Frame design

[kzoldman]

Posted these pics on my build but have a design question. Because of a front cross member and rear end assy. I already have I built a 90 degree off set in the frame at the rear to make it all work out instead of straight rails. I plan to put an angle brace on the outside of both sides. I may have to notch the cross member as well or even put a U in it to clear the drive shaft later. Also adding a cross member for shocks.

Any thoughts on how strong this will be under flex and will it be strong enough with this off set? Keep in mind I am using 3/16 tubing and not racing.......much

 

[donsrods]

Looks like a pretty long wheelbase.......what are you shooting for? And yes, I think it will flex at that offset, and also the frame is going to sag in the middle if the wheelbase is as long as it looks in the pictures.

Don

 

[kzoldman]

Looks like a pretty long wheelbase.......what are you shooting for? And yes, I think it will flex at that offset, and also the frame is going to sag in the middle if the wheelbase is as long as it looks in the pictures.

Don

It does look long but it is only going to be about 120". An extra cab Ranger is 126". I have extra rail out front that I will cut off after I mount my sway bar. In the pic the cab will sit 1" in front of the crossmember and stop where the front side of the 4' level is at. I could move the front cross member back to shorten the wheel base but I like the look of the front tires kind of centered around the engine fan area.

 

[donsrods]

120 is very long, especially using only 3 inch tubing.........even if it is 3/16. The one in the picture was built by my Son for his fordor and it had a 120 wb, and it too sagged in the middle. He ended up taking a sawzall to it because of that happening. He found out when the doors wouldn't open and close right that the middle had sagged. He knew it would never do, so even though it was all painted and done, it went in the dumpster.

Just trying to save you some problems before you get too far along. You mention the Ranger 126 wb, but compare the stoutness of both and you will see why theirs holds up.

Don

 

[Sam_Fear]

It's not the thickness of the tube as much as the height. a 4x2 is way stronger than a 3x2, vertically that is. A full car body can handle a bit lighter frame than a truck cab - the car body distributes some of the weight to the ends, were as a truck cab is all in the center of the frame.

Get 1500lbs worth of friends to stand on the frame in the approximate placement of engine, cab, driver, etc. and bounce in unison. That offset is no different than a series of butt welds - lateral stress wise. How much do you trust your welding?

 

[kzoldman]

Thanks a lot Don, sorry about that for son, it looks like a lot of work went into it. It really looked good. I still have time to play around with the wheel base a little and get the look I want. Rather do it now than wish I had later. I would like to see some pics of rat rod trucks with rear leafs and wheel base numbers though. What is a "good" wheel base for this type of build?

 

[kzoldman]

"It's not the thickness of the tube as much as the height. a 4x2 is way stronger than a 3x2, vertically that is. A full car body can handle a bit lighter frame than a truck cab - the car body distributes some of the weight to the ends, were as a truck cab is all in the center of the frame."

Thanks Sam, had to use the 2x3 to work with the front cross member I already had.

 

[Sam_Fear]

... That offset is no different than a series of butt welds - lateral stress wise..

I don't think 'lateral' is correct there, but hopefully you get what I mean - stress along the rail front to back.

 

[kzoldman]

I don't know. I rolled some stuff around and threw in some old tires and the 120" just "looks" right. That gives me 32 inches from the fire wall to the center of the tire. I may could go 115". Going to have to think on this a few days. [S

 

[Sam_Fear]

Really there won't be much difference in deflection between 115" and 120" wb. Here are a couple deflection calculators to give you a rough idea how things compare.

This one is fairly straight forward, but is for sq. tube only. 2.7" sq is close to 3x2 and 3.65 is fairly close to 4x2.
http://metalgeek.com/static/deflection.php

This one, you need to manipulate the #'s a bit. Hollow Rect. Beams. You need to divide both the Length and Force by 2 for it to work for a car frame.
http://www.calculatoredge.com/civil%20engg%20calculator/beam.htm

Also keep in mind, they are using a 'point load' on only 1 rail.

 

[donsrods]

If you are concerned about matching up with your front crossmember, you can still use 4 inch and cut a tapered section out of it and reweld it so it is 4 inches near the firewall and 3 inches at the front crossmember. You can see the tapered frame in front of the firewall on my 29 rpu in the picture below.

Don