Bicycle Geometry Explained
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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00:00 (bike engine revving)
00:03 Now, the geometry of a bicycle is incredibly important
00:07 and it's something to study really carefully
00:09 if you're thinking about buying a new bike.
00:11 Geometry very much defines the personality of a bicycle,
00:15 whether it's fast and flicky and nimble,
00:17 or if it's stable and feels particularly comfortable
00:20 at lower speeds.
00:21 The problem is that different brands
00:23 provide different lists of numbers.
00:25 So for example, Giant might provide one list of numbers
00:29 and Specialized might provide another
00:30 and they might not be directly comparable
00:33 unless you're looking at exactly the right metrics.
00:36 We're gonna look at the important metrics
00:38 that you need to look at
00:39 if you are thinking of buying a new bike.
00:42 So before we go into the details of geometry,
00:45 just gonna spend a second focusing on the difference
00:48 between geometry for handling and geometry for fit.
00:52 Now, if you're buying a new bicycle,
00:54 first, you need to look at the geometry for handling
00:57 because you wanna determine if this is the right bike
00:59 for you.
01:00 The numbers you're gonna look at are stack and reach,
01:03 wheelbase, relationship between the front center
01:05 and the rear center, particularly the chainstays,
01:08 also the head angle and trail.
01:10 We're gonna look at all of those in this video.
01:13 So once you've chosen a bike based on handling,
01:16 then you're going to look at a bike based on fit,
01:18 but the two really do need to interact.
01:20 Be honest with yourself
01:21 about the kind of rider you really are,
01:23 about your strength, about your flexibility,
01:26 and the sort of speed you're gonna be riding on,
01:28 as well as the kind of roads you're gonna be riding on.
01:30 Now, if you are comparing a bike that you currently have
01:33 with a bike you are considering buying,
01:35 there are some really, really useful websites out there.
01:39 So it's geometrygeeks and bikeinsights.com.
01:42 Both of those include huge databases
01:44 of various different bicycles
01:46 and have all the geometry details
01:48 so you can compare different bikes.
01:50 (upbeat music)
01:54 (upbeat music)
01:56 So stack and reach is relatively new
01:59 in terms of bike geometry charts.
02:01 However, there's a very, very good reason for its emergence.
02:05 In the past, brands would use top tube and head tube
02:08 as a marker of how aggressive, so long and low,
02:12 or relaxed, high and short, a bicycle was.
02:16 The problem with these metrics
02:18 is that top tube could be measured
02:20 from various different points.
02:21 You could have effective top tube
02:23 or centered center top tube or actual top tube.
02:26 And it was never entirely clear
02:28 which brand was using which metric,
02:30 which made them not very comparable.
02:32 The problem with head tube
02:33 is that fork lengths now vary quite a lot,
02:36 in part because of the emergence of disc brakes
02:39 and also tire clearance as well.
02:41 So neither of those metrics
02:42 were actually particularly useful for customers.
02:45 So now we use stack and reach.
02:48 So in order to explain this,
02:49 I'm gonna need the post-production guides
02:51 to draw some fancy lines on a bicycle.
02:53 Stack is the vertical distance
02:55 from the center of the bottom bracket
02:57 to the central point at the top of the head tube.
03:00 Reach is the horizontal distance
03:02 from the center of the bottom bracket
03:03 to the middle of the head tube.
03:05 So a long reach and a short stack
03:07 will put a rider in a position
03:09 that feels aggressive and fast.
03:11 They'll also be more aerodynamic,
03:13 which is why it's often favored by racers.
03:16 Of course, in order to maintain that position,
03:18 you need to know that you have the flexibility
03:20 and the strength to stay there without getting injured.
03:23 A shorter reach and a higher stack
03:25 will feel much more comfortable
03:27 and will suit riders that are gonna be out all day,
03:29 riding usually at slightly slower speeds.
03:32 For example, a 56 centimeter specialized Tarmac
03:35 has a stack of 555 millimeters
03:37 and a reach of 398 millimeters,
03:40 versus a 56 centimeter specialized Roubaix
03:43 at 611 millimeter stack and a 381 millimeter reach.
03:48 Those are quite notable differences.
03:50 So some people like to use stack and reach ratio,
03:53 which is stack divided by reach.
03:56 As a rule of thumb,
03:57 a lower number is gonna be more aggressive
03:59 and a higher number is going to be
04:01 more relaxed and comfortable.
04:03 It sounds perfect, right?
04:04 But there are caveats to bear in mind.
04:07 Stack and reach does not take into account
04:09 various different components on the bicycle.
04:12 For example, the stem length and also handlebar reach.
04:16 A very good example of this
04:17 is the difference between a Canyon Aeroad
04:19 and a Trek Madone in H1.5 fit.
04:23 The Aeroad in a large comes with a reach of 403 millimeters
04:27 versus the Madone's 391 millimeters in a 56.
04:31 However, the Madone uses a much longer reach handlebar.
04:34 And that is a factor that would not be immediately obvious
04:37 from the geometry chart.
04:38 You can often see the handlebars reach
04:40 if you actually go to the spec sheet
04:42 for that particular component.
04:44 Alternatively, at that point,
04:46 you are looking at speaking to bike fitters
04:48 or also calling in a test bike,
04:50 which is something I always, always recommend
04:52 if you're really looking seriously
04:53 at investing in a new bike.
04:55 So thankfully, some brands are actually starting
04:57 to take this into account.
04:58 For example, Canyon has started listing
05:00 stack plus and reach plus,
05:02 and that does take into account the stem
05:05 and also the handlebar reach.
05:07 Do remember though, that you need to look at the bike
05:09 in your specific size.
05:11 That is because at times bike designers
05:13 will make the bike more or less aggressive
05:16 in smaller or larger sizes
05:18 using the middle size as a median or average.
05:21 This is often a cost cutting method
05:23 because it allows them to use the same mold
05:25 for various parts of the frame across different sizes.
05:28 So one final thing that you should also bear in mind
05:31 is that stack and reach does not take into account
05:33 the effect of spacers.
05:35 So if you are running all the spacers
05:37 below your handlebar, it will bring the stack up
05:39 and it will also bring the reach very slightly back
05:42 because of course you've got a diagonal line here.
05:45 (upbeat music)
05:48 Fork offset, sometimes called rake,
05:53 trail and head angle are intrinsically intertwined.
05:57 It's quite impossible to look at one
05:59 without looking at the others.
06:01 So head angle is the angle at which a line
06:04 would travel through the head tube
06:06 to the steering axis to hit the floor.
06:08 Steeper head angles will handle more quickly.
06:11 It'll take less input from the rider
06:13 in order to get that bicycle to turn quickly.
06:16 However, they can feel a little bit twitchy at slower speeds
06:20 whereas a slacker head angle will feel
06:23 a little bit less nippy and a little bit less twitchy
06:26 when traveling at fast speeds,
06:28 but will feel more stable at slower speeds.
06:31 So this is a really interesting one to look at
06:33 when we're talking about the new crop of gravel bikes
06:36 because it will really show you
06:38 whether this bike is tending towards
06:40 the mountain bike end of the spectrum
06:42 or more towards the road bike end of the spectrum.
06:46 So the very perfect example of that is the Evil Shammy Hagger
06:50 which has a head angle of 66.67 degrees.
06:54 That is really, really slack
06:56 and it's very much in the arena of mountain bikes.
07:00 Now on most road bikes, we would generally see
07:03 a head angle of around 70 to 71 degrees
07:06 as being quite relaxed and more in tune
07:09 with a sportive or endurance bike,
07:11 whereas 72 to 73 degrees is something
07:14 that you would see on a race bike.
07:16 Those sound like really small differences,
07:19 but they're definitely not
07:20 and they do make an incredibly big difference
07:22 to the way a bike rides and handles.
07:25 There is a caveat to this in that in very small bikes,
07:28 sometimes we see the head angle becoming slacker
07:31 in order to prevent toe overlap
07:33 simply because the wheels are very large on the frame size.
07:37 Brands have gone some way to counteract that.
07:39 As an example, Canyon releases its much smaller women's bikes
07:43 with 650B wheels.
07:45 So offset or rake is how far the front axle is offset
07:50 from the steering axis.
07:52 Imagine a straight line from the center of the steerer tube
07:54 to the ground.
07:55 It's the gap between that and the front axle.
07:57 Trail is the distance between the tire's contact patch
08:01 with the ground and where the head angle hits the floor.
08:04 It's a product of the offset and head angle.
08:07 So a short trail number is gonna involve
08:09 a steeper head angle and less rake,
08:12 whereas a larger trail number is going to involve
08:16 a slacker head angle and more rake.
08:19 So as a rule, around 55 to 59 millimeters
08:22 is the kind of trail that you would see
08:24 on a fast, quick handling bike,
08:26 whereas 60 millimeters plus is more what you would expect
08:29 from an endurance bicycle.
08:31 So as an example, Trek Zimonda in a size 56
08:34 has a trail of 58 millimeters and a head angle of 73.5.
08:39 And that of course is a fast handling race bike.
08:43 Comparatively, the Dimane has a trail of 61 millimeters
08:47 and a head angle of 71.9 degrees.
08:51 And that is very much an endurance bike.
08:54 Another one to bear in mind is also seat tube angle.
08:57 Now in most road bikes, this will vary between 72 to 74.
09:02 And it generally remains fairly consistent across the sizes.
09:06 This can actually represent a problem for taller riders.
09:10 Because the seat tube is longer,
09:12 it means that the saddle is pushed further back.
09:15 Of course, you can push it forward on its rails,
09:17 but it can only go so far.
09:20 This can be counteracted with an inline seat post
09:23 and that can help to get the saddle further forward.
09:26 However, it'll only take you so far.
09:29 So if you wanna understand what kind of effect
09:31 that seat angle has, it's worth having a look
09:33 at the TT bikes you get on the market.
09:35 So both the Specialized Shiv and the Giant Trinity
09:38 have a seat angle of 77 degrees.
09:41 That is much, much steeper,
09:43 and it will put the rider much further forward,
09:45 up and over the bottom bracket,
09:46 allowing them to get that fast, flat back,
09:48 aerodynamic position, and really to push the power
09:51 through the pedals.
09:59 Wheelbase is another marker of how quick-footed
10:02 or stable a bike will feel.
10:04 A longer wheelbase has a larger turning circle,
10:07 whereas a shorter wheelbase has a shorter turning circle.
10:10 Wheelbase can be divided into front-center,
10:13 the difference between the center of the bottom bracket
10:16 and the front axle, and rear-center,
10:19 the distance between the center of the bottom bracket
10:21 and the rear axle.
10:23 Unfortunately, not all bike brands do list this.
10:26 However, they do generally list chainstay length,
10:29 which will, of course, give you a pretty good idea
10:31 of rear-center.
10:33 So in both those cases, a longer number,
10:36 so a longer front-center and a longer rear-center,
10:39 will feel more stable at slower speeds,
10:42 whereas a shorter front-center and rear-center,
10:45 or chainstay length, will feel much more nippy
10:48 if you're riding at faster speeds.
10:50 Of course, the chainstays, or rear-center,
10:53 do need to be long enough to accommodate different tires,
10:56 which is why often on gravel bikes,
10:58 as well as them providing more stability,
11:00 we also see longer rear ends,
11:03 because this does allow for more tire clearance.
11:06 However, there are some caveats to that.
11:08 So as an example, Specialized Diverge
11:11 has shorter chainstays,
11:12 because it helps get the rider's weight up and over
11:15 when they're scrambling up those really short,
11:17 steep climbs that you find off-road.
11:20 So in the past, disc brakes have interfered
11:23 with this status quo.
11:24 That's not the case anymore.
11:26 Group sets have changed,
11:27 so we don't need longer chainstays
11:29 to account for disc brakes.
11:31 And we do see that in newer bikes.
11:33 As an example, newest iteration of the Canyon Aeroad
11:36 had shorter chainstays again,
11:39 which allowed it to keep its intended personality.
11:42 So last but not least is bottom bracket drop.
11:51 This is how far the BB drops from the two-wheel axles.
11:56 You can also look at bottom bracket height,
11:58 which is the BB height from the floor.
12:01 However, this is influenced by tire volume,
12:04 so I tend not to focus on it quite as much.
12:07 So a larger BB drop lowers the rider's center of gravity.
12:11 It's something you would still want
12:13 on a fast-handling crit race bike,
12:15 because you are gonna be leaning hard into those corners,
12:18 and you want a low center of gravity.
12:20 The problem is, if you go too low
12:22 and you've got problems with pedal strike
12:24 when you are leaning over into the bends,
12:26 and pedal strike can have some pretty nasty consequences,
12:29 I can tell you.
12:30 It's also a key measure that we look at
12:36 when we're looking at off-road bikes,
12:38 because of course, pedal strike and also clearance
12:41 is something that becomes really, really important.
12:44 So using Specialized as an example,
12:47 a 56-centimeter Specialized Tarmac
12:49 has a BB drop of 72 millimeters
12:52 versus 76 millimeters on the Specialized Roubaix.
12:56 However, on the cyclocross bike, in the same size,
12:59 so still a 56-centimeter frame,
13:01 it's 69 millimeters on the Crux cross bike.
13:05 However, on the Diverge, it's 80 millimeters.
13:08 Now that is a really, really big difference.
13:10 It's going to make the Diverge feel
13:12 an awful, awful lot more stable on gravel terrain.
13:16 However, it gives you a lot less clearance,
13:18 whereas the cyclocross bike is much more tailored
13:20 to that really thick mud and obstacles
13:23 that you might find on a cyclocross course.
13:26 So there you have it.
13:28 I've tried to cover off the key metrics
13:30 that you need to look at
13:31 when you are considering buying a new bike.
13:34 In short, if you're after a fast, aggressive bike,
13:38 then look for a long reach, a short stack,
13:41 a steep head angle, and a shorter trail number.
13:44 Alternatively, if you're after a comfortable ride
13:47 in a more endurance position
13:48 for those long days out in the saddle,
13:51 then look for a bike with a shorter reach,
13:53 a higher stack, a slacker head angle,
13:56 and a longer trail number.
13:59 Comparison sites are your friend.
14:01 Compare bikes that you know that you like the handling of
14:03 and new perspective bikes,
14:05 'cause they can be really, really useful.
14:08 I hope you've enjoyed this video.
14:10 If you have, hit like and subscribe
14:12 so you'll see more of our videos in the future.
14:14 If you have any questions,
14:15 pop them in the comments down below.
14:17 I will do our best to answer them.
14:18 [SWOOSH]