- Matt and Dan recently performed a tire width test
on the cobbles of Flanders to find out whether wider tires
really are faster over rougher roads.
Now they filmed it on a particularly cold day in Flanders
so I thought it'd be a good idea to delve a little deeper
into the subject but in the dry and the warm.
So, let's geek out and find out
why different tire widths could lead
to different speeds.
And maybe even why you might not quite've
seen the results that you were expecting.
Now if you haven't watched that video yet
then do make sure you go and check it out.
(techno music)
The methodology was to use the same bikes,
wheels, and tire pressures.
Meaning that the only controlled variable
was the width of their tires.
Now there was of course no accounting for weather conditions
or in fact for Dan and Matt, more to the point.
But they did ride the same section of cobbles
three times at the same set power.
Minimizing those variables as best they could.
Any speed difference therefore
would have to come from different tire widths.
Or you'd hope so, anyway.
Now here are the results of that test.
As you can see, Matt was slightly faster
as the tires got wider.
And Lloydy, not really any change at all.
So statistically, excuse me, not significant results then.
Sorry about that chaps.
But it was not a wasted day out in the cold, not at all.
It is interesting, nevertheless, because we might've
expected to see faster times with wider tires.
So, what is actually going on?
Well, we're going to break this down into three parts.
Contact patch, both hysteresis and slip angled,
and aerodynamics.
Why might they've been faster then?
Well, wider tires mean that the contact patch
between the tire and the ground is shorter and wider
compared to a narrower tire.
When the inflation pressures are equal.
The shorter the contact patch
the less rolling resistance it will generate
and therefore, the faster you go.
Now the reason being, that while the contact area
is the same between the widths
the longer and narrower shape of the narrower tire
requires the tire to deflect more as you roll.
Deflection that requires energy
in a process called hysteresis.
Basically as the rubber in the carcass of the tire
deforms and rebounds as you roll
an amount of energy is lost in the form of heat.
A wider tire therefore suffers less hysteretic loss.
Now, as an aside when you couple wider tires
with wider rims like the new Bontragers that they were using
the contact patch becomes even shorter.
Although, we have to say that given that was consistent
across all the tests it won't actually
have influenced the results.
But I though it was interesting anyway.
Now, another advantage of that wider contact patch
is when the test sector requires side to side grip,
so like cornering or getting on and off
the crown of the road.
Now this is due to what is called the slip angle.
Which is the angle between a wheel's actual
direction of travel and the direction it's pointing.
Now, it is a super complicated picture
but effectively a shorter contact patch
reduces the slip angle.
Meaning that the tire should effectively grip better.
More grip means faster through turns,
shorter time taken for the run.
You would've thought.
What about aerodynamics then?
A narrow profile is faster, right?
Well yes, unless it's too narrow.
So, let me explain.
To shorten the contact patch of the tire
rims have been getting wider internal dimensions,
which we touched on earlier.
But they have also been getting wider
to improve aerodynamics with wider tires.
Which actually means now, that if a narrower tire
is fitted to a wide rim, the rim itself will literally
stick outside of the tire and a lip like that
is obviously really bad aerodynamically.
A wider tire, meanwhile, will cause smoother air flow
over the edge of the rim.
However, it's only true up to a certain point.
Once a tire gets too wide, no matter how wide the rim
the increased frontal area of the tire negates
the benefit of improved rolling resistance.
So, this is why professional riders
aren't riding around on slick mountain bike tires.
In fact, when I asked Zip during the 650b video
they estimated that a 47 millimeter wide tire
would need about 20 watts more than a 30 millimeter tire.
So imagine what it would be
compared to that 23 mil. tire right there.
So, from all of that we would expect to see
some speed improvements as the tire got wider in our test.
Assuming the 28 mil. tires are not over the tipping point
of being too wide.
And put it this way, estimation from Enve
when they released their latest super wide
SES 4.5 AR wheels suggested that a 28 compared to
a 25 was literally just a couple of watts.
Would that though be enough to offset
any improvements from the contact patch?
Well, I don't know.
But it may explain why there was
a mixed picture from our data.
Or maybe, it's because as with all GCN science
we needed more of it, more data.
And to be honest, it's probably because we controlled
one aspect of setting up the tires
that actually needed to be a variable, tire pressure.
One of the main reasons for using wider tires
is that you can run them at lower pressures
without increasing the risk of pinch punctures,
which are especially an issue on the cobbles of Flanders
and indeed Northern France.
And then, why would you wanna run
lower pressures in the first place?
Well, that is the most important thing.
And it is a topic for a future video.
Literally because Dan and Matt went back
and repeated their test but varying
the tire pressure instead.
The differences in results there will be due to
reduced suspension losses relative to hysteretic loss
and aerodynamic drag.
It is a hugely important componant to reducing
road resistance and we will get into it next week.
Until then, geek out.
See what I did there?
Anyway, do make sure you check out Dan and Matt's video.
If you haven't seen it already
you can get through to it just down there.
Now please give this video a big thumbs up
if you like me, like a bit of geeking out.