The SAE coordinate system
The SAE coordinate system
(OP)
As we analyze a dynamics problem, we occasionally realize that our choice of coordinate system was poor and we are forced to start anew. I doubt if we'll be changing the SAE coordinate system very soon, but I do wish more thought had been devoted to its conception.
It's reasonable to have the origin at the CG, of course. And, I'm certain it was thought equally reasonable to align its axes with the chassis. But, when you consider the dynamics involved...excluding, for the time being, the driver as part of the system...it really makes little sense. The only function of the chassis...apart from being a convenient place to which the wheels can be attached...is to provide the major portion of the inertial force. A more reasonable system would have the XZ plane perpendicular to the axle without steering.
Consider roll "understeer" and roll "oversteer." Since tire loadings are unaffected, these matters do not merit consideration when analyzing the dynamics of the car BY ITSELF. But, by the names which they have been given, they serve to cause a great deal of confusion. If the XZ plane was perpendicular to the rear axle, roll steer would never be mistakenly included in the understeer budget. (Measurement of steer angle would, of course, be more complicated.)
This is not to say roll steer effects are to be ignored, for, when the car-driver system is to be analyzed, they certainly do merit consideration. I just believe it would be better to consider them AFTER the understeer budget has been completed.
Any thoughts?
It's reasonable to have the origin at the CG, of course. And, I'm certain it was thought equally reasonable to align its axes with the chassis. But, when you consider the dynamics involved...excluding, for the time being, the driver as part of the system...it really makes little sense. The only function of the chassis...apart from being a convenient place to which the wheels can be attached...is to provide the major portion of the inertial force. A more reasonable system would have the XZ plane perpendicular to the axle without steering.
Consider roll "understeer" and roll "oversteer." Since tire loadings are unaffected, these matters do not merit consideration when analyzing the dynamics of the car BY ITSELF. But, by the names which they have been given, they serve to cause a great deal of confusion. If the XZ plane was perpendicular to the rear axle, roll steer would never be mistakenly included in the understeer budget. (Measurement of steer angle would, of course, be more complicated.)
This is not to say roll steer effects are to be ignored, for, when the car-driver system is to be analyzed, they certainly do merit consideration. I just believe it would be better to consider them AFTER the understeer budget has been completed.
Any thoughts?





RE: The SAE coordinate system
eg
http://en.wikipedia.org/wiki/Bundorf_analysis
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: The SAE coordinate system
RE: The SAE coordinate system
Next, we'll add some roll oversteer. The driver is forced to turn away from the oncoming traffic and, according to Olley's definition, the car is oversteering. But, as far as the tire patches are concerned, nothing has changed. All that's happened is that the chassis has been rotated relative to the car's path and the driver has had to turn the steering wheel to compensate. Since tire loads haven't changed, there is no inherent instability, critical speed, etc. associated with the introduction of roll oversteer.It all comes back to the arbitrary initial definition of the coordinate system. If the X axis is defined as perpendicular to the rear axle and the steer angle changed to maintain its angle relative to the X axis as the axle rotates under the chassis, a more realistic definition of oversteer is obtained.
Again, roll steer is an important consideration, but ONLY when the system under analysis is expanded to include the driver.
RE: The SAE coordinate system
RE: The SAE coordinate system
With that matter cleared up, it should be obvious that the point I am making is that the straight line path involves the same tire patch loads, whether rear axle roll oversteer is present or not. The roll steer accomplishes nothing more than a rotation of the chassis relative to the path.
RE: The SAE coordinate system
Lastly, the use of understeer, especially rear axle understeer, is primarily concerned with increasing the system bandwidth (shortening response times). The understeer raises the system natural frequency. In so called "Neutral" cars, system respnses are sluggish (1st order) and require exceptionally high cornering stiffness tires to obtain good subjective response quickness. Understeer lets you use more comfortable tires however with more overshoot in the response (which is generally agreed to as being undesireable in excess). Any good simulation will reveal these traits.
RE: The SAE coordinate system
You have two identical neutral cars traveling at a constant speed on the right side (US side) of a crowned road. Car A has a rear axle which remains parallel to the SAE Y axis. Car B has a rear axle which, when viewed in the positive Z direction, is rotated slightly clockwise relative to the axle location in Car A. (Don't worry about how it got there. It's simply fixed in that location.) Using the SAE coordinate system, Car A has a steer angle of zero. Using the same SAE coordinate system, Car B has a steer angle equal to the axle location difference.
Cars A and B will follow the same straight path, angling away from the crown, and the tire loadings will be identical.
That's all I'm saying. Well, I'm also asking why, when this identical condition is caused by rear suspension geometry, it is called roll "oversteer," when, for instance, there is no critical speed involved. I'm also maintaining that, while it might be impractical from a measurements standpoint, a coordinate system with the Y axis parallel to the rear axle would eliminate some of the confusion about roll steer.
RE: The SAE coordinate system
RE: The SAE coordinate system
Wait minute! Now, I "get it." You're taking my example Car B and making a real car out of it. I thought I was making it clear that it merely represents the effect of roll steer. I realize there are some oval track constructors who shorten up the wheelbase on one side, ending up with a car that looks like a dog that's chased one too many cars. As for high traction applications, I was involved with a factory dragrace team and this certainly never crossed our minds. We were more interested in asymmetric suspensions (to cancel driveshaft torque effects).
Anyway, it seems we now are talking the same language.
RE: The SAE coordinate system
Primary understeer affects not only the steering wheel angle, but also the slip angles at the tires. It is controlled using things such as:
Weight distribution
Lateral load transfer distribution
Camber
Tire properties
Secondary understeer does not change the slip angles at the tires, but does change the steering wheel position. Examples of things that control secondary understeer are:
Roll Steer
Lateral force compliance steer (LFCS)
Imagine a vehicle going around a constant radius turn at a constant speed with no roll steer and no compliance steer. The force at the front and rear axles is known, and the tires will generate those forces by running at some slip angle.
Now take the exact same vehicle and add a spring in the intermediate shaft. (Effectively adding front LFCS) Go around the same corner at the same speed and what you will find is that the road wheel angles are the same as in the case above, the slip angles are the same as the case above, but the steering wheel needs to be turned significantly further to get the same road wheel angle at the same speed. This feels like understeer to the driver, but it did not change the slip angles or road wheel angles for a steady state turn.
I have some philosophical beliefs about when to add what type of understeer…
-Joe
RE: The SAE coordinate system
That "feels like" business is when you start considering the driver as part of the system. Brings to mind the little trick that was played with the '57 Chrysler products. If you'll recall, the torsion bar front suspension was a big deal that year. But, the handling wasn't really as different as the magazine testers made it out to be. The big difference...as far as driver "feeling" is concerned...was at the other end of the car, where the leaf springs had been changed to shorten the length from the housing forward. The increased roll oversteer made the driver "feel like" he was driving a car with far less push.
I'm presuming this is the sort of thing included in your "philosophical beliefs." Hey, I'm not disagreeing! Give the customer what makes him feel good.
RE: The SAE coordinate system
Primary understeer works in the linear range, and at the limit.
Secondary understeer will not work at the limit. Once you write a check the tires can't cash, roll steer or compliance steer will do nothing.
-Joe