I asked this once before, but I could not get a good answer...I'm trying to figure out how much Positive Caster to go with when I get my allignment...has anyone noticed a difference between low P. Caster and high postive caster....what do you recommend I go with...P4, P5, P6??

 

Depends on how you use the car. Street or Street/strip, get a factory setting. Drag use only wants more caster.

https://www.thirdgen.org/newdesign/f...tml#suspension

 

You drive the car on the street regularly? (Daily driver) Or is it specifically built for one purpose use?

Putting too much postive caster into a lowered car without bumpsteer provisions will make the car wander all over the place on every road imperfection. Too little makes the car twitchy and very reactive in steering response (not stable) Every car has a general range they should be set within. 3rd gens range is about +4 to +6 range. Street being +4 drivers side and +4.5 pass side. The .5 less on the drivers side will make the car steer slightly towards the left which keeps the car centered on most crowned roads. Roads are crowned in the center divider for rain drainage.

 

If too much positive caster in a lowered car make it wander...then what happens with too much positive caster on a stock height car

 

Try doing a web search for steering geometry and caster angles.

http://hunterengineering.com/pub/undercar/2573T/

 

wow, thats a lot of technical stuff....so let me ask this....i use my car just on the street, maybe i drive it twice a week...I'm just interested in straight line performance and stable conditions at higher speeds (75-100)...and I don't care if my wheel is harder to steer.....so what would you recommend for a Positive caster setting?

 

Stock setting

 

1) Is the car stock height or lowered?
2) what is the suspension travel (spring rates, and shock dampers) over the biggest bumps at speed? In other words, is the travel about stock with 3-4" compression travel or is the suspension stiffer with only about 2" travel.
3) do you have bumpsteer kits installed if the car is lowered to correct the tierod angles back to stock geometry?

 

my suspension has not been modified in any way....the car is not lowered....the body and suspension has not been modified...only some engine mods...but if i could adjust the alignment setting a bit for better straight line stability, then i would like to do that....I'd like the car to stay "straight" without pullying to one side or the other...(there is nothting really wrong with the alignment as of now....)

 

Try bumping the caster up to +4.75* left/+5.25* right.
Stock is +4* left/ +4.5* right.

 

Is the Purpose of having more right caster to keep the vehicle from verring to the right? If not, then what happens if i can equal caster on both sides...Will it cause more wear by having the same amount of caster on both sides?

 

Caster is not a tire wearing angle

 

 

ok, that helps me a bit, but i still don't know how the car will handle differnetly between 4, 5, and 6 positve caster...Toe and camber are easy for me to understand, but the caster thing is still throwing me off...

 

the more caster you have the harder it is to turn the steering wheel because the inside wheel/tire presses downward and in essence has to lift the inside of the car upward in order to turn the wheel. Hence why the car will straighten itself easier with more positve caster when coming out of a corner.

Also, more positve caster will induce more steering camber bite when the wheels are turned. There is straight line camber and there is turned wheel camber. Caster affects turned wheel camber.

 

thanks...i understand it better now

 

so more positive caster helps increase camber gain(negative) when the wheels are turned? Is there any downside to running extra caster? If i can get 6 degrees would that be beneficial?

 

Laiky, the downside is when the wheels are turned with that much caster, the tierods are already stretched to the outer limit of articulation. Then you hit a bump and further enhance bumpsteer due to poor tierod geometry and walk all over the road at high speeds.

This is the only part of my car that I do have trouble with is there is one particular overpass that is very bumpy where my suspenion will unload and then aboiut bottom out with good lateral steering force applied at about 80-85 mph through it. I get very scary bumpsteer. This is the only section of road i have ever experienced this problem with my suspension and it is due to the 2.25" lowered stance combined with +5.5* caster on that outside of the car. I due have bumpsteer corrections, but they do not have quite enough adjustment provisions that I get it on this very rough high speed freeway transition.

For anyone local to me in So Calf, you'll know this freeway transition as the 55N to 91 W overpass. There is not a car on the face of this planet that doesn't rock and roll through that changeover at speed. It loads the front right bigtime

 

so basically if i wanted to run a little extra caster (lets say P5)....then i would just notice that my steering is a bit more difficult? (my chassis/suspention has not been modified)

 

This is a great read, but I'm a little confused on something. The chart in the FAQ section shows more positive caster for road racing or heavy street use, but I would assume a road racing car would be lowered. If more positive caster on a lowered car causes it to wander a lot, wouldn't this be detrimental? I would think less would be better in this situation, and would keep the tires flat on the road due to less steering camber. Am I missing something?

 

Cruisin, Yes the steering would technically be slightly heavier to turn at low speeds, but its very minor change and on a stock suspension car it is probably not even noticible to go from +4 cvaster to +5 caster unless cranking the wheel standing still getting read to do a u-turn.

Cayse, (Haven't seen you in a while, maybe I am just missing seeing your posts?) The cause of wandering with more caster is only a possibility. It really is dependant on just how much a car is lowered and with what parts (bumpsteer correction kits, overall motion of travel range, etc). The thing that causes a car to wander of course is bumpsteer. This is basically directly relivant to the articulation angles (swing radius up and down archs that shorten the distance of the arm from centered or level geometry distance). The more a car is lowered, the more the starting articulation point is near or past center and thus the swing is only upward where a dramatic bump will shorten the angle length of the tierod an cause that side to toe-in making unwanted steer.

Now heres where more positive caster comes into play. The tierod not only moves upward in articulation, but it also moves rearward. The more positve caster, the more rearward it moves and thus the more potential of shortening the radius swing even greater causing bumpsteer.

When a Strut suspension car is lowered, without moving the strut mount plate the caster will increase positive because the length of the strut from spindle to mount shortens increasing the angle (or caster) towards the rear of the car. Hence why a lowered car can yield higher caster adjustment ranges thanb a stock height car. Stock may yeild a range of say +2 to +5 range, a lowered car (take mine for instance that is lowered aprox 2.25") can yeild a potential Caster range of +3.5 to aprox +7 because the strut is sitting a shorter more inclinated angle.

See pic- Note the blue line shows more of a stock height car strut angle (equivilant to mine fully extended). The green line represents the more positive caster angle of the lowered stance of the strut

 

thats a good pic....i can see the difference now...

 

Hey Dean! I didn't realize it was you until I actually read the sidebar stuff in your posts.

I understand now how the tie rods move up and back with more positive caster, but it still makes me wonder why more would be better for road racing. I'm basically trying to achieve a well-handling car (straight line is not a concern with my 305... lol) on public roads for a more enjoyable drive and to be able to carve some exit ramps for the occassional heart-pumper. Unfortunately, public roads in NJ are wrought with uneven surfaces, potholes, and construction of all sorts. I plan to drop the car with Prokits and will probably end up with Moog strut mounts and Koni reds.

It's probably right in front of me and I'm not getting it, but I still don't see why more positive caster would be advantageous on a car that's hitting the twisties.

 

Cay, Its more a speed rated thing rather than a performnace thing. Caster settings is basically one more "tool" in balancing the feel of a car for performance driving.

Its a compromise feel between high speed stabilty and low speed turning effort.. More caster at low speeds will affect steering lightness and reaction feeling, yet at high speeds it can make a car feel very stable, and visa versa.

To put it simply, the more caster you can get into the car the more stable it will feel at high speeds UNLESS you hit a bump. Its like pushing a car compared to pulling a car(towing)- when you pull a car the front wheels will track straight. Try and push a car from behind without anyone steering and it will immediately start to wander to the left or right. Caster in essence helps straight line tracking in a different but somewhat same senerio as I just stated about towing.

A cars alignment specs is setup from the factory for highway speeds and u-turns in parkinglots (0-70mph range). A race car is is more geared towards higher speed driving (20- to say- 130mph - or ?) Hence why more caster than factory setting will make a production car feel better at higher than designed speeds, but steer heavy at slow speeds- its a give-take compromise for intended driving speed.

In conclusion, you want as much caster as you can get without causing bumpsteer. Even at low speed, as long as you can still steer the car quick enough under hard braking into a corner, the extra caster will give a car more camber gain into a corner when the wheel is cranked towards full lock. For example, Lets take a senerio where a car runs front Camber at -2.5* with the wheels pointed straight. At full lock to the left with Caster at lets say +4* may yeild a camber gain of total -3*. Now with Caster at +6* instead of 4, the Camber gain at lock may grow to more like -3.5*.

This is great when you are comming in hot and overshooting or pushing into a corner and under heavy braking trying to recover. What the larger caster does to the Camber under an understeer with full lock steering situation is promote more weight transfer and tire flex kicking that now -3.5* comtact patch over to a flat 0* contact under hard tire flex and promote the car to grip and turn- again when coming in too hot. If the car is too loose under this circumstance, then most likely the rear swaybar is too large.

 

Good examples and info....Thanks a lot