Castor and the corner

[Tony]

Typical Castor values range from 0 to +10 degrees.

 

Camber Change - If a wheel has positive Castor, then the top of the wheel leans into the corner whichever way it is steered. The change in Camber is approximately:

 

Camber Change = Castor x steer-angle / 60 (measured in degrees)

Example: Camber Change = 6 degrees

Steer-angle = 10 degrees

Then:- Camber Change = 6 x 10/ 60 = 1.0 degree

 

The change in camber that results from positive Castor is beneficial to the grip of both front wheels during cornering, providing it isn't excessive. A negative Castor (top of the steer-axis leaning forwards) changes the wheel Camber angles in the 'wrong' direction during cornering, this is not desirable.

 

Self-Centering phenomenon

Castor has a self-centring effect that is similar to that of SJI/KPI. As the wheel is steered away from straight-ahead its Camber angle changes and more weight is carried by one edge of the wheel. This shifts the wheelprint centre sideways and the offset force then acts to return the wheel to straight-ahead. As with the SAI/KPI self-centring this effect is proportional to the Castor and to the radius of curvature of the tyre.

 

Self-centring exists even if the tyre is narrow, although it is almost non-existent at small steer-angles. It is very evident on wide, flat cross-section tyres with stiff sidewalls.

 

Castor Trail

The common argument for the trail is wrong and it aggravates me since the example has become a seed for most attempting to explain the trail.

 

wim : explanation:- Sometimes people refer to a stabilizing 'Castor effect' when the effect is actually due to the trail. This confusion is probably due to the naming of wheels under furniture (Castor's) Castor by itself has only a small stabilizing effect, whereas trail by itself has a large stabilizing effect. When we increase the Castor on a cars wheel we usually also increase it's trail, this may contribute to the confusion.

[Roger440]

Couple of minor points. Increased castor does not help the inside front on cornering, at least not from a tyre wear point of view. I was under the impression (im gonna check) that the inner edge will suffer more wear.

 

As for castor trail, this gets back to one of my previous questions on castor. Does the castor trail actually create the self centering effect? Hence if a wheel double the diameter was fitted (i know, it can't be done!) would the self cenering effect be increased? assuming of course that the KPI was adjusted to suit said new wheel?

 

What is the common argument for trail, as opposed to yours?

[Tony]

Couple of minor points. Increased castor does not help the inside front on cornering, at least not from a tyre wear point of view. I was under the impression (im gonna check) that the inner edge will suffer more wear.

 

As for castor trail, this gets back to one of my previous questions on castor. Does the castor trail actually create the self centering effect? Hence if a wheel double the diameter was fitted (i know, it can't be done!) would the self cenering effect be increased? assuming of course that the KPI was adjusted to suit said new wheel?

 

What is the common argument for trail, as opposed to yours?

Taken to the extreme the Castor will indeed wear the tyres, not forgetting this would need to disclude Camber and KPI from the equation.

 

The most common argument is the trail is the (do all) in steering centering, alone the trail is worthless. Don't forget any sideways movement of the tyre-print centre allows the vertical force to create a torque about the Castored steer-axis.

 

High cornering forces cause a lateral distortion of the tyre that shifts the tyre-print centre towards the inside of the corner (relative to the rest of the wheel). The resulting lateral movement of the vertical force acts to turn the wheel towards straight-ahead. This excludes KPI?

 

Your reasons for measuring that particular car will different size wheels on was a gallant one....But we have a communication problem since i cannot establish how your system generates and image/increment for the vehicle to be measured... The good old X, Y, Z, again..... excluding 'F' ..... Can you upload a link to 'photo-bucket' or somewhere so we can view the equipment.