I figured before we move on to discuss Roll Centers, we should wrap up our conversation on steering geometry.
All of us know that each front suspension geometry setting affects the overall picture ... and to a degree ... each other. As a race car Designer & Crew Chief, I can't look at just one individual setting. I have to look at the spindle KPI/SAI, Caster, Caster Gain/Loss, Camber, Camber Gain/Loss as a team ... a team of geometry devices that I need to work together in harmony to improve how the tires contact the road dynamically.
Having done this for years, designing, building, tuning & racing a lot of cars, in just about every type of racing, has given me some firsthand insight into how all these things work. So now it's a little easier & quicker for me to "get a set-up there" to the sweet spot. I have a process that gets me there quickest ... with the least back-n-forth.
First … some guidelines:
Zero scrub radius with long control arms & deep backspaced wheels is optimum, but many race series rules prevent us from achieving this, with rules on the LCA. If the rules allow, we're running long control arms & a low KPI spindle on deep backspaced wheels & achieving low to zero scrub radius.
But when the rules restricting our LCA choices prevent us getting the ball joints "out there" … making us choose between track width & scrub radius … we are “usually” going with wheels with less backspacing to achieve the maximum track width allowed by rules … then running higher KPI spindles to reduce the scrub radius as much as we can.
I've had people ask me why don't we give up track width to achieve a lower or zero scrub radius ... but in the big picture ... track width trumps scrub radius, up to a point. Everything has its limits & there are exceptions for everything. Tight AutoX courses sometimes favor narrower cars with narrower track widths. But most anything faster than that favors a wider track width. When you have no rules or limitations, you making everything optimum. When rules prevent that, you’re shooting for the best overall compromise.
My process:
A. I have to work out the Spindle KPI/SAI with the length of the A-arms, tire width & wheel backspacing ... to end up with maximum track width & a desirable scrub radius.
B. Once I know the spindle KPI/SAI … that guides me on how much caster I need to build in to achieve a KPI/Caster Split favoring the caster.
C. I need to figure out if we want caster gain* … and if so, how much … or for the caster to stay the same, as the front suspension compresses fully under braking & turning (called "Dive"). I have seen poorly set-up cars with caster loss in their set-up, but it is not desirable.
D. If I can get the caster I want ... statically and/or with gain ... so the KPI/Caster Split favors the caster ... I can run less total Camber, which is the goal. If I can NOT get the the KPI/Caster Split favoring the caster … we will need more total camber. Once I know how much total camber I need for that combo, I work out how much of it is going to be static camber & how much is going to be camber gain. I like to get 50%-60% of the total camber desired … through camber gain.**
E. If I can get the camber gain I want ... I can run less static Camber, which is the goal. We're always going to run some static negative camber … because it improves initial turn-in response***. Depending on the application, I like -0.75 up to -2.0 … again, as long as I can get the camber gain AND the KPI/Caster Split favoring the caster. But if I can not … I will need more static camber.**
F. 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.
All of this is to optimize both front tires’ 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.
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*Caster gain is achieved when the front A-arm geometry is set for anti-dive. 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.”
** The more static camber you have to run to optimize the outside tire, the more you’re hurting the inside tire. I like to get 50%-60% of the total camber desired … through camber gain. Here is why: The suspension on the inside of the corner is not compressed as far as the suspension on the outside corner. So effectively … the inside tire is not getting as much negative camber gain to fight & overcome.
*** Don’t get greedy with static camber. Yes it improves initial turn-in steering response, which is good. But two things:
1. Camber helps the outside tire & hurts the inside tire. If you run too much static camber, you can’t get the inside tire to work optimum.
2. There are a LOT of other things that help turn-in response … so use some of those … & don’t get greedy with static camber.
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You know how people say a little information can be dangerous? This is one of those times.
If your car has a large-ish scrub radius … you have to be careful with how much caster you can put in the car … because caster combined with a high scrub radius creates a “jacking effect” when you turn the wheels. Dynamically, this jacking effect “de-wedges” the car … loading the inside front & outside rear tires more … while also unloading the inside rear tire & increasing the degree the car diagonally rocks & loads the outside front tire.
All of this helps the car to turn better. But go too far … and the car will get loose on entry. This is where track tuning comes into play. If you have a large-ish scrub radius … sneek up on the caster you put in the car … until you get the car “free” on entry … then reduce the caster a tick … or tune something else to allow you to keep that amount of caster, so the car turns well in the middle. But don’t keep a set-up that makes the rear step out on corner entry.
Make sense?
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Brian Hobaugh SCCA National Tour June 2014
First Hemi 'Cuda Convertible Ever Built
Short clips: Goodguys Pleasanton autocross and pit videos
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