This article first appeared in the October, 2015 edition of SportsCar Magazine. SCCA members can read the current and past editions of SportCar digitally here after logging into their account; To become an SCCA member and get SportsCar mailed to your home address monthly in addition to the digital editions, click here.
Words by Randy Pobst
Photo by Kenneth Hawking
OK, esteemed readers, especially you SCCA National Champions: All else equal, what’s the number one factor in a fast lap time? Why does a Miata sometimes pass my Mustang halfway down a short straight (although if it does, I’m probably not a Runoffs winner)? What did we use before Traqmate, AiM, and MoTeC on-board data recorders? What single factor affects our car a larger part of the time around a circuit than any other? That’s correct: Our paint job!
Let’s face it, if we don’t look great, we’re gonna be slow. Or look slow, which is even worse. Paint matters around every inch of the 2.54 miles at Road Atlanta. Even in the paddock – it isn’t how fast we go, but how sexy we look out there.
What, no? It’s not a car show it’s a car race? So you prefer function over form, I gather? Well, if you insist, let’s think about the function that affects our lap time more than any other. Though I recommend an open trailer to display awesomeness and dishearten any unfortunate hopefuls we blow by on the way there.
Exit speed. The National Champs had it from hello. Pre-data, we’d look down at the tach when the wheel was straight at the track-out point. Po’ folks still do. Our velocity as we finish a corner determines our speed all the way to the next one, a long, dang time. This is true in Solo, too. But let’s be more specific. What affects exit speed? A combination of two elements, blended together by the friction circle: apex speed (that topic is for another column); and acceleration (that’s where we’re going today).
Putting power to the ground, kids! Traction, my road race engineers call it. Forward bayht (Yankee translation: bite), NASCAR style. The more power you have, the more important it becomes. Wheel spin looks cool but gets you nowhere on pavement. I am so into traction (my speed secret) that a favorite engineer used to call me (cover your ears First Gear presented by SafeRacer kids) a traction whore. I’d do anything for it.
It’s tricky, because when we first roll into the throttle to leave a corner, our tires are loaded with lateral g-forces. Now we ask them to add longitudinal. If we don’t change something, it’ll be too much, and the driven tires will break free and slide. Too much slide is slippery. If we lose grip, we lose time. If we apex late enough, we are straightening the steering, smoothly cornering less, so we can smoothly accelerate more without going over the limit of the tires. Notice I wrote smoothly twice. Make that three times. That’s how important it is.
As we add power, weight transfers back, and that load adds grip to the rear wheels. More weight back, more rear grip, so more power, so more weight back, so more grip. Beautiful, ain’t it? Unless we’re driving the front wheels. Oh, yeah, that. That’s ninety percent of the cars on the street, and twenty-five percent of the race paddock. Leaning into the go-pedal on a Civic transfers weight rearward, too – off the drive wheels. More power, less weight on the front, less grip up front, what?
Which brings us to FWD Pet Peeve 1: Even with your OS Giken limited slip, the front tires will not pull you through the corner. Though I will give you this, the resulting reduction of inside wheel spin will likely improve on power steering grip over an open diff. The real trick with a front-driver is to put so much cornering load into the rears that it wants to oversteer, so when we kick into VTEC the surge will return the chassis to balance (see any RealTime Racing Acura TSX YouTube vids). The entry looseness exhibited is too much for a street Joe, so most factory cars push strongly. Brake torque vectoring helps a lot, until they overheat (Ford Focus). That’s why it’s common to see stiffer springs and bigger swaybars on the rear of FWD racecars; or even better, a little rear toe out or bump steer. The good news is that front drives also have a lot more weight on the front anyway; engine, trans, and final drive all on that end. The challenge is, the stronger the powerplant, the lighter the front gets. I have great respect for the Pirelli World Challenge KIA Optima turbo front-wheel-drive team in GTS for how well they make that KIA work against Camaros, Mustangs, and Porsches.
Which brings me to Pet Peeve 2: all-wheel drive. I raced Pirelli World Challenge two years in the Champion Racing Audi RS6 (winning the championship in 2003), and five years in the K-PAX Racing Volvo S60 (becoming the 2010 champion). Counter to advice I often hear, we cannot just floor it at apex and let the all-wheel drive pull us through the corner. Why? Because the tires cannot corner and accelerate hard at the same time. The same rules of the friction circle apply. We absolutely must sacrifice some cornering in order to crank up the speed. The critical trick with all-wheel drive is to split enough torque to the rear wheels. It takes a good 65-70 percent to have a balanced chassis under power. An even 50/50 split means power understeer, and I hate that. If you have a street-based all-wheel-drive car – most are that way – set it up like a front-drive for optimum balance.
For quicker lap times, focus on getting power to the ground, accelerating off the corner. Most racers are rear-drive, and this means transferring weight to the rear quickly when we roll on the loud pedal. Soft front rebound allows the load to shift more quickly off the front tires, and stiffer rear compression adds it to the rear in a hurry. Softer rear springs reduce the cornering load relative to the front, so there is room for the weight transfer without going into a slide so soon. A stiffer front swaybar does the same thing, adding corner load up front and leaving more potential for the rear tires to accept the power. Some describe this as holding the inside tire down, a misnomer, but a useful visualization. Anti-squat geometry adds rear load, within reason, a common tune in GT-1 and Trans Am.
But wait, there’s more! A special bonus! If we reduce wheel spin, either end, our tires last longer, and we will be faster near the end of the race – and exactly when does that checkered flag fall?
