tIre pressure

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.

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.

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

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.

Re: tIre pressure

“gpickett00” <gpickett00@NoEmail.Message.Poster.at.Unicyclist.com> 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.

Ken

Hmmm, speaking of tire pressure…
http://gallery.unicyclist.com/albuw98/john_slide_1

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

I run 15-20 psi.

john

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!

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!

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.

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

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. However, I noticed today that I was not comfortable with the tire rolling over - at all. I have run my current tire at 20 pounds for dozens of miles without bottoming out, but 30 pounds seems to be the “sweet spot.” Can 30 pounds on one tire be “like” 20 on another?

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 :frowning:
/I’d try a 3.0 tire again tomorrow, but I hear rain outside now.

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?

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)

http://www.trials-online.com/forum/viewtopic.php?t=31
–"run low pressure, i have 20 in my hutchison and i probably should drop it to even lower. "

http://biketrials.com/cgi-bin/ikonboard312a/ikonboard.cgi?;act=ST;f=1;t=5187;hl=psi
–“I run about 18-20 psi”

http://biketrials.com/cgi-bin/ikonboard312a/ikonboard.cgi?;act=ST;f=1;t=4994;hl=psi
–“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”

http://observedtrials.net/forum/viewtopic.php?t=1629&highlight=pressure
–“runnin about 25psi”
–“at the moment it is at 15PSI”

http://observedtrials.net/forum/viewtopic.php?t=1466&highlight=pressure
–“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

speaking of tire pressure

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

http://gallery.unicyclist.com/albuv08/uni2

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

Im like 6’6" and 240lbs…is that good for the tire??

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

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.

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

Re: tIre pressure

In article <m2fzaujec6.fsf@cs.cmu.edu>,
Ken Cline <ken.cline@cs.cmu.edu> wrote:
)“gpickett00” <gpickett00@NoEmail.Message.Poster.at.Unicyclist.com> 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

Re: tIre pressure

doosh@inl.org (Tom Holub) writes:

> In article <m2fzaujec6.fsf@cs.cmu.edu>,
> Ken Cline <ken.cline@cs.cmu.edu> 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.

> 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