[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.