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Peter Nilges did his
graduate dissertation at the German College of Physical
Education, Cologne, and researched the subject of
rolling resistance. The article was published in the
mountainbike-magazin.de.
Peter measured rolling
resistance under varying conditions. Three different
tyres (Schwalbe's Fast Fred, Racing Ralph and Albert
Brothers) in three different widths and at four
different pressure levels (1.5, 2.0, 3.0 and 4.0 bar -
c. 21, 28, 43 and 57 p.s.i.). The test course is an
uphill grade, 460 m in length, with side by side road,
gravel and a meadow surfaces. Nearly 350 test rides
totalling a distance of 150 kilometres.
Effect of tyre tread
pattern on rolling resistance
The Albert Brothers, with
their coarse tread pattern, were the hardest to roll on
all surfaces. Surprisingly enough studded Racing Ralph's
fine pattern rolls easier on-road than Fast Fred, a
semi-slick, while positions are reversed off-road. The
explanation is likely to be found in the structure of
the tread pattern and the carcasses' flexibility
(Evolution Design). Racing Ralph's studs protrude
significantly further but in the tread area are spaced
more closely so that they nearly form an uninterrupted
centre ridge. This lets the tyre roll more smoothly on
firm ground, as opposed to continually descending into
the gaps between individual studs, only to have to rise
again. Off-road on the other hand flexibility has a more
significant influence than the tread pattern. The
thinner rubber layer on the carcass of a semi-slick can
adapt more easily to an uneven surface.
Effect of tyre width
on rolling resistance
The test covered widths
from 2" to 2.4", or 50 to 62 mm. While on-road there is
no marked difference between a narrow and a wide tyre,
off-road the wide tyre is proven to roll more easily!
The rougher the ground, the greater the advantage, as
the data obtained on grass proves. The wider tyre's
contact area is wider, but shorter. Moreover wider tyres
have larger diameters, and again that improves rolling.
Off-road rolling resistance decreases significantly with
increased tyre width. For instance on grass the wide
mountain bike tyres required 15.41 W less rolling
resistance power than their narrower equivalents.
Effect of tyre
pressure on rolling resistance
As soon as you leave the
road, reducing tyre pressures does not just leave
rolling resistance more or less unaffected, as can be
heard here and there, but actually reduces rolling
resistance! This is true even on level paths of fine
gravel, but the rougher the ground, the greater the
effect, as the grassy ground shows. Reducing tyre
pressure from 4 to 1.5 bar (57 to 21 p.s.i.) can save an
averaged 20 W! The main reason for this is the
unevenness of the ground. A tyre with less inflation can
adapt to unevenness more easily. The total system needs
to be lifted to a lesser degree and less frequently.
Resistance is reduced, less power is required. Off-road
a reduction of tyre pressure reduces rolling resistance.
In a meadow for instance going back from 4.0 to 1.5 bar
(57 to 21 p.s.i.) can save remarkable 18 Watts of power.
On an identical course
and at exactly the same speed, the widest of the tyres
tested here at 1.5 bar (21 p.s.i.) requires a solid 50 W
less power than a narrow tyre at 4.0 bar (57 p.s.i.).
Rolling Resistance
Conclusions
Anyone who wants to ride
really fast off-road needs to decrease tyre pressure.
The rougher the ground, the more pronounced the effect.
In addition traction and comfort increase, too. Due to
their thin and flexible structure, semi-slicks offer the
best start-up values for minimizing rolling resistance
off-road. With a reduction in pressure, however, the
risk of a flat increases. And traction with the
semi-slick is limited. So the answer to the question of
which width is best off-road clearly reads 'fat tyre'
both for superior traction and snake bite prevention.
For cross-country-races
and marathons involving only a small percentage of
tarmac a wide tyre with low pressure is recommended. The
most overestimated aspect here is the frequently
criticized extra weight of the wider tyre. To accelerate
a pair of tyres with an extra weight of 500 g from 0 to
25 kph in 4 seconds requires an additional 4.2 W power.
On the other hand the wider tyre on a grassy surface
saves you 15.5 W against a narrower specimen, and this
at the low speed of 9.5 kph. Moreover the rolling
resistance reduction has a continuous effect while
lighter weight is only of relevance during acceleration.
Full translated download
of the
Rolling Resistance
Test thanks to
www.bicicletta.co.za
Lennard Zinn on
rolling resistance
"During any rolling
resistance test, you obviously have to keep tire
pressure constant so that you can compare apples to
apples. Beyond that, tire pressure should come down when
the road is wet or rough, and not just for shock
absorption. If the road is rough, the tire has less
rolling resistance when softer since the small bumps and
gravel chunks are absorbed into the tire, rather than
throwing the entire bike and rider up and back as
happens with high pressure, costing energy and requiring
re-acceleration of the bike. It’s the same reason for
why the rolling resistance of a mountain bike is reduced
with suspension and low-pressure, tubeless tires – the
“sprung weight” is reduced."
Taken from
Technical Q&A with Lennard Zinn – Rolling resistance
redux
Other Rolling resistance links
MTB Tires; rolling resistance, snakebite resistance etc.
- an analysis based on tire test on Bike 08/2004 |
