Safe Driving with Driverthink

Independent Rear Suspension, or “IRS” for short, can take turns, curves and throttle steering to a far higher level.  A quick Google of the subject will suggest it’s not a topic of insignificant interest — to serious drivers.

Last week we reviewed the basics.  We looked at how IRS can introduce that Negative Camber thing to our rear wheels, producing a much more stable “squat”.  That’s when the bottoms of those rear wheels tilt out.

It’s important to note that those rears don’t just squat all the time.  At rest, or even on a straight of way, they’re going to act pretty much like any other system.  In other words, they’re going to rest or run with neutral Camber.  They’re going to be running straight up and down.

It’s on curves, turns and bumpy roads that those IRS wheels just “wanna have fun”.  On the rough road, the ability of one wheel to absorb shocks without jolting the entire car will contribute to a smoother ride and more control.  On curves the IRS really comes alive.  It’s all about that squat – both recognizing it when it happens, and creating it when you want some.

We can induce squat to those rear wheels by introducing torque (force) to them through the drive train.  Torque is what converts the power from our engine to power that makes the wheels turn.  Since this torque must come through the drive system and since we’re talking about “independent rear wheels here, control over squat is mostly about rear wheel, or to a point, some all wheel drive vehicles.

We can increase “forward force”, making the wheels go faster, or what I’ll call “back force”, making the wheels slow down.  Either will make the wheels squat to a degree.

Think of a hockey skater headed for the boards.  In making a left turn to avoid hitting those boards, he’ll stick his right leg (his rear wheel) way out and pushes with it.  This gives him the added stability and balance he needs to complete the turn.  To then make a right turn, he’ll straighten out and push off on his extended left leg.  This is kind of similar to what we can do with our IRS rear wheels.
 
So we’re leaning into a right hand curve at about sixty.  With the original IRS systems, that lateral force to the rear wheel would have pushed the bottom of the wheel in toward – and under the car, creating body roll and a possible roll-over.  The way to correct for this was to apply heavy torque to the wheel.  The additional torque to the wheel would cause it to flatten out and squat.  If we applied forward torque, we called it “powering through the curve.”  We were “pushing our leg out.”

The modern IRS system has sway bars and other good stuff so the wheel can’t easily fold in under the car any longer.  As a result, “powering through the curve” still works — but with far less risk of rollover. 

The idea of speeding up on a curve to decrease your rollover potential may go against natural instincts but once you get used to it, it’ll become quite natural – and safe.

If you decide you want to slow down in the turn, letting off hard on the gas will create back force torque to the wheel and also tend to squat it out.  You want that force to run through the drive train though, not the brakes.

Letting off the gas or even a good downshift will do the trick.  You’re making the rear wheels slow down the car.  If you brake, most of that stop force goes to the front tires.  The car will lean forward, reduce the weight on those rears, spring them back to neutral camber and introduce a very uncomfortable body roll.
 
The bottom line, then, is applying either forward or back force directly to the rear wheels through the drive train, will tend to squat the car and make it more stable in the turn.  But how do you do this with a front wheel drive vehicle?

You really can’t.  Even without the ability to torque your rear wheels though, the IRS is still there in your front wheel drive. The rears will still move independently over rougher roads and they’ll still squat on a harder turn.  The response just won’t be as controllable.

You won’t find IRS on trucks or “go straight” drag racing cars.  IRS isn't as strong as the straight axel.  IRS will also slide on slippery roads just as quickly as another system so all of this squat stuff assumes a dry, sticky road surface.

Getting to know your IRS — and what it can do for you, will allow you to safely approach turns and curves with confidence.

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