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December 2, 2020

SportsCar Feature: Low Power, Fast Car

SportsCar Feature: Low Power, Fast Car

This article first appeared in the July, 2020 edition of SportsCar Magazine. Everyone can read the current and past editions of SportCar digitally here. To become an SCCA member and get SportsCar mailed to your home address monthly in addition to the digital editions, click here.

Hunting for speed in a C Street, Solo Spec Coupe, or Street Touring Roadster autocross setup, I called the person everyone else talks to, Brian Karwan

How do you set up a low-powered, rear-wheel-drive autocross car to go fast? That was what I wanted to know. Specifically, I wanted the secrets to speed for C Street (CS), Solo Spec Coupe (SSC), and Street Touring Roadster (STR). My search began as it always does, by reaching out to numerous top finishers in those categories. And they all offered great information. But when you’re told by more than one of those top finishers the equivalent of: “When I get stuck, I call Brian Karwan” – well, I called Brian Karwan.

Karwan is a three-time Solo National Champion, the defending STR champ, and the founder of Karcepts, which specializes in this type of car. In other words, he knows his way around car setup. More importantly, he’s willing to share his knowledge with friends, competitors and, now, all of us.

First things first, he says: know the rules of your class. Options are limited in SSC, there is limited flexibility in a Street class, and Street Touring opens the door considerably. The trade off, of course, comes in resources required – both mental and physical.

Because of the rule sets, Karwan recommends working on alignment before anything else – which includes toe, camber, and caster. The idea is to get the most amount of rubber on the ground as possible. A larger contact patch equates to more speed through the corners, the most critical variable in any form of racing. Most cars can accomplish this in the rear, but the front may be another story – limited either by the car or the rule set.

“Most of these cars are going to want to see more than 4 degrees of negative front camber,” Karwan says. “I know that may sound extreme to some, but we know this by also observing long-term tire wear in race trim. A CS Miata or Honda S2000 will be limited to around two degrees of negative front camber, so they will wear their tires’ outside shoulders aggressively, and not see as much wear on the inside of the tires. Where STR and STX have more options in their rules, they can get in the negative 4.1 to 4.5 range of front camber, which is what you will find the fast guys running.”

That’s critical information, because everything else in the setup relates directly to this.

“With the Street classes, people will characterize those cars as generally being pushy, so most adjustments will have to be made on the setup to make those cars rotate,” Karwan explains. “But then when comparing to an SSC or ST class, you have more contact patch with the camber, plus wider tires, and those cars naturally become more oversteer prone – especially ST cars. This will affect the whole approach to the setup.”

This is where paths may begin to diverge.

In a class like C Street, the parts you choose are pretty much limited to two main areas. There are many swaybars available with various ranges in adjustments and sizes; shocks have multiple options ranging from stock to a cost-effective Koni with rebound adjustment only, with the options continuing through a high-end shock with double adjustable dampers to control rebound and compression independently.

A Street Tuning class like STR is even more open. These liberal rules appeal to many autocrossers who love to tinker and adjust to find the parts that work best for them. The great news for those who are chasing the competition is that it’s fairly easy to talk to others in the class and find a baseline for parts. Those who choose to start from scratch should plan to test parts and pieces until they land upon the best balance, which can include a dramatic investment in both time and money.

We Have Contact

Let’s skip ahead and work under the assumption that the car has a swaybar, shocks, wheels, and tires, all within the rules for a particular class. Where to start tuning? Karwan’s recommendations sound obvious when he lays them out but may not be initially intuitive.

“It is important to realize which variables will have the greatest effect on the handling of the vehicle and start with those items first,” Karwan says. “The variables that have a lesser effect can be used to fine tune the overall setup.”

Of course, basic alignment has been discussed already. That front-end camber should come first as a general rule, Karwan explains, but up next is swaybars and spring rates. This fits together because, as Karwan tells us, “Both the springs and swaybars work together, and both have a drastic ability to manipulate the handling of a chassis – more so than any shock change, alignment change, or tire pressure change – so you want to start with adjusting these before anything else.”

If spring rate is fixed for your car, that eliminates one step. Running the maximum wheel and tire width for a class will maximize the grip, bringing a need for roll and compression resistance. The lowest ride height possible also lowers the center of gravity but remember to take into account whether or not you plan to drive this car on the street.

“My theory on selecting spring rates is to run the softest spring rates possible that still enable you to run the lowest ride height,” Karwan reveals. “The softest possible may still end up being a fairly stiff spring, as the additional parameter to satisfy is to not allow the suspension to over-travel, where you start slamming shocks into bump stops or tires into fender wells.”

The goal is to keep the tire in contact with the ground as much as possible.

“Assume a case of springs so stiff that they don’t even compress the suspension when hitting a bump,” he explains. “In that case, the tire gets airborne and skips over the surface, eliminating grip.”

Once you’ve met the requirements for a low center of gravity and suspension travel, you can predict the expected bias of the car. More front spring relative to rear will produce understeer; soft fronts and a stiffer rear spring will product more oversteer. There is not necessarily a right or wrong answer here, but rather personal preference for the driver.

Sway Control

With springs set, Karwan next looks at the swaybar all the while breaking the car’s characteristics into three categories – entry, mid-corner, and exit – plus transitions like a slalom. The dominant characteristics of the car mid-corner are typically based on the spring stagger, but swaybars will impact entry, mid-corner, exit, and transitions.

“In my experience,” Karwan says, “corner entry to mid-corner behavior is mostly dictated by the front swaybar. The front swaybar also highly dictates the transitional stability. The rear swaybar primarily affects the corner exit behavior of the chassis.”

He does caution, however, that this is a general rule. Adjustments on the front will affect the rear of the car, and vice versa.

“Sometimes people use front bar to help corner exit,” Karwan acknowledges. “And sometimes you can notice the rear bar affect corner entry behavior. All of that is true, but I feel it is best to focus on utilizing the specific adjustment that has the most influence on the behavior you are trying to address.”

As always, the setting and size of the swaybar is a balance. Karwan focuses on the slalom for setup. A large front swaybar allows for more aggression through a slalom but may cause too much understeer in longer or tighter corners. A softer front swaybar helps the corners, but the lack of stability in the front may put a driver behind through a slalom. This is why a good, adjustable bar is important – it’s pretty much impossible to swap bars between runs, but it is possible to make an adjustment.

“My general method of setting the front swaybar is to set the front bar as soft as possible yet have it still comfortable to attack the fastest slalom on the course,” Karwan says.

Once the car will run quickly through sweepers and transition through slaloms, the focus can be on the rear bar setting. Jump back on the throttle with a bar that is too stiff, and it can be hard to put the power down, yet a bar that is too soft in the rear will exhibit understeer behavior.

“If you have a highly adjustable rear swaybar that you can test and tune different settings, you will realize quickly that this adjustment is one of the best adjustments in finding overall speed on the clock,” Karwan said. “The first time I realized the importance of a rear swaybar setting was mind blowing. The difference in time between a car with a proper rear bar setting and an improper rear bar setting can easily be a half-second on a 60-second course.”

The Fine Tune

With the majority of the car set, Karwan uses shocks to fine-tune settings. In a limited class like C Street, many will use shocks to manipulate the handling of the car simply because there isn’t much else to adjust.

“Ideally, the shocks should be used to keep the tires in contact with the ground as much as possible,” Karwan says. “That is how you would want to use the shocks in STR trim, as you have unlimited spring rate and swaybar rate options. However, in a class like CS, you end up with a pushy car and so this is an instance where shocks may be used to actually keep the tire more off the pavement, such as the rear tires, by inducing extra rear rebound or rear compression in order to get the car to pivot more on corner entry or corner exit.”

Longtime autocrossers are already yelling about one trick that hasn’t yet come up: tire pressures. There is a reason for that. Today’s tires are really, really good.

“You can have 5psi differences from another competitor and still have the same speed, so nowadays being in the ballpark is good enough,” Karwan says. “That doesn’t mean to not adjust it and test and tune to see what you like, but I’m just saying that the overall speed in the car is less affected because the operating window is so large on these newer tires.”

And, like that, I had it: the secret to speed in C Street, Solo Spec Coupe, and Street Touring Roadster. But even as I hung up the phone, I had a feeling like knowing all of Karwan’s secrets was still not going to be enough to beat him. But hey, it’s a start.

SSC, Specifically

A Solo Spec Coupe package for the Subaru BRZ and Scion FR-S is sold by Tire Rack, meaning all competitors have basically the same parts for their cars. Tires, shocks, alignment, and suspension pieces are all dictated by the rules, giving competitors a narrow window for adjustments.

There are some adjustments, however. For instance, last year’s Tire Rack Solo National Championships SSC runner-up finisher Jimmy Vajdak feels the most important part of setup in the class is driver comfort.

“In my opinion, SSC is setup agnostic,” Vajdak says. “It doesn’t seem to really care. Adam Benaway is running the stiff front bar, and I’m on the soft front bar. I’m running more rear camber than most. At the end of the day, we’re still within a few tenths and thousands of a second of each other, so I think it comes down to setting the car to your driving style and whatever you’re happy with.”

But Benaway, the two-time SSC National Champ, isn’t so sure. While most were running the softer option for the front swaybar, he was almost a lone ranger with the stiffer bar.

“I drove Matthew McCabe’s car afterward, and his car felt light and nimble,” Benaway says. “Maybe I’m describing it wrong, but it felt as if the contact patch was smaller. It responded very well. When I turned the wheel, it was very ‘skate-y.’ Some people may like that. It felt light on its toes. My car almost felt heavy and planted. To me, that builds confidence in a car. But no one wanted to try my setup.”

Was Benaway’s setup correct? Did it make a difference? It did to Benaway, but it’s hard to tell if that was a personal style, as Vajdak believes, or if it’s truly quicker.

Benaway thinks it is truly quicker, which is actually a humbling statement. The other option is that setup truly doesn’t matter, or even that the accepted setup is quicker. If that’s the case, Benaway is even faster than his fellow competitors behind the wheel.

“To me, if I was coming into the class and the winner has a setup, I’m going to try that setup until it has proven me wrong,” Benaway reveals. “I think setup really did help. I ran a little bit of tow-out in the front and a little bit of tow-in in the rear, which kind of seems contradictory. But the car felt so stable. If you watch my video, I play with the wheel at a very minute level. I’m not chasing the car, where I felt like a lot of other people did.”

Words by Reece White
Photo by Rupert Berrington

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