tIre pressure

From what I hear, you want to run the lowest you can get
away with. That is the lowest you won't bottom out with.

I run mine a bit higher then that just so it's a bit less
dead when I'm riding around.

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gpickett00 said:

Today my dad brought up the issue of tire pressure. How
come you can hop higher with "ideal" tire pressure instead
of maximum? My dad made a comparison with lower tire
pressure as sand, and higher as concrete. I didnt know
what to tell him, but the highest tire pressure centainly
isnt the best for hopping.

It has to do with how much energy the tire can add to your
hop. A high pressure tire won't compress much when loaded,
so it can't add much spring to your jump.

Ken

I know i want it low, and i know it lets you bounce higher,
but how come?

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In article <[email hidden]>, Ken Cline
<[email hidden]> wrote: )"gpickett00"
<[email hidden]>
writes: ) )> Today my dad brought up the issue of tire
pressure. How come you can hop )> higher with "ideal" tire
pressure instead of maximum? )> My dad made a comparison
with lower tire pressure as sand, and higher as )> concrete.
I didnt know what to tell him, but the highest tire pressure
)> centainly isnt the best for hopping. ) )It has to do with
how much energy the tire can add to your hop. A )high
pressure tire won't compress much when loaded, so it can't
add )much spring to your jump.

This is the right effect, but the wrong explanation.

The amount of energy put into the tire is exactly the same
on a low pressure or high pressure tire. That energy will be
returned *more* efficiently on a high-pressure tire. That's
why a basketball with a lot of air in it will bounce higher
than one with not much air in it. If you dropped two
unicycles straight down from 200 feet up so that they landed
on their tires, the one with more air in the tire would
bounce higher.

So while the higher-pressure tire will compress less, the
amount of energy it stores is just as high, and it returns
that energy more efficiently than the lower-pressure tire.
So that's not the explanation for why lower pressure is
easier to hop with.

I think the answer is simply in the timing. With a high-
pressure tire, the energy you put into it is returned almost
immediately, too quickly to fluidly press down and then pull
up to get a good jump. The fat, low-pressure tire takes a
lot longer to compress, giving you a longer window of
opportunity to *use* the return of energy. -Tom

A better comparison might be a trampoline or springs.
Concrete and sand is a bad analogy. If you have springs to
tight on a trampoline it is like jumping on concrete. Were
as you loosen them up it allows for more action. For every
action there is an opposite and equal reaction.

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(Tom Holub) said:

In article <[email hidden]>, Ken Cline
<[email hidden]> wrote: ) )It has to do with how
much energy the tire can add to your hop. A )high pressure
tire won't compress much when loaded, so it can't add
)much spring to your jump.

This is the right effect, but the wrong explanation.

Bzzzt. My explanation is correct. Read on for the gory
details.

Quoted message said:

The amount of energy put into the tire is exactly the same
on a low pressure or high pressure tire.

After reading your examples, I can see why you disagree,
unfortunately, those examples don't reflect the dynamice of
a unicycle hop. The ingredient in the hop that makes all the
difference is the upward acceleration of the rider and
resultant force on the pedals. This is a very different
situation than dropping a ball or unicycle, where there is
an impulse of energy at the bounce, rather than the hopping
force seen by the rider's push.

In other words, the energy imparted to a bouncing ball is
exactly the kinetic energy of the ball. Dampening scrubs
energy and dulls the bounds, more and more as the air
pressure gets lowered.

The physics of the unicycle wheel is different: As the rider
pushes off, he imparts a force, f, to the wheel which
compresses and storse energy. Assume for the moment that the
wheel acts as a spring with constant k. Then the amount of
energy stored is f^2/2k, which increases as k decreases
(i.e. as pressure is lowered). The wheel is not a perfect
spring, and dampening plays a role, at some point
overwhelming the benefit of lower pressure.

Ken

Hmmm, speaking of tire pressure....
gallery.unicyclist.comjohn slide 1Open ↗

note: that picture was taken after i had landed and
before i pushed to hop. So thats just my weight on
the uni. When I went to hop, it bottomed out more,
if thats possible.

john

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I run 15-20 psi.

john

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I wish someone would do a study on the subject.
Logically, I believe the "low pressure" equals much
higher hops is a myth.

Examples:
1. Golf balls are high pressure and bounce well from
concrete.

2. Low pressure racquet and tennis balls bounce less than
high pressure ones.

3. Weight is an issue. Obviosly if you bind a weight to a
racquet ball, and drop it in such a way to attempt to
make it bounce, it will not bounce when there is too
much weight.

4. Skill is an issue. I would bet Dan Heaton can hop higher
using a tire with 65 pounds of pressure than most
unicyclists can with a low pressure tire.

5. I haven't heard any mountain bikers or bike trials (or
anyone else) guys lowering pressure to get higher hops
(low pressure has other benefits).

6. Skateboarders can ollie (hop) onto cars (higher than most
unicyclists) with wheels, board, and truck system that
has very very little elasticity.

7. Test the wheel. Take the unicycle and bounce it with
various pressures. Does it bounce MUCH higher at
different pressures?

There could be some unexplained scientific phenomena that
causes a unicycle to hop an extra 6 inches(?) with lower
pressure. Perhaps it's psychological?

Has anyone actually changed increased the pressure and
measured how much height is LOST when hopping?

Some of you guys make it sound like if I take 10 pounds of
pressure out of my tire and I will have to worry about
hitting my head on high tree limbs!

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ChangingLINKS.com said:

*I wish someone would do a study on the subject.
Logically, I believe the "low pressure" equals much higher
hops is a myth.*

I think of it this way:

With a high pressure tire, the unicyclist must hop/jump from
what is effectively a hard surface.

With a low pressure tire, the unicyclist can use the tire as
a (small) spring baord (picture a spring baord like used in
gymnastics.

Some of your examples are of "bouncing objects", not
"springing people". Consider: diving board, trampoline, pogo
stick, bed, spring baord, ..

But hey, You do what works for You!

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ChangingLINKS.com,

Also consider, compared to most muni/trials riders, you are
running a relatively narrow and low profile tire (2.125",
and the profile is particularly low on a wide rim). So your
tire is never going to "spring" nearly as much as a 24x3 or
20x2.5 does.

You're also a pretty big guy, so you can't lower the
pressure very much before you bottom out that tire.

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duaner said:

*Some of your examples are of "bouncing objects", not
"springing people". Consider: diving board, trampoline,
pogo stick, bed, spring baord, ...*

Good examples. However, all of the things you mentioned
have 5-6" of elasticity. I am wondering how much >2" of
spring helps.

duaner wrote:*

Quoted message said:

Also consider, compared to most muni/trials riders, you
are running a relatively narrow and low profile tire . . .
So your tire is never going to "spring" nearly as much as
a 24x3 or 20x2.5 does. You're also a pretty big guy, so
you can't lower the pressure very much before you bottom
out that tire.*

I wasn't including myself or my current tire as a case
study. I am just starting to care about hopping - so I am
still focused on consistently hopping up curbs and stairs.
Currently, I don't know what tire works best for hopping,
however unlike many of the other unicyclists, I *will* be
trying several different tires to find out the truth.

/sadly the tire I loved is no longer available 🙁

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The weight of the rider has as much to do with this equation
as the pressure in the tire. The heavier the rider the
better they can compress a tire with more pressure. If
someone weighs 130 pounds, they are not going to compress a
tire with 30 pounds nearly as easily or as much as someone
weighing 250 pounds. So for lighter riders, lower pressure
makes a lot of sense.

When riding Muni I like lower pressure so I absorb more of
the roots and small rock that throw you off with higher
pressure. Unfortunately this also makes the tire more bouncy
for the stretches were I am trying to get maximum speed. I
know there is a point were you can lower the pressure so
much it no longer gives you any bounce or spring. The other
side is true as well, too much pressure no bounce.

Changing links the other examples you gave golf ball, tennis
ball... What is the pounds per square inch being exerted
against the object. I know in golf the the club head speed
is pretty fast. Also the weight of the object is pretty
light, so if it was not dense you start to have issues with
air resistance. An example is the light plastic golf balls
used to go only 50 feet on a hit that would send a regular
ball 300 yards. The air slows it down. That really isn't an
issue on a unicycle.

Where is our resident physicist?

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Your ‘Dad’ is in fact correct in the approach that he has
taken to thinking about it, but impractical in this
situation.

It is to do with stored energy. In engineering stored energy
takes the form of springs, compressed air systems, and
raised loads etc, etc. In a pneumatic tire energy can be
‘stored’ in the tire and ‘released’. ‘Stored’ by you
performing a pre-hop and released by the main hop, the
‘release’ of energy assists in the raising of the load, the
unicycle and passenger in this case. If you could ride the
unicycle with a, we will call it an ‘energy storage assist
mechanism’ you would ride the tire at the maximum working
pressure. This ‘energy storage assist mechanism’ would be a
person handing you a 100 kg weight just as you land on the
pre-hop and removing it once the energy from this weight is
stored in the tire, split micro-second timing would be
required and you’d gain significant advantage. Since we live
in the real world (some do) you are the ‘energy storage
mechanism’ (note removed the assist) your weight is related
directly to the potential to store energy within the tire.

Allegorically we are talking about Compressed Air Energy
Storage (CAES) which to gain the maximum efficiency we must
ensure that we can storage the energy within our system with
minimum loss. Hence the reason why the maximum tire pressure
is not the most efficient means of increasing ones hop, you
as a rider can not ‘store’ energy in it. You can ‘store’ the
energy when the tire is at a lower pressure. One could plot
a graph of tire pressure relative to passenger weight with a
third dimension of the height they drop down from on to the
saddle, in doing so you would yield a result providing you
with the most competitive tire pressure for the individual
rider. Alternatively you could just fiddle with it all and
get what’s best for you.

JJ (Engineer)

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Quoted message said:

5. I haven't heard any mountain bikers or bike trials (or
anyone else) guys lowering pressure to get higher hops
(low pressure has other benefits).

trials-online.comviewtopic.phpOpen ↗ --"run
low pressure, i have 20 in my hutchison and i probably
should drop it to even lower. "

tinyurl.com35ltbOpen ↗ --"I run about 18-20 psi"

tinyurl.com2269zOpen ↗ --"im about 175 pounds ride
generally high pressure for trials. about
35/40 i guess" -and in response: --"35-40psi is ridiculous.
i'm running 17, i know people who run almost half that."
--"i run a michelin comp24.1 with 10psi." --"I was
running it at about 18psi" --"165 pounds run 21 psi" --"i
run 20 psi" --"Running about 20-25 psi and weigh ~200
lbs" --"the rest of the night i didnt come near flatting,
didnt come near dinging my rim once, and was doing fat
gaps, and this is at 20 psi"

tinyurl.com2vc5jOpen ↗ --"runnin about 25psi" --"at the
moment it is at 15PSI"

tinyurl.com2gpo2Open ↗ --"225 pounds? Good tire, 20 psi"
--"I like squish without a lot of rim hitting so I run low
20s to 30psi." --"i weigh ~165, fide[ride] very poorly, and
i have like 10 psi" --"15PSI with minion ST 2.5 on rear "
--"13psi in front and 15 in the rear with Tioga 2.3."

john

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I run my tire pressure 25-30 but my tire still looks like
this when i hop

gallery.unicyclist.comuni2Open ↗

Do NokianGazz's normally flex like they do in that pic?

Im like 6'6" and 240lbs...is that good for the tire??

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A lower pressure tire just acts like a spring. Theres
nothing more to
it. And to the ChangingLINKS.com guy, I can feel the
differenc in a high/low pressure tire and have
experimented plenty. Of course I can only speak for one
person, but it seemed so natural to experiment that I
can imagine most other riders have done it too.

David

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A spring it is what you observe, but way more complex than
that, the tire compressed like a spring stores energy, but
with the tire it’s the whole tire that stores the energy, 3
dimensional, not two-dimensional.

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also, you used skateboards and trials bikers in comparison.
Both of those have pivot points for when they jump, unlike
unicycles. unicyclists jump straight up directly above the
wheel, while skateboards pivot on the back wheels and then
level out(same with bikes) I think using those to compare is
pointless. Also, the lower pressure in the tires of bikes
makes it MUCH easier to jump high. I used to run 60-65 psi
in front and back and my highest jump was 15-16", after I
lowered the pressure, I have been able to get ovet 2'
without going all out.

john

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