What is acceptable scrub radius?

The distance between the center of the tire’s contact patch and the point where that line touches the road is the scrub radius. If the line touches the road inboard of the contact patch, scrub radius is positive. If the line touches the road outboard of the contact patch, scrub radius is negative (see Figure 2).

Is positive scrub radius bad?

Extreme positive scrub radius can make the steering very heavy, too, and it was only really viable on old cars with very thin tyres. Most of us have negative scrub radius on our cars because it tends to go hand in hand with MacPherson strut suspension.

What does positive scrub radius mean?

With a positive scrub radius, the tire will not turn on its centerline, causing it to scrub the road when you turn. For you, this means exerting more effort. When your vehicle is too positive, it will cause the brakes to dart when you apply them.

How much does scrub radius affect handling?

When scrub radius is zero, tension on the steering system is reduced and vehicle handling will tend to be unstable. Zero scrub radius also causes the tires to scrub in the opposite direction of rotation during turns (tire squirm). This has a negative effect on handling in turns and affects overall tire wear.

Why is the radius of a scrub important?

Keeping the scrub radius small will make the car easier to handle at low speeds. It also reduces the risk, that a loss of traction on one front wheel during braking, will cause the car to change direction. If the scrub radius is small then the contact patch is spun in place when parking, which takes a lot more effort.

Can scrub radius be measured?

Really just the distance from that point to the central point of the tyre contact patch. Two schools of thought on measuring scrub, radius, the classic way is as above to measure at ground level.

Do wheel spacers affect scrub radius?

Fitting a wheel spacer will increase the positive scrub radius at the front wheels which has some unfavourable consequences. If your car already has a bit of positive scrub radius then the wheel spacer will increase it.

How important is wheel offset?

Proper offset assures your new package has enough clearance so nothing rubs against the suspension, brakes or vehicle body (like fenders, bumpers and mud flaps). It’s also important for driving safety, since the wrong offset can reduce vehicle stability or interfere with braking.

Does wheel offset effect turning radius?

Overall, a change in tire size does not have a significant effect on the turning radius. … However, if the tires are too big, offset and rims may need to be altered as well, thus influencing the turning radius.

What is negative steering?

With a negative steering offset, (see figure) the track point lies outside of the center, and with positive, inside. If the trace point lies dead center, its value is zero. Positive steering offset nowadays, is found in the motor car only if the construction makes it necessary.

How will positive scrub radius affect the steering of the car?

Positive Scrub Radius will increase steering effort, torque steer (Making the car harder to control during turning and cornering) and kickback on bumps to a considerable degree. At the same time,a blowout or a failure of one front brake could yank the wheel hard enough to pull it out of your hands.

Can you increase offset?

Yes it’s possible. If you want to increase the number of positive offset (ex. from +30 to +35), the back of each alloy wheel has to be machined down evenly using a special CNC machine. Note that not all offset changes can be achieved.

How do you calculate the radius of a scrub?

From a front view of the car, draw a line between the ball joints and make it intersect the ground. The distance between the centerline of the tire and where this line meets the ground is your scrub radius.

What is king pin inclination?

Swivel pin or kingpin inclination is the lateral inward tilt (inclination) from the top between the upper and lower swivel ball joints or the kingpin to the vertical (Figure 10.3).

Whats is offset?

What is Wheel Offset? – YouTube

What is SAI on a car?

The (SAI) Steering Axis Inclination angle is created and measured in degrees. It runs from the lower ball joint through the upper ball joint, or on most front-wheel vehicles, through the center of the strut mount.

What is negative caster?

Negative Caster – If the line slopes towards the front of the vehicle then the caster is negative. Negative caster will allow you to steer less around turns, but may cause you to drift if you are driving straight forward.

What is negative camber angle?

A negative camber refers to the angle placement of the wheel in a way that the bottom is further out than the top of it.

Do spacers lower offset?

Wheel spacers simply move the wheel away from the hub. Adding wheel spacers will have the effect of running a wheel with a negative offset. They give you room for larger tires, thus solving fitment issues.

How much wheel offset is ok?

It’s best to allow an 1/8-inch of play for potentially wider tires. See all 11 photos Offset not only affects how far the wheel/tire sticks out of the fender well but also its proximity to suspension components like knuckles, and control arms.

How do you know what offset you need?

Subtract the smaller number from the larger number. That gives you the offset of the wheel. If the centerline number is smaller than the hub measurement, offset is positive.

How much wheel offset difference is OK?

If the new wheels are the same width, the new offset should ideally be within 5mm of the old offset in either direction. If offset must be substantially different, avoid using more positive offset at all costs. If the new wheels are wider, backspacing must be calculated.

Is 10mm offset a big difference?

10mm isn’t a huge amount. On some cars it makes little difference. Tough you say. But if you want exact fit…see if the dealer still make it right.

Do wider tires increase turning radius?

The shorter and wider a tire is, the better the cornering and handling. Other benefits of wider tires include improved turning radius, ability to accelerate and stopping power. These larger tires also influence other ways that a vehicle responds to the road, including traction control and stability.

Is a bigger turning radius better?

Turning circle radius gives an indication of the space required to turn a particular vehicle. Hence, the higher the value of turning circle radius, the higher is the space you need to turn the vehicle completely and vice versa.

What is a good caster angle?

Positive caster angles run between 3 – 5° on modern vehicles. This gives a good mix of highway stability and steering feel. For a more performance oriented setup on a MacPherson strut you can add a degree or two to have more favorable camber gain at high steering angles.

What will too much caster do?

If you don’t have enough positive caster (forks too little angle toward the front) the car will wander and feel unstable. If you have too much positive caster, the vehicle will be harder to turn. However, just like with camber, vehicle pull is not caused by too much or too little caster, but by side to side imbalance.

What causes bad caster?

It was common in older vehicles that don’t have power steering to help lessen the steering effort. Negative caster improves steering but diminishes handling and stability when driving in a straight line. A vehicle that pulls to one side when moving in a straight line may have a negative caster.

What is bump steer caused by?


Bump Steer is when your wheels steer themselves without input from the steering wheel. The undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages. Most car builders design their cars so that the effects of bump steer are minimal.

What is kingpin offset?

Kingpin offset

The king pin offset is defined as the distance between the center of the wheel and the crossing point of the king pin axis and the wheel spindle axis.

What is my factory wheel offset?

Your wheel offset is the distance from where you mount your wheel to the wheel’s center. The centerline is the literal “center” of the width of your wheel.

Can you reduce wheel offset?

Can I change offset without changing wheels? Yes. Companies make wheel spacers with varying thicknesses which allow you to change the offset of your wheel quickly and easily. The wheel spacers essentially reduce the distance from the center of the wheel to the hub, thus reducing positive offset.

How do you adjust tire offset?

Lay the wheel on the floor face up. Measure the distance from the floor to the bottom of the center of the wheel. Add the distance the tire is to be moved in to this measurement. This is the new offset.

What is scrub torque?

The scrub torque measurement test applies a normal load to a surface–wheel interface, and then measures the force required to rotate a surface contact plate. The relative motion between the test surface and wheel emulates the wheel scrubbing (pivoting) on the test surface.

What is a scrub person?

Scrub has been slang for an “insignificant or contemptible person” since the 1500s. Over the centuries, the term was used as a slur to denigrate prostitutes and even Black people for a time.