The Ford Mustang rear suspension from Fox bodies all the way through the last SN95 chassis that rolled from the Rouge in 2004, is an art form that can only be considered abstract. The 4-link setup has always had bad manners in the Mustang and when you add big power it only gets worse. The design provides little control over axle windup, lateral motions, and wheel-hop. Ford did their best to tame the rear end by installing horizontal dampers known as the “quad-shock”. It helps to slow the jerks, sways, and hops. While the quad-shocks don’t stop the axle from doing evil things, they slow its motions down such that you usually don’t notice them.
Our 2001 Roush Mustang has about 30,000 miles on the clock. In the last 5000 miles it has been starting to loose its composure under hard acceleration. We have noticed increased axle hop while accelerating over rough pavement or on curves, a bucking action that starts and can only be stopped by letting off the gas. On lane changes where we accelerate to pass the rear end has become squirmy, feeling like it’s on the edge of kicking out. Even straight line acceleration runs have started to become an exercise in steering corrections to keep the car going where we want it.
All these are symptoms of worn quad-shocks - one of the most overlooked maintenance areas of the newer generation Mustangs. Our hunch was proven correct. After removal and hand testing, we found that our quad-shocks were indeed shot.
For replacements, we chose an upgrade from KYB. Their gas charged dampers are available for around $35-40 bucks each from most online and brick mortar retailers. A side-by-side comparison between the KYB and factory dampers show some physical differences that need to be considered on some cars.
When installing them it is best to have the car chassis on jack stands and allow the axle to drop. This gives you the precious room you will need to get wrench leverage on the nuts and bolts. Both a 15mm and 18mm wrench or socket are required for removal and replacement of the dampers.
KYB provides two metal bushings with each unit. You must install both bushings on the inboard side of the rear mounting post before installing the new damper. This provides the needed clearance between the thicker tube of the KYB damper and factory rear shock. Without them, the KYB and your rear shock would bind and rub together.
Once installed, we checked both sides for proper clearance with brake lines and the tires. Our car has 18x9 rear wheels with 295/35/18 rubber. We had clearance to spare between the tires and new KYB dampers. One of the metal brake lines had to be gently nudged to prevent rubbing on the new KYB damper. Overall the job only took about an hour and was only about a 2-3 on the hammer scale, 5 being major all-day mess and 1 being a snap.
On the road we noticed a big difference in the composure of the rear suspension. Under hard acceleration over the expansion joints of our local highway, the rear end no longer jolts and see-saws. Now when speeding off from a stop sign and turning at the same time, the inside tire no longer hops like a nervous poodle. Best of all, the car just feels planted like the day it did when we bought it.
The only negative aspect of the KYB axle dampers is that the clearance between them and the rear factory shocks is still less than it should be. In the long term they could still potentially bind or rub in some cases. We’ll report back if this is the case.
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