From: dab@vuse.vanderbilt.edu (The Nashville Flash)
Subject: Re: How does a dyno work?
Originator: dab@space3
Organization: Vanderbilt University School of Engineering, Nashville, TN, USA
Date: Wed, 15 Apr 1992 13:37:34 GMT

In article <1992Apr14.223117.28744@umbc3.umbc.edu> bmatthew@umbc5.umbc.edu (Bud Matthews) writes:
>O.K., second try.  How does a rear wheel dyno get a HP estimate
>from that roller?  Is there some sort of resistance gradient that is
>being overcome by the rear wheel?  What, exactly, is getting measured?
>How is HP at a given RPM estimated?

Since nobody else has chosen to walk into the rotisserie on this one, I'll 
put up the opening bid.  Up front disclaimer: I'm an EE, not an ME.  
Furthermore, my only dyno experience was more than ten years ago and my 
memory is fading like drum brakes coming down Pike's Peak.

As I recall it works like this: you pump your rear tire full of as much air 
as you dare.  Then with the bike strapped down on the dyno and in gear, the 
rear wheel spins the drum. The mechanical resistance in the system to which 
the drum is connected is increased.  When the throttle reaches WFO, you 
record a series of data points consisting of ordered pairs of RPM and "HP" 
loading.  Plotting this data as HP vs. RPM gives a fairly accurate picture 
of the power delivery curve for your engine's current state of tune.  
HOWEVER, the absolute HP reading is probably very inaccurate because of 
calibration errors in both the dyno itself and the "system" which includes 
the bike's complete drive train.

If you put your bike on a dyno and your buddy puts his bike on the same 
dyno the same day, any small differences between the absolute power readings 
is irrelevant.  Large differences are relevant.

The dyno I used had the roller drive an impeller than pushed a fluid which I 
seem to recall was water.  By twisting a knob an increasing amount of 
restriction (or back pressure or something) was introduced thus putting a 
"heavier" load on the engine.  This is akin to riding WFO with the rear 
brake on.  Hence the term BRAKE Horse Power (BHP), coming from the method of 
loading (braking) the engine.  There is some equation somewhere that 
relates the amount of fluid moving through the resistance set by the knob
to "horse power."  So they put "HORSE POWER" on the meter instead of what
it really measures.  I don't remember what that is, but it's something
like gallons per minute thru a pipe of x diameter.

The reason that the rear tire is inflated a bunch is that it tends to get 
very hot very quickly at "normal" inflation pressures.  The guy's dyno I was 
using exploded the rear tire on his GPz550 the first time he tried it out.

By checking a bike on a dyno and establishing a baseline for that bike on
that dyno at some set of atmospheric conditions, you can then examine the 
effect of changes.  For example, swap exhaust systems and see if the peak 
power is higher and the band (above some level) narrower than stock or what.  
Maybe change to K&N filters and see that effect.  If you OWN a dyno you can 
eventually raise and broaden the powerband through a series of trial and 
error tests.  Keeping a notebook helps a lot.  And while you're at it, don't 
forget to record temperature, humidity, and barometric pressure (convert to 
relative air density, RAD, for convenience).  If you don't own a dyno, any 
incremental gains you make may pass unnoticed because of system changes in 
the lapses between tests.  You may also lose a shit-pile of money buying
dyno time.  That's why Brian bought a dyno in the first place.  (He's an
ME.)

OK.  Now you know just about everything I know about dynomometers.  Any 
mechanical engineers want to step into the breach to correct my 
misremembered misconceptions?
============================================================================
 "No hour of life is lost that is spent in the saddle." - Winston Churchill
    The Nashville Flash	  -   dab@vuse.vanderbilt.edu   -   DoD # 412