Cycling culture is full of little idiosyncrasies. Some of them we love and respect, like the fact that a water bottle is a water bottle in all walks of life except when on a bicycle. Others can be infuriating – like the inconsistencies within geometry charts.
The geometry of a bike is hugely important when it comes to selection. Geometry creates personality - making the bike either more fast and flicky, or comfortable and stable.
The geometry of a bike is hugely important when it comes to selection. Geometry creates personality - making the bike either more fast and flicky, or comfortable and stable.
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00:00Now, the geometry of a bicycle is incredibly important and it's something to study really
00:08carefully if you're thinking about buying a new bike.
00:12Geometry very much defines the personality of a bicycle, whether it's fast and flicky
00:16and nimble, or if it's stable and feels particularly comfortable at lower speeds.
00:22The problem is that different brands provide different lists of numbers.
00:25So for example, Giant might provide one list of numbers and Specialized might provide another
00:31and they might not be directly comparable unless you're looking at exactly the right
00:35metrics.
00:36We're going to look at the important metrics that you need to look at if you are thinking
00:40of buying a new bike.
00:42So before we go into the details of geometry, I'm just going to spend a second focusing
00:47on the difference between geometry for handling and geometry for fit.
00:52Now, if you're buying a new bicycle, first you need to look at the geometry for handling
00:57because you want to determine if this is the right bike for you.
01:00The numbers you're going to look at are stack and reach, wheelbase, relationship between
01:04the front center and the rear center, particularly the chainstays, also the head angle and trail.
01:10We're going to look at all of those in this video.
01:13So once you've chosen a bike based on handling, then you're going to look at a bike based
01:17on fit, but the two really do need to interact.
01:20Be honest with yourself about the kind of rider you really are, about your strength,
01:24about your flexibility and the sort of speed you're going to be riding on as well as the
01:28kind of roads you're going to be riding on.
01:31Now if you are comparing a bike that you currently have with a bike you are considering buying,
01:36there are some really, really useful websites out there.
01:39So it's geometrygeeks and bikeinsights.com.
01:42Both of those include huge databases of various different bicycles and have all the geometry
01:48details so you can compare different bikes.
01:57So stack and reach is relatively new in terms of bike geometry charts.
02:01However, there's a very, very good reason for its emergence.
02:05In the past, brands would use top tube and head tube as a marker of how aggressive, so
02:11long and low, or relaxed, high and short, a bicycle was.
02:16The problem with these metrics is that top tube could be measured from various different
02:21points.
02:22You could have effective top tube or centered top tube or actual top tube and it was never
02:27entirely clear which brand was using which metric, which made them not very comparable.
02:32The problem with head tube is that fork lengths now vary quite a lot, in part because of the
02:37emergence of disc brakes and also tire clearance as well.
02:41So neither of those metrics were actually particularly useful for customers.
02:46So now we use stack and reach.
02:48So in order to explain this, I'm going to need the post-production guys to draw some
02:51fancy lines on a bicycle.
02:54Stack is a vertical distance from the center of the bottom bracket to the central point
02:58at the top of the head tube.
03:00Reach is a horizontal distance from the center of the bottom bracket to the middle of the
03:04head tube.
03:05So a long reach and a short stack will put a rider in a position that feels aggressive
03:10and fast.
03:11They'll also be more aerodynamic, which is why it's often favored by racers.
03:15Of course, in order to maintain that position, you need to know that you have the flexibility
03:20and the strength to stay there without getting injured.
03:23A shorter reach and a higher stack will feel much more comfortable and will suit riders
03:28that are going to be out all day, riding usually at slightly slower speeds.
03:32For example, a 56 centimeter specialized tarmac has a stack of 555 millimeters and a reach
03:38of 398 millimeters versus a 56 centimeter specialized Roubaix at 611 millimeters stack
03:45and a 381 millimeter reach.
03:48Those are quite notable differences.
03:51So some people like to use stack and reach ratio, which is stack divided by reach.
03:56As a rule of thumb, a lower number is going to be more aggressive and a higher number
04:00is going to be more relaxed and comfortable.
04:03It sounds perfect, right?
04:05But there are caveats to bear in mind.
04:07Stack and reach does not take into account various different components on the bicycle.
04:12For example, the stem length and also handlebar reach.
04:16A very good example of this is the difference between a Canyon Aeroad and a Trek Madone
04:21in H1.5 fit.
04:23The Aeroad in a large comes with a reach of 403 millimeters versus the Madone's 391 millimeters
04:30in a 56.
04:31However, the Madone uses a much longer reach handlebar, and that is a factor that would
04:35not be immediately obvious from the geometry chart.
04:39You can often see the handlebars reach if you actually go to the spec sheet for that
04:43particular component.
04:44Alternatively, at that point, you are looking at speaking to bike fitters or also calling
04:49in a test bike, which is something I always, always recommend if you're really looking
04:53seriously at investing in a new bike.
04:55So thankfully, some brands are actually starting to take this into account.
04:58For example, Canyon has started listing stack plus and reach plus, and that does take into
05:04account the stem and also handlebar reach.
05:07Do remember, though, that you need to look at the bike in your specific size.
05:11That is because at times bike designers will make the bike more or less aggressive in smaller
05:17or larger sizes using the middle size as a median or average.
05:22This is often a cost cutting method because it allows them to use the same mold for various
05:25parts of the frame across different sizes.
05:29So one final thing that you should also bear in mind is that stack and reach does not take
05:33into account the effect of spaces.
05:35So if you are running all the spaces below your handlebar, it will bring the stack up
05:39and it will also bring the reach very slightly back because, of course, you've got a diagonal
05:44line here.
05:52Fork offset, sometimes called rake, trail and head angle are intrinsically intertwined.
05:58It's quite impossible to look at one without looking at the others.
06:01So head angle is the angle at which a line would travel through the head tube to the
06:06steering axis to hit the floor.
06:09Steeper head angles will handle more quickly.
06:11It'll take less input from the rider in order to get that bicycle to turn quickly.
06:16However, they can feel a little bit twitchy at slower speeds, whereas a slacker head angle
06:22will feel a little bit less nippy and a little bit less twitchy when traveling at fast speeds,
06:28but will feel more stable at slower speeds.
06:31So this is a really interesting one to look at when we're talking about the new crop of
06:35gravel bikes, because it will really show you whether this bike is tending towards the
06:41mountain bike end of the spectrum or more towards the road bike end of the spectrum.
06:46So the very perfect example of that is the Evil Shammy Hagger, which has a head angle
06:51of 66.67 degrees.
06:55That is really, really slack and it's very much in the arena of mountain bikes.
07:00Now on most road bikes, we would generally see a head angle of around 70 to 71 degrees
07:06as being quite relaxed and more in tune with a sportive or endurance bike, whereas 72 to
07:1373 degrees is something that you would see on a race bike.
07:17Those sound like really small differences, but they're definitely not, and they do make
07:21an incredibly big difference to the way a bike rides and handles.
07:25There is a caveat to this in that in very small bikes, sometimes we see the head angle
07:29becoming slacker in order to prevent toe overlap, simply because the wheels are very
07:35large on the frame size.
07:37Brands have gone some way to counteract that.
07:40As an example, Canyon releases its much smaller women's bikes with 650b wheels.
07:46So offset or rake is how far the front axle is offset from the steering axis.
07:52Imagine a straight line from the center of the steerer tube to the ground.
07:56It's the gap between that and the front axle.
07:59Trail is the distance between the tire's contact patch with the ground and where the head angle
08:03hits the floor.
08:04It's a product of the offset and head angle.
08:07So a short trail number is going to involve a steeper head angle and less rake, whereas
08:13a larger trail number is going to involve a slacker head angle and more rake.
08:19So as a rule, around 55 to 59 millimeters is the kind of trail that you would see on
08:25a fast, quick handling bike, whereas 60 millimeters plus is more what you would expect from an
08:30endurance bicycle.
08:31So as an example, Trek Zamanda in a size 56 has a trail of 58 millimeters and a head angle
08:38of 73.5 degrees.
08:41And that, of course, is a fast handling race bike.
08:44Comparatively, the Damane has a trail of 61 millimeters and a head angle of 71.9 degrees.
08:51And that is very much an endurance bike.
08:55Another one to bear in mind is also seat tube angle.
08:57Now, in most road bikes, this will vary between 72 to 74 degrees, and it generally remains
09:04fairly consistent across the sizes.
09:06This can actually represent a problem for taller riders because the seat tube is longer.
09:12It means that the saddle is pushed further back.
09:15Of course, you can push it forward on its rails, but it can only go so far.
09:20This can be counteracted with an inline seat post and that can help to get the saddle further
09:25forward.
09:26However, it'll only take you so far.
09:29So if you want to understand what kind of effect that seat angle has, it's worth having
09:33a look at the TT bikes you get on the market.
09:36So both the Specialized Shiv and the Giant Trinity have a seat angle of 77 degrees.
09:41That is much, much steeper, and it will put the rider much further forward, up and over
09:46the bottom bracket, allowing them to get that fast, flat back aerodynamic position
09:50and really to push the power through the pedals.
09:59Wheelbase is another marker of how quick-footed or stable a bike will feel.
10:05A longer wheelbase has a larger turning circle, whereas a shorter wheelbase has a shorter
10:09turning circle.
10:11Wheelbase can be divided into front-centre, the difference between the centre of the bottom
10:15bracket and the front axle, and rear-centre, the distance between the centre of the bottom
10:21bracket and the rear axle.
10:23Unfortunately, not all bike brands do list this.
10:26However, they do generally list chainstay length, which will of course give you a pretty
10:31good idea of rear-centre.
10:34So in both those cases, a longer number, so a longer front-centre and a longer rear-centre
10:39will feel more stable at slower speeds, whereas a shorter front-centre and rear-centre or
10:45chainstay length will feel much more nippy if you're riding at faster speeds.
10:50Of course, a chainstay, or rear-centre, do need to be long enough to accommodate different
10:56tyres, which is why often on gravel bikes, as well as them providing more stability,
11:00we also see longer rear ends, because this does allow for more tyre clearance.
11:06However, there are some caveats to that, so as an example, Specialized Diverge has shorter
11:11chainstays because it helps get the rider's weight up and over when they're scrambling
11:16up those really short, steep climbs you find off-road.
11:19So in the past, disc brakes have interfered with this status quo.
11:24That's not the case anymore, group sets have changed, so we don't need longer chainstays
11:29to account for disc brakes, and we do see that in newer bikes.
11:33As an example, newest iteration of the Canyon Aeroad had shorter chainstays again, which
11:39allowed it to keep its intended personality.
11:49So last but not least is bottom bracket drop.
11:52This is how far the BB drops from the two wheel axles.
11:56You can also look at bottom bracket height, which is the BB height from the floor, however
12:02this is influenced by tyre volume, so I tend not to focus on it quite as much.
12:07So a larger BB drop lowers the rider's centre of gravity.
12:11It's something you would still want on a fast handling crit race bike, because you
12:15are going to be leaning hard into those corners, and you want a low centre of gravity.
12:21The problem is, if you go too low, and you've got problems with pedal strike when you are
12:25leaning over into the bends, and pedal strike can have some pretty nasty consequences I
12:29can tell you.
12:34It's also a key measure that we look at when we are looking at off-road bikes, because
12:38of course pedal strike and also clearance is something that becomes really, really important.
12:44So using Specialized as an example, a 56cm Specialized Tarmac has a BB drop of 72mm versus
12:5376mm on the Specialized Roubaix.
12:57However on the cyclocross bike, in the same size, so still a 56cm frame, it's 69mm on
13:03the Crux cross bike.
13:05However on the Diverge it's 80mm, and that is a really, really big difference.
13:11It's going to make the Diverge feel an awful, awful lot more stable on gravel terrain, however
13:16it gives you a lot less clearance, whereas the cyclocross bike is much more tailored
13:20to that really thick mud and obstacles that you might find on a cyclocross course.
13:27So there you have it.
13:28I've tried to cover off the key metrics that you need to look at when you are considering
13:33buying a new bike.
13:35In short, if you're after a fast, aggressive bike then look for a long reach, a short stack,
13:41a steep head angle and a shorter trail number.
13:44Alternatively, if you're after a comfortable ride in a more endurance position for those
13:49long days out in the saddle, then look for a bike with a shorter reach, a higher stack,
13:55a slacker head angle and a longer trail number.
14:00So, if you have any questions, pop them in the comments down below and we'll do our best
14:18to answer them.