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.

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

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.

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.

quote:

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Originally posted by britishtrident
2 mm tube takes a lot longer to cut

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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]

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.

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

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]

For a bike-engine car, light weight is extra important because of the low torque output of those engines.

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.

quote:

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Originally posted by kb58
Technically it'll be fine, but the chassis will be 25% overweight for no good reason.

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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.

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]

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.

quote:

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Originally posted by JoelP
If you dont feel able to correctly analise the chassis,

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not as bad as anal lysis! (lysis meaning splitting )

quote:

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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.

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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.

quote:

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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]

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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

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.