Why so many different Spindles? Tall ones, short ones, large offset, small offset. Different kingpin inclination (KPI) and scrub radius.
We have a step-by-step process that we follow, with the goal being optimizing the contact patch of both tires, while turning hard on tight corners of tracks or Road Courses.
- We have to work out the Spindle KPI/SAI with the length of the A-arms, tire width, & wheel backspacing ... to end up with a desirable scrub radius.
When we are working out a car for class rules, those rules often limit what you can do for spindles & wheels. In more unlimited series there are less rules. On the street, there are no rules, except the ones you impose based on your desires, budget, & priorities.
If we have a scenario where we are running front wheels with a lot of backspacing … that gets the KPI/SAI closer to the center of the tire … reducing Scrub Radius. "Zero Scrub" is optimum, but challenging to achieve for packaging reasons. It requires very wide wheels & deep backspacing. You can go too far ... with REALLY DEEP back spaced wheels & HIGH KPI/SAI spindles … you can get into a situation of negative Scrub Radius, which isn't desired. Reminder: Your goal is not zero scrub radius static ... because it will result in negative Scrub Radius. The goal is zero scrub radius (or close to it) dynamically with the car in dive, roll, & the tire at slip angle.
We recently designed a front end with 12" wide wheels & 10" of backspacing. A 8-10 degree KPI/SAI spindle would have put it into negative Scrub Radius. The spindle design & hub design also play a part on the packaging with the distance from the steering axis to the wheel hub face. With the hub we used & a 5 degree spindle, it created a zero scrub radius package (dynamically).
- Once we know the spindle KPI/SAI that guides us on how much "static" caster we need to build in to achieve a KPI/Caster Split favoring the caster.
Optimally, we need the 1.0-3.0 degrees more caster than KPI/SAI angle. A lot of Ford passenger cars run 8.0 KPI, so we're shooting for 9.0+. A lot of GM spindles are 8.75 KPI, so we're shooting for 9.75 +/-. If we can run a lower KPI/SAI spindle … like in the example above using a 5 degree KPI spindle … we'll need 6.0+ degrees of caster.
- We design the suspension so the car is caster neutral.
When the front suspension compresses, if the angles of the lower control arm and/or upper control arm combine for no change in the ball joint locations … that is considered Caster Neutral.
- If I can get the caster I want ... statically so the KPI/Caster Split favors the caster at least 1.0+ ... I can run less Camber, which is the goal.
We're always going to run SOME static camber (negative) … say –0.5 at a MINIMUM … and we always want SOME Camber gain … but if we don't end up with a KPI/Caster Split favoring the Caster by 1.0+ degrees … then we need to make the difference with Static Camber.
If we can get to the optimum angle with a KPI/Caster split favoring the caster by 1.0-3.0 degrees … we'll run all the camber we need to … to make that outer tire WORK.
- Once we know how much camber we need for that combo, we work out how much of it is going to be static camber & how much is going to be camber gain.
We always want some static camber. -0.5 is like the minimum. I like -1.0 to -2.0 … IF everything above falls into place. But even this depends on how much camber gain we end up with.
- How we get the camber gain, & how much, affects the car's static & dynamic roll center ... so they have to work as a team too.
Our desired roll center plays a role in this decision, because all the A-Arm angles creating Instant Centers determines where the static Roll Center is ... and where it goes dynamically in dive.
It should be clear now why we don't start with Camber.
All of this ... is to optimize both front tire's contact patch with the asphalt ... In the car's dynamic states when it's driven HARD ... meaning turning, braking, rolling, unwinding & accelerating to the limits of the car, tires & driver ... sometimes beyond.
I realize this is a lot to digest. Don't be afraid to ask questions where ever I wasn't clear.
*Caster neutral is achieved when the front A-arm geometry is set: zero caster loss or gain is achieved with zero anti-dive. Caster loss is achieved when the front A-arm geometry is set for the opposite of anti-dive … called "pro-dive."