Dr. Diandra: The weak (rear toe) link

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The Next Gen car’s composite body stands up to impacts far better than Gen-6 metal bodies. No longer do bumps with the wall come high probability of a cut tire a few laps later.

Instead, a new word has entered the NASCAR lexicon: rear toe link. A broken rear toe link can put a driver out of the race just as fast as a cut tire. Let’s learn what the rear toe link is, what it does, and why making it more robust would actually be worse for drivers and fans.A graphic of a car with no toe

Toe

Look down at a passenger car from overhead. All four tires would be oriented parallel to a line drawn down the length of the car, as shown at right.

Racing — especially racing on banked ovals — requires setting up the car to keep as much of the tire in contact with the track as possible. Toe is one of the adjustments a team can make to help optimize the tires’ contact patches.

Toe is the deviation between a tire and the car’s centerline. The diagram below shows toe in (positive toe), zero toe and toe out (negative toe). Remember that you’re looking at the car from overhead in this graphic.

A graphic showing the variations of toe possible on a car

Toed out means that the fronts of the tires are farther apart than the backs of the tires. Toed in means the opposite. Engineers specify toe either by an angle, or a distance — usually the difference between the fronts and rears of opposing tires.

Toe Links

The rear toe link (shown below) pulls one side of the wheel/tire closer to the car’s centerline or pushes it further away.

A graphic showing the rear toe link
Adapted from NASCAR rule book

The rear toe link, fabricated by Visser Precision to NASCAR’s specifications, is a rod with a connector on each end. The side with the circular connector attaches to the rear upright and hub assembly (on which the wheel and tire are mounted), while the fork-like piece connects with the upper control arm.

 

The graphic below shows the rear toe link (in white) as part of the rear suspension.

  • The upper and lower control arms attach to the chassis.
  • The turquoise element is the shock absorber.
  • The mustard-colored rod is the rear anti-roll bar.
  • The burgundy piece that’s just barely visible is the weight jacker assembly — that’s the part being adjusted when a crew member put one of those long wrenches into a hole in the rear windshield.

A graphic from the NASCAR rule book showing how the rear toe link attaches to the car.

NASCAR rule book

Teams adjust shims to change the effective length of the toe link. Even if you don’t want any toe, you still need the toe link there to fix the toe at zero.

Strengthening the rear toe link

Considering the stoutness of the other components in the rear suspension, it’s not surprising that the rear toe link is the most likely component to break when a car hits the wall.

That’s not an accident. The rear toe link is one of the easier parts in the suspension to replace since it has only two connections. As Chase Elliott‘s team showed at Charlotte, teams can even replace the rear toe link on pit road. It’s not a fast fix, and probably puts the team laps down. But they’re still in the running. I suspect it’s also one of the least-expensive parts in the suspension.

If a suspension part has to break, the rear toe link is the best part to have break.

It’s safer for the driver if a part on the car breaks because it takes energy to break things. Energy used to break the car is energy that can’t get through to the driver.

A race car going 160 mph has about the same energy as is stored in one kilogram (2.2 pounds) of TNT. Because it’s impossible to create or destroy energy, all that energy has to go somewhere when a car stops. Every crumpled fender, squished piece of foam and screeching tire transforms energy. Energy dissipated by the car doesn’t reach the driver. The rear toe link is one element in a chain of sacrificial components.

The Next Gen car is stiffer than the car it replaced. Much of that stiffness is due to reinforcements that protect the driver from debris coming into the cockpit. Greater stiffness dissipates less energy, which means more energy reaches the driver.

“The little slaps against the fence that may not look that big,” Joey Logano said on SiriusXM NASCAR Radio’s “The Morning Drive”, “you feel them way more than we used to, for sure.”

That makes the role of intentional ‘weak links’ like bumpers, foam and rear toe links even more important. Safety elements that protect drivers in potentially serious crashes sometimes come at the cost of them feeling the smaller impacts a little more.

Indestructible cars make for worse racing

Leaving safety aside, there’s a strategic rationale for not making the Next Gen car indestructible. If a driver can hit a wall and just keep going, the importance of skill decreases. Racing turns into bumper cars and drivers become more aggressive because they can do so without penalty. In the worst case, that leads to more accidents — and potentially more serious accidents.

The Next Gen car demands drivers employ a different skill set. The edge between ‘fast’ and ‘in the wall’ is thinner than ever before. Drivers must combine finesse behind the wheel with aggression. Like Denny Hamlin did last week at Charlotte, drivers must balance speed and passing with preserving their car.