- We get hit with statements about the superiority
of disc brakes all the time.
Bike manufacturers say that discs are the future.
A lot of bike riders, to be fair, say that discs
are the future.
And several GCN presenters, in fact, feel that discs
may well be the future.
But do we actually know why?
We get told that discs give better braking performance,
but then critics will say we don't need better brakes
on road bikes, you need better tyres that give you
more grip, because you can lock up your wheels
very easily with rim brakes, should you so wish.
Today's critics oh they say a-ha, it's not about power,
it's about modulation.
And then I realised I actually don't have any facts
to back that up, it's all based on feel,
and feel is, of course, completely subjective.
So we wondered whether or not you can actually analyse
the performance of disc brakes versus rim brakes.
Generate some real data for both wet weather and dry
to find out just whether discs are actually
better at braking.
So how are we gonna do a super science, then?
Well firstly, the bikes.
We've got two Orbea Orcas here.
One with SRAM RED eTap HRD disc brakes,
and one with SRAM RED mechanical rim brakes.
Now, while the bikes aren't completely identical,
the important points are, so the geometry
of the frames is the same.
My position on the two bikes is also the same,
and then the contact patch, the tyres are also the same.
They're Continental GP 4000 IIs, 25mm wide,
and run at the same pressures, so 75 PSI.
And then, what are we gonna do with them?
Well, we brought them to the Alta Badia region
in the Dolomites, and we are currently stood on top
of the incredible Passo Pordoi.
Down there in the background lies nine kilometres
of sinuous tarmac.
One of the most amazing descents I have ever ridden,
and I'm gonna get to ride it another four times,
twice on each bike.
Once in the dry, and then once again in the wet.
Then, in terms of the data we're gonna collect,
firstly we've got our GPS data that we're gonna get
on our Wahoo ELEMNT.
That will be able to tell us, obviously, our actual time,
but then also we have to drill down and see
specific entry and exit speeds on corners.
Now so far that might not sound too sciency,
but we're gonna back it up with powermeter data.
And we have a heart rate monitor, but I'm not really sure
that's gonna tell us anything.
But then, it's gonna get really really interesting
with this, which may well just be a phone,
but it has an app on there which turns it into
a really accurate accelerometer.
So all being well, this will tell us just how hard
I can brake on a dicc bike versus a rim brake bike,
and also how hard I can lean into corners.
See?
Science, big data.
Right then, let's go.
Disc bike first, in the dry.
Recording, here we go.
(dramatic music)
So run number two, rim brakes.
I'm a bit more familiar with the descent,
so that might go in their favour.
Right, run number three.
Rain with a little bit of snow, on discs.
Ready, off we go.
Oh God, the things we do for science.
Right then, run number four.
Rim brakes in the wet.
(brakes squeal)
Right then, time for some data.
And there is quite a lot of it, actually.
We logged 688,000 points of data alone
from our accelerometer.
So it's take a little bit of time and more than
a lot of help from GCN team legend and brain,
Daffyd Thomas, so thank you very much to him.
Right then, the million dollar question:
were disc brakes faster?
Yes.
And also no.
In the dry, there wasn't much to separate the two at all,
and in fact rim brakes came out slightly faster,
by two seconds, in fact.
In the wet, though, results were flipped.
Disc brakes were significantly faster, by eight seconds.
If you translate that out into the road,
at 50 kilometres an hour, that works out as about
120 metres of gap between the two.
So that's not to be sniffed at, I think.
But, that's just the top line.
Let's have a closer look at the data, shall we?
We do have enough of it, after all.
We've isolated a key section of road.
It's 2.2 kilometres long.
It's got 11 corners on it, some of them super fast
and others really tight.
If we overlay the speed of each bike through that section
in the dry, then we can again see that there isn't much
to separate the two.
Disc brakes are in blue, and rim brakes are in red.
Now the GPS data is a little bit patchy in places,
but if anything, it does look like the disc bike
goes slightly faster on the straights,
but also slightly slower 'round the corners.
The power data was the same, the average was 175 watts
for each run, and the maxes were also about the same,
804 watts with discs and 811 watts with rim brakes.
To see whether or not we really were going fast
on the straights and slow on the corners,
we've taken the velocity data and plotted it on a histogram.
So that is where each bar represents the number of seconds
on each run where the bike was going a certain speed.
So if the bike was consistently going faster speeds,
we expect to see taller columns on the right-hand side
of that graph.
And we can see that on that short twisty section, again,
there isn't really anything to separate the two.
But then, if we look at the data over the whole run,
we can actually see that the rim brake bike
consistently hits much faster speeds.
And that's kind of puzzling.
So, we thought about the bigger picture.
Although the bikes are almost identical,
you can actually guess that the rim brake bike
has the aerodynamic advantage.
If nothing else, it's got the slightly faster Zipp 404 NSWs,
as opposed to 303s.
So, potentially then, a significant part
of the faster velocities could actually be down
to pure aerodynamics.
What about in the wet, then?
Well again, disc brakes seem to go a little bit faster
on the straights and then a little bit slower
in the corners.
But this time when we actually plot that data
on a histogram, we can actually see that that is
indeed true, disc brakes did seem to go a little bit faster
on the straight.
So that's food for thought, and we will come back to that
in just a moment, after we've had a look
at our acceleration data.
Now I'm not gonna lie, this was a massive pain
in the backside.
I was a little bit concerned initially that the data
would simply be too noisy, given that the top tube
of the bike at 70 kilometres an hour is probably going
to be quite bumpy.
And sure enough.
Not to be deterred, though.
We took the data from the y-axis, which records force
in the direction of travel, and then Daff set about looking
into one particular corner with a super fast entry.
Can we see the difference?
Eh.
One thing, though, those oscillations at the end
of the graph, they were the only pattern that was repeated
throughout the whole of the descent.
And we can see it quite frequently.
So we plotted that against GPS data, and it turns out
that's actually me getting out of the saddle
and sprinting back up to speed.
Kinda cool.
Not very relevant to braking, but nevertheless,
kind of cool.
Anyway, through the noise I think you could argue
that actually there is definitely more pronounced activity
on the disc brake bike on the entry into the corner.
It looks like I brake harder compared to the rim brake bike
where there is less activity, until midway through
the corner when I grab a handful of brake.
Which is not exactly textbook stuff, I know.
And then those big spikes, by the way, that is actually
bumps in the road, like that one.
So if that is the summary of the data, then,
what does it all really mean?
Well, there isn't much to separate disc brakes
and rim brakes when it comes to performance in the dry.
There is nothing from any of our data, in fact,
that can really set them apart,
other than perhaps our rim brake bike performed
slightly better aerodynamically.
In the wet, though, things are much more interesting,
thanks to the fact that there is actually a difference
between the two, particularly the faster top speeds
on straights, which I think are probably down to the fact
that the braking in the wet on disc brakes
is much more controlled.
We can see from our accelerometer data that I was able to
brake much harder, and therefore slow down faster.
And given that I obviously didn't have any more grip
from the tyres, I think that purely comes down
to more predictable braking.
Do that for each corner, then, and you can see where time
is really gonna add up.
The reason I was going slow around the corners
on the disc bike in the wet was probably a result
of it being my first run down the mountain in the rain.
And therefore in the second run, I was a bit more familiar
with the level of grip that the road was offering.
And actually the same is true in the dry,
the disc bike went first and the rim brake bike
went second.
Although it wasn't my first run down the mountain full stop,
I was a little bit more familiar with the road,
particularly on a couple of key sections where I knew
that I could let off the brakes completely.
Now in the intro to this video I said that anything
we ever really talk about in the disc brake debate is feel.
And so having spent ages collecting data and then
going through it, I do feel slightly sheepish about the fact
that we are now gonna come back to feel.
But then maybe actually that is one of
the most important things when we ride our bikes.
I had more confidence descending on a disc brake bike
in the wet, and actually I had a little bit more confidence
in the dry, as well, if I'm being completely honest.
And clearly I wasn't making the most of the bike.
I wasn't pushing the tyres to their limits.
And I think that comes down, then, to the limitation
of the test.
I didn't do enough runs to remove any kind of bias
about run one versus run two, and being more familiar
with the road on that second run.
So, more time needed actually doing tests.
And then whilst we're at it, why not do tests
on different descents, as well, that may well change
the results, as would different riders.
What happens if you weight 60 kilos,
as opposed to my 73 kilos, and then what happens
if you're 80 kilos, or even 90 kilos?
It certainly brings home the difficulties with actually
trying to measure the differences between bikes.
Often what we perceive as being significant
is actually almost impossible to quantify out on the road.
Finally, before I go, there is one thing that definitely
needs to be said, and that is that our little test track
of the top five and a half kilometres of the descent
of the Passo Pordoi in the Dolomites
was one of the best descents I've ever ridden.
And on each bike regardless of brakes, it was fantastic fun,
and I think that is very important to mention.
Now, obviously we wanna hear your thoughts on this.
Have we missed something from our data?
And in fact which side of the fence do you sit on
in this great debate: disc brakes versus rim brakes.
Let us know in the comment section down below.
Also make sure you subscribe to GCN
before leaving this video.
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Just click on the globe, and then if you want
some more content now, we've got two old
but very good videos about disc brakes.
Are they ready for road bikes?
That one is just there.
And how much faster can you stop with disc brakes?
That one is just there.