"archer" <ns_archer1960@ns_hotmail.com> wrote in message
news:[email protected]...
> > If you're an engineer then you should be interested in
> >
> > Martin JC, Milliken DL, Cobb JE, McFadden KL, and Coggan AR. 1998. Validation of a mathematical
> > model of road cycling power. J Appl Biomech
> > 14:276-291.
> >
> > This was a test of how well the laboratory results and theoretical
calculations
> > match up with the real world. The answer is: very well indeed.
>
> Yes, I really am an engineer (EE to be specific) Is that paper available online? I'd like
> to see it.
Don't think so. You can see the abstract on-line, but not the paper itself. In any event, the model
in that paper is pretty close to the classic model in Whitt & Wilson, with a yaw component
correction for wind direction. The fit was very good, and the SE of the estimates was on the order
of 3 watts over the range of conditions they were able to test.
> > > The power equation doesn't take into account any effect that the different riding position
> > > might have on the overall mechanical efficiency of the bike/rider system.
> >
> > If you're an engineer then I'm presuming you're using efficiency in an engineering sense and
> > you'll probably want to know that biomechanical efficiency doesn't appear to depend very much on
> > position.
>
> That's exactly what I was referring to; is that discussed in the above paper?
Nope. Biomechanical efficiency during cycling usually sits in the range of 20-25%, and there really
isn't much you can do to improve that. There are things you can do to make it worse, however...
> > In any event, why would one care about efficiency anyway? Just eat another energy bar.
>
> If you're already at max output, then better efficiency means more road speed, and another energy
> bar won't help you.
Then you're not talking about efficiency of the rider. You must be talking about efficiency (i.e.,
resistance losses) in the bike, or perhaps whether positioning helps the rider to produce more power
(who cares about efficiency in those situations? If your efficiency goes from 23% to 24%, all it
means is that you don't have to eat another candy bar, not that your power output goes up). Rider
position doesn't affect bicycle efficiency in any meaningful way. Rider position does appear to
influence power production and my understanding is that thigh-torso angle is the biggest of these --
if you're too cramped up you won't be able to breathe right. On the other hand, simple rotations of
the body so that you're supine rather than upright *given the same thigh-torso angle* don't appear
to have much effect, nor does having a thigh-torso angle larger than, say, 90 degrees--your gut
can't really use anything wider than that. And I think we're talking about sustainable power:
positioning definitely has an influence on your ability to do things like sprint, but I don't think
max output is relevant since it only lasts for a couple of seconds.
As for the bike itself, slightly longer chain runs may make a small difference, but chain losses
aren't really all that big anyway. Mostly, the losses occur in the bending of the chain (as around
the derailleur rollers) so it may not be all that different between uprights and recumbents if they
both use conventional derailleurs (depending on how convoluted the chain path is on a recumbent, of
course). A slightly larger area of difference is in tire rolling resistance. All other things being
equal, smaller diameter tires tend to have slightly higher resistance since they often have to
deform more. They can have lower aero resistance, but presumably that gets picked up in the CdA.
For conventional uprights, a rule of thumb is that overall drivetrain losses are going to run around
5%. Even with the longer chainruns of a recumbent and (typically) smaller tires, the overall losses
couldn't be more than a couple of percent higher. At 250 watts, we're talking a difference of maybe
5 to 10 watts between the two platforms. Since power scales with the cube of velocity, for all "real
world" purposes that difference isn't going to be important if what you're trying to answer is which
is faster.
So, the two things that dominate are the aero drag force and the gravity drag force. The only
published data I've seen were several years old, from the (German) Tour magazine, and they showed
that most of the advantage in aero drag for recumbents comes from the use of a fairing. My memory
may be failing me, but IIRC unfaired recumbents (depending exactly on the type) had aero drag
numbers better than MTBs, better than upright riders on bartops, not quite as good as upright riders
in the drops--basically, in the same ballpark as upright riders on the brakehoods. Faired recuments
were a completely different story. I don't think any of the tested bikes were lowracers. In any
event, I'm not sure why putting a rider closer to the ground would affect CdA. If you took exactly
the same bike and held it up on a stick in the air, do you think the CdA would change?
The bottom line is that it doesn't make sense to compare Fast Freddy Markham in a fully-faired
recumbent with an old fat guy whose max output is 200 watts on an upright MTB, just as it doesn't
make sense to compare an old fat guy on a BikeE with Chris Boardman on a Lotus Superbike. Those are
"real world" comparisons, but it just feeds the endless debate. A guy I occasionally ride with
thrashes my butt, and he rides a red upright bike. I ride a black upright bike. Would you claim that
red bikes are faster? The debate in this thread is equivalent to that. The thing to ask is what's
the CdA with a standard-sized rider on each type of bike, and its total weight. From this you can
figure out exactly under which conditions each will do better or worse.
