the terrible (?) truth about tailboxes

[Paul Worden]

Could some experienced bent designer/rider add some light to this.

It's claimed by the builders of the M5 trike that their tailbox reduces energy requirements by 28%
All the figures that I've seen suggest that a tailbox only increases average speed by about 2 - 5
kph ( 1 - 3 mph) Tailboxes that I've attached to my MR Swift increase my average speed over a
hilly 25 kms (15 miles) by about 2 kph and my max rolldown on a known short hill from 58 to 63 kph
(35 - 38 mph.)

How does an energy saving of nearly 30 percent only create an added 2 kph? Aaaarghhhh...... maths
was never my strong area.....

Paul Worden - MR Swift

 

[Tom Blum]

Paul Worden screv:

"It's claimed by the builders of the M5 trike that their tailbox reduces energy requirements by 28%
All the figures that I've seen suggest that a tailbox only increases average speed by about 2 - 5
kph ( 1 - 3 mph) How does an energy saving of nearly 30 percent only create an added 2 kph?"

Because power required increases by the square of velocity. (or is it cube??). 1.1 * 1.1 = 1.21.
1.1*1.1*1.1=1.33.

See????

Tom

 

[Paul Worden]

>Thank you Tom.....1.1 * 1.1 = 1.21. 1.1*1.1*1.1=1.33.
that's much clearerer (!)

it makes me glow with admiration for a wedgie sprinter that can top 64 kph......

Paul W ...Swift by name, but obviously not by nature.....

 

[Fred Klingener]

"Tom Blum" <[email protected]> wrote in message news:[email protected]...
> Paul Worden screv:
>
> "It's claimed by the builders of the M5 trike that their tailbox reduces energy requirements by
> 28% All the figures that I've seen suggest that a tailbox only increases
average
> speed by about 2 - 5 kph ( 1 - 3 mph) How does an energy saving of nearly 30 percent only create
> an added 2
kph?"
>
> Because power required increases by the square of velocity. (or is it cube??). 1.1 * 1.1 = 1.21.
> 1.1*1.1*1.1=1.33.

Drag force goes as the square of the velocity, power as the cube.

> See????

Yep.

Fred

 

[Rcpinto]

>Could some experienced bent designer/rider add some light to this.
>
>It's claimed by the builders of the M5 trike that their tailbox reduces energy requirements by 28%
>All the figures that I've seen suggest that a tailbox only increases average speed by about 2 - 5
>kph ( 1 - 3 mph) Tailboxes that I've attached to my MR Swift increase my average speed over a
>hilly 25 kms (15 miles) by about 2 kph and my max rolldown on a known short hill from 58 to 63 kph
>(35 - 38 mph.)
>
>How does an energy saving of nearly 30 percent only create an added 2 kph? Aaaarghhhh...... maths
>was never my strong area.....
>
>Paul Worden - MR Swift
>
>

Hi Paul

I'll give it a try.

The power required to overcome rolling resistance and aero resistance can be viewed as two
separate figures, and added together at a given speed.

Rolling resistance force, and the power to overcome it, varies directly with your speed... if
you go twice as fast, it takes twice the power.

Aero drag FORCE varies as the square of your speed, so if you go twice as fast, you have four
times the aero drag (aero drag force varies as speed squared)

Distance traveled comes into the equation for power though (P= Force X Distance/Time) so
going twice as fast requires eight times the power (power to overcome aero drag varies as the
speed cubed)

As the speeds approach those in the M5 SRM measurements (43-46km/hr) the aero drag force
starts to dominate the equation, and very small increases in speed result from large
decreases in aero drag or large increases in power.

Hope this helped.

Rich Pinto
Bacchetta Bicycles

 

[Tom Sherman]

RCPINTO wrote:
> ... Rolling resistance force, and the power to overcome it, varies directly with your speed... if
> you go twice as fast, it takes twice the power....

This will almost never be true. Riding faster will create more heat due to the flexure of the tire
casing and friction between the casing and inner tube. The temperature increase will change the air
pressure and/or the rolling diameter and/or width of the tire. The elastic modulus and coefficient
of restitution of the elastomers in the tire and inner tube will also likely very significantly with
temperature.

Tom Sherman - Recumbent Pedant

Arguing with an engineer is like mud wrestling with a pig... You soon find out the pig likes
it! - Unknown

 

[Paul Bruneau]

Tom Sherman wrote:

> will also likely very significantly with temperature.

vary

 

[Rotofool]

[email protected] (RCPINTO) wrote in message news:<[email protected]>...

> Rolling resistance force, and the power to overcome it, varies directly with your speed... if
> you go twice as fast, it takes twice the power.
>

Rich,

Don't you mean:

Rolling resistance force varies directly with your speed and the power to overcome it varies as the
square of your speed.

Len Thunberg

 

[Steve McDonald]

Aerodynamic resistance doesn't necessarily increase by a factor of 4 with an increase in speed. It
really can't be calculated without actual testing, using a full-sized model or actual working
vehicle. This is because different airframes have variations in how well they perform at certain
speeds. Some airframes have a terminal velocity that is lower than others, because the airflow
becomes turbulent enough at that speed to greatly raise the drag. Other designs might allow a smooth
airflow to be maintained at much greater speeds. Some designs might have poor airflow
characteristics at lower speeds, but when pushed faster the airflows might smooth out into more
laminar patterns. The formulas that some claim can be used to calculate drag on a proposed design,
are theoretical only. Only when proven designs are only slighly changed, can these theories come
close to matching actual performance.

The reason the Boeing engineers may have been able to design the 777 completely by computer, is
because they had previously designed and proven the 757 and 767 in the old-fashioned way and
had all the data from them to feed their computers.

That 2 or 3 kph increase in speed from a tailbox sounds pretty good to me. I've worked hard
over designs to squeeze out far less extra speed than that.

Steve McDonald

 

[John Foltz]

rotofool wrote:
> [email protected] (RCPINTO) wrote in message news:<[email protected]>...
>
>> Rolling resistance force, and the power to overcome it, varies directly with your speed... if
>> you go twice as fast, it takes twice the power.
>>
> Don't you mean:
>
> Rolling resistance force varies directly with your speed and the power to overcome it varies as
> the square of your speed.
>
No, Rich had it right. Rolling resistance, i.e. tires and bearings, is proportional to speed. It is
only aerodynamic forces that increase at a higher rate.
--

John Foltz --- O _ Baron --- _O _ V-Rex 24/63 --- _\\/\-%)
_________(_)`=()___________________(_)= (_)_____

 

[Dave Lehnen]

John Foltz wrote:
> rotofool wrote:
> > [email protected] (RCPINTO) wrote in message
> > news:<[email protected]>...
> >
> >> Rolling resistance force, and the power to overcome it, varies directly with your speed... if
> >> you go twice as fast, it takes twice the power.
> >>
> > Don't you mean:
> >
> > Rolling resistance force varies directly with your speed and the power to overcome it varies as
> > the square of your speed.
> >
> No, Rich had it right. Rolling resistance, i.e. tires and bearings, is proportional to speed. It
> is only aerodynamic forces that increase at a higher rate.

While I'm sure Rich understands it perfectly, he didn't type it quite right. Rolling
resistance force is nearly a constant, and since power is force times velocity, power to
overcome rolling resistance is proportional to speed. As he stated, if you go twice as fast,
it takes twice the power.

Dave Lehnen

 

[Rotofool]

John Foltz <[email protected]> wrote in message news:<[email protected]>...
> rotofool wrote:
> > [email protected] (RCPINTO) wrote in message
> > news:<[email protected]>...
> >
> >> Rolling resistance force, and the power to overcome it, varies directly with your speed... if
> >> you go twice as fast, it takes twice the power.
> >>
> > Don't you mean:
> >
> > Rolling resistance force varies directly with your speed and the power to overcome it varies as
> > the square of your speed.
> >
> No, Rich had it right. Rolling resistance, i.e. tires and bearings, is proportional to speed. It
> is only aerodynamic forces that increase at a higher rate.

John,

If you'll reread my post you'll see that I said rolling resistance is proportional to speed. My
point was that the power required to overcome that resistance varies as the square of speed, power
being a constant times resistance times speed.

Len Thunberg

 

[Rcpinto]

Steve MacDonald wrote;

> Aerodynamic resistance doesn't necessarily increase by a factor of 4 with an increase in
> speed. It really can't be calculated without actual testing, using a full-sized model or
> actual working vehicle. This is because different airframes have variations in how well they
> perform at certain speeds. Some airframes have a terminal velocity that is lower than others,
> because the airflow becomes turbulent enough at that speed to greatly raise the drag. Other
> designs might allow a smooth airflow to be maintained at much greater speeds. Some designs
> might have poor airflow characteristics at lower speeds, but when pushed faster the airflows
> might smooth out into more laminar patterns.

Steve

The difference between airplanes (Cd's under .1) and bluff bodies like unfaired bicycles (Cd's
from ~.5 to about .9) is that almost all of the drag on airplanes is from skin friction, and
almost all of the drag on unfaired bicycles drag is due to pressure drag.

Cd's are just a measure of the streamlining of any object, per unit of frontal area. The lower
the Cd, the more streamlined the object.

Airplanes also have much higher speeds, and complicated laminar/turbulent flow control problems
under various yaws, lifts, etc. In general, a much more complicated problem than the normal
turbulent and separating air flow of unfaired bicycles.

The range of speeds of interest for bicycle aero drag calculation's is also much slower at
20-35 MPH, and in a stable Cd range.

Cd's of various objects can change based on speed, size, and fluid viscosity (which the
dimensionless "Reynolds number" describes) and suddenly drop by a factor of four over a very
small range of velocities, as air is tripped into turbulence and reduces the separated area
(low pressure) behind the object.

I've never seen bicycle Cd's change over the normal range of wind tunnel velocities though, so
the aero drag force does scale with the velocity squared for the normal range of interest.

Rich Pinto
Bacchetta Bicycles

 

[Archer]

[email protected] (Steve McDonald) wrote in message
news:<[email protected]>...

> The reason the Boeing engineers may have been able to design the 777 completely by computer,
> is because they had previously designed and proven the 757 and 767 in the old-fashioned way
> and had all the data from them to feed their computers.
>

OT here, but... 15 years ago NASA engineers were able to calculate (accurately) the aerodynamics of
an airframe. Only trouble was it took several days on a Cray 2 to get the results. Nowadays I
imagine that a few hours on a modern supercomputer will do the same thing. Really, computation fluid
dynamics (CFD) was a solved problem years ago. The only reason for looking at the old data is to
double check the results of the simulations.

-- archer

 

[A&B]

Weren't these the same folks who divided inches by centimeters on the way to Mars. bill, where is
that stinkin telescope, g

archer wrote:
>
>
> OT here, but... 15 years ago NASA engineers were able to calculate (accurately) the aerodynamics
> of an airframe. Only trouble was it took several days on a Cray 2 to get the results. -- archer

 

[Emmett M. Hood]

[email protected] (RCPINTO) wrote in message news:<[email protected]>...
> >
> >RCPINTO wrote:
> >> ... Rolling resistance force, and the power to overcome it, varies
> directly
> >> with your speed... if you go twice as fast, it takes twice the power....
> >
> >This will almost never be true. Riding faster will create more heat due to the flexure of the
> >tire casing and friction between the casing and inner tube. The temperature increase will change
> >the air pressure and/or the rolling diameter and/or width of the tire. The elastic modulus and
> >coefficient of restitution of the elastomers in the tire and inner tube will also likely very
> >significantly with temperature.
> >
> >Tom Sherman - Recumbent Pedant
> >
> >Arguing with an engineer is like mud wrestling with a pig... You soon find out the pig likes it!
> >- Unknown
> >
> >
>
> To the nearest watt of power, and for the sake of explaining it to someone who was having a
> hard time grasping the concepts, the power to overcome rolling resistance is directly
> proportional to speed.
>
> And now for the "mud wrestling with a pig" portion of the explanation ;<)...yes, there are
> very small second order effects to bicycle tire rolling resistance which is described by this
> equation;
>
> Crr= Cro + (Crv X V)
>
> or, the total rolling resistance= base Crr + (a VERY small # times the velocity in M/S)
>
>
> The significance of this speed dependent rolling resistance effect for bicycle tires can be
> judged by its lack of mention in almost every rolling resistance measurement study of
> bicycle tires you can find, with only one exception that I can find, in "Bicycling
> Science", where it is briefly mentioned.
>
> As far as temperature effects on rolling resistance, of course changes in the tire and tube
> temperature, and the increases in tire pressure will cause changes in Crr. But all else being
> equal, at a given temperature and tire pressure, to a very close approximation...rolling
> resistance power is proportional to speed.
>
>
> Rich Pinto
> Bacchetta Bicycles
>
> I'm going to take a shower now!! - Unknown...after mud wrestling :<)

Hey 'M&M(Marked Man) Rich, Are we having fun yet? Emmett

 

[Archer]

a&b <[email protected]> wrote in message news:<[email protected]>...
> Weren't these the same folks who divided inches by centimeters on the way to Mars. bill, where is
> that stinkin telescope, g
>

Nope. That was the, ahem, fine engineers down at Lockheed in Colorado Springs. And it was ft-lbs/sec
vs NM/sec, IIRC. CFD work is done at Moffitt NAS in Sunnyvale, CA. -- a