C
On Sun, 01 Oct 2006 15:51:13 -0400, Paul Hobson
<[email protected]> wrote:
>[email protected] wrote:
>> On Sun, 01 Oct 2006 14:39:47 -0400, Paul Hobson
>> <[email protected]> wrote:
>>
>>> Kinky Cowboy wrote:
>>>> On 30 Sep 2006 08:44:26 -0700, "john" <[email protected]> wrote:
>>>>
>>>>> So if I were to follow this link
>>>>> (http://www.schwalbe.com/gbl/en/tech_info/rollwiderstand/) to it's
>>>>> logical conclusion, I'd be riding a 6" wide tire. Some tiny hint of
>>>>> intuition says: 'I don't think so!'
>>>> And we all know intuition beats science any day.
>>>>
>>>> Assuming you could get a 6" tyre with a more or less round cross
>>>> section with the same case/tread construction, and inflate it to the
>>>> same pressure, as the 35mm tyres under discussion, the change in
>>>> curvature of the tread/casing required to form a flat 2sq. in. contact
>>>> patch would be very small indeed, and the hysteresis would be
>>>> correspondingly tiny.
>>> But here's where I'm confused: Is Schwalbe claiming that their fat
>>> tires are *faster*, or just have lower rolling resistance?
>>>
>>> Since aerodynamic drag is proportional to the square of the relative
>>> velocity, I imagine there's an RR-AD break even point. That's why the
>>> fat tires are good for the average bloke (who rides well below that
>>> point) and racers ride skinny tires (above the break even point). Am I
>>> thinking about this correctly?
>>>
>>> Reason I ask: My commute through Atlanta's stop/start downtown traffic
>>> averages about 21 km/hr (13 mph). The new bike I'm looking at comes
>>> with 38 mm tires, but I'm accustomed to my "fast" 25 mm tires. Based on
>>> this thread, I'm inclined to say the 38 mm tires would be better for
>>> several reasons. Right?
>>> \\paul
>>
>> Dear Paul,
>>
>> A $5,000 bicycle has no speed advantage over an armchair when they're
>> both sitting at a stop light.
>>
>> The random variation in how long you wait at the traffic lights is
>> much larger more than any small gains or losses available from
>> fiddling with tires and aerodynamics.
>>
>> Over a ridiculously long period, you might eventually notice a tiny
>> speed improvement, but in general delivery trucks keep up quite nicely
>> with sports cars in stop-and-go traffic.
>>
>> In fact, bicycles often keep up or even pass sports cars in heavy
>> downtown traffic, whether they're rattling old rust-buckets or snazzy
>> new carbon-fiber racing bikes.
>
>Right, but here's my point: my cruising speed on flat ground w/ no
>traffic is about 20 mph with my current set-up (48x17, 25 mm tires).
>The reason why my stop-n-go *average* is so high is b/c I can sprint
>quickly up to my cruising speed to catch green lights instead of
>stopping at each and every one. Also, the first half of my commute[1]
>is considerably less dense with stop lights, though much hillier.
>
>The article and portions of this thread seem to suggest that 35 mm or 38
>mm tires would be better suited for my practical need for speed and of
>course, comfort. Care to comment on where you feel my sweet spot might be?
>
>[1]
>http://toporoute.com/cgi-bin/getSavedRoute.cgi?routeKey=AYFVODGCJOEIPNC
>(I don't know what that huge spike is between 5 and 6 miles - ignore it.
> But you get the idea)
>Thanks,
>\\paul
Dear Paul,
Sorry, but I doubt that the tires will much speed difference.
If you averaged 20 mph for 9 miles with no stopping, it would take you
27 minutes.
You mention that you're averaging about 13 mph, so it takes you about
41 to 42 minutes (9/13 x 60).
So you're spending about a third of your time braking for lights,
sitting at lights, and pedaling back up to speed from lights.
Here's a calculator:
http://www.kreuzotter.de/english/espeed.htm
Choose hands-on-tops, blank out the default 160 watts, put in 20 mph,
page down, and put in 9 miles.
I get 172 watts, 20.0 mph, and 27:00.0.
Change to the worst possible tires, off-road mtb, blank the speed, and
recalculate, using the 172 watts that gave us 20.0 mph with the
default narrow tires.
I get 18.3 mph and 29:30.5. That's two-and-a-half minutes slower from
the very best to the very worst tires that can be reasonably expected.
With a two-way commute, you'd save 5 minutes per idealized day
But that's a flying start and finish, no traffic lights, no hills, no
wind, no sensible slowing down for safety near pretty girls.
In terms of acceleration, it's mostly the mass, not the wind drag or
rolling resistance, that matters. You can test this by flipping your
bike upside-down and cranking the rear wheel up to 20 mph in about
three seconds with a few turns of one arm--very little mass, very high
acceleration, just as you can easily flip a small rock 20 to 30 mph
with a casual toss of your hand, but a bowling ball would scarcely
move.
In practical terms, remember that--sorry--your uncouth 25 mm tires are
not narrow racing tires. They're hog-fat, compared to narrow 23 and 21
mm tires. Nor are the 38 mm modern touring tires that you're looking
at likely to be as awful as the calculator's generalized knobby mtb
tires. So you probably wouldn't even suffer the full 2.5 minute daily
loss that the calculator predicts for generalized default tires.
Comfort might improve with wider tires and lower pressures, but I
can't say that my daily 15-mile ride on 26 mm tires is uncomfortable
on a touring bike at a similar 20 mph speed, with only one stoplight
and about 6 miles of bike trail that some have been unkind enough to
call badly weathered.
Flats probably aren't going to be much of a problem with either kind
of tire. Any improved flat resistance from thicker casings is likely
to be offset by the 50% increase in width--the wider tire sweeps a
broader path that will obviously hit more of those pesky little pieces
of glass, nails, chips, thorns, and so forth.
That astonishing elevation spike in your route could be just a sign of
a topological typo, where data from two old maps was combined and one
disagreed with the other.
Or it could be that the route includes passing a skyscraper.
Basically, that graph shows you in free fall for 300 feet.
Cheers,
Carl Fogel
<[email protected]> wrote:
>[email protected] wrote:
>> On Sun, 01 Oct 2006 14:39:47 -0400, Paul Hobson
>> <[email protected]> wrote:
>>
>>> Kinky Cowboy wrote:
>>>> On 30 Sep 2006 08:44:26 -0700, "john" <[email protected]> wrote:
>>>>
>>>>> So if I were to follow this link
>>>>> (http://www.schwalbe.com/gbl/en/tech_info/rollwiderstand/) to it's
>>>>> logical conclusion, I'd be riding a 6" wide tire. Some tiny hint of
>>>>> intuition says: 'I don't think so!'
>>>> And we all know intuition beats science any day.
>>>>
>>>> Assuming you could get a 6" tyre with a more or less round cross
>>>> section with the same case/tread construction, and inflate it to the
>>>> same pressure, as the 35mm tyres under discussion, the change in
>>>> curvature of the tread/casing required to form a flat 2sq. in. contact
>>>> patch would be very small indeed, and the hysteresis would be
>>>> correspondingly tiny.
>>> But here's where I'm confused: Is Schwalbe claiming that their fat
>>> tires are *faster*, or just have lower rolling resistance?
>>>
>>> Since aerodynamic drag is proportional to the square of the relative
>>> velocity, I imagine there's an RR-AD break even point. That's why the
>>> fat tires are good for the average bloke (who rides well below that
>>> point) and racers ride skinny tires (above the break even point). Am I
>>> thinking about this correctly?
>>>
>>> Reason I ask: My commute through Atlanta's stop/start downtown traffic
>>> averages about 21 km/hr (13 mph). The new bike I'm looking at comes
>>> with 38 mm tires, but I'm accustomed to my "fast" 25 mm tires. Based on
>>> this thread, I'm inclined to say the 38 mm tires would be better for
>>> several reasons. Right?
>>> \\paul
>>
>> Dear Paul,
>>
>> A $5,000 bicycle has no speed advantage over an armchair when they're
>> both sitting at a stop light.
>>
>> The random variation in how long you wait at the traffic lights is
>> much larger more than any small gains or losses available from
>> fiddling with tires and aerodynamics.
>>
>> Over a ridiculously long period, you might eventually notice a tiny
>> speed improvement, but in general delivery trucks keep up quite nicely
>> with sports cars in stop-and-go traffic.
>>
>> In fact, bicycles often keep up or even pass sports cars in heavy
>> downtown traffic, whether they're rattling old rust-buckets or snazzy
>> new carbon-fiber racing bikes.
>
>Right, but here's my point: my cruising speed on flat ground w/ no
>traffic is about 20 mph with my current set-up (48x17, 25 mm tires).
>The reason why my stop-n-go *average* is so high is b/c I can sprint
>quickly up to my cruising speed to catch green lights instead of
>stopping at each and every one. Also, the first half of my commute[1]
>is considerably less dense with stop lights, though much hillier.
>
>The article and portions of this thread seem to suggest that 35 mm or 38
>mm tires would be better suited for my practical need for speed and of
>course, comfort. Care to comment on where you feel my sweet spot might be?
>
>[1]
>http://toporoute.com/cgi-bin/getSavedRoute.cgi?routeKey=AYFVODGCJOEIPNC
>(I don't know what that huge spike is between 5 and 6 miles - ignore it.
> But you get the idea)
>Thanks,
>\\paul
Dear Paul,
Sorry, but I doubt that the tires will much speed difference.
If you averaged 20 mph for 9 miles with no stopping, it would take you
27 minutes.
You mention that you're averaging about 13 mph, so it takes you about
41 to 42 minutes (9/13 x 60).
So you're spending about a third of your time braking for lights,
sitting at lights, and pedaling back up to speed from lights.
Here's a calculator:
http://www.kreuzotter.de/english/espeed.htm
Choose hands-on-tops, blank out the default 160 watts, put in 20 mph,
page down, and put in 9 miles.
I get 172 watts, 20.0 mph, and 27:00.0.
Change to the worst possible tires, off-road mtb, blank the speed, and
recalculate, using the 172 watts that gave us 20.0 mph with the
default narrow tires.
I get 18.3 mph and 29:30.5. That's two-and-a-half minutes slower from
the very best to the very worst tires that can be reasonably expected.
With a two-way commute, you'd save 5 minutes per idealized day
But that's a flying start and finish, no traffic lights, no hills, no
wind, no sensible slowing down for safety near pretty girls.
In terms of acceleration, it's mostly the mass, not the wind drag or
rolling resistance, that matters. You can test this by flipping your
bike upside-down and cranking the rear wheel up to 20 mph in about
three seconds with a few turns of one arm--very little mass, very high
acceleration, just as you can easily flip a small rock 20 to 30 mph
with a casual toss of your hand, but a bowling ball would scarcely
move.
In practical terms, remember that--sorry--your uncouth 25 mm tires are
not narrow racing tires. They're hog-fat, compared to narrow 23 and 21
mm tires. Nor are the 38 mm modern touring tires that you're looking
at likely to be as awful as the calculator's generalized knobby mtb
tires. So you probably wouldn't even suffer the full 2.5 minute daily
loss that the calculator predicts for generalized default tires.
Comfort might improve with wider tires and lower pressures, but I
can't say that my daily 15-mile ride on 26 mm tires is uncomfortable
on a touring bike at a similar 20 mph speed, with only one stoplight
and about 6 miles of bike trail that some have been unkind enough to
call badly weathered.
Flats probably aren't going to be much of a problem with either kind
of tire. Any improved flat resistance from thicker casings is likely
to be offset by the 50% increase in width--the wider tire sweeps a
broader path that will obviously hit more of those pesky little pieces
of glass, nails, chips, thorns, and so forth.
That astonishing elevation spike in your route could be just a sign of
a topological typo, where data from two old maps was combined and one
disagreed with the other.
Or it could be that the route includes passing a skyscraper.
Basically, that graph shows you in free fall for 300 feet.
Cheers,
Carl Fogel