Stim.Tech Articles

Reinventing E30 RTABS

August 15, 2017 Suspension 0 Comments

As promised, we are well on our way to full production of a component we are quite excited for. A component which I myself, have been thinking about for years, since the very first time I built an e30 and modified that first subframe. It was a much simpler time then, 400hp was “too much” and Holset HX35’s were being strapped to anything with spark plugs.

Quality is possibly the most timeless characteristic of an automobile product. So, better late than never, we bring to you, a proper rear trailing arm solution for the e30.

Possibly the most challenging, well not possibly, definitely, the most challenging part of this project has been sourcing a bearing that would even make this solution possible. Why would sourcing a bearing be so difficult you ask? Well when you measure the bores of the e30 rear trailing arm link points it is quickly apparent the radial distance between the mounting bolts and the arm is extremely minimal.

Fortunately we were able to source a perfectly sized bearing for this application and we were also able to attain these bearings made in a Heat Treated Motorsport Spec material. You could hang a stacker race trailer from one of these bearings alone… probably.

After our initial design calculations and bearing selection was completed, we moved right into a working model.

The model for this assembly, like any other spherical bearing sleeve set, is fairly simple. The challenge would lie ahead of this step when the dimensioning and tolerancing of the bearing sleeve would be determined. The assembly consists of the Bearing Sleeve, Spherical Race Bearing, two Tapered Bearing Cones and a heavy duty Spiral Lock retaining ring (for safety measures only).

Typically when you create an assembly like this, much more room exists between the suspension arm and the bearing itself. In the common spherical bearing solution, you will see nice large machined aluminum bearing sleeves with healthy amounts of press-fit interference and very little risk of bearing pinching. Here, however we had no such luxuries.

With such minimal room for our Bearing Sleeve, we had to drive the geometry with two design considerations. The first being sleeve growth. Due to how thin the sleeve is, we knew that the outer diameter of the sleeve would grow by the amount of interference between the bearing and the inner diameter of the sleeve (almost 1:1 growth occurred). The second consideration we had to design for, was bearing pinching. This is a condition where an interference press fit is so great that the load it applies to the outer diameter of the bearing causes the bearing to seize up or operate with abnormal levels of friction.

After completing our design, we turned out a few sets of Bearing Sleeves and Tapered Cones. The first sets were turned from a high strength chromoly steel. This steel has great strength properties but for long term use it must be coated to hedge corrosion. For our full production run we will be utilizing a Precipitation Hardened stainless steel.

Moving right along we got into assembling our first sets of bearings to prepare for testing. Taking a step back, we must remember the press interference tolerances of these components is very critical. Now perhaps, is a good time to offer a brief explanation why these press loads are so critical.

To keep it simple, almost all suspension links see both radial and axial dynamic loading. Radial loading is in the direction perpendicular to the bearing bore (i.e. hanging a stacker trailer from your bearing) where axial loading is the load the bearing sees through the bore. The easiest way to picture axial loading is to picture pressing a component perfectly straight into a housing, this is pure axial loading.

So, back to press-fits, our interference fit of both the bearing into the sleeve and the sleeve into the trailing arm must always surpass any axial load these assemblies will see during operation.

We could have cranked these bearings in, given them a thumbs up and cracked a diet, caffeine-free, bacon flavored cold pop and called it a day, but that isn’t the STIM.TECH way. We busted out the data logging load cell to verify our calculations and look for improvement. This press set-up allows us to log and plot what axial load the bearing sets can resist. Of course, we always use the linear press fixture we built to ensure we are assembling parts perfectly straight as even slight side loading can skew results and deform components.

With our first set of trailing arms assembled we are ready to hit the track. As always we will be putting our test pieces through some abuse to ensure they operate flawlessly.

In our next post regarding these components we will review the installation tools we have designed and produced that will be included with these bearing kits.

Stay tuned for the release of this product!

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