- Matt and Dan recently performed a tire width test
on the cobbles of Flanders
and they did so in order to try and show
just how much faster a wider tire would be
on rough roads.
Except the results weren't quite
what you might have expected
and we went into a little bit more detail as to why
that was the case, in our previous geek edition video
which there is a link to on screen now.
Fundamentally though,
the reason why the tires weren't faster in this case
was because Dan and Matt kept the tire pressures the same
for all the widths of those tires
thus negating any effect of suspension.
So, undeterred, they went back to see whether or not
changing tire pressures would therefore lead
to significant results and make Matt and Dan faster
and more comfortable over rough roads.
They went back to the rough and brutal Pavé
of Northern France.
(electronic music)
Now let's have a quick run through
of the testing conditions first.
Matt and Dan chose a section of the five star
Carrefour de L'Arbre cobbles for their test.
Now, these are some of the toughest cobbled
in all of Paris-Roubaix
and they're often where the winning move goes.
Now, they rode the section three times
at a set F level
although it was different between the two of them
and they used a set of 28 millimeter continental GP 4000s.
They dropped the pressure for each run,
they started 100 PSI, so 6.9 bar,
then 70 PSI, 4.8 bar
and then finally, 45 PSI, which is 3.1 bar.
Each run was timed, but then they also used Dan's
exceedingly high tech vibration meter.
An open water bottle, filled with water
and which ever run left the most water in the bottle
would be the comfiest tire pressure.
It probably surprised only Dan
that if you do ride with an open water bottle
across some of the toughest cobbles in professional cycling,
then no matter what your tire pressure,
there wouldn't be much water left in it at the end.
But the other results are on screen now and as you can see,
both Matt and Dan were significantly faster
each time they dropped their tire pressure,
which is a relief
because that was what we were expecting to see
but why were we expecting to see that?
And is it of any relevance to you and your riding?
First, let's talk rolling resistance, shall we?
We touched on that in the previous video.
One of the main causes of rolling resistance
for a rubber tire is hysteresis,
or more specifically, elastic hysteresis.
Now this is a property of a deformable material
whereby the energy of deformation is greater
than the energy of recovery.
So basically, excuse me,
what that means is that actually
the energy required to deform the tire in the first place
has got to be greater than the energy you get
when the tire bounces back.
So if you think of it in terms of a ball, for example,
you drop it from a height,
it is never going to bounce quite back up to the height
that you dropped it from.
The energy will go in the form of heat, or possibly,
(tire bounces)
a little bit of noise as well.
Now, tire hysteresis is more pronounced
when the tire has to deform more,
so on bumpy roads, or when the pressure is less
and so the tire is literally deforming more in your hat.
So you would think therefore that in order to reduce
hysteretic loss you need to whack a load more pressure
in your tire and therefore you would go faster.
And you know what?
For many years, that was the conventional wisdom,
you want to go fast, you stick a load of pressure
in your tire.
And actually on super smooth roads
and like a velodrome for example,
that is absolutely the case.
However, on anything less than a super smooth road,
and in fact of increasing significance,
the rougher the road gets,
you have another thing at play.
Suspension loss.
When you ride your bike down any stretch of road
your body is going to be vibrating slightly
and the worse the section of road,
the more it will vibrate.
As we can see here with Dan,
vibrating quite significantly.
As he rides his bike across the Pavé
and his skin, his tendons, muscles, bone
will all be rubbing against one another
and that will be generating friction.
Friction which therefore turns to heat
which is energy lost.
And actually the amount of energy that can be lost
is really, really quite significant.
So according to Bicycle Quarterly,
there was a study carried out in 1986,
by the US Army to try and find out which seats
they should use in their tanks.
And in doing that research they discovered
that a tank driver may have to absorb up to 2000 watts,
2000 watts!
Yeah, so no section of Pavé
is ever going to be quite that bad,
but it does illustrate the point.
So, the worse the vibrations get in your body,
the more power that you will need
to overcome them and the more power therefore
that you need to put out in order to keep moving forwards
down the road.
So, to combat those vibrations you let
a bit of pressure down in your tires
and you can actually feel this just out on your local road
you don't need a section of Pavé.
Whack 120 PSI in and ride down a rough road
and then let 40 PSI out and do it again.
You will notice the difference.
There is a problem though, isn't there?
Because at what point does hysteretic loss
exceed suspension loss?
Well, to find out exactly is actually quite difficult
and to be fair, how much pressure is the optimum pressure
will vary depending on the exact road surface
that you're riding on,
although you don't need a power meter
you can do it on feel.
Basically, if you start letting
a little bit of tire pressure out,
you can feel the vibrations reducing
each time you ride across that section of road,
up to a point where the bike then starts feeling sluggish.
So that should be the point at which hysteretic loss
exceeds suspension loss.
Either that, or you get to a point
where you bottom the tire out all the time
and you get a pinch flat.
Which brings us neatly on to the reason why
wider tires perhaps should have been faster over the cobbles
in the first place.
The wider the tire, the more force is needed
in order to actually bottom it out.
So that's the point at which your rim strikes the road
and you may well end up with a pinch flat.
Now, that isn't actually due to the width of the tire at all
it's due to the increased height of the tire
that a wider tire will actually give you.
And why does that matter?
Well, it means that you can therefore run lower pressures
get the benefit of reducing their suspension losses
with less chance of that rim striking the tarmac
and you getting punctures.
Now unfortunately, in spite of being armed
with all of that knowledge,
there is no universal panacea
for tire width and tire pressure,
it is completely dependent on your body weight
and the types of roads that you ride.
Hence the kind of guidelines about running 25 width tires
on smooth roads and 28 and 30 mm wider tires
on bumpy roads.
Or in fact, as we found out last year,
if you ride over cobbles,
you probably just want to ride a mountain bike.
Oh yeah, if you want to see that little experiment we did
in fact you can get through to it just down there.
Please do also make sure you give this video a thumbs up
and if you haven't seen Matt and Dan's experiment
about tire pressures
you probably want to watch that right now
and that one is just down there.
Geek out.