- These days, everyone is talking about reach,
they're talking about offset, they're talking about trail,
but why on earth do you need a steep seat angle,
and what exactly is long, low, and slack?
Well, today, I've got a bike behind me here,
and I'm going to take you through everything you need to know
about basic geometry, angles, and measurements,
and we're going to tell you exactly what they mean
in relation to your riding.
(industrial ambient music)
Okay so we're going to start with the basics first.
Now I've got behind me here a classic double diamond frame.
Now it's called that reason literally
because is two diamonds meeting in the middle.
Now most mountain bike frame designers today
will still resemble this even if their full suspension.
You might see a bit of a linkage going on here.
You might see a shock placed in the middle.
They're essentially the same concept.
So for ease of illustration
this is what I've chosen for this video.
(wheel bearings rattle) Okay so the first one
we're going to look at is the top tube.
That's this fellow right up here.
Simply called that because it is the top tube of the bike.
Now originally top tubes were quite flat in their profile.
You'll notice this one is quite sloping,
and that essentially is to give you
a bit of undercarriage clearance.
No one wants to be too close to the top tube
when you have to get off the bike.
Now originally the length of the top tube
was quite important in terms of bike fit,
but actually by today's standards is very inaccurate
for a mountain bike, and the reason being
that is only relevant when you're sat down
on a bike for comfort.
That is not important on a mountain bike
because the most important stuff you do on a mountain bike
is when you're stood up, it's all about control.
So these days we don't really talk about
a top tube length in terms of bike fit,
however, it is good to know these things.
There's two ways of measuring the top tube length.
There's the actual measurement which is measured
from the center of the seat tube
to the center of the head tube,
and then there is the effective top tube length,
and the way that is measured, and this is basically
because some frame designs are quite wild
so it's not always applicable to here to measure it
like that, you'll measure from the center of the head tube
and you'll draw a horizontal line to where it meets
with the seat tube, in fact, the seat post
extending from that and that will be
your effective top tube length, but again is not
that important in terms of mountain bikes these days.
It's only really a comfort-based measurement.
It doesn't affect the performance on the bike
which is what you need to know about.
Next up is the head tube.
That's this fellow up the front here.
Now that joins the down tube and the top tube
to the rest of the frame, and it also houses the bearings
that in turn house the steerer tube and the stem
and basically the controls of the front end of your bike.
Now originally these used to be quite thin.
You'll notice this one's quite fat in size.
It's 'cause it houses the bearings directly into it.
Original head tubes were a lot smaller and a lot thinner.
You'd actually have to press cups into them
into which the bearings would be pushed.
So it's quite a different system these days.
Now what is quite important to notice is on older frames,
in particular 26-inch models, you might see that head tubes
are quite long on the bike.
The reason for that is a thing called stack heights.
Now this is the height a bike is.
Now if you think on a bike of very small wheels
if you've got a very low front end, it's going to be
an awkward position to ride on, and to keep the position
the same equivalent for a tall rider as it would be
for a short rider, you get have to have different lengths
in the actual head tube length.
These days though which we'll get to you later on again,
it's not that important because we have increased
wheel sizes which has changed things.
Next up is the down tube.
Of course that goes from the head tube
down to the bottom bracket area of the bike.
Doesn't do anything fancy.
Obviously it's a structural part of the bike,
but there's nothing else you really need to know
about the down tube.
It's not often referred to in terms of measurements
or any sort of geometry that actually indicates
the way a bike would ride, but importantly
is the bottom bracket which is housed under here.
Now the bottom bracket is a shell that essentially meets
a bit of a hub of the bike.
This part of the bike is really important to be stiff
and well designed, and the reason for this
is when you're pedaling, all those forces go through
that part of the frame, and if it's weak, if it's flexible,
if it's not very well designed, you're going to get movement
down here which translates as an awkward feeling bike
and also a loss of power.
Now something with the bottom bracket itself, the shell,
there's a few different styles on the market.
The classic ones are threaded into which you would thread
a bottom bracket shell, but you also get press fit style
bottom brackets that just push straight in,
and they don't require threads to be in place
on that shell.
Now you can also get some fancier bottom brackets.
It cost a lot more money on certain frames where those cups
are actually machined as part of the frame.
Now whilst this is an excellent system,
it's very expensive to manufacture like that.
So they're not that common.
The classic threaded bottom bracket shells are really
probably still the best option.
Seat tube, that is the one essentially divides
the front half of the bike and the back half of the bike.
You'll notice that the seat tube itself houses the seat post
which you sit on, and the height of the seat tube
therefore actually correlates to the sizing of the bike.
Now this used to be much more important on bikes
'cause you would have literally a 20-inch seat tube
would be on a size 20-inch bike, and so forth,
and you would still see the same thing on road bikes today,
but on mountain bikes, modern geometry is starting
to change things a bit.
They're starting to size bikes in length rather than height
which actually is the correct way.
We're going to get to that when we talk about
specific measurements and geometry
a little bit later in this video.
These are the seat stays.
These are the upper rear stays that connect the rear axle
to the main frame, and these are the chainstays.
So they're opposite, these are the lower parts of the frame.
Now typically you'll see short chainstays
give a nice agile handling, and longer chainstays
give more stable handling,
but we're going to talk about that next.
(wheel bearings rattle)
Right, so now let's get into some specific angles
and measurements now we've had a bit of an overview
of a typical frame layout.
(upbeat music)
Now first up, let's talk about the seat tube angle,
also known as the seat angle.
Now this is the acute measurement when you put a line
all the way through the seat tube straight through
the bottom bracket to the ground
between that and the ground effectively.
Now originally on rigid road frames
and early mountain bike frames, a slack seat tube angle
was seen as a good thing, and the reason for that is it
takes more weight off your hands when you're sat down
because you'll put your weight further back on the bike.
Now if you are touring or you're gravel riding
or something like that, then this could be a good thing.
However for modern mountain biking it's not really that good
because you don't spend that much time sat down.
You don't need the comfort from the frame anyway
because most bikes have suspension in the form
of bigger tires and also in rear suspension
and front suspension you find on modern mountain bikes.
Now in terms of road bikes you as I just listed there
you had those slacker seat tube angles for comfort
which we used to see on touring bikes and stuff.
Now if the completely extreme of that would be
a time trial bike which they're nearly vertical,
the seat tube angles, and what that does is it engages
different leg muscles, stronger muscle groups
that can give more power over a shorter time,
and it basically loads up your arms to be
in an aerodynamic position on the bike.
Now it's a really efficient use of the body,
but only for a short time, and it's not comfortable at all.
So if you look at a typical road bike
or a typical mountain bike, cross-country frame today,
it's somewhere in the middle of those,
and the reason for that is it gives the best of both worlds
and it makes a nice transition between the seated
and the stood-up positions, therefore your muscles work
equally as well sat down and stood up,
but you're using different muscles all around the bike,
and it's a generally efficient position to be in.
Now these days you might hear steep seat tube angles
being referred to as a good thing on mountain bikes.
Now, this is something that we know is not comfortable
on road bikes because of that extreme angle,
but on mounted bike this is a really good thing
because it puts you further between the two wheels
when you're climbing on a bike, and the advantage of that
is you get traction on the rear wheel which of course
you need, and you also give weight on that front wheel,
and the reason that is a good thing is you're not fighting
the front wheel trying to climb,
you're not putting yourself in an unnatural position.
You can just sit there and keep on cranking,
and the bike is going to go where you want it to.
As soon as the front wheel goes light when you're climbing,
you have to really load that front end of the bike,
and it becomes an additional chore
when you're climbing a bike.
So modern mountain bikes tend to have a steeper
seat tube angle for this very reason,
and we still haven't found the limit.
They're getting steeper and steeper.
Essentially what you have to remember
with mountain bikes is our riding position
changes constantly so therefore we need to rely
on our stood position and our seated position
as two different things.
We can't rely on it like we do on a road bike
whereas it's all about efficiency.
So it's very different on mountain bike terms
because it's about handling.
Now you might also hear the term effective seat angle.
Now this is a way of calculating the seat angle on a bike
if you have a peculiar frame design
that doesn't have a consistent seat tube.
Now the way to measure it essentially is still through
the bottom bracket axle at the bottom and through the middle
of the seat post at the top irrelevant of whatever
is going on in the middle of the frame.
You take that measurement,
not what is going on anywhere else.
It's that simple.
There's nothing else you need to know about that.
The effective seat tube angle is really where your weight
is on the bike despite the optical illusion
of some frame designs.
(upbeat music)
Next up is the head angle of the bike,
also known as the steering angle.
Now if you draw an imaginary line through the stem,
through the center of the stem and the center
of the head tube all the way down to the ground
and measure the acute angle against the ground,
you will get your head tube angle or your steering angle.
Now the steeper the angle, typically the more nervous
and twitchy the handling which is why bikes
that used to have steep angles used to have longer stems
'cause they would calm that effect down.
Also look at road-cycling bikes, you look at
typical road-racing bikes, they have very steep responsive
head angles on them, and to calm that steering down,
they have a longer stem.
So there is one way round of it, but that's not
the optimum way for off-road riding because we need
more stability at speed, but you need low-speed response
which is why we opt for a slacker head angle
with a shorter stem so it changes your riding position.
Now having this position also really increases
your comfort and stability and to degree you could say
your confidence off-road because your handlebar position
is further behind the rear wheel axle as opposed to
in front of it which means when you're handling
on a very steep terrain, it feels like you're less likely
to be pitched over the handlebars.
(upbeat music)
Okay so next up is your BB height and drop,
also known as bottom bracket height
and bottom bracket drop.
Now your bottom bracket height is essentially how high
your bottom bracket is off the deck, off the ground.
So that is that measurement there,
and your bottom bracket drop is how low
your bottom bracket is, the axle, so the center of the axle
below the line of your two wheel axles, is that simple.
Now the lower the bottom bracket is on your bike,
the more stable your bike is going to feel,
especially when cornering.
You're going to feel really low and in the bike.
This is why 29ers are adopted by a lot of riders.
It's not necessarily size of the wheels.
It's the fact that the wheel axles are so high on a 29er
in relation to a normal bike, that to keep your
riding position ascend, the bottom bracket has to be lower
so to return your body position, and one of the effects
of that is you feel that you're sat in the bike
a lot more on a 29er.
One of the downsides of having a low bottom bracket,
okay, so you got the stability, but it also means your feet
are a lot closer to the ground so you could
bottom your feet out on rocks and roots as you're pedaling.
So to make the most of this, you end up having to have
shorter cranks which arguably can make things
a little harder when you're pedaling a lot.
A higher bottom bracket doesn't have that effect,
and it also has a positive effect of making a bike
feel more agile, but at the same time
it can feel a little nervous.
So there's a fine line between too high and too low.
Some riders love them high, some love them low.
There's no right or wrong.
(upbeat music)
Reach and stack are the two next measurements.
Now we'll do a stack very briefly first
because it's slightly less important on a mountain bike.
It's much more relevant on a road bike
where your frontal view and basically keeping
nice and low on the bike is more important.
Of course we have to factor in ground clearance
and the amount of suspension travel we have
so the stack does change on frame.
So reach is the one we're going to look at here
because this is much more important.
Reach is a realistic way of measuring
a modern mountain bike for fit.
You don't need to look at the seat tube length.
The only reason you need to know that
is for your inside leg measurement to make sure
you can actually fit on the bike and fit a dropper post
on there with enough room for it to extend and compress.
What the reach does is essentially tow the distance
between your bottom bracket and your handlebars
which of course on a mountain bike all your important riding
the handling of the bike happens when you're stood up.
Basically you want that measurement to be nice and long
if you're tall and a bit shorter if you're shorter.
Now I'm going to make this quite easy to understand.
If you look at two wheel axles on a bike,
what you're looking for for any sized rider really
is the illusion of an equilateral triangle.
So your body weight wants to be at the top
in the middle of the two wheels.
If you've got a bike that's got too short reach,
you could be more like an isosceles triangle,
and the effect of that is you're going to be like
a big pendulum swinging around up top,
and you're going to have to move your body weight fore and aft
much more when you're going up and down
for different reasons.
When you're going up, you need to move further forward
to weight the front wheel, keep traction on the rear wheel
to move around, and that becomes more physical
on your arms and on your legs, and when you're descending,
it can be terrifying if you're too far up on the bike
because you've got too much weight on the front wheel.
You need to return that weight to the rear wheel.
So if you look at the reach on the bike, this is really
where you're going to calculate the correct length for you.
Of course it's different for everyone,
but it doesn't mean you should get carried away
looking at geometry charts.
Once you find a length that works for you,
you can figure things out from there.
The way this distance is measured is literally
a vertical line going up from the bottom bracket shell
off the ground and then that is met all the way to the top
in the middle of the head tube there.
So you'll also get the stack height there by doing that,
and the reach is the horizontal.
Nice and simple.
Just a little thing to touch on with reach and stack
is the measurements given by frame manufactures
are the frame reach and the frame stack.
You can also have your bar reach basically on the bike
and your bar stack because everyone's going to have
a slightly different preference.
I, for example, like a nice short stem,
and I like a really high bar, so my stack and my reach
accordingly will be a little bit different
to what the frame manufacturer specifies.
Something I found makes a quite a cool way of setting up
a new bike is to, once you find out your reach and stack
that you're comfortable with, make a measurement
from the end of your bars down to the ground,
and also make a measurement from the end of your bars
to your bottom bracket axle, and you'll find that
pretty quickly you can sort the sort of bar rise,
a bar width, and a stem length and a stem height you need
on your bike to get in a position where it feels
very similar to what you were riding previously.
It just makes the transition between bikes a little easier.
(upbeat music)
Chainstay length, This is the measurement
from the rear wheel axle to the bottom bracket of the bike.
Now it's generally seen that a short chainstay length
is a good thing and a long chainstay length is a bad thing.
Opinions vary, but this is not accurate information.
So a long chainstay can give you better traction,
and it can put your bottom bracket closer to the middle
of the bike neutralizing your position.
So that is a positive thing.
However, having a long chainstay can weight
the front wheel more which can be a good thing
and a bad thing.
It can mean you get more weight to that front wheel
which is a great thing if you have a long bike.
You actually need a long backend to keep things balanced,
but if you're a sort of person that likes to manual
and pick up the front wheel a lot, you're probably going to
prefer a short chainstay which has the opposite effect,
not so much weight on the front end
because there's more weight on your rear end.
It's all about balance,
and don't go getting carried away with this.
It is literally, there's so
many different factors within a bike,
and the next one we're going to roll straight into
is the front center.
That is the measurement from the bottom bracket
to the front wheel axle, and again these do correlate
to each other, and they correlate to the total wheelbase
which is the axle to axle.
So there's three measurements in one there,
and you'll often see chainstays mentioned,
and you'll see total wheelbase mentioned,
but you don't often hear the front center mentioned.
(upbeat music)
Fork offset is next up, and this is something you'll hear
talked about quite a lot because fork offset is something
that people are now starting to experiment with
rather than what was just specified on bikes.
There's a lot of factors in this, and we're going to go into it
in another video soon that deals with that specifically,
but fork offset, what is it?
It's simply the measurement of your fork axles
in front of your steering angle.
So if you take that steering angle pane all the way down
to the ground, you draw another one on that same angle
all the way through your fork axle there,
is that measurement between the two.
Nice and simple.
(upbeat music)
And the last one you need to know about
is your trail measurement.
So this is in direct relation to the fork offset
and your steering tube angle.
So if you take that steering angle all the way down
to the ground, the trail measurement is the measurement
between where that meets the ground
and your front tire contact patch.
The shorter that measurement is the more agile
or perhaps nervous you're steering can feel.
The larger it is the more stable your handling can feel.
Now the reason for the offset change, recent times,
is basically with the advent of 29-inch wheels come in.
Because the higher your fork axle is off the ground,
it changes that contact patch there.
So you get a larger trail measurement naturally
when you have a larger wheel because the axle goes higher.
So when the 29-inch wheel boom really kicked off,
frame and fork manufactures started specking forks
with a larger offset number in order to reduce
the trail back to the same equivalent that they were getting
on their 27 1/2-inch wheel bikes, but these days
they're actually going the other way around.
They're trying to increase it again for better handling,
but we're going to get to that in another video,
and the reason this measurement is called trail
is because it trails behind the steering axis.
Simple as that. (wheel bearings rattle) Well, there we go.
That was sort of geometry 101, I guess you can call it.
All the sort of measurements you need to know
about what they are and what they mean
and what they do for your riding.
If you have any questions, let us know underneath,
but we have three more videos planned,
slightly more advanced, telling you how to make the most
out of some of these measurements on your bike.
They're coming up soon.
So if there's anything you want us to direct look at,
let us know before we start scripting them,
and for a couple of videos, I'm going to throw you down here
to an e-bike challenge with Jonesy over on EMBN.
Good fun videos.
Got of a bit of an appearance with Olly Wilkins
and Brendan Fairclough in that one,
and I'm going to throw you down here to Henry's video
with some world cup mechanics at Red Bull Hardline.
If you want to know anything else about geometry,
as I said let us know in the comments underneath.
Give us a thumbs up and hit that subscribe button.