Spindle length

[colinmseries]

Can I ask for some help in understanding the role of spindle length. By this I mean when viewed from the front, that horizonal distance at hub height from the road wheel centre line to the steering axis centreline.
I can see how the same scrub radius can be achieved at the road by using different KPIs and wheel offsets. So I am wondering where in the design priorities spindle length is found.
My reason for asking is while trying to analyse the front suspension of my car I can see there is a positive scrub radius of 60mm from a spindle length of 100mm and when driving a feeling of kickback or 'liveliness' in the steering when one front wheel drops into a pot hole or hits a bump. I am very sure the car does not suffer from an obvious degree of bump steer as this has been checked with the wheel moved through it's range.
Thankyou, Colin.

 

[GregLocock]

60 mm of scrub radius is a lot these days, so I'm not really surprised you get kickback.

As you say, once you set the rolling radius, the scrub and the KPI then this dimension, probably better called the FVHO (front view horizontal offset) is an outcome, and it is not as 'useful' as looking at scrub and KPI by themselves.

In effect you are trading off KPI (more is bad for steering feel and other stuff) against scrub radius (more is bad for kickcback, split mu braking, good for rolling parking efforts).







Cheers

Greg Locock

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

 

[colinmseries]

Thank you for those comments. From reading Milliken I note three areas they suggest will contribute to kickback. They mention a gyroscopic effect induced by large camber changes, a force in proportion to spindle length acting at the wheel centre from changes in wheel rotation speed and one said to be due to the offset at the contact patch.
I can appreciate the first source and seem to remember the effect being obvious when holding and tilting a spinning top, but I am struggling to differentiate between the later two. Can you help explain or direct me where I may read how these forces are described. By this I am thinking of the extremes of a set up with zero spindle length but an offset at the contact patch, and vice versa.
Thankyou, Colin.

 

[GregLocock]

I'd have to read Milliken, the second one sounds a bit odd, as though they are trying to differentitate betweeen braking and traction and other longitudinal forces. The third one is just the scrub radius effect, and so far as I am concerned is the important one.

The first one is easy to calculate once vou can remember the equations for a gyro, for 'normal' amounts of camber gain it is pretty small, but not negligible.

Cheers

Greg Locock

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

 

[colinmseries]

Ok thanks for your thoughts, the comment is on pg711.
Colin.

 

[GregLocock]

Good, yes he is differentiating between those forces reacted at the wheel centre, and those at the contact patch.

From a practical perspective we tend to minimise both scrub radius and spindle length by using negative scrub and positive KPI, and vaguely try and minimise both(there are practical problems that limit the alternatives for a production car). As he points out, you don't really know where the CP point is on rough roads, and to be honest finding the instantaneous steer axis is not straightforward either, so there is a large element of suck it and see if you are trying to optimise stability/kickback on rough roads.

I've done it in the past by running a road profile under the suspension and looking at the tie rod forces, having just thought about it I'd be inclined to move the notional CP point around laterally to see what the sensitivity is.




Cheers

Greg Locock

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

 

[colinmseries]

Just one last go at trying to understand this. If the steering is centrepoint how does the length of the spindle introduce kickback? By this I mean the loads come in through the CP which in this case has no moment. I can see how if the wheel was struck at hub height it would have the spindle length as a moment but they are not meaning that.
Thanks for your patience, Colin.

 

[GregLocock]

That's the difference between forces at the CP (braking and lateral) and traction type forces, which act at the wheel centre. I understand his point, on the rear wheels (in a RWD) it is an important effect, but I must confess I'm a bit dubious that in practice that forces on an undriven wheel ever really act at the wheel centre alone.

I suppose the simple explanation for them is if we imagine a general force vector acting at the CP, even with zero scrub, then it will have a moment about the wheel centre (unless it fortuously points directly at the WC), and so it will generate a torque that can only be resisted by the steering arm (as that is the only way of resisting torques about the steering axis). I'm happy enough with that bit as an explanation, that's just mechanics.

Cheers

Greg Locock

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

 

[colinmseries]

Ok, case closed then. I did speculate in my mind, if with a negative scub setup the two forces would cancel to some extent, since then they act either side of the steering axis.
Thanks for listening, Colin.

 

[GregLocock]

Thought experiment - which would have more kickback (with the same KPI)

1) zero spindle length, -60 scrub

2) 60mm spindle length, zero scrub?

Five bucks to a penny that (2) is the nicer drive.



Cheers

Greg Locock

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

 

[patprimmer]

My money is very firmly on 2

Regards
Pat
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