Blank Slate: Rear Suspension Upgrade To a Four-Bar Coil Over System

Our 1969 Camaro, Project Blank Slate, was purchased as an incomplete dream project that the previous owner had stripped down. This beauty was a true “Blank Slate” to start working from. This bare chassis allowed us to dream a little bigger and take on the task of making the old unibody into a solid Pro-Touring machine. We’ve already upgraded the front suspension with a full Chris Alston Chassisworks’ Bolt-on g-Machine front clip kit. Next, we also wanted the adjustability of modern four-link rear suspension systems. 

To accomplish our goal, we knew that getting an old pro to help was going to be crucial. Enter Chris Alston, a long time hot rodder and Pro-Touring guru before Pro-Touring was even a word. Alston is also the man behind Chris Alston’s Chassisworks, a premier suspension company that provides suspension kits for the DIY guys.

The cradle assembly is the foundation for this upgrade. Once the cradle is fastened to the chassis, all of the other components can bolt in.

The cradle assembly is the foundation for this upgrade. Once the cradle is fastened to the chassis, all of the other components can bolt right in. No fabrication is required but the cradle will need to be stitch welded in place.

According to Alston, anyone that owns a car can become a do-it-yourself builder with his kits. “This is nothing new,” says Chris Alston, “we’ve been doing this type of chassis upgrade for years. We just decided to make it easier for the regular Joe with a simple kit that is easy to install.”

“Nobody needs to fear how to install a four-bar suspension in their first generation Camaro,” adds Alston. The parts manufactured by Alston’s Chassisworks are “better than stock, and fabricated from exact chassis measurements as we scanned them from the original Camaro chassis. These are more exact than the factory originally produced because of GM’s wider tolerances.”

g-Machine Rear Suspension

Chris Alston Chassisworks’ Bolt-on g-Machine 4-Bar Rear Suspension (Part # 7701)

What’s In The Kit:

  • g-Link Pivot Ball style lower control arms
  • Canted Upper Bars
  • VariShock Coil-Over shocks
  • VariSprings
  • Chassis cradle
  • Adjustable shock mounts

Options:

  • FAB9 direct-fit rearend housing
  • Anti-Roll Bar, sliding link style
  • Anti-Roll Bar, spline end style
  • Double Adjustable shock upgrade

 

Body On Frame Versus Unibody Construction

The first generation Camaros were constructed in the F-body platform, a unibody construction similar to the X-body platform that GM used in the compact line. The F-body was designed for the sports models and had a couple of advantages over the traditional body-on-frame construction. They were lighter and cost less to manufacture.

“Decide whether you need a back-half modification or a full chassis,” says Alston. “The advantage to the back-half conversion is the price, but weight distribution becomes a problem. Because the rear is so much lighter than the front, it’s hard to make the car perform consistently.”

The trade-off for the cost and weight savings came at the expense of durability and ride quality. The larger the vehicle was, the worse the ride quality became in unibody construction. This begs to question why anyone would want to turn a unibody car into a Pro-Touring machine. However, you can modify the front and rear suspension, add some chassis stiffeners and have the best of both worlds; body-on-frame durability with unibody weight.

According to Alston, “You can start out with our rear-suspension conversion kits now, and install one of our bolt-on front clips later, turning your unibody car into a full, Pro-Touring style chassis.”

Switching to a 4-bar rear suspension has the added benefit of creating more space for wider tires tucked into the chassis – especially with a mini-tub kit. More rubber in the wheel wells means more traction at the track. Combine this with the adjustability of a four-link rearend and it’s almost a forgone conclusion. You’d be foolish not to make the switch.

Before we got too far along, we installed the Chis Alston Chassisworks' mini-tub kit. We ordered Alston's narrowed rearend housing so that we could use wider wheels/tires. This required us to install a mini-tub kit to keep the tires in the wheel well.

Getting Started

Our project had been stripped by the previous owner, so our prep work was very limited. There was a stock rearend and a set of stock type wheels on the car, but not much more than that – just enough to actually roll it around the garage. Because the Chassisworks conversion kit replaces most of the existing rearend components, the entire rearend must be removed from the vehicle before you start installation.

We ordered the Chassisworks’ g-Link Canted 4-Bar Coil-Over Rear Suspension Conversion kit for 1967-1969 Camaro and Firebird (Part #5804-F10), a Chassisworks FAB 9 reared housing and a complete strange rearend internals. 

Once the stock rearend was removed and the mini-tub kit installed, we took a few minutes to clean and paint the car's underside.

Once the stock rearend was removed and the mini-tub kit installed, we took a few minutes to clean and paint the car’s underside.

Removal of the rearend can be accomplished with the car resting on jackstands, but we used our two-post hydraulic shop lift to make things a little easier. Alston’s instruction manual directs the installer to remove the rearend along with the exhaust system in the back, the driveshaft and any brake and fuel lines that could be damaged during installation.

Chassisworks also recommends a decent chassis inspection once the components have been removed. Metal fatigue, broken welds and torn sheet metal will have to be repaired before the installation can begin. We were pretty safe with our chassis since the prior owner had it stripped and cleaned so we were able to get a good look at things.

Once the cradle assembly was put together, we did a test fit in the chassis prior to welding.

The first real step in the installation process is assembling the cradle assembly. Attaching the frame brackets to the upper shock crossmember with the provided hardware completes the cradle assembly. 

Next the framerails need to be prepped for welding into the chassis by removing the zinc coating on the rails with a fine grit pad and disc sander. The bolt/lightning holes can be cleaned with a die grinder to remove the zinc coating. Zinc is often used in manufacturing of metals and alloys. Inhalation of zinc oxide fumes can occur when welding zinc coated materials, causing “metal fume fever.” The time spent in prep by removing the zinc coating will pay off when you get ready to weld.

The driver and passenger side frame brackets are then C-clamped on the frame to hold them in place. Next we installed the cradle in position and ensured that it was square to the chassis. On the driver side and passenger side, you must measure from the flat vertical section of the undercarriage sheetmetal to the lowest corner of the shock mount. Our vehicle measured 10.50 inches, which we wrote down and recorded as directed by the instruction manual. This measurement should be equal from one side of the vehicle to the other.

The framerails need to be prepped with a sanding pad or scotch-brite pad in preparation for welding. You will need to stitch weld along the cradle bracket’s seams where the framerails are making contact.

We finished prepping the edges of the crossmember tube by grinding a chamfer on the outside edge in preparation for welding. After a good modified beveled edge was ground on the crossmember tube, the crossmember was positioned between the blocks and pressed as tight to the floor as possible. To secure the placement, we tack welded the crossmember in place so that the rearend housing could be test fit for clearance.

Getting The Mounts In Place 

The forward mount tabs are designed to go through the toughest part of the unibody chassis – right behind the rear seat is a triple-layered floor panel. The tab is held flush to the mount and secured to the chassis with a hex bolt (3/8-24 X 1 3/4) and a flat washer. If there is a gap between the mount tab and the chassis, Alston’s team has thoughtfully included extra fender washers to use as shims.

Everyone loves to weld. Our shop crew fought for the privilege of welding the cradle to the frame. We used our Lincoln Electric Power MIG to install the Alston kit.

Our crew then marked the weld-prep areas onto the undercarriage by scribing guidelines onto the surface of the framerail. It was important to scribe the overall outline of the bracket, the holes along the inside of the framerail, and finally the eight oval holes along the bottom of the framerail. The bolt holes were scribed so that 3/8-inch diameter holes could be drilled along the bottom of the frame rail. 

The cradle assembly was unclamped and removed so that our team could continue with the welding prep. The holes that were drilled in the bottom of the frame rail were slightly enlarged with a cone grinding attachment on a die-grinder while the rest of the surface metal that was scribed earlier was dressed up with a scotch-brite disc pad.  

Welding And Mounting

Our crew welded around the sheet metal reinforcement cap and rosette weld the 4-holes on the side of the frame rail, then ground the welds flush to allow the frame bracket to seat tightly against the bottom of the frame rail. We reinstalled the cradle assembly, clamped it securely in place and then installed the forward mount bolts.

Other than welding these seams for a stronger rearend, the complete installation did not require any additional fabrication.

Remembering the measurement that we took earlier (10.50 inches), we remeasured the shock bracket channel edge to the flat vertical section of the undercarriage. Once again, we measured 10.50 inches. Using the method outlined in Alston’s instruction manual, we installed the square U-bolts into the frame brackets on both ends. Once these are tightened down, the cradle is pretty much installed in the proper position, awaiting a final torque on the mount bolts.

With the exception of welding these seams for a stronger rearend, the complete installation did not require any fabrication. What makes this conversion kit practical and offers a reliable placement in the chassis is Alston’s “Self-Positioning” installation system. The g-Bar chassis cradle uses existing factory mounting features and top shock mount factory bolt holes for reference and direct location installation. 

A fresh layer of paint on all the bare surfaces helps prevent corrosion problems.

Protecting The Chassis

After giving the welds a chance to cool, our crew sprayed primer and paint on the metal for protection. Leaving the surfaces with bare exposed metal would invite corrosion problems later down the road.

Chassisworks also includes some chassis protection with their kit. One such item is the cover plates that cover the factory shock mount holes. Since we were switching over to the Chassisworks coil-over system, we would not be using the factory shock mounting holes. A quick bead of silicone and installation of the cover plates ensures a seal against water seepage into the trunk area.

Installation of the cover plates that seal the factory shock mount holes are a nice touch from Chassisworks. These cover plates help keep water out of the trunk area.

Chassisworks’ Direct Fit FAB9 Rearend Housing

While the Chassisworks’ 4-Bar conversion kit will readily accept a factory rearend housing, we opted to step up a few notches and make life easier by ordering Alston’s Direct Fit FAB 9 reared (part #84F10-801). This option gave us room to narrow up the flange to flange measurement and mount up some wider wheels and tires yet keep them tucked in the wheel wells.

Our wheel selection would be tailored to a Forgeline 18-inch by 12-inch wide wheels with a 5.50-inch backspace. This would allow for a 315/35/18 Mickey Thompson traction grabbing rubber radial for those road courses we envisioned driving. We also opted for the Mini-Tub option which Chassisworks moves the lower axle brackets and control arms inward to allow more room for deeper backspaced wheels.

Chassisworks direct fit FAB9 rearend getting prepped for the third member installation.

The diversity and flexibility of this conversion kit relies on the adjustable suspension geometry incorporated in the control arms attaching the rearend housing. Upper and lower control arm attachment points are manufactured with multiple mounting holes to adjust chassis anti-squat and to optimize the vehicle’s handling characteristics. Both upper bars are length adjustable to set pinion angle and preload. The g-Link lower arms are also adjustable for wheelbase variations.

The actual FAB 9 rearend housing is engineered to accept all 9-inch Ford-style differentials. These housings include a fully-welded center section with internal gussets, 3-inch axle tubes, and Ford big-bearing late-model Torino or small-GM housing ends. We chose to go with the Torino housing ends for different brake options. All of the housings are manufactured in-house utilizing Chassiswork’s state-of-the-art robotic spray-arc welder. Weld penetration, and quality are absolutely outstanding and will provide consistent, reliable performance. The center sections are fully CNC-machined after welding to provide an excellent third-member-seal surface and extremely tight tolerances on the remaining housing features.

Optionally, a folded back brace assembly can be factory welded to your FAB9 housing, substantially strengthening the housing without adding significant weight. Standard housings are constructed from mild steel, but can be upgraded to 4130 chromoly. This is recommended for vehicles weighing over 3500-pounds or developing 650-plus horsepower.

Strange’s 9-inch third member dropped right in and the unit was ready to bolt into the cradle assembly. Our Strange ST Iron third member was equipped with Eaton Truetrac worm gear differential.

Strange Engineering Third Member

Strange Engineering offers a complete line of performance Ford 9-inch center sections for practically any type of application. For our Pro-Touring Camaro with the upgraded front and rear suspension, Strange Engineering’s Pro Iron PRF120 center section for performance street applications fit the bill nicely. Strange’s 9-inch Pro Iron center sections have been tested in the harshest racing environments for several years making it a great choice to use as a foundation. The nodular iron construction with Daytona iron support makes this a tough unit.

Our shop crew installed the axles into the FAB9 rearend and prepared to mount the brakes and wheels.

Our shop crew installed the axles into the FAB9 rearend and prepared to mount the brakes and wheels.

The TrueTrac is a worm-type posi-unit, also known as a torque-sensing differential, allows a transfer up to 2.50 times the amount of torque to the opposite wheel when the inside wheel begins to lose traction. Using cornering as an example, the inside tire has limited traction due to weight transfer. Torque is transferred to the outside wheel, helping to push the car around the corner. Torque transfer is seamless and quiet which is ideal for street and autocross use.

Topping off the center section internals is a 3.50:1 Motive gear set. The 3.50:1 gear should be perfect for highway driving and with the right transmission, this gear set will do very nicely on the road courses. Combine the Motive gear set with a set of Strange’s S/T axles with 35 splines manufactured from 1550 steel for an exceptionally high degree of hardness, even before heat treatment. The S/T axles come with bearings and 1/2-inch wheel studs that are designed to handle the street performance loads and torsion of Pro-Touring cars. 

We selected Strange’s upgrade to a Chromoly yoke (OPRF07) which is a great upgrade for the small price-tag. For just under four sawbucks (two-Jacksons), you can get a little extra piece of mind with a strong, dependable drive yoke.

Finishing It All Off

Alston’s Chassisworks are built with consideration for other components like Wilwood brakes. For our rear suspension upgrade, the folks at Chassisworks recommended the 12.19-inch rear disc brake kit with 4-piston calipers (Part #WW140-7140-D) for massive clamping power. An added bonus with this brake kit is the fact that it includes a parking brake provision.

Installation of the brake rotors and calipers and simple to install and do not require anything beyond normal mechanical ability.

The caliper mounting bracket serves as part of the parking brake assembly. Installing the mounting bracket on the housing ends with the supplied T-bolts guarantees perfect positioning for the parking brake assembly. For the rest of the brake installation, the axle assembly is installed after the caliper mounting bracket and the bearing retainer is installed to keep everything connected to the axle tube housing ends. The rotors and calipers can be installed after that. Much like the rest of Chassisworks rear suspension conversion kit, the brake assembly is simple to install and doesn’t require anything beyond normal mechanical skills.

To read the full story on our brake installation, check out: Our ’69 Camaro gets Chassisworks/Wilwood Brakes.

All together and in the chassis, this is a stout Chris Alston rear suspension that we can take out and thrash on any street or course.

The Best Part 

You’d be hard pressed to find anyone that isn’t happy after making a major upgrade like this rear suspension conversion. There is a feeling of real accomplishment when the car is back on the ground at ride height and the final adjustments are being made. Running the suspension through full compression, full extension and full roll really validates the work that has just been done. However, the real payoff comes when you get to sit behind the steering wheel and do a little road testing.

We can’t wait to see Project Blank Slate in action, and more so – we can’t wait to actually put some rubber to pavement and tear up some autocross, road, and track courses soon enough. Stay tuned!

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
Read My Articles

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