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Here's how all those numbers can tell you about the fit on offer, and give you an indication of the handling you can expect

Geometry plays a huge part in determining a bike's character and behaviour as well as its fit, so checking out those tables is crucial before making your buying decisions.

You can look at bike geometries on manufacturers' websites and, if you're old school, brochures. The exact information contained in those tables varies a little between brands but they all give you tube lengths, frame angles, wheelbase, chainstay length, stack and reach. 

Merida geometry table - 1

 

Here's what the most important variables mean and how they influence the bike's fit and feel.

Stack and reach

Let's start with stack and reach because these are terms that get bandied around all the time and they're important. Here's what we're talking about...

Stack The vertical distance from the centre of the bottom bracket to the central point at the top of the head tube.

Reach The horizontal distance from the centre of the bottom bracket to the central point at the top of the head tube.

Stack and reach - 1

So stack and reach, expressed in either millimetres or centimetres, are measurements between the same two points, one being the vertical component and the other being the horizontal component. 

These figures allow you to assess quickly whether a bike is likely to offer the ride position you're after before you dig deeper (we're not covering how to determine the best stack and reach figures for you here; we'd suggest you get a professional bike fit for that).

Frame size used to refer to the length of the seat tube (and it still does in some cases) but it gives you no more information on the ride position you're likely to get. Then sloping top tubes came in and complicated matters further by shortening seat tubes, and there's the massive variation in top tube and head tube lengths between different models to consider... A 54cm Cannondale SystemSix race bike has a very different fit from a 54cm Cannondale Synapse endurance bike, for example.

For all these reasons, it's impossible to look at any particular frame tube length and get a good idea of fit. Comparing stack and reach makes things easier by giving you a straightforward way of comparing different bikes.

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Of course, a lot of other factors influence your riding position: seatpost extension and layback, the position of the saddle on its clamp, the size and number of headset spacers you use, the length and angle of the stem, handlebar width, reach and drop... All of these can all be altered by either adjusting an existing component or swapping to a new one.

A frame's stack and reach, though, can't be changed (although, admittedly, Argon 18 uses what it calls a 3D System on some of its bikes to allow you to adjust the height of the head tube). These figures refer to the frame only, leaving components out of the equation.

For a given reach, a greater stack puts you into a more upright riding position; your hands will be moved upwards and your body position will rise more towards the vertical.

If the stack remains the same and the reach is increased you'll get a longer ride position; your hands will move forwards and you'll be more stretched out.

Orro Pyro Evo 105 Hydro - stem.jpg

You can fine-tune your ride position by adding or removing headset spacers, swapping your stem, and so on, but only by so much, which is why stack and reach are so important.

Say that it's time for a new bike. If you're happy with the ride position of your current bike, comparing stack and reach allows you to determine whether you'll be able to get a similar position on any new model.

On the other hand, if you find your current position a little too aggressive – you feel that your body position is too low and/or too stretched – you might want to consider a model with a shorter reach and/or a higher stack.

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Take Specialized's Tarmac road race bike, for instance. The 56cm model has a stack of 565mm and a reach of 395mm.

Those figures might not mean much to you in isolation but they allow you to compare the Tarmac with other bikes.

2020-roubaix-comp_15209 (1)

The 56cm version of Specialized's Roubaix, which is an endurance road bike designed with a focus on comfort, has a stack height of 605mm and a reach of 384mm.

In other words, the top of the head tube is considerably higher and a little closer to the saddle on the Roubaix than on the Tarmac. All other things being equal, your ride position is going to be much more upright on the Roubaix. That's good news if you want to put less strain on your back and neck for long-ride comfort, but bad news if you want to keep drag to an absolute minimum. Horses for courses.

 

Two bikes might have exactly the same stack and reach figures but if one has a slacker seat tube angle the distance from the saddle to the handlebar will be slightly greater, all other things being equal.

Stack and reach don't take into account the headset system (external, integrated, etc) used either; some add more height to the front end than others (and bring the front end slightly closer to the saddle in the process). 

Plus, all of those equipment choices mentioned above (seatpost layback, stem length, headset spacers, handlebar type and so on) have a massive effect on a bike's fit and handling.

Stack+ and reach+

Some bike brands give additional information about handlebar position. Canyon, for example, has introduced what it calls stack+ and reach+ measurements which are not the same as stack and reach.

Here's the explanation from Daniel Heyder, one of the product engineers behind Canyon's Grail CF bike:

Stack+ describes the vertical distance between the bottom bracket and the contact point on the TOP of the handlebar tops . 

Reach+ describes the horizontal distance from the bottom bracket to the CENTRE of the handlebar tops. 

This system enables the comparison of each road bike, while taking all relevant measurements into account (spacer count supplied, cockpit angle, stem length etc).

For example, the Grail CF ships with 15mm (1x 5 mm, 1x 10mm) of headset spacers, whereas an Ultimate or Endurace ships with 27mm of spacers (2x 10mm, 1x 5mm and 1x 2mm) plus a 10mm headset top cover.

Seat tube angle

A bike's seat tube angle – often called 'seat angle' – is simply the angle between the horizontal and a line that starts at the bottom bracket and goes up through the seat tube. 

Seat tube angle.jpg

Like stack and reach, seat tube angle refers solely to the frame. It doesn't take into account a seatpost's layback (the position of the clamp in relation to the centre of the seatpost) or whether the saddle is pushed forward or backward on its rails. 

The seat tube angle influences but does not define saddle setback, which is the horizontal distance between the front of the saddle and the centre of the bottom bracket.

A steeper seat tube angle moves the saddle forwards – putting you more directly over the bottom bracket – and a shallower seat tube angle moves it back, shifting more of your weight from the handlebar to the saddle.

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Giant's Contend is fairly typical of road bikes in having seat tube angles of 72.5° to 74°, depending on the frame size. 

Specialized Tarmac DIsc Sport.jpg

Most of Specialized's Tarmac frames have seat tube angles from 73° to 74°, although the very small 44cm model has an unusually steep seat tube angle of 76° which moves the saddle closer to the handlebar for less of a stretch.

A steep seat tube angle allows racers and anyone else who wants to ride fast to achieve a flat-backed position for aerodynamic efficiency without the need for too tight a hip angle (which can be uncomfortable and restrict both breathing and power).

Canyon Speedmax CF SLX 9 (1).jpg

Time trial and triathlon bikes tend to have very steep seat tube angles for similar reasons. Canyon's Speedmax, for instance, has a seat tube angle of 80.5° across all sizes, and a seatpost with an adjustable clamp that allows you to push the saddle even further forward. 

Head tube angle

Head tube angle – often shortened to 'head angle' – is measured from the horizontal to a line running down the centre of the fork steerer tube. 

Head tube angle (1).jpg

Mountain bikes with front suspension usually have head tube angles in the 62-73° range while road bike head tube angles tend to be above 70°, commonly between 72° and 74°. 

A slacker head tube angle (a smaller number) will move the front wheel further forward and lengthen the wheelbase and the front centre. The alternative is for a bike designer to fix the wheelbase at a set measurement and then a slacker head tube angle will shorten the top tube.

Smaller sized road bikes sometimes have slacker head tube angles to reduce the amount of wheel/toe overlap and avoid problems when steering.

At the risk of insulting your intelligence, the fact that the head tube is angled rather than perpendicular means that adding headset spacers moves your handlebar horizontally as well as vertically.

Say your road bike has a 73° head tube angle and you add 25mm of headset spacers. As well as lifting your handlebar about 24mm, this will move it about 7mm horizontally towards the saddle.

Trail

Trail is the distance that the centre of the front tyre's contact patch trails behind the point where the bike's steering axis (a line through the fork steerer tube) intersects with the ground.

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Trail is a function of the head tube angle, fork offset/rake (the distance the front hub is offset from the steering axis) and wheel/tyre size. Steepening the head tube angle decreases trail, all other things being equal, as does increasing the fork offset and decreasing the wheel/tyre size.

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Many bike brands list trail (but not mechanical trail – the perpendicular distance between the steering axis and the front tyre's contact patch – which would be more informative). 

As a rule, more trail results in more stable steering and a greater ability to hold a line on rougher terrain. Less trail makes for a bike that steers more sharply, but too little can result in a twitchy feel.

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Trek's Madone road race bike (we'll talk about a 56cm size here) has a head tube angle of 73.5°. This, along with a 40mm fork offset (and the wheel/tyre size) results in a trail of 58mm.

Trek Boone 7 Disc 2020.jpg

In contrast, Trek's Boone cyclocross bike has a slacker head tube angle of 72°, a 45mm fork offset and a much higher trail of 67mm.

The idea is that the Madone offers the quick, agile steering required in road racing while the Boone's higher trail will help keep the steering straighter on the rough terrain you'll encounter when riding cyclocross.

Trek Domane SL4 2020 (1).jpg

The Domane, the endurance road bike in Trek's range, has a slack head tube angle (for a road bike) too – 71.9° – a 48mm fork offset and 61mm of trail. It is designed to offer more stability than the Madone.

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Trek's Checkpoint gravel bike – designed for surfaces that are rougher than tarmac but not as extreme as you find in cyclocross – has a 72.2° head tube angle, a 49mm fork offset and a 61mm trail, each figure sitting between those of the Madone road bike and the Boone cyclocross bike.​

It's worth pointing out again that bike designers sometimes have to increase trail on smaller bikes to prevent toe overlap with the front wheel. The 44cm version of Specialized's Tarmac road bike, for example, has 71mm of trail compared with 55mm on the 56cm model.  

WHEELBASE CHAINSTAY LENGTH FRONT CENTRE

Front centre

Front centre is the horizontal distance from the centre of the bottom bracket to the middle of the front wheel axle. Some brands list it on their geometry charts – Cannondale, for example – but most don't.

Beware that a long frame with a steep head tube angle could have a similar front centre measurement to a shorter frame with a slack head tube angle, yet they'd be two very different bikes.

A short front centre means more potential for toe overlap (where the tyre hits your foot when you turn the front wheel), particularly when combined with large tyres.

Chainstay length

Chainstay length is the distance from the centre of the bottom bracket to the centre of the rear wheel axle, and it influences wheelbase. The chainstays obviously need to be long enough to provide clearance for the range of tyre sizes that are likely to be fitted to a particular bike.  

Performance orientated road bikes tend to have short chainstays while adventure bikes, for example, usually have longer chainstays, giving you more heel clearance if you fit panniers. That said, Specialized’s Diverge gravel/adventure bike has short (415mm) chainstays to tuck the back wheel in for more traction on steep climbs. 

Wheelbase

Wheelbase is the distance between the front axle and the rear axle (or the distance between the middle of the tyres' contact points with the ground). 

Wheelbase is an important factor in determining your bike's stability. All other things being equal, a bike with a longer wheelbase will be more stable and better able to keep moving in a straight line, whereas shorter wheelbases are generally said to make a bike feel more nimble.

A 56cm Trek Madone SL 6 Disc road race bike, for example, has a 98.3cm wheelbase because agility is important here, while a Trek Domane endurance bike in the same size has a longer 100.8cm wheelbase. Trek's Checkpoint SL 6 gravel bike has a wheelbase that's longer again – 102cm for a 56cm model. 

BBDROP AND HEIGHT

Bottom bracket height

This is the distance from the ground to the centre of the bottom bracket. Some brands, such as Specialized, list it for both road bikes and mountain bikes; some brands, such as Trek, list it for mountain bikes only; and some brands, such as Giant, don't list it at all!

A lower bottom bracket height moves your centre of gravity downward and makes a bike a bit more stable. A higher bottom bracket offers greater cornering clearance. 

You also need enough clearance that the pedals and frame don't strike any obstacles that come your way. That's obviously a far greater concern in mountain biking than on the road, especially when suspension is involved. When the suspension is compressed (and it will 'sag' as soon as you get on board), the bottom bracket moves closer to the ground.

Bear in mind that the 700 x 33mm tyres fitted to a cyclocross bike, for example, will lift the bottom bracket upwards more than a road bike's 700 x 25mm tyres.

A 54cm Specialized Tarmac road bike has a 268mm bottom bracket height, while a medium sized Specialized Hardtail Expert mountain bike (with front suspension only) has a 309mm bottom bracket height, and a medium Specialized Epic Expert Carbon (with both front and rear suspension) has a bottom bracket height of 332mm. 

Bottom bracket drop

Imagine a line drawn directly between the centre of your bike's front and rear hubs. The bottom bracket drop is the vertical distance that the BB sits below that line. 

A greater bottom bracket drop lowers your centre of gravity and provides more stability, but that needs to be balanced against the need for pedal clearance.

An ML sized Giant TCR Advanced 3 road bike has a bottom bracket drop of 65mm, for example, while a Giant TCX SLR 2 cyclocross bike has a 60mm bottom bracket drop across all sizes. ​
 

Thanks to Dom Mason of Mason Progressive Cycles for giving parts of this feature a quick once-over prior to publication. Any errors are road.cc's own.

Mat has been in cycling media since 1996, on titles including BikeRadar, Total Bike, Total Mountain Bike, What Mountain Bike and Mountain Biking UK, and he has been editor of 220 Triathlon and Cycling Plus. Mat has been road.cc technical editor for over a decade, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a winner of the Cycling Media Award for Specialist Online Writer. Now pushing 50, he's riding road and gravel bikes most days for fun and fitness rather than training for competitions.