How To Overcome The Low-Range Push
Ever wheel a Jeep with an automatic and a 4:1 transfer case? Most require a ton of brake pedal effort to stop—unless you pop the transmission into Neutral. This scenario can be nerve-racking at first and then downright annoying once you get used to it. We contacted a few of the industry’s better-known Jeep builders to see how they go about resolving the issue on customer rigs. The answers we received were as varied as the customers. As you might expect, each solution has pros and cons. In this story we will explain each method and why you should consider them to overcome torque multiplication compounded by extreme low-range gearing. Check it out.
How It Happens
To understand why low-range creep is a problem, you first need to understand a few basic principles about torque multiplication, clamping force, and friction. Let’s establish the fact that hydraulic braking systems amplify driver effort when pushing on the brake pedal. Now, pretend that your Jeep’s engine is a massive gasoline-powered breaker bar on the end of a huge ratchet. The crankshaft is the business end and the flywheel or flexplate is a massive socket. The more power your engine develops, the longer the breaker bar is. On automatic-equipped vehicles, when rotational force is applied to the breaker bar, the flexplate engages the internals of the torque converter, which governs how much rotational force can be transmitted to the transmission. The torque converter is basically a giant liquid pump that uses a stator (series of blades) between the engine-driven impeller and turbine. Think boating. Just as the diameter, spacing, and angle of propeller blades on a boat influence acceleration and top speed on the water, the diameter and angle of the blades inside a torque converter impeller, stator, and turbine affect how much and how quickly torque is transmitted to the transmission. As the stator redirects fluid from the impeller, it also multiplies torque by a ratio from anywhere between 1.8:1 to 2.5:1.
Inside the transmission, torque makes its way through a series of planetary gears, where further torque multiplication can be achieved. Next, the transfer case enters the picture. In high range, torque travels straight through to the driveshafts. In low range, however, the engine torque is multiplied further still. Rubicon models have a 4:1 planetary which allows the transfer case to quadruple the torque output for a given engine rpm. This is like adding four times the leverage to the breaker bar mentioned earlier.
The differential gearing multiplies this effort even further and when the torque finally reaches the wheel mounting surfaces, a staggering amount of brake friction force is required to overcome it. Most factory braking systems are optimized for high-range gearing and stock tires and wheels. Multiplying the mechanical advantage of the engine takes a serious toll on the effectiveness of the braking system, and if you add bigger tires to the equation, the brake system has to overcome the additional rotating mass. When you add it all up, it’s easy to understand why stock brakes have trouble controlling the creep.
One way to address the creep problem is to increase the performance of the braking system. There are basically two ways to go about it: The first and most common method is to increase the amount of hydraulic pressure the master cylinder can supply to the calipers. This increases the amount of friction between the pads and rotors. The second method increases the size of the rotor (diameter) and adds additional clamping force to the pads by way of larger brake calipers with bigger or additional pistons. Big brake kits are typically the most expensive way to resolve the creep issue. Increased rotor diameter typically requires a bigger wheel to provide appropriate clearance between the outside of the brake caliper and inner diameter of the wheel. So, unless you plan to swap in a larger set of wheels, big brake kits are better left to guys with big budgets.
On power brake arrangements, the additional power comes from either engine vacuum as is common to OEM applications, or hydraulic power (hydroboost) driven by the power steering pump. Both types have drawbacks and while neither is exactly cheap, both will improve braking performance noticeably over a manual arrangement. Vacuum-assist requires a somewhat-bulky diaphragm to amplify the driver’s physical effort on the master cylinder. This can create challenges in terms of packaging under the hood and because the system is always vacuum dependent, it won’t work on diesel engines or those that lack a steady, reliable source of vacuum. Hydroboost setups produce almost four times the operating pressure of vacuum assist and come in a compact package that can be juiced by the factory power steering box. One downfall to hydroboost is its dependency on engine rpm. Should the engine fail, the system has a small accumulator that will allow for one or maybe two power assisted brake applications prior to defaulting to manual brakes—which is better than vacuum boosted brakes can claim.