Re: Speed record

On Mon, 17 Sep 2007 16:37:28 -0500, Ben C <spamspam@xxxxxxxxx> wrote:

On 2007-09-17, carlfogel@xxxxxxxxxxx <carlfogel@xxxxxxxxxxx> wrote:
On Mon, 17 Sep 2007 06:38:22 -0500, Ben C <spamspam@xxxxxxxxx> wrote:

On 2007-09-17, Chalo <chalo.colina@xxxxxxxxx> wrote:
Ben C wrote:
[...]
I know from experience that when I ride my motorcycle down the highway
at speed with a large windshield mounted, the wind pushes me forward.
The faster I go, the harder I am pushed forward. It wouldn't surprise
me if drafting the right sort of vehicle at the right distance allowed
a cyclist to maintain his speed without energy input at the pedals.

Yes, you're probably right. The other thing to consider is that although
we need quite a lot of power to overcome rolling resistance (1300W or so
based on Cr of 0.005-- but probably you only need half that or less for
a special LSR bike), that's still quite a small force at these high
speeds. The airstream only has to be pushing you along with about 5N,
which is perfectly believable.

Dear Ben,

This page makes two points:

http://www.canosoarus.com/08LSRbicycle/LSR%20Bike02.htm

First, the 138 mph Abbott single-gear bicycle is shown being pedaled
in a parking lot.

Yes, I'd forgotten about that. So much for the idea that it's impossible
to get moving on these things.

Second, "these bikes" (meaning the 138 mph Abbott and the 152 mph
Howard) are fixed gear, so something must be causing the crank and
legs to spin fast enough to overcome drag.

Well, if the vortex was dragging the machine along the ground, that
would cause the cranks and legs to spin around.

The analogy that Jobst usually provides is of a piece of paper or a
passenger's hair being blown forward behind a deeply curved
convertible's windshield at highway speeds.

The surface-to-mass ratio of a piece of paper fluttering in the breeze
is enormously higher than the surface-to-mass ratio of a bicyclist
sitting on a 43 pound bicycle.

Indeed, although the force required to overcome RR at high speed is not
enormous. My estimate of 5N is based on a rider/vehicle mass of 100kg
and a coefficient of RR of 0.005, which is the default value in
Velocity.java, one of those bicycle speed/power calculators.

That's presumably about right for a normal tyre on a road bike. I
assumed it might be lower for an LSR bike, but it may be higher since as
you point out they are using motorcycle tyres.

It's only a crude estimate anyway-- it's not as simple as just RR.

There's also the drag of road shocks. The course is "smooth," but the
fastest bike used heavy motorcycle front suspension to absorb the
shock of hitting small irregularities at 166 mph:

http://www.fredrompelberg.com/en/html/algemeen/fredrompelberg/record.asp

That page lists gearing as 70x13 connecting to 60/15, giving 114.2
feet of travel per pedal cycle with motorcycle slicks on 18 inch rims.

70/13 x 60/15 = 5.3846 x 4.00 = 21.5-to-1 gearing to small-diameter
tires.

When I plug those figures into my spreadsheet, a bike with 21.538-to-1
gearing, a rear wheel circumference of 1616 mm, and a cadence of about
128 rpm will be going about 166 mph.

Interestingly, the huge chain rings were chosen because they allow
using larger and more _efficient_ "rear" sprockets, even though they
require building up to a frantic racing cadence at top speed.

Very good point.

If the expensive mammoth gears didn't produce power and the frantic
cadence was merely cosmetic, then smaller cheap stock sprockets could
have produced reasonably equivalent gearing:

52 x 11 and 50 x 11 = 21.488

70 x 13 and 60 x 15 = 21.538

55 x 12 and 52 x 11 = 21.667

53 x 11 and 53 x 11 = 23.215

55 x 11 and 55 x 11 = 25.000

As that list shows, if no useful power was being supplied by the
rider, a pair of stock 53x11 or 55x11 would have let him spin just as
uselessly, but much more comfortably and slowly.

There's also the question of what kind of wind drag would be produced
by the spokes on those small wheels. In still air, the outermost
sections are acting as fan blades at about 160 mph.

Yes, although that just tells us we need more power, not that it
necessarily has to come from the rider rather than from the vortex.

Dear Ben,

Near the bottom is a table of land speed records by speed (not date):

http://www.fredrompelberg.com/en/html/algemeen/fredrompelberg/record.asp

Speeds up to 96 mph were achieved with motorcycles in 1950. The
shields pulled by motorcycles presumably produced far less draft than
the shields pulled by cars at the same speeds.

So did the bicyclists behind motorcycles have to pedal harder than the
bicyclists behind cars?

The same question applies to numerous motorcycle pacings in the 60 mph
range, where the two locomotive pacers are listed.

Here's a French site with pictures showing how small some of the
motorcycle and car shields were:

http://lepetitbraquet.free.fr/chron13_rdvmort.html

Vanderstuyft, for example, seems to be doing 120 kmh, over 70 mph,
behind an early pacing motorcycle with no shield at all. Right now,
Magnus Bäckstedt is hoping to break the _hour_ derny record, which
stands at 66 kmh (41 mph), roughly 30 mph slower--which suggests that
sprinting behind the comparatively feeble draft of a derny can put a
rider past Mile-a-minute-Murphy behind a gigantic shield built on the
back of a train:

http://www.cyclingnews.com/news.php?id=news/2005/sep05/sep30news

In all cases that I've seen, the bicycles are drafting close enough to
need a safety bar behind the pacer, whether it's a locomotive, a car,
or a motorcycle.

If Jobst's theory is correct, a shield on a motorcycle can provide
enough draft to tow a free bicycle effortlessly, so what would the
effect of a much larger shield on a car or locomotive be?

Perhaps there is no increase in draft with a larger shield at the same
speed for the pocket occupied by the bicycle?

Perhaps the bicyclists automatically back-pedalled harder to counter
the greater draft, keeping silent about the reversed effort? Or
changed their posture to reduce the huge wind at their backs?

Perhaps the theory that bicycles will roll along at 60 to 160 mph
without power behind pacers is mistaken?

Sam Whittingham pedalled the Varna Diablo II to over 80 mph with no
drafting, so it seems strange to argue that Mile-a-minute Murphy and
others could not pedal usefully while drafting behind a train going 20
mph slower. Whether they _need_ to pedal is really the question.

Cheers,

Carl Fogel
.

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