece so you can cut it and check the weld penatration) TIG welding would best if you can

just watch out that you don't create an overly stiff point in the middle of the tubes, all that does is shifts the problem further along the tube



[Edited on 24/10/2012 by mcerd1]

[Edited on 24/10/2012 by mcerd1]

The Black Flash - 24/10/12 at 10:30 AM

OK, so what about this:



I make a plate with bush welded on (left), then weld onto the chassis rail. (right) The RH side of the bush I'll just weld into the hole, the LH (outside) will have a nice big plate.
If I did this, should I weld only the ends of the plate, or all the way around it?

ETA better pic

[Edited on 24/10/12 by The Black Flash]

sebastiaan - 24/10/12 at 11:12 AM

I'd weld all around the perimeter of the oustide plate if you go for that option. (and deliver a big bill to where it needs to go....)

MikeRJ - 24/10/12 at 11:17 AM

quote:
Originally posted by chillis
The integrity of the box section has been lost, the only safe way to repair it is either to plate each side or replace that part of the box section, neither of which look like easy jobs.


Welding in the bush will restore the integrity of the box section. I agree with BT etc. that the outer flange should be made closer to the wall thickness of the tube though.

Mr Whippy - 24/10/12 at 11:41 AM

tbh your orginal idea with the big boss thing is just fine, I'd put a chamfer round the edge to help with welding but it's well strong enough, your meant to use a crush tube when putting bolts through rails anyway, that's just a mighty fine crush tube. Larger box chassis (landy ones for example) are often full of webs & tubes to help stiffen them up, there's no reason it all has to be the same thickness throughout.

britishtrident - 24/10/12 at 11:56 AM

quote:
Originally posted by The Black Flash
OK, so what about this:



I make a plate with bush welded on (left), then weld onto the chassis rail. (right) The RH side of the bush I'll just weld into the hole, the LH (outside) will have a nice big plate.
If I did this, should I weld only the ends of the plate, or all the way around it?

ETA better pic

[Edited on 24/10/12 by The Black Flash]




Rather than a rectangular plate it would be easier and better to start with a large washer, even a 14mm id standard industrial MS washer should be OK as long as the welds are good but if you want you could start with a large od repair washer and trim it down with an angle grinder after welding.

Because a washer has curve edge where it is welded to the tube it is slightly less likely to fail at the welded edges.

[Edited on 24/10/12 by britishtrident]

britishtrident - 24/10/12 at 12:27 PM

14mm id washer




The Black Flash - 24/10/12 at 02:46 PM

OK, so I'm basically back to the bushing then, which is fine. I can easily chamfer the edges so that's no problem.
The reason I'd like to use a turned bush rather than a simple crush tube + washer is so that I can tap a thread on the inside of the bush. Then I'll bolt straight into that (with locktight) and not have to have nut + bolt sticking into my footwell.

T66 - 24/10/12 at 05:20 PM

Your second idea works better for me.


Personally I would plate all four faces of the chassis, well beyond the hole, however my fabrication is always belt & braces.



Reflecting on it, if you want belt & braces without removing the body, slip over a 3 sided box over the lower of the chassis, get the side plates as high up the hole as you dare, and weld everything.


4x2 box with the top cut off might slip over, and would be simple.

[Edited on 24/10/12 by T66]

coyoteboy - 24/10/12 at 06:37 PM

Personally I'd replace the member and find a better solution rather than supporting something requiring a chuffing great bush mid-span if at all possible.

T66 - 24/10/12 at 07:06 PM

quote:
Originally posted by coyoteboy
Personally I'd replace the member and find a better solution rather than supporting something requiring a chuffing great bush mid-span if at all possible.




The whole member doesnt need replacing, that is a large clean hole in a otherwise clean chassis, if the access for welding is workable, theres no reason why a nice tight fitting U section couldnt be welded ontop. Bear in mind you could have it 30cm either side of the hole if space/shape permits.

coyoteboy - 24/10/12 at 07:25 PM

Doesn't NEED to be, no, but it's a bit of a botch to plug it. I just like a neat clean solution rather than a patch that'll probably take more work to do than doing a direct swap.

[Edited on 24/10/12 by coyoteboy]

T66 - 24/10/12 at 07:56 PM

I dont think my suggestion is a botch, its a solution. By the time its painted most wont notice it. ....




The classic car guys repair far worse than that

coyoteboy - 24/10/12 at 08:25 PM

I do. We disagree. Not the end of the world though, I don't disagree it would work but it introduces lots of issues (vastly different wall thicknesses being different stiffnesses, harder to weld together properly (getting enough heat into the larger part to get good penetration) etc. Plated it'll stand proud so the bodywork around it won't fit as well. Just loads of little reasons I'd replace it fully rather than fix, but I'm a bit perfectionist.

The Black Flash - 24/10/12 at 08:35 PM

quote:
Originally posted by coyoteboy
Doesn't NEED to be, no, but it's a bit of a botch to plug it. I just like a neat clean solution rather than a patch that'll probably take more work to do than doing a direct swap.

[Edited on 24/10/12 by coyoteboy]


Well, the guy who designed the chassis said that a welded bushing would be "quite a nice solution" and that replacing the whole member was really not required.
If it were, I'd do it. But at this point, to do that I'd have to remove the floor, scuttle, and arches, which would all probably come off ok. But then also probably the inside rear panels, which would also mean the rear bulkhead panel, and the inner & outer side body panels. Removing these would be destructive or damaging and I'd probably have to replace some or all of them. I could really do without that scale of setback if possible.

Having trouble to muster the enthusiasm to do any work on it at all at the moment, it's gutting to be chopping bits out

T66 - 24/10/12 at 08:40 PM

quote:
Originally posted by coyoteboy
I do. We disagree. Not the end of the world though, I don't disagree it would work but it introduces lots of issues (vastly different wall thicknesses being different stiffnesses, harder to weld together properly (getting enough heat into the larger part to get good penetration) etc. Plated it'll stand proud so the bodywork around it won't fit as well. Just loads of little reasons I'd replace it fully rather than fix, but I'm a bit perfectionist.



Boring world if we all agreed



but in respect of you being a perfectionist, I am most definitely the opposite

mcerd1 - 25/10/12 at 08:50 AM

quote:
Originally posted by The Black Flash
Well, the guy who designed the chassis said that a welded bushing would be "quite a nice solution" and that replacing the whole member was really not required.

can't argue with that, he's a first class old school engineer

The Black Flash - 25/10/12 at 11:22 AM

Made some test pieces last night from smaller tube, and tested them against normal tube and tube with big holes drilled in it via the extremely unscientific method of a large hammer. I was pleased to confirm that a welded bush doesn't seem to noticably weaken the tube, (unless the weld is crap of course.)

daviep - 25/10/12 at 11:31 AM

quote:
Originally posted by The Black Flash
Made some test pieces last night from smaller tube, and tested them against normal tube and tube with big holes drilled in it via the extremely unscientific method of a large hammer. I was pleased to confirm that a welded bush doesn't seem to noticably weaken the tube, (unless the weld is crap of course.)


Good stuff, looks like you are making are nice job of the build.

Cheers
Davie

p.s. I'm sure some of our more theoretical builders will be along to tell you that your test results are not valid as the problem will be "cyclic stresses" or something else with bigger words.

Minicooper - 25/10/12 at 11:37 AM

Just weld it up, easy, 10 minutes and your done.

Cheers
David

mcerd1 - 25/10/12 at 12:03 PM

quote:
Originally posted by daviep
p.s. I'm sure some of our more theoretical builders will be along to tell you that your test results are not valid as the problem will be "cyclic stresses" or something else with bigger words.

translation: hit it lots of times with a smaller hammer and see what happens

sebastiaan - 25/10/12 at 06:54 PM

quote:
Originally posted by mcerd1
quote:
Originally posted by daviep
p.s. I'm sure some of our more theoretical builders will be along to tell you that your test results are not valid as the problem will be "cyclic stresses" or something else with bigger words.

translation: hit it lots of times with a smaller hammer and see what happens


No, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no, no!

You NEVER use the smaller hammer

On a more serious note; if you are comfortable with the welded in bush solution AND you did a succesful test then go for it. Our contraptions are actually quite crude in design, lots and lots of stress raisers and "incorrect" spaceframe constructions everywhere (well, on a locost at least; I'd assume a DAX is not THAT much better) yet we see very few chassis failures. Just keep an eye on the repair at regular intervals and it'll be fine.

The Black Flash - 25/10/12 at 09:36 PM

quote:
Originally posted by daviep
quote:
Originally posted by The Black Flash
Made some test pieces last night from smaller tube, and tested them against normal tube and tube with big holes drilled in it via the extremely unscientific method of a large hammer. I was pleased to confirm that a welded bush doesn't seem to noticably weaken the tube, (unless the weld is crap of course.)


Good stuff, looks like you are making are nice job of the build.

Cheers
Davie

p.s. I'm sure some of our more theoretical builders will be along to tell you that your test results are not valid as the problem will be "cyclic stresses" or something else with bigger words.



Oh, I'm under no illusions that it's a valid reflection of the real stresses in a frame. But it made me feel better because it seems as resistant to bashing as an undamaged tube, which I can assume will translate to reasonable strength elsewhere. Plus I got to bash something
Off for the weekend, but one side now stripped, so I'll get to welding it next week. Thanks all for the help, if nothing else it's been good to talk about it.