Why a live rear axle is better for drag racing – Jalopnik

Where do you want G-forces? You can choose from four directions: while accelerating forward, while decelerating, while turning left or while turning right. If your car is experiencing persistent vertical G’s, something has gone wrong. The problem is that hardware that improves performance in one direction can hurt performance in another, and perhaps this concept is best illustrated by your back end. I mean, the back of your car. And no, I’m not talking about styling, like when people hated the Honda Civic’s butt so much they fixed it in Photoshop. I’m talking solid rear axles and independent rear suspension, or “IRS” if that doesn’t spark thoughts about tax returns.

Special drag cars usually use live rear axles, or solid axles if you prefer, while cars designed for winding road routes generally feature IRS. This makes sense because solid axles have fewer small, delicate parts that can be damaged by continent-shifting torque, and independent suspension reduces unsprung weight and allows both rear wheels to respond to bumps without upsetting each other. That said, there are cars that buck these trends. To drag racing with IRS, everything must be reinforced to prevent axle jumps or broken half axles. Motor trend spoke with Corvette IRS towing specialist Rodney Massengale of RPM Transmissions, who said, “It’s the entire drivetrain that needs to be strengthened to support this level of performance. It’s like saying a chain is only as strong as its weakest link.” For proof that IRS can work, look no further than Mark Carlyle and the 6.58-second quarter he ran in his IRS C6 Corvette.

As for solid-axle cars that buck the trends, Car and Driver found the 2013 Mustang Boss 302 Laguna Seca performed so well that they had to triple check to see if the engineers hadn’t sneaked in an independent setup.

Top Fuel Dragsters have the simplest setups: no suspension at all. Watch a video of a Top Fuel dragster running and you’ll see the tires deform, then the car teleports to the other side of the track without crouching.

A less simple (but still simple) solid axle suspension uses leaf springs and a pair of shock absorbers. Although leaf springs can twist under force, tie rods can soften this. More complex fixed axle setups use multiple links, usually three or four, to prevent front-to-back twisting. Panhard rods keep the axles from moving back and forth, although four-link triangular arrangements also work. To reduce flex even further, some solid axles use torque arms, which are essentially rods that connect from the center of the differential to a location near the transmission. And, as the aforementioned Mustang Boss 302 proves, stiffer springs and bushings, thicker anti-roll bars and fully adjustable shocks can do wonders to improve handling and reduce mid-corner jumpiness.

Eliminating squat with stiffer springs and shocks can help on the straights and on launches, but it will also hinder the suspension’s ability to absorb bumps. In addition, the car can feel unstable when cornering due to the stiff suspension (which also applies to cars with standard wishbones or equal length struts). In that case, it might be easier to use a live axle that will always keep the tires perfectly vertical, especially if all you care about is drag strip dominance.

IRS has a lot of pros and cons, but one of the inherent problems with IRS when it comes to drag racing is that it is prone to axle jumps. The tires can switch between “all traction” and “no traction” very, very quickly because the differential bushings and suspension subframe flex. Semi-shafts of equal diameter can bind and release at the same rate, making jumping worse. Therefore, the C6 Corvette ZR1 used asymmetric half-shafts (33mm on the left, 40mm on the right). During the offset of the axle, there is a twisting and relaxing of the suspension components. This oscillation can destroy everything from the ring and pinions to the differential housing, and even break the welds of the control arms.