[Ron Sutton]

Originally Posted by Panteracer

Ron,
I understand the decoupled 3 link... is that why
you did not do an IRS rearend?
Or too many things to break or worry about with halfshafts, etc

Bob

Originally Posted by gofastwclass

I'm curious about this too Ron.

I've always thought IRS was a good concept on paper but torque limited in reality. This was my own personal opinion after reading a few things, but I could be completely off base.

Hey Guys !

As you know, just about everything has pros & cons. There is no perfect anything, let alone a rear suspension. Some are better than others in a specific situation. I've designed & raced a lot of cars with & without IRS.

Some factors that guide my designs & builds are:
A. IRS does NOT add more rear grip. If it is setup optimum ... as opposed to "Hey! It fits! Let's just weld it in here" ... we can get the roll center optimum, rear steer either positive, neutral or negative, toe in or out ... to HELP/Assist the car to turn better.

B. Some cons are the roll center is NOT quick & easy to tune, getting the geometry optimum is just as complex as front suspensions & if you change the ride height, the geometry goes wacky. Overall they add weight to a car. They cost more & have durability issues to overcome.

C. If I'm designing & building a rear engine car, an IRS is practically mandatory. If the car is going to be LOW ... like 1/4" to 1" off the ground ... and the front end can not travel very far (due to the low ride height) ... we can't get very much "pitch angle" under braking. This limits how much weight transfer we'll get to the front tires, so having IRS really helps us to get this car to turn better. If I'm designing & building a rear engine car with a 1/4" to 1" ride height ... I WANT an IRS to help the car turn better. Regardless of cost or complexity, the IRS will be best in these applications, like Formula cars, Indy Cars, Prototypes, etc.

D. If someone has their car at some kind or "normal" ride height like 2.5" to 6" ... but run the front springs super stiff where there is very little front end travel in dive under braking ... we are not getting very much "pitch angle". So for them, they would benefit from IRS helping their car to turn better.

E. If I'm designing & building a front engine car, IRS is not mandatory, just an option. If the car is going to have some kind or "normal" ride height like 2.5" to 6" ... and we can travel the front end anywhere from 2.5" to 4"+ ... we'll achieve a good degree of pitch loading the front tires. So the IRS is not an advantage here, at least performance wise. On rough streets & highways, IRS can offer superior ride quality. But we're talking about running race tracks here, which are vastly smoother than rough streets & highways.

F. So if we're designing & building a front engine car of 2.5" to 6" ride height with a 2.5" to 4"+ front end travel strategy .. IRS is not a performance advantage ... so the question is which straight axle suspension is best.

G. Truck arms & leaf springs are the low end have many "cons." 4-Links are OK, as long as they don't bind in the articulation range we need. Triangulated 4-Links need to have the top links go from outside onth ehosuing to inside on the chassis ... or the roll center is too high. IMHO torque arms are the best suspension is someone is keeping their back seat. But in track cars, race cars & Pro-Touring cars that want to perform at the top level, nothing beats a 3-Link. Just gotta run a roll bar/cage & eliminate the back seat to do it right.

H. The basic 3 link articulates a ton & can have a quick & easy top link adjuster. If we offset the top link the correct distance, we can neutralize & eliminate torque steer. If we can place the top link mount high enough on th e housing, we can increase the percentage of torque that transfers load to the tires. And lastly, if we decouple it, we achieve something that NO OTHER rear suspension can. We separate the corner entry tuning from the corner exit tuning. With ALL other suspensions, the anti-squat percentage defines the rear tire grip on corner entry & exit.
* Higher A/S % = More rear grip on exit / Less on entry.
* Lower A/S % = Less rear grip on exit / More on entry.
It's always a compromise. A decoupled 3-link is the best Racing rear suspension for front engine cars with 2.5"-6" ride heights .. because the corner exit grip is tuned independently with the "Accel Link" (the lower one) and the corner entry grip is tuned independently with the "Decel Link" (the upper one). There are cons to this style of suspension too.

I. Regardless of type of link rear suspension, the other decision is Watt's link or Panhard bar. Since I'm a constant fine tuner, I like panhard bars that are fine tunable on both sides. For my clients, the Watt's links work best, because they're automatically neutral. And with our version, VERY quick & easy to adjust. Literally 20-30 seconds.

Hope that sheds some light on when & why IRS is better or a straight axle with some form of link suspension better suits the application.