I found this and thought it was important to share: http://www.subaruwrxsti.org/caster-angle-explained-adjust-caster/ Caster angle is something that affects your daily drive more than you can imagine. It’s also something that is easy to adjust and can improve your drive in more ways than one, especially for those fighting the steering wheel without power assist. What is Caster? Caster angle is a geometric measurement in the front suspension. It is the differential, measured in degrees, of the steering axis (red) and the vertical axis of the wheel (blue). The picture above shows the typical caster axis for a MacPherson strut style front suspension. It should be noted that the axis is drawn through the upper and lower ball joint on a double wishbone style of suspension which doesn’t always line up with the shock absorber / damper assembly. Why do Cars have Caster Angle? When you drive your car down the road you don’t want to constantly be correcting the steering to drive in a straight line. Caster makes the front wheels straighten out and the vehicle stabilizes in a straight line. Without it the car would wander aimlessly. This question is also answered by understanding the main effect of caster; align torque. When you push a basket at the grocery store the front wheels tuck back and the basket goes straight (or vibrates and wanders if it’s a crappy Walmart basket). Without caster the front of the basket would wander all over the place and not go where you wanted it. The front wheels do this because of the large amounts of caster angle (positive caster in this case) that creates a centering force called “align torque”. This align torque is the result of mechanical trail allowing the side force of the tire to act with leverage. The greater the caster angle, the greater the mechanical trail, the greater the mechanical trail the greater the leverage, and with increased leverage you have an increase in align torque. Steering Effort vs. Caster Angle A secondary function of caster angle is it’s affects on steering effort. While there are more things at play than caster, it does have a significant affect on steering effort and feel. In the modern era of power steering and high performance vehicles the caster found in virtually every vehicle on the road today has a fair amount of positive caster angle. A side effect of large amounts of positive caster is an increased amount of steering effort needed to turn the wheels. With power steering this steering effort is minimized and not apparent to the driver. However in the old days, prior to the proliferation of power steering systems vehicles ran very little, or even negative caster angles. Typically they ranged between +3° to -1° in angle (compared to +7° today) in order for their non-assisted steering racks to retain a light steering feel. This had the side effect of making the steering very prone to wandering and vague feeling. So in conclusion, adding positive caster will make the steering heavier and removing positive caster angle makes the steering effort lighter. Benefits of Positive Caster Angle Improved Straight Line Stability Improved Camber Gain During Cornering Faster Steering Return Weight Shift with Steering (Oversteer) Reduced Front Grip (Beneficial for Counter-Steering) Extra Caster can Improve Net Caster even with High Pneumatic Trail Straight line stability is improved because a positive caster angle creates a centering effect at speed. This occurs because the lateral grip force of the tire generated during turning pushes the tire in the opposite direction of the steering angle. It does not over-correct because as the tire rotates back to straight ahead the amount of lateral force generated is diminished. The greater the caster angle, the more leverage the lateral force has to center the wheels. This is why cars today run large amounts of caster, to maintain highway stability and avoid wandering. In a car with very little to no caster angle, there is very little centering force due to the lack of leverage. The car will wander and tend to pull when it encounters road variability (tramlining). The same centering force that creates active steering stability is the same force that will quickly return the steering wheel to center. Faster steering return is important for car control; especially in a skid or slide condition where you need quick steering inputs to control the car and avoid a spin, loss of control, or a wreck. Without it the driver must quickly and accurately return the wheel when the vehicle returns to center. The more involved the driver is the greater the chance of over-correction which is the leading cause of spin-outs. A common issue with MacPherson strut front suspensions is the lack of camber gain during cornering. When you compress the suspension the individual components go through a range of motion which varies their lengths depending on the geometry of the suspension. This is traditionally referred to as suspension travel or bump travel. Camber and toe angle both adjust during this travel. Dynamic shifts in camber are referred to as “camber gain” and changes in toe are called “bumpsteer.” Ideally, you want as little toe change as possible and an increase in negative camber angle. By increasing the negative camber angle of the tire during suspension compression you will ensure the tire remains horizontal on the road surface and evenly distributes the weight load across the entire face of the contact patch. This improves peak lateral G (handling) as well as ensuring even wear across the tire for track cars. By increasing the positive caster angle you will also increase the amount of camber gain as the vehicle turns. This happens because as the suspension pivots the outer wheel will gain negative camber. The converse happens to the inside wheel, it will gain positive camber. This is considered one of the best mods to do to a MacPherson strut suspension because it is largely a “free lunch”. The benefits are much better than the negatives. Typically you want to aim for about +6 to +9 degrees of caster angle. Going excessively far into caster angles above +9 starts to have diminishing value because of an overall reduction in steering feel at the grip limit. However, when you are counter-steering the camber gain goes the opposite direction. It actually reduces negative camber and reduces peak lateral grip of the outer tire, which is the most vital tire in any cornering situation. This can be beneficial from a car control aspect because the front and rear will progressively gain grip as the vehicle slows down. If the front gained grip as you counter-steered then the front could bite and shoot the car off in a very rapid and abrupt manner. The idea here is to scrub speed and allow the inertia of the car to slide in a controlled manner to give a predicted path and minimize driver input, and chance for error. A nice, solid 4-wheel drift is also incredibly fun.
cont. Due to the geometry of the caster the front end will raise as you increase steering angle in either direction. This raising motion is geometric rather than due to the spring, but it does have an affect on weight shift during cornering. By increasing the amount of positive caster you will increase the amount of weight shift that occurs. As an example, during a left hand turn the corner weights of the left front and right rear tires will increase. This means that an increase in positive caster will increase the chassis’s tendency to oversteer. For a car that understeers, adding caster will increase it’s peak handling ability and make it more neutral. Another effect that causes centering torque is called pneumatic trail. Caster itself creates what is referred to as mechanical trail. Mechanical trail is the distance between the steering pivot axis and the tire centerline axis. Pneumatic trail is the distance created between the contact patch and deformation from the resulting tire slip. The image below will make more sense. (Image Source: Haim Kotler) Pneumatic trail should be taken into mind when you are setting up the car, at least in brief. The stronger the side force, the greater the slip angle, narrower and taller the tire, generally the longer the pneumatic trail will become. How do I Adjust Caster? Caster is adjusted by a couple of different methods depending on the type of suspension you have. For most vehicles, caster is not directly or independently adjustable without some sort of aftermarket part however some do use washers or spacers from the factory. MacPherson Strut For this type of suspension the best way to adjust caster is when you have a forward link that can be elongated or shortened, or by installing camber / caster adjustable top plates. If you have adjustable lower control arms they can be asymmetrically lengthened to add small amounts of caster, but this will also affect camber and other geometries. Double Wishbone Caster is derived differently than MacPherson setups because the pivot axis is a line drawn through the center of the upper and lower ball joints (vertically). Adjusting the shock will not alter camber or caster like a MacPherson setup. Your main option is to either install off-set bushings or run adjustable control arms that allow you to asymmetrically adjust the lengths. Another less available option is an adjustable ball joint fixture that allows you to move either ball joint forwards or backwards. Is Caster part of a “Front End Alignment” or “Complete Alignment”? Most Hunter alignment machines and other commercially available machines will measure caster. Unless it is significantly out of spec it is very unlikely that the alignment tech will adjust it, and even if it is out of spec, your vehicle may lack the necessary adjustment to make the changes. Typically if your caster is significantly out of spec you have a bent control arm or some other suspension issue that needs to be addressed. If your vehicle has adjustable caster due to aftermarket parts it would be in your best interest to seek out a performance oriented alignment specialist in your area. They can help you get the most out of the alignment since most chain stores and mom & pops are going to put factory specs on your car, which are never ideal for a modified setup. Please note that adjusting caster can adjust other measurements depending on the suspension setup. Simply making a “quick adjustment” can mean having to completely re-align the front end in some cases. Does Caster cause Pulling? Caster can cause pulling if it is not the same on both sides. I recommend keeping caster within half a degree to ensure you won’t get heavy amounts of pulling. The car will tend to pull towards the side with less positive caster angle. Does Caster cause Tire Wear? Typically no. Caster can cause accelerated tire wear in very limited situations. Because it affects camber during cornering, particularly a positive cambering effect during counter-steering, it can cause accelerated shoulder wear on the outside edge of the tires during drift events.