Question about automotive suspensions

[krekquel]

hello :)

hopefully i will be able to explain my question clearly.

i am trying to understand the effect that caster angles and steering axis inclination have on the camber angle of a wheel. imagine that we are looking at the left front wheel and turning it 90 degrees to the right (theoretically). the way i imagine it to work...

1. every degree of SAI (steering axis inclination) i have will translate to the same number of degrees of POSITIVE camber when the wheel is turned 90 degrees.
2. every degree of positive caster i have will translate to the same number of degrees of NEGATIVE camber when the wheel is turned 90 degrees.

correct so far?

if so...

let's assume the following about the front wheels of a vehicle:
camber = -2.5 degrees
castor = +1.5 degrees
SAI = 17 degrees

now... if i turn the wheels of the car are turned 30 degrees... i would expect the following to happen:

i would expect a +5.6 change in camber because of the steering axis (17 x 30/90)
i would expect a -0.5 change in camber because of the castor (1.5 x 30 / 90)

resulting in +2.6 degrees of camber when the wheel is turned 30 degrees.

but that can't be right. when i look at the wheel... it most definitely does not look 2.6 degrees tilted out.

so what am i missing here?

 

[krekquel]

perhaps i am being impatient... anyone? :(

 

[ravenprp]

Hello,

Great question! The effect of caster angles and steering axis inclination on camber angle can be a bit tricky to understand. You are correct in your understanding that every degree of SAI will result in the same number of degrees of positive camber when the wheel is turned 90 degrees, and every degree of positive caster will result in the same number of degrees of negative camber when the wheel is turned 90 degrees. However, the key factor you are missing in your calculation is the wheel's kingpin inclination.

The kingpin inclination is the angle between the steering axis and the centerline of the wheel. This angle is determined by the vehicle's design and cannot be adjusted. When the wheel is turned, the kingpin inclination also affects the camber angle. In your example, with a SAI of 17 degrees, a caster of 1.5 degrees, and a kingpin inclination of 5 degrees, the resulting camber angle when the wheel is turned 30 degrees would actually be around +2.2 degrees (not accounting for any suspension geometry changes).

Additionally, other factors such as suspension geometry, tire flex, and road conditions can also affect the camber angle. Overall, the effects of caster and SAI on camber angle can be complex and may require further calculations or measurements to accurately determine the resulting camber angle at a given wheel position.

I hope this helps clarify your question. If you have any further questions, please don't hesitate to ask. Happy learning!