AdamR
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| posted on 22/12/04 at 07:52 PM |
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mix & match tube thicknesses
I'm all set to start work on my chassis over the holiday period, but I've now got my hands on some extra steel that throws my design up in
the air.
The bulk of the chassis will be made from fairly standard 2mm thick 25mm box section. However, I've read a lot recently about replacing some of
the main tubes with 1.5" x 1" RHS (or other similar dimensions) to add extra strength where heavy engines will be used such as my V8.
To this end, what I now have in addition to a big pile of square section is some 25mm x 50mm, both in 1.5mm thick and 3mm thick derivatives. If I had
2mm thick stuff then I'd have no problems using it for tubes C, E, K1, K2, N1, N2, J1, J2, G1, G2 and R as per the Cymtriks 'heavy
engine' mods, as it would be the same thickness as the rest of the tubes and the fact it is 1/2" bigger section than Cymtriks suggests
would only add to the strength.
However, I'm not sure whether either the 1.5 or 3mm stuff would be suitable for this, bearing in mind the rest of the chassis will be built from
the 2mm thick square stuff.
Would the 1.5mm be too weak (it's actually described as 18 guage on the receipt but it was sold to me as 1.5mm)? Would using mismatched tube
thicknesses in the chassis be a bad idea from a structural point of view if I went for the 3mm stuff?
Any input much appreciated!
Cheers,
Adam
PS. The extra steel was an impulse buy and I got rather ripped off on it (let's just say that next time I'm asked "do you want to
check the prices before I cut it for you" I'll have a different answer) so I'd quite like to use it if possible - but obviously not
if the result is a dodgy chassis design! The alternative is to build the whole thing out of the 2mm thick square section as originally planned.
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britishtrident
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| posted on 22/12/04 at 08:55 PM |
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The book chasis is designed around 25x25 16 swg that is 1.6 mm or the now more commonly found 1.5mm . This thickness is fine for a lot of the
chassis even with a big engine --- 2 mm thickness can be put to good use is some parts as per Cymtriks ---- but don't take it too far, 3 mm
thickness is just too much.
18 swg is 1.2 mm wall thickness --- although a handful of tubes on my own chassis are 18 swg I would not use as the general scantling size for he
chassis even though this was the thickness used on the original Lotus 7.
Apart from strength issues 1.6mm is the easiest size to cut and weld ---- 1.2 needs a care and very fine pitch hacksaw blade to cut cleanly and
cutting 2 mm is very hard work without a chop saw.
With thicker material the problem of cracks and lack of penetration in welds has to be taken more seriously --- with 1.6 mm it is quite easy to tell
if a weld is a good weld by just looking at it.
When welding 1.5 to 2 mm it is tricker to get a good weld than same thickness wellding but it can be done.
The Rover V8 isn't much heavier than a Pinto nor is it massively powerful unless have one of the bigger version or have loads demon tweaks so
don't build a tank . While strength is important around direct loads such as the engine mounts think more interms of adding extra stiffness
than strength and the best way to add stiffness is extra tubes bracing tubes to triangulate the chassis.
Using 25x30mm mixed with 25x25mm requires planning --- for example welding 25x25mm to the 30mm side of 25x30mm isn't going to be as
strong/stiff as 25x25 welded to 25x25.
It will also involve a rethink of a lot of the book dimensions and mitre cuts.
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AdamR
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| posted on 22/12/04 at 09:30 PM |
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quote:
Originally posted by britishtrident
18 swg is 1.2 mm wall thickness ---
Thanks for clarifying that. Measuring the thickness of my new tube, it is definitely more than 1.2, and looks on the thick side of 1.5 (although this
was done with a tape measure rather than a caliper guage!), so looks like my receipt is in the wrong rather than the steel.
quote:
Originally posted by britishtrident
Apart from strength issues 1.6mm is the easiest size to cut and weld ---- 1.2 needs a care and very fine pitch hacksaw blade to cut cleanly and
cutting 2 mm is very hard work without a chop saw.
Amazing how a couple of tenths of a mm can make such a difference. Luckily I have both hacksaw and chopsaw options available.
quote:
Originally posted by britishtrident
While strength is important around direct loads such as the engine mounts think more interms of adding extra stiffness than strength and the best way
to add stiffness is extra tubes bracing tubes to triangulate the chassis.
Agreed, and extra triangualtion/bracing of engine mounts is all in the plan, as per Cymriks' mods. But he does also suggest using the
rectangular tubes for the tubes I listed above.
quote:
Originally posted by britishtrident
Using 25x30mm mixed with 25x25mm requires planning --- for example welding 25x25mm to the 30mm side of 25x30mm isn't going to be as
strong/stiff as 25x25 welded to 25x25.
It will also involve a rethink of a lot of the book dimensions and mitre cuts.
Regarding the strength of the joint, my gut feeling is that I agree, but exactly why should it be any weaker? Probably something to do with stress
risers, or some other structural issue that I have a less than limited understanding of!
As for the mitre cuts, thankfully they are no different to standard for the tubes that I mentioned, as they are all positioned vertically 'on
end'. The only knock on effect of using rectangualr section instead of square is that the verticles (H tubes etc) will have to be a bit shorter
to compensate.
Cheers,
Adam
[Edited on 22/12/04 by AdamR]
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Mark Allanson
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| posted on 22/12/04 at 09:43 PM |
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Quote:
"Using 25x30mm mixed with 25x25mm requires planning --- for example welding 25x25mm to the 30mm side of 25x30mm isn't going to be as
strong/stiff as 25x25 welded to 25x25."
Why?
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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Rorty
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| posted on 23/12/04 at 05:12 AM |
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quote:
Originally posted by AdamR
....to add extra strength where heavy engines will be used such as my V8. quote:
The RV8 is actually lighter than the Pinto engine.
Cheers, Rorty.
"Faster than a speeding Pullet".
PLEASE DON'T U2U ME IF YOU WANT A QUICK RESPONSE. TRY EMAILING ME INSTEAD!
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britishtrident
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| posted on 23/12/04 at 07:38 AM |
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quote:
Originally posted by Mark Allanson
Quote:
"Using 25x30mm mixed with 25x25mm requires planning --- for example welding 25x25mm to the 30mm side of 25x30mm isn't going to be as
strong/stiff as 25x25 welded to 25x25."
Why?
Think of a "T" joint --- if both tubes are 25x25 the walls of both tubes line, but if the cross bar of the the T is 30mm at best
only one wall of the 25mm tube will line up.
Less of a problem with 2mm wall thickness but with 1.5 mm its weaker and more likely to fatigue.
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Mark Allanson
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| posted on 23/12/04 at 05:08 PM |
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Nice theory, but a little off the mark
If you T joint 25x25 onto 25x30 with one side aligned, you will have a 5mm land on the nonaligned side (less the radius of the corner). This will give
you a nice place to lay down a fillet weld. The welded surface area will be massively greater than the butt weld and hence a great deal stronger.
The only down side is if the joint is constantly subjected to bending forces, it will focus the movement to the aligned, weaker weld and may fatigue
it more.
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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