1/8 or 3/16 wall for A frame??

So Im going to the metal yard to pick up materials for my Model A frame this weekend...I am getting 2x3, but not sure if i should get 1/8 (11ga.) or 3/16 wall... I would think that the 11ga. would suffice, based on the dimensions of a stock frame. Someone told me i should use the 3/16, but it seems way overkill for my goal weight of under 2,000lbs. What are your thoughts? I will be using a factory spec Buick 340 and ST 400, so its not exactly going to be a power-house (220hp, 350 lbs torq.)...

 

1/4 is overkill, 3/16th is what you want.
1/8th isn't even a consideration

 

^^^^^^like fm says.

 

I always use 3/16 because it only costs a little more than 1/8 and it gives so much more wall thickness to drill and tap holes for non structural items like line clamps, etc. Some people do use 1/8 on something like a T bucket, but while you are at it you might as well go bigger. 1/4 is way too thick.

Don

 

Thanks for the advice! I hadn't really given the drill and tap aspect much thought, but that is definitely an added benefit. I read on another thread tonight that 1/8 is plenty thick,and on yet another thread that its too thin.. I kind of get the feeling this may be one of those topics that is never reallt agreed upon...either way I'm going to gusset the hell out of it and make inserts for all my joints, so even if i go 1/8 it should be plenty strong. Im also going to be making the crossmembers out if 3" round... Is 11ga. good here?

 

I have used 11g on 2x4 frame rails but 11g 2x3 is a bit springy and twisty (engineering tech terms) to my liking. For the weight difference I think you'd be better off with the 3/16. Another option is to get one stick of 2x3 for the front and rear and a second stick of 2x4 for the long center section. You can fab up taper transition plates for the 3" to the 4" and make a very pretty frame that's also strong. Here is a couple shots of one for a 31 A coupe we made that actually goes down to 2x2 for the part behind the rear suspension since it just holds sheet metal and not subject to any torque.

 

Mine has 3/16 2x4 and is solid as a rock!

 

It depends some on what kind of bracing you incorporate. Since the frame is the foundation of your entire build, I'm in the bit overkill pot and advise using the 2X3X .188 (3/16). And as Don points out, there are many other small benefits as well. For the small difference in weight, why risk using the 1/8", especially since you are using 2X3.

 

Per what others have said, for 2 x 3 use 3/16. If you were using 2 x 4 1/8 would be reasonable.

 

Mine is 2x3 3/16 zero twist with a hot 289 and still straight after a roll over

 

As you have found out, people all have different opinions of what is right and wrong to use for building a frame. Really, it all boils down to personal preference.

I like 2 x 3 x 3/16 on something like a T bucket size car and 2 x 4 x 3/16 on bigger cars, like a model a, etc. That is just my preference. I have seen 2 x 3 x 1/8 deflect under weight and actually sag to the point where doors will chafe when opening, so we stay away from it and just go with 3/16.

If you want to use 2 x 4 you might consider doing what I did on my rpu frame. I used 2 x 4 but didn't want it to look bulky at the front crossmember so we tapered it down to 3 inches at the very front by cutting a long wedge shaped piece out of it and then rewelding the cut. It was kinda a lot of work cutting that long of a piece on the bandsaw and then welding 4 cuts, each 3 feet long, but it did give a little cleaner profile. Just a thought.

Don

 

On anything with a direct-mount body, in other words no soft body mounts to compensate for twist, just welt, I use .188". With .125", you risk stress-cracks in the body, paint, and or filler.

 

Make sure you've got a welder that'll do 3/16ths too.

 

definitely go the 2x4 with the taper.
Closer to a stock model a frame.
TP

 

Ok, I'm definitely going with the 3/16 wall. It's stronger, more convenient for mounting things to, and only cost an extra 20 bucks compared to the 11ga. Will the 3" round be strong enough in 11ga, or should I go thicker here as well?

 

I used 1/8" for mine, 1 1/2 x 3, but then I used a slightly different design to make up for it.

And the car is not exactly massive.

 

Ok, I'm definitely going with the 3/16 wall. It's stronger, more convenient for mounting things to, and only cost an extra 20 bucks compared to the 11ga. Will the 3" round be strong enough in 11ga, or should I go thicker here as well?
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No, that will be fine. Most of the commercially available round front crossmembers (Total Performance, etc) were 1/8 inch wall.
Don

 

3/16 all the way bro. if you do you'll never regret it, cutting corners to save $$$ will only bite ya in the a$$ in the end. I've always been told the most expensive tool (or part) is the one you have to buy twice.............. and if you don't have time to do it right the first time when will you ever find time to fix it right later. Now .......... on with the pics and updates lol

 

I used 1/4" for my round front tube. Lots of stress in the centre there.

 

Interesting discussion.

 

3/16 is the only option. 1/8 will start to crack around the welds in stress areas down the road.

 

do they sell 2x5 ?

 

I used 2x3x3/16 and had to restructure it after a while do to frame flex while driving. Its sitting on 140" wheel base so it is long compared to the stock wheel base. It depends on what wheel base you want. The longer it is the more flex and stress you are gona have.


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I built the frame for my truck out of 2x4 11GA. with minimal center bracing. I tapered the front rails. I have beat on it with a Blown 327 and 4 speed since 2005 with no problems at all. Drag raced it, Auto crossed it, whatever Its been fine. I am pretty sure its stronger than the stock chassis was.

 

I always go with the heavier steel on light weight street cars.

1. A few extra pounds down low = better ride
2. Pease of mind things will not flex …. leading to cracks and failures.

 

That's my rationale. One too many cracks in bodywork covered by multi-thousand-dollar paint jobs, or worse, the frame itself.

 

I looked up some numbers for metric equivalents:

50x100x3.0 RHS (=2"x4"x⅛") weighs 6.54kg/m; section modulus 20900mm³
50x100x4.5 RHS (=2"x4"x3/16") weighs 9.42kg/m; section modulus 28400mm³

The thicker wall gives 36% better beam strength for 44% more weight. That's a bit less elegant, but that is only considering beam strength. Torsional rigidity is another matter, and that is where we might expect the thicker wall to shine.

Keep in mind that the amount of torsion coming through the suspension from road irregularities and inertial forces can be several times the maximum engine torque, even if the latter is multiplied by a short first gear and a high-stall converter. And the entire wheelbase is subject to suspension torsion, while only the distance from the engine mounts to the rear axle handles engine torque (depending on how the various driveline components are mounted).

 

Cars from the 20s and 30s that did not depend on the body for support (wood frame bodies) had C channel frames 7" deep for that wheelbase, on some cars 8".