How does Caster play with Camber

[CTX-SLPR]

I've been reading and rereading lots of threads and articles on suspension aspects and thier effects. Probably the one I have the hardest time following is how caster affects camber. While I think I can see the physical driving points behind it (i.e. how the leading bolt on the cross shaft has more shims under it causing it to cause camber gain) but how is this called caster?



Thanks,

[RSX302]

I've been reading and rereading lots of threads and articles on suspension aspects and thier effects. Probably the one I have the hardest time following is how caster affects camber. While I think I can see the physical driving points behind it (i.e. how the leading bolt on the cross shaft has more shims under it causing it to cause camber gain) but how is this called caster?

Thanks,

Camber and caster is directly connected. Caster will increase/decrease camber as the tire turns or rotates on the king pin or steering inclination angle. The more caster the more camber change throughout steering lock to lock. Example: higher caster 5 deg(more SIA angle) - while turning, the inner wheel will become more positive camber while the outer wheel will have more negative camber. Straight initial camber can be much less (<1 deg) because of the higher caster.

I'm sure many on this board can add much info on this topic.

I'm sure there are many on this board that can provide much input.

Systems with lower caster (mustang II suspensions- 1 - 2 deg caster) will require more initial camber (1-2 deg camber)because there will be less camber change throughout steering lock to lock.

[exwestracer]

Referring to your description of a typical cross shaft upper A arm with shims; the difference in shim stacks will angle the A arm either to the front or rear of the car. This moves the upper ball joint ahead or behind relative to the lower ball joint. The angle of a line between the ball joints in side view is caster. More shims in front would result in LESS caster angle, as the upper ball joint is further forward.

Camber gain is affected more by the height of the upper A arm mounts in relation to the lower ball joint.

[UMI Tech]

On a side note, I remember caster as fore and aft location of the upper ball joint relative to the lower. Like the motion of CASTing a fishing line.

I remember camber as the upper ball joint being closer or further away from the CAMshaft in the engine.

The caster camber relationship is somewhat tricky to envision. The common caster measurement method of turning the front wheels 20 degrees, zeroing the gauge, turning the opposite 20 then reading caster has always freaked me out.

[exwestracer]

The common caster measurement method of turning the front wheels 20 degrees, zeroing the gauge, turning the opposite 20 then reading caster has always freaked me out.

Easy there, big guy... Nothing to go into therapy over, LOL.

All that process is doing is finding the average of 2 angles. Sort of like naval warfare. Shoot one long, shoot one short, then split the difference...

[Norm Peterson]

The caster camber relationship is somewhat tricky to envision. The common caster measurement method of turning the front wheels 20 degrees, zeroing the gauge, turning the opposite 20 then reading caster has always freaked me out.

You can think of the 20° as being little more than a fortunate coincidence. There's a little math involved, and 20° ends up being pretty close to making caster equal to 1.5 times the difference in camber (which is what zeroing the gauge turned 20° the first way and turning to 20° the other makes easy when the 1.5 factor is built into the scale on the caster vial) . . . plus 20° is an angle that nearly any car can get the wheels steered around to.



CTX - picture a right side wheel with its alignment set to 0° camber and some amount of positive caster (say +10°). Now suppose you could steer that wheel 90° into a left turn - think crosswise in the car ahead of the ball joints. Now the wheel would then be sitting at -10° camber looking from the side of the car, which would now put you looking in the plane of the wheel (just like from the front with the wheel not steered at all). At any angle less than 90° steered, you'll pick up less than than that 10°, as some function of just how many degrees you steered it (there is a slight complication that doesn't matter at 90° steered that's best left for later discussion).


Norm

[RSX302]

Also more caster will allow the car to track better at speed. Cool to recover from a power slide as the steering wheel will snap back to straight just by letting go.

[CTX-SLPR]

Ok, I think I'm starting to get my head around this. There are 3 areas where I can see this effect being adjustable.
1. Shims behind the cross shaft on the upper A-arm
2. Incline of the cross shaft compared to level
3. Length of the two legs of the upper A-arm

Of these with my compromise of not altering the chassis side suspension points, I can mess with 1 and 2. Stock the car calls for 2º possitive caster with radial tires, all done by shimming the A-arm. Since I have the opportunity to design in caster and to adjust it with adjustable A-arm legs and shims, what should I shoot for in a perfect world of static geometry? 5º, 10º? My current camber curve looks like ~-0.4º static camber with a camber gain of ~-0.5º per inch of compression for the first 1in and gaining ~-0.15º on top of that per inch afterwards (0.5º, 0.65º, 0.8º, 0.95º). Thats with 12.00in upper and 19.15in lower control arms on a C5/6 spindle with a Kingpin axis of 8.8º. I've not measured the stock inclination of the upper A-arm but I can't change that. The car is rear steer and the goal is 70% open road comfort and confident handling (think grand touring or sports sedan) and 20% road course/autocross.

Thanks,

[Norm Peterson]

With only a few exceptions, GM was pretty conservative with their caster specs for RWD cars. About 1973, the Monte Carlos went to a much higher caster spec, but I don't remember exactly what it was.

While it's perhaps not directly comparable, the current Mustang runs +7.1° caster with a tolerance of ±0.75° from there, and has a pretty good reputation for handling.

I'm thinking maybe somewhere in the +5° to +7° range . . .


Norm

[CTX-SLPR]

That seems to line up with the C5/6 "tire wear" specs of very little static camber and around 6-6.5º of caster

[RSX302]

Ok, I think I'm starting to get my head around this. There are 3 areas where I can see this effect being adjustable.
1. Shims behind the cross shaft on the upper A-arm
2. Incline of the cross shaft compared to level
3. Length of the two legs of the upper A-arm

Of these with my compromise of not altering the chassis side suspension points, I can mess with 1 and 2. Stock the car calls for 2º possitive caster with radial tires, all done by shimming the A-arm. Since I have the opportunity to design in caster and to adjust it with adjustable A-arm legs and shims, what should I shoot for in a perfect world of static geometry? 5º, 10º? My current camber curve looks like ~-0.4º static camber with a camber gain of ~-0.5º per inch of compression for the first 1in and gaining ~-0.15º on top of that per inch afterwards (0.5º, 0.65º, 0.8º, 0.95º). Thats with 12.00in upper and 19.15in lower control arms on a C5/6 spindle with a Kingpin axis of 8.8º. I've not measured the stock inclination of the upper A-arm but I can't change that. The car is rear steer and the goal is 70% open road comfort and confident handling (think grand touring or sports sedan) and 20% road course/autocross.

Thanks,

I'm far from a suspension guru, but I think there is a little more to it then just shimming caster to what you would like. I think placement/angles of upper and lower a-arms are critical to how much caster you can run. Also more caster will make the front wheels have more weight on them during cornering due to the extreme camber change. Power steering will be worked much harder so that will need to be addressed as well.

[exwestracer]

One other thing to consider: The side view inclination of the upper control arm shaft (#2 above) affects caster gain, but also controls brake anti-dive effect. Anti-dive geometry is designed into an OEM chassis for the original nose weight and tires. Removing a lot of weight from the front end, and/or going to a much stiffer sidewall tire can result in excessive anti-dive. This can cause front end instability and understeer under heavy braking (commonly referred to as a "brake push").

Too much caster gain with suspension travel will also cause the steering effort and feel to change as the suspension compresses in a corner. If you are planning on limited travel, you won't see much caster gain without a LOT of upper arm angle.

[CTX-SLPR]

Ray,

I must be doing something bad wrong with my modeling, now granted it's about half eyeballed but I'm getting a stock antidive measurement of between 60-80% depending on CG height. I've estimated between 24 and 30in high for the CG height and a 67% front brake force. I'll take some better measurements today but what sounds reasonable for a 60's GM car and what is reasonable on todays modern rubber with a 45-series sidewall?

Thanks,