Mar 29, 2017 update to the Ford Mustang GT project car

Making a Solid Rear Axle Perform

Solid axles get a bad rap. There’s nothing inherently “wrong” with solid rear axles. It’s just that their implementation in performance cars usually has many masters to serve, and ends up being a series of compromises. So it’s no surprise that properly setting up a solid-rear-axle car can be a transformative undertaking.

Such was the case with our project 2012 Mustang GT. If there’s any magic happening in the setup, it’s mostly happening out back.

The keys to making solid axles work properly for cars that like to turn is controlling their motions. We’ve discussed this subject at length in the magazine articles dealing with this end of the car, but suffice to say that when the axle moves, you want it to move in as straight a line as possible. And you also want the links that control its motion to be positioned in such a way as to not cause weird leverages when that motion occurs. For example, merely by replacing our short upper third link with a torque arm from Cortex Racing, we greatly reduce the motion of the rear instant center during cornering. This results in a more predictable feeling car.

Additionally, by replacing the stock panhard rod setup with a Cortex Racing watts link, we constrain vertical motion of the axle to a single vector, as well as precisely controlling side to side movement under load.

The result is a high-horsepower car that doesn’t drive like a dirt sprinter. You can actually get more power to the ground, and throttle your way out of corners without looking like you’re on a moonshine run through the Georgia woods. Sure, power-on oversteer is still available, but it’s much more progressive, predictable and controllable. And with the ability to adjust the height of the rear roll center by moving the watts link pivot, handling balance can be further fine-tuned.

Complete discussion of our magic rear end is available in the print edition of Grassroots Motorsports. Don’t have a subscription? You can get one right here.

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View comments on the GRM forums
HapDL New Reader
3/30/17 1:16 p.m.

Cortex makes nice stuff. I just wonder about all the extra weight that's going into an already heavy car.

dean1484 MegaDork
3/30/17 1:34 p.m.

Dare I say that is a very nice looking rear end?

Seriously though that is really cool!!!!

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