Theory craft / Rear suspension geometry custom IRS adaptation

[TOlsenAK]

Got a bit of an interesting concept im trying to figure out, not sure if anyone with more knowledge than me has any suggestions, not 100% suspension related but there is a few factors of it.

I'm going to be installing a integral-link rear end suspension system on my 67 Camaro that has a track width of 64.9in. I'm installing mini-tubs and plan on running 315s in the rear on 18x12s.

Stock Track Width: 60''
New Track Width: 64.9"

This would push a stock wheel out 2.45", which basically means If I wanted to run the same size wheel people get away with stock I would need around 2.5" more backspacing. I've seen a lot run a 275/40 on the rear with 18x9.5 and around 5.5 backspacing. so in theory if I wanted to run with the 64.9" track width without shortening it, I'd have to go for an 18x9.5 with 8" backspacing (which is probably hell on the bearings and not feasible) or with an 18x12 I would need 9.38" of backspacing to get the same fender clearance. I'll have to do some mocking up and testing here, which should happen in the next month or so to get some more measurements.

Picture for better idea:


My thoughts were to (with the help of a friend who has done this stuff) is to go through and get shorter axles made, shorten up all the control arms, and try to get around 1.5"-2" off each side. I haven't found any integral-link suspension calculators, all the ones are for dual-wishbone or the macpherson style setups (front suspension is the QA1 setup). Pretty complicated situation, but it potentially lead to a very cheap alternative to people wanting to go the IRS route over the $9k+ kits.

[Craig510]

The geometry of the S550 rear suspension will not like narrowing. Probably best just to flare the car and run wheels with 8 or so inch backspace. Check out some of the late model mustang forms and see what the guys are using to fit 335s

[dhutton]

Here’s a more costly option with a lot less effort:

http://www.artmorrison.com/multilinkirs.php

http://www.artmorrison.com/compactirs.php

I have one of these in my 69 Camaro. It is awesome. I don’t think I’ll own another solid axle car.

Don

[astroracer]

Craig is correct. Narrowing an integral link suspension, without re-calculating every pick up point and articulation angle will be next to impossible to get right. Controlling toe and caster are requisite to a good handling suspension and once you change the link lengths and pivot points all of that engineering goes right down the toilet.
Like Craig suggested keep the offsets manageable and do a wide body style flare to keep the tires covered.
You will also want to check how wide the rim can be before you start hitting other suspension components like shocks and trailing links.
Mark

[67King]

THe only way I would consider narrowing that would be to cut the width out of the middle. If you shorten the control arms and don't address the upper mount, you are going to have a camber curve that will go positive, rather than negative, under travel. You may have a change in toe as well. And I don't know if taking that much out of the middle is even possible. But that's the only way I'd consider it.

[TOlsenAK]

Here’s a more costly option with a lot less effort:

http://www.artmorrison.com/multilinkirs.php

http://www.artmorrison.com/compactirs.php

I have one of these in my 69 Camaro. It is awesome. I don’t think I’ll own another solid axle car.

Don

The price difference however when you have access to a CNC machine shop and a friend who used to do race car fabrication work is huge, I can probably be into modifying and buying this set up for around 60 hours of work and less than $2,000 vs. the $13,850. We basically could recreate one if we had the dimensions and had thought about going that route. the $11,000 dollar difference is worth the effort to me.

[TOlsenAK]

THe only way I would consider narrowing that would be to cut the width out of the middle. If you shorten the control arms and don't address the upper mount, you are going to have a camber curve that will go positive, rather than negative, under travel. You may have a change in toe as well. And I don't know if taking that much out of the middle is even possible. But that's the only way I'd consider it.

I think the biggest issue in it will be getting the angle and bend for the upper camber link, the rear sway bar part is easy, not much changes on the geometry with that as the length is the only factor for that portion.

The only pieces I see needing to be shortened are the sway bar (non factor as the mount points will be much the same), the Lower H-arm (build a shortened tubular style on a jig), Upper Camber Link(Jig and build shortened version with appropriate curvature and distance) and the Toe Link (shortened adjustable tubular arm).

I know there is a lot of software out there you can input in your suspension geometry for stuff like wheel load transfers, spring properties, roll calculations, and the forces that will be exerted on your wishbones and springs. I've also found a few links that allow to get a decent idea of changes in handling characteristics for double wishbone and macpherson style strut suspension setups where you are able to adjust the mount criteria for the frame points as well as the control arm lengths and inclination

[dhutton]

The price difference however when you have access to a CNC machine shop and a friend who used to do race car fabrication work is huge, I can probably be into modifying and buying this set up for around 60 hours of work and less than $2,000 vs. the $13,850. We basically could recreate one if we had the dimensions and had thought about going that route. the $11,000 dollar difference is worth the effort to me.

I don’t think you are assigning any value to the engineering design effort that went into the Art Morrison setup and that you will have to do....

Don

[TOlsenAK]

THe only way I would consider narrowing that would be to cut the width out of the middle. If you shorten the control arms and don't address the upper mount, you are going to have a camber curve that will go positive, rather than negative, under travel. You may have a change in toe as well. And I don't know if taking that much out of the middle is even possible. But that's the only way I'd consider it.

Since I will be running just a coilover in the rear and not the shock/spring setup the standard S550 has stock, the H-arm can be built differently with the mount point taken into consideration as well as spring rate and the length of the coilover.

Mostly the biggest piece of the puzzle I need to figure out is the effects of shortening them all on the travel of the wheel hub during zero and full load on the suspension.

[TOlsenAK]

I don’t think you are assigning any value to the engineering design effort that went into the Art Morrison setup and that you will have to do....

Don

Oh I agree there is a huge amount of value to their engineering design, I'm not at all thinking I will be getting a result as amazing as the Art Morrision setup, I understand there is a reason its priced as it is and I imagine its worth every penny. I fully plan to at least run things through a dynatune suspension design software (example of what sort of stuff you can do with it below) to compare it to the stock S550 rear suspension, which I'm 100% sure the Art Morrsion setup is probably leagues better than, but im just going for better than solid axle handling in all reality and on a budget I can afford (I have the time to do this luckily, and the time it would take me to earn the $9,000 dollar difference between this build vs. the AME setup at what I make is much more massive).

Basically im trying to build a good inbetween that may work for people that want IRS but don't have the budget to go with the AME setup. This will probably be something that will cost around $2500 to put together and will work as effectively as a slightly improved S550 rear end in regards to handling performance as opposed to the solid axle routes.



I know this is massively outside the box and I may be overestimating my ability to calculate the vectors and forces applied with my limited experiences that is only from a few classes in engineering, but I think from what I've seen and researched so far its doable.

[TOlsenAK]

Also, with the suspension mocked up in the rear (with stock rx8 wheels installed to get a base of measurement and to get a rough check of clearances), it looks like I may be able to run 18x12 +65 wheels on 315s as one option. This gives 30mm of push to the outside and 60mm extra on the inside (have plenty of room with the mini-tubs and a coil over relocation) Fenders will be most likely pulled/rolled to give a little extra clearance. With this in mind, to get it to be flush as the RX8 wheels are, I would only need the 30mm shortening of the rear track width (1.2in or so on each axle).


Mocked up with the S550 suspension side view:



I do see it being a possibility to build a subframe that is shorter to accommodate not having to change any control linkage lengths as well, it would though alter a lot where the mount points would be.

[TOlsenAK]

Craig is correct. Narrowing an integral link suspension, without re-calculating every pick up point and articulation angle will be next to impossible to get right. Controlling toe and caster are requisite to a good handling suspension and once you change the link lengths and pivot points all of that engineering goes right down the toilet.
Like Craig suggested keep the offsets manageable and do a wide body style flare to keep the tires covered.
You will also want to check how wide the rim can be before you start hitting other suspension components like shocks and trailing links.
Mark

Another option I haven't really considered is to leave the geometry of the mount points and arms all the same and fabricate up a new subframe that is not as wide, the shocks/struts are going to be installed after and calculations will be run to determine the best mount point to maintain clearance. Basically the part I'm having to focus on is what you said, the toe and caster impacts with suspension travel, which i'm thinking I'm going to bite the bullet and spend a little on some software to be able to run simulations through to see what kind of impact I will have on those. Toe though i'm not as worried about as the toe link will be made to be adjustable.