Do I need trailing arms on my custom IRS chassis?

[ironworks]

Originally Posted by Ron in SoCal

I know at least one (or two) that will laugh at this, but these are words of wisdom in my book Roger. Your direct style is appreciated by many, even when it goes against the 'conventional' grain

If your going to ask me a question I'm going to answer it. Don't ask a question you can't handle the answer to.

[ironworks]

Originally Posted by garickman

I know you say these types of threads are a waste of time, but some where along the line there is always some good advice offered. And you are right, I have had a rough go, I have been burned by a couple of builders. I'm sure you are familiar with one of them.

When you ask for free advice, you get what you pay for. And on the internet, lots of guys who have done nothing more then watch Nascar will tell you all day long how it really is.

Most people are only going to say nice things because that is what we are taught to do. The other 90% of people really have no idea what they are looking at. Except the nice welds and the parts are super cool.

I will agree this suspension does not need "toe links" as set up, but I really think the spindle as used in this design will be a weak link with the lower plate bolted to the spindle. I also think the cantilever arm could upon droop of the suspension go over center and and lock up the suspension when it is allowed to travel over center of the link. I also think that the positioning of the push rod on the spindle will limit your rear tire width unless your running like a 24" rear wheel. If that push rod link was connected to the lower control arm it would allow you to have a much longer link and allow you to run a much wider rear wheel. Plus the link would not be as apt to go over center on droop. I hope I'm totally wrong and look like an idiot when it is all done, but that is my opinion.

I'm done

[onevoice]

Originally Posted by garickman

Here is a few pics of my chassis for my 1967 Chevelle. Someone over on PT suggested that I need trailing arms on the rear lower control arms. Any input or advice you can offer would be much appreciated. If interested here is the link to build thread with alot more pics.

https://lateral-g.net/forums/showthread.php4?t=34813

The top link needs to be triangulated (like a vette) or an upper trailing link has the same effect. Something needs to take the brake reaction forces at the top of the spindle. Picture trying to rotate the spindle looking at it from the side, the lower link will resist all the force by twisting. It can't handle it.

[onevoice]

Do what you want, but a viper and corvette both have upper links that provide fore and aft locating of the spindle (both use A arms). All IRS's I have ever seen have either triangulated control arms (a arms), trailing links, or trailing arms, or a combination thereof. Nobody makes an IRS rear with a single, unbraced upper link

I would take their engineering over a street rod builder.

[bdahlg68]

Originally Posted by garickman

... Taking the shock off the lower control arm has reduced the load on the spindle. .... The push rod mount on the top of the spindle is being redesigned to take some of the angle out of the push rod. He wanted me to add (as some people have already pointed out) it is difficult to know exactly what is going on by just looking at the few photos I have posted.

I disagree that it will take load off the spindle. It has changed the loads, yes, but not taken them off. The lateral and fore aft loads are still applied through the spindle but now a moment has been added about the attachment points with now a shorter moment arm. In addition, the "vertical" load from the shock / spring has now been moved from the lower ball joint to the upper ball joint. However, the ATS spindles are 6061-T6 and OE Vette spindles are A356-T6. So you are gaining some strength and elongation there which is a big plus. Whether or not you'll have an issue depends on so many other things like vehicle weight and distribution, tire diameter, and on and on, that really no one can tell you for sure if this is an issue within pro-touring operating use.

Good to hear the push rod angles are being optimized. It was very hard to tell from the pics whether or not it could be an issue, but it looked like there was a good chance that at least you'd see undesired variation in spring rates.

Originally Posted by barraza

Do what you want, but a viper and corvette both have upper links that provide fore and aft locating of the spindle (both use A arms). All IRS's I have ever seen have either triangulated control arms (a arms), trailing links, or trailing arms, or a combination thereof. Nobody makes an IRS rear with a single, unbraced upper link

I would take their engineering over a street rod builder.

I think the OP was mainly concerned with function, and not whether or not this was the optimum IRS style. It will work without toe links. Yes, most other IRS's have a toe link, but there are not many "swing arm" style IRS's out there, but there are some good ones. Check out the previous BMW X5 rear suspension for instance. Quite different from the OP's suspension and does have two upper links, but not really a toe link. Nor are the upper links all that effective at completely limiting roll / twist as they have rubber bushings.

Lots and lots of things that may or may not achieve ones goals.

Good luck to the OP and his builder!

[onevoice]

Originally Posted by bdahlg68

I think the OP was mainly concerned with function, and not whether or not this was the optimum IRS style. It will work without toe links. Yes, most other IRS's have a toe link, but there are not many "swing arm" style IRS's out there, but there are some good ones. Check out the previous BMW X5 rear suspension for instance. Quite different from the OP's suspension and does have two upper links, but not really a toe link. Nor are the upper links all that effective at completely limiting roll / twist as they have rubber bushings.

Not sure what you are getting at here. I wasn't talking about toe links. I said all IRS suspensions control the fore and aft movement of the upper part of the spindle from the top , think constraint of caster. All IRS suspensions also control the toe, whether with a toe link(like a vette or viper), or the physical layout of the locating arms like this design.

From the side view, the spindle as designed only resists rotation from the lower control arm. This is no way to design a performance suspension, unless the lower arm is super strong, because the forces will constantly be twisting the lower arm.

This design would be much better by spreading the upper link inner attaching point into two pieces, making it an A-arm. Same effect by using an upper trailing link.

Also, I'm not a big fan of modifying such a critical part as a spindle to use the steering arm location mounts as lower suspension mounts. Without obvious overkill design, it is foolish to do so without engineering analysis by someone familiar with the design and strengths of the original part.

[bdahlg68]

Originally Posted by barraza

This design would be much better by spreading the upper link inner attaching point into two pieces, making it an A-arm. Same effect by using an upper trailing link.

This is what I was getting at. I agree it could be better designed for performance applications. That just didn't seem to be the intent of the original question.

[Vince@Meanstreets]

Originally Posted by barraza

Nobody makes an IRS rear with a single, unbraced upper link

I would take their engineering over a street rod builder.

I prefer a short arm long arm configuation myself.

[onevoice]

Originally Posted by Vince@MSperfab

I prefer a short arm long arm configuation myself.

Both of those setups have inboard brakes

Big difference in the spindle loads.

The lower is similar to a jag setup, and doesn't even have an upper link, it uses the axle as the upper link. A design from the 1960's, that wasn't known for great handling even back then, isn't something I would emulate for a modern scratch built performance vehicle.

I don't know who designed the other one, but it is not a modern, robust design. Fairgrounds street rod it is fine, otherwise, get something that incorporates the best design practices. Those IRS designs are the equivalent of updating a 30's solid front axle with an independent 50's design that uses kingpins. Or comparing a DSE front subframe to a street rod mustang II setup.

A bad IRS is worse than a good design solid axle.

Edit: I just saw the linked picture, and I stand corrected, that is obviously a recent rear axle. Explorer maybe? It is however very different from the OP design. The linked pictures show a lower control arm that is VERY wide to take the loads involved. The inner mounts of that arm are about twice the diameter of the brake disc, probably 18-20" apart. The lower arm attachment to the spindle is also spread apart what looks like a brake disc width, call it 10 inches, and the attachment of the arm to the spindle is very close to the axle center-line, minimizing the moment. In short, that arm is designed to take the loads I was saying the OP design isn't. Certainly not the same case as using a modified front spindle with the steering arm mounts as the lower arm attachments.

[Vince@Meanstreets]

well you said it....
lol