andyd
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| posted on 13/9/05 at 10:10 AM |
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Tube dimension query
Would anyone advise against using 25mm x 25mm x 2mm square tube for a chassis?
Reason being that my local steel stockist doesn't stock 1.6mm, nor do they do 1" x 1". I can see that per metre length the 2mm wall
thickness will be 0.3kg (roughly) heavier. Does anyone actually use the imperial sized stock anyway or do you use 25mm stuff instead?
Any comments most welcome.
Andy
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Locost?
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| posted on 13/9/05 at 10:20 AM |
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Hi
I used 25x25x2 mm for chassis. It is a bit heavier but that shouldn't be a problem if you arn't going racing. 2mm thickness means it is
less likely that you will burn holes in it if you are new to welding. One thing that I would suggest though is that you try to get the shiny rhs
rather than hot rolled with all the mill scale and grease on it. The shiny stuff (don't know what the technical term for it is) is easier to
prep for painting, but I assume it will rust quicker if like me you take a couple of years completing the chassis 
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craig1410
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| posted on 13/9/05 at 11:43 AM |
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Andy,
I too used the 2mm thick stuff and I got on just fine. Okay it's heavier but not by that much and as mentioned, is easier to weld and should be
marginally stronger. I didn't use the shiny stuff and didn't have too many problems painting it. I used etch primer after keying and
degreasing the tubes.
One other thing - make sure you clean the tube at all weld sites before welding. You'd be amazed how much difference it makes to the quality of
the weld. A bit of 80 grit emery works well or a flap wheel or rotary wire brush if you are doing lots of weld sites.
Cheers,
Craig.
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britishtrident
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| posted on 13/9/05 at 11:44 AM |
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1.6 mm is the stuff to use if for no other reason it is a lot easier to cut and weld.
2 mm tube takes a lot longer to cut and also with 2mm tube you start running into penetration issues with welds so your welding has to be just a bit
better.
General steel stockholders don't carry 1.5/1.6 you need to go to a tube stockholder such as your nearest branch of Tyco tubes.
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craig1410
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| posted on 13/9/05 at 11:56 AM |
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quote:
Originally posted by britishtrident
2 mm tube takes a lot longer to cut
I cut all my tubes by hand with good quality hacksaw blades and hardly broke a sweat. How much easier can 1.6mm tubing be to cut? I never had any
problems with weld penetration either and yes I did cut open some of my sample welds to check this.
Cheers,
Craig.
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kb58
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| posted on 13/9/05 at 02:04 PM |
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Technically it'll be fine, but the chassis will be 25% overweight for no good reason. It'll carry this weight around its entire life
which must be accelerated and decelerated 1000s of time.
Having said that, do you care? If you're only making a fun car for Sunday drives, on the street, great. But if you're making a real
sports car there's no reason to make it heavier. Do you care that when you take off from a light next to a Lotus, and you lose by an inch, that
it's because of that weight? 25% of the total frame weight isn't a lot, but weight is the enemy of perforance for a sports car, and I
know it would always bug me that the car is needlessly overweight.
[Edited on 9/13/05 by kb58]
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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VinceGledhill
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| posted on 13/9/05 at 02:18 PM |
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Take a look on Yell.com for metal supermarkets. They supplied all my steel for about 60 quid. They do the right stuff too.
I wouldn't make it heavier for any reason. Can't see the point. Metal supermarkets will deliver it too if you can't collect.
Regards
Vince Gledhill
Time Served Auto Electrician
Lucas Leeds 1979-1983
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iank
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| posted on 13/9/05 at 02:42 PM |
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Metal supermarkets
http://www.metalsupermarkets.com/
they have a product list and store finder on their site, including getting a quote from your local outlet. I've heard you pay a little extra
for the convenience (delivery, cutting to size, no minimum order) compared to more 'traditional' suppliers.
I do wonder what the guy on the frontpage is going to do with that tube though 
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andyd
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| posted on 13/9/05 at 03:39 PM |
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Thanks guys for all the comments. Yes, the extra weight would bug me as I'm going to be building a mid-engined bike powered car. I'm
looking to make something very similar to an Elise so would like it to be as light as possible without it being flimsy.
Unfortunately the nearest "Metal Supermarket" is in London (about 80 miles away). Even if they deliver I'd bet it'll cost way
more. I've emailed them anyway to ask costs.
I may have to try a few more local (but further afield) places to try to find 1.6mm walled RHS.
Can I assume that 25mm x 25mm would be ok as opposed to 1" x 1"? After all the 0.4mm difference would make the entire chassis lighter!
[Edited on 13/9/2005 by andyd]
Andy
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kb58
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| posted on 13/9/05 at 03:49 PM |
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For a bike-engine car, light weight is extra important because of the low torque output of those engines.
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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pbura
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| posted on 13/9/05 at 04:16 PM |
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If anything, the weight-conscious builder might want to consider 18 gauge (1.2 mm) for cross-bracing and other areas that won't be subject to
bending. I've also wondered if 18 gauge 32mm tube might not work out better for the main tubes (more stiffness at about the same weight). I
believe this is what the US Stalker V6 has.
Still wouldn't want to weld a chassis bracket to anything less than 16 gauge, but that's not a lot of tubes.
Pete
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craig1410
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| posted on 13/9/05 at 07:33 PM |
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quote:
Originally posted by kb58
Technically it'll be fine, but the chassis will be 25% overweight for no good reason.
Just to put this in perspective, we are only talking about something like 25% of 75Kg's which is less than 20Kg's of excess. This would
represent less than 3% of the weight of a typical 650Kg completed vehicle. Unless you are going ultra light in every other area including
wheels/tyres, floor, interior, seats and running gear then it won't make a blind bit of difference.
Of course if you can get hold of the "right stuff" then all well and good but don't delay starting your project for a minor detail
such as this - you will find many more important details to keep you scratching your head later on!!!
Cheers,
Craig.
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kb58
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| posted on 13/9/05 at 08:22 PM |
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True, which is why I said it's only important if he cares.
About the weight not mattering, that line of reasoning is why cars often end up being so much heavier then they could be. If 25% in the weight of the
chassis isn't a big deal, then buying wheels that are "only a little" heavier then light ones doesn't matter either. And
larger tires won't really matter because it's just a little heavier. Sure a small light battery could have been used, but this larger one
is only 5lbs more, right?
See where this ends up? Once the decipline of always looking to cut weight is ignored, it opens the door to design laziness, and the entire car ends
up weighing at least 10% more then it could. It does matter, at least to me. Building a heavy car would be embarrassing, but that's just me.
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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andyd
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| posted on 13/9/05 at 09:11 PM |
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Ok well I do care about the weight because even though my preferred engine is the Hayabusa, I still want it to be as lightweight as I can without it
being unsafe.
So what about making the majority of the chassis from round tube? 19mm 16swg round weighs about 64% of 25mm 16swg square section. Would a chassis
consisting of mainly round tube be as rigid as one of square? The suspension mount points would be square section however in order to facilitate a
stronger connection.
[Edited on 13/9/2005 by andyd]
Andy
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JoelP
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| posted on 13/9/05 at 09:18 PM |
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you have to balance it. Spend some time looking at a model, and work out which members are stressed, and which are less stressed. Then pick tube sizes
to suit. You are looking minimise weight, whilst maintaining both normal use strength (ie against twisting etc) and abnormal use strength (ie, ramming
a tree sideways).
Balsa models do help!
If you dont feel able to correctly analise the chassis, just play it safe and stick to book plans.
Beware! Bourettes is binfectious.
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Stu16v
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| posted on 13/9/05 at 09:32 PM |
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quote:
Originally posted by JoelP
If you dont feel able to correctly analise the chassis,
Sounds painful...
Dont just build it.....make it!
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JoelP
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| posted on 13/9/05 at 09:57 PM |
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not as bad as anal lysis! (lysis meaning splitting )
Beware! Bourettes is binfectious.
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craig1410
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| posted on 13/9/05 at 11:14 PM |
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quote:
Originally posted by kb58
True, which is why I said it's only important if he cares.
About the weight not mattering, that line of reasoning is why cars often end up being so much heavier then they could be. If 25% in the weight of the
chassis isn't a big deal, then buying wheels that are "only a little" heavier then light ones doesn't matter either. And
larger tires won't really matter because it's just a little heavier. Sure a small light battery could have been used, but this larger one
is only 5lbs more, right?
See where this ends up? Once the decipline of always looking to cut weight is ignored, it opens the door to design laziness, and the entire car ends
up weighing at least 10% more then it could. It does matter, at least to me. Building a heavy car would be embarrassing, but that's just me.
That's exactly the point I was making when I said that chassis weight only mattered IF he was planning to keep all the other components light
weight as well.
There is a downside to always being on the lookout for some weight saving but you might only realise it when you experience a chassis failure. There
has been much discussion in the past about how much safety margin exists within the "book" chassis design and I think the general
consensus was that there wasn't much. So much so that dropping from 1.6mm to 1.5mm tubing (again due to what that particular person had to hand)
was not recommended. I could have got 1.5mm shiny tubing in 25x25mm but chose to go to 2mm to avoid a potential compromise to my chassis integrity.
What I might have done if I did it all again is to build a "crumple zone" at the front from 1.5mm wall tubing back as far as the ends of
the "G" tubes and then use 2mm tubing everywhere else.
In general though I agree that if you are going to use a bike engine then mass is the enemy whereas I am using a V8 so I couldn't care less as
it will scare me $h1tless no matter if it weighs 400 Kg's or 700Kg's...
Cheers,
Craig.
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britishtrident
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| posted on 14/9/05 at 07:39 AM |
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Lotus used 1.2 mm for the chassis on all its spaceframes which were a lot sparser than the book chassis-- so the difference between 1.6 and 1.5
isn't going to errode the margins of the book chassis so much it is going to fail in fact just about every Locost or Locostlike car on the road
uses 1.5mm and how many chassis failures have we heard of ? Pretty remarkable in view of the crazy engines some guys fit and the DIY welding.
My chassis uses 1.5mm but has some 1.2, 2.0 and one bit of 2.5 for the seat belt mounts.
The danger of using thicker tube throughout is it can introduce a mind set that confuses strength with stiffness. The whole concept of a Seven style
cars is that they are minimalist, so lets leave the heavy metal for the the Cobra guys.
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timf
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| posted on 14/9/05 at 02:19 PM |
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quote:
Originally posted by andyd
Unfortunately the nearest "Metal Supermarket" is in London (about 80 miles away). Even if they deliver I'd bet it'll cost way
more. I've emailed them anyway to ask costs.
I may have to try a few more local (but further afield) places to try to find 1.6mm walled RHS.
Can I assume that 25mm x 25mm would be ok as opposed to 1" x 1"? After all the 0.4mm difference would make the entire chassis lighter!
[Edited on 13/9/2005 by andyd]
try isg steel in ipswich
they stock 1"x1" 16 and will deliver any quantity for £15
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mark chandler
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| posted on 14/9/05 at 05:40 PM |
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Andy,
I got my steel from www.isg-steel.co.uk
Costs as follows:
6m lengths:
1" erw 16swg round tube £6.44
1"x1" erw 16swg square tube £6.85
sheet
Steel 2m x 1m x 1mm £15.56
Ali 2.5m x 1.25m x 1.2mm £37.19
Delivery £15
+ VAT
And they are near you, sheet came on a pallet and its all nice and shiny, not a bit of old tat from the yard covered in surface rust.
Regards Mark
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andyd
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| posted on 14/9/05 at 11:08 PM |
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Good info guys, I'll give ISG a go.
Mark, what quantities did you get and how long ago was that the price? I've been quoted £18.50 + VAT for one single 7.5m 25mm x 25mm x 2mm and
the price dropping to about £9.50 for 10 or more lengths. I can't see that 1" x 1" 16swg in 6m would be that much cheaper to
produce so I'm thinking the prices I've got are way too high.
Andy
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