Author Topic: Geometry calibration, what is it and why?  (Read 21969 times)

Offline wheels-inmotion

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Geometry calibration, what is it and why?
« on: August 06, 2010, 09:51:17 PM »
 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


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Offline stealthwolf

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Re: Geometry calibration, what is it and why?
« Reply #1 on: August 06, 2010, 09:54:09 PM »
Can't help but feel pics/diagrams would greatly benefit your otherwise informative post.

The GTI isn't just a machine. It's very much a living, breathing thing.

Offline RedRobin

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Re: Geometry calibration, what is it and why?
« Reply #2 on: August 06, 2010, 10:27:45 PM »

Can't help but feel pics/diagrams would greatly benefit your otherwise informative post.


.... x 2



^ Though to be fair, this applies more to your other technical post. Not everyone here is so technically minded.
« Last Edit: August 06, 2010, 10:30:20 PM by RedRobin »


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Offline wheels-inmotion

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Re: Geometry calibration, what is it and why?
« Reply #3 on: August 08, 2010, 09:56:46 AM »
Can't help but feel pics/diagrams would greatly benefit your otherwise informative post.

Agreed, is there a "how to guide" here you can point me to?

Offline stealthwolf

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Re: Geometry calibration, what is it and why?
« Reply #4 on: August 08, 2010, 10:00:55 AM »
Agreed, is there a "how to guide" here you can point me to?

You mean with uploading pictures? This websitr doesn't have the facility to upload them. You need an accout with sAy photobucket (free). Upload the pics there and it gives you the address to copy and paste. As long as it has
Code: [Select]
[img][/img] tags, you'll be fine.

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Offline wheels-inmotion

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Re: Geometry calibration, what is it and why?
« Reply #5 on: August 08, 2010, 12:33:50 PM »
Reads like a lot of work.... Maybe i'll be more explicit in the posts and allow members imagination to do the rest.

Offline Teutonic_Tamer

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Re: Geometry calibration, what is it and why?
« Reply #6 on: August 30, 2010, 02:12:44 PM »
I understand it all . . . .   :P

So what kind of prices are we talking about for you to do a full check and adjust?  :smiley:
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Offline RedRobin

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Re: Geometry calibration, what is it and why?
« Reply #7 on: August 30, 2010, 03:18:06 PM »

I understand it all . . . .   :P


....Hello smart arse! :evilgrin: - Good to see you back, buddy  :drinking:


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Offline stealthwolf

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Re: Geometry calibration, what is it and why?
« Reply #8 on: August 30, 2010, 11:12:20 PM »
+1. T_T's back in town! :pomppomp:

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Offline the bruce

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Re: Geometry calibration, what is it and why?
« Reply #9 on: January 10, 2011, 12:36:35 PM »
+1. T_T's back in town! :pomppomp:


T O E



Toe in (= positive); toe out (= negative)


C A M B E R



Stock - 1° front and - 1°40' rear. 1° is 60 minutes (' ). 1.5° = 1°30'.


C A S T O R





sorry, I don't know the english word for 'Spreizung'




again, I don't know the correct word for 'Lenkrollradius'





C E N T E R  O F  R O L L



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Offline Kiwi

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Re: Geometry calibration, what is it and why?
« Reply #10 on: January 10, 2011, 06:04:31 PM »
 :signIWS: What he said.

Offline s2martyn

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Re: Geometry calibration, what is it and why?
« Reply #11 on: February 26, 2011, 01:03:39 PM »
can anyone point me in the direction of the front track settings please..done a search to no avail..car will be set up on supertracker(in mm not degrees)thaks in advance martyn

Offline RedRobin

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Re: Geometry calibration, what is it and why?
« Reply #12 on: February 26, 2011, 03:03:36 PM »
^^^^
Don't know if this info will help, but here goes:

If you are having a laser alignment on say a Hunter rig, one of the presets is called: "Volkswagen : Golf V 2004-09 : Sport Suspension GTI (G08/1JE)".

Ranges are:

FRONT : LEFT & RIGHT

Camber: -1º14' - -0º14'
Caster: 7º17' - 8º17'
Toe: 0º00' - 0º10'




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Offline rich83

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Re: Geometry calibration, what is it and why?
« Reply #13 on: February 26, 2011, 03:05:10 PM »
oops.. i think i got the USA setting by accident!  :popcornsoda:

Offline Teutonic_Tamer

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Re: Geometry calibration, what is it and why?
« Reply #14 on: February 03, 2012, 11:38:19 AM »
sorry, I don't know the english word for 'Spreizung'

'Spreizung' is German for 'steering axis inclination' (SAI) - basically the modern term for the old skool 'king pin axis inclination' (KPI).  The blue line is just the vertical plane (as measured at the centre line of the roadwheel/tyre), the red line is the steering axis inclination, and the degree of angle between the vertical (blue) and the SAI (red) is the value.


again, I don't know the correct word for 'Lenkrollradius'

'Lenkrollradius' means 'steering roll radius' - no normally measured.  Using the upper picture, this is measured as the distance from the road surface to where the vertical (blue) crosses the SAI (red).

There is a final measurement (again, not normally stipulated) - the distance widthwise on the road surface between where the vertical and the SAI 'touch' the road.  From the deepest bowels of my memory, I think this is called something like 'scrub offset'.


Its interesting to note that no one has actually discussed the actual effect each setting does . . .  :booty:
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