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Jun
29
comment How many miles of riding require the same effort as one mile of running?
@heltonbiker: The data were downloaded from the Ironman site. The plots were done with R, using the basic graphics function "pairs" which creates the scatterplot matrix. A slightly custom function was used to draw the smooth red line in the upper plots.
Jun
29
comment What is preferred lacing for PowerTap wheel build?
+1 for the reference to the PT FAQ. The number of crossings will also depend on the depth of the rim: you may not be able to get a 3x on a deep rim (though you shouldn't need to -- a relatively shallow training rim and a wheel cover will work fine for almost all triathlons). To get consistent readings from the PT's torque tube you should not use radial lacing on the left side.
Jun
21
comment Are there devices to moderate speed on mountain descents without braking?
Regarding point #2 about lowering tire pressure from 120 psi to 105psi: measurements on smooth surfaces show that Crr decreases as pressure increases, so you're right: lowering pressure will increase rolling resistance. The conundrum is that on not-quite-so-smooth surfaces, measurements show a "V"-shaped relationship between pressure and Crr so that sometimes lowering pressure actually reduces Crr. Interestingly, the "V" appears not to be symmetric, so being a bit over "optimal" pressure increases Crr more than being a bit under. Unfortunately, "optimal" depends on the road surface.
Jun
18
comment How can one estimate drag for a bicycle?
Heltonbiker, the problem is that terminal velocity not only has poor precision but also only gives you ordinal ranking for comparisons (that is, under the best conditions you can tell that A has lower drag than B but not by how much) which means you don't get an estimate of CdA. Similarly, people have tried rollout distance. Moment-by-moment modeling works much better.
Jun
18
comment How can one estimate drag for a bicycle?
The "tow rope" method is discussed in the Debraux et al. article linked elsewhere. It doesn't have good precision. The deceleration method works well if you have a way to record moment-by-moment speed, such as with one of the increasingly popular Garmin bike computers. A method for doing this is discussed at forum.slowtwitch.com/cgi-bin/gforum.cgi?post=3590389#3590389 and, when done on a calm day without passing cars or other traffic, it has produced results in agreement with wind tunnel estimates.
Jun
18
comment How can one estimate drag for a bicycle?
Sadly, that article focuses only on one component of drag that cyclists experience (viz., aerodynamic drag) and answers the question "how can one estimate drag?" with "in a wind tunnel."
Jun
18
comment Are there devices to moderate speed on mountain descents without braking?
@Reid, Basically, ask something like this: "I want to estimate either the amount of drag force when coasting down a hill at a certain speed, or the amount of power needed to ride on the flat or up a hill at a certain speed. All of the online calculators either assume coefficients of rolling resistance or aerodynamic drag or assume that I have estimates for them. How do they make those assumptions, or how can I make those estimates myself?"
Jun
17
comment Are there devices to moderate speed on mountain descents without braking?
Well, the short answer to your question is how to convert drag force into power, and that's simple. Convert the drag force into Newtons, then multiply by speed in meters/sec. A watt is a Newton-meter/sec, so if you're traveling at 10 m/s and experiencing, say, 3 kg of drag "force", you'll just convert to Newtons then multiply by speed. That's 3 * 9.8 m/sec^2 * 10 m/s = ~ 300 watts. The longer question is how to estimate drag area on a bike. The answer is long (and I have fewer than 10 characters of explanation left) so that probably deserves its own question. Ask it and I'll answer.
Jun
17
comment Why is the rear wheel of my tandem bike skidding?
What kind of brakes do you have on this tandem? In particular, do you happen to have a rear drum drag brake?
Apr
22
comment Performing on long and tiring tour
There's not quite enough information in your question to give a targeted answer. Was this a typical distance for you, or had you not previously attempted a ride like this? How steep were the climbs? How often did you stop, what did you eat and how often, were you riding alone at your own pace (like a randonee) or riding with a group that set your pace (like an audax)?
Apr
20
comment Setting up a bicycle for someone with restricted knee articulation
Here's a similar product.
Apr
20
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
@heltonbiker: Fair enough. I'd upvote your answer if it were modified to explain that you're giving an explanation about "typical" reasons why power drops with lowered position but that in this particular case the loss is atypical and may be the result of a different cause.
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
@heltonbiker: exactly, it appears to be atypical, which is why I would think that a "typical" answer (that is, closing of the hip angle) isn't the issue, especially since he's seeing it with a 0.5 cm change not only in bar height but also in reach. That's why I haven't been able (yet) to upvote your answer.
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
There's a current thread over on the timetriallingforum which includes more "typical" losses with drop; the post author is reporting something like 30% for .5 - 1.0 cm change in drop.
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
What about the change in reach?
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
I don't think the hip-torso angles explain it. He says a 5 - 10 mm change in height or in reach has a large effect on speed. I've seen small changes in height or reach affect aero drag and power -- but never as large as observed here.
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
To clarify, are you saying that you can climb a 10% grade at, say, 10 kph while if you lower your bars by a quarter inch you climb the same hill at 7 kph? Without knowing your exact weight or that of your bike it's impossible to make an exact calculation but that would probably be in the neighborhood of 1 watt/kg difference (like, from 3.3 watts/kg to 2.3 watts/kg). That's a huge difference for a change in bar height of that size.
Apr
19
comment Why such a dramatic drop in power when the handlebars are lower than the seat?
3+ kph slower up a 10% grade from a 5-10 mm change in bar height is pretty extreme. What is your "base" speed from which this 3+ kph loss is observed?
Apr
17
comment Why can't Yellow Jersey change hands on last day of Tour De France?
In 1947, the first post-WWII Tour, Jean Robic started the final stage in 3rd place on GC, about 3 minutes down on the Italian rider Brambila. The stage was long and flat, from Caen in Normandy to Paris, but the absence of race radios meant that Brambila lost track of Robic. Robic finished about 13 minutes ahead of Brambila.
Apr
17
comment Why is it safer to bike with traffic, instead of facing oncoming cars?
Oh, I'm pretty happy with what I wrote. The original question did not make any mention of presence or absence of cycling infrastructure, I linked to a report from a governmental research institute that discusses the exact reasons and conditions under which riding against traffic is a safe activity, I mentioned the NHTSA/FARS data in other situations, and I showed that the real world is actually quite complex. I'm willing to take downvotes on this.