paul the 6th
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| posted on 20/12/08 at 10:55 AM |
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Building board slightly wonky at the back - buggery.
I've just noticed the tube br 12 (most rear bottom rail on the initial chassis foundation part) can pivot slightly across the width of the
building board. If I push down on what will be the offside, the nearside rises by about 2mm. What tolerance is considered acceptable?
Properly pissed off since I took great care and used around 15 metres of timber to brace the thing, and then about another 15 metres to make sure the
whole table frame was strong/sturdy etc...
If it's an ABSOLUTE NO NO, then it's obviously better to take it all to bits and start again. If a few millimetres won't make much
of a difference, what potential issues could there be?
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David Jenkins
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| posted on 20/12/08 at 11:06 AM |
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A few mm is permissible in the finished chassis, and you'd be doing well to achieve that after welding. If you want to check how the
professionals do their work, try taking a tape measure to your tin-top - you'll be surprised at what you find...
It is best to start off with a flat board - but you'd be pushed to get a totally flat surface without moving over to a metal table! You could
try using thin bits of ply under every junction point, which would overcome any irregularity in the board. If you cut the pieces (100mm square) out of
1 sheet of ply then you should get an equal uplift all around the chassis.
[Edited on 20/12/08 by David Jenkins]
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speedyxjs
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| posted on 20/12/08 at 11:09 AM |
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I dont think a few mm will make that much difference but if i were you, i would get it sorted as you are not very far into your build 
How long can i resist the temptation to drop a V8 in?
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speedyxjs
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| posted on 20/12/08 at 11:10 AM |
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quote:
Originally posted by David Jenkins
try taking a tape measure to your tin-top - you'll be surprised at what you find...
Did i read somewhere that the proper mini was about four inches longer on one side?
How long can i resist the temptation to drop a V8 in?
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mr henderson
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| posted on 20/12/08 at 11:11 AM |
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There's no way a wooden table is going to be flat unless deliberate steps are taken to make it flat.
What you need to do is to identify the high and low points and use firm packing (not cardboard which can compress) and rectify it.
Select a straight piece of square tube, unless you have a long spirit level, and use it to check that you now have a flat surface. You will know
immediately if it isn't flat because if the middle bows up the tube will rock, and if it sags in the middle the tube will stay firm
John
PS bear in mind that timber moves in responses to changes in its moisture content, and that will depend on the amount of moisture in the air
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paul the 6th
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| posted on 20/12/08 at 11:15 AM |
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cheers for the replies guys - I know it makes sense to sort it now since I haven't even tacked everything in place. I'll see what I can do
to sort it out
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David Jenkins
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| posted on 20/12/08 at 11:19 AM |
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For future reference - if anyone's going out to buy a sheet of something to build a locost chassis on, try and get blockboard. It's
lengths of square timber glued together side-by-side, with a skin of thin ply on each side.
It's very strong, and reasonably flat (as flat as timber will ever be, anyway). It's also VERY heavy and quite stable.
[Edited on 20/12/08 by David Jenkins]
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Peteff
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| posted on 20/12/08 at 12:53 PM |
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quote:
Originally posted by speedyxjs
Did i read somewhere that the proper mini was about four inches longer on one side?
I don't know if that's true, it sounds a lot but I know the wheelbase on my Mini was 3/4" longer one side than the other.  On a
self build chassis you should get within 3mm easily but the thing I would check is the diagonals. You will be able to correct a lot with the bracket
positioning when you attach your suspension and wishbones.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Bigheppy
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| posted on 20/12/08 at 01:14 PM |
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Also dont forget to check that the tube is straight you'd be suprised how much bend can be in it 
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James
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| posted on 20/12/08 at 01:19 PM |
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My precision got better as the project went on... when it was too late!
If you start with a wonky chassis then it'll only get worse as the project goes on. Best to get is sorted now so you're building on good
foundations IMO.
ATB,
James
------------------------------------------------------------------------------------------------------------
"The fight is won or lost far away from witnesses, behind the lines, in the gym and out there on the road, long before I dance under those lights."
- Muhammad Ali
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iank
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| posted on 20/12/08 at 01:20 PM |
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quote:
Originally posted by speedyxjs
quote:
Originally posted by David Jenkins
try taking a tape measure to your tin-top - you'll be surprised at what you find...
Did i read somewhere that the proper mini was about four inches longer on one side?
That isn't true (certainly not 4 inches - the car is small enough that you'd be able to see it), but IIRC one of the old french things had
a different wheelbase side to side by quite a margin.
I do remember a story from a guy who was installing the robots at jaguar (many years ago) where one of the big problems was that the front wing on one
of their cars was a few mm longer than than the other side. Had never been a problem since the guy that installed them just hit it harder with his
rubber hammer than the other side and it bent into place nicely. Bitch to automate that kind of thing.
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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mark chandler
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| posted on 20/12/08 at 01:28 PM |
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It will move about as you weld it anyway, the secret is not to completely weld each join at once, but keep working your way around and flipping the
frame over. backfilling the gaps. It keeps everything a bit cooler as the heat shrinks the metal.
In the ideal world the top and botton assembles will be a perfect fit either way up.
When you weld in the uprights again it will move, opposite opposite upraights and braces will pull it back to shape.
If you get within 3mm accuracy you will have done very well indeed
Regards Mark
[Edited on 20/12/08 by mark chandler]
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liam.mccaffrey
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| posted on 20/12/08 at 01:49 PM |
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I was lucky enough to be able to build a steel table (from oil storage tank floor plate) with a 3/4" super dense chipboad top.
It was incredibly flat straight and true.
I did however have a tube which was not perfectly aligned. I subsequently had to dismantle the chassis for another reason and I'm sooooooooo
glad I did because I'd be waking up in the middle of the night thinking about that 1 tube.
Make sure its right
[Edited on 20/12/08 by liam.mccaffrey]
[Edited on 20/12/08 by liam.mccaffrey]
Build Blog
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the_fbi
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| posted on 20/12/08 at 01:53 PM |
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FYI I noticed my local Wickes last weekend had an 1.8m spirit level for around £12.
Whilst it isn't locost exactly, at least you've got an engineered straight edge and due to its cross section/profile its not going to bend
too easily.
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Dick Axtell
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| posted on 20/12/08 at 03:00 PM |
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quote:
Originally posted by speedyxjs
quote:
Originally posted by David Jenkins
try taking a tape measure to your tin-top - you'll be surprised at what you find...
Did i read somewhere that the proper mini was about four inches longer on one side?
According to an old college lecturer, (who had served his apprenticeship at Longbridge), they had to allow +/- 3/8 inch adjustment for the old rod
actuated brakes, simply because of tolerance build-up on bodies of that era (early 50's).
Work-in-Progress: Changed to Zetec + T9. Still trying!!
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martinq357
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| posted on 20/12/08 at 07:02 PM |
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The front suspension location points on my 'professional' Tiger Chassis are miles out.
Sounds like you're doing OK at that tolerance, but as you say, it may be easier to cut it out now and redo that piece.
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paul the 6th
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| posted on 20/12/08 at 07:14 PM |
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managed to sort it in the end started to realise that everything is made to a 'tolerance' (even the pro-level stuff) so a mm here and
there isn't a mssive problem, although it's obviously better to get everything as straight and true as possible
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prawnabie
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| posted on 20/12/08 at 07:17 PM |
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I get it sorted purely for the fact that everyone will know you as "the guy with the bent locost" for years to come if you don't!
Shaun
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blueshift
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| posted on 21/12/08 at 07:20 PM |
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A couple of mil is fine as said, another thing to note is that the shape of the chassis isn't critical, it's the suspension pickup points
that define the suspension geometry and handling. You put those on last, so long as you position them carefuly relative to each other (not just
relative to the local chassis positions) you can compensate for some chassis wonk.
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paul the 6th
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| posted on 22/12/08 at 05:22 PM |
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Quality "Chassis Wonk"
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kb58
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| posted on 22/12/08 at 08:03 PM |
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The table matters for the mock-up but once welding starts it cannot possibly hold everything true. Welding heat distortion is incredibly strong so the
key is to alternate welding on the sides of the chassis else it'll warp like crazy.
Mid-engine Locost - http://www.midlana.com
And the book - http://www.lulu.com/shop/kurt-bilinski/midlana/paperback/product-21330662.html
Kimini - a tube-frame, carbon shell, Honda Prelude VTEC mid-engine Mini: http://www.kimini.com
And its book -
http://www.lulu.com/shop/kurt-bilinski/kimini-how-to-design-and-build-a-mid-engine-sports-car-from-scratch/paperback/product-4858803.html
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