Spindle length
Spindle length
(OP)
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.
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.





RE: Spindle length
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
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RE: Spindle length
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.
RE: Spindle length
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spindle length
Colin.
RE: Spindle length
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spindle length
Thanks for your patience, Colin.
RE: Spindle length
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spindle length
Thanks for listening, Colin.
RE: Spindle length
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Spindle length
Regards
Pat
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