Whiteline ALK

[TMcRally]

Hi

I am trying to make some sense of the technicalities of the Whiteline ALK or as they now call it WALK. Not sure if you are aware of it but it lowers the rear height of the front lower control arm by about 20mm. It also moves the arm away from the centre of the car to increase Castor. I'm Ok with the castor side of it but the lowering of the arm I am having some trouble understanding.

I'm not sure I agree with the working out of the SVIC and I'm not sure if/how this is providing a benefit.

The kit will also increase the SAI which is already around 15 degrees.

I can only see a softening of the effective spring rate which provides better front end grip, and we get more castor (maybe good maybe bad) and a firmer bush.

Am I missing something.

Thanks
Dave

 

[GregLocock]

If you start with that much ad and al then a reduction has to be a good thing. Stupid bound-up suspensions.

I haven't checked the SVIC method but why do you think the SAI will increase?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[JohnLear]

If I'm understanding this correctly, lowering the rear end of the 'wishbone' will alter the angle of it's pivot axis (up toward the front / down toward the rear, to state the obvious). With a Mac' Strut this will (to some degree) lessen caster increase with bump motion and lessen caster decrease with droop motion.

With a more or less horizontal 'wishbone' pivot axis the caster angle changes with suspension motion because the ball joint rises / falls (more than less) vertically, but because the top of the Strut doesn't move (i.e. it's 'movement' differs from that of the ball joint) this causes the line between the top of the strut and the ball joint to change angle (i.e. caster angle change with vertical ball joint motion).

Only if the wishbone pivot axis is at 90° to the caster angle (in side view) will the caster remain constant with suspension motion, and lowering the rear of the wishbone moves the geometry closer to this condition, thus decreasing caster change.

Similar bump / droop caster change can occur with SLA suspensions if the pivot axes of the upper and lower wishbones are not identical.

The 'ALK' will I suspect also (to some degree) promote 'dive' motion under brakes, which would make sense since one of the 'benefits' they are promoting for this product is a decrease in 'lift' when accelerating (i.e. 'ALK' = 'anti lift kit').

 

[TravWood]

Yes, "anti lift kit" ie it adjusts the amount of Anti lift (and anti Dive). As I think Greg has explained to me, somewhere else on here, its not a trade off between Dive and Lift.
For some information from the source - See attached File
Certainly the firmer bush helps reduce unwanted movement under load for more motorsport orientated use.
Extra front Caster is usually a good thing right?

 

[JohnLear]

TravWood wrote:"Extra front Caster is usually a good thing right?"

Please preface all comments with 'IMO':

It depends on the details of 'extra' and 'usually', and for what purpose the car is being used by what drivers.

If trying to improve the handling capabilities of a production car (where for the original set up a high degree of handling prowess may not be the Prime Directive, and which may have a relatively small stock caster angle) then greater caster angles do create some desirable geometric effects (i.e. desirable steered camber changes at both front wheels, increased mechanical trail, an increase in the steered jacking effect that loads the IF and unloads the IR (more useful for FWD rhan RWD)).

However, assuming a well designed geometry with little bump steer in stock form, with more than a fairly small caster angle change (whether a static or dynamic change...) there will almost inevitably be a significant increase in the amount of bump steer, which would need to be addressed (i.e. this bump steer is created by the change in the vertical position of the tie rod end that will occur as a result of changing the caster angle).

With a Mac Strut, the more static caster angle there is the more dynamic caster angle change there will be with suspension motion. This won't be the case with an SLA suspension where both 'wishbones' pivot on parallel axis.

I would think that in principle it's more likely than not to be somewhat desirable to prevent (or lessen rather than allow to increase) any dynamic caster angle change. Such caster change (I suspect) is likely to create at least some degree of 'noise' in the steering 'feedback'.

However, it may be that even with a Mac Strut the positives of using a larger caster angle might outweigh the negatives...

 

[GregLocock]

I doubt there is any real benefit in more than 4 degrees of castor for a reasonably conventional car, you just get locked into more by packaging and other things.

For a circuit car where grip rather than feel is paramount then the castor should be set by the amount of inboard vs outboard camber you want in your most important corner. Obviously there is a tradeoff with your camber gain curve and staic camber settings.

In practice I doubt anyone bothers to change it for different circuits, who reads these pages.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

JL

He did say USUALLY, not always and he did pose it as a fairly rhetorical question not a stated fact.

Regards
Pat
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[JohnLear]

PP,
I may have got it wrong, but I read it asking for comment / confirmation of a statement, not as a rhetorical question, i.e. I interpeted it like this;

"Extra front Caster is usually a good thing. Right?".

 

[patprimmer]

I expect only the OP knows what was between the lines, but I interpret it like this.

"Extra front castor is USUALLY a good thing right? ;-)"

I also agree more than about 4 deg is starting to go past optimum in many cases, but I expect most cars have considerably less than 4 deg. A lot also depends on camber and king pin inclination and car weight over the front and steering ratio and rear suspension settings etc etc Oh and of course intended use and drivers tastes.

Regards
Pat
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[TMcRally]

Hi everyone

I apologise for not coming back sooner, I have changed my email address and didn't stop to think the forum wouldn't be able to notify me. Feeble excuse.

GL, your right the SAI won't change, I have had a proper look at the lower arm and I notice the ball joint won't move out as I thought it might as the rear pivot point moves out.

JL, I can follow your post with the reduction in caster change. I can't see why the ALK won't create both extra lift and extra dive though. If the SVIC is lowered further from the COG then won’t the mechanical advantage in braking and accelerating increase the load/unloading put through the spring ?

For all, The road going STi's have 3.9d of static caster and the competition base model has 4.9d. I have tried increasing from the 4.9 to 7 and didn't like it, however at that time I was running the rear clutching diff with a lower pre-load and didn't like the unloading of the inside rear wheel which allowed the diff to slip on uneven ground.

Can you confirm for me that the SVIC is found as shown in the Whiteline documents by running a line through the 2 lower control arm pivots and a line taken at right angles to the pivot line at the point where the strut is fixed at the top.

Why does the acceleration line come from the centre point of the wheel and not the contact patch.

Thanks
Dave

 

[GregLocock]

If 4 degrees of castor is OK then I can't think why 7 would be better.

The tractive effort is supplied by a force forward at the wheel bearing, whereas in outboard braking it is supplied by a torque about the wheel bearing, which resolves out as a force at the contact patch. Draw free body diagrams of the spindle to see why.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

Perhaps this is intended for optimizing performance at autocross, tight turns taken in 2nd gear at high lateral acceleration, with some desire to avoid excessive static negative camber settings (straight line braking and acceleration consequences). Top cars are trailered to the event, so street driving behavior is not a major consideration.


Norm