June 25, 2001

Suspension at home
Home made adjustable pan hard rod

Notice the adjustable nu

F-Bodies leave the GM Assembly Plant with a fixed-length/non-adjustable PHB.  The purpose of a PHB on a live axle car is to center the differential/rear under the car.  The PHB mounts to a fixed location on the chassis/frame above the differential/rear and attaches to the differential using rubber bushings at both ends.  The factory PHB measures 43-inches bolt hole center to center, or 45-1/8 inch end to end.

A common problem with a fixed length PHB is there is no way to “adjust” (i.e. shorter or longer) it to compensate for differences in factory manufacturing tolerances.  If the rear is not centered under the car (i.e. centered - the left and right tires are the same distance between the wheel wells), you need an adjustable PHB.  Even from the factory the wheels are rarely centered.  Lowering the car will accentuate the problem – pushing the rear closer to one side or the other.  Tire clearance is especially critical when using wider rims/tires and during hard lateral driving such as autocross.

Aftermarket adjustable PHBs cost well over 100 dollars.  A low cost alternative is to make your own.  Using the stock f-body PHB a 1”x 3-1/2” Grade 8, fine thread bolt/nuts and 14 gauge sheet metal; all you need is a welder, hand grinder and hack saw.  Purchase the bolt/nuts (one jam (i.e. thin nut) one regular nut) for 10 dollars at any specialty bolt shop.  A welding shop can supply and cut the mild steel sheet metal to length for another 10 dollars.  Or you can cut the sheet metal using a jigsaw with a metal cutting blade.

Are your wheels/tires centered??  To check – have you car level and normal tire inflation.  Make a “plumb bob” using a length of string and a nut attached at one end.  Hold the string to the center of the wheel well and measure the distance between the string and the tire using a ruler.  Repeat the process on the other side of the car making sure you place the string and measure from the same relative places.  The difference (if any) is how much your wheels are not centered.  Don’t be alarmed if the “difference” seems great.  For example to fix a 1-inch difference at the tire is only ½ inch of rear adjustment, which is less than ¼ inch of adjustment at the PHB.  For my car the total adjustment of the PHB was about ¼ of an inch (longer) to get the wheels back to center.

Assuming the wheels on your car are centered prior to lowering it, after lowering the car, they will not be centered.  To get them back to center you will need to “shorten” the PHB.  Most times a “small” amount of adjustment on the PHB is all you need to get the wheels back to center.  Because of this a 3-1/2 inch bolt is more than sufficient length to provide necessary adjustment.  Also as it just so happens – this is the longest all-threaded 1” fine thread bolt the shop had.  A shouldered bolt (one that does not have thread all the way to the end) does little good, since you cannot use it for adjustment.  You may find a metric equivalent, or go to a slightly less diameter, but I found 1 inch to be a good round number.  (Writer’s note: I’ve had my homemade PHB to 90 mph during 20 “paced laps” around Bristol Motor Speedway and 30+ ¼ mile passes – no failures or signs of cracks/fatigue.  But I still recommend the largest bolt that fits – in this case 1inch.)

Start by cutting he factory PHB 3-inches from the center bolt hole, or where the “U” shape of the PHB stops and it becomes flat.  The bar is the same at both ends, so cut either end.  Reason you cut it away from the bushing is two fold: The PHB will not interfere with the rear mounting gusset, and; during welding you will not transfer too much unwanted heat to the rubber bushings.  To weld the bolt to the small cut end, use a clamp/vise to hold the bolt parallel to the PHB.  Tack weld it in place.  Check for straightness and make sure the bolt will be about 1/8-inch above the PHB when fully screwed into the receiver nut.  Wrap a wet rag around the rubber bushing (as a heat sink) and finish weld it to the bolt.  Weld intermittingly and do not overheat rubber bushing.  Allow plenty of time between welding to cool the metal around the bushing.  Do not over heat the bushing.

To allow for the bolt head and nut thickness you add to the length and allow for adjustment of the PHB you must cut 2-1/2 inches from the other end of the PHB.  Also you need to “spread” the nut end of the PHB to allow clearance for the 1” bolt to screw into the nut.  Use an old ½ inch drive 7/8” (or whatever you have) to “spread the PHB - a couple-three good whacks with a hammer will do the job.

Screw the bolt all the way into the nut to align it on the other end of the cut PHB.  Tack weld the nut to the PHB and check for clearance between the bolt and the bottom of the “U” in the PHB.  The clearance is not too critical, but the bolt must move freely in and out of the PHB.  Make sure the bolt turns and does not protrude above the top of the PHB’s “U.”  Weld the bolt securely to the PHB.

Finish the PHB by “boxing-in” the top of the “U” with the 14 gauge mild sheet metal.  This adds strength to the PHB.  You can sticth-weld or weld the entire length of the PHB.  Finish by grinding the welds and de-burring the PHB.  Paint using a good quality enamel/epoxy.

Install by adjusting the PHB to factory length, than check as discussed before.  Drive the car between adjustments to ensure the suspension/rear has “settled-in.”


Photography:  Eric Barger

Web Author: Eric Barger  help@installuniversity.com
Copyright © 1999 - 2002  Eric Barger.  All rights reserved.
Revised: June 07, 2007.