Lower gears can offset the downside of larger and heavier tires and wheelsPosted in How To: Tech Qa on May 29, 2016
If you have been reading the last few issues of Jp, you know that we have been making upgrades to regain the acceleration and braking we lost after adding a set of taller, heavier 35-inch Toyo Open Country M/Ts to our ’06 Jeep Wrangler Unlimited Rubicon. Our next upgrade is to regear the axles with new Yukon gears to restore the engine to the proper powerband for the taller tires. Some might say that this is the first upgrade we should have made, and they would not be wrong. Regearing is not an easy or inexpensive endeavor, though, and we wanted to first see what gains could be made by other upgrades that require less time and money.
Our Jeep Wrangler came with 31-inch tall tires and 4.10 gears, the deepest (numerically highest) gear ratio offered from the factory. They likely would have been adequate with 33-inch tall tires, but were not enough for our 35s, as evident in the drop in 0-60 times from 18.6 to 29.6 seconds. The 35-inch Toyos are 13-percent taller than the factory tires, so a 13-percent lower gear ratio of 4.63 would theoretically put our engine back in the same powerband as stock. The 4.56 gears are the nearest available ratio, but we went the next step lower to 4.88 to account for the added weight of the new tires that must be factored in, as well as the tire height.
Randy’s Ring & Pinion had everything we need to regear our Jeep, including master install kits with high-quality Timken bearings and the proper “thick” 4.88 Yukon ring-and-pinion gears necessary to work with our Rubicon’s factory lockers. Since we were tearing into our axles, we also took the opportunity to add Yukon’s 4340 chromoly axleshafts to the front and rear of our LJ. While these do nothing to improve our acceleration, they do ensure we don’t have to walk home due to a broken axleshaft as a result of our 106-pound tire and wheel combination. Once we had the parts in hand we headed to Bayshore Truck for the installation. While the new axleshafts are a bolt-in affair that could be handled in the driveway, ring-and-pinion setup is something that requires specialized tools and equipment and is typically best left to the professionals. Bayshore mainly focuses on over-the-road trucks, so our little Jeep didn’t pose any challenge for them. When we needed to replace some seals and bearings due to wear, they notified us about the issues and had what we needed in stock to ensure our Jeep remains reliable no matter what situation we find ourselves in.
Once the gears are properly broken in and we have performed our next upgrades (a full exhaust system and performance engine tune from the DiabloSport Trinity tuner), we will run another set of acceleration tests to see just how much of what we lost have been gained back from the new gears and the engine mods. Stay tuned.
Our LJ is a Rubicon model so it came equipped from the factory with TFS air lockers and 4.10 gears. Naturally we wanted to retain the lockers, but they require special “thick” gears due to position that the ring gear mounts on the carrier. Fortunately, Yukon Gear had everything we needed.
The Rubicon’s factory TFS air lockers use very low air pressure in the 3-5 psi range. Applying the sort of pressure you would use to lock an ARB Air Locker (50-70 psi) will scatter the internal components of the TFS locker. One nice thing about the rear locker, though, is that it has a limited slip when it is unlocked, unlike most selectable lockers that default to an open differential.
The first thing Bayshore did was remove the carrier and cleaned out the housing thoroughly. Note that the mating surface for the diff cover was also cleaned off to ensure a reliable seal after the new gears are installed.
Dana 44s have a carrier break at 4.10, which means that there are two different carriers in order to place the ring gear the proper distance from the pinion. A typical ring gear is on the left, but that wouldn’t work with our air locker so we required a thicker gear (right
The 4.88 gear ratio we chose have 39 gear teeth on the ring gear and 8 teeth on the pinion, while the old 4.10 gears had 41 gears on the ring gear and 10 gears on the pinion. Note that the shims and bearings are still on the old pinion, which provide a starting point for setting up the new gears.
The Yukon front axleshafts accept full circle snap rings, which greatly reduce the chances of spitting out a U-joint cap and breaking the ears of the axle yoke. We used Spicer 5-760X U-joints with needle bearings since our Jeep doesn’t have front hubs. Yukon’s 4340 chromoly Super Joints are much stronger than the Spicers, but they are not recommended for full time operation.
Yukon’s axleshafts offer a bolt-in strength increase. While the ’shafts have the same diameter and spline count as the factory axleshafts, the Yukon ’shafts are constructed from 4340, making them 39 percent stronger than the factory 1040 axleshafts. They also have more material around the ears/yokes where the U-joints mount for even greater strength.
Michael Sage at our Bayshore Truck handled our gear installation. He started by removing the old ring gear, bearings, and shims from our factory lockers. Sage noted the importance of measuring the shim packs on the carrier and pinion as a starting point for setting up the new gears.
The Yukon master install kits come with high quality Timken bearings. When performing something as labor intensive as regearing, we appreciate the peace of mind that American-made Timken bearings provide.
A case spreader isn’t always necessary for gear installations, but it is critical when working on a Rubicon Wrangler for a couple of reasons. First, the shims on the Rubicon carrier install on the outside of the bearings, rather than under them like traditional Dana 44s. Secondly, you do not want to damage the locker engagement indicator.
This little plastic button is used to illuminate the dash light for the factory lockers when they are engaged. Prior experience has taught us that this engagement indicator is very easy to damage and the entire carrier has to come out to replace it.
One of the myriad little steps that Bayshore performs to ensure that a gear set runs cool and quiet is lapping the gears. This involves taking a flap disc to each gear tooth to smooth it out and remove any ridges left from the machining process. The gears were then cleaned thoroughly.
The location of the carrier shims on the outside of the bearings is a two-edged sword. The upside is that the bearings can be pressed on to the carrier once with no need for setup bearings. The flip side is that the shims are not retained as securely and can be spit out if the housing flexes.
Our LJ Rubicon uses rear disc brakes with a small drum brake within the disc that acts as the emergency brake. This entire backing plate assemble must be removed and transferred to the new axleshafts. We replaced the axle bearings and seals at the same time.
TJs and LJs use a semi-floating rear axle with a bearing retainer that is held on by these four bolts. This is a far better design than the c-clips inside the carrier that are used to retain Dana 35 axleshafts, but not as strong (or as heavy) as the full floating axle design found on 3/4 and one ton trucks.
Our LJ Rubicon has Dana 44 axles front and rear, so the setup is very similar. In fact, they use the same gear sets and master install kits from Yukon Gear. Like most mechanical operations, doing gears the second time is faster than doing it the first time.
Michael Sage only needed one attempt to get a perfect pattern on our Yukon Gears. Moving the pinion gear closer to the carrier will move the drive-side pattern closer to the flank and slightly toward the toe. The coast pattern will move deeper toward the tooth flank and slightly toward the heel. Moving the pinion away from the carrier will move the drive pattern toward the face and slightly to the heel. The coast side will move toward the face and to the toe.
Rather than carrying a hundred different axleshafts for a hundred different applications, Yukon cleverly designed a cut-to-length axleshaft with extra-long splines. Note how the shafts have three bolt patterns—5 by 5.5, 5 by 4.5, and 5 by 5—to fit most Dana 44 Jeep applications.
Measure twice, cut once? We measured at least three times before taking the chop saw to our new Yukon axleshafts. The 4340 chromoly axleshafts are not easy to cut through, so you need to be patient and take your time.
We failed to purchase wheel studs, axle bearings, or backing plates from Randy’s Ring & Pinion when we ordered up all of our parts. Fortunately, Bayshore had what we needed in stock so our Jeep wasn’t on the lift for any longer than necessary.
Note the access hole on the factory axleshafts that is used to bolt on the backing plate and retainer once the axleshaft is installed. Our new Yukon axleshafts didn’t have this hole, but they have since added the access hole to their cut-to-fit shafts, which should simplify installation.
The last step involved programming our new gear ratio into our DiabloSport Trinity programmer in order to calibrate the speedometer, odometer, and transmission shift points.
After properly breaking in the gears and swapping the gear oil we hit the trail. The lower gear ratio not only improved our acceleration and fuel mileage on the pavement, it also provided more gear reduction on the trail for slow speed, technical crawling.