Cycling physics questions

Hi all,

A couple of cycling physics questions:

1. When I'm cycling into a headwind, I usually bend as low as possible to
reduce the wind resistance. Does it make sense to keep my back as upright as
possible when I have a tailwind behind me (so as to maximise the area of my
body that is being 'pushed')? I ask, because I still find that being hunched
over the bars is a more comfortable position here as well.

2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb a 20%
hill with the same amount of effort, I should use a 19x20 combination?
(Experience tells me that there is no such linearity, but I was thinking
that since the second case meant doubling the amount of work I do, then I
should double my efficiency.)

Cheers,

--
Akin

aknak at aksoto dot idps dot co dot uk

Epetruk wrote:

1. When I'm cycling into a headwind, I usually bend as low as possible to
reduce the wind resistance. Does it make sense to keep my back as upright as
possible when I have a tailwind behind me (so as to maximise the area of my
body that is being 'pushed')? I ask, because I still find that being hunched
over the bars is a more comfortable position here as well.

It depends on relative velocity. If you're going faster than the
tailwind then you still have a net headwind, so you're better in a
crouch. If, OTOH, you want to freewheel with sind power then the
greater the area of "sail" the better. Any amount of pedalling will
probably mean that a crouch is more efficient, but if you /really/ want
to do this properly get a velomobile or other faired recumbent.

2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb a 20%
hill with the same amount of effort, I should use a 19x20 combination?

The effort (work) required to get up a hill, irrespective of gradient,
is basically the height of the hill plus a bit more for frictional drag
along the road. What gearing you use, along with the cadence,
determines how fast you'll get up it, not the total amount of effort
you'll need. Trundling up something in 10 minutes in an ubergranny
actually requires the same amount of work as mashing it in top, but the
latter requires a lot of force applied on a few pedal strokes rather
than hardly any applied over very many. Steeper hills feel harder
because basically they require you to gain the height more quickly, so
your work /rate/ is quicker.

To get up hills best, just ask your legs and cardiovascular system what
works best for them. They'll be telling you with puffing and lactic
acid buildup...

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/

Epetruk composed the following;:
Hi all,

A couple of cycling physics questions:

1. When I'm cycling into a headwind, I usually bend as low as
possible to reduce the wind resistance. Does it make sense to keep my
back as upright as possible when I have a tailwind behind me (so as
to maximise the area of my body that is being 'pushed')? I ask,
because I still find that being hunched over the bars is a more
comfortable position here as well.

With a tailwind a lot depends upon if you're going faster than the wind.
If so, then you're still riding into a headwind, so a crouch is better.
If you're riding at the same speed as the tailwind then an upright
stance should help. I just go with what feels comfortable, rather than
trying to analyse it ..

2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb
a 20% hill with the same amount of effort, I should use a 19x20
combination? (Experience tells me that there is no such linearity,
but I was thinking that since the second case meant doubling the
amount of work I do, then I should double my efficiency.)

Climbing a hill uses the same amount of work however you do it. If
you're spinning then you use lots of fast, little bits of work, if
you're grinding in a high gear you're using fewer, slower, bigger bits
of work, for the same net amount of total work.

Again, personal preference dictates, for me, how I climb. Some days I
grind, some days I spin.

--
Paul ...
http://www.4x4prejudice.org/index.php
(8(!) Homer Rules ...
"A tosser is a tosser, no matter what mode of transport they're using."

Peter Clinch wrote:

The effort (work) required to get up a hill, irrespective of gradient,
is basically the height of the hill plus a bit more for frictional drag
along the road. What gearing you use, along with the cadence,
determines how fast you'll get up it, not the total amount of effort
you'll need.

Well, it also depends how fast you're going up the hill. The faster
you go, the more work done to overcome wind resistance. However, if
like me you head up hills pretty damn slowly, the additional work done
is likely to be negligable compared to the work done getting up the hill.

There's also the issue that if you go very slow indeed, there's likely
to be a degree of "wasted" effort from the increased wobbling :-)

R.

Peter Clinch wrote:
2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb
a 20% hill with the same amount of effort, I should use a 19x20
combination?

The effort (work) required to get up a hill, irrespective of gradient,
is basically the height of the hill plus a bit more for frictional
drag along the road. What gearing you use, along with the cadence,
determines how fast you'll get up it, not the total amount of effort
you'll need. Trundling up something in 10 minutes in an ubergranny
actually requires the same amount of work as mashing it in top, but
the latter requires a lot of force applied on a few pedal strokes
rather than hardly any applied over very many. Steeper hills feel
harder because basically they require you to gain the height more
quickly, so your work /rate/ is quicker.

Actually Peter, when I said "same amount of effort", I actually meant the
same amount of exertion, i.e. huffing and puffing, muscle strain, etc. I'm
sure you'll agree that if you do a hill in low gears, this exertion is less
than if you do it in higher gears (even though you may go slower). Perhaps
the word I'm looking for is 'power' rather than 'work'.

Richard wrote:

There's also the issue that if you go very slow indeed, there's likely
to be a degree of "wasted" effort from the increased wobbling :-)

Trike with a supergranny and parking brake: no wobbles, stop whenever
you want without taking your feet off the pedals...

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/

Epetruk wrote:

Actually Peter, when I said "same amount of effort", I actually meant the
same amount of exertion, i.e. huffing and puffing, muscle strain, etc. I'm
sure you'll agree that if you do a hill in low gears, this exertion is less
than if you do it in higher gears (even though you may go slower). Perhaps
the word I'm looking for is 'power' rather than 'work'.

Probably, though note that if you spin up at half the speed you have to
do it for twice as long and just because the power output is lower you
/do/ need the stamina to keep it up.

A case in point: on the flat Roos is just as quick as me, she's got
similar power. Put us up a big hill (say, over 50m ascent and at least
10%) and because I've had a lifetime of going up hills and she's spent
most of her life in NL my extra hill stamina makes a very tangible
difference, and I can keep putting out that power for appreciably longer.

Also note that speed is not absolute in a gear, because you can vary
your cadence. It may be better to spin a lower gear at a higher rate to
get a certain speed. Because human muscles and joints and their
associated cardiovascular fueling systems aren't neat linear machines
you'd probably be better off trying to experiment empirically rather
than do mechanical thought experiments.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/

Epetruk wrote:
Hi all,

A couple of cycling physics questions:

1. When I'm cycling into a headwind, I usually bend as low as possible to
reduce the wind resistance. Does it make sense to keep my back as upright as
possible when I have a tailwind behind me (so as to maximise the area of my
body that is being 'pushed')? I ask, because I still find that being hunched
over the bars is a more comfortable position here as well.

Only if the following wind is actually blowing faster than you are
cycling, which is rare to nonexistent IMO.



2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb a 20%
hill with the same amount of effort, I should use a 19x20 combination?

Yes, assuming by "effort" you mean force on the pedals (or equivalently,
power at a given cadence). As others have said, the total work done
depends on the height of the hill, not its gradient, but I don't think
that is what you are talking about.

James
--
If I have seen further than others, it is
by treading on the toes of giants.
http://www.ne.jp/asahi/julesandjames/home/

Epetruk wrote:

Hi all,

A couple of cycling physics questions:

1. When I'm cycling into a headwind, I usually bend as low as possible to
reduce the wind resistance. Does it make sense to keep my back as upright as
possible when I have a tailwind behind me (so as to maximise the area of my
body that is being 'pushed')? I ask, because I still find that being hunched
over the bars is a more comfortable position here as well.

In most situations you'll still be travelling faster than the average
tailwind, so a crouch is still in order. However, it's less critical to
crouch if the resultant headwind is only 5mph.

2. When I'm climbing a 10% hill, I can just about cope using a 38x20
combination of front/rear chainrings. Should this mean that to climb a 20%
hill with the same amount of effort, I should use a 19x20 combination?
(Experience tells me that there is no such linearity, but I was thinking
that since the second case meant doubling the amount of work I do, then I
should double my efficiency.)

For a given pedal rpm, it's true that 19x20 will give you the same force
on your knees than 38x20 on the shallower hill. You'll climb at half
the speed though.

Peter Clinch wrote:

Also note that speed is not absolute in a gear, because you can vary
your cadence. It may be better to spin a lower gear at a higher rate to
get a certain speed.

Unless you're on a fixie, when you must ATTACK! I've done 1 in 6 hills
in 48 x 18, and it's surprisingly easy. If your knees are well
conditioned, it's sometimes easier, at least for short hills, to stomp
up them in a big gear. Disclaimer = this isn't advisable all the time,
or you get fat heavy legs.