Caster geometry is the measurement of the steering pivot axis measured in degree against the vertical axis. The casters angle can be used to refine steering feel, vehicle stability and optimize the handling performance
- Learn what is caster
- Caster effects on driving
- How to tune it to improve performance
- Ways to measure caster
- How to adjust caster
What Is Caster
The caster angle is a suspension geometry that relates to the self-steering attribute. It’s easiest to visualize a McPherson struts front suspension from the side.
The difference between the vertical axis the and steering axis is the caster angle measured in degrees. The top of the strut has the pivot point for the steering rotation. If you draw a line from this point through the axle bottom ball joint to the ground this gives you the steering axis. If this steering axis crosses the vertical line to a point on the ground in front, it is known as the lead point, resulting in positive caster.
The opposite steering axis crossing a point to the rear is called negative caster
If the steering axis is in line with vertical it’s called zero caster.
If you’re wondering how it works for other suspensions like double wishbone etc. the steering axis is measured from the top hub bearing/ball joint to the bottom ball joint.
The best way to remember positive camber is to think about a big bald biker with a long beard on an American chopper.
Choppers have big long forks stretched out in front, this shows an exaggerated steering axis.
The chopper’s lead point is on the ground in front of the vertical axis and tire contact patch. This causes a stabilizing self-centring steering. Allowing our big bald bearded biker to ride with no hands and keep the bike going straight.
Where does self-steering come from?
The steering axis intersects the ground in front of the tyre contact patch where the tyre cornering force is located.
Abit of physics regarding mechanical trail and bomb! Between these two points, a torque/moment arm is generated. Resulting in the rotation force twisting the wheel back to the centre. This torque arm creates a self-centring steering/self-aligning attitude of the caster.
The torque arm generates a steering feel too. The longer the torque arm created the gather resistance resulting in a heavier steering feel.
In common English, Positive caster stops our cars’ from wandering all over the road, like a lost dog, while adding to steering feel.
The most simple to remember, as the steering axis is in line with the vertical axis.
Neutral or Zero caster has no self-steering properties which makes steering light and easy to use.
It’s as simple as the opposite. The steering axis is on the ground at the rear, behind the contact patch becoming a trail point.
The negative caster causes the wheels to steer in whatever directional force is applied to the wheel, reducing stability
How Caster Affects Driving Dynamics
Out of the 3 types of caster mentioned above, positive caster is the most widely used, as it has a desirable effect.
We will quickly cover negative caster 1st as it’s least desirable and not all that useful for vehicle steering.
Then skim through zero caster, as you’ll get more benefit from going in-depth on positive caster to help you find performance.
Negative Caster Effect
Negative caster has one main advantage: it requires very little effort to turn.
Except for this, it isn’t very useful for race vehicles of any type, as it has little steering feedback and under braking it increases making steering unstable in corner entry.
Yes it’s unstable exactly when you want steering to be most practised.
Zero Caster Effect
Neutral or Zero Caster has the same advantage of being very easy to turn, without compromising stability too excessively.
With the steering axis and vertical axis being perfectly aligned there will be camber change with steering input. Depending on the situation it can help or hinder.
Now I hear you asking camber change on steering? Well, this leaves us at a good point to dive into deeper explanation of positive caster.
Positive Caster Effects
Visualise the car from a slight offset bird’s eye view, looking directly down the steering axis, as you turn the steering wheel to the right. The left wheel gains a slight amount of negative camber and the right wheel gains a slight amount of positive camber. The opposite occurs when the steering turned left.
Caster Causes Camber Change
The camber change is a result of the steering axis being offset to the vertical axis, as the wheels move in more than one dimension causing this camber change.
Advantage of gaining negative camber on the outside wheel and the inside wheel gaining positive camber aids the tyre to maintain maximised contact patch during cornering. Increasing grip.
Even slightly offset the undesirable camber change during body roll. Effectively leaving the tyre into the corner like a motorbike.
Lean more about camber, it’s effects in depth here Camber Geometry – Tuning Wheel Alignment
Negative of positive casters camber change. For drifters the lead wheel during a drift will gain positive camber and reduce the contact patch of the leading to understeer. so a higher static camber is needed to overcome this.
Self Steering Torque
Positive caster generates a self-steering/selff-aligning torque, best displayed by watching a drifting onboard when they turn the wheel in a big over steering moment let go with both hands completely let the steering wheel spin itself then catch it to holding an awesome tire smoking slide.
Search Drifting POV on youtube you will see exactly what i mean.
So what is this self-aligning torque good for? Increase vehicle stability
Advantage is the wheel wants to self centre, this increases straight line stability. This self aligning torque creates a heavier feel to the steering that provides the driver confidence as it’s firm and stable.
On corner exit the wheel wants to return to centre allowing the driver to be much smoother and practised with steering control.
2 key points here
- Corner exit is the most important part of cornering. it doesn’t matter how much of a hero you are late on the brake or how much mid corner speed you carry. If you’re not fast out the corner you’re nowhere.
- “When I look fast, I’m not smooth and I am going slowly. And when I look slow, I am smooth and going fast.” Alain Prost,
Disadvantages to positive caster
The self aligning torque required more effort to turn the steering wheel, especially at low speed.
However, this is easily overcome with modern power steering and those race cars that don’t come with power steering are usually lightweight single seaters. Just like the historic 1985 Mclaren MP4/2 F1 car Prost and Senna pushed to the limits in thire championship battle.
Before we go on, there is another steering axis factor to be aware of. Understand its effect before turning caster alignment setup
King Pin Inclination honourable mention.
King pin inclination (KPI) also known as steering axis inclination (SAI) is another part of the steering geometry. When we look at the caster it is from a side profile, KPI is from front/rear profile, looking at the steering axis in relation to the centerline of the tyre.
The distance between where the steering axis intersects with the group and tyre centre line is called scrub radius. The larger this distance the harder it is to turn the wheel at slow speed. Affecting steering feel and something to be considered.
The steering axis is unusually inclined “tilted” to reduce the scrub radius, as the suspension pivots have to be mounted outbound of the wheel. The inclined axis when rotated will move the wheel in an ark raising and lowering the wheel resulting in two effects.
- Push one wheel into the ground while lifting the other known as jacking. If you sit in a car and go lock to lock you will notice the front inside side raises and the front outside fall. As steering is applied the jacking creates a self-centering force as the wheels wish to return to level. (straight steering position)
- It causes the wheels to change camber with the inside wheel gain negative camber and the outside wheel loading negative camber
Yes, that’s the opposite of what we want for cornering.
The opposite of the camber changes casually by the caster somewhat offsetting each other.
KPI’s effect on the steering and driving dynamics needs to be considered when changing the caster
Learn more about king pin inclination, how it works and affects performance here: (article coming soon)
Read more about KPI later,
Tap the link within your finger,
Let’s get back caster,
So our setup drives faster.
Yes I did take the time, to make a little rhyme.
Positive caster being the most widely used and beneficial to vehicle performance we will focus on what condition increases it will be an advantage to perform and some symptoms when you may want to decrease positive caster to enhance vehicle performance.
Not to sound repetitive some of the effects of increase positive caster might sound similar to the positive caster effect section, as going from zero to positive essentially increase positive caster
Increase Positive Caster
The advantages of increasing positive caster are as follows.
- Straight line High Speed Stability – if the car wanders on high speed straight, adding caster increases self aligning torque keeping the vehicle straight
- Self Centering Speed – if the driver feels they need to work the steering too much on corner exit, increasing positive caster can help with a smoother corner exit transition.
- Heavier Steering Feel – adding positive caster increases the self aligning torque, any steering input requires more effort to over the torque resulting in a heavier feel.
steering feel is subjective, tyre width, wheel offset, KPI, tire scrub radius and slip angle etc are all continuing factors to be considered.
- Increase Dynamic Camber Change – Some suspension systems are limited to how much camber can be gained during dynamic motion. A increase to caster can increase dynamic camber gain in cornering, reducing static camber required improving braking while mentation cornering grip
Reduce Positive Caster
Reducing positive caster could be so advanced to perform in the following situation.
- Mid Corner Understeer – As a result of the camber change that positive caster generates, if the outside wheel gains too much negative camber it will not use the full contact patch during cornering reducing front end grip mid corner. Caster usually affects slower speed corners, as more steering lock is required to feel this effect.
Having a lot of steering lock on during a high speed corner means 1 of 2 things.
- You have brown stains in your pants as you desperately try to steer away and avoid hitting the tyre wall.
- Your Drifting, as drifting requires a hell of a lot more steering lock than circuit racing, getting a good caster alignment set up is important.
- Too Heavy Steering Feel – if the driver arms are aching. reducing positive caster reduces the self aligning torque, any steering input requiring less effort over the torque resulting in a lighter feel and easier to turn.
Please note: the steering feel change is a result of shorting the torque arm, on some suspension setups it’s possible to do this without changing caster although rare.
How to Measure Caster
Before we can change and dial in a good set up we need to measure the caster so we actually know what we’ve got to work with.
A popular method tool measuring caster
- Turn plates and a bubble or digital gauge
- 4-wheel alignment machine
Turn Plates & Gauge
Very popular methods in motorsport due to portability and only require a few pieces of equipment.
1st a flat floor is needed as the car needs to sit on a level datum to take measurements. As caster angle is the measurement better the steering axis and vertical axis, if the car is slanted it causes the vertical axis to off, throwing all your reading out.
Race teams usually carry their own flat floor as it’s also needed for other chassis set requirements.
Don’t have a flat floor don’t worry, most race tracks usually have one in the scrutineering bay or the whole area will be a flat floor, on a test/track day it will be open to use to set your car up.
Turn plates are placed under the wheels, once the car is level. the plates will give a reading in degree how much steering angle has been applied.
The castle gauge is then fitted to the wheel. Digital and Bubble gaugese will both work on the same principle. The steering is turned about 15-20 degrees out; the degrees turn varies with gauge manufacturer. Gauge is then zeroed or bubble set level. Then the steering is turned inwards the same amount of degrees. Then a digital gauge may ask spirit bubble to be levelled before displaying the reading.
Each camber gauge will have its own set of Instructions on its correct application. However, they will be very similar to the example given above.
4 Wheel Alignment
Most road car garages will use a large 4-wheel alignment machine to adjust wheel alignment. The car will often be driven onto 4 post ramps. Then attach an alignment machine to each wheel, the machine will have instructions on how much the wheels are to be turned, utilities sensors to read the caster. Providing a reading on the display screen.
Before a wheel alignment check Setting all the tyres to the target pressure increases the consistency of measurement.
Also inspect buckled wheels. The buckle in the wheels will provide a false reading as equipment is attached causing the alignment to be set wrong.
How To Adjust Camber
Caster angle is normally fixed on road based cars, there are many components that can be fitted to unlock caster adjust ability. It may vary on different vehicles, this section will cover the most common type. so you can get stuck in and adjust yours
These are used on McPherson struts. The rotation axis is at the top of the sturt, where it mounts to the body by moving its mounting location towards the front of the car it can reduce positive caster, moving it rearward increases positive caster. there will be sliding plates or elongated hole that enables the mountain point to me moved,
Loosening the bolt the bottom plate will be free to slide along the slots moving the top of the strut changing steering pivot and steering axis changing the caster ready bolted tight in place. Adjustment range is limited due to the size of the slots,
Adjustable Tension Rod
Used on the front McPherson Strut suspension. The tension rod can be used to move the wheel forward in the arch increasing positive caster or rearward reducing positive caster.
These adjustable arms replace the stock suspension link. Often being stronger and lighter than OEM. Designed with threads section that can be screwed out to lengthen, or in to shorten the arms total length. changing the wheel caster.
These are an excellent upgrade if you’re converting road cars to push the limits on the track.
Particularly for Drift as having control over self steer and camber change as a result of the caster enable you to dial in a fine setup.
On a lot of purpose built race cars like kanus… cough cough I mean single seater. As they often run a double wishbone setup the top pivot will be the start of the steering axis. Having adjusted hub’s allowed the offset of the top pivot to move forward or backwards changing the caster angle.
Now you’ve learned the effect of changing the wheel’s caster, where to measure it, and how to adjust it. You can apply it to enhance your driving with a custom wheel alignment.
Another thing to consider is the wheel toe & camber angle, and how that can be optimised to enhance grip, learn how it works and how to tune it here