Modifying the suspension, or even replacing your stock wheels with different ones changes the vehicle’s suspension geometry. Contrary to popular opinion, car modifications are far more complex than people realize.
Some of these modifications are done purely for aesthetic purposes, but even then the performance capabilities of a car are changed. Scrub radius is one of the aspects that a lot of people haven’t heard of, but plays a significant role in the way a car handles.
In this article, you’ll find all the information about scrub radius, what it means, and how it affects a car’s performance. Let’s jump right into it.
What is Scrub Radius?
Scrub radius is the distance between two imaginary lines, and where they touch the road. The two imaginary lines include the wheel’s centerline and the steering axis when looking at the car from the front.
If you replace your stock wheels and tires with aftermarket ones that are of different sizes and offset, you directly affect the scrub radius, and your car’s handling, stability, and tire wear. You’ll agree that all of these are important factors, and that scrub radius is far more relevant than people realize.
There are three types of scrub radius – negative, positive, and zero scrub radius.
A positive scrub radius is when the two lines we talked about meet underneath the ground. A positive scrub radius is most commonly found in rear-wheel-drive, performance, and race cars. This allows the driver to feel the road much better and react to understeer or oversteer.
A negative scrub radius also improves road feedback, but much less than a positive scrub radius. It’s most commonly found on front-wheel-drive cars.
A zero scrub radius is the safest option and provides the driver with the easiest handling at lower speeds. At higher speeds, especially during hard braking, a zero scrub isn’t the best choice, because it can lead to unpredictable reactions.
Why Simple Changes To Your Car Affect Scrub Radius?
Now that we know what scrub radius is and what types of scrub radius exist, it’s time to cover why a simple change can affect this.
Imagine replacing your factory-fitted wheels with same-sized wheels, but with a different offset. Your new wheels will either stick out more, or will be tucked underneath the car more, but one essential thing changes – where the center of the tire is compared to the steering axis. Offset alone won’t affect scrub radius drastically, but combined with spacers, and lowered suspension, the scrub radius can easily go from negative to positive, completely changing the way your car drives and handles on the road.
This is one of the reasons why it’s important to check the suspension geometry regularly, especially if you do any modifications to your car such as lowering it or fitting aftermarket wheels that are different compared to your old ones.
Even if you just replace one of your suspension components due to regular wear and tear, the suspension geometry can change, causing different scrub radius on each wheel. This shouldn’t be ignored, especially if you notice a difference in how your car handles on the road. Having your suspension geometry adjusted properly is an inexpensive process that will improve your car’s handling and tire wear, saving you money in the long run.
The Effects Of Scrub Radius
It’s time to cover the effects of different scrub radius types, their advantages, and their disadvantages.
The positive scrub radius is most common in rear-wheel-drive cars, double-wishbone suspension designs, and race cars. A positive scrub radius keeps rear-wheel-drive cars straight, but the downside is that it requires more effort to turn the wheels. If the positive scrub radius causes issues, there are a few things that can help with reducing its effects.
The first, and most affordable option is using wheel spacers which will move the tire centerline from the center of the vehicle, reducing the tire scrub radius. Another option includes getting a new set of wheels that are wider than the wheels originally found on the car. This is recommended when
A negative scrub radius is common in front-wheel-drive cars and it goals hand-in-hand with MacPherson strut suspension. Negative scrub helps keep front-wheel-drive cars going straight, no matter whether you run through water or anything else that might affect handling.
Zero scrub radius is the most uncommon out there, because it makes cars unstable during cornering, and because the entire width of the tire is contacting the surface. This means that your tires will wear out much quicker compared to a car with either a positive or negative scrub radius. A zero scrub radius will also make it extremely difficult to turn the wheels, especially when stationary.
Which Scrub Radius Is Better For Daily Driving?
Unlike popular opinion, zero scrub radius isn’t ideal, and it doesn’t do any favors to either front-wheel-drive cars, rear-wheel-drive cars, or all-wheel-drive cars. It will make them unstable while cornering, and the tires will wear out significantly faster.
A positive scrub radius is acceptable and recommended if you drive your car on a daily basis in urban areas. This allows you to turn the wheels easier at slow speeds but isn’t a great option when it comes to higher speeds or hard braking. A negative scrub radius requires a bit more effort when turning the wheels, but keeps your car controllable at higher speeds and when braking hard. Modified cars with lower suspension and wider wheels tend to have negative scrub radius by default, however, when it comes to this, more isn’t necessarily better. Too much scrub radius, whether it’s positive or negative significantly affects the driveability of the car, and how fast suspension components and tires wear out.
To sum this up, there’s no right or wrong when it comes to scrub radius on your car. Each option has its pros and cons, but the underline is that whichever you choose, you should never go to extremes. The most important thing to keep in mind is that every modification on your car regarding the width and size of your wheels, or suspension will alter the scrub radius and other aspects of the suspension geometry, so make sure you always have it checked when you change a component on your car.