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How does frame geometry affect a bike?

Road bike cornering skills
Max Glaskin
31 Aug 2018

A frame's geometry may affect the way a bike handles, but can you really predict how well a bike will ride just with the tube angles?

Warning: in the next few paragraphs you may experience wobble, flop and twitch. Don’t be alarmed – they are terms that will help you to better understand the complex subject of bicycle handling. Put simply, handling is the input required to control a bike’s direction, and the biggest factor is the bicycle’s geometry.

It has taken a century to get close to fully comprehending the dynamics but Assistant Professor Arend Schwab of Technical University of Delft, Netherlands, is one of a team of physicists that claims to have eventually cracked the problem in 2011.

‘In our equation there are 27 parameters,’ he tells Cyclist. These include tube angles, dimensions, fork rake, bottom bracket height and centre of gravity. Change just one element and the handling changes.

Decades of cumulative experience have led framebuilders to a general geometry for road bikes that’s an acceptable balance between weight, comfort, efficiency and handling.

This means that, today, most of the tweaks for handling are made with the head tube angle and fork offset, sometimes called rake, because they determine ‘trail’.

To get an idea of trail, imagine a beam of light shining down through the centre of the head tube. It will touch the ground ahead of the point where the front tyre touches the ground.

The distance between these two points is the trail and it’s one of the main factors in how a bike handles.

‘Trail affects the stability of the bicycle,’ says framebuilder Richard Craddock of Craddock Cycles in Bromsgrove.

‘All bicycles are more stable the faster they move but more trail makes it easier to stay upright. On the downside, more trail means more wobble when you’re out of the saddle and it requires more effort from the rider to steer when it comes to cornering.’

It’s this last point that energises Anders Annerstedt of Luxembourg bike brand Rolo.

‘A bike with a longer trail is more difficult to keep on the ideal trajectory around a corner. So on a hairpin you’ll have to steer through the turn with maybe two or three different inputs.

'You have to be forceful because the geometry wants to keep the bike in its existing trajectory,’ he says.

With less trail the rider doesn’t have to expend as much energy to make those corrections and can do so quicker, Annerstedt adds.

He tunes his geometry to eliminate excessive ‘wheel flop’ – low-speed instability – and all framebuilders aim to adjust the trail to provide the kind of handling most suited to the intended use of the bike.

So a touring bike might have a lot of trail to provide a stable, predictable ride, while a race bike might have a small amount of trail to offer quick, sharp cornering. 

There are two main ways a framebuilder can adjust trail: by changing the angle of the head tube or by adjusting the rake (offset) of the fork. To reduce the trail, you can make the head tube steeper, increase the fork rake, or both.

Road cyclists have generally come to favour the handling of bikes where the trail is 50-60mm. A head angle in the region of 73° with a fork offset of around 45mm tends to acheive this.

Many framebuilders are cagey about their precise figures, but if you know two of the three variables you can calculate the third at the website. 

Road bike descending skills

There is an extra complication with small frame sizes because the front wheel is closer to the bottom bracket, which brings a risk that it will be clipped by the rider’s toes.

To increase the clearance either the head angle must be made shallower or the fork offset increased, either of which will affect handling unless other design changes are made.

Weights and measures 

Unfortunately, checking the geometry chart to assess trail won’t tell you all you need to know about handling, because there are other factors involved.

Framebuilders cite bottom bracket height and wheelbase as the next two most influential geometrical elements for handling.

Further changes to handling can be made by altering the length of the stem, the width of the handlebars, and the type of wheels and tyres.

Heavier wheels and tyres, for example, have more inertia and require more effort to turn the bars.

The way a bike handles is also down to the stiffness of the frame and forks, which means materials and tube shapes play their parts.

‘The choice and use of material allow a bike manufacturer to achieve more specific bike-handling characteristics within a defined geometry,’ says Tim Hartung, design engineer at Trek Bicycles.

‘This is where composite materials have a major advantage over isotropic materials such as steel and aluminium.’

He likes how carbon allows him to control and tune stiffness precisely through lay-up patterns, tube thicknesses and cross-section shapes.

Crete descending

Modern developments such as disc brakes on road bikes have also had an effect on the way a bike handles, according to Hartung: ‘Typically, thru axles are much thicker [than standard quick releases], commonly 12mm or 15mm.

'The thicker axle is stronger and stiffer and also creates a stiffer system connection between frame and wheel. It can create a scenario where the front and rear wheels are not in sync with each other.

'The rear wheel and frame will tend to stay more in the same plane with each other, which can be bad when trying to hold a constant line in high speed cornering.

'You can end up with a bike that does not have enough “give” or flex in the system to hold a good line in a corner or allow you to successfully counter-steer accordingly.

‘This could mean your tyre contact patch with the ground in the rear could be compromised. Or you end up hitting the brakes to adjust your line in a high-speed corner on the road,’ Hartung adds.

‘Neither of these are desirable in race situations. Keep in mind that this level of detail is really only noticed by elite cyclists and some of this effect can be mitigated by changing your frame stiffness parameters.’

The organic element

No matter what geometry and stiffness levels have been dialled into a bike, there will always be one variable that is impossible for the framebuilder to accurately predict: the rider.

Body shapes and riding styles are going to affect the dynamics of handling, and Arend Schwab of Technical University of Delft has definite views on how the rider’s position relates to the handling.

‘The pose or position of the rider isn’t so important when it comes to handling but the way you hold the handlebars is very important,’ he says.

‘The steering structure is very light and if you add the big mass of arms and upper body, it can influence the system and its response.’

Schwab suggests that, theoretically, holding the bars with straight arms at the same angle as the head tube would be best for stability because then your arm and upper body mass has minimal influence on the steering system.

London-based framebuilder Tom Donhou adds, ‘You really want to keep the centre of gravity in the right place, particularly when you’re riding at speed. Speed wobble is down to weight placement so you need to get the right distribution of weight between the wheels.’ 

For a framebuilder, trying to balance all the geometric elements against the rider’s physiology can be a daunting task, especially when customers can be remarkably demanding, while simultaneously unclear, about the kind of handling they want.

‘People use words like responsive, reactive, agile, nimble, aggressive,’ says Tom Rodi of American bespoke builder Parlee Cycles.

‘There are 20 or 30 adjectives that you’ll hear. Sometimes someone will talk about a bike feeling “nervous”. It could be too steep at the head tube or the fork being too rigid.

'There are a lot of pieces to look at. There’s the torsional rigidity of the head, top and down tubes, the torsional responsiveness of the whole fork assembly and there’s the vertical compliance – the ability of those things to bend in reaction to road abnormalities.’

What is apparent is that there’s no simple solution to predicting a bike’s handling. It’s something that framebuilders will continue to tinker with and refine as much through trial and error as through science.

And once you’ve got your frame, the parts you attach to it will also have an effect too, but mostly the way you carve through that hairpin bend will be down to you.

The way a bike handles is largely down to the person handling it.

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