1971 Experimental F750 Racer

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wakeup said:
Then I found a definition

Well done, you get an "A"; someone else on the other hand needs to do a bit more homework.......

I read through a bit of Phil Irvings Tuning for Speed last night to see how he presented the aerodynamic material and I can see how someone might get focused on (misdirected towards) cross sectional area reduction rather than drag coefficient. Using Irving's example cross sectional area values indicates a 2" reduction in overall height will yield maybe 6-8% reduction in drag whereas reducing the drag coefficient by 0.0001 will yield a 12.5% reduction in drag.
 
Very educational reference figures Dances, Best is a combo of small frontal area plus streamlining features. If ya dig in on some of the Norton full race fairing you will find statement that the front around engine was left flat to allow air to pile up to allow some stream lining of 'bluff' shape. Aim of exhaust plume is also worth some sucking drag reduction. Who know about the VW vs 924 testing results w/o a roll down test and changing stuff to see what matters, beside tire rolling drag. To me the sideways stream lining is more important than bee line designs, which is where the dust bins had real issues to avoid. Peel suffered side ways wind / ground effect lift/drop issues and d/t her incredible isolation, [as in unbelievable separation meaning] of every force of air and surface texture, I know where the forces struck the most and what/where they ended up begin felt in handling upsets, so I'll start with fork eddies lessening, then venting in middle and pressure build at rear, one side at a time when leaned good. I cut my mc teeth on too low a bike to lean, so know both the bee line sprinting speed advantages and its handling down sides. Peel will have it both ways to go faster and slow faster too i hope with ass end squated. On the fly spring rate changes too. Peel never got completely liffed off surface by air gusts, just the front/forks that would wiggle/jggle through frame but my SuVee has twice lifted both tires/whole bike up and out at the tangent on pitching into down hill sweepers with some strom winds left, never ever again I assure you as only lack of oncomming at the time allowed room to recover. Part of my motto Throw Yourself at the Ground and Miss" is d/t the air compression rebound lifting her back up w/o my effort. Its unworldly sensation and one of the few unpredicable things her flex-ness helps take up to make use of.
 
All you have to do is fit a bigger twist grip, and the aerodynamics mean nothing.

1971 Experimental F750 Racer
 
Dances with Shrapnel said:
Well done, you get an "A"; someone else on the other hand needs to do a bit more homework.......
<snip>
Using Irving's example cross sectional area values indicates a 2" reduction in overall height will yield maybe 6-8% reduction in drag whereas reducing the drag coefficient by 0.0001 will yield a 12.5% reduction in drag.

When someone throws jargon into the conversation, THEY need to define it.
Or folks can be talking at croseed purposes = confusion forever more.

If someone reworks the aero so the drag coefficient goes from .40 to .30, and "0.0001 improvement yields a 12.5% reduction in drag",
does that mean they get a 12500 % decrease in drag ???
 
It's called initiative and diligence. Some people try and fail and some succeed.

Drag coefficient I referred to is from Phil Irving's book and probably some obscure imperial based unit, something like 0.0008. If you have his book, look it up. I am on the road so cannot help you in your quest.

Your reference to 12500% reduction in drag is off a bit. I hope you recognize that.
 
Using your more common units of coefficient, reducing the coefficienct from say .40 to .35 yields a 12.5% reduction in drag. Phil Irving presented a coeeficient of 0.0008 and I arbitrarily picked 0.0007 (net 0.0001) which yields the same percentage reduction. I have a hunch Irving may have combined a few terms; don't know for sure without digging into an old fluid dynamics text and studying it a bit.

So design or redesign a complete bike for 2" lower and maybe 5-8% reduction in drag or reduce the coefficient by 10-12 or more percentage points. It could be be as easy as adopting a different tuck in riding position or a modified tail piece.

Irving was pragmatic. Testing to measure results could/would be done on a flat out and read the tach.
 
Dances with Shrapnel said:
Your reference to 12500% reduction in drag is off a bit. I hope you recognize that.

Not from the examples/numbers YOU supplied ..... !!!
Until you explain what the units of drag coefficient you are quoting are, we are none the wiser...

For example, the chart that hobot linked to quoted the drag coefficient for a 924 Porsche as 0.31 (or thereabouts).
That would seem to be whole orders of magnitudes different to your numbers.
AND ABOUT WHAT WE WOULD EXPECT TO SEE.
Even Wiki quotes similar http://en.wikipedia.org/wiki/Drag_coefficient
Never the twain shall meet. ?
 
MY confusion ??

Explain your units of drag coefficient, to the wider audience here.
You still seem to be a whole order of magnitude awry...
 
Your confusion, yes, with a little help from me. No great masses here have chimed in along the lines of obssesing over something they don't fully grasp. I expected more. Do you have a copy of Phil Irving's Tuning for Speed? If yes, do a little home work, if no, then maybe you should try to ring him up and ask.

Let me digest this for you: easier to knock several percentage point off of a drag coefficient for our beloved vintage racers than it is to achieve same by redesigning a bike with less cross sectional area. That is the forest, really not interested in discussing the trees while traveling. I've explained it or indicated where to look for the details in previous posts. I've given you and others a reasonable explanation but it is up to you if you want more - not in my spoon feeding mode right now.
 
From Phil Irving (my recollection):

F= Drag force (lbf)
A= Area (ft^2)
C= Coefficient of drag
V=Velocity (ff/sec)

Where

F=A×V^2×C

Where an example value given for C was 0.0008
and
An example value for A was 5.0

Plug and chug.
 
Your explanation is totally good.
Your math was somewhat flawed back there.

As I pointed out back there.

5.0 is still not even in the ballpark.

I'll refer folks again to Wiki version of Aerodynamics 101
http://en.wikipedia.org/wiki/Drag_coefficient
Just look at the little shapes and numbers, to begin with...
 
I am pretty sure the math is good. The value of 5.0 is straight out of Phil Irving's text and represents a typical frontal area projection of a period motorcycle (in sq ft). It seems reasonable to me but I have not spent much effort confirming since this is more about where to make gains in terms of ratios (percentages). Absolute values become somewhat moot. Coefficient tables you cite are what I am familiar with.
 
A man seated fully upright on a bike is said to have a coefficent of around 1.0 - which is worst case -
which still seems to be a long way from your 5.0

??
 
Agreed but the value "5" has the unit ft^2 and is used by Phil Irving as a typical frontal projection of a motor bike. I am saying 5 sq ft and I am not saying a coefficient of "5".
 
The penny has finally dropped.
You are using all imperial figures.
The coefficient of drag is different in imperial ??
 
Yes, imperial, just like Phil in the good old days. We are discussing British Iron, are we not?

Coefficient of drag is cited as dimensionlaess but not sure if free to use interchangeably between SI and Imperial. Furthermore, when you read about the common coefficient of drag, it embodies mass density of the fluid of interest as well as cross sectional area whereas Phil Irving's reference must factor it in ( ie. composit coefficient number)....just a hunch.

For fun and games if you run out the equation I presented above and instead of squaring the velocity, you cube it and divide the result by 550, you get the air drag horsepower(ie. power required to push at speed.)
 
hehe its the peddle bikers that really work up some hot wind on this and that to the nth degree. Here's some data points on them to consider for us, especially the height of bars etc, no fairings of course.
http://www.analyticcycling.com/RiderAeroStudy.html

aero drag calculator search results I've time to time played with so maybe plug in some known bike measures just for comparison sense of values.
https://www.google.com/#q=aerodynamic+calculator

Rather informative to me Motorcycle physics article on every factor, though light on frame height aspects,
http://www.motorcyclejazz.com/motorcycle_physics.htm
 
Dances with Shrapnel said:
I am pretty sure the math is good. The value of 5.0 is straight out of Phil Irving's text and represents a typical frontal area projection of a period motorcycle (in sq ft). It seems reasonable to me but I have not spent much effort confirming since this is more about where to make gains in terms of ratios (percentages). Absolute values become somewhat moot. Coefficient tables you cite are what I am familiar with.

Wiki knows better than Phil Irving ? There might be some stoichiometry involved. 'A little knowledge is dangerous'.
 
acotrel said:
Dances with Shrapnel said:
I am pretty sure the math is good. The value of 5.0 is straight out of Phil Irving's text and represents a typical frontal area projection of a period motorcycle (in sq ft). It seems reasonable to me but I have not spent much effort confirming since this is more about where to make gains in terms of ratios (percentages). Absolute values become somewhat moot. Coefficient tables you cite are what I am familiar with.

Wiki knows better than Phil Irving ? There might be some stoichiometry involved. 'A little knowledge is dangerous'.

Stoichiometery involved? Really? 'A little knowledge is dangerous' - I think you just showed us the way and shot yourself in the foot! :lol: :oops:
And who is this Wiki and what does he or she know anyway? :P
 
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