interpreting tests

[John Riley]

When doing roll down tests, what does it mean when top speeds are similar, but one configuration
rolls further than the other? This came up with TE fairing tests, and I seem to recall some issue
like this with Rotator tests as well.

John Riley

 

[B. Sanders]

"John Riley" <[email protected]> wrote in message news:[email protected]...
> When doing roll down tests, what does it mean when top speeds are similar, but one configuration
> rolls further than the other? This came up with TE fairing tests, and I seem to recall some issue
> like this with Rotator tests as well.

Variations in wind speed?

-Barry

 

[Bentnut]

Different C/D at different speeds is my best guess. i.e. a fat teardrop shape is the most
efficient at lower speeds, a skinny teardrop at higher speeds(a gross generalization I know). The
"fat teardrop" may have a lower top speed but roll out further at low speeds. This ties into
something else... The whitehawk was not all that fast at Battle Mountian yet holds the one hour
record. Perhaps the aerodynamics are optimized for around 50mph? Looking at the top speed (67.480)
vs. hour time (52.81) for the whitehawk a ratio of approx. .782 and applying it to the Varna top
speed of 81 gets you
63.342 miles in one hour! That is IF the Varna is as efficent at lower speeds. Hmmmmm.

Gabe

"John Riley" <[email protected]> wrote in message news:[email protected]...
> When doing roll down tests, what does it mean when top speeds are similar, but one configuration
> rolls further than the other? This came up with TE fairing tests, and I seem to recall some issue
> like this with Rotator tests as well.
>
> John Riley

 

[Jeff Wills]

John Riley <[email protected]> wrote in message news:<[email protected]>...
> When doing roll down tests, what does it mean when top speeds are similar, but one configuration
> rolls further than the other? This came up with TE fairing tests, and I seem to recall some issue
> like this with Rotator tests as well.
>
> John Riley

This is one method of doing a roll-down test. The problem is entering the roll-down area with
exactly the same speed every time. Another way is to find a hill that ends in a straight, level
road. You coast down the hill over and over with different configurations- whichever coasts the
farthest has the least drag. It's tricky to get reproducible results, though.

Jeff

 

[Dave Lehnen]

John Riley wrote:
> When doing roll down tests, what does it mean when top speeds are similar, but one configuration
> rolls further than the other? This came up with TE fairing tests, and I seem to recall some issue
> like this with Rotator tests as well.
>
> John Riley

Were the similar top speeds reached at the same point on the course? A bike with lower drag but also
lower mass would reach its top speed later on the course.

If tires or pressures were enough different to matter, changes in rolling resistance could account
for some difference. A bike with lower air drag but more tire rolling resistance might have the same
top speed as another bike, but would slow down faster as the speed drops, and rolling resistance
becomes a bigger fraction of the total drag.

Probably more important than all of this is the difficulty of duplicating test conditions: wind
conditions, rider position, rider steering technique, identical path on the road, and probably some
I haven't thought of.

Dave Lehnen

 

[John Riley]

I think the Rotator tests were different wheel sizes, but bentnut did some TE tests where the only
variable was the fairings. These are fabric, so weight shouldn't have been an issue.

JR