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To explain....

 

wim has agreed to train Pete Mather online and this will be visible to all as we undertake this opportunity.

 

Currently Pete works for a fast fit chain and he desperately wants to advance his understanding of chassis dynamics to advance his work.

 

Let's begin.

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Over to you Pete.....

 

I thought WIM was doing the training, how is this working then...Pete asks the questions and you answer?

 

It has to start somewhere..... We have a huge pool of information in wim web/forum and i have no idea of the level of understanding Pete has so where do you start?

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Over to you Pete.....

 

I thought WIM was doing the training, how is this working then...Pete asks the questions and you answer?

 

It has to start somewhere..... We have a huge pool of information in wim web/forum and i have no idea of the level of understanding Pete has so where do you start?

 

That's a good point actually...I suppose the best place to start though is the beginning...why is a geometry needed, what are the advantages over tracking etc?

 

Even though you will both know the answer and the regulars on here...for newbies they will get the full picture by starting at the beginning :lol:

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Over to you Pete.....

 

I thought WIM was doing the training, how is this working then...Pete asks the questions and you answer?

 

It has to start somewhere..... We have a huge pool of information in wim web/forum and i have no idea of the level of understanding Pete has so where do you start?

 

That's a good point actually...I suppose the best place to start though is the beginning...why is a geometry needed, what are the advantages over tracking etc?

 

Even though you will both know the answer and the regulars on here...for newbies they will get the full picture by starting at the beginning :lol:

 

Well I suppose "Geometry" is used to achieve, as near as possible, the optimum tyre posture when the vehicle is in motion.

Is that a good, if slightly simplistic, definition ?

As for tracking, by checking only the front wheels across a single plain, it makes several unfounded assumptions which can, and probably will, render any subsequent remedial work (at best) counter-productive. A Full Geometry check looks at the position of all 4 wheels across all relevant plains and can also indicate suspension defects

 

I'd say about 85% of my "knowledge" of suspension/geo is from years of Gran Turismo and Forza tweakage. :lol:

 

Anyway, been through all the Techno Info and have questions!

 

1: Is KPI applied to steered axles only ? If so, what are the implications as to 4-wheel steer ?

Or is it applied to the front axle only ?

 

2:Do manufacturers allow a tolerance to be applied to Included Angle ? That is, assuming camber is within specs, the tyre shows even wear but IA is not to spec ? Or is tyre wear/poor handling inevitable given such readings ?

 

3:Could you define bump steer ? And what are the causes ? I've never been able to pin-down a clear definition.

EDIT: Look what I just found:http://www.wheels-inmotion.co.uk/forum/index.php?showtopic=2335 ("Positive Ackerman theory, I have a cunning plan")

 

4:I'd also be interested in the implications of changing wheel offset (ie using spacers, different wheels) and also Rolling Radius.

 

 

I have more (ofcourse) but these are the most relevant and will do for starters!

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In reply to question 1

 

KPI or King Pin Inclination is now known as SAI or Steer Axis Inclination.

 

SAI is the position of the lower pivotal point, the lower joints pin measurement is perpendicular to the vertical, plus since most cars have positive castor the pins position will be laterally set-back.

 

One of the goals with SAI is to allow the cars weight to migrate across the tyre during a turn in conjunction with the castor sweep and since the rear doesn't require a castor trail then SAI on a 4ws is not needed.

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In reply to question 2

 

The Included Angle is just an angle and there's no OEM target for it.

 

The IA is used as a diagnostic tool to determine if there's a bend in the chassis or suspensions sub components. As you should know the IA is the SAI + or - the cambers + or - position.

 

For example

NSF

Camber -30

SAI 11d

IA 10d 30'

 

OSF

Camber -1d

SAI 10d 30'

IA 9d 30'

 

And the data said the osf camber was out of tolerance then it's a fair bet the bend is within the un-sprung chassis namely the wishbone. If the IA was even but the camber still outside of OEM positions then the bend would be in the sprung chassis.

 

IA is never + or - it's just an angle.

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1) Okay, got that. That's straight forward (uh, no pun intended!).

 

2) So an asymmetrical SAI indicates a bend somewhere within the chassis as a whole. The IA can be used to determine wether it's within the sprung or unsprung areas. Ineffect, narrow down the search. I assume simply correcting camber would be illadviseable ?

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1) Okay, got that. That's straight forward (uh, no pun intended!).

 

2) So an asymmetrical SAI indicates a bend somewhere within the chassis as a whole. The IA can be used to determine wether it's within the sprung or unsprung areas. Ineffect, narrow down the search. I assume simply correcting camber would be illadviseable ?

 

If the camber's adjustable i would adjust it so long as the SAI's end up within two degrees of each other. if the cambers fixed then the IA will help find the bent component.

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In reply to question 3

 

Bump steer means as it says, the wheel hits a bump and the car steers without Yaw.

 

Study this image

post-2-1220993288.gif

 

What you can see is the suspension in compression/ release and the sub-systems uniform radii, this radii will not interfere in the intended path.

 

What if the sprung area has been lowered?

 

Study this image

post-2-1220993516.jpg

 

This car has been lowered and as you can see the through pin on the track-rod-end is pulled away from vertical despite the fact that the suspension is at rest.

 

Dynamically as the suspension compresses both the wishbone and steering arm will migrate but since the TRE through pin was fully extended statically, dynamically it has no option other than act on the steering during transition and steer the wheel on the bump.

 

Bump-steer is normally a consequence for the modified car and there's after market kits available to correct the problem.

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In reply to question 4

 

Are you ready for this yet without concluding 2 and 3?

 

Just one quick point to Q4. If aftermarket TREs were fitted would that require bespoke toe settings ? Or would factory suggested settings still apply ?

Otherwise I'm good. I actually quite enjoyng this ^_^

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In reply to question 4

 

Are you ready for this yet without concluding 2 and 3?

 

Just one quick point to Q4. If aftermarket TREs were fitted would that require bespoke toe settings ? Or would factory suggested settings still apply ?

Otherwise I'm good. I actually quite enjoyng this ^_^

 

It's easy isn't it when you can ask the questions relivent to you.

 

Generally if the car needs aftermarket TRE's then it's highly modified and the OEM toe wouldn't apply anymore. if the intention is for the track then most times there would be 0 toe f/r (on bump) in an effort to increase saturation limits.

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I understand Westfields are notorious bump-steer'ers due to poor location of the steering-rack.

 

Incidentally, remember our hyperthetical MX5 scenario ? Had that today. Facelift Porsche Caynne (Jeep affair thing). To look at the tyres I'm immediatly thinking Camber. Smooth wear, localised on outer edge on all 4 tyres to varying degrees. Slapped it on the rig and all primary angles were near enough spot-on, save for Steer Ahead which was wandering -/+ 2 minutes. Was at loss but given it was still under warranty I advised get it back to Porsche with a copy of our report and one of the tyres in the boot. Maybe a bad bush somewhere ?

 

Y'know the more I look into this the more I'm getting frustrated at KF. I'm just skimming the surface and, as my Hunters are taking longer as I respect ride-height pre-checks, pre-loading etc I'm under more pressure. And I'm only measuring primary angles at the moment too. That rig is wasted on KF.

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How far down the sidewall did the wear go..... was it as far as this?

post-2-1221080948.jpg

 

I understand your frustration about being under pressure to cut corners..... points to remember is you are in fast-fit and your manager doesn't understand what you are doing so he won't be sympathetic toward your desire to do the job properly.

 

On that note is he aware of your training?

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Back on topic then......

 

Most times the German marques use a very long castor, low camber and low SAI. if you read the actual values often you will see the castor migration on lock -V- ackerman will exceed expectations.

 

Point to remember "green" on the screen doesn't nessacerally mean it's ok when you add dynamics to the figures.

 

A little test for you..... measure your next car and with a live screen turn the steering (20 degrees or so) and watch how the cambers and castor migrate, let me know what you find?

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Off untill Tuesday now so it'll take a while before I do another Hunter.

 

I've noticed the primary angles give no indication of SAI. Haven't looked for a while but is there a Seconday procedure specifically for SAI (such as there is for TOOT etc) ?

I'm going to try and "sneak in" some secondary measurements in the future as I'd never even heard of them prior to WIM.

 

edit: Could you get into more detail in defining Ackerman please ? Especially how it differs from TOOT ?

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