Now let’s go over caster. As you should know by now, caster is the longitudinal inclination of the steering axis. One of the major effects of caster is that it changes the camber of the wheel as it turns, we can use this to help us find the caster angle. By defining the relationship between camber change and caster, we can solve for the caster angle by taking camber measurements at multiple wheel positions.
For those of you curious, this is the actual equation to calculate caster. Do you see the problem yet? The equation requires you to know the SAI to calculate the caster! Don’t worry, we can sort of fix that with a few assumptions.As long as you’re not a member of the cambergang, it’s a good bet that the camber angles you’ll be working with are very small. This allows us to assume that the cosine of the camber angle will be very close to 1. For example, if you have a worst case scenario 6 degrees of camber like me, the cosine of 6 degrees is 0.9945, which is only off by 0.55%. Making this assumption allows us to greatly simplify the equation.Next, we assume that the toe will be equal in both directions. That means when you make your camber measurements, you will start at zero toe, and ensure that you turn it to the exact same angle in both directions. Thanks to the trig opposite angle identities, making this assumption nullifies the whole right side of the equation containing the SAI component. So there is your simplified equation for calculating caster!
It is critical to understand the two assumptions that are made when calculating the caster. The accuracy of your calculation is proportional to your camber, the more camber you have, the worse the accuracy will be, do not forget that.
Before you can start making measurements, you need to ensure that you have a good zero toe reference from which you are measuring your turn forward and backwards. If your toe is unaligned and out of whack, you would need to skip forward and string up the car to make sure you can get the toe close to zero. You also need to make sure that your rear toe is spot on, because the toe used in this caster equation is actually supposed to be the toe relative to the thrust line (the direction that your rear wheels are pointing). If your rear toe is uneven, then your front toe measurements will not be relative to the thrust line, so your car will tend to pull to one side. However, you don’t have to set your rear toe before making caster measurements so long as you ensure that you make it even afterwards.
Once I have the wheel straight, I tape down the zero toe reference by using the edge of the tile as a guide.Now here comes the clever part. I turn the wheel, and the tile moves with the tire. I can then use a protractor to check the toe angle by measuring the angle between the tile and the reference edge of the masking tape. I usually turn the wheel to 15 degrees. I’ve found that at higher angles, the tile starts to peel off the tire and slip. If you hear the adhesive peeling heavily, stop and check that your tile is still aligned by returning the wheel to zero toe, and checking if the tile edge still lines up with the masking tape.Now I take my first camber measurement. I measured 2 3/16” or 2.1875”, which when plugged into our camber equation yields -6.462 degrees of camber.
ATAN in excel returns the result in radians by default. Convert it to degrees by changing your formula to =DEGREES(ATAN((SIN(RADIANS(B64))-SIN(RADIANS(C64)))/(2*SIN(RADIANS(B65))))
Great article and a very clear explanation of the string method for alignment but one query. You stress the importance of the bars being parallel to each other but presumably they don’t actually have to be square to the car – making a parallelogram out of the string and the bars is sufficient and they don’t have to make a rectangle. Would you agree?
This is correct, a perfect rectangle isn’t necessary. An isosceles trapezoid or parallelogram is okay. In either case, the string is spaced evenly from the hub cap on each side.
10 comments
Thanks for the guide but I have a problem.
I entered your shortened caster formula in Microsoft Excel, and got a totally different value.
=ATAN((SIN(RADIANS(B64))-SIN(RADIANS(C64)))/(2*SIN(RADIANS(B65)))
=0.05481679
Instead of 4.2
ATAN in excel returns the result in radians by default. Convert it to degrees by changing your formula to =DEGREES(ATAN((SIN(RADIANS(B64))-SIN(RADIANS(C64)))/(2*SIN(RADIANS(B65))))
This worked, thank you.
Great article and a very clear explanation of the string method for alignment but one query. You stress the importance of the bars being parallel to each other but presumably they don’t actually have to be square to the car – making a parallelogram out of the string and the bars is sufficient and they don’t have to make a rectangle. Would you agree?
This is correct, a perfect rectangle isn’t necessary. An isosceles trapezoid or parallelogram is okay. In either case, the string is spaced evenly from the hub cap on each side.
Thanks!
Thank you for the great article and explanation, you’ve inspired me to give it a try!
Is this an empirical formula? If not, could you provide the source? I’d like to know how it was derived.
You do know the average driver don’t want to change tires every 25 miles
Do you know the tires won’t wear out in anything close to 25 miles even with the most extreme racing settings?