Russ Crofton's Handmade Full Suspension

Feb 7, 2019
by Eric Isaacs  
Russ Crofton and his creation A steel full suspension.

Introduction
In the spring of 2018, rumors of a steel full suspension created by a Michigan Tech student were flying in the small town of Houghton, MI. The source of the buzz turned out to be a project that Russ Crofton, a Mechanical Engineering Technology (MET) student, and his senior design team were working on. Fortunately I was able to track down Russ and the bike to get the scoop on the bike and take some photos while it was (mostly) built up.

The Project
When Russ had the opportunity to participate in designing and building an e-bike to fulfill his program’s senior design requirement, he did exactly what we all dream of: he volunteered to design and build the frame he wanted to ride, and let his teammates figure out the “e” part.

The project was a continuation of a previous team’s work, and there was lots of room for improvement. The previous design was heavy, sluggish to ride, uncomfortable, and outdated. It was quite literally a department store bike that had been cut in half and welded back together. Russ had something a little more svelte in mind, and convinced his team to start from a blank canvas.

The previous design.
A render of Russ and his team's design.

Design
The geometry and suspension travel (130mm rear, 150mm front) of the bike was inspired by the 2017 Transition Scout. This was chosen as an industry leading design that values modern geometry and durability over weight concerns.

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When choosing the suspension design, cost and manufacturability were the primary constraints. This led Russ to consider using a flex stay design in order to eliminate a pivot and maintain clean aesthetics. Unfortunately, the placement of the shock on the seat tube would have had to be much higher in order to limit the deflection of the stays to an acceptable level based on the fatigue properties of steel (See the Swarf Contour, which has significantly less rear travel).

Brian's workshop—where most of the magic happens.

Other issues quickly surfaced with the flex stay design, primarily related to the limited stroke of the shock available for the project. Instead, Russ chose to go with a modified single pivot design that uses a one-piece rear triangle. The one-piece design simplified the fixturing and alignment of the rear triangle and made it feasible to build without a dedicated fixture.

Russ designed the suspension around a 110% antisquat value at 30% sag and a highly progressive leverage ratio that is controlled by a rocker and swing link.

Construction
Local frame builder Brian Eggart, of Equinox Bicycleworks, graciously donated the use of his frame jig and tube mitering equipment for the project. The rest of the work was performed in the School of Technology machine shop at Michigan Tech. Russ had previously built a hardtail under Brian’s guidance and wanted to step it up a notch, knowing that he had the backing of both resources.

The frame was TIG welded using ER70-S2 filler, except for the headtube reinforcment rings and the seat tube collar. These were silver soldered in order to create clean joints without putting as much heat into the frame.

The fully assembled frame (with shock) came out to just shy of 8.5 lbs (3.8kg).
While the frame lacks polish its elegance stands out.
While the frame lacks polish, its elegance stands out.

The swing link, rocker, and thru axle were machined from 6061-T6 aluminium using a CNC mill. In addition, custom soft jaws had to be machined in order to hold the rocker for its second milling operation. The main pivot, rocker pivot, and dropouts were machined as well, but from 1020 mild steel.

The rocker required extensive machining, including custom soft jaws to hold it in the mill.
Several operations were needed to produce the finished part.

Russ and his teammate Jacob Vuillemot spent most of a semester in the machine shop, learning how to program and operate CNC equipment. The pair regularly were at the shop when it opened at 6AM in the morning and were some of the last to leave when the shop closed at 8PM. They took turns watching over the machines and attending classes. Over the course of the semester, amid a small pile of broken endmills and failed parts, the frame emerged piece by piece. The rocker and soft jaws alone took over 40 hours to machine.

The raw materials for the frame portion of the project were backed by steel giant Arcelor-Mittal. The budget for rest of the project was a mere $300.

The bones of the full-suspension, ready for assembly.
The bike was created by Russ as a project for Michigan Technological University with the support of Arcelor-Mittal for materials and Brian Eggart s framebuilding experience. It bears the badge of Brian s personal label Equinox.
The bike bears the badge of Brian's personal label, Equinox.

Lessons Learned
The bearings and hardware used in the pivots were cheap standard parts due to the limited budget. The use of Enduro MAX bearings and dedicated pivot bolts would improve durability greatly.

A lot more heat than necessary went into the frame during welding, something that will only get better with practice.

Additional bracing would likely be needed if this frame were to be used extensively. A brake brace and seatstay brace could be added in the rear triangle and a seat tube brace at the top tube junction could be added in the front triangle.

The bike in it s finished state.
The bike in its finished state.

The frame is unfinished and has developed a characteristic rust patina.
The frame is unfinished and has developed a characteristic rust patina.
Due to clearance issues with the shock the linkage had to be machined thinner than initially expected. This could be easily resolved in the future.
Due to clearance issues with the shock, the linkage had to be machined down.

The machined steel seatstay yoke main pivot is a thing of beauty.
The machined steel seatstay yoke/main pivot is a thing of beauty.
Details of the suspension design.

The single-piece rear triangle.
The single-piece rear triangle.

What Next?
As far as the full suspension goes, it is destined to collect dust in a lonely closet at Michigan Tech, at least until the next young mountain bike enthusiast comes through senior design. Russ is finishing up his degree this semester, but is anything but done building bikes. He has teamed up with Brian, his framebuilding mentor, on a venture involving steel hardtails… check out moonlightcycles.com.


2 Comments

  • 1 0
 That's a really cool project. It would be cool to see him be able to finish the design some day with an existing brand where they could attach this story to the bike.
  • 1 0
 Russ did an amazing job. He did apply for work at some smaller manufacturers but there really aren't a lot of openings for engineers fresh out of college. There will be more from him in the future, at least as hobby projects.

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