STREET CLASS AUTOCROSS: How to win with just 3 mods.

Street Class Autocross

I am on a budget. Why? Because I’m a normal human being living a normal life. Since I can’t afford to buy a vehicle’s maximum potential in a Modified class, I’m happy to compete in a class where maximum potential can be reached without much money. So it makes sense that I race Street class autocross.

Having become a Street class autocross champion last year, I have a lot to stay about the necessary upgrades required to win races. You don’t need any power adders, expensive brake parts, or sneaky cheater parts. (See how I feel about motorsport cheating here)

All you need are three modifications. So let’s jump right into it.



Tires are the absolute number one upgrade for any performance car. If you don’t believe me, listen to 2016 ProSolo champion/Mazda Shootout driver Julian Garfield. He says the same thing in Episode 014 and Episode 015 of the Racers HQ Podcast.

Power, torque, and even top end suspension mods mean nothing without excellent tires. There are only four points which connect your car to the driving surface. If you don’t optimize them, you’ll be slow.

A good set of performance autocross tires are worth 1-2 seconds per lap for a talented driver. There is more to gain from tires than any other modification.

Let’s have a quick look at the tire allowances for Street class autocross in section 13.3 of the SCCA Solo rulebook.

You can see that tires are only limited by two main factors:

Factor #1

They must be DOT approved for highway use and have 200 UTQG rated or higher. A UTQG rating is a measurement of how soft a rubber compound is.

street class autocross

A long-lasting highway tire will have a higher rating because the rubber is packed tighter to wear over a longer period of time. Higher rated tires aren’t as pliable and can’t conform to imperfections in the road, meaning they can’t grip as well.

A lower UTQG rating is better for performance because a softer rubber can conform to imperfections in the driving surface. This means that the tire is locking into the surface and can give better traction. (Learn more about compounds and how molecular adhesion relates to traction.)

To keep costs down, 200 is the lowest rating allowed in Street class autocross.

Factor #2

The other main factor is the wheel size. As you can see, the rule is written to allow any size tire that fits on a legal wheel. 


So the next question is: what is a legal wheel? No problem.

Street class autocross rules are very clear about wheel specs.


So, a legal wheel:

  • Is the same width as the OEM wheel.
  • Is within one inch diameter of the OEM wheel. So if a car comes with a 17×7 inch wheel, the only legal wheels would be 16×7, 17×7, or 18×7.
  • Must be within 7mm offset of the OEM wheel. This means no deep dishes or wheels that widen the track more than 7mm. Make sure you do your research. Any reputable seller/website will have this information readily available.

Ok, back to the tires.

Again, you can use any tire that fits on a legal wheel. Even if a tire has with a wider section width or aspect ratio than the OEM size.

You should also be aware that you must use tires that are available to the general public. Custom made tires aren’t allowed because they aren’t equally available to everyone. If you buy the best tires on the market you shouldn’t have any issues with that, so I wouldn’t be too concerned about it.


In 2016 the two best tires on the market were the Bridgestone Potenza RE-71R and the BFGoodrich Rival SYou can see the national results by clicking here. These two models were clearly used by more competitors at the national level than any others. 

I also like the idea of supporting these brands because they offered national level contingency sponsorships for grassroots autocrossers. That’s a big deal. 

BFG is currently reworking the Rival S to be even more competitive. Be on the lookout for more information about that.

My personal favorite tire, the Dunlop Direzza Zii Star Spec is outdated and no longer the fastest tire. Unfortunately, it’s time to move on. 

There is a lot of tire news right now going into the 2017 season. Pay attention, there’s still plenty to learn before purchasing your tires for this season. The latest news can be found in this recent post on the Racers HQ Facebook page.


2. Anti-Roll Bars (Also referred to as sway bars)

The physics involved with anti-rolls are beautiful. To put it simply: anti-roll bars minimize body roll at the end of the vehicle they’re installed. Knowledge of weight transfer is one of the things that separates good drivers from amazing drivers.

An aftermarket anti-roll bar isn’t as critical as tires. Get your performance tires first.

Here are some generalities to keep in mind about anti-roll bars.

  • A stiffer anti-roll bar at the rear of the car decreases body roll at the rear. This increases oversteer and decreases understeer.
  • A stiffer anti-roll bar at the front of the car decreases body roll at the front. This increases understeer and decreases oversteer.

I’m currently racing a Subaru BRZ, which has a natural tendency to understeer. Because that’s not my preference behind the wheel, I was able to use a larger rear anti-roll bar to remove the understeer and achieve a neutral steering car.

If you didn’t already know, you can tune both tire pressures and your anti-roll bar by monitoring your tire temperatures.

In Street class autocross you’re allowed to change or entirely remove one of your anti-roll bars and the links. That’s excellent because it usually only takes one bar to radically improve the handling of your vehicle.

You’re also allowed to use an adjustable bar, adjustable links, polyurethane link bushings, or bearings that act as bushings. The bar can be solid, hollow, and can use interchangeable sections. 


Are you new to racing and feeling out the handling characteristics of a vehicle? If you are, you absolutely cannot tune a vehicle if you’re not being consistent and putting down laps that are within .2 seconds of each other. For more about that here’s a great post with an article about when you should start tuning and why jumping the gun can do more harm than good. 

One of the best ways to figure out what your car needs is to let one of the fast guys drive it. Their experience and feedback will give you a solid plan to improve your car and lap times.


3. Shocks

A set of performance shocks can further increase grip for your Street class autocross car. While not as critical as tires or an anti-roll bar, shocks can get you a little more. They’re also more expensive than tires or bars, typically.

Get your tires first and then your anti-roll bar. If you have money left over, grab a nice set of shocks.

A good set of performance shocks can help reduce lateral weight transfer and body roll. By slightly reducing the spring’s ability to compress, and more quickly returning the spring to it’s natural position, the car spends less time in body roll and gets back to neutral in less time. These minuscule time savings add up and cause more grip and lower lap times.

If you aren’t up to par on your shock knowledge, here’s how it all works. 

Street class autocross shocks are the bolt in type, making them easy to install. The great thing is that several manufactures make tuned shocks for most popular autocross cars. These shocks are specifically designed to optimize performance in each model.

You aren’t typically allowed to change springs in a Street class autocross car.


Instead, you’ll want a set of performance shocks that designed to work with the factory spring. Shy away from shocks that are made to pair with lowered springs. The spring and shock rates won’t match. Learn more advanced magic about shocks and dampening rates here.

You won’t have to think about rates or tuning if you buy a reputable model designed to enhance the factory spring. The manufacturer has taken care of that for you with science, testing, and real-world results.


Modifications to avoid in Street class autocross

Let’s briefly touch on some common mistakes that racers tend to make. There are a few popular vehicle upgrades that I would recommend staying away from. They just don’t produce a big enough advantage to justify the investment.


Brake Pads

As per the SCCA Solo Street class rules, you are allowed to change brake pads without restriction.


Pads are an upgrade that many autocrossers are obsessed with. I think it has less to do with performance than they’re affordability and ease of installation. You might make small gains, but they’re aren’t necessary. Even at the national level.

Julian Garfield revealed on the Racers HQ Podcast that his 2016 ProSolo championship was earned while using the Madza MX-5’s OEM brake pads. If Julian can win a national championship without upgrading pads, you don’t need them to be fast.

Save the money for something more useful like your next set of tires or any of Ross Bentley’s books.


Exhaust Upgrades

Since the rules in Street class autocross don’t allow you to improve engine output, it’s tempting to spend your hard earned cash to improve engine efficiency with a cat-back exhaust system.


Even with the best aftermarket system and the best drop in air filter, your power gains with be almost negligible. Sure, your car will sound awesome. But it won’t be much faster. Save the money for your next set of tires or a Carroll Smith book.


Even seemingly harmless modifications can bump you into another class.

I’ve referenced the SCCA Solo Rulebook several times in this article. I would recommend spending the time to be  more familiar with it. If you understand what you’re allowed to do, you won’t accidentally make a modification that will bump you into a higher class.

For Example:

  • Carbon fiber hood? Bam, you’re in Street Modified class.
  • Polyurethane bushings? Now you’re in Street Touring.
  • Camber plates? Street Touring again.
  • Non standard engine swap? Modified

Competing in a higher class isn’t necessarily a bad thing, but it needs to be thoroughly planned. The problem with being bumped into a higher class is this: you only have one mod that puts you in that class. Everyone else has carefully maximize their cars with every allowable mod in that class. There’s no way you’re going to be competitive.

Be careful what you do to the car if you want to keep your car in Street class autocross.



Street class autocross is a great way to enter the world of motorsport. It’s affordable, competitive, poses minimal risk for cars and people, and goes all the way up to the national level. But to truly be competitive you’ll need a few upgrades. Remember: tires are the number one upgrade to improve traction everywhere. Once overall grip is improved, an anti-roll bar can help balance the car and get rid of understeer or oversteer. If you still have a few bucks to spend, a good set of shocks can reduce lateral weight transfer and make the car even quicker through the corners.

Real motorsport isn’t about how much money you spend. It’s about cultivating a thorough knowledge of racecraft and advanced vehicle control. The best drivers in the country are the ones who can drive slow cars fast, not those who spend the most to have the fastest car. Street class gives aspiring drivers an arena to battle with their knowledge and raw skills.



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