just say i was making a custom crossmember and lower control arms for a 1200, what sorta steel would be a good idea?

tube or RHS?

they both need webbing over the joins, they both require the same wall thickness and the only difference i can pick is that tube comes in a lot of different materials (chromemoly, alloy, mild steel, aluminium) and RHS has a limited selection (mild steel and alum only that i've found).

is there a benefit to using tube over RHS or vice versa? i've seen a lot of custom drag rear ends use both with success on both fronts.

It depends a lot on what the car will be used for. RHS/Pipe is generally more heavy walled than tube and can add weight to a car unnecessarily. A round tube is tortionally stiffer than a square section tube. Chrome Moly can be thinner walled and lighter without sacrificing strength. Though it can be more difficult to weld. For a crossmember I would probably use mild steel but fabricate it in pieces folded from sheet. It really comes down to how much your willing to pay for the materials. My racecar has Stainless steel tube bottom arms and it's only cause it looks good polished and theres heaps of it lying around at work. If the car is to be steet driven it would be wise to add .5 to 1mm more in the wall thickness for durability. Racecars get caned but don't run up the mileages like a road car. If you look at the thickness of material in a 1200 crossmember you'll see that it doen't have to be thick to be strong.

yeah i understand the thickness vs stength. joins and lengths just have to be ribbed / webbed correctly and the load is spread evenly instead of concentrated. like you said, it doesn't have to be heavy to be strong.

so tube is stronger torsionally than RHS? they would both (to me) seem to be equal - SHS (square hollow section) wouldn't be too great but i thought RHS (rectangular hollow section) would be...?

Stirlingmac is spot onn. It will depend on the exact design of your parts as to what materials you use. Chrome Moly is the way to go if you can justify the cost and you are good with a TIG and have other machine tools available to use - tube notchers etc.

yeah that's my biggest headache right there - tube notchers. the design is to be as lightweight and as space saving as possible, so i think stirlingmac has it with chromemoly tubing. are tube notcher's big machines or are they like an attachment that fits a drill press or something?

it's not hard to custom make something like this with the crossmember out of the car, just bolt the mounts in place and make a "jig" of the mounting points and measure your clearances. the trouble is, there's so many options of materials to make it out of i really have to decide what i'm making it out of first before i can go any further.

thanks for your help fellas

hey 240,
to my way of thinking, use both. The crossmember will generally be holding weight in compression with your mounts out near the chassis rails anyway so I'd use SHS or RHS here as it is easier to change direction (for sumps etc) and you have a flat, regular surface for eng mounts etc. Control arms however are subjected to multi directional loads hence a tube is a little better as it has no 'flat' to form a possible weak spot and it is easier to mount your rod ends etc. I'm certainly no expert but that's how I would go. Good to save weight and go lighter and lighter, but without a design program or years of experience I would err on the side of strength - not everyone is disciplined enough to regularly check or crack test light weight components . Good luck with the project !!
Cheers
P.S. You can scallop out for round joins using a grinder or plasma (make join as neat as possible!). Tube notcher as a tool is a little pricey for a 1 off project and a holesaw on the end of a drill will not work as you only ever take half a hole or so each join.

If you have access to a mill or a drill press you are sweet. A tubing notcher is just a hole saw and a vice incorporated. You can do a pretty good job with a vice bolted to the drill table. If you have a mill you can use a similar diameter end mill and machine the tube ends. I made a tubing notcher which I copied from photos in a catalogue. Chrome Moly requires care with the heat that is used on it's welding and may require heat treatment dependant on the grade used.

chromemoly would be TIG territory for sure. i spose you 'could' MIG it, but the extra heat control is worth so much more than the extra initial co$t.

re: the engine mount points - you could always make a flat 'podium' or 'stool' using chromemoly plate and tube up to it.

Gawd, guys, please dont play with chromemoly unless you really know what you're doing.
I dont know what I'm doing, but I still know it has to be preheated, you have to use the right filler rod (definately not a job for the mig) and you have to post heat it.

Chrome moly sounds good, but it's not really a practical thing to do in my opinion, unless MAYBE on a full-on no expense spared race car where the last hundredth of a second was important. As mentioned welding is more difficult than mild steel and the cost is much higher.
But it does sound cool... do it!