JBR Motor Mounts

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1 - JBR Motor Mounts

Introduction

James Barone Racing is a relatively new name to the Mazda aftermarket. Jamie has been making quality products for the Mazda market since September 2009. Today, we will be reviewing a full set of his 80a durometer motor mounts for the Mazdaspeed 3 (Gen 1 and Gen 2). Since there has already been a MSF review done on the CP-E mounts, this review will only have limited coverage of the installation process. In addition to the installation tutorial in the existing MSF review, very thorough instructions are available here at the JBR support website:

Support : James Barone Racing, Custom Fabrication

The main goal of this review is to show the reader how the successive installation of the JBR RMM, TMM, and then the PMM affect the performance, drivability, and ride comfort of their car. This review was written to answer the common questions that people have about how their car will be affected by upgrading the three different kinds of Mazdaspeed 3 motor mounts.

Technical Background

There are three Mazdaspeed 3 motor mounts: the Rear Motor Mount (RMM), Transmission Motor Mount (TMM), and Passenger Motor Mount (PMM). This section will cover the design of the physical dimensions and materials that make up the mount bushings, and the importance of engine mount placement. The TMM and PMM are commonly collectively referred to as the side mounts. It is important to note that these two side mounts are the load-bearing mounts. When at rest, the engine sits on these two side mounts. This is why the engine does not need to be supported when doing a RMM install. Here is a picture of roughly where the stock side mounts are located:



When the engine is turning, the rotation of the crankshaft induces a circular rotational force (as seen in the picture above) that rocks the engine back and forth in place toward the front and back of the car when the rotation of the crankshaft accelerates and decelerates. The side motor mounts are designed to support the weight of the engine pushing downward and the horizontal forces exerted by the rotational force of the crankshaft. The placement of the PMM at the top of the rotational arc and the TMM at the bottom of the rotational arc allow the isolation of the circular forces generated by the crankshaft to a single horizontal plane at these strategic locations. These forces are shown in the following pictures, which are taken as if the camera were aimed from the driver’s side while the mount was installed:





This gives absorption points to deaden the rotational forces generated by the crankshaft at both the top and bottom of the arc it traverses. The bushings of the stock side motor mounts were designed to deaden forces in both a vertical and horizontal direction, so this is why they are both shaped in a roughly conical shape:







This conical shape is designed to only be secured (attached to the frame of the car) on one side of the bushing (with the weight of the engine pushing on this point), with the other side of the bushing swinging loosely to absorb engine vibration and rotational energy in both a vertical and horizontal direction. The advantage of this complex bushing geometry is that it allows for the absorption of both horizontal and vertical forces, but its disadvantage is that it allows more deflection, and does not age as well as a more standard bushing design. This differs from the RMM, which is designed to deal with only the horizontal rotational forces, which are roughly parallel to the ground at the point where the RMM is mounted. Remember: the stock motor mounts were designed with daily driving comfort as a high priority, so that is why Mazda has used rubber bushings with complex conical bushings and extra cutouts to allow for added comfort while driving and idling. The follow picture shows the JBR 88a dogbone RMM, but its bushing orientation is the same as the stock RMM:



A common change that aftermarket mounts make to the OEM design is to make the mount bushings solid (as opposed to having the weird cutouts that the OEM RMM and TMM have) to increase the hardness of the bushing material. The durometer of the bushing gives a measure of the hardness of the material. The harder the material, the more force it is able to absorb for a given amount of deflection (and the more uncomfortable it is for the ride quality). Aftermarket mounts also commonly change the hardness (durometer) of the specific blend of polyurethane or rubber that they use to achieve different levels of performance vs comfort that fit the needs of their users. Generally, the better the mount performs, the more the ride comfort will suffer. (Sorry, no free lunch here…)

The physical dimensions of the bushing also play a large role in the performance and comfortable characteristics of the mount. A bushing with a small diameter will put a lot more stress on the bushing material, whereas a larger bushing will have much more bushing material to absorb the force applied by the engine. This means that all things equal, a large bushing does not need to be as hard as a smaller bushing to maintain the same deflection with the same applied force. Additionally, a larger bushing (of appropriate hardness) eats up more high frequency (low amplitude) vibration while still providing good deadening against large movements than a smaller bushing. These performance and comfort advantages are why JBR uses some of the largest bushings available on the market.

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