- The major downfall of MacPherson struts on a passenger car in a racing application is that the strut geometry dynamically loses negative camber under load. So, a generous helping of static negative camber is necessary to maximize cornering performance. This is a relatively easy modification that is arguably safer and less complicated than running a second camber bolt in the lower strut body position.
Step 1: Preparation
Begin by measuring the distance between your strut bolts and determining the desired change in camber. The distance between the strut bolts becomes one leg of a right triangle, with the amount you slot as the other leg. Using b = distance between camber bolts, theta = the change in camber you desire, and x = the slotting distance,
x = b * tan (theta).
Usually it works out to around 1 mm for 1 degree.
Step 2: Measure Twice, Cut Once...
Mark the amount to slot on the lower strut bolt hole using your preferred marking tool and your preferred measuring tool. It is important that all four slotting operations (two for each lower strut bolt) remove very close to the same amount of material. Be extra sure you are removing material on the correct side of the strut body! On a 2008 WRX, this is to the outside of the strut body.
Step 3: Slotting the Struts
Remove the material using a Dremel (TM) high speed rotary tool with a grinding bit. Try to keep the same radius at the end of the slot so that the bolt fits tightly against the end of the slot. Check the fit often with one of the bolts.
This is generally easier with the strut body in a bench vise, but can be done on the ground. Please wear safety glasses!
Step 4: Reassembly Is Not Quite the Reverse of Disassembly
When you are reassembling the suspension, the hub must be loaded on the outside with a jack to force the hub into the maximum negative camber position. With this technique, the upper eccentric strut bolt can still be used to fine tune the camber between sides. It is very important to look up the torque specifications for your car - on a 2008 WRX, it is well in excess of 100 ft-lbs! High torques are necessary to prevent slippage of the bolts.
Step 5: Aligning the Car and Securing the Camber
Measure the camber and make sure it is sufficiently equal between sides. Use the upper camber bolts to adjust the camber. Typically after drastic changes to camber, the toe will be out. You must align the car now to prevent excess tire wear. Remember that if a shop adjusts camber as well, they must again apply an upward load to the outside of the hubs prior to tightening the bolts. It may be a good idea to learn to align the car yourself (many guides and techniques are available online) to get it close, so that a shop only needs to dial in the toe.
After a successful alignment, it is recommended to paint the bolt-strut joints to prevent slip. Mark the rotation of the upper camber bolts, then loosen the upper and lower bolts while applying a vertical load on the hub with a jack. Spray paint in between the bolt head and strut body, and between the nut and the strut body, then reassemble and torque quickly. The thin layer of dried paint will greatly increase the shear force necessary to dislodge the bolt.