- We've just released a video over on GCN
where Dan and I finally get to check out
this new trend of using smaller 650b wheels
but fitted with bigger, fatter tyres.
Now, there is a lot in that video.
But then we also had to leave quite a lot out as well,
not least the fact that we both really
nearly stacked it on the very first corner of the day.
Something you've probably already seen.
But if you haven't, you've just been able to see it again.
Clearly, too good to leave out.
Anyway, we thought that we would take the opportunity
over here on the tech channel to just
delve into the subject a little bit more.
To basically really geek-out.
All right, first things first, the top line.
A 650b wheel has a smaller diameter
than a traditional 700c road wheel.
It measures 584 millimetres across,
whereas a 700c wheel measures 622 millimetres across.
However, if you actually do put a tape measure
across your wheel, you will see
(grunting)
that it doesn't measure 622 millimetres, at all.
And that's because the 622 refers to what's called
the bead seat diameter.
So that's like, the innermost bit of the rim
where your tyre sits.
Now, you could be forgiven for asking,
why on Earth they are called this.
And it is a very good question.
And it basically stems from an old French system
of measuring wheels, where 650 and 700
referred to the overall diameter, not just of the wheel,
but of the tyre as well.
And then, the letter refers to the width of the tyre.
To A, B, and C.
Now with the fattest tyres, C,
you therefore needed a rim that was smaller
in order to keep that overall diameter
exactly the same.
Now, as you can see, that is a pretty crappy system.
Or at least, I think so.
And that's probably why we are now
down to just two major choices.
Or are there?
Because there is also, 650c.
And which measures 571 millimetres across.
Remember, back in the day, that would have had
a fatter tyre on it, to help it measure up
to 650 millimetres total diameter.
Now, it is a little-used size of wheel,
normally on smaller road bikes,
but it can't be that little-used, to be fair,
because Continental still makes 650c road tyres
and there are other options out there as well
if you start to look.
And Emma Pooley, in fact, won her
World Time Trial Championship aboard a Cervelo P3,
designed around 650c wheels.
Unfortunately, though, Cervelo don't make that bike anymore
and they are starting to fall out of use.
But they are now being replaced by 650b.
Oh, yeah.
Probably because they have been made popular
in recent times by mountain bikers.
More on that in a second.
(upbeat music)
Now while Emma Pooley's World Time Trial Championship
victory is still fresh in our minds,
let's have a think about performance, shall we?
Because it's a commonly-held belief that
bigger wheels roll better,
more efficiently than smaller wheels.
And it's something that mountain bikers
are all too familiar with.
They've been sold a concept for like, 10 years, now.
A 29er, which, incidentally, uses the same
diameter rim as our 700c road ones.
So, 622 millimetres across,
will roll better across bumpy terrain, so faster,
more efficiently, than the original mountain bike
size wheel which was 26 inches.
Apologies for mixing my Imperial with my Metric, there.
But let's have a quick look at why, shall we?
A hypothetical obstacle on a trail, let's call it a rock,
measures five centimetres high.
Now, when that smaller diameter 26-inch wheel hits it,
the angle created between the contact patch
of the tyre on the trail,
and the obstacle, is 128.7 degrees.
When the larger 29er wheel hits said obstacle,
the angle of attack is also larger,
up to 131.1 degrees.
And it's that larger angle that means
that the larger wheel is able to roll over
the obstacle more easily.
Now, you might not think that
that difference adds up to very much.
It is just 2%, after all, but as anyone
who's ever ridden a 29er can attest to the fact
that they do roll over almost anything.
and they're significantly better than 26ers.
And there's actually a stack of empirical evidence
as well to say that 29ers really are faster.
However, that sense that bigger wheels
are more efficient also exists
in our skinny tired world as well.
But when you think about it, we don't encounter
five centimeter-high obstacles that much, thankfully.
When we do, they tend to be quite catastrophic.
What we do encounter all the time are those much,
much smaller micro-bumps, a few millimetres high,
that contributes to that high frequency road buzz
that we feel on the bike.
And when you think about angles of attack there,
the difference between a 700c wheel,
and a 650b wheel, it's almost nothing.
Microscopic, in fact.
And actually you could probably get away from them
simply by letting 10 PSI out of your tyres.
And then, when you add into the mix
the fact that 650b wheel is 7% more aerodynamic
than a 700c according to Zip,
and it's also 12% lighter, there's actually
a case to say, that smaller wheels could well
roll faster than 700c wheels on smooth tarmac, anyway.
On that GCN video though,
only some of it was on tarmac.
'Cause as you can see, our 650b wheels
have been rather fat, very trendy 47 millimeter-wide tyres.
Meaning, that we can go off road.
Like, proper off road.
And with these tyres, the angle of attack question
is largely academic too because the diameter
of these tyres is almost identical
as the diameter of our Cyclocross tyres
on a standard 700c wheels, as well.
Although, given what I've just been talking about,
you could quite rightly ask the question:
why would you not just put
a mountain bike tyre on your road wheel
to make it into a 29er?
And then you'd be faster.
Valid question.
For me, the answer lies in geometry
because wheel diameters have a hugely
significant effect on the way a bike feels to ride.
But it's also the way the bike
has to be designed around them.
To ride 29er wheels, the bike's gonna
have to be longer and bigger in order to fit them in.
And that is gonna seriously affect
the handling characteristics.
For a start, it's likely to feel more stable,
forward slash, ride like barge.
And it's also gonna raise you up more,
raising your position.
But then so that you don't feel like you're
12 feet off the ground, probably the front
and the bottom bracket will be lower.
Now, on a mountain bike, this is a really good thing.
It makes you feel like you are in the bike,
not on the bike.
But so again, to exacerbate that position problem.
'Cause while you may not feel like you're
12 feet off the ground anymore,
your handlebars probably are 12 feet off the ground.
So the theory then, is that you use 650b wheels,
but with larger tyres,
and you can keep the geometry and your position
exactly the same as on your road bike.
So you get the same handling, same position.
You have the same chaser length, same fork length.
You have the same angles, you've got the same trail.
You've got the same wheelbase, you've got the same--
(upbeat music)
So, does it feel identical, then?
No, no it doesn't, actually.
I mean, it feels really similar
when you first get on the bike
and you accelerate out the saddle on the road.
And then it feels similar when you're riding at slow speeds
like around the car park at the bike shop.
But when you get faster, and you start cornering
at faster speeds, it feels very, very different indeed.
And that is because of the width of the tyres
and something called pneumatic trail.
You see, larger tyres have larger
contact patches with the road.
So literally, more rubber is in contact
with the tarmac at any one time,
and that's particularly true if you also make the most
of the fact that you can run lower pressures
because they are wider tyres.
Now, this gives you more grip, yay!
But it can also make the bike feel
more sluggish, because of that
aforementioned pneumatic trail.
Now, you can literally feel this at the handlebars
because as you turn, the front tyre
naturally wants to straighten up
away from the direction of the turn.
Meaning that you, therefore, have to put more force
into the handlebars in order to keep going
in the direction that you want to.
And that is because of force that builds up
at the rear of the contact patch.
And it generates what's called a self-aligning torque
literally trying to straighten up your front wheel.
And it is genuinely really noticeable,
even on 30 millimeter-wide tyres, as well.
You can combat it by increasing the mechanical
trail of the bike, so tweaking the geometry.
But then on a bike where you regularly swap
between thin tyres and fat tyres,
you have to strike some kind of a balance.
And there is also, actually, a short-term fix as well,
by inflating your tyres really hard,
you will therefore decrease the size of that contact patch.
If, having watched the GCN video and also
got a bit more info on the tech side,
you are keen to give 650b wheels a go,
you're probably gonna be wondering
how you're gonna be able to do that.
What's compatibility like?
What bikes can they fit?
So, the bikes that we rode in the video were
kindly lent to us and they're made by Rondo.
They've been designed specifically to fit
wider tyres on 650b wheels and also to be swapped
as you saw, with 700c wheels as well.
However, you don't necessarily need bikes
specifically designed for the purpose.
I have been told, that quite a few Cyclocross bikes
would also be able to fit them in.
Or at least, the more modestly-sized
47 millimeter-wide WTB ones that we've got here.
Including, apparently, my own TREK Crockett Cyclocross bike.
Now, I haven't tried it
and TREK don't say explicitly that you can.
Although I'm not sure they say explicitly that you can't.
But GCN viewer Adam Walter was kind enough
to let me know on Instagram that he
has been doing exactly that, to great effect as well.
So, you will need tyre clearance,
and that you will also unfortunately need
disc brakes as well.
Because of course, rim brakes will not be able
to cope with the difference in rim diameters
between 650b and 700c.
Oh, and then of course, you also need to borrow
some 650b wheels, but at this point,
if you've got through axles,
you could just nick a pair from your mountain bike friends.
That's right.
Watch their face when they see you riding around
on a fully rigid drop-handlebar bike.
Priceless.
Anyway, do make sure you give this video
a big thumbs-up if you like geeking out,
and as well as letting us know your thoughts
on this subject, in the comment section.
Also let us know if there are other subjects
that you wanna nerd-out on,
get involved in a little bit more science,
and we will try and do our best.
We'll also rope in the services of
Mr. John Cannings as well.
And then, of course if you have haven't already seen
that GCN video, I suggest that you head over there, now.