Liam
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 posted on 12/3/03 at 06:36 PM |
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Aaaaarghh! Bent tube
Hello...
Chassis fully welding is almost complete and giving it a look over I noticed a slight bend in one of the tubes! My chassis is quite different to the
book but its the equivalent of F2 in the book running from the end of the drivers footwell to the rearmost bottom wishbone mount. I guess things have
moved around a little with the welding and this tube has developed a slight bend - it's about 1-2mm higher in the middle than at the ends over its
70-80 cm length. Boo-larks. Questions are:
How much does this matter? From an engineering point of view i can see it's not exactly optimum, but is it a problem?
Or do I need to remove it (which will be a right pain in the arse at one end. Maybe I could remove one end, cut it close to the awkward end, and weld
in a replacement?
Or maybe I could cut it in the middle with a hacksaw and the width of the cut will shorten the tube enough for it to sit straight, and weld it back
together. Hmmmm.
Bit of a git. Almost didn't notice it - kind of wish I hadn't. Any suggestions anybody?
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Liam
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| posted on 12/3/03 at 06:40 PM |
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Ooops posted before completion - and it wont let me edit it (??). Anyway, where was I...oh yeah:
Cheers,
Liam
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Jon Ison
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| posted on 12/3/03 at 07:24 PM |
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or maybe you could just leave it ???
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Metal Hippy™
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| posted on 12/3/03 at 08:00 PM |
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1-2mm sounds pretty minimal in the big scheme of things...
President of the Non-conformist Locost Builders Club. E-mail for details...
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Mark Allanson
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| posted on 12/3/03 at 09:14 PM |
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If you cut it out, the forces making it bend in the first place will be unleashed and you will probably have several bent tubes! You need to find out
what is causing the distortion, a probable cure would be to heat the tube causing the bend to cherry red to relieve the stress, you will probably find
the bent tube will settle back to its original position.
On the other hand, the deflection is not massive, do some cross triangulation measurements or a drop test and see if it is making any appreciable
difference
Personally if it aint broken, dont fix it!
Mark
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Liam
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| posted on 12/3/03 at 10:04 PM |
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Thanks for the thoughts guys. I am leaning in the direction of leaving it cos it's a rather small bend.
Mark - cross triangulation measurements and drop tests sound like things any self respecting engineer should know about - would you mind enlightening
me?
I guess tube 'a' must have pulled the side in a bit shortening the hypotenuse (so to speak) tube 'b' making it bend upwards (or downwards in this
photo cos the chassis is up-side-down. So do ya reckon heating tube 'a' will let the side come back out a bit and relieve some of the compressive
force in tube 'b'?
Here is a shot attempting to show the bend...
It's a bit hard to tell. I may just leave it alone. Hmmm...
Liam
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Viper
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| posted on 12/3/03 at 10:19 PM |
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Looking at your last pic i can't see that any welds you have done could have that effect, i am not being funny but was the tube straight before you
fitted it, i know a lot of tube like this is not allways straight, or perhaps something was dropped on it or even the chassis dropped???
just a thought..
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stephen_gusterson
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| posted on 12/3/03 at 10:37 PM |
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Liam
If a tiny deflection like that is worring you, you are in for a mental breakdown before you finish your build.
go to TOL
and search for posts by Ken Walton. Ken built many cars in his life, and raced em. His philosophy to chassis building will make you feel better and
more confident.
atb
steve
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fastenuff
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| posted on 13/3/03 at 06:31 PM |
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if you want a quick result type "banana" bananashape"
Ingmar
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Liam
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| posted on 13/3/03 at 07:10 PM |
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Oh righty ho - maybe I should consider myself lucky then! At least my chassis still sits flat on it's base. Why on earth would you fully weld the
thing before putting any triangulation in it? Surely you can expect it to distort if you do that? Oh well.
Cheers for the reassurement guys.
Liam
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Mark Allanson
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| posted on 13/3/03 at 07:21 PM |
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Drop checks and cross triangluation
A drop check is what bodyshops used to do before chassis jigs were invented (or cheap enough to be viable!). Use a plumbline to mark on the floor
several reference points on the chassis, use chalked string pulled very taught across the marks on the floor and then twang the string. This leaves a
fine chalk line on the floor. Any distortion will be picked up by a lack of symetry in the chalk lines.
Cross triangulation is just measuring all the diagonals to make sure they are the same, I used a 3m length of 1" RHS marked with a scriber line at
one end to check that I had no distortion. I used B1 as a datum and checked that every joint was the same distance from its opposite end of B1, it
works in 3 dimentions as well.
It is imposible to tell from an image, but the length of the weld at the front of the tube is twice the width of the tube. If you have put more amps
into the upper or lower weld, or ground off more from the top than the bottom, this will pull a twist which may exhibit itself as a beld. If it
worrying you, hacksaw through the weld and see which way the tube jumps - if it doesn't, you just have a bent tube!
Mark
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Stu16v
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| posted on 14/3/03 at 06:58 PM |
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And remember, the chassis is only there to locate the suspension points. As long as these are in the right place, it's not important.
Cheers, Stu.
Dont just build it.....make it!
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stephen_gusterson
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| posted on 15/3/03 at 12:01 AM |
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it stops your bum from dragging on the floor too!
 
atb
steve
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