(sound effect)
- Nervous, relaxed, aggressive,
stable, agile, boring, fast or slow
however you want to describe the handling of a bike,
it does seem that it is always lumped
into one of two categories based on its geometry,
the combination of angles and dimensions
is probably the single most important thing
that governs how a bike feels to ride.
Now I don't know whether the slip is down
bike manufacturers actually building bikes
to fit into distinct categories
like race bikes or endurance bikes
or whether it's down to people whose job is
to describe the way those bikes feel.
Now as you're struggling to do so
but you are still needing to summarize it for our benefit.
Now I did think that despite having forgotten everything
I ever learnt in school about geometry,
I did understand bikes, atleast I understood what I liked
which is aggressive, fast handeling,
responsive bike, with short wheel bases
and short chain stays.
But then a few months back, this bike turned up
and its got me really thinking everything
that I thought I knew.
(upbeat music)
so, I'm off to get a lesson
and I thought you might be interested in joining
because to help us pick this subject apart
we one to go and visiting Tom Sturdy,
a Jedi of bike building
and conveniently based just down the road
at the bicycle academy where he teaches people
how to hand build bike frames as well as making his own.
(pop music)
Tom I have brought you two very different bikes,
I love them both equally but they are
so different from one another
the Canyon Aeroad I guess you call
your typically fast feeling bike, aggressive geometry,
well it's not confined to a race track
I mean I have ridden it for 400 kilimeters,
this happily is possible to run 400 kilometers
and also I would not be angry you know
its not just a out in out Aeroad bike.
The Trek meanwhile at the opposite side of the spectrum,
would be your quinticential stable bike on paper
but yet it's also hugly fast and also incredibly fun as well
and coincidentally both bikes went to victory
at the Tour of Flanders so they got peddled
in the same difficult race, so I was really hoping you would
able to help us kind of unpick this whole fast versus slow
geometry thing, I mean what actually does govern the feel
of a bike and can it affect how fast or slow it is?
- Sure, so what we'll do is we'll have a look
at each of these bike individually
and take some measurements to work out
what the geometry is and so we can look it out in detail.
- Great stuff!
(up beat)
- Okay Tom, let's go over
some of the most important dimensions then
that we're gonna be discussing
in respect to bike handling then.
We've got one Bottom Bracket Drop, that's one of the keys?
- Yes, so we gonna measure how low the Bottom Bracket
is relative to the axles of the wheels
so that's what we're referring tp with Bottom Bracket Drop,
so we've got a laser line here which is
at the same horizontal height as the center
of the front and the rear axle,
and I can measure to the outer most edge
of this circle here
and get a Bottom Bracket Drop of 73mm.
- Okay, and what does Bottom Bracket Drop
actually affect when we're talking about bicycle handling?
- So what we are doing with the Bottom, your riding position
is all going to be referenced from that Bottom Bracket
center and so by lowering the Bottom Bracket further down,
you're going to the ultimately lower the saddle
and the handle bars correspondingly
so you're going to lower the center of gravity
of the rider and the bike which is going
to put the center of gravity
closer to its foot print
or the vehicles foot print
which is contact part of the tyre.
- Okay cause commonly if you look at high school physics
text book which I have to read regularly
to try to understand things,
it will say that a higher center of gravity
makes an object more stable, but in the contect of a bike
actually, the opposite is true.
- So, it can be..
It all depends on how the object is
standing or balancing
because the way that a bike stays up right,
is actually by connstantly nearly falling over
so as its center of gravity moves outside of its
contact patch or foot print,
the vehicle will start to fall
in one direction and then the steering geometry
will allow the wheel to rotate
in that direction which allows its foot print
to get back underneath its self
a bit like balancing something on your fingers,
you have to constantly move.
- Next one then, we're gonna get for Chain Stud length
sometimes called rear center,
- That's right, so on most geometry charts that you
might read from published by a manufacturer
the Chain Stud Length is going to be a measurement of
a simple straight line measurement of the center
of the Bottom Bracket here
to the center of the rear axle here.
Another alternative measurement that people might use
is that distance but in a horizontal direction
although that's less common.
- Now chain stud is something that I have tended
to latch on to as a measure of whether
or not I think I might like a bike
but what does it actually affect when it comes to handling?
- It's important to consider the Chain Stud length,
with some other dimensions as part of the total wheel base
and indeed compared to its opposite measurement
the Front Center but you're controlling how close
the rear wheel is up underneath your position
and so by having a short chain stud,
the bike is generally going
to feel more snappy, more lively.
When we look at the Steering geometry
the key things that we're looking at,
one of the first ones is the Head Tube Angle,
so that's the angle the steering tube or the steering axis
is relative to a horizontal
so most Road bikes sit some where between 72 and 74 degrees
and that is quite standard for most Road bikes.
The other thing that we're looking at is what you mentioned
the Rake or the offset of the Fork,
and that is how far forwards typically
of that steering axis the axle is held.
And what those two dimension do
is they create a projected measurement along the ground
which is called Trail or somethimes referred
to as Mechanical Trail and that gives
the front wheel predictable rotational stability
so by having that measurement there,
it means the Contact patch of the tyre
will always trail behind the steering axis of the bike.
The more Trail you have, the more quickly
the front end of the bike
will react to any change
so arguably that's more stable
but we really are interested in here
in terms of how the vehicle feels
is that the trail is largely
or the magnitude of the trail
and the weight that acts over it,
really controls what you feel through the bike.
(upbeat music)
- Right, back with our two original bikes, Tom
so lets talk through the differences then
and why they do feel so different
but yet ultimately still deliver the same thing
which is fast and fun
- So these are actually quite significantly different
if you were to take the standard and of fast Reap road bikes
that, what most people would refer to
that's what you'd get as the Canyon
and the Trek has got quite significantly longer wheel base,
its also got a lot of Bottom Bracket,
a slacker Head Tube Angle
but interestingly it has much more Rake in the fork.
- [Interviewer] so its got those hallmarks of a stable bike
so, long wheel base, long front center,
low Bottom Bracket, long chain stays
but that Trail begins to offset that
so that's why it feels both stable
and yet also responsive at the same time.
- [Tom] Yah that's right so interms of
the kind of vehicle dynamics this is definitely
has got much more directional stability
from the longer wheel base
and you're right, the slacker Head Angle,
the combination of the slacker Head Angle
and more offset in the Fork means
that the Front Center is quite long
and so yes you're looking at something
that on paper is much more stable
but the sensation that you get from riding it
isn't necessarily fully in line with that
and a lot of that is to do with the fact that
they have done that iin combination with less trail
which effectively gives you a little bit less feedback
through the handle bars puts you more in control
of the handle bars rather than the bike doing things.
- So not a conventionally stable bike, it was designed with
the particularly needs of fabric and (mumbles)
so it's a stable bike
that also required rider input
if we take in the Canyon,
it's our traditional race bike geometry
why do you think we've arrived at this
collection of angles and dimensions
that requires that rider input
I mean it feels fast?
- [Tom] I think that really hits the nail on the head
is that a big part of riding a bike
and enjoying riding a bike is getting
the right feeling from that bike
and a bike with a short wheel base
or relatively short wheel base
is going to be one that it deliberately
doesn't have have much directional stability
so it will change direction very readily, very quickly
and that's much what we expect of racing road bike
very short Chain Stays typically road bikes will have
the shorter chain stays as possible
again snappy, fast accelerating this is all things
that we expect to feel when we ride a race bike
slighlty steeper head angle, which means
that there is less Wheel Flop as the bike leans
from one side to the other
sort of faster steering if you like
so everything it feels quite fast and snappy and racy
and yah you hit the nail on the head
really is that it's creating a certain
sensation if you like for the rider
which is what we have come to expect of a race track.
- [interviewer] I tend to use that
because so much of what we are told about
why a bike has been designed in a certain way
is with speed in mind, so for example if we take that Canyon
it's an air dynamic bike which means it is literally faster
you can go faster for the same effort
and it is light weight which also means you can
go faster and it's stiff and whatever else
but the geometry has almost been designed
separately to reward the rider in a different way
so the geometry makes you feel like your going fast
even if you're not actually going fast
is that a fair assessment?
- I think so, yeah and that doesn't necessarily
make it a good or a bad thing, it's just a thing.
So for some people the fact
that they feel like the're going fast
might actually makes them go faster
and the really complicated thing about a bikes
is the human riding them actually
and how they ride a bike what they enjoy doing
when they ride a bike how they produce power,
how they steer, how they brake,
all of these things will have a really meaningful impact
on the overall experience
and sometimes creating a certain sensation
is what makes a bike a good or bad bike
and it could be very different
for one individual to another.
- Is it fair to say then that when you look at bike geometry
as a bigger picture, actually there is as much
philosophy there as there's actual engineering
once you've got your basic parameters
whether the heand Angle is 72 or 74
and the Trail is 43 or 49, whatever
is there a case of actually more about the feel of the bike
that the speed, bike geometry has no effects
on air dynamics presumably
and its got no effect on how fast you can ride in a sprint?
- No, I mean there're ways in which
it could affect it slighlty so for example by lowering
the Bottom Bracket you have the facility
lower the whole rider which will have a small impact on
the kind of frontal area which is a key driver
for the Coefficient of drag
so there're small things
that the geometry could provide but by and large
what's safe to say is that what you are influencing
by changing the geometry of the two bikes
is what the rider is going to feel,
how how that vehicle is going to handle
and there fore what experience the rider is going to get
from being out on the road and climbing
or decending at a speed or just plowing along in the flat.
The weight of the rider is always going
to be the most significant weight on the vehicle
and small changes in where that rider is
and it's normally the most powerful measurement
is where your saddle is in relation to the Bottom Bracket
anything that you do that moves the riders Center of Gravity
forwards or backwards is gonna have a big influence
on how the bike handles
so you could have exactly the same bike
and if you were for example to switch between
a seat post that had lay back and one that didn't
and adjust the position on exactly the same bike accordingly
the feed back that you would get from that bike,
the way that bike would handle out on the road
would be significantly different
simply because you've moved your weight
so yes if the rider is if one rider has a
particularly long torso and therefore has more
weight over the front of the bike,
any given bike would feel different
to someone whose the opposite.
- Okay, I feel like I've come here in search of answers
and I've opened a can of worms
so rather than leave the viewers with this grappling
with the philosophical question,
what about some practicle advice
and if someone is looking to buy a new bike
and before they're taken for a test ride,
or if that's not possible and they're looking
at geometry charts what are the actual key things
that you can look for to say,
okay that I think is a boring effect,
I'm gonna enjoy riding.
- So it can be quite a daunting question
I think for any viewers who are asking that question
it's most easy to do if you have
some sort of frame of reference
so let's say you have an existing bike
and you know how that feels to ride,
you can then look at some geometry charts
and look at some of those key measurements
that we talked about like the wheel base
the bottom bracket drop, the front center
the rear center, the head angle, the trail
all of those numbers most of them are readily available
on the geometry chart and you can compare that
to what you know, compare that
to what you have currently
and that will enable you to make some decisions
so for example, if the bike that you have currently
you find that it is too agile, too twitchy
you want something with more stability
then you're looking for something with a longer
wheel base, lower bottom bracket,
a longer front center,that sort of thing.
So you can use the geometry
there're certain knowns there's no way that a bike
with a longer wheel base is more agile
than the one with a shorter wheel base for example
but ultimately, unless you have a frame of reference
as to how that feels for you versus for myself
or for someone else, then it's quite difficult
to make that decision, so the short answer
is there's no substitute for doing it, for having it,
having a go on the bike you ultimately want to buy
but if you have that frame of reference
then that's a really good starting point.
- Cool, Tom as always,
thank you very much. - [Tom] Your welcome.
- Have in my hands, a follow up video
these are two frames that were built here the past (mumbles)
and they have almost identical geometry
except with the relevant forks plugged into the front end,
will have wildly different trails
so when built up, we're gonna be able to neatly illustrate
one the points that Tom has just been talking about.
So what this space, in the mean time
give this video a big thumbs up,
say thank you very much to Tom
and if you would like to see another video,
filmed right here actually, where we throw
a little bit doubt over the question
of frame stiffness and mechanical efficiency,
then click on screen now.