Ralph & Diane Barone said:In article <[email hidden]>,
Phil Holman' piholmanc@yourservice said:"Ralph Barone" <invalid@not_real.ca> wrote in message
news:invalid-F754D9.22555224022007@shawnews...
Quoted message said:In reading the ad copy for bike trainers, everyone makes a big deal
about how they generate resistance, but with the exception of the Kurt
Kinetic road machine, nobody seems to care much about generating
realistic inertia effects. When riding the inexpensive trainers I'm
willing to shell out money for, there is a very unnatural feel caused
by
the speed varying every time I push down on the pedals. I've done the
math and I've figured out how large of a flywheel I need to get into
the
ballpark (a 20 cm diameter, 2.3 cm thick solid steel flywheel should
do
it).
What is everybody else's thinking on the subject?
Is it something you just get used to with time?
Do I just have terrible pedaling form?
Or does everybody else find it annoying too?
PS: The equation for the flywheel size needed to approximate normal
riding conditions is
Mf = 2 * Mb * (D_roller^2/D_flywheel^2)
where
Mf = mass of flywheel
Mb = mass of bike + rider
D_roller = diameter of roller on trainer
D_flywheel = diameter of flywheel on trainer
units aren't important as long as you are consistent.
An 80kg bike and rider travelling at a modest 32 kph has approx 31600
Joules of energy invested. Your 7kg flywheel radius of gyration would
need to be travelling at 108 kph to have the same inertia.
1/2*80*9.8*8.9^2 = 1/2*7*9.8*30^2
Will it have a gear mechanism to obtain this speed?
Phil H
Yes, so to speak. The ratio between the flywheel diameter and the
roller diameter provides the "gearing". In the calcs I did, the
flywheel was 20 cm diameter and the roller on my trainer is about 3.6
cm. This means that at your 32 kph, the rim of the flywheel is
travelling at 177 kph. Throw in an adjustment factor of 0.707 to
account for the fact that the radius of gyration is not at the edge of a
solid disk, and we're in the ballpark (I think we used different numbers
for the density of steel).
Check <http://www.kreitler.com/content/trainer_wattage.pdf>
I've got 2 of their flywheels, and the "killer headwind fan", and use
various combinations of them and fan settings for different types of
workouts. All 3, even with a low setting on the fan requires a very
low gear, and is a very hard workout, even with granny gears, giving
limited WU/CD ability, but with 2 flywheels you get really good
coasting, and more realistic petaling, giving better petaling
technique training, imo. To attain 18 mph with my Challenger 4.5"
drums, e.g., I need to put out 106 watts + 76 watts (2 flywheels) + 88
watts (1/4 open fan) = 270 watts, which is fairly realistic for a
slightly breezy, generally uphill outdoor course, and with great
coasting. at 20mph it's 126+94+103=323watts, which is hard to hold
for long (like into headwind going up a 1% grade). With higher fan
setting, and only one pulley it's more like a very windy day (more
variable wind resistance) but not so good coasting. E.g. one pulley
and 1/2 open at 18 mph is 106+38+133=276watts, or a 15 mph headwind on
a flatter course. Those wattages are very closely approximate my
powertap readings. And, btw, the Kreitler stuff is mostly > 15 years
old, and will outlive me, for sure.
Nice setup, but too bad it mostly drives me nuts indoors and I go out
at almost any cost, like into the deep snow today on studs
Bill Westphal
