Your Opinions: Caster Vs. SAI(kpi for the old-schoolers)

[RidingOnRailz]

First of all let me say I'm lucky to be seeing this screen and posting on here. My cookies have been cleared but it seems that I'm able to log in from only certain sections of A.F. Weird! I sent a letter to the webmaster about my plight but haven't heard back.

But to the topic:

I know what Caster & SAI are, the angles they establish relative to the ground and the vehicle. I've seen a trend over the last 20 years of perusing alignment specs in cars from many countries that the amount of SAI designed into cars has almost doubled, while caster remains dependent upon application(family sedan vs. sports coupe). In some cases, amount of caster have gone down while SAI has gone up.

If I were designing a car from ground up, I would design in as much SAI as possible and keep caster under 4 degrees positive. My reasoning is that high amounts of positive caster(6-10deg) becomes highly negative caster when backing the car up a long ways. I would keep caster minimal bacause I want quick reaction when I turned, but at the same time want the weight of the car - via SAI - to center those wheels as quickly as possible out of a turn. I know that there are scrub radius issues with SAI that need to be accommodated, but is my thinking right?

Also - and this is a question for which I have yet to receive a direct answer either way:

Which - Caster or SAI, plays the dominant role in straight-ahead stability and quickly returning the steering-wheels to center from a turn?

If this is not a simple question to answer, please feel free to explain the ramifications dictating the decision on how much of each goes into a platform at the design level.

Thanks, and - hope all of you can see this!

[curtis73]

Inclination and caster are good topics to discuss. The short answer is that inclination has very little to do with wheel centering other than its effect on scrub radius. Caster, ackerman, and pneumatic trail are the more important factors in self-centering and high speed stability.

You'll also have to design both based on IC and RC on both planes. A stiffly sprung car with tall spindle will have much less body roll and a more ideal camber curve than a short-spindle loose spring car. What is important to remember is that inclination will always camber IN during a turn, which is good for the outside wheel and bad for the inside wheel. Caster will cause the outside wheel to camber in, and the inside wheel to camber out; both favorable for keeping the tires planted flat. Using all four (caster, inclination, body roll, roll center) you can design a suspension that will keep the tires a little closer to ideal.

But... too much caster has increased jacking effect. As you turn, the jacking effect of caster unloads the outside spring and loads the inside spring... the opposite of what you want. In stiffly sprung cars its less of a practical issue, but still something to consider. Another thing to consider is whether or not the car has power steering. Lots of caster makes steering a b**ch. I personally set up my Impala SS with stiff lowering springs and 5* caster. It has a great on-center feel, but it has done two things; 1- it has caused some premature wear of steering components and there is a lot of slop in the steering box, and 2- I threw a serp belt while making a 30mph turn losing power steering. 5* caster + 3700-lb car + 275mm rubber = narrowly avoiding an oncoming dump truck.

If you really want the heavy scoop, pick up a copy of Milliken's Race Car Vehicle Dynamics. Its a 900-page engineering textbook that does an excellent job of talking to nearly anyone. There is some very involved calculus, but for the most part you can skip past it and just read the text on why it happens. Its been out of print for several years, but its available used on Amazon. Truly a fine read. I was so absorbed, I read it cover to cover over a 5 day period on a train trip.

[RidingOnRailz]

Originally Posted by curtis73

Inclination and caster are good topics to discuss. The short answer is that inclination has very little to do with wheel centering other than its effect on scrub radius. Caster, ackerman, and pneumatic trail are the more important factors in self-centering and high speed stability.

.

Wow - that is a profound statement! Everywhere else I've read states that SAI lifts the front end of the car as the wheels are turned to one side or another, and that that weight, when the steering wheel is either let go or slide through your hands, forces the wheels to return to straight ahead.

Caster, OTOH, raises one side of the front end and lowers the other, creating the camber conditions for stability & grip in turns.

They both contribute to straight ahead, but it looks like it takes at least many times more degrees of SAI than caster to accomplish the same thing.

[MagicRat]

Originally Posted by RidingOnRailz

Wow - that is a profound statement! Everywhere else I've read states that SAI lifts the front end of the car as the wheels are turned to one side or another, and that that weight, when the steering wheel is either let go or slide through your hands, forces the wheels to return to straight ahead.
.

A friend of mine was taught the same thing in his auto shop class. As far as I know, this is an incorrect statement.
If it was correct, you would get the same self-centering effect going in reverse as well as forward. Anyone who has done some adventurous high-speed driving in reverse knows that, when backing up, the steering wants to fall to one side... just the opposite of self-centering.

[RidingOnRailz]

Originally Posted by MagicRat

A friend of mine was taught the same thing in his auto shop class. As far as I
know, this is an incorrect statement.
If it was correct, you would get the same self-centering effect going in reverse
as well as forward. Anyone who has done some adventurous high-speed driving
in reverse knows that, when backing up, the steering wants to fall to one side...
just the opposite of self-centering.

Ahh - you refer to the effect of strongly positive caster becomng highly negative
when moving in reverse. Think about it, when you're backing up, the top of the
steering axis is leading in the direction of travel. This is opposite of what happens
when driving forward, where the lower end of the steering-axis leads.

[curtis73]

You seem to have a good foundation on the topic. Reading Milliken will simultaneously show you how much you haven't learned yet, AND effectively teach it to you.

Its an epiphany in hardback.