Bicycle Geometry Explained

Cycling Weekly

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.

Transcript

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

02:36in part because of the emergence of disc brakes and also tire clearance as well so neither of those

02:42metrics were 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 a bicycle stack is the

02:54vertical distance from the center of the bottom bracket to the central point at the top of the head tube

02:59reach is the horizontal distance from the center of the bottom bracket to the middle of the head tube

03:05so a long reach and a short stack will put a rider in a position that feels aggressive and fast

03:11they'll also 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

03:31for example a 56 centimeter specialized tarmac has a stack of 555 millimeters and a reach of 398 millimeters

03:39versus a 56 centimeter specialized roubaix at 611 millimeter stack and a 381 millimeter reach

03:48those are quite notable differences so some people like to use stack and reach ratio which is stack

03:54divided by reach as 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 it sounds perfect right but there are caveats to bear in

04:06mind stack and reach does not take into account various different components on the bicycle for

04:12example the stem length and also handlebar reach a very good example of this is a difference between a

04:19canyon air road and a trek madone in h1.5 fit the air road in a large comes with a reach of 403 millimeters

04:27versus the madone's 391 millimeters in a 56 however the madone uses a much longer reach handlebar and

04:34that is a factor that would not be immediately obvious from the geometry chart you can often see

04:39the handlebars reach if you actually go to the spec sheet for that particular component alternatively at

04:46that point you are looking at speaking to bike fitters or also calling in a test bike which is

04:50something i always always recommend if you're really looking seriously at investing in a new bike

04:54so thankfully some brands are actually starting to take this into account for example canyon has

05:00started listing stack plus and reach plus and that does take into account the stem and also handlebar

05:06reach do remember though that you need to look at the bike in your specific size that is because at

05:12times bike designers will make the bike more or less aggressive in smaller or larger sizes using the

05:18middle size as a median or average this is often a cost-cutting method because it allows them to use the

05:24same mold for various parts of the frame across different sizes so one final thing that you should

05:30also bear in mind is that stack and reach does not take into account the effect of spacers so if you are

05:35running all the spacers below your handlebar it will bring the stack up and it will also bring the

05:41reach 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

05:58impossible to look at one without looking at the others so head angle is the angle at which a line

06:04would travel through the head tube to the steering axis to hit the floor steeper head angles will handle

06:10more quickly it'll take less input from the rider in order to get that bicycle to turn quickly however

06:17they can feel a little bit twitchy at slower speeds whereas a slacker head angle will feel a little bit

06:24less nippy 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 spectrum

06:42or more towards the road bike end of the spectrum so the very perfect example of that is the evil shammy haggar

06:50which has a head angle of 66.67 degrees that is really really slack and it's very much in the arena of mountain bikes

06:59now on most road bikes we would generally see a head angle of around 70 to 71 degrees as being quite relaxed

07:08and more in tune with a sportive or endurance bike whereas 72 to 73 degrees is something that you would see

07:15on a race bike those sound like really small differences but they're definitely not and they do make an incredibly

07:21big difference to the way a bike rides and handles there is a caveat to this in that in very small bikes

07:28sometimes we see the head angle becoming slacker in order to prevent toe overlap simply because the wheels

07:34are very large on the frame size brands have gone some way to counteract that as an example

07:40Canyon releases its much smaller women's bikes with 650b wheels

07:45so offset or rake is how far the front axle is offset from the steering axis imagine a straight line from the center of the steerer tube

07:54to the ground it's the gap between that and the front axle

07:59trail is the distance between the tires contact patch with the ground and where the head angle hits 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

08:12whereas a 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 the kind of trail that you would see on a fast quick

08:26handling 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 back

09:15of 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

09:45over the bottom bracket allowing them to get that fast flat back aerodynamic position and really to push the

09:51power through the pedals wheelbase is another marker of how quick-footed or stable a bike will feel a

10:05longer wheelbase has a larger turning circle whereas a shorter wheelbase has a shorter turning circle

10:10wheelbase can be divided into front center the difference between the center of the bottom bracket

10:16and the front axle and rear center the distance between the center of the bottom bracket and the

10:22rear axle unfortunately not all bike brands do list this however they do generally list chainstay length

10:29which will of course give you a pretty good idea of rear center so in both those cases a longer number

10:36so a longer front center and a longer rear center will feel more stable at slower speeds whereas a shorter

10:43front center and rear center or chainstay length will feel much more nippy if you're riding at faster

10:49speeds of course the chainstays or rear center do need to be long enough to accommodate different

10:56tires which is why often on gravel bikes as well as them providing more stability we also see longer

11:02rear ends because this does allow for more tire clearance however there are some caveats to that so as

11:09an example specialized diverge has shorter chainstays because it helps get the riders weight up and over

11:15when they're scrambling up those really short steep climbs you find off-road so in the past disc brakes

11:22have interfered with this status quo that's not the case anymore group sets have changed so we don't

11:28need longer chainstays to account for disc brakes and we do see that in newer bikes as an example newest

11:35iteration of the canyon air road had shorter chainstays again which allowed it to keep its intended

11:41personality so last but not least is bottom bracket drop this is how far the bb drops from the two wheel

11:55axles you 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

12:21if you 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

12:45as an example a 56 centimeter specialized tarmac has a bb drop of 72 millimeters versus 76 millimeters on

12:54the 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

13:08really really 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 to

13:20that really thick mud and obstacles that you might find on a cyclocross course

13:27so there you have it i've tried to cover off the key metrics that you need to look at when you are

13:32considering buying a new bike in short if you're after a fast aggressive bike then look for a long

13:40reach a short stack a steep head angle and a shorter trail number alternatively if you after a comfortable

13:46ride in a more endurance position for those long days out in the saddle then look for a bike with a shorter

13:53reach a higher stack a slacker head angle and a longer trail number comparison sites are your friend

14:01compare bikes that you know that you like the handling of and new perspective bikes because they are

14:06kept they can be really really useful i hope you've enjoyed this video if you have hit like and subscribe

14:12so you'll see more of our videos in the future if you have any questions pop them in the comments down

14:16below and we'll do our best to answer them

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