Do You Know What’s in Your 3rd Member?
We headed over to Currie Enterprises, an industry leader in this department, so we could get an inside look at what exactly a 3rd member (rear differential) is comprised of. This is one of the most important parts of your vehicle from the time you leave home in your chase truck to the time you cross the finish line in your race truck. The 3rd member is made up of a few parts, but those few parts are important to get exactly right. For instance, if you do not have the proper gear ratio you’ll lack power and torque and that could be the difference between winning and losing a race or being able to make it up the Cajon pass on your way to Vegas or Barstow for the race!
We took a look at one of the most common 3rd members of all time, the Ford 9″ design. This is used in about 80% of offroad vehicles that have a solid rear axle. The big brother of this, the 10″ system, is used in most Trophy Trucks. The biggest upgrade to do right off the bat is to use one of Currie’s custom “N” nodular iron cases. We picked out this N “Sportsman” case based on the rules in the Trophylite spec class. Currie also offers a 9+ Race Case that is used in even more extreme conditions and bigger, heavier vehicles. These cases use a bigger carrier bearing and a reinforced pilot bearing bore which increases the strength. Just by looking at a “stock” 9″ housing and a “N” housing side by side the bearing size differences are clear.
The next part that is a must have for racing applications is the big bearing nodular iron pinion support. Once again the biggest difference is clearly seen by looking at the size difference of the bearings. On the front side it also features additional ribs built in for additional support.
Next you need to pick your gear ratio. This is one of, if not the most important selections you’ll make. This makes a difference between being able to pull 3rd gear up that sand wash or being stuck on the rev limiter in 2nd gear. This 3rd member is being built for a Trophylite truck that runs an Ecotech 2.4L motor with a Turbo 350 transmission behind it. A 6.00 gear was selected to go into this setup.
Next you’ll need to select what type of carrier to install the gear onto. In your typical street car, most have an “open” 3rd member which means that if you jack both drive wheels up in the air only one of them will be under power. An upgrade from an “open” 3rd member is a “LSD” or limited slip 3rd member. This transfers power to both drive wheels under certain situations but not all of the time. The most common carrier in any type of race application is a “spool”. Using a spool, both drive wheels are under the same power at all times. In racing applications you never know when both wheels will be on the ground at the same time so you always want full power going to either wheel when its on the ground (which in off-road racing could be a very limited amount of time). This is the best out of all of the types of carriers for a race application, they have the least amount of moving parts so there is less to fail and are the strongest at the same time.
Once you have chosen all of the above parts, it’s time to put it all together. This is a fairly simple process, but a very technical part in the setup. The backlash needs to be set just right. Backlash is the adjustment that meshes the ring and pinion gear together. To tight and it will wear out quickly, too loose and it can cause failure in the gear set. Currie Enterprises is an expert in this department and have built many rear ends over the years for all different applications so just be sure and tell them what the application is and they will have a perfect backlash setting.
Once the backlash is set and everything is tightened down, Currie will “paint” the gear set and run it to make sure the settings are ideal.
Currie also has a 3rd member dyno in house that they can use if needed for certain applications.
Once everything is tested and working like it should, it’s time to put the final bolts in with some loctite and button it up. Torquing everything to spec is a standard procedure at Currie.
Once installed in the vehicle, it’s time to fill up the rear end with gear oil. Currie Enterprises recommends using their own 9+ 85/140 weight Racing Gear Oil.
Now you know what’s in your 3rd member. Next time you’re in the market for one call or visit Currie Enterprises and they’ll get you setup for anything from your stock street car to extreme race vehicle. Currie also manufactures other items to go along with your 3rd member such as axles, and rear end housings. For more information you can visit their website at www.currieenterprises.com or give them a call at 714-367-2676