Stick Axle vs IRS Debate – Which is Better?

Regarding the Stick Axle vs.  IRS debate in the application of heavy front engine high powered cars, I have given seminars about this, spoken about it, and written about it so many times I thought I had an official document on it, but I can’t find one in any of my files.  I have also proven my ideas in this regard dozens of times on track.  I guess I should write one.  This is the best I can do in the few minutes I have.  (Explaining the physics behind this would require and/or videos that I will make when I publish my book… yeah, some day…)

Everything I refer to here as a “Stick Axle” is meant to be as we geometrically lay out a GR40 stick axle system, with correctly designed TorqueArm, two lower trailing arms, and widely spaced springs and shocks or Coil overs, and a Panhard, (track bar) or Watts Link.  It also refers to cars in the vicinity of 2800 lbs.  or more operational weight, front engine, rear drive.

Claimed Benefits of IRS

• Less un-sprung weight
• Adjustable camber allows for tuning of tire contact patch
• Adjustable Bump Steer allows chassis tuner to affect vehicle behavior
• Better under car packaging for aero down-force design

Arguments against the above

• If one takes the time to actually measure and calculates the un-sprung inertia in a properly tuned, heavy, high hp/torque output GT car, this difference is usually negligible, unless sacrificing reliability (strength) and/or cost effectiveness. More expensive material to save weight may improve this, but the reward usually does not warrant the trade off in other factors listed elsewhere in this blog.
• Static camber requirement is dependent on body roll; but, in a stick axle camber change due to body roll is nonexistent. With a stick axle we can apply all static camber required by making the housing so.  We do this in most of the Hybrid 8.8/9” axles we build.  Only enough static camber is required to compensate for tire carcass distortion under lateral load.  With an IRS, much more static camber is required than in a stick axle, due to the body roll.  This reduces tire contact substantially in a straight line.  Again, this is not so with a stick axle.  Of course, making the springs and shocks really stiff will negate the high static camber need; but then the car is much harder on drivers and tires, and is very sensitive to bumps, (skittish).  Optimizing all this for given conditions is difficult and takes a team.  Not so with a stick axle.
• This enters an additional variable that along with the other variables that IRS introduces is most difficult for any but the best engineers to work with.

• Packaging under-trays for high down force is difficult with a stick axle. IRS works better here only on tracks where very high speed is obtainable for longer periods, and the car has serious down force, since this requires a really stiff setup, which the IRS needs to reduce body roll affecting applied camber.  Such stiffness reduced traction and bump compliance at lower speeds.  Still the stick axle car will be far more forgiving at the limit than an IRS unit given the same down-force and weight, due to the more consistent tire camber and loading.

So, the claimed benefits are nullified. 

And there are additional arguments against IRS:

• It is most difficult to lay out more than 100% anti-squat geometry, or rise in an IRS without severe compromise to other geometric characteristics.
• During changes in power induced acceleration or heavy braking, subsequent lifting and dropping of the sprung mass as the torque reacts through the chassis geometry, the subsequent torque induced loading or unloading of both front and rear springs uniformly yields a much more stable car when transitioning left and right, or topping hills, --any time the ride height is changing due to lateral or vertical accelerations of the CG. This can be achieved with a stick axle without other detrimental effects, but cannot be achieved with an IRS.

As an example, a Viper or Corvette or IRS Mustang under acceleration lifts its nose, and squats in the rear.  This reduces camber in the front when accelerating out of a corner, and increases rear camber, reducing tire contact.  Also a large amount of the weight transfer is applied through the springs.  A proper TorqueArm layout on a stick axle lifts the car at the CG, more uniformly removing weight from all four springs, distributing the weight to the center of the rear axle, therefore evenly distributing to both rear tires independent of body roll, or ride height changes.  This yields a more consistent cornering balance through transitions of power without the use of stiff springs.

Also the camber change in the rear under acceleration, while the ride height varies causes the center of load and thrust of the rear tires to vary likewise across the tire.  This causes instability under power at the limit and the dynamic variation is exacerbated by wider tires.

Basically, all these things make the rear tires scrub against each other in varying ways, which can only be corrected by limiting the travel through stiffer and stiffer springs, and bars.  This results in what is uncomfortable at best, hard on the tires, and though may be fast for a while, yet unforgiving when pressed on all but the fastest and smoothest sweepers, which when road racing in America is generally not the case.

And, the limit to the amount of evenly distributed rearward transfer is when all 4 springs unload completely.  This cannot happen in an IRS, so more over all weight is available for transfer evenly to both rear tires, again without change to camber.  Result: a stick axle potentially has more consistent forward bite, as the tires can handle more power with the driver experiencing “less sawing at the wheel” to maintain control.  This is why every sprint car and super modified in the world uses similar geometry.

• Non-adjusting and low roll center. To balance roll couple distribution, a low roll center requires increasing roll stiffness through even stiffer springs, and/or use of a heavy rear anti-roll bar.  To an extent this defeats the “independent” action of the rear suspension, which compared to a stick axle limits extension travel mid-corner in most cases.  This can work the differential harder by unloading the inside rear tires on bumps and other surface irregularities, especially when cresting hills while turning.  Whereas the stick axle can have a roll center easily adjustable and placed where little or no rear anti-roll bar is needed.  Again the spring rates can be reduced, maintaining compliance and therefore better cornering adhesion and stability.
• In a sports or passenger car, it is not possible to package the IRS linkage so that the anti-squat geometry remains near constant to both rear wheels regardless of body roll. This can be a cause for a nervous car under power, but is not the case with a stick axle.
• Higher cost, both in design and maintenance, as well as tuning. Optimizing to conditions can require more than one engineer on sophisticated GT cars.  What club racer can afford this? Also, half-shafts, control arms and uprights all cost much more than a stick axle to maintain.  Plus tire wear is generally higher due to the scrub.

Arguments against a stick axle:

• Packaging under-trays for high down force designs is difficult compared to an IRS. But, IRS works better here only on tracks where really high speed is obtainable for longer periods, and the cars has serious down force since this requires a really stiff setup, which the IRS needs.  The stick axle car will be far more forgiving at the limit that an IRS unit given the same down force.
• Perception in the market as being archaic or unsophisticated.

To Summarize:

Any IRS is better than a stick axle mounted with leaf springs, a four link, or a short 3 link.  However with a properly designed and tuned stick axle rear axle with loads controlled by a TorqueArm with Watts link or Panhard, (or even truck arms), the higher the power, the wider the tire, and the rougher the track, with the stick axle car the advantages over IRS are:

• It is less expensive to build.
• It is more reliable with fewer moving and/or highly loaded parts.
• Requires less maintenance, and associated operational cost.
• The wider the tire the better/greater the performance advantage over IRS.
• The more torque applied to the rear tires, the greater the performance over IRS.
• It will be more forgiving and therefore faster in most driver’s hands especially over time than IRS.
• It will be more of a joy to drive, requiring less or no electronic driver aids to control at the limit than IRS.
• It will usually yield less rear tire performance degradation and longer rear tire life than IRS.
• In a production based car it usually easier to adjust for optimal chassis balance as ambient conditions change, and therefore requires less track side manpower and data management than IRS.
• Has no significant difference in ride quality in a track prepared vehicle.

These are the thought of Bruce Griggs quickly written for this discussion.  This is not intended to be an engineering treatise, and does not include all lesser details of the argument but only those major points of consideration for chassis design and/or preparation at the club level of high power GT car.