Geometry-chassis calibration

[Tony]

What is it?

Simply put Geometry is the X,Y,Z axis that forms a dynamic realm expressed by the suspension, steering and the cars parallelograms.

 

Why have it?

The actual tyre contact patch is about 1/3 of the total tyre width. During the suspensions transitions there is a need to maintain the position of the contact patch without distortion or saturation but still allow for comfort and indeed tyre preservation.

 

How?

Geometry has two areas that concern us after the mechanical engineers offer the final product.

Static:- This is the inert position of X,Y,Z measured during a calibration.

Dynamic:- This is the "expected" gains of X,Y,Z whilst in motion.

 

The true realm of Geometry is dynamic, a fluid 3D environment applying Geometric forces that are dependable during Yaw, maintaining the contact patch..... The holey grail of chassis calibration is realizing the gains whilst the cars chassis is static... Not an easy task.

 

The angles and forces

Of the many angles we will just concentrate on three.

 

Camber:- Is the vertical position of the wheel, it's duty is to position the cars weight to the correct area of the tyre contact patch. If the vertical position is incorrect then the vehicular weight will be disproportionate and accelerate tyre wear.

 

Camber force:- The tilt of the camber deforms the circumference of the tyre sidewall, this forces the tyre into a conical profile that wants to roll into it's conical centre...... So if i were to describe the camber force it would be "compressive".

 

Toe:- Is the longitudinal position of the tyre relative to the direction of travel. In a straight line a need to calculate the type of momentum albeit front-rear or four wheel drive -V- the actual rolling resistance generated between the tyre contact patch and the road makes Toe an infinitesimally difficult position to exact outside of the theoretical.... Nevertheless the ultimate aim for all drives is a dynamic 0 toe.

 

Toe force:- Toe exerts no force unless aggressively displaced, then the car will feel unstable off the bump.... Toe cannot make a car pull.

 

Castor Is an extension of the front wheels steer axis... By design the lower steer axis is off-set to the wheel centre and the upper rotational axis.

 

If an imaginary line was drawn through the pivotal points it would fall in front of the tyre this is called the "trial distance", by design the tyre has no option other then to follow the trail unless interrupted by Yaw.

 

Castor force:- Is compressive adding weight to the steering feel.... My best description is the castors energy is gyroscopic resiting any attempts to deviate from dead ahead.

 

Time to turn

Now we have a basic understanding of the angles lets see what happens when you turn....This explanation will not include inertia or transfer properties.

 

Example taken at the front wheels on a 10 degree right lock

At 10 degrees the front camber positions will change from / \ to / / the near side castor will reduce and the offside will extend.

 

This action lowers the cars upper parallelogram on the offside corner, diagonally modifying the camber contact patch, the castor trail and the acceptance of toe on the inner wheel.

 

Most believe the recovery of the lock is due to the castor extension or gyroscopic laws, in fact this is not true?

 

During any lock transition the cars upper parallelogram is supported by the lower King-pins (lower swivels) as the inner castor sweeps forward the inclination of the king-pins is off-set so that the inner offside front is more vertical than the relaxed nearside front.

 

Since the position of the king-pin is perpendicular a higher position holds more vehicular weight and since the kingpins are connected via the steering rack a natural equilibrium insists the steering will be returned.

 

Those settings

All manufacturers offer a static setting range, this allows for wear since the driven car is subject to road trauma and progressive deterioration of the suspension, so we have a range.

 

On a fully adjustable chassis a range means nothing... Optimum positions can be achieved and you should expect nothing less if you want to experience the true splendor of the chassis.

 

That's it..... I hope my explanation made some sense and offers a little understanding for you and what to expect at the shop from the Geometry calibration.

 

Thank you

Tony

[Sam@TDi]

Tony that is brilliant, a really good succinct list of definitions. Bit like a geometry dictionary.

 

I followed you whilst reading this but I can imagine that to some who perhaps don't have an in depth knowledge of the terminology involved they might struggle to understand the meaning and context of some of these terms.

 

What could really bring this to list to life in the future might be some nice clear diagrams, combined with these descriptions that could be excellent.

[Tony]

Tony that is brilliant, a really good succinct list of definitions. Bit like a geometry dictionary.

 

I followed you whilst reading this but I can imagine that to some who perhaps don't have an in depth knowledge of the terminology involved they might struggle to understand the meaning and context of some of these terms.

 

What could really bring this to list to life in the future might be some nice clear diagrams, combined with these descriptions that could be excellent.

 

Thanks Sam@TDi... The post was part three of a series of topics in the MX5oc forum. I used your superb explanations for part one and two>

http://www.mx5ocforum.co.uk/viewtopic.php?t=56033

http://www.mx5ocforum.co.uk/viewtopic.php?t=56287

 

As the lead for this final part 3..... Unfortunately i don't have image privileges in the MX5oc........ But i do in wim so as suggested i will add some.

 

This is something i wanted to do for the MX5 club but it would have been an impossible task without part 1&2 to explain why we need Geometry as the final modifier....

 

Needless to say explaining Geometry in a pure text format is incredibly hard... Part 3 took about two hours