Behind the Design - Not All Steel Frames Are the Same
The thought process behind the selection of high-quality steel tubes at Panorama Cycles.
You may have already noticed that some steel bikes are heavier than others. Or that some bikes, still made of steel, are more 'flexible' than other models made of the same material. This is because there are several types of steel tubing, which differ in their level of quality and properties.
At Panorama, we exclusively source from the English company Reynolds for their high-quality steel tubes. And we select the range according to the characteristics we want to give to each bike.
In this article, we discuss the characteristics of different types of tubing, what we consider in the design of our bikes, and why not all steel frames are of the same quality.
Words : Simon Bergeron, Owner and Frame designer at Panorama Cycles
Photo Credits: Serge Gouin
The Properties of Steel
For several decades, steel has been the material of choice for bicycle frame construction. However, significant innovations in tube manufacturing now allow bicycle manufacturers to fine-tune the properties of the frame according to desired objectives.
A bicycle frame, regardless of the chosen material, must meet two main criteria: it must be sufficiently rigid to be ridden properly and sufficiently strong to support various loads.
Stiffness: The Importance of Tube Shape, Not Steel Grade
First, it's important to understand that all types of steel have the same rigidity (Young modulus = +/-200 GPa). Whether it’s the complex Reynolds steel alloys (953/853) or the common construction steel used for fences or frameworks for example, all have the same rigidity.
The rigidity of steel in a structure like a bicycle frame therefore comes from the tube profile, which includes the diameter and/or wall thickness. Comparatively, steel is much stiffer than aluminum or titanium, which is why tubes are often slimmer on a steel bike.
The desired rigidity for a bicycle frame (the tube profile) is primarily determined by the intended use, depending on whether we are looking to maximize power transfer or to prioritize comfort, and whether we want the bike to be able to handle a heavy load or not.
But, since the rigidity is the same for equal tube profiles, regardless of whether the steel is purchased from a hardware store or a renowned brand like Reynolds, why pay for high-end steel tubes then?
Ultimate Strength: What Makes All the Difference
The parameter we play with by using high-end Reynolds steel tubes is the ultimate strength, which is the maximum stress that can be applied to the tube before it fails.
Graph of Different Reynolds Ranges in relation to Their stiffness and strength:
At Panorama, we use either Reynolds 525 tubes, which increase strength to 800 MPa (versus 250 MPa for conventional steel), or Reynolds 725, which more than quadruples strength to 1080 MPa.
This drastic increase in the ultimate strength of Reynolds tubes versus standard mild steel therefore makes it possible to minimize weight, by using thinned tubes that can be both sufficiently rigid and strong for the intended use.
In fact, the tubes used on our bikes have variable thickness ("butted"), meaning the thickness varies along the length of the tube. The tube is thicker at the welds and thinner in the middle, where less stress is exerted.
We work with Reynolds for our tube selection because this company, founded in 1898, offers an immense choice of tube profiles, allowing us to find the exact profile that provides the precise frame rigidity and strength properties we aim to achieve.
What to Consider for Touring and Bikepacking Bikes: Yield Strength vs. Ultimate Strength – or Why Reynolds 853 Is Most Likely Not the Best Choice
Yield strength is a factor not to be overlooked, especially for adventure bike frames. Yield strength is the stress that can be applied to a tube before it deforms permanently without breaking.
The biggest issue is that the yield strength for the Reynolds 853 or 953 grades is too close to the ultimate strength. These very high-grade alloy steels are often considered brittle.
On a trip, if the rider needs to make an emergency repair on the bike (let's say cut or drill tubes) or install a narrower hub (thus bending the rear triangle), it can become more difficult, if not impossible, without exceeding the ultimate strength and breaking the frame.
Another aspect we consider in our tube selection is dent resistance, or when the yield strength is reached on the tube. We always keep in mind that the force needed to dent a tube is affected by the use of extra-thin tubes (despite the increased strength properties in high-end steels like 853/953).
So here are the main considerations we take into account when selecting tubes for our bikes.
For our gravel and MTB bikes, our selection of Reynolds 725 steel tube profiles offers, in our opinion, the best compromise between weight, ride comfort, strength, and field reparability.
For our touring frames designed to be much stiffer, the margin in terms of ultimate strength obtained by default is much greater. Since the priority for these models is not to save a few grams (for a bike that will carry 50+ lbs of gear in touring) but rather to have bikes that are as resistant as possible to loading and easily repairable, Reynolds 525 is the best choice.
