Granny gears

davek said:

Jon Senior said:

The motor car (And its gearbox) leads many
people to directly equate gears with speed.

Quoted message said:

Surely most motorists understand the need to select a lower gear when
going up a steep hill? Although, having said that...

But on a bike with fashionable derailleur gears, you have to stop
pushing the pedals for a few moments to change. And if the only reason
you're moving is because you're standing on the pedals, if you change
gear you'll fall off. Or stop. And you can't change gear if you're
stopped.

The only way out of this is thinking ahead. It wouldn't surprise me if
that's one of the things they've dropped from the National Curriculum,
along with school dinners and playing fields.

--
Chris Malcolm [email hidden] +44 (0)131 651 3445 DoD #205
IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK
[http://www.dai.ed.ac.uk/homes/cam/]

Jon Senior (jon_AT_restlesslemon_DOTco_DOT_uk) wrote:
: Peter Clinch [email hidden] opined the following...
: > Indeed. I used to be rather afraid of using them for some reason.
: > Whatever it was, it was a daft and unreasonable reason, and I'm glad
: > I've left it behind. But it does seem to be quite widespread.

: I suspect a large part of it may boil down to cyclists who dont
: understand the physics.

On the subject of not understanding physics ... if I've the option of
two gears which work out at the same overall ratio does it make any
difference which one I use. If one uses the small front ring and small
rear sprocket and the other uses the large front ring and a large rear
sprocket am I getting more leverage on the cranks using the smaller
chainring or is it simply cancelled out by the smaller rear gear?
Blair.

B.G. Finlay IT Services said:

On the subject of not understanding physics ... if I've the option of
two gears which work out at the same overall ratio does it make any
difference which one I use. If one uses the small front ring and small
rear sprocket and the other uses the large front ring and a large rear
sprocket am I getting more leverage on the cranks using the smaller
chainring or is it simply cancelled out by the smaller rear gear?

In this case both of them are bad. The simple ratio of teeth to teeth
front to rear is altered by deviation from a straight chainline, and the
straighter the chainline the more efficient it will be (and also less
wear on the transmission). So in the above case you should actually use
middle to middle rather than large to large or small to small.

There will still be some overlap combinations where you've effectively
got an even choice, in which case it's mainly determined by where you're
coming from/going to. Front changes tend to be more traumatic than rear
ones and if something's going to go wrong it'll be at the front, so with
all else equal change at the back. But you need to be predictive, so if
you're about to start a [censored] Hill then get into the small ring early
on when you're still doing a reasonable speed so you don't have to
subsequently do a front change with a fair bit of tension in the
transmission.

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 [email hidden] http://www.dundee.ac.uk/~pjclinch/

Peter Clinch said:

There will still be some overlap combinations where you've effectively
got an even choice, in which case it's mainly determined by where
you're coming from/going to.

I understood the OP to mean this rather than the use of the extreme big-big
and small-small combinations. Something like 53-19 versus 39-14. In which
case I would think that there /is/ a small difference in efficiency.
However, one study has said that bigger cogs are better as the chain doesn't
have to articulate through as great an angle, while another said that
smaller cogs are better because of something to do with the chain tension,
which made no sense to me...

Overall, I reckon the answer is "it depends" :-)

--

Dave Larrington - http://www.legslarry.beerdrinkers.co.uk/
World Domination?
Just find a world that's into that kind of thing, then chain to the
floor and walk up and down on it in high heels. (Mr. Sunshine)

Peter Clinch ([email hidden]) wrote:
: In this case both of them are bad. The simple ratio of teeth to teeth
: front to rear is altered by deviation from a straight chainline, and the
: straighter the chainline the more efficient it will be (and also less
: wear on the transmission). So in the above case you should actually use
: middle to middle rather than large to large or small to small.
: There will still be some overlap combinations where you've effectively
: got an even choice, in which case it's mainly determined by where you're
: coming from/going to. Front changes tend to be more traumatic than rear
: ones and if something's going to go wrong it'll be at the front, so with
: all else equal change at the back. But you need to be predictive, so if
: you're about to start a [censored] Hill then get into the small ring early
: on when you're still doing a reasonable speed so you don't have to
: subsequently do a front change with a fair bit of tension in the
: transmission.

Not sure you've understood what I mean. I know about chain crossover and
too much pressure when changing gear. What I'm after is whether there is
any advantage using the small/small combo over the big/big combo if the
overall gearing is the same for both ... no gear changes involved.
Blair.

B.G. Finlay IT Services said:

Not sure you've understood what I mean. I know about chain crossover
and too much pressure when changing gear. What I'm after is whether
there is any advantage using the small/small combo over the big/big
combo if the overall gearing is the same for both ... no gear changes
involved. Blair.

Isn't there some ad(d?)age that one extra tooth on the back is worth two off
the front, or something? I've no idea why or, indeed, whether this is the
case.

B.G. Finlay IT Services said:

Not sure you've understood what I mean. I know about chain crossover and
too much pressure when changing gear. What I'm after is whether there is
any advantage using the small/small combo over the big/big combo if the
overall gearing is the same for both ... no gear changes involved.

My thinking is "not much, if any", but having said that gear changing
will eventually come into it so you may be better off with a combo that
lets you do your next change at the back more sensibly than the front
when the time comes. Or in other words, it makes no difference right
now, but might do in a minute when it comes time to select something else.
Beyond that, bigger sprockets wear a little less at the same speeds
becuase there's more teeth to take the overall wear, but OTOH they weigh
more. Like Dave said, "it depends"

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 [email hidden] http://www.dundee.ac.uk/~pjclinch/

Surely the only real change is in chain tension and velocity? That might have a fractional effect on efficiency, but I bet it's so little you can ignore it.

Peter Clinch [email hidden] opined the following...

Quoted message said:

/Another/ thing that's good about recumbent bikes is they force one to
abandon this sort of thing on the grounds that wobbling and then falling
over makes one more obviously appear Silly than trying to honk away in a
100" gear...

The invoice for my replacement keyboard will be arriving shortly. :-)

Jon

Chris Malcolm [email hidden] opined the following...

Quoted message said:

Most people buy bikes for fashionable reasons. Lots of gears are
fashionable. So is no mudguards. Can't think of any other explanatiom
for why on rainy days university students cycle round Edinburgh with
mud stripes up their backs sogging the lecture notes and textbooks in
their back packs, which of course are not proof against water being
hurled *upwards* at them under the flap.

For many years a friend used to have a white jacket with a permanent
brown stripe up it. He now has a Flinger detachable mudguard (And the
jacket was retired after one of his unintentional dismounts!).

Jon

B.G. Finlay IT Services said:

On the subject of not understanding physics ... if I've the option of
two gears which work out at the same overall ratio does it make any
difference which one I use.

Use the one that keeps your chain in a straighter line - ie don't use
opposite extremes of gear combinations, because it will wear your chain
out quicker.

d.

Peter Clinch said:

Or in other words, it makes no difference right
now, but might do in a minute when it comes time to select something else.

For precisely this reason, when I'm on the hybrid, which has a triple, I
try to keep it on the middle front ring as much as possible, and only
change up or down to avoid using the largest or smallest cogs on the rear.

(Well, that's the theory - I actually find that the hybrid is a little
under-geared for road riding so I end up spending a lot of time on the
big front ring.)

Quoted message said:

Beyond that, bigger sprockets wear a little less at the same speeds
becuase there's more teeth to take the overall wear, but OTOH they weigh
more.

But shurely if you've got them on your bike they weigh the same whether
you're using them or not?

d.

davek ([email hidden]) wrote:
: B.G. Finlay IT Services wrote:
: > On the subject of not understanding physics ... if I've the option of
: > two gears which work out at the same overall ratio does it make any
: > difference which one I use.

: Use the one that keeps your chain in a straighter line - ie don't use
: opposite extremes of gear combinations, because it will wear your chain
: out quicker.

No, I'm not bothered about chain line, gear changes etc.
OK, I'll make things simpler. Two single speed bikes - one has a large
chainring and large sprocket, the other has smaller chainring & sprocket
but both have the same overall gearing. Do I gain anything with the extra
leverage the crank has on the smaller chainring?
Blair.

B.G. Finlay IT Services said:

davek ([email hidden]) wrote:
: B.G. Finlay IT Services wrote:
: > On the subject of not understanding physics ... if I've the option of
: > two gears which work out at the same overall ratio does it make any
: > difference which one I use.

: Use the one that keeps your chain in a straighter line - ie don't use
: opposite extremes of gear combinations, because it will wear your chain
: out quicker.

No, I'm not bothered about chain line, gear changes etc.
OK, I'll make things simpler. Two single speed bikes - one has a large
chainring and large sprocket, the other has smaller chainring & sprocket
but both have the same overall gearing. Do I gain anything with the extra
leverage the crank has on the smaller chainring?
Blair.

It's about the ratio. 40:20 or 30:15 - the same. There is no extra
'leverage'. If you want more leverage get longer cranks.

In article said:

davek ([email hidden]) wrote:
: B.G. Finlay IT Services wrote:
: > On the subject of not understanding physics ... if I've the option of
: > two gears which work out at the same overall ratio does it make any
: > difference which one I use.

: Use the one that keeps your chain in a straighter line - ie don't use
: opposite extremes of gear combinations, because it will wear your chain
: out quicker.

No, I'm not bothered about chain line, gear changes etc.
OK, I'll make things simpler. Two single speed bikes - one has a large
chainring and large sprocket, the other has smaller chainring & sprocket
but both have the same overall gearing. Do I gain anything with the extra
leverage the crank has on the smaller chainring?

The one with the larger gears will wear both the gears and the chain more
slowly, but nothing to do with "extra leverage". On the downside, it will
weigh a little more.

mike ("mike"@mikemc.demon.co.uk) wrote:
: It's about the ratio. 40:20 or 30:15 - the same. There is no extra
: 'leverage'. If you want more leverage get longer cranks.

Thanks (finally!).
Blair.

[email hidden] (B.G. Finlay IT Services) wrote in message news:<[email hidden]>...

Quoted message said:

OK, I'll make things simpler. Two single speed bikes - one has a large
chainring and large sprocket, the other has smaller chainring & sprocket
but both have the same overall gearing. Do I gain anything with the extra
leverage the crank has on the smaller chainring?

No, because whatever torque you gain at the smaller chainwheel you
lose again at the smaller sprocket. Increasing the crank length while
keeping the chainwheel/sprocket tooth ratio the same, however, will
effectively give you a lower gear. As usual Sheldon has the answer:
<http://www.sheldonbrown.com/gain.html>.

--
Dave...

" mike" <" mike"@mikemc.demon.co.uk> wrote in message news:<[email hidden]>...

Quoted message said:

B.G. Finlay IT Services wrote:

Quoted message said:

Quoted message said:

No, I'm not bothered about chain line, gear changes etc.
OK, I'll make things simpler. Two single speed bikes - one has a large
chainring and large sprocket, the other has smaller chainring & sprocket
but both have the same overall gearing. Do I gain anything with the extra
leverage the crank has on the smaller chainring?

Quoted message said:

It's about the ratio. 40:20 or 30:15 - the same. There is no extra
'leverage'. If you want more leverage get longer cranks.

Blair is right. There is extra leverage at the chainwheel. The effort
is at the pedal, the load is at the chainwheel teeth and the fulcrum
is the bottom bracket. Using a smaller chainwheel moves the load
closer to the fulcrum and further from the effort and therefore
increases the leverage. However, the sprocket is also a lever. The
effort is at the sprocket teeth, the load is at the rim and the
fulcrum is the axle. Because the gear ratio is the same, the sprocket
must also be smaller. This moves the effort closer to the fulcrum and
further from the load meaning less leverage. These changes cancel each
other out resulting in no difference in the net gain from pedal to
rim.

--
Dave...

Dave Kahn said:

Using a smaller chainwheel moves the load
closer to the fulcrum and further from the effort and therefore
increases the leverage.

Gosh, yes, I remember that from physics GCSE. I hadn't considered
applying it to riding a bike, though.

Interesting, thanks.

d.

"David Glover" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

My five mile commute to work, as I have previously mentioned, involves a
particularly unpleasant steepish hill, right at the start of the journey.
It's probably an entirely pleasant hill for other cyclists, but I'm unfit.
Anyway, I digress.

Normally I force the bike up it, wheezing and generally cursing (me, not

the

Quoted message said:

bike), and then complete the rest of my journey feeling pretty terrible.

Today, as an experiment, I used my otherwise unused granny gears, peddling
like a fool while crawling along.

Surprisingly, it didn't take much longer to get to the top than it

normally

Quoted message said:

does, and I actually got to my destination *quicker* than I normally do,
since I felt great for the rest of the journey and managed a higher

average

Quoted message said:

speed.

The wonders of granny gears!

Well, that's what the granny ring is for, so if you find it useful, use it!