From Prototype to Production… A Case Study of the Evolution of a Design


Lean Machine pioneered the technology for affordable low-inertia resistance exercise in the home-user market.  Exploring the various stages of Lean Machine’s development will highlight the interrelationship of technology, design, manufacturing, and marketing.  Corporate strategy and identity also play key roles, both in the types of products a company develops, and in the way in which a particular product idea is rendered by its designers.

Why Low-Inertia Exercise

Traditionally, resistance exercise (weightlifting) relies on heavy weights to provide the resistance. But iron weights are costly to manufacture and ship, and the weight machine must be built to withstand the mass and impact forces of the moving weight stack.  And more importantly, weights also introduce the counter-productive phenomenon known by fitness experts as the adverse effect of inertia.

The adverse effect of inertia is caused by the forces that result when the heavy weight changes direction or speed during an exercise movement.  A 100-pound weight, for example, delivers 100 pounds of resistance only when the weight is at rest or moving at a steady-state speed.  But during exercise, weights are almost always either accelerating, decelerating, or reversing direction.   So a 100 pound weight rarely delivers 100 pounds of resistance.  Sometimes the resistance is far less (letting the barbell drop from the chest on the negative stroke of a curl, for example), and sometimes the resistance is significantly greater (bringing the barbell to a quick stop at the bottom of a curl).  Click on Figure 3 in the left column for a graph of the resistance fluctuations during a correctly performed (slowly) bench press on a weight machine.

Not only does the adverse effect of inertia reduce workout benefits, inertia problems are also responsible for many weightlifting injuries.  And because weight systems are limited to slow movements, it is virtually impossible to do aerobic exercise on a weight machine.

Shortfall of Conventional Elastic Resistance Machines

The adverse effect of intertia can be eliminated by replacing weights with elastic elements (springs, rubber cords, etc).  But simply making the switch to elastic resistance does not, by itself, result in a proper low inertia exercise machine.  In fact, it introduces a new problem that is even more counter-productive.

All elastic elements have an intrinsic characteristic know as “spring rate.”   The spring rate is the rate at which tension increases as the element is deflected.   When elastic resistance is directly applied, this characteristic makes it impossible to maintain a particular resistance level over the exercise movement.  As the element is deflected, resistance goes up, and as it is relaxed, resistance goes down.  Because of spring rate, users are forced to select a resistance that will be within their strength capability at maximum deflection, or the exercise movement cannot be done at all.   As a result, during the first 50 percent or more of the movement,  resistance is too low to produce much benefit.  Figure 4 shows the resistance curve of a popular exercise machine that uses rubber elements for resistance.

The Lean Machine Solution

Inertia effects are solved in Lean Machine’s design by using inexpensive springs instead of weights to provide the resistance.  The negative effect of spring rate is eliminated by using a patented spring-rate-canceling cam to deflect the springs.  When the springs deflect, the cam guides the connection point inward toward the axle at a rate that precisely cancels out the increase in tension.   In this way, the cam maintains a consistently uniform level of resistance throughout the entire range of both positive and negative exercise movements (click on Figure 5 for an illustration).

Precise resistance selectivity is provided by a secondary idler arm (the counterforce arm in Figure 5) having an adjustable input connection point.  When the input is positioned nearer the pivot, resistance is lower.  When it is positioned farther from the pivot, resistance is greater.  Resistance is thereby infinitely adjustable up to 200 pounds; or greater, depending on the design of the springs. A scale on the side of the arm shows the selected resistance in pounds.

First-Generation Production Model

One of the keys to pioneering a new product is to minimize early investment before the market has been  proven.  The design of the first-generation production version of Lean Machine was therefore dictated mainly by financial constraints, which were a result of marketing uncertainties.  The primary marketing uncertainty of Lean Machine emerged from the fact that it was based on new and unfamiliar technology – and the fact that weightlifting (at the time) had significant macho overtones.  Management was uncertain as to whether a “real man” would feel as good about “pumping coils” as he would about pumping iron, regardless of the greater benefits.

To minimize expenses for tooling and fixtures, the first production version was designed around cut-and-weld manufacturing processes and off-the-shelf components.   Resistance springs, for example, were stock garage door springs.  The plastic housing that contained the springs was cut from 20-foot lengths of sewer pipe.  The large round pulley and elliptical cam were hand fabricated from laminated blanks of black acrylic.  Lean Machine was put into production on a budget of just slightly above zero for tooling, fixtures, and equipment.

Early direct response marketing brochures are shown in Figures 6 and 7.  Once a sales history had been established, Lean Machine was quickly slated for redesign, and a budget for tooling and fixtures was allocated for the new model.

The Lean Machine Pro

The primary objectives for the design of the second generation product were to simplify the resistance system, reduce manufacturing costs, and improve the product’s appearance.   Focus groups had indicated that consumers were turned off by exposed pulleys and cables. So a covered upper pulley system with a self-retracting cable was also on the wish list.

New Styling

Prior to the Lean Machine Pro, styling was almost an afterthought  in the design of fitness equipment.   Our sense of it was, like the body the machine is designed to create, the equipment itself should also appear strong, graceful, and well proportioned – a look of elegance in form and function.  But while it is easy to make an elegant statement about appearance, transforming the idea into a real product is more difficult.

A full-size non-working mockup of the new design was delivered within six weeks.   The architecture and ergonomics of the design were already well established.  But detailed engineering still had to be done onthe resistance package, which had been reduced to a simple canister containing two torsion springs, one on each side of a centrally located cam.

Simplified Resistance System

On the first Lean Machine, the cam pulled against a pair of extension springs via a cable.   So the spring rate was cancelled on the input side.  Linear torque was then transferred through the frame to a round output pulley on the other side where a second cable transferred torque to the idler arm.   With the new design, the round pulley was eliminated, springs were connected directly to the cam, and the cam was connected by a single cable to the idler arm.  Rate-canceling was therefore handled on the output side rather than the input side.  This reduced the entire resistance system to a relatively simple, self-contained canister (see Figure 9 to the left).   The system was cycle-life tested to one million cycles.

Self-Retracting Upper Cable

One of the most challenging tasks was the request for an upper  cable system with a self-retracting cable.   Early on, the option of a spring-loaded reel was discarded as too costly and too prone to malfunction under the loads experienced in the application.   Ultimately, however, the challenge yielded to the most elegantly simple solution; one that was trouble free and cost only about 50 cents to manufacture.   We simply attached a weight to the fixed end of the cable, just behind the point where it exits the frame, and let the falling weight pull the cable inside the frame.  To connect the upper cable to the idler arm, the user simply grabbed onto the connector and pulled it toward its connection fixture at the end of the idler arm.  To stow the cable, the user simply unsnapped it from the arm and let go of it.  As the weight fell, the cable disappeared inside the frame.

A commitment to production was made in June, just weeks after the mockup was delivered.   By September, parts procurement was already underway, even though some components were still being designed.  With December delivery, The Lean Machine Pro quickly became a success with consumers.    Switching from direct response advertising to retail dealers, a network of 400 retail dealers was established within the first six months.

SCA Health and Fitness

The Lean Machine Division of Inertia Dynamics Corporation (now Ryobi) was ultimately purchased by SCA Wolff System (tanning bed distributor) as the beginning step in the formation of SCA Health and Fitness Corporation.

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