Bringing up the rear on our project car build from Factory Five Racing (FFR): in this chapter we show you the installation of a full floating rearend using components from Moser Engineering and Baer. The front suspension and steering were installed in the prior article, and the project is coming along quickly on this build-it-yourself kit. The Challenge Car is Factory Five’s road racing version of the Mk4 Roadster kit, and while both cars can be taken to the track, the Challenge Car offers a bit more strength in the chassis as well as some extra options specific to road racing, such as a lowered stance and increased cabin protection with a more structural cage.
While both the Mk4 Roadster and the Challenge Car offer similar suspension and drivetrain options, for our Project FFR Cobra Jet Challenge we opted to install Baer’s Tracker full floater rearend conversion on our custom-built Moser Engineering housing. This decision was made for the overall strength and performance benefits, as well as a little bit of our desire for showing our readers some of the cool products available in the performance aftermarket.
A conventional rearend in most rear wheel drive cars is considered a semi-floating rearend: both axles have flanges at the outer end that the wheel bolts to, and the inner end is splined to match the differential. The weight of the vehicle is supported on that flanged axle, and while that is not a bad thing for conventional driving situations, it is not as strong as a full floater.
A full floating rear axle differs in several ways, and the benefits are truly worth it for a track car. Rick Elam from Baer tells us, “The biggest factor about a full floating axle conversion comes down to cost. It is a bit more expensive, but for someone who is racing competitively that extra cost could mean better lap times.” We’re going to explain why in this article with the help of Baer and Moser Engineering.
You many not think you’ve ever seen a full floater rearend, but it is likely that you’ve seen tons of them – “tons” being the operative word here. Many large trucks utilize the full floater rearend because of its strength and ability to carry a heavy payload. This type of rearend is more easily identified by the hub that extends through the center of the rear wheel which carries its own bearings and end plate to cover the axle end.
We can make the full floater a very strong unit and at the same time make it very compact and lightweight. -Rick Elam
Rick Elam from Baer tells us, “A full floater can be used on a street car if the owner desires, but it is a bit overkill. The real benefits are out on the track where the car is being pushed harder in the turns; that’s where a full floater serves a better purpose.” That’s not to say that the Challenge Car requires a full floater for racing, just that the benefits of a full floater are better served on a car that is used in road racing. “We can make the full floater a very strong unit,” he continued, “and at the same time make it very compact and lightweight.”
A full floater housing differs from a conventional, semi-floating in a few ways. For both types, the axles are splined at the differential end. But the full floating axle is also splined at the outer end, which fits into a hub that is bolted to the housing. Inside that hub is a pair of bearings that ride inside the hub and support the weight of the vehicle. For a conventional rearend, the wheel is bolted to the axle flange, which rides on a single bearing. The full weight of the vehicle at that corner is supported by that single bearing and axle.
When pushing a car hard in the turns, a conventional flanged axle can actually bend or bow from lateral forces; this is called axle deflection. As the lateral forces increase, this deflection is more noticeable. Another side effect from this deflection is what is known as brake pad knock back, and that will affect the the braking feel at the pedal.
Knock back is when the brake rotor and axle flange are affected by the deflection of the axle. This deflection causes the rotor to knock the brake pad back into the caliper, displacing fluid and causing the driver to reapply the brakes, or pump the pedal, for a firmer feel. If you’ve ever driven a car with a warped rotor and pressed hard on the brakes, you have experienced what knock back feels like – it doesn’t instill confidence during hard braking.
Another side effect to axle deflection is that lateral grip is affected during hard cornering, which can decrease cornering speeds and cause uneven tire wear, as well as affect tire temperatures. Additionally, axle deflection can decrease bearing life on a conventional flanged axle bearing, which can also increase that knock back and affect handling. The full floating axle addresses these concerns by removing the axle from the flange to reduce the axles purpose to one thing: driving the wheels.
A full floating axle doesn’t support any weight or lateral forces because it rides inside the hub at the outer end. The wheel bolts to the hub, and that hub rides on a pair of roller bearings that support the full weight of the vehicle. That hub also absorbs all of the lateral forces, and because the hub isn’t a part of the axle, there isn’t any bowing or deflection that can occur.
The wheel is mounted to the hub, and the hub is bolted to the housing, providing a better support system for lateral forces. Additionally, the brake rotor is attached to the hub, not the axle, and this helps eliminate knock back because the hub cannot bow or deflect from lateral forces.
The Build From Moser
Instead of starting from scratch on this build, the best choice was to build off of a proven differential and modify it from the start. We chose a Fox body Mustang 8.8-inch rearend that we picked up locally, and we sent it out to Moser Engineering for the modifications to be done.
The FFR kit is designed to use the Fox body Mustang rearend, and since we were going to use new internals and the Baer Tracker Full Floater system, the rearend was in capable hands at Moser.
The Fox body rearend has brackets attached for the factory four-link suspension setup, but the two inner mounts will not be used. The inner mounts can be cut off, but since they’re not in the way of anything, it’s just additional, unnecessary work. If customers request them to be removed, Moser can do that and clean it up a little bit.
The rearend is converted to a three-link setup, with additional brackets included from FFR. The two outer brackets are attached to the stock outer trailing arm brackets, and a center, third link is attached to the passenger-side axle tube.
These three brackets can be bolted into place, but FFR highly recommends that they are welded to the housing, especially for racing. The center bracket is located with a separate dog leg support to bolt to the center section, however, Moser provided us with their own bracket for this application which was welded to the rearend.
The three-link setup will be kept in place by the panhard bar installed between the rear drop-down bracket on the chassis and the passenger side rearend bracket.
In order to get the dimensions correct, Moser needs to know the overall width of the rearend. They work with Factory Five on these kits, and since the kits are built around the Fox body 8.8-inch rearend, that is the starting point. Rear wheels have specific offset requirements for this set up, so it’s important to stay with that offset for this application in order for everything to fit.
We spoke to Jeff Anderson at Moser Engineering about the conversion to the full floater. “We have a formula that we work with for this conversion, so we start with the basic 8.8-inch Ford and we know where to make our cuts based on the dimensions we have,” he told us.
Moser Engineering is familiar with just about every modification that’s done to these popular rearends, and spends much of their time testing out their own products on the street and at the track.
Moser techs put the Ford housing into a jig and went to work cutting off the ends of the axles at the proper location so they could install the Baer Tracker full floater housing ends. The ends were welded into place on the rotating jig, completing a bead around the housing ends to secure them into place. Moser also reinforces the center section and tube by welding the two together to help strengthen the housing assembly.
We chose the Detroit Truetrac for this application because of it’s overall driveability and for longevity. It’s a torsion type differential that has no wearing parts; the helical gears rely on friction to provide the limited slip effect, and for that reason Eaton recommends that conventional gear lube is added to the differential instead of synthetic. Additionally, unlike many limited slip differentials, the Truetrac should not receive any type of friction additive or modifiers.
Adding to the overall strength of the rearend housing is the Performance Cover that Moser added to the housing. Anderson said, “This is probably the most beneficial component you can put on the rearend and add strength to it.” The cover is made from 356-T6 aluminum and it’s designed to not only look great but to add strength by providing support for the bearing caps inside the differential.
The two bolts on the cover are kept loose while the cover is installed over the third member, and once the cover has been tightened the two support bolts can be tightened. These two bolts make contact with the bearing caps and help reduce ring gear deflection under load. This additional cover is only about six pounds but will help improve the life of the ring and pinion gear.
Baer Tracker Full Floater Assembly
The assembly of the Tracker full floater is like putting together a model kit – there are a few pieces and some hardware to assemble it. Patience and attention to detail are important here, and it will require some measuring and use of spacers for both the axles and the brake calipers. The brakes that are chosen for this build will depend on the buyer’s needs, and are not included with the Tracker full floater.
Installation began with the brake backing plates, which bolted to the end of the housing. This backing plate incorporates the parking brake shoe that will fit inside the brake rotor hat. This plate comes assembled with the wheel cylinder and adjuster installed.
The wheel hub and the brake rotor hat are bolted together on the bench, and then the brake rotor was bolted to the hub assembly. We used thread locker on the many small bolts to assure that they won’t back out with vibration. We used extended ARP wheel studs that were provided with the Tracker kit.
The wheel bearings were greased up and inserted into the hub, and the grease seal was carefully pressed into place. The whole assembly was then placed onto the housing end, with the bearings riding on the modified end of the housing, instead of being pressed onto the axles. This type of bearing setup will keep the full floater mounted to the housing and eliminates brake pad knock back that we mentioned earlier because the axle is separate from the hub, and therefore deflection is eliminated.
You’ll see a cog, or gear teeth, on the inside of the hub assembly, and this is for ABS applications. For our application, we’re using these components for the traction control, so the small retainer on the backing plate will be for the harness/sensor.
The hub assembly was placed onto the housing end, followed by a locking ring and the retainer that threaded onto the housing. A special tool that comes with the kit allowed us to tighten down the retainer to proper torque specs. After we torqued the retainer, the tabs on the locking ring were bent up to keep the retainer from rotating, much like a crown nut on a front spindle.
Inserting the axles is where the measuring takes place, the first axle is inserted and the end cap is bolted into place. The end cap retains the axle inside the housing. Because the axles are floating between the differential and the hub, they need to be retained without sliding completely inside the carrier, and that’s where the nut and bolt on the end of one axle is used.
The axles only need to contact the splines on the carrier, and engage to the depth of the splines on the Eaton differential, they should not slide all the way through to the center, there needs to be spacing between the axle ends.
Anderson mentioned that they have an updated axle now with a groove that allows for a C-clip to retain the axle, but early versions like ours use the bolt to provide the spacing. With one axle in place, we measured the distance from the inside end of that axle to the outside hub on the opposite side. Then we measured the axle and adjust the bolt to make up that length.
The nut is used as a jamb nut to lock the bolt into place; Anderson recommends that the space between the two axles is approximately 1/8-inch with the bolt in place. On some other applications, a puck, or spacer, is used inside the differential to keep the axles in the proper location, the bolt works in a similar manner.
With the spacing figured out we were able to insert the second axle in the same manner and attach the end caps to retain both axles in the housing, and then we installed the Baer Brakes 6P rear calipers. The process was much like we did with the front brake calipers installed with the front suspension. The calipers mounted similarly using mounting brackets provided with the kit.
To mount the calipers properly, we bolted them to the mounting brackets without brake pads, and measured the distance on the inside and outside between the rotor and the inner caliper housing. Half of the difference between both measurements is the approximate size for the spacer to mount the caliper.
With the spacers in place, we mounted the calipers and measured a second time to make sure that we had the calipers centered. Without this alignment, uneven pad wear and rotor warp can occur, so this is an important step. The only thing we were left to do was to install the rearend in the car, and that was a little bit tricky because of our modified suspension set up.
We had an additional brace mounted to the drop down for the panhard bar, so that meant we couldn’t just lift the rearend into place. It actually had to be fed through the opening and attached to the control arms.
The coilover Koni shocks were installed and that concluded the rearend installation. With both front and rear installation installed, we were ready to get the drivetrain and interior going, and those will be in forthcoming updates that can be found on our Project FFR Cobra Jet Challenge Car build thread.