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

Good to know!! I will have a 108" wheel base. I went with the 2x3x3/16 and 3"x1/8th round. Now I just gotta go get some new discs for the chop saw and we're in business!! Still need to find a rear end as well... gonna hit up Pick n Pull Saturday for that, they have lots of 70s Ford trucks usually.

 

Pickup trucks. That sheds some light.

How deep is a typical late-model pickup truck frame? They tend to be channels rather than RHS, and seem to be about ⅛" more often than not. And while the pressed steel body and bed assemblies add strength to the respective parts of the frame, there is a break between the two around the middle of the wheelbase, where the frame needs to be strongest. Moreover I'd expect the effective point load at that point to be a lot more than the equivalent of the distributed load represented by a typical hot rod. On the other hand modern pickups have structurally useful overhangs front and rear.

Maximum bending moment varies directly with wheelbase - leaving aside the effects of overhangs. Section modulus varies (generally) with the square of beam depth. For any given distributed load, doubling the wheelbase requires only a 41% increase in beam depth. Halving the wheelbase allows only a 29% reduction in beam depth. Of course it isn't quite this simple but it gives us an idea of proportion.

So, going from 100" wheelbase to 108", you would need 8% more section modulus compared with something known to work at 100" (probably less, because the car isn't going to be 8% heavier.) That means you want roughly √1.08 = 1.039 times the beam depth. If that isn't so deep within the safety factors etc. involved that it pretty much comes down to the same thing, I'd say you've got bigger things to worry about

I think the general consensus is that it's things like joints and screw-fixings and fabrication issues that are going to determine wall thickness, rather than sheer beam strength. The extra 35lbs won't make much difference on a 2000lb car.

 

That seems like a good argument for keeping Henry's original suspension design. The buggy spring/wishbone arrangement directs the torsional suspension loads to the centerline of the chassis, whereas independent designs and coil over shock components direct the load to the sides of the frame, inducing more twist. Seems logical. Is it?

Bob

 

To the center of the front and rear crossmembers and then to the frame thru the crossmember connections.
I think it still pretty much works out to be the same thing as far as forcing the chassis into a twist.

I would GUESS that the big thing for a stiff chassis on an independent car is to simply make the suspension better able to do the things its capable of...eg work independently.
No matter how stiff the frame...straight axles simply can't move one wheel without affecting the other.

Regardless...I'd still much rather the looks of a buggy spring suspension over the marginal improvement in handling (for me) of a full independent setup.

 

1/8" wall is fine, been building chassis in it for years without any problems if you know what your doing.

 

One more consideration- what trans are you using- I had a t bucket frame which would
flex like crazy with a stock chevy clutch- was fine with an auto. Jim

 

Unless the sprung mass is infinite, resulting in infinite inertia, any suspension system will have one wheel affecting the others. When a wheel goes over a bump, part of that motion goes into the spring's compression; the rest goes into moving that corner of the sprung mass upwards. The ratio between the former and the latter depends on the spring's stiffness. So independent suspension isn't really all that independent: a bump on the left will affect camber on the right - due to the entire car moving - moreso if the springs are stiff and less so if the springs are soft.

An old girlfriend of twenty-odd years ago taught me all about good reasons and real reasons. The good reason for ifs was set out in GM's Knee Action print ads in the '30s. The real reason was about improving ride comfort by increasing polar moment of inertia by shoving the mass of the engine into the space between the front wheels. This was not only far less amenable to description in snappy advertising copy, it had the (arguable) disadvantage of much reduced yaw response (which is fortunately not amenable to description in snappy advertising copy either).

 

the guy that built mine did it out of 2x3 1/4, is that bad??

 

Maybe a little heavy, but no harm done. You will have a lot of meat there to tap threads into the frame with.

Don

 

OK...missed the "bold and blue" didn't ya!?!? LOL

 

This is an interesting topic.

I am still in the planning stages of building an extended 29a Model Pickup (I have included a pic of it below). I have been told by the guys down under here that I cannot use the design which was from the USA (chassis). I have been told by them I have to use a Model AA Truck chassis which I have not been able to get 1 in good condition. The next thing is to build 1 from the measurements I have been given. Howver, I have been told by the guys down here that I have to use 1/4 inch plate (6mm) for my chassis. I have got all the materials I need to get my project up any running for the chassis rails only. Can anyone tell me if the Model AA Truck chassis is 1/4 inch thick? Cheers in advance.

Mtw fdu.

 

I have one and I'm fairly certain it's 3/16" at best, if it was 1/4" the thing would weigh a ridiculous amount.

 

I suppose but rereading that paragraph now, the chassis rigidity thing is really about maintaining the proportion or bias of roll stiffness front to rear, and this applies equally whether the suspension is independent or not.

Let's say you want to load an outside-front wheel to counteract an oversteering tendency brought about by other factors, like a rear weight bias, for instance. It would do no good to increase the front spring rate and/or fit a front antiroll bar if the frame weren't rigid enough to maintain the resulting load bias at the front, especially against the centifugal bias at the rear.

If independent suspension does tend to increase the need for rigidity, it is because most practical independent suspensions put the roll centre quite close to the road. Solid axles, depending on how they're located and sprung, are more likely to put the roll centre near the middle of the axle. Total lateral load transfer therefore tends to be more about roll stiffness and less about roll centre height with independent suspension, and vice versa with solid axles. So, with independent suspension you might need more rigidity because you'd need more roll stiffness to maintain any given roll angle at given lateral acceleration.

Roll axes close to - or on - the road have other advantages, however.

 

I have no idea, but chances are the chassis was U channel originally. If you plan to box it, if you are allowed to, then that triples its strength and stiffness.

All this talk about heavy-as-hell chassis material is well and good, but how many guys hang their suicide front end off a small boss or bracket welded into the 16 gauge hairpins? 80 year old hairpins at that.

Have any of them ever snapped or cracked? There are several forces at work, in various directions, but I'm sure those hairpins were never designed to have the weight of the whole front end of a car on them like that.

Just for example, but I've seen far bigger engined cars set-up the same way:

 

This.

 

Great discussion. This is exactly what I was looking for. I'll be ordering 2x4x3/16 tomorrow.

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replacing channel with tubing---wouldn't the tubing be superior in strength to begin with ?

 

According to the TAC guys here "down under", it is not. Makes you wonder sometimes!!!

Mtw fdu.

 

I am using an AA truck chassis and it is 3/16......heavy

 

@Turns What are you building??? It must be a longer than standard wheel base???

3/16 is 4.5mm or 5mm??

Mtw fdu.

 

3/16" is 0.1875"

0.1875" is 4.7625mm

When you are over, round up. 5mm

 

Now I'm completely unsure. Ido have about 60 ft. Of 2X4,.125 I would like to use.
However my thought was to use 1/4" for interior gussets at the weld joints. My thoughts were to use a 2-1-2 plug weld and butt weld at the joint.
Any opinions?

 

Ya know, .120 wall has been industry standard for basic frames for many many years, so I would go ahead and use what you have. That said, the last couple of frames I have done have been out of 3/16" stock at the customers request, and I really have to say that it does help quite a bit flex over the length of the frame. As far as torsional twist, I still advocate a good X-member whenever possible, and would if I was working with .120 wall stuff today.

 

I have actually been told by the SA TAC guys here down under to use 6mm and that is what I have purchased to do the job. 1mm extra over the entire chassis length weight wise is probably an over-kill but will still use it anyway just to keep the TAC guys happy.

Mtw fdu.

 

Little late to the party but I prefer 3/16". HRP

 

Have built other structures out of less-than 1/8" wall rectangular tubing and found these Weld Nuts to be very useful for attaching whatever to the wall....as long as you can get inside the tube. But, they could also work outside the tube if need be; then drill out the tube wall and finish the tapped hole through the wall.




McMaster-Carr catalog page 3189




For inside application, a hole matching the boss is drilled on location in the tube. Then another hole in the tubing just to the side, marked "X" for plug welding......can be the same size hole. Deburr both holes. Fish the Weld Nut into the tube and screw in a short bolt to draw it up tight and hold in position, then weld. Works great.

 

It won't hurt. It is just weight. Best to keep the lawmen happy.

 

If it's boxed .120 wall is totally fine. Use gussets and fish plates and build a suitable k member. My coupe I used .120 tubing. Zero flex. Ultimate rigidity.

 

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This one was 3/16. Went plenty fast for many years...Small block with a 'glide on alcohol