Yukon Gear and Axle's Spool

To Spool or Not to Spool?

When discussing the options available for traction aiding differentials, the conversation is usually dominated by automatic lockers, manual lockers, and limited slips. Eventually, spools are mentioned but quickly dismissed as being too hardcore for street use. To some extent, this is true but for some of us, they might be the perfect balance between functionality and price.

What's the Purpose of a Differential?

Before I go any further, I'll quickly explain why automobiles come equipped with some form of differential. When going around a corner, the outside wheels always follow a wider arc than the inside wheels. This means that the outside wheels travel a longer distance than the inside wheels in the same amount of time. A differential (this includes automatic lockers, open differentials, and limited slips) allows the left and right wheels to turn at different speeds to account for the different arcs.

Randy's Ring and Pinion Yukon Gear and Axle Website: www.yukongear.com Phone: 1-800-292-1031 Fax: (425) 347-1440 Randy's Ring & Pinion 11630 Airport Rd. #300 Everett, WA 98204 Yukon Gear and Axle spools are available for the following applications: - Dana 44, Dana 60 - Toyota 4-cyl and V-6 axles - Ford 9" - Chrysler 8.75"

The most common type of differential is the "open" differential. This is a gear-driven differential that using torque balancing to allow the wheels to travel at different speeds. While this design works beautifully on pavement, it fails miserably in the dirt. Why? Because it balances the torque based on the wheel with the least torque. For instance, if one side of the axle was on rock and the other was on ice, the ice side would have the least torque available to it because the slippery ice provides no resistance to torque against. The open differential balances this torque by applying the same amount to the wheel with good traction (the one on rock). The result: the wheel on ice spins helplessly while the wheel on rock barely moves, if at all.

Limited slip differentials were designed to overcome this weakness. Usually using clutch packs, these differentials use some of the wheel spin from the poor traction side to apply torque to the good traction side. Unfortunately, they can only provide a fraction of the available torque. For extreme terrain, the limited slip falls short.

The next step up in the food chain of traction aiding differentials is the automatic locker. This differential's default state is to lock both left and right wheels together. When driving around corners, the outer wheel is allowed to ratchet and turn faster than the speed of the inner wheel. Hence, when going around corners on pavement, the inner wheel usually does the driving because the outer wheel is driven faster by the pavement. The automatic locker works beautifully on the trail but on pavement, the locking and unlocking action creates noticeable side-to-side jerking when applying or releasing the throttle while going around a corner. Other side-effects include ratcheting and banging sounds, and with some, jerking when coasting at slow speeds. (note: I've found the Detroit Softlocker to exhibit the least of these characteristics)

At the top of the locker food chain sits the manual locker. This differential can be forced to act like an open differential or, with the flip of a switch, lock up and become a spool. The only drawbacks to this device is its cost and, sometimes, the method by which it is switched. Also, because it is more complex than the other systems, its applications are more limited.

A spool, which replaces the differential, locks both wheels together so that they must always turn at the same speed. When going around a corner, the inside wheel usually chirps as it tries to turn as fast as the outside wheel, even though it is travelling a shorter distance.

Spools (top) compared to do-it-yourself lockers (bottom). The simplicity of the spool is also the reason for its tremendous strength. No moving parts = brute strength.

Pros and Cons

Now that we know how the various traction-aiding differentials work, let's look at the disadvantages of a spool:

1. Because it allows no rotational differentiation, the left and right axle shafts are subjected to a lot of stress. Add a high-traction surface like pavement to this situation and you have a perfect recipe for twisted axle shafts.
2. You will be serenaded by barking and chirping tires whenever you drive around a corner. Again, because the two axle shafts are always locked together, something else has to allow for the fact that the left and right wheels are travelling different distances. In this case, the tires will slip, causing the barking sound and resulting in premature wear.
3. Since the wheels always want to turn at the same speed, there will be a lot of resistance to turning, especially on a high traction surface such as pavement. Spools in the front axle will making steering very difficult, especially when trying to negotiate tight corners.

Those disadvantages sound pretty bad, don't they? So why would anyone even consider using a spool? Here's why:

1. Spools are cheap. You can pick one up for under CAD$200. Yes, it'll cost more than welding your carrier's spider gears in place, but the spool will be virtually unbreakable AND you will still have the option of re-installing your original carrier if you don't like the spool. The spool is far cheaper than the cheapest locker or limited slip.
2. Spools always work. A spool has no moving parts needed to allow for differentiation. Therefore, there is virtually nothing that can go wrong with them. They are much more reliable than the best locker.
3. Spools are strong. No moving parts means there are no little parts to worry about breaking.
4. Spools are predictable. Although there will be resistance when driving around a corner, there will be no side-to-side jerking. The "locker lane change" is not a side-effect of a spool.

Conclusion

As you can see, the spool has some pretty good advantages, especially for the budget-minded fourwheeler. But what about the potential for axle shaft damage? Well, for off-highway use, it's not that much of a concern. The typical off-highway trail will allow for enough tire slippage to reduce axle shaft stress. It's the high-traction surfaces that you have to watch out for. For most dual-use vehicles, that means pavement. Ben Olson recently installed a Yukon Gear and Axle spool into his "Airzuki" and came up with a way to minimize the stress of pavement driving. He keeps his tires aired up around 40 psi which reduces the tires' contact patch This allows them to more easily slip on pavement. Ben has had the spool in his Suzuki LJ80 for a few months now and is pleased with the results. A short wheelbase vehicle like his Airzuki is very sensitive to the "locker steer" effect common to automatic lockers. The spool's predictable characteristics have made the Suzuki much easier to drive on the street. Of course, there is still the issue of faster tire wear but since the Airzuki is not Ben's daily driver, it is a good solution. If you're looking for a very affordable and reliable way to lock the differential on your weekend fourwheeler, you should have a look at Yukon Axle and Gear's spool.

Installation

Installing a spool is identical to installing a new carrier or ring and pinion gears:

Ben's "Airzuki" runs a very custom Ford 9" rear end in the back. Its auto locker had died so this was a perfect opportunity to try out Yukon Gear and Axle's spool.	Once the third member was removed, Ben was able to do the autopsy to see what went wrong with its locker.	The locker's pins (circled) had sheared. Bits of the broken pins had also produced collateral damage. Replacing this locker with a spool with result in a virtually bullet-proof system.
The spool replaces the existing carrier so it was necessary to transfer the old carrier's bearings and ring gear to the spool. Because of the ring gear's tight fit, Ben first heated it up in some hot water.	Once it was heated, the ring gear easily slipped onto the spool.	After transferring the ring gear and bearings to the spool, Ben applied some marking compound to check the contact pattern.
Adjustments were made to achieve the correct contact pattern and backlash and the third member was re-installed into the 9" housing.