Cletus Emmett Wheelwelker
Join Date: May 2013
Location: A², Michigan
iTrader: 3 reviews
While a PHB does a far better job of centering the rear axle laterally than does two upper control arms, it alone is not an ideal solution. A stock 99-04 GT has a pretty high roll center defined by the upper control arms. After installing a PHB, a new, lower, roll center is defined. As a result, the upper control arms are forcibly made to comply with the new roll center of the PHB, causing the stock rubber bushings to deflect and bind as they are now operating outside their normal range of motion. This symptom is referred to as roll bind.
Maximum Motorsports did a study on roll bind in several rear suspension configurations. They found that by adding a PHB with stock upper and lower control arms, rear wheel rates increased by 47 lb/in in the first inch of suspension travel. This number decreased to 30 lb/in between 2-3" of travel. That's a significant amount of wheel rate added to the rear, all because of bind in the upper control arms. Despite this increase in upper control arm bind, MM still recommends a PHB because of the increased predictability and stability. By eliminating one upper control arm, however, roll bind–and by extension snap oversteer–is reduced (this is the main reason S197 Mustangs came equipped with a single upper control arm with the factory PHB).
So why doesn't MM overtly recommend the PM3L (poor man's 3-link)? Because it isn't a true or durable (or even safe, for that matter) solution to eliminating upper control arm bind–it's merely a trick road racers on a budget came up with to combat snap oversteer. The main concern with the PM3L is the resulting damage to the bushings in the remaining upper control arm. On a stock rear suspension car, the forces from engine torque, acceleration, braking, and handling are transmitted through two upper control arms and their respective chassis mounts. Remove one of those arms, and now all those forces are passing through a single control arm, a single set of bushings, and a single chassis mount. This will destroy the bushings, and worse the upper control arm mount, in short order. A lot of MM customers drive their cars on the street, so you can only imagine the danger involved if a bushing or chassis mount fails while cruising down the road.
To safely utilize the PM3L, the chassis mount needs to be reinforced, and the stock rubber bushings in the remaining arm should be replaced with spherical heim joints. A few people have designed their own bushings out of steel. This is quite a bit of work, especially for someone that doesn't competitively race their car. The best solution for a street driven car is to leave both upper control arms in place with the PHB, or better yet, remove them entirely with a torque arm.