Let’s say you run across a lightweight, inexpensive, easy to manufacture metal that you are considering for a new bike frame. Beyond the functional considerations of strength, size, and finish options what else do you consider?
Is it durable? If it fails how does it fail (a shattering a bicycle frame would not be good, for example). You may also consider how the bicycle will be used and stored. What stress will the frame experience over its lifetime?
The Stress versus Strength
The concept of stress/strength curves come to mind. At the moment you have the material’s data sheet which describes tensile and bend strength plus a range of other material properties. Once formed into a frame there are a range of tests to conduct to determine how the system once built will perform. Basically, these tests will help estimate the strength curve.
What about the stress curve?
Where does that come from? How strong is strong enough? If the frame is able to withstand 100 shock loads similar to hitting an 8-inch high concrete curb, is that good or not? How many times will riders in this market strike a curb over the life a bike?
The process to estimate or determine the range of loads is different than evaluating the strength part of the stress/strength relationship. We need to consider the rider’s behavior plus the environment.
Creating Stress Curves
For a specific model of bicycle typically used by a specific type of rider, we may very different sets of stress conditions and behaviors.
A child’s first bike may experience very low power exerted to the pedals, maybe10 to 100 watts; while a world class competitive rider may impart close to 1,000 watts at times. The force across the frame is dramatically different.
A combination of observations, measurements, simulations, and studies allows your team to build a set of expected stress conditions for different classes of products. The weight, frequency of riding, distance traveled, and the many other factors that shape the set of stress conditions are different for each family of bicycles.
One approach is to understand the limits of expected stress and design a product to always meet those limits. Designing a child’s bicycle based on the competitive rider’s set of stresses is not practical.
Designing, building and testing a product to meet the customer’s set of stress conditions is a common approach.
Collect the Stress Conditions
Each bicycle frame has a target rider in mind.
An avatar or typical user of the specific style of bicycle. At first, the team may make some educated guesses about the behavior of the rider and where and when they ride or store the bicycle.
Over time and with some investment you gather information about the ranges of specific stresses that impact the performance and durability of the bicycle family of designs. Minimum and maximum, and the distribution of temperatures during riding and storage, for example.
The list of stresses that apply may include weather related conditions, like temperature, humidity, rain, snow, ice; plus conditions related to use, such as shocks due to street curbs, drops, or transport on a car rack. Stress also varies by the style of riding. For example, a recreational rider may only lightly stress the mechanical loading of the frame, yet a professional rider will notice the slightest change in flex across the frame as it impacts power transfer and handling.
Gather the data related to performance, to durability, and to reliability.
What sets of stress may incrementally damage the materials, embrittle metals or welds, fade paint, or impede smooth operation? Characterize the stress with minimum, maximum, and nominal values. Plus work to determine the distribution of values across the range (stress curve).
Benefits of an Environmental Manual
The environmental test manual tabulates and describes the stress curves for the various types of riders for each family of products.
The document provides a repository for the conditions the product may experience during transport during the sales process or during use, plus storage and use conditions.
The testing focuses on measuring the strength curve related to a specific stress curve. This permits the testing to make a conclusion. It the strength of this design sufficient to withstand the expected stress curve initially and over time.
An added benefit of tabulating the stress curves is it provides the entire team with a common set of conditions, clearly spelled out and supported. It helps avoid the issue where one designer believes the design will be strong enough when mistakenly considering only the average weight of riders, and not the proportion of heavier riders.
The benefit of having distributions rather than limits alone is the ability to balance the expected failure rates with the proportion of customers that may experience failures.
While it may be uneconomical to create a recreational bicycle frame that performs well for a person on the maximum power (force on the pedals) end of the scale, there highly are not many 500+ watts (high sustained power) bicycle riders in the recreational market.
Each product is not unique when you are serving the same market. Relearning or discovering the set of expected stresses is not necessary. Create an environmental profile and the associate strength testing for each distinct market. As materials, processes, and markets change, update the manual.
An environmental test manual is like an internal standard, for your products based on your understanding of your customers.
Publicly available standards provide a starting point, yet will not provide sufficient insight and information for your product in the hands of your customers.
Use and Environmental Profiles (article)
Reliability Testing (article)
Basic Approaches to Life Testing (article)
James Bergman says
I certainly hope that there is a good computer program for figuring all of this out based on the metal’s properties. There are a lot of variables that have to be considered. However, I do agree that regardless of how the stress curve is figured out, it is important to figure it out. I just don’t envy the person doing the math.
Fred Schenkelberg says
Thanks for the comment James, and yes the math can be a bit much at times, yet worth the effort as you and team will be able to make better decisions going forward. cheers, Fred