2. Will installing TRAK+ Wheel Spacers wear out my wheel bearings faster than normal?
No. TRAK+ Wheel Spacers will not cause premature wear of your wheel bearings. Inspect your bearings to make sure they are in good working condition before installing TRAK+ Wheel Spacers.

3. Can I test fit TRAK+ and exchange them if needed?
Once spacers have been installed, H&R can no longer accept them for return. To ensure you get the correct spacer the first time, view How To Buy

Scrub Radius. by Roger Jackman.

This man walks into a bar... "NORM!" He proceeds to belly up to the bar and begins to talk about the daily grind and the latest in his relationship with his wife. The bartender, a former sports icon, talks to a patron at the other end of the bar about the specifics of modifying the suspension on his Corvette. He was told that after changing to an aftermarket wheel and tire combination, he needs to modify the scrub radius a bit and wants to know what in the world they were talking about. Cliff is trying to explain that scrub has been in use since early man invented the wheel. He claims, "in order to get the perfect lateral friction to rolling resistance ratio, you must periodically 'scrub' the tires to expose the unused material."

Does this scenario sound like you and your car buddies at the local watering hole after a club meet or romp through the countryside? I know I have heard it somewhere before. Now, I was able to get through school with a little intuition and a lot of B.S., but these qualities do not always work when I am dealing with inanimate objects that are not humored by my B.S. and theory. When dealing with suspension design and dynamics, little things that are easily overlooked like scrub radius can make a greater difference in your car's handling and feel than all your theory and fervent tuning can accomplish.

To understand scrub, you must first know about Steering Axis Inclination (SAI). The steering axis is the line between the top pivot point of your hub and the lower ball joint of your hub. On a MacPherson strut, the top pivot point is the strut bearing, and the bottom point is the lower ball joint. On a suspension using upper and lower control arms, the pivot points are where the upright connects to the control arms. The inclination of the steering axis is measured as the angle between the steering axis and the centerline of the wheel, so if your camber is adjustable within the pivot points (i.e. Volkswagen) you can change the SAI.

Now back to the subject at hand. The scrub radius is the distance on the ground between the centerline of the tire contact patch and the point at which the SAI intersects the ground. If these two lines intersect at ground level, then you are said to have zero scrub. If the SAI intersects the ground at a point inside or outside of the centerline of the contact patch, you are said to have positive or negative scrub respectively.

You now know what elements make up scrub, now you may ask what it has to do with the price of tea in China? Well, the point at which the steering axis line contacts the ground is the fulcrum pivot point on which the tire turns. The location of this point within the contact patch has a great effect on steering effort, feel, and stability. If you have not already guessed, the easiest way to change scrub is by changing your offset with either new wheels, or hub centric wheel spacers.

If the scrub is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

Positive and negative scrub radii have benefits in different types of suspension. A MacPherson strut assembly typically performs well with a lot of SAI and caster, a system negative scrub works well in. Because both SAI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort. Negative scrub also helps reduce torque steer in front wheel drive cars. Positive scrub radius works well with suspensions that use dual control arms that use less caster and SAI to optimize geometry.

As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. When you have excessive scrub, whether it be positive or negative, steering effort increases and road 'feel' increases, as the steering is more susceptible to road shock. Additionally, if you plan on doing some homework on, and modifying your scrub radius, you must take into account the amount of sidewall flex your tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the SAI and can make a slightly negative scrub radius become zero.

Well, now you know all about scrub, so the next time it comes up at the watering hole you have the confidence in knowing that your drunken compatriot is having more fun with creative banter than tuning and driving his car. That is not such a bad thing though. Order another beer and go with it. I will bet you can come up with something better.

Scrub radius and SUV handling
A new wheel end concept was designed and tested at Dana Corporation to offer sport utility vehicles and light trucks the possibility of achieving a negative scrub radius. Computer analysis and performance data from instrumented vehicles determine its effects.

Scrub radius is defined by SAE as kingpin offset, which is the distance between the two points created by the inter- section of the wheel centerline (centerline of tire patch) and the steering axis with the ground plane. The steering axis is determined by the kingpin index angle, which is the plane projected through the upper and lower ball joints and the wheel shaft joint. A zero scrub radius exists when the wheel centerline and the kingpin index intersect exactly at ground level. Positive offset occurs when the wheel centerline is located laterally inboard from the steering axis at the ground plane, while negative offset occurs when the steering plane intersects the wheel centerline above the ground plane.

Choosing a negative or positive scrub radius for a vehicle's suspension has emerged in recent years as a philosophical debate. For a passenger car strut suspension, achieving a negative scrub radius is at least a viable option. However, on SUVs and light trucks with 4WD and short long-arm (SLA) suspensions, packaging the wheel end components and achieving a negative scrub radius has been difficult or impossible. Demand for off-road capability, working payload capacity, and the required brake component size needed, makes achieving even a small positive scrub radius difficult or requires compromising brakes, steering linkages, or other suspension components.

Configuring the CV Hub wheel end design with several scrub radius settings was an attempt to show that it is feasible to achieve even a negative scrub radius in an SUV. Opinions about whether negative scrub in a light truck is what the average driver wants are mixed. However, there is a trend to provide improved vehicle-handling dynamics, which has increased the focus on reducing scrub radius to a small positive number.