Speed Vs Stability - GCN at The Bicycle Academy

We have recently worked with the GCN youtube channel to produce a video about road bike geometry.  They asked a few questions that we address on a weekly basis here at The Bicycle Academy. We were keen to provide some further reading and a little clarification of the topics covered in the video. Simon made a small mistake in the video, see if you can spot it.

We were asked to shed some light on some of the terminology used to describe the performance of a bike by its rider. How much does a bikes geometry affect it's handling? Can it have a big impact on your overall speed? On the face of it these questions are fairly straightforward but the answers are, in fact, quite complex. Simon brought two of his personal bikes to The Bicycle Academy in order for us to help him understand their individual handling characteristics and to draw some comparison between the two.

To address the question we have to first look at some key dimensions that carry influence over vehicle dynamics.  This is not an exercise in understanding how a bike functions but a look at the relevant dimensions that can be changed, within the relevant constraints of bicycle design, in order to alter its dynamic performance.

The complex part comes when considering rider interaction with these dimensions and how they might be interpreted.  One of the challenges here is that it can become difficult to isolate all of the overlapping influences to attribute various vehicle behaviour and rider experience to specific dimensions.  To assist with this we have produced two seperate frames that change only one of the major dimensions outlined below. We chose to isolate trail as it is an interesting example of a dimension that produces what, in static terms, is a relatively simple behaviour (see below for more on that) but actually contributes in a more complex way to the rider’s experience of the bike. Stay tuned for a future video on this.

Wheelbase

This is the distance between the two contact patches of the tyres.  It has influence over the directional stability of the vehicle. The greater the distance between the two contact patches, the better the vehicle will continue along its current path (straight line or otherwise).  This dimension is very powerful in terms of its influence over the handling of the bike, and riders will typically be able to notice a change as small as one or two percentage points. In the context of the video the Trek has a wheelbase of 1029mm which is 2.8% longer compared to the Canyon’s 1001mm. As a result we can conclude - as Simon has from riding it - that it has greater directional stability than the Canyon.

Bottom Bracket drop

This is the vertical measurement between the wheel axles and the centre of the bottom bracket.  This dimension has influence over the height of the rider, more specifically the centre of mass relative to the ground. This contributes to the stability of the vehicle.

In the video Simon refers to a high centre of mass being more stable than a low one (that's the little mistake he made). If considering an isolated static system this is not actually correct, and an object will be more stable as its centre of mass approaches is ‘footprint’ or point of support.  This is, of course, true also of a bike - however the picture is a little more complex than an assessment of the static stability. We must instead think about the dynamic system of rider and machine combined, supported by a 'point’ contact, and how the reality is that the system is one that must be able to iterate towards a stable solution.  In this instance what would be deemed to be static instability can improve a human’s ability to control the system and thus improve the ‘control logic’, which is often what is referenced by the rider as stability.

To illustrate my point here imagine (or try) standing a hammer on one end.  If you were to stand it on a table it would be significantly more stable if the handle were above the heavy head. This is because its centre of gravity will be lower which improves stability in this static situation, there is no question about this.  Now balance the hammer in a dynamic situation by placing it on the end of your finger. You will notice that it is easier to balance with the head above the handle. This is not because it is more stable in a static sense, but the dynamic system of you controlling the movement of the point support of the hammer produces what could be perceived as a more stable result.  This is because the greater distance between your finger and the centre of mass means that larger movements are required from your hand to regain static stability when the hammer starts to fall. This in turn means that the system is inherently less sensitive to small movements, which makes it easier for us to coordinate iteration towards a stable solution.

During real world use there are constantly changing forces on the bicycle meaning that it is never (or very rarely) perfectly balanced.  The fact that it has the ability to move its ‘footprint’ - via turning the front wheel - means that it can continue to remain upright by adjusting it’s contact patch to be underneath its centre of mass (for example; as the bike leans to the left the bars will flop to the left and the bike will head left).   So, in the real world, when our contact patch is always moving around slightly due to the various forces exerted on it by both rider and the environment, having a sufficiently high centre of mass is actually beneficial to effectively dampen this movement and provide a dynamic system that has better ‘logic’ for the user.

In practice the bottom bracket height from the ground (and therefore drop relative the the axles) is also limited by our need to pedal and lean the bike without our feet striking the ground.  This limit tends to make the centre already sufficiently high to work as described above and so it becomes reasonable to assume that a lower bottom bracket will generally result in a more stable vehicle overall.In this comparative example the Trek’s BB sits 8mm lower relative to its axles than the canyon representing a 10.3% change from 70 to 78 mm.  This will again contribute to the greater stability reported by Simon riding the bike.


Rear and front center

These are the portions of the wheelbase measured from the rear and front axles to the bottom bracket respectively.  These measurements can be taken as directed straight line measurements or, perhaps more usefully, effective horizontal ones.  The relationship between these two dimensions will dictate what percentage of the riders mass is supported by which wheel. It is actually more relevant to measure these from the centre of mass of the vehicle but when considering a bicycle frame in isolation  that is not an easy measurement to take. By referencing the bottom bracket center we are assuming that the rider’s centre of mass remains a relative constant from here, which given the nature of the bikes and how they are ridden is a reasonable thing to do.

There is a bit of cross-over to consider here with the structure of the frame;  a short tube will deflect less in absolute terms along its length for a given load and so one easy way of reducing the extent to which the chainstays will twist under load is to make them short. This is often a motivation for keeping their length to a minimum. A short chainstay also places more weight on the rear wheel, this improves traction and positions the rider toward the back of the vehicle which tends to improve the steering logic.

The front center has influence over the handling of the vehicle in a slightly more complex way, as it ties in with some key aspects of the steering geometry.  Most notably the front center dictates the amount of weight that acts over the ‘trail’ of the front wheel which is outlined below.

In this example the longer wheelbase of the Trek comes as a result of increasing both the front and rear centres.  What is important to note here is the relative change of both and interestingly these are very similar between the two bikes;

Model

Horizontal component of rer center as percentage of wheelbase

Horizontal component of front center as percentage of wheelbase

Trek

40.7%

59.3%

Canyon

40.3%

59.7%

This means that, assuming that Simon’s position is the same on both bikes, he will have a very similar amount of weight distribution over the wheels of both bikes.

Trail and wheel flop

Trail is the measurement taken along the ground between the contact patch of the tyre and the intersection of the steering axis of the front wheel. This dimension ensures that the contact patch of the tyre will always trail behind the steering axis so that any external force on the wheel or rider driven movement of the steering axis results in predictable front wheel rotation.  Mechanical trail is an alternative measurement that is taken between the contact patch of the tyre and the steering axis at 90 degrees to the steering axis.

The influence of trail over steering geometry is affected by its magnitude and the weight that acts over it.  A bike with more trail and/or more weight over the front wheel is sometimes loosely referred to as having ‘heavy’ steering, it will have a greater response to any force that moves the contact patch away from its position trailing the steering axis.  This force could be an external force like uneven terrain or a gust of wind or a rider driven force like a change in direction. As the contact patch moves from behind its steering axis (relative to the movement of the vehicle) the trail creates a force that tends ‘straighten’ the wheel returning the contact patch to its trailing position.

Wheel flop refers to the tendency of the wheel to ‘fall’ or ‘flop’ to one side as the bike is lent over in that direction.  It is sometimes referred to as a measurement of the vertical component of mechanical trail. It is thanks to this that we can steer the bike by leaning it to one side or the other, just like when you push your bike by holding saddle and steer by leaning it.  Wheel flop and trail are intrinsically linked and work together to allow the vehicle a degree of autonomous stability.

It is important to note that trail and wheel flop are a function of both head angle and fork offset.  People often reference head angle alone as a measure of how ‘fast’ or ‘light’ the steering of a bike might feel.  It is true that a steeper head angle might produce this sensation as there is less ‘wheel flop’ but it is too simplistic to consider head angle alone. The combination of head angle and fork offset which produce trail and wheel flop all work together to create a certain steering characteristic for a given weight distribution.

In this specific case it is where we can see some of the largest differences between the two bikes:

Trek

Canyon

Percentage change

Head angle (degrees)

72.8

73.3

0.68%

Fork offset (mm)

53

44 (this is adjustable but the 44mm setting was used with other measurements

16.98%

Trail, with 700*25c tyre. (mm)

49

55

10.91%

Wheel flop, with 700*25c tyre. (mm)

14

15

6.67%

We can see here that the trail produced by the steering geometry of the Trek produces a significantly lower trail and wheel flop.  This means that the wheel will have less tendency to straighten itself. At face value this could be interpreted as a bad thing, but consider here that each time the trail acts the rider will feel the force acting through the handlebars.  If the bike is being ridden over rough terrain the contact patch will be constantly moving and there will be many external forces on the wheel that will create a response from the trail, each of which will be felt by the rider. So, much like the consideration for the height of the center of mass, it could be that by reducing the self straightening tendency of the front wheel the rider is better able to control the steering over rough terrain simply because they don’t get such large inputs from the external forces trying to turn the wheel. For the rider this is likely to produce a sensation that is sometimes described as ‘lighter’ steering. The bike has less autonomous control over the bars and so the rider will get less feedback through the bars from any steering input change to the direction of the bike.

The can of worms

During the video Simon concludes that he has opened a can of worms by investigating steering geometry. Simon asks what are the key things a person buying a new bike can look for on a geometry chart.  The short answer is that ‘it depends’. Some viewers commented that maybe it just doesn’t matter. It absolutely does matter and indeed the geometry of the bike is the most powerful way that we can change the riders experience of the bike. To think only in terms of the static measurements outlined above and their influence is to misunderstand the question: really what we are being asked is bigger than that as it's about what the rider feels and whether or not they would describe the bike to be a certain way. 

The big variable that is difficult to quantify is the riders expectation and preference. Some riders will feel more confident handling a bike which has more directional stability and as a result will ride it faster, others will prefer having a bike that is more agile and responds faster to their input and they will ride it faster as a result. 

It is also very difficult to draw conclusions about a bike’s performance based on someone else’s experience of riding it:  If you are a short rider and read a review of a bike written by a tall journalist you are not reading a review of the same geometry; even with attempts to scale geometries to account for size variation the physical constraints of wheel size, crank lengths etc etc mean that it is not possible to create like for like geometries across a size range.  Even for two riders of the same height this is a difficult thing to do; if one has a long torso relative to their leg length and rides with less saddle setback they will have a higher percentage of their weight on the handlebars and therefore front wheel (irrespective of the front center). This will change the weight acting over the steering geometry and create a different experience for two people riding what is otherwise the same bike. 

The easiest way to understand and interpret geometry is to give yourself a frame of reference that is specific to you;  your existing bike for example. From here by looking at some of the critical dimensions outlined above, you can draw comparisons between different geometries that are more relevant to you and what you like in a bike. In the following paragraph I have compared both of Simon’s bikes as an example of this process.

The comparison

As several can attest, both the Trek and the Canyon can be ridden fast, and have been ridden to victory in similar races.  The Trek is an interesting example of a bike that was developed for the specific requirements of a specific rider (Fabian Cancellara) with specific events in mind (spring classics) which gives us the opportunity to consider why it has departed from the more ‘standard’ road geometry.

Spring classics feature very variable terrain and surfaces that need to be tackled at speed often in a tight bunch of riders.  Normally riding in a peloton is one of the key requirements for a short wheelbase and therefore a bike that will change direction quickly and allow safe navigation through a pack.  However when the terrain gets rough it could be advantageous to have a bike with greater straight line stability that will not get knocked off course by the terrain, or indeed other riders.  Looking at both the longer wheelbase and lower bottom bracket we can conclude that the Trek would achieve this over the Canyon. Another element of successfully holding a line over rough terrain is the steering geometry itself;  here Trek have opted to reduce the trail and wheel flop significantly so that there is less ‘feedback’ through the bars from the inevitable highly variable forces put through the wheel by the rough terrain. This low trail could also, although this point is open to individual interpretation, help to offset the sensation of loss of agility from the longer wheelbase to potentially help handle the bike in a tight pack of riders and restore some of the sensations that are familiar when riding a ‘race bike’.  Critically this change was made without altering the relative weight distribution over the wheels compared to the Canyon. This helps us draw conclusions with greater confidence as the relative weight acting over the steering geometry is not having a further influence.

It is very difficult to state, at face value, which bike might be better for a specific individual. We can, however,  see that by looking at some critical relative dimensions we can start to understand how one vehicle might behave compared to another and build up a picture of whether those changes are a good or a bad thing for the task that we are asking of that bike. 

Want to know more?

Many of these concepts can be tricky to fully understand and visualise.  At The Bicycle Academy we have started building a series of bikes for students to trial so that the effect of these individual components can be experienced first hand. The opportunity to ride almost identical frames back to back is a very powerful way to get a real understanding of what that one dimension can change.

One of these ‘pairs’ will feature in an upcoming video with Simon where he will have the opportunity to experience the influence of mechanical trail on steering geometry. This is a good example of a dimension which has, on the face of it, a simple role to play in providing stability to the front wheel yet one that has quite a complex impact on the experience of the rider using the bike.

As any good frame builder can testify, it is near impossible to truly predict the riding experience that a frame will provide, the human component of the bicycle is so varied. What we do know is that every rider will respond favourably to certain frame geometry characteristics. A skilled and experienced frame builder can help a rider to identify these particular characteristics. Teasing out these details and considering them in parallel with the specific function of the bike is an integral part of the fit and design consultation process. This is exactly where our frame building courses begin; we use our experience and knowledge to help students identify their desired handling characteristics. Using this information we work together to design a bicycle frame tailored to meet the unique measurements and requirements of each student. A frame building course at TBA is a truly bespoke and personal learning experience.

 

Words by Tom Sturdy - Head of Education