Looking for input on my custom IRS chassis

[garickman]

Here is some pics of my custom chassis with a custom IRS set-up. I was hoping everyone can take a look at the pics and offer some input on the design and if they think it needs improvements. I have been getting different messages from members saying that I may need trailing arms, the load path looks suspicious, how do you adjust for caster. I am a novice at this, and although I have been with the builder helping build this, I don't always know the right questions to ask. I will be going back to the shop Monday and I was hoping people could raise some questions they think I should address with the builder.

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[Poopy]

Very nice clean work...With the 4 heims on the lower arm does it move around at all? <||> like that way...forwards and backwards?

One more question, is the spindle strong enough to take the load of the coilovers as well?

[Bryce]

my post on lat-G:

There are two questions to ask:

Is this a fully constrained system?
Is the setup strong enough?

The system is fully constrained as is. Some IRS setups use a ball joint top and bottom this allows for more adjsutments in the future. Toe, caster, camber. If you adjust the LCA to move forward then you will have more positive caster.

Without know material thickness and properties its hard to say if this is strong enough. I think that lower plate welded to a tube and bolted to the spindle needs some gussets. Also what car is that spindle from. That load path looks suspicious, is that the best place to attach the spindle?

[garickman]

my post on lat-G:

There are two questions to ask:

Is this a fully constrained system?
Is the setup strong enough?

The system is fully constrained as is. Some IRS setups use a ball joint top and bottom this allows for more adjsutments in the future. Toe, caster, camber. If you adjust the LCA to move forward then you will have more positive caster.

Without know material thickness and properties its hard to say if this is strong enough. I think that lower plate welded to a tube and bolted to the spindle needs some gussets. Also what car is that spindle from. That load path looks suspicious, is that the best place to attach the spindle?

I responded to your post on lat-g and was hoping to hear back from you. Here was my reply;

Bryce,

Thanks for your input, these are some questions I will definatley bring up with the builder. I have been at the shop almost daily helping with the chassis build. I try to ask as many questions as I can so I can learn as I go, but being a novice in suspensions, I don't always know the right questions to ask. For instance, what is a load path and what makes it suspicious?

Oh, the spindles are ATS spindles originally for a 2nd gen camaro.

[garickman]

Very nice clean work...With the 4 heims on the lower arm does it move around at all? <||> like that way...forwards and backwards?

One more question, is the spindle strong enough to take the load of the coilovers as well?

Two more questions I will ask the builder, thanks for the input.

[keithq69]

The caster can be adjusted with the joints on the lower control arm. Allowing you to move the lower pivot forward or backward.
My thoughts were this, the upper locating link is a single link. Since the lower pivot uses two mounting locations at the spindle the upper link doesn't need to be an A arm but I would feel better if it was on my car.
The spindles are ATS spindles with the lower ball joint pivot machined off. The lower control arm is connected to the steering arm location, I assume since the steering arm connects here and this area transfers the load from the ball joint to the hub that it is strong enough to carry the load.
My concern is that machining the Lower ball joint flange off shortens the spindle substantially.
The normal pivot of the ball joint would be center of the lower control arm, maybe an inch or more below the normal ball joint mount.
I would guess the pivot now shortens the spindle by almost 2". The shorter spindle coupled with longer than normal control arms may make for a poor camber gain.

Did the company who's building it run the dimensions on a suspension program to check everything before they built it.
I'm not a suspension designer and am only offering my opinion because it seems like you have some concerns.
Hope everything works out, the workmanship is incredible.

[Poopy]

Here's another custom build for something for you to look at....


57872

[MrQuick]

The caster can be adjusted with the joints on the lower control arm. Allowing you to move the lower pivot forward or backward.

....are you talking about the front of the rear? I am still not seeing it. Caster adjustment would require tipping the spindle fore and aft and not side to side. Adjusting the lower arm heims would change toe.

[exwestracer]

....are you talking about the front of the rear? I am still not seeing it. Caster adjustment would require tipping the spindle fore and aft and not side to side. Adjusting the lower arm heims would change toe.

Quick,
Adjusting the inner heims on the lower arm would angle the entire arm forward or backward, changing caster. (Not really a GOOD solution, I'll grant you, but it does allow some adjustment...)

My concern with the system is caster control under heavy braking. I've never been a big fan of a single lateral upper link for that reason.

[2gofaster]

Also, I'd want to really give some thought on that inner upper link bracket. That is a long lever arm with a load path off axis. Maybe it's welded to that crossbar that the shock reservoir's are bolted too, which would shorten the lever arm. I can't see that from the perspective of these pictures.

[garickman]

Also, I'd want to really give some thought on that inner upper link bracket. That is a long lever arm with a load path off axis. Maybe it's welded to that crossbar that the shock reservoir's are bolted too, which would shorten the lever arm. I can't see that from the perspective of these pictures.

Thanks for all the input guys, this is just what I was looking for in regards to this thread. These are just some questions I can bring up with the builder that I would have not known to ask. Yes the inner upper link bracket is welded to the crossbar.

[ace_xp2]

Hard to estimate this stuff from pictures alone, but it looks as though there is very little camber gain.

[MrQuick]

Quick,
Adjusting the inner heims on the lower arm would angle the entire arm forward or backward, changing caster. (Not really a GOOD solution, I'll grant you, but it does allow some adjustment...)

My concern with the system is caster control under heavy braking. I've never been a big fan of a single lateral upper link for that reason.

I see it now...its like tipping a ladder.

[exwestracer]

I see it now...its like tipping a ladder.

Not really a good idea either...lol.

With big brakes and a lot of tire, I could see those lower "A Frames" flexing and allowing the knuckles to rotate forward.

BTW, Basically what you have going here is a lightened up version of a T-bird IRS...
57945

The T-bird version controls caster with a VERY heavy duty lower control arm.

Garickman, any idea what the rear spring rate is? I don't see you gaining much motion ratio back with that rocker arm design. The less the spring moves in relation to the wheel, the stiffer it has to be. The push rod is way out there, which is good; but at about a 45deg angle, which is giving up a lot of motion. Just something to think about...

[garickman]

OK, I brought up several questinos to the builder so I will summarize what he told me. I was taking notes as he was explaining things to me so keep in mind that some of what he was telling me I didn't quite understand. First and foremost, looking at the pics as they were posted by me are about 75-80% complete. The reason he machined off the lower ball joint mount was because in order to get the ride height of 5 1/2 inches and the correct A-arm angle he needed to build much stronger lower control arms. He has used this style of rocker arm and single upper link in the past with no problems. He has redesigned the lower control arms to control the caster under heavy braking and take more load. It was designed to get a good shock ratio. Taking the shock off the lower control arm has reduced the load on the spindle. To get the control arm angles where he needed them, he moved the pivot point up .250. 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. He also said as the rear suspension nears 100% completion, there may be some small changes.

Both the builder Leonard Lopez of Dominator Street Rods and I want to thank everyone for their input and interest in the build.

[2gofaster]

Moving the shock off the lower arm only removes the load from the lower side of the spindle. Now he's got the load of the shock and spring pushing out on the top of the spindle instead. Plus the attachment between the lower spindle mount plate and that tube it's welded to are in peel. Were it me, I would have machined a plate to encompass all four of the hub bolt holes(rather than just two) and some sort of gussetting to that tube. Then mounted my shock pushrod to the lower arm. Maybe I'm off base. I'm just not thrilled about that connection point from the spindle to the lower control arm as it's designed now.

[keithq69]

I'm not sure I understand what you meant when you said "he machined the lower balljoint mount off to get the 5.5" ride height". Machining the mount off and moving the pivot up moves the center of the wheel closer to the lower pivot which will raise the car.

It's really hard to look at a half finished project and criticize, we don't know what his total plan is.
But it's always good to get opinions, sometimes a fresh set of eyes can make all the difference.

[MrQuick]

there's a thin line between criticism and positive opinion. I believe the 5.5 Greg is referring to is the ground clearance.

Im one for letting him finish the job. Im confident the performance will do the talking. Call me Greg.

[andrewb70]

I am no engineer but I am worried about how the center section is attached to the chassis and in turn the lower control arms are hung from the center section. All of the load on the suspension is being carried by the center section and the point where it attaches at the center. Maybe I am missing something...

Andrew

[garickman]

I am no engineer but I am worried about how the center section is attached to the chassis and in turn the lower control arms are hung from the center section. All of the load on the suspension is being carried by the center section and the point where it attaches at the center. Maybe I am missing something...

Andrew

Yeah, Andrew

The center section area is not quite finished. There will be bracing that goes underneath the center section which will attach the bacing for the control arms together and a lower cradle will be fabricated to attacth to the frame.

[Motorcitydak]

Also with the upper control arm angled down as it goes out to the wheel, that will cause camber loss with suspension up travel. That is not a good thing for performance driving, you want to opposite. Looks like the only decent option to change that in your setup is to have the upper link's frame mount lower down by a couple inches or some how get that link to mount to the top side of the knuckle

If you notice, the front end of your car is different and better. The lower arm looks about level and the upper arm angles up