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 geometry very much defines the
00:13personality of a bicycle whether it's fast and flicky and nimble or if it's stable and
00:19feels particularly comfortable at lower speeds the problem is that different brands provide
00:24different lists of numbers so for example giant might provide one list of numbers and
00:29specialized might provide another and they might not be directly comparable unless you're looking
00:34at exactly the right metrics we're going to look at the important metrics that you need to look at
00:39if you are thinking of buying a new bike so before we go into the details of geometry just going to
00:46spend a second focusing on the difference between geometry for handling and geometry for fit now if
00:52you're buying a new bicycle first you need to look at the geometry for handling because you want to
00:57determine if this is the right bike for you the numbers you're going to look at are stack and
01:02reach wheelbase relationship between the front center and the rear center particularly the chain
01:07stays also the head angle and trail we're going to look at all of those in this video so once you've
01:14chosen a bike based on handling then you're going to look at a bike based on fit but the two really
01:19do need to interact be 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 kind of
01:29roads you're going to be riding on now if you are comparing a bike that you currently have with a bike
01:34you are considering buying there are some really really useful websites out there so it's geometrygeeks
01:40and bikeinsights.com both of those include huge databases of various different bicycles and
01:47have all the geometry details so you can compare different bikes
01:50so stack and reach is relatively new in terms of bike geometry charts however there's a very very good
02:03reason for its emergence in the past brands would use top tube and head tube as a marker of how aggressive
02:10so long and low or relaxed high and short a bicycle was the problem with these metrics is that top tube
02:19could be measured from various different points you could have effective top tube or centered center top
02:24tube or actual top tube and it was never entirely clear which brand was using which metric which made
02:30them not very comparable the problem with head tube is that fork lengths now vary quite a lot in part
02:37because of the emergence of disc brakes and also tire clearance as well so neither of those metrics
02:43were actually particularly useful for customers so now we use stack and reach so in order to explain
02:49this I'm going to need the post-production guys to draw some fancy lines on bicycle stack is a vertical
02:54distance from the center of the bottom bracket to the central point at the top of the head tube
02:59reach is a horizontal distance from the center of the bottom bracket to the middle of the head tube so a
03:06long reach and a short stack will put a rider in a position that feels aggressive and fast they'll
03:11also be more aerodynamic which is why it's often favored by racers of course in order to maintain
03:17that position you need to know that you have the flexibility and the strength to stay there without
03:22getting injured a 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 for example a 56 centimeter
03:34specialized tarmac has a stack of 555 millimeters and a reach of 398 millimeters versus a 56 centimeter
03:42specialized roubaix at 611 millimeter stack and a 381 millimeter reach those are quite notable differences
03:51so some people like to use stack and reach ratio which is stack divided by reach as a rule of thumb a lower
03:57number is going to be more aggressive and a higher number is going to be more relaxed and comfortable
04:02it sounds perfect right but there are caveats to bear in mind stack and reach does not take into
04:09account various different components on the bicycle for example the stem length and also handlebar reach
04:16a very good example of this is the difference between a canyon air road and a trek madone in h1.5 fit
04:23the air road in a large comes with a reach of 403 millimeters versus the madone's 391 millimeters in a 56
04:30however the madone uses a much longer reach handlebar and that is a factor that would not be immediately
04:36obvious from the geometry chart you can often see the handlebars reach if you actually go to the spec
04:42sheet for that particular component alternatively at that point you are looking at speaking to bike
04:47fitters or also calling in a test bike which is something i always always recommend if you're really
04:52looking seriously at investing in a new bike so thankfully some brands are actually starting to take this
04:57into account for example canyon has started listing stack plus and reach plus and that does take into
05:04account the stem and also handlebar reach do remember though that you need to look at the bike in your
05:10specific size that is because at times bike designers will make the bike more or less aggressive in
05:16smaller or larger sizes using the middle size as a median or average this is often a cross-cutting method
05:23because it allows them to use the same mold for various parts 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 into
05:33account the effect of spacers so if you are running all the spacers below your handlebar it will bring
05:39the stack up and it will also bring the reach very slightly back because of course you've got a diagonal line here
05:45fork offset sometimes called rake trail and head angle are intrinsically intertwined it's quite impossible
05:58to look at one without looking at the others so head angle is the angle at which a lion would travel
06:05through the head tube to the steering axis to hit the floor steeper head angles will handle more quickly
06:11it'll take less input from the rider in order to get that bicycle to turn quickly however they can
06:17feel a little bit twitchy at slower speeds whereas a slacker head angle will feel a little bit less
06:24nippy and a little bit less twitchy when traveling at fast speeds but will feel more stable at slower
06:30speeds so this is a really interesting one to look at when we're talking about the new crop of gravel bikes
06:36because it will really show you whether this bike is tending towards the mountain bike end of the
06:42spectrum or more towards the road bike end of the spectrum so the very perfect example of that is
06:48the evil chamois hagger which has a head angle of 66.67 degrees that is really really slack and it's very
06:57much in the arena of mountain bikes now on most road bikes we would generally see a head angle of around
07:0470 to 71 degrees as being quite relaxed and more in tune with a sportive or endurance bike whereas 72 to
07:1273 degrees is something that you would see on a race bike those sound like really small differences but
07:19they're definitely not and they do make an 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 becoming slacker
07:31in order to prevent toe overlap simply because the wheels are very large on the frame size brands
07:37have gone some way to counteract that as an example canyon releases its much smaller women's bikes with 650b
07:44wheels so offset or rake is how far the front axle is offset from the steering axis imagine a straight
07:53line from the center of the steerer tube to the ground it's the gap between that and the front axle
07:57trail is the distance between the tires contact patch with the ground and where the head angle hits
08:03the floor it's a product of the offset and head angle so a short trail number is going to involve
08:10a steeper head angle and less rake whereas a larger trail number is going to involve a slacker head angle
08:17and more rake so as a rule around 55 to 59 millimeters is the kind of trail that you would see on a fast
08:25quick handling bike whereas 60 millimeters plus is more what you would expect from an endurance bicycle
08:31so as an example trexamonda in a size 56 has a trail of 58 millimeters and a head angle of 73.5 degrees
08:40and that of course is a fast handling race bike comparatively the demane has a trail of 61 millimeters
08:48and a head angle of 71.9 degrees and that is very much an endurance bike
08:54another one to bear in mind is also seat tube angle now in most road bikes this will vary between 72 to 74
09:02degrees and it generally remains fairly consistent across the sizes this can actually represent a
09:08problem for taller riders because the seat tube is longer it means that the saddle is pushed further
09:14back of course you can push it forward on its rails but it can only go so far this can be counteracted
09:21with an inline seat post and that can help to get the saddle further forward however it'll only take
09:27you so far so if you want to understand what kind of effect that seat angle has it's worth having a look
09:33at the tt bikes you get on the market so both the specialized shiv and the giant trinity have a seat
09:39angle of 77 degrees that 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 and really to push
09:50the power through the pedals
09:59wheelbase is another marker of how quick-footed or stable a bike will feel a longer wheelbase has a
10:06larger turning circle whereas a shorter wheelbase has a shorter turning circle wheelbase can be divided into
10:13front center the difference between the center of the bottom bracket and the front axle and rear center
10:19the distance between the center of the bottom bracket and the rear axle unfortunately not all
10:25bike brands do list this however they do generally list chainstay length which will of course give you
10:30a pretty good idea of rear center so in both those cases a longer number so a longer front center and a
10:38longer rear center will feel more stable at slower speeds whereas a shorter front center and rear center
10:45or chainstay length will feel much more nippy if you're riding at faster speeds of course the chainstays
10:52or rear center do need to be long enough to accommodate different tires which is why often on gravel bikes
10:58as well as them providing more stability we also see longer rear ends because this does allow for more
11:05tire clearance however there are some caveats to that so as an example the specialized diverge has
11:11shorter chainstays because it helps get the riders weight up and over when they're scrambling up those
11:16really kind of short steep climbs you find off-road so in the past disc brakes have interfered with this
11:23status quo that's not the case anymore group sets have changed so we don't need longer chainstays to
11:29account for disc brakes and we do see that in newer bikes as an example newest iteration of the canyon
11:36aeroad had shorter chainstays again which allowed it to keep its intended personality
11:49so last but not least is bottom bracket drop this 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 this is
12:02influenced by tire volume so i tend not to focus on it quite as much so a larger bb drop lowers the
12:10rider's center of gravity it's something you would still want on a fast handling crit race bike because
12:15you are going to be leaning hard into those corners and you want a low center of gravity the problem is if
12:21you go too low and you've got problems with pedal strike when you are leaning over into the bends and
12:26pedal strike can have some pretty nasty consequences i can tell you
12:34it's also a key measure that we look at when we're looking at off-road bikes because of course
12:39pedal strike and also clearance is something that becomes really really important so using specialized as
12:45an example a 56 centimeter specialized tarmac has a bb drop of 72 millimeters versus 76 millimeters
12:54on the specialized roubaix however on the cyclocross bike in the same size so still a 56 centimeter frame
13:01it's 69 millimeters on the crux cross bike however on the diverge it's 80 millimeters now that is a really
13:09really big difference it's going to make the diverge feel an awful awful lot more stable on gravel
13:15terrain however it 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 so there you have
13:28it i've tried to cover off the key metrics that you need to look at when you are considering buying a new
13:33bike in short if you're after a fast aggressive bike then look for a long reach a short stack a steep
13:42head angle and a shorter trail number alternatively if you are after a comfortable ride in a more
13:48endurance position for those long days out in the saddle then look for a bike with a shorter reach
13:53a higher stack a slacker head angle and a longer trail number comparison sites are your friend compare
14:01bikes that you know that you like the handling of and new perspective bikes because they are kept they can
14:06be really really useful i hope you've enjoyed this video if you have hit like and subscribe so you'll
14:12see more of our videos in the future if you have any questions pop them in the comments down below and
14:17we'll do our best to answer them