Camber change and dynamic trail

[frips]

I have a quick enquiry about drawing the front suspension of a vehicle

When considering the centreline of a wheel to calculate scrub radii and track width, is camber ignored as per the second picture? Or is the center of the actual contact patch the more important dimension? I understand that the shape and position of the contact patch is significant when considering dynamic trail and steering feedback.

I imagine that a picture is worth more than a thousand forum posts.

 

[GregLocock]

The usual construction is that the CP location is at the point where the ideal wheel disc hits the ground. As you say, that is not really the effective point of application of the tire forces. I think that's why production cars have scrub radii of more than 10mm or less than -10 mm, so that at least the sign of the torque produced by Fx doesn't change. If you happen to have a plot of Fx and Mz in a braking test then that would be very interesting, it is not data I routinely see. It may be possible to back it out of a proper carpet plot.

Cheers

Greg Locock


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[cibachrome]

You should take into consideration the Mx properties of the tire, especially if the vehicle has a high torque, high horsepower motor. Otherwise, you can get into some steering wheel kick situations during full power launch. I've direct experience with 3 brands/constructions of tires with +-20mm of pneumatic scrub (a tire's dynamic scrub radius contribution). If the tire has a positive component, it will effectively cancel out a good portion of the geometric negative term. Calculate Mx/Fz at slip and camber angles in driving and cornering inputs to directly see the magnitude of this 'issue'.

Mx is an often ignored tire property that is quite important during large Ax and Ay maneuvers.

 

[GregLocock]

Are you in SAE tire cordinates? I'm in vehicle!

Cheers

Greg Locock


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[GregLocock]

Oh I see I was confused by "Mx/Fz", thats right, the pneumatic scrub radius.

I hadn't realised it could be as much as 20mm.

Cheers

Greg Locock


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