thewheelman New Reader
July 1, 2012 12:22 p.m.

I've been thinking about this for the past few days, and wondering what everyone's thoughts are on this subject.

My car (Mazdaspeed 6) has SPC adjustable upper ball joints installed, which can allow for camber and/or caster adjustments if they are configured properly in the upper control arm. I currently run about 1.8 degrees negative camber on the front of the car, and the stock caster angle which around 2.5 degrees positive. I was wondering if it would be worth my time/money to have the ball joints adjusted to allow for a little more positive caster while maintaining my current camber angle, or slightly less. I'm pretty sure I could get another degree or so out of each side.

I've noticed on some more performance-oriented vehicles, that they run a good amount of positive caster, and it makes sense to me; when turning, negative camber on the outside wheel would increase. Now I know that there is more engineering involved (strut vs. double wishbone suspension deign, tire width, spring rate, sway bar size, damping, etc.), but in general, wouldn't I be better off with as much positive caster as I could muster? I'd be sure that the caster angle is equal on both sides to prevent the car pulling more to either side. What are the side effects? I'd imagine that the car might feel a little more "darty" on the road, but some of that could be corrected for by adjusting toe (I run 0 toe front and rear right now with no issues).

I'm currently lowered, but will be installing factory height, but stiffer springs. I will, of course, be having an alignment done - which is why I'm asking the question. If I'm going to be realigning the car, why not explore the options I have?

fast_eddie_72 UltraDork
July 1, 2012 12:26 p.m.

When you say "upper ball joint" I assume that means the car has A Arms? I have no experience there, but can tell you on my MR2, caster is a wonderful thing. I run about -3 camber and about +5 caster on the front of it.

However, I'm also in the middle of adding power steering to the car. Loads of positive caster makes it kinda hard to steer.

Knurled SuperDork
July 1, 2012 12:26 p.m.

Caster is for stability, nothing more. At the steering angles that you will see while driving, the camber change is negligible.

I moved the subframe in my Mazda an inch forward for more caster. It's not a bad thing to have, but it's important to understand just what exactly is going on.

fast_eddie_72 UltraDork
July 1, 2012 12:36 p.m.
Knurled wrote: Caster is for stability, nothing more.

I don't have any data to back this up, so it's just my seat of the pants opinion, but...

I'm certain that I gained front grip when I added the caster. I'm not sure it's as simple as you say. But I'm sure the effect is dependent on suspension geometry.

thewheelman New Reader
July 1, 2012 1:07 p.m.

In reply to fast_eddie_72:

The car does have upper A arms. In addition to the adjustability provided by the ball joints, Mazda makes a pair of replacement control arms that add another .5 degree of positive caster, should I choose to go that route.

Knurled SuperDork
July 1, 2012 2:35 p.m.
fast_eddie_72 wrote: I don't have any data to back this up, so it's just my seat of the pants opinion, but... I'm certain that I gained front grip when I added the caster. I'm not sure it's as simple as you say. But I'm sure the effect is dependent on suspension geometry.

Oh, it's entirely possible. When you change caster, you're also minutely changing the corner weighting changes as you turn the wheel, as well as altering the bumpsteer curve.

Just like anything else, when you make a "simple" change, you're affecting three or four other factors, some in your favor and some against you, and the net gain or loss will depend on the exact situation at hand.

If you move caster two degrees (which is a major change, maybe double for most cars), then if you could turn the steering ninety degrees you'd see two degrees of negative camber. But if you're only turning the steering ten degrees, how much camber are you gaining? Full lock, in comparison, is rarely more than 40 degrees.

iceracer UltraDork
July 1, 2012 4:58 p.m.

I found that after I increased the caster by a sizeable amount , I could run a little less camber. Worked great.

curtis73 SuperDork
July 1, 2012 9:21 p.m.
Knurled wrote: Caster is for stability, nothing more. At the steering angles that you will see while driving, the camber change is negligible

Not sure I agree totally, but I see your point. I guess it depends on the car and the driving. I know it depends on the tire. A tall skinny tire will respond more to the camber gain than a short fat one. Optimum camber angle is partly a function of tire deflection.

Too much caster and you can get adverse weight jacking/roll.

Many race cars (for instance when Nascar turns both ways like at Watkins Glen) run 7-9 degrees of caster, however their spring rates and tire grip mean that there is much less body roll, weight jacking, and steering angles to cause those adverse effects.

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