IRS vs Solid axle. Why one over the other?

[Over2tonsoffun]

I have seen articles about swapping to from one to the other and vice versa. So what is the advantage with going to a IRS vs Solid axle rear and the other way. Art Morrison has the option in their chassis. So why would one go with one over the other.

Tim

[raustinss]

Cost is the biggest deciding factor for most people... solid axle is way cheaper , easier to tune the suspension for the less knowledgeable people ,as well as easier to package within the car .

Independent is heavier, more bulky in size and harder to tune for those that aren't damn near engineers lol. But, as for grip and performance you can't beat what a I.R.S. has to offer over a solid axle

[Blake Foster]

Hopefully Ron will chime in on this one as well. it is an interesting conversation, Ron and I have been talking about it. His explanation will be better than mine but he is a BIG proponent of the Torque Arm and Solid axle for all round performance. His explanation was that "IF" you have the Aero downforce on a big track the IRS will be better "provided the geometry is correct and the toe steer is set up correctly. He was telling me how much improvement the Corvette Rear needs to work to its potential. if that is the case and discounting all the negatives listed above not to mention Exhaust routing. limited width options and on some cars the difficulty of being able to get the wheels on and off. Unless you HAVE to have it the Torque arm with a solid axle is probably the best choice.

[Ron Sutton]

The short answer is ...

* The IRS doesn't offer more grip across the board. If set up correctly, it offers more grip than some suspension set-ups & less than others.
* IRS offers 50/50 torque distribution under acceleration. 50 pushing the car/50 planting the tires
* In racing, we don't pick IRS for more grip. Again, if set up correctly, it helps a car turn better/easier.
* For rear engined cars, IRS is the practical packaging solution.
* For cars with ultra low ride heights (1/4" to 1") ... and therefore ... little to no front suspension compression travel under braking ... the IRS is preferred to help the car turn better. But hopefully the rules allow aerodynamics, as we'll need some rear downforce at speed.

IRS has several pros & cons different than solid axle suspensions.
Pros:
* Can produce more grip on entry or exit than "low mount" solid axle suspensions.
* Can be set up to have positive, neutral or negative roll steer.
* Can be set up to have toe in, zero toe or toe out.
* Can be set up to negative camber the outside tire, increasing its grip.
* Can make the rear roll center migrate a positive direction, helping tire loading on cornering.
* All of these require the geometry to be worked out optimally.

Note: Torque distribution can vary on solid axle linkage style suspensions, depending on the length of the link mounts.
* 3-Link & 4-Link rear suspensions offer 50/50 torque distribution only if the upper & lower mounts are the distance from the rear axle centerline.
* 3-Link & 4-Link rear suspensions will have less than 50/50 torque distribution if the upper mount(s) is closer to the rear axle centerline than the lower mounts.
* 3-Link & 4-Link rear suspensions will have more than 50/50 torque distribution if the upper mount(s) is farther from the rear axle centerline than the lower mounts.
* Truck Arm & Torque arm suspensions always offer 50/50 torque distribution, same as IRS.
* I prefer 3-Link rear suspensions, for many reasons. One of which is we can the top link mount taller than the lower mounts & INCREASE the loading on the rear tires under acceleration.
* But you can't package a 3-Link or 4-Link optimally under the factory floor. The top link ends up mounting too low ... reducing the amount of torque distribution planting the tires. Plus the top link(s) end up short, causing severe pinion angle changes during travel.

Cons:
* Does not produce as much grip on entry or exit as "high mount" solid axle suspensions.
* If the geometry is not worked out, we have no idea what's going on with he roll steer, camber, roll center height & migration.
* If set up for positive roll steer for autocross, will be looser on higher speed road course corning ... unless countered with significant aero downforce.
* If set up for counter roll steer for more rear grip on road course cornering, will be tight/pushy on tighter corners like autocross.
* Rear roll center is not easily adjustable, especially without affecting camber & steering attributes.
* More complexity to deal with.
* More parts to manage/prevent breakage.

My opinion:
* When I'm building a front engined car with 2.5" or more ground clearance, I prefer a solid axle with a linkage suspension.
* When I'm building a rear engined car, IRS is practically required, regardless of what we want.
* When I'm building a low ride height car (1/4" to 1"), I prefer IRS to help the car turn better/easier ... but we need some aero downforce for high speed tracks.
* We can make any of them work well, if we know what we're doing.
* IRS requires more knowledge & work to set up correctly.
* Solid axle rear ends & suspensions are easier to setup optimally.
* If I'm building a solid axle car for autocross & road course action ... and the back seat can be deleted & the floor reworked ... I much prefer the 3-Link. (Offset 3-Link is optimum)
* If I'm building a solid axle car for autocross & road course action ... and the back seat needs to stay ... I prefer the Torque arm ... because it provides us 50/50 torque distribution without loosing the rear seat.





P.S. Yes ... that was the short answer.

[Blake Foster]

I told you he would have a better explanation then mine

[raustinss]

Hate to read the long answer

[David Pozzi]

I drove the Art Morrison test Camaro with IRS when it first came out and just loved it. The lower unsprung weight helped the car ride over track bumps and undulations without being as upset. I translate that into increased car stability and feel, which is what I was looking for, and I absolutely love the ride on the street, - just dial the shocks down. It's also a big improvement for street cars, which is the target market for Art Morrison, and Mary has proven it works for autocrossing, she's not that aggressive a driver on a road course but the car has felt great there. The downside is it takes some smarts to change alignment settings on it. We tend to not mess with it once we got it set well.

Prior to this I was thinking of swapping a Corvette IRS into my 67 Camaro but I was worried about how much build time it would eat up designing and fabricating it, and would it break? What needed beefing up?

When the Art Morrison IRS came out, I saw right away it is much stronger than the Corvette unit all the way through, needed no beef up, the lower links links are Johnny joints, and it has a Strange Dana 60 third member, durability was a big factor for me in selecting this IRS, plus it was already engineered.

[Ron Sutton]

Originally Posted by David Pozzi

I drove the Art Morrison test Camaro with IRS when it first came out and just loved it. The lower unsprung weight helped the car ride over track bumps and undulations without being as upset. I translate that into increased car stability and feel, which is what I was looking for, and I absolutely love the ride on the street, - just dial the shocks down. It's also a big improvement for street cars, which is the target market for Art Morrison, and Mary has proven it works for autocrossing, she's not that aggressive a driver on a road course but the car has felt great there. The downside is it takes some smarts to change alignment settings on it. We tend to not mess with it once we got it set well.

Prior to this I was thinking of swapping a Corvette IRS into my 67 Camaro but I was worried about how much build time it would eat up designing and fabricating it, and would it break? What needed beefing up?

When the Art Morrison IRS came out, I saw right away it is much stronger than the Corvette unit all the way through, needed no beef up, the lower links links are Johnny joints, and it has a Strange Dana 60 third member, durability was a big factor for me in selecting this IRS, plus it was already engineered.

I think you nailed the summary of the IRS. It provides a great ride, good tight corner turning (autocross). You just want to pick one that is built strong & the geometry has been worked out ... or be prepared to work it out yourself. You also need to be prepared to write the large check. LOL

I remember talking to Mary initially and the set-up had a bit too much rear steer for her tastes. Once they/you dialed that down, she loved it. And of course was & is fast with it, winning often.

[David Pozzi]

I made a mistake early on. The first thing I did after installing it was add more neg camber. This changed the rear clocking (caster) of the rear hubs, lowering the toe link rod end height. From then on, the Art Morrison recommended cam settings we had for toe steer were off, and it was getting a lot more toe out than expected. Also Perf Trends software will not correctly model this suspension. We get numbers, but they are wrong as far as roll center height.

[Erickson Performan]

Originally Posted by raustinss

Hate to read the long answer

HAHAHA I would love to read the long answer. Ron you are a wealth of knowledge.