1600 G's on a Schrader valve?
carl...@comcast.net
about 140 mph, using Akront motorcycle rims and wide tires?
Is 1,600 G's a reasonable rough estimate?
***
I'm asking because in an unrelated thread I wandered off into the
familiar account of how the Schrader valve in the rear tire of John
Howard's land speed bicycle deflated at about 140 mph:
" . . . on the fourth run, around noon, Howard was smoothly pedaling
past 140 mph when suddenly he had to fight to control his handlebars.
They jerked in his grip as the rear wheel fishtailed wildly. He was
forced to fall behind the car's slipstream . . ."
"His rear tire was flat. When his mechanic, John Vaneeckhoutte
[really, Vaneeckhoutte], took the tube out of the tire, he was
surprised to see that the tube still held air."
"Doug Malewicki, an engineer, figured out that the culprit was
centrifugal force [cue excited arguments about centripetal]. He
calculated that, at the speed Howard was traveling, air inside the
tube pushed out through the valve, which lacked a cap. [No, the wheel
spun so fast that the Schader valve pin itself was thrown outward,
which opened it, but the author gets a lot of these things wrong.] By
chance, the front tire had a valve cap. The rear tube was put back in
the tire, which was pumped up. Howard borrowed another cap."
--"Pushing the Limits," John Howard with Peter Nye, p. 224-5
In another account, Howard said that Doug Malewicki:
" . . .calculated that at 150 m.p.h., the outer surface of my speed
bike's tire wanted to fly away from the axle impelled by a force
greater than 1,600 times normal gravity--or 1,600 g's. Now, while
1,600 g's represents no real problem for V-rated tires normally (and
incidentally, less than half that stres would pull a human body
apart!) the valve stem spring is designed so that it can be compressed
with just a fingernail touch, which meant that the 1,600 g's was more
than enough to squeeze the spring and deflate my tire!"
--p.124-5, "Racing the Wind," Cecil Cripps
A quick test with my electronic scale shows a short Schrader valve
starting to open (against no air pressure) at about 3000 grams.
I couldn't figure out how to test the long-style Schrader valve.
The short pin weighed only 3 grams, while the long pin weighed 4
grams:
http://i39.tinypic.com/302x9gg.jpg
My guess from this slightly angled and foreshortened photo with
half-inch chain links as a crude reference is that the valve pin was
rotating around a circle with a ~7 to ~8 inch radius:
http://i43.tinypic.com/2i6nh4y.jpg
(One book mentions that Howard was 6'2". The tires aren't terribly
small, but the thick diameter and the heavy Akront motorcycle rims put
the Schader valve closer to the axle.)
For a 17.8 cm to 20.3 cm (7~8 inch) radius, this calculator predicts
that we need 2834 to 2654 RPM for 1600 G's:
http://www.centrifuge.jp/cgi-bin/calc-e.cgi
1600 1000
G G
tire rollout radius RPM RPM
17.8 cm = 2834 2241 7" valve stem radius
20.3 cm = 2654 2098 8" valve stem radius
20" 1596 mm 25.4 cm = 2373 1876 (141~111 mph)
22" 1759 mm 28.0 cm = 2260 1786 (147~117 mph)
24"x1.75 1888 mm 30.0 cm = 2183 1726 (154~121 mph)
(Alas, the calculator's upper radius limit is only 30 cm.)
It looks as if the tire-valve radius, the valve-pin mass, and the
high-speed RPM work out to produce a G-force "weight" of well over the
~3,000 grams needed to open a 3-gram Schrader-valve pin (1,600 * 3 =
4800 grams, 1,000 * 3 = 3,000 grams).
But I'm wondering if there's anything seriously wrong with my rough
calculations.
I'm certainly suprised by the enormous G-forces involved at bicycle
land-speed-record speeds.
Coasting down a steep hill at a far more sedate 43 mph, the small
24"x1.75 ATB tires are doing about 500 RPM and producing a G force of
only 83 G's.
(Alas, the calculator's lower limit is 500 RPM.)
Cheers,
Carl Fogel
carl...@comcast.net
wrote:
>On Wed, 18 Feb 2009 22:44:42 -0700, carl...@comcast.net wrote:
>
>>A quick test with my electronic scale shows a short Schrader valve
>>starting to open (against no air pressure) at about 3000 grams.
>
>Wouldn't the centrifugal force push the seat of the valve closed
>rather than open ?
Dear Still,
No, you push a Schrader valve's pin down into the valve to let the air
out.
When the spinning tire acts as a centrifuge, the G-force throws the
pin away from the axle, as if you were pushing it in against the
spring with your thumbnail.
Cheers,
Carl Fogel
bjwe...@gmail.com
> What are the G forces on a Schrader valve on a land speed bike doing
> about 140 mph, using Akront motorcycle rims and wide tires?
>
> Is 1,600 G's a reasonable rough estimate?
>
Carl,
Don't rely on online calculators as a substitute
for the physics involved. This is very basic stuff.
A copy of Halliday and Resnick or similar intro
physics text will explain centripetal acceleration
clearly.
The equations are extremely simple.
a = v^2 / R, or a = omega^2 / R,
where omega is the angular speed in e.g.
radians per sec (that is, in radians, not in
revolutions).
Ben
Ryan Cousineau
Still Just Me <no...@nope.com> wrote:
> On Wed, 18 Feb 2009 23:27:32 -0700, carl...@comcast.net wrote:
>
> >>
> >>Wouldn't the centrifugal force push the seat of the valve closed
> >>rather than open ?
> >
> >Dear Still,
> >
> >No, you push a Schrader valve's pin down into the valve to let the air
> >out.
> >
> >When the spinning tire acts as a centrifuge, the G-force throws the
> >pin away from the axle, as if you were pushing it in against the
> >spring with your thumbnail.
> >
>
> I still see it the other way - You push the pin towards the hub, this
> moves the movable valve seat towards the hub and away from the fixed
> seat, then air can escape. If there is centrifugal force on either the
> movable valve seat or the pin, they would be pushed in the opposite
> direction - towards the rim - closing the seat tighter.
You have the valve action exactly backwards.
> Please explain your suggestion in more detail.
A schrader valve (or for that matter, a Presta valve) has a pin that is
pushed rim-wards (to the outside of the wheel) to open the valve to air.
Presta valves don't have the same vulnerability simply because they
incorporate a screw-down closure.
--
Ryan Cousineau rcou...@gmail.com http://www.wiredcola.com/
"In other newsgroups, they killfile trolls."
"In rec.bicycles.racing, we coach them."
carl...@comcast.net
Dear Ben,
The calculators are awfully nice for checking that you have things
right (at one point, I noticed that I'd performed my usual trick of
forgetting that the radius is only half the diameter) and for doing a
lot of extremely tedious (and error-prone) calculations and
conversions.
Take a moment and show all your work for calculating the RPM for 1400
G's for a tire with a rollout of 1775 mm. Then find the speed in mph
for a 1625 mm tire that provides 1400 G's.
Cheers,
Carl Fogel
carl...@comcast.net
wrote:
>On Wed, 18 Feb 2009 23:27:32 -0700, carl...@comcast.net wrote:
>
>>>
>>>Wouldn't the centrifugal force push the seat of the valve closed
>>>rather than open ?
>>
>>Dear Still,
>>
>>No, you push a Schrader valve's pin down into the valve to let the air
>>out.
>>
>>When the spinning tire acts as a centrifuge, the G-force throws the
>>pin away from the axle, as if you were pushing it in against the
>>spring with your thumbnail.
>>
>
>I still see it the other way - You push the pin towards the hub, this
>moves the movable valve seat towards the hub and away from the fixed
>seat, then air can escape. If there is centrifugal force on either the
>movable valve seat or the pin, they would be pushed in the opposite
>direction - towards the rim - closing the seat tighter.
>
>Please explain your suggestion in more detail.
Dear Still,
No, you push a Schrader valve-pin _away_ from the hub, toward the rim,
and into the valve.
Etch-a-sketch artwork:
http://home.comcast.net/~carlfogel/download/schrader.JPG
Pushing the pin into the valve and away from the hub lets you inflate
or deflate the tire.
Maybe I'm thinking of standard Schraders and you have some really
interesting version in mind? Have a look at a car tire.
Cheers,
Carl Fogel
mike.a...@gmail.com
wrote:
2 Pi (2 * 3.1415926) radians per revolution.
jobst....@stanfordalumni.org
>>> What are the G forces on a Schrader valve on a land speed bike
>>> doing about 140 mph, using Akront motorcycle rims and wide tires?
>>> Is 1,600 G's a reasonable rough estimate?
>> Don't rely on online calculators as a substitute for the physics
>> involved. This is very basic stuff. A copy of Halliday and
>> Resnick or similar intro physics text will explain centripetal
>> acceleration clearly.
>> The equations are extremely simple.
>> a = v^2 / R, or a = omega^2 / R,
>> where omega is the angular speed in e.g.
>> radians per sec (that is, in radians, not in
>> revolutions).
> 2 Pi (2 * 3.1415926) radians per revolution.
Make that: a = r * omega^2
Acceleration increases with radius at the same angular velocity. The
valve on most bicycle wheels faces inward toward the axle. The
illustration on Wiki shows the centrifugal release effect:
http://en.wikipedia.org/wiki/Schrader_valve
Jobst Brandt
bjwe...@gmail.com
Yes, thank you Jobst. I made a typo.
a = V^2 / R
v = omega * R
and so equivalently, a = omega^2 * R
These corrected formulae are pretty much all
that goes into calculating centripetal/centrifugal
G-force in a centrifuge, or this wheel/valve sysem.
Ben
Doug McLaren
| The equations are extremely simple.
| a = v^2 / R, or a = omega^2 / R,
| where omega is the angular speed in e.g.
| radians per sec (that is, in radians, not in
| revolutions).
The equations are even simpler than that, because you don't need to
figure out angular speed. All you need is simply --
acceleration = v^2 / r
Where v is the speed of the bike -- 140 mph, was it?
Google calculator will even do all the heavy lifting for you.
http://www.google.com/search?hl=en&q=(140+mph)^2+%2F+14.5+inches+in+g
gives a figure of 1084 g for a 29" diameter tire (which I think is
typical for the outside diameter of a 700c tire?)
HOWEVER, this assumes that the valve mechanism on your Schrader valve
is right at the outer part of the tire -- that it's touching the road.
The reality is that it's perhaps 2" in (it depends on the specifics of
your tire and tube, of course) which lowers the g forces
significantly.
Assuming that we've got a 29" diameter tire (14.5" radius), and the
valve stem mechanism is 2" away from the road, and the bike is going
140 mph, I work out that the g forces are `(140 mph * (12.5/14.5))^2 /
(14.5-2) inches in g', or 935 g's -- pretty far away from 1600 g's,
but still significant.
Though really, rather than just switch to Presta valves, I'd probably
at least look to see what the motorcycle racers are doing, at least if
I had a reason to keep using Schrader valves.
--
Doug McLaren, dou...@frenzied.us Anything worth doing is worth overdoing.
Nick L Plate
So with Fogel's 3g short pin, that'll be 2800g force and he measured
3000g that's less than 10% I'll take it a typical shcrader valve will
lift off at above 140mph on a motorcycle wheel. Makes sense that cars
were limited to 135mph, expensive, those sealing rings. When the
authorised dealer mangles the cap, or fits that one off the workshop
floor that fell of his mothers mini he was doing a free service on an
hour earlier.
TJ
Nick L Plate
> Nick L Plate <tj-j...@blueyonder.co.uk> considered Fri, 20 Feb 2009
> so it's not such a problem for them.
Because they were limited to satisfy CO2 reduction policy, limit
insurance premiums and save on the higher speed rating of tyre.
TJ
carl...@comcast.net
Dear Doug & Trevor,
Since John Howard's wheels and tires were rather smaller than 29"
wheels, the RPM and G-force when he was doing that 140 mph was higher.
See the original post for the photos and estimates of the radius of
the circle described by his valve stem.
Cheers,
Carl Fogel
Doug McLaren
|>>
|>> >> Google calculator will even do all the heavy lifting for you.
|>>
|>> >> ? ?http://www.google.com/search?hl=en&q=(140+mph)^2+%2F+14.5+inches+in+g
| Dear Doug & Trevor,
|
| Since John Howard's wheels and tires were rather smaller than 29"
| wheels, the RPM and G-force when he was doing that 140 mph was higher.
|
| See the original post for the photos and estimates of the radius of
| the circle described by his valve stem.
Ok, 20" rather than 29" tires. Looking at the pictures, that sounds
about right.
You still seemed to not properly take into account that the valve stem
isn't right at the road rather than inwards somewhat.
Assuming that the tires have a 20" diameter, and the speed is 140 mph,
and the valve is 2" in from the outer edge, the acceleration at the
valve is 1258 g's.
Assuming that the valve stem is 1.5" in, it's 1337 g's.
Assuming that the valve stem is 1.0" in, it's 1415 g's.
Assuming that it's right at the road (0" in), it's 1572 g's.
Note that the speed might be higher than 140 mph. Your post mentions
150 mph, and there's stuff online about him going 152 mph. In
particular, the `over 1600 g' figure quoted by Doug Malewicki appears
to be related to 150 mph. (And if he used 20" tires, he must have
taken into account that the valve isn't at the outer edge of the tire.)
The formula is simple, especially if you let google do the work for
you --
http://www.google.com/search?hl=en&q=(140+mph+*+((10-1.5)%2F10))^2+%2F+(10-1.5)+inches+in+g
Where 140 mph is the speed of the bike, 10 is the outside radius of
the tire in inches, and 1.5 inches is the distance in from the outer
edge of the tire of the valve.
Considering that he's not really very worried about aerodynamic drag,
but rolling resistance is likely to be a very significant issue, I'm
surprised he didn't use larger tires. Larger tires would have less
rolling resistance and the G-forces would be smaller, making the risk
of a catastrophic failure smaller (all else being equal, of course,
which it never is.)
Though I imagine that everything has been well thought out, and he has
good reasons for going with smaller tires that I'm just not aware of.
Perhaps it's just because that's what's available and well tested,
being what motorcycles use at that speed on a regular basis?
--
Doug McLaren, dou...@frenzied.us
"Nostradamus told me this would happen. Smug bastard."
Chip C
[...]
I totally believe that at some speed, the G forces will open a
Schrader valve enough to let air out. Whether it's at 1600 G or 935 G
or whatever, I believe it.
What I'm having trouble with is the screw-on dust cap on the front
wheel containing the pressure in it...perhaps the G forces on the cap
press it down hard enough on the valve to form a gas-tight seal?
Chip C
russell...@yahoo.com
Doesn't Mad Dog come with a screw cap? If it can hold the precious
liquid in the bottle, the valve cap should do OK for the air. And he
was not riding for hours on end at that pressure, just a few minutes,
so not much would leak out even if it was not completely air tight.
carl...@comcast.net
Dear Chip,
A plastic dust-cap just keeps mud out of a Schrader valve.
A good metal Schrader valve cap has the valve-core wrench built into
one end and a sealing washer inside.
When you screw the cap down, the washer easily gives an air-tight
seal.
I just inflated a tubeless trailer tire to ~60 psi, removed its
Schrader valve core, screwed the metal Schrader cap with its inner
seal onto the valve before much air was lost, and took this photo
while standing on the tire:
http://i40.tinypic.com/v6ktix.jpg
The valve cap doesn't leak. The removed valve core is on the rim, near
another valve cap with a red inner seal.
Cheers,
Carl Fogel
carl...@comcast.net
wrote:
>Quality caps come with a rubber gasket inside the top of the cap that
>seals against the top of the threaded portion of the schrader. This
>would likely prevent catastrophic air loss. I'd think it would likely
>leak down, but probably over a couple days.
Dear SJM,
Such doubts are understandable, but groundless, like worrying about a
tubeless tire.
A Schrader valve cap with an inner seal forms a routine air-tight
seal. It's just as good at holding pressure as you get with a tubeless
tire or a normal Schrader valve with its valve core installed. The cap
on a blind run of plumbing pipe works about the same way.
Here I'm standing on a ~60 psi tubeless trailer tire with only a valve
cap holding the air in. You can see the Schrader valve core lying on
the rim, near another metal valve cap with a red inner seal:
http://i40.tinypic.com/v6ktix.jpg
And here's the same Schrader valve-cap (still with no valve core)
passing the glass-of-water valve test:
http://i44.tinypic.com/30k62x1.jpg
And no, I didn't give a mighty heave to twist the cap on tight--no
need for that. The metal-threaded cap tightens down on the seal and
works just fine without much trouble.
Of course, no one removes the valve core except for a demonstration
like this or to empty or fill a large tire quickly. The seal is
actually over-built for what it does, namely keeping dirt and water
out and any slight leakage from the valve core in.
Cheers,
Carl Fogel