On Wed, 16 Aug 2006 21:20:15 -0700, jim beam <
[email protected]>
wrote:
[snip]
>2. i was also surprised to discover that low spoke count wheels can be a
>substantial benefit when riding in strong cross winds. i proved that to
>myself again the other weekend. had a flat so quickly grabbed a spare
>32 spoke wheel and used that on my commute across the golden gate
>bridge. it was startling how much difference the extra spoke count made
>to cross-wind handling. next day, back to normal, 24 count front on the
>mavic cosmos, much less wrestling required. try that for yourself some
>time.
[snip]
Dear Jim,
My almost flat, non-aero front rim (not even box section) and 700c x
26 front tire and rim are about 38mm thick.
Most posters would have thicker tire and rim combinations.
At a circumference of about 2100mm, that's 38 x 2100 = 79,800 mm^2 of
tire and rim, as viewed by a side wind.
With about 290mm of my 36 straight 2mm spokes exposed, that's 36 x 290
x 2 = 20,880 mm^2.
Some crude theoretical calculations:
290mm 2mm 38mm 36 32 28 24
spoke spoke rim+tire total spoke spoke spoke spoke
count mm^2 mm^2 mm^2 change change change change
----- ------ -------- ------- ------ ------ ------ ------
36 20,880 79,800 100,680 100.0% 102.4% 104.8% 107.4%
32 18,560 79,800 98,360 97.7% 100.0% 102.4% 105.0%
28 16,240 79,800 96,040 95.4% 97.6% 100.0% 102.5%
24 13,920 79,800 93,720 93.1% 95.3% 97.6% 100.0%
So on my front wheel, it looks as if the simple sideways area changes
only about 5% between 32 and 24 spokes.
My wheel, of course, maximizes this difference with thick 2.0mm
straight spokes and thin rim+tire combination.
With thinner spokes and a thicker box-section or even aero rim, the
effect on area of a spoke count reduction from 32 to 24 would be even
smaller.
With straight 1.8mm spokes (a rough approximation of the kind of thin,
butted spokes often mentioned) and a rim+tire thickness of about 45mm
for a modest box-section with mild aero profile, reducing the exposed
spoke-length to 283mm (like a wheel hanging in my garage) . . .
2100 x 45 = 94,500 mm^2 rim+tire side-view area
36 x 1.8 x 283 = 18,338 mm^2 spoke area
283mm 1.8mm 45mm 36 32 28 24
spoke spoke rim+tire total spoke spoke spoke spoke
count mm^2 mm^2 mm^2 change change change change
----- ------ -------- ------- ------ ------ ------ ------
36 18,338 94,500 112,838 100.0% 101.8% 103.7% 105.7%
32 16,301 94,500 110,801 98.2% 100.0% 101.9% 103.8%
28 14,263 94,500 108,763 96.4% 98.2% 100.0% 101.9%
24 12,226 94,500 106,726 94.6% 96.3% 98.1% 100.0%
This suggests that most posters would see a 3.7% drop in side-view
front wheel+rim+spoke area if they dropped from 32 to 24 spokes (or
gain 3.8% if they added 8 spokes to a 24-spoke wheel).
Aerodynamics is a tricky matter, so raw side area may somehow be
deceptive
But the 8-spoke count reduction sounds like less than a 3% change.
Moving from one kind of rim to another in my poorly stocked garage can
reduce side area from 94,500 mm^2 to 79,800 mm^2, which is 8.4%.
almost three times as much.
The winds on the Golden Gate vary more than that much every day. They
also blow sideways against the handlebars, fork legs, levers, hands,
and arms of the rider.
So I'm curious what the spoke thicknesses and exposed lengths are on
the two wheels that you rode, as well as how thick the tire plus rim
is on each wheel.
The weight would be interesting, too. At speed, a heavier tire and
wheel have more angular momentum and should be more stable in gusting
winds.
Maybe someone will have a link to wind tunnel tests, or an explanation
of how the spokes have a much greater effect than would be expected.
Cheers,
Carl Fogel
