Inside the Design of the FTR Watts Link

We recently opened up pre-orders for our new SN95 Watts Link. The purpose of this post is to go through the design step-by-step and discuss the engineering behind what we’ve created.


Part I: Conception & Goal

In 2014, we first started development on our Watts Link. There are currently a few SN95 watts link’s on the market, but they all lacked a few features that we wanted to see. Our #1 concern was the following problem: no current Watts link on the market was able to locate the car’s rear roll center lower than a Panhard rod. While the Panhard rod is good solution for rear axle lateral location, it compromises on a couple areas. First off, a Panhard rod, by nature, does not have identical cornering characteristics left and right. When compressed, the axle will be laterally offset to the left, and when decompressed, the axle will be offset to the right. When cornering, this issue becomes even more complex, as the mounting point on the chassis is moved relative to the axle and the axle deviates from its proper location.

A Watts Linkage, in contrast, provides identical cornering characteristics left and right. This concept was invented by James Watt himself (hence the name).

“Watts Linkage.gif” by Van Helsing is licensed under CC BY 3.0

 

 

Clearly, the Watts Linkage is an ideal method of laterally locating the axle, due to its consistent horizontal location throughout articulation.

Our goal is to combine the best of both worlds: Pair the superior cornering characteristics of the Watts Link with the lower roll center of the Panhard rod, previously impossible to obtain with a Watts Link.


Part II: Differential Mounting Plate

We’ll start at the center of the part. Our differential plate mounts to a Strange R5234 differential cover. This cover is part of the “Ultimate 8.8″ axle package, and has built-in mounting holes intended for an axle brace. We thought this would be the perfect cover to accommodate our mounting plate.

Prototype rear assembly, red anodized bell crank not shown

This plate is secured to the differential housing at four different places. One bolt in each corner. We also put in clearance holes for the axle girdle bolts, so that the plate is not being secured to the differential using the girdle bolts and introducing unwanted stress to the the differential itself. This way, the user does not run the risk of shearing their axle girdle bolts.

The differential backing plate has five different bell crank mounting positions, allowing a 4.5″ range of roll center adjustment. When mounted on 315/35/17 tires, this comes out to a 7.5″ roll center at the lowest setting, and a 12″ roll center at the highest setting.

This is the widest and most flexible range of adjustment of any SRA lateral-locating device on the market.


 

Part III: Bell Crank

The initial design for our bell crank was a 1/2″ threaded aluminum plate. This bell cranked function correctly, however we have since redesigned it for even more strength, rigidity, and overall sexiness.

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The bell crank is available with three different bushing packages, depending on application:

Street: Polyurethane center bushing to combat vibration and noise
Performance: Delrin bushings and PTFE rod ends for stiffer movement
Race: Self-lubricating bronze bushings and bronze rod ends for ideal stiffness where noise is not an issue.


 

Part IV: Chassis Brackets

Our chassis brackets are laser-cut, TIG welded, and powder coated, providing maximum strength and a clean appearance. We have also added red anodized ID plates with laser-engraved part number, serial number, and QR code. The QR code allows us to keep careful track of all production data including date of fabrication, materials used, machine settings, etc.

wattsrender_final


 

Part V: The Results

In July of 2014, we sent our prototype watts link down to Brett at BTM Autosport in San Diego. Brett had previously ran a competing part, but by switching to the FTR part he found even more grip due to the increased rigidity and lighter weight of the FTR piece.

“Going from the Fays2 Watts Link to the Fully Torqued Racing Watts Link changed the car’s handling characteristics drastically. We used the same roll center height we had on the Fays and the car had so much rear end grip it produced understeer. ” – Brett Madsen, BTM Autosport

Since the installation of the FTR watts link prototype, Brett’s team has achieved the following victories:

2014 CP National Tour Champion
2015 SCCA CP National Tour Champion
2015 SCCA CP ProSolo Champion
2014 SCCA SU Regional Champion

The prototype version of the Watts Link on BTM Autosport’s car differs from the production version, which includes a more robust bell crank, laser cut brackets, and a lateral support. However, we told Brett to be as hard on the Watts Link as possible, and he has provided. After each race, the Watts Link has been inspected and has shown no signs of wear, flexing, or cracking.

Here is a video of the prototype in action:

As you can see, there is a bit of flex in the bell crank, so we have since upgraded it. However the function and strength has proven to be quite sufficient.


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