where do you guys start when designing a rear trailing arm?

lets say for a live rear axle that will have coilovers/bypass shocks mounted on it....

Serg

desired wheel travel must be known.determines where the mounts go on the link.

length of shock would also be helpful

i wouldnt have thought lenght of shock would be the first thing. there are so many options when it comes to shocks that i would have thought u could get them to suit...

obviously lenght of trailing arm is important... but what else?

Serg

Length of shocks is determined by desired wheel travel, desired motion ratio, and sometimes the motion ratio and thus the shock length are determined by the cab position if you don't want the shocks in the cab.

To get it all to work properly the first time you need to account for, and design around everything before building anything. The whole rear suspension must work as a system, it's not complicated, but does take a little time to figure it out and think it through a couple times to be sure it will work right.

Too bad you're in Oz., there's enough knowledge floating around on the board to find the perfect link length for the common desert trucks in the US, but with different trucks available, different motor and tranny combos and different terrain over there it gets really tricky.

ok lets look at something a little more simple:

a trailing arm 40inches long, using a standard horizontal bush at axle end and a pin bush at frame end. NO shocks mounted on arm.

i see that most of the rockcrawlers use round tube, and most of the desert racers run fabed box section that tapers at ends. im guessing the reason behind it for the desert guys is that you want them as lite as possible and strong, most of the stuff is chromoly right?

so if i wanted to build a box section trailing arm out of mild steel and GMAW where do i start to decided the design size and material thicknesses etc?

i know it would probably be easier to use tube, but thats not the point.

cheers, Serg

For the most part you will have to worry about 2 types of loading; bending and buckling. Links don't often fail in tension as long as the welds are good and it isn't stupid thin. For both types of loading you need the thickest, tallest, and widest section to be right in the middle (right where desert racers cut a hole to stuff shocks in which just makes matters worse). Calculate what the loads will be, draw up a bending moment diagram, apply a factor of safety, and calculate the I value that you need. From the I value pick a cross section.

For the most part you will have to worry about 2 types of loading; bending and buckling. Links don't often fail in tension as long as the welds are good and it isn't stupid thin. For both types of loading you need the thickest, tallest, and widest section to be right in the middle (right where desert racers cut a hole to stuff shocks in which just makes matters worse). Calculate what the loads will be, draw up a bending moment diagram, apply a factor of safety, and calculate the I value that you need. From the I value pick a cross section.

wooooooooooooo, slow down there mate :flipoff2:

ok so #1 Calculate what the loads will be: how do i do that?

Cheers, Serg

wooooooooooooo, slow down there mate :flipoff2:

ok so #1 Calculate what the loads will be: how do i do that?

Cheers, SergYou start with some basic physics knowledge, some SWAGs (scientific wild ass guesses), some WAGs, some assumptions, and some PFM (pure f'n magic). In some cases it is easier to put something on the vehicle based on what other people have done and if/when it fails calculate the load based on that or toss some instrumentation on it and beat the crap out of it.

I have put some of my SWAGs and assumptions into my link calculator on the "Materials" tab. The text document that is in the zip lists (many of) my assumptions.

You start with some basic physics knowledge, some SWAGs (scientific wild ass guesses), some WAGs, some assumptions, and some PFM (pure f'n magic). In some cases it is easier to put something on the vehicle based on what other people have done and if/when it fails calculate the load based on that or toss some instrumentation on it and beat the crap out of it.

I have put some of my SWAGs and assumptions into my link calculator on the "Materials" tab. The text document that is in the zip lists (many of) my assumptions.

basic physics knowledge.....hmmmm yeah ok :homer:

ill come at it from another angle. what thickness is the chromo used in the desert racer trailing arms?

how much stronger than mild steel is it?

Serg

Basic for an engineer:flipoff2:

Much of the stuff I have seen is 1/8" thick with some either doubled up or 3/16".

Here is one I designed for a Ranger with 3/16" 4130 top and bottom plates, everything else 1/8" 1430, and they will be heat treated to bump the strength up
http://www.pirate4x4.com/forum/attachment.php?attachmentid=360050&stc=1&d=1205909592

Strength can be all over the place. Go to www.matweb.com and look some up. Your best bet however would be to get spec sheets from your material supply place for stuff they can get without too much trouble.

You might also want to have a look at these threads:
http://www.pirate4x4.com/forum/showthread.php?p=8083317
http://race-dezert.com/forum/showthread.php?t=31417
http://race-dezert.com/forum/showthread.php?t=4050