wheels-inmotion

For this superb club and it's members i would like to offer a better understanding of Geometry or in it's true terminology "chassis dynamics".

Rather than saturate everyone with a 2500 word thread, i feel it's best i brake this topic into weekly sub-sections.....Hopefully yourselves and the moderator team will agree.

The topics are constructed by (wheels-) (Torque Developments International ) with a Pro-drive accreditation.

Before we can consider chassis dynamics, first we need to understand "US".

The demands on manufacturer's to deliver faster, better handling cars has been surpassed by our inability to drive them. The evolution of systems like ABS and interactive suspension, to mention a few allows the cars intelligence to replace our miss givings.... "Why"

Part 1 Humans
The driving process:

• Step one: A driver looks at the oncoming road and estimates the required inputs to complete the task at hand, we make this estimate based on a model of the car held in our memory, the more accurate this model the more accurate our initial guess.
• Step two: An “Open Loop” input is made according to our guess
• Step 3: Then “Closed Loop” trimming modifies these inputs to take account of emerging conditions or errors in the initial guess

There are 3 main channels of feed back used for “Closed Loop” driving

1. Visual feedback
2. Inner ear (yaw rate, lateral acceleration etc) feedback
3. Muscle tension or Steering wheel torque feedback

It is through these communication channels that a car can speak to a driver.

Ok

firstly, the drivers eyes….

Processing visual stimuli is actually quite a complex procedure and incurs a delay of around 600msec in most people between seeing and understanding, so that’s a maximum update rate of around 1.8Hz
Driving by your eyes alone is only possible if no quick action is required.

• If the roads and bend radius's are known and learned
• And of course as long as no chassis control task’s are required

The inner ear….

Processing the yaw and information from the inner ear is a far simpler process and is therefore faster but even so this feed-back process still suffers a delay of around 300msec so this loop can actually update at around 3.2Hz which is pretty fast.

Muscle tension in the drivers body….

The task of moving the steering wheel is a function carried out by the brain stem, the brain stem works automatically to maintain the correct steering angle by varying your arms muscle tension. Changes in the steering resistance cause the brain stem to get confused and flag up the problem to a higher part of the brain that is responsible for processing the overall task of driving the car. This form of feedback is extremely fast at around 100msec so this can provide you feedback at 10Hz (ten times per second).

Some interesting points to note:

• A driver’s in-head car model is almost exclusively linear
• Excessive anxiety in a driver can easily cause the steer torque mechanism to be completely “swamped” by muscle tension and therefore useless.
• Inexperienced drivers are frequently unable to interpret inner ear signals or are too confused to act on them so they are discarded by the brains automatic information filters in the brain stem
• A surprising level of confusion exists in the inner ear between yaw and chassis roll, this lead some drivers to “dislike” body roll
• Many drivers are found to revert back to an entirely visual driving strategy in emergency circumstances
• Many drivers are overwhelmed and lose control even though the car was capable of completing the maneuver
• A maximum practicable hand steering wheel rate is around 1100 degrees / second –but not for long
• Typical hand wheel rates are much slower - < 100 degrees / second in “Sensible” driving.

As you can read our abilities are somewhat primitive compared to our manufacturing requests.....

That's it part 1 "Human".

Notes
The information platform is born from Pro-drives chassis dynamics course.

This encompasses chassis dynamics, suspension calibration, dynamic indexing, kinematics..... to mention a few.

The papers are not available to the public since the course costs many thousands of pounds and attracts designers from around the world.

Since we can only absorb a small amount of new information the papers from the study run into the hundreds....Fortunately belayed questions are answered in the papers so you don't need a Pro-drive "top-up"...... For a few years anyway.

My intentions here was to explain why the MX5 MK1 and 2, 2.5 has such particular needs Geometrically.

I decided that to just simply blabber on about Geometry doesn't offer the complete picture. This would belittle the validity of the thread and probably over complicate my intentions.

Hopefully gentle, progressive steps into the realm of chassis calibration will offer members a more imbedded understanding encompassing the entire picture not just the frame.

I would like to acknowledge Sam at TDi who's grammar construction regarding these topics is far superior to mine (as you may notice)