Beat The Creep With Bigger BrakesPosted in How To: Transmission Drivetrain on March 1, 2012 Comment (0)
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.
Friction, Clamping Force, and the Missing Link
Many people think that simply adding clamping force will improve brake performance. However, pad material also has a lot to do with the system’s effectiveness. Pads are the final point of the system and come in a staggering array of compounds for different applications. To understand the creep issue completely, it’s important to understand how the different compounds affect brake performance when hot and cold. All brake pads are required by law to have a published edge code that tells consumers what the average coefficient of friction is over a given temperature range. These ratings, once decoded, can tell you a lot about why a braking system on a Jeep with 4:1 gearing fails to stop the vehicle in low range. Chrysler designs all brake systems to work in a variety of real-world driving scenarios. Slowing the vehicle to a stop while in low range idling over boulders is not a major concern to Chrysler’s R&D team—such activity generates very little heat in comparison to a life-threatening panic stop on the interstate.
For example, the JK Rubicon front brake pads have the following alphanumeric sequence: FF0821MR. The FF portion of this code tells us that the OEM pad has an average coefficient of friction between 0.35 and 0.45 for both low and high temperature ranges. The higher the number, the more friction you get. These numbers are not particularly high when compared to pads found in the racing industry. However, high-performance pads perform best when hot. High brake heat is not typically associated with low-speed rockcrawling, so regular pads with higher friction at lower temperatures would in theory grab better when rockcrawling. We think the industry needs to develop special brake pads for Jeepers with dedicated trail rigs that employ a compound that delivers a high coefficient of friction at below average brake temperatures. If such a brake pad existed, it probably wouldn’t stop well at high speeds, but it might prevent the creep from happening off road.
Torque Converter Upgrades
Often overlooked by Jeep builders, the torque converter is probably the best place to spend money when attempting to reduce low-range creep. To do so, you need to raise the stall speed of the torque converter. The stall speed is the point at which the engine rpm begins to transmit torque to the transmission. Manufacturers tune factory torque converters so that they don’t transmit very much torque to the transmission while at idle. This is why you don’t have to press the brake pedal very hard while sitting at a stoplight in 2-Hi. Think of stall speed as the point at which the clutch begins to transmit power to a manual transmission on a stick shift rig.
Stall speed is determined by several factors, including the distance between the impeller and the turbine and the design of the stator. Stock JK torque converters are set up around the 2,200 rpm range. By modifying variables of the torque converter’s internal components, manufacturers can alter the stall speed and create a torque converter that engages at a higher or lower engine rpm. Raising the stall speed allows the engine to spin faster before transferring power to the transmission. The negative effect of a higher stall speed is increased heat in the ATF, as the engine will need to rev higher to propel the vehicle from a stop. However, with only a slightly higher stall speed, say 300 to 400 rpm over stock, the effect is minimal. In low range however, the additional rpm afforded by the higher stall speed can help significantly as the braking system isn’t trying to fight gobs of torque at idle.
Remember how we told you that torque is multiplied by the transmission and transfer case gearing? Well, if a higher stall torque converter is used, less power (at idle) is sent to the transmission in the first place. If less torque is present downstream of the torque converter the factory brakes have a better chance at controlling the creep. Luckily, replacement torque converters are available for several Jeep models and are not super expensive ($250-$350) and with a virtual smorgasbord of suppliers across the country, you won’t have to wait long to get one. The only bummer we see in swapping in a torque converter with a slightly higher stall speed is the fact that the transmission and transfer case have to be removed to gain access. Additionally, you don’t want to tow heavy loads with one because it will cause excessive heat.