Frame Materials

Titanium

Torus titanium bicycles are fabricated from plain gauge titanium alloy.  The main frame, chainstays and seatstays all use Ti-3Al-2.5V tubing, while the headtubes, bottom bracket shell and dropouts are fabricated from the Ti-6Al-4V alloy.  Why?  In our opinion, this provides the perfect compromise between weight, strength, stiffness and cost. 

So why not use butted tubing?  There’s no need.  Titanium is not as dense as steel and, therefore, can be fabricated from thicker non-butted tubing without suffering a massive weight penalty.  It’s also stronger than aluminium, so the tubing does not need to be made any thicker near the stress points. The resulting plain-gauge frame should be strong with comparable stiffness, but still lighter than steel.  In our opinion, therefore, the performance advantage of butted tubing does not justify the additional expense.

 

Steel

Torus Bicycles use a selection of the finest tubing available to fabricate its steel frames.  All our frames are fillet brazed to provide frames that are light, stiff and also aesthetically pleasing.

The choice of steel tubing depends very much on the intended application:

For our road frames, we selected Columbus Spirit triple-butted niobium steel tubing.  The wall thickness in Columbus Spirit tubing is reduced to just 0.38 mm, providing a very lightweight tubeset, while the impressive breaking yield and fatigue characteristics of the niobium steel alloy maintains an excellent strength/weight ratio.  This results in a lightweight, but strong, frame that’s the ideal starting point for a top-end bicycle.

For our mountain bike frames, we needed something a little more robust.  We selected Reynolds 520 butted chrome-moly tubing, which allows us to fabricate frames which are light and stiff, but still competitively priced.  Reynolds say “for cycling, our tubes are mandrel butted for accurate profiles and available in a wide range of shapes. Weight savings from butting provide lightweight framesets.  The Reynolds "520" range uses the same alloy, made under license in Taiwan and subject to the same quality standards”.

Why not Reynolds 853?  Most manufactures producing high end steel frames use Reynold’s 853 tubing, which is an excellent air-hardening tubeset.  Reynolds say “the benefits of air-hardening steels are particularly noticeable in the weld area where, unlike conventional steel alloys, strength can actually increase after cooling in air immediately after welding”.  Since our steel frames are all fillet-brazed, and not welded, the key benefits of 853 are lost and simply do not justify the additional cost.  By fillet-brazing Reynolds 520 butted tubes, we can produce frames of outstanding performance and quality, at a competitive price.

 

Which material is right for you?

When it comes to choosing a frame material for your bike, there is no right or wrong answer.  The choice is as much about personal preference, as it is about material performance.

Let me try to explain.  There are currently four major materials used in frame construction: steel, titanium, aluminium and carbon fibre (in addition to a host of less common alternatives).  You’ve probably heard numerous suggestions, such as: steel is heavy; aluminium gives a harsh ride; titanium is too flexy; and carbon fibre just isn’t robust.  These are all falsehoods (or at least gross exaggerations, based on a bad experience with a poorly designed product). 

The performance of a bicycle frame lies more in its design than it does on the physical properties of the material it’s constructed from.  To demonstrate, let us consider some of the material properties that are relevant to frame design.

These properties, which are outlined in the table below, describe the critical performance parameters required of a bicycle frame, namely: weight, strength and stiffness.

Material	Modulus of Elasticity (GPa)	Yield Strength (MPa)	Tensile Strength (MPa)	Fatigue Strength at 50,000 Cycles (MPa)	Density (kg/m3)
Aluminum – 6061-T6	72	193-290	241-320	75	2,700
Aluminum – 7005-T6	72	290	350	~75	2,780
Steel - 4130	205	800-1,000	650	250	7,800
Titanium – Grade 9	91-95	483-620	621-750	250	4,480
Carbon Fibre	275-415	Varies	Varies	Varies	1,800

Ref: Forrest Dwyer, Adrian Shaw and Richard Tombarelli, Material and Design Optimization for Aluminium Bike Frames, Worcester Polytechnic Institute, 26th April, 2012.

 

Stiffness is determined by a property of the material called the modulus of elasticity, which describes how “elastic” a material is - i.e. how much it will bend or stretch and still return to its original position.  The modulus of elasticity is relatively independent of the quality, quantity or number of alloying elements in the material.  Stiffness is an important material property for bicycle frame design as it can affect the “ride quality” of the bike, whether in terms of comfort, control (handling) or feedback.

Strength determines how robust a frame will be, whether it will be fit for purpose, and provides a measure of how long it can be expected it to last.  The strength of a material can be determined from its yield, fatigue and tensile strength values.  It should be noted that strength generally has no effect on the ride quality.

• Yield strength is defined as the amount of stress that a material can undergo before moving from elastic deformation into plastic deformation – i.e. permanently deforms.
• Tensile Strength is defined as the stress limit at which the material breaks.
• Fatigue strength is defined as the level of stress that a material can endure for a given number of loading cycles before failure – i.e. resistance to repetitive use.

Weight is determined by the density of the material – i.e. the mass of a given volume of material.  The weight of a frame is often a major concern for the performance oriented cyclist looking for a racing advantage, but also for those of us you would rather not pedal excess weight up those hills. 

Each material has a different combination of these properties.  So how does this affect the performance of a bicycle frame?  To answer this question, let’s compare frames that are constructed from these materials, with an identical design, size and tube profiles.  Based on the data in the table above, we could make the following predictions:

• Frame weight would follow: carbon fibre < aluminium < titanium < steel
• Stiffness would follow: carbon fibre > steel > titanium > aluminium
• Strength would follow: steel > titanium > aluminium (carbon fibre can not be classified, since it would depend on type and lay up)

i.e. carbon frame would be lighter than steel, and stiffer than aluminium, but not as strong as steel.  We know that this does not describe the whole picture – aluminium frames can be built to be very stiff and plenty strong enough to last, while some cheaper carbon frames are as heavy as quality steel frames.  Hence, the performance of a bike frame lays more in its design than its material properties – the tube lengths, angles and profiles (shape, diameter, wall thickness and butting); the way in which these materials are used to create the frame.

 

You can’t really judge a frame by the material it’s made from alone  -  you need to consider the design as a whole, the material used and the way it's constructed. 

However, we can make some hints and suggestions about the various advantages and disadvantages of frames that are constructed from these materials (with a typical design), while avoiding the all too common BS, like light or stiff or fast or comfy, that can be true of any well designed bike.

Aluminium

• Easily formed (e.g. hydroformed) into custom profiled shapes
• Relatively good value for money
• Relatively easy to repair

Steel

• Tried and tested tubing and designs
• Susceptible to corrosion
• Relatively easy to repair

Titanium

• Great balance of weight / stiffness / strength
• Corrosion resistant
• Relatively expensive to produce / buy
• Relatively easy to repair

Carbon Fibre

• Easily laid up (moulded) into custom shapes
• Frame characteristics (e.g. strength or stiffness) easily controlled
• Corrosion resistant
• Relatively less resistant to large impacts
• more difficult to repair

 

So, in that case, why do we construct our frames from steel and titanium?  Well, that because it’s what we know and where our experience lies.  Our chief frame designer, Justin Burls, has over fifteen years experience in the design of custom geometry bicycle frames, for a multitude of disciplines, and is also accomplished in the art of fillet-brazed steel.  More recently, Justin has moved towards titanium and has produce over 300 custom titanium frames.  It is this experience and knowledge that makes us confident that we produced a range of bicycle frames, in both steel and titanium, which are amongst the best frames currently available.

That’s not to say that we’ll not consider carbon fibre or aluminium.  We’re working on that as you read this!