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Old 09-27-08, 10:41 PM   #1
Freakonwheels
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Do firm tyres increase speed and make climbing easier?

Silly question maybe but I have to know - do firm tyres go faster and make it easier to climb?

The problem is, I did my first Time Trial last week around a course of part of my city (beautiful by the way - I get to bike along this piece of scenery - aren't I lucky?! http://rcd.typepad.com/photos/cycle_shots/img_0300.JPG)

Anyway, yeah, I did my first Time Trial last week and I clocked in about 45 minutes 33 seconds. Today, a week later I did my second one, and I clocked in about 40 minutes 25 seconds. It's a good improvement on the surface, but my tyres were pumped up just a few minutes before, which means they would've been more firm than last week. I've been training throughout the week but I'm not sure how much I've improved or if it was just because of the difference in my tyres...lol
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Old 09-27-08, 10:52 PM   #2
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harder tires do make the bike roll easier, but there's a reason that there is a maximum pressure stated on the tire. i've over filled a time or two by accident and had belts showing on the sides after a few rides. you should be able to find some real good tires that take pressures in the 100's if you don't already.
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Old 09-27-08, 10:52 PM   #3
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Yes, less rolling resistance due to less surface area of the tire touching the ground at any given time. Not sure by how much. Probably a combination of improvement and tires. You can always experiment for fun.
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Old 09-28-08, 12:03 AM   #4
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have it at the ideal pressure compared to the tyre width and your body weight.

an over inflated tyre will bounce around when it hits a rough patch instead of conforming to it like a properly inflated tyre would.
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Old 09-28-08, 01:19 AM   #5
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As stated should help but IMO it's likely more in your legs.
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Old 09-28-08, 01:50 PM   #6
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on the road yes until it is really steep like 15 deg or more, then a little softer helps dig in better

on a mountain bike maybe, depends on terrain
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Old 09-28-08, 01:50 PM   #7
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Yes, less rolling resistance due to less surface area of the tire touching the ground at any given time.
Seems to me that's not actually true, according to a study a few years ago.
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Old 09-28-08, 03:59 PM   #8
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Seems to me that's not actually true, according to a study a few years ago.
Well he is technically pretty much right, he just said it differently.

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The primary cause of rolling resistance is hysteresis:

A characteristic of a deformable material such that the energy of deformation is greater than the energy of recovery. The rubber compound in a tire exhibits hysteresis. As the tire rotates under the weight of the vehicle, it experiences repeated cycles of deformation and recovery, and it dissipates the hysteresis energy loss as heat. Hysteresis is the main cause of energy loss associated with rolling resistance and is attributed to the viscoelastic characteristics of the rubber.

-- National Academy of Sciences[3]

Thus materials that flex more and bounce back slowly, such as rubber, exhibit more rolling resistance than materials that flex less, such as steel, or that bounce back more quickly, such as silica. Low rolling resistance tires typically incorporate silica in place of carbon black in their tread compounds to reduce low-frequency hysteresis without compromising traction.[4]
Basically the more air in your tire = less deformation = greater efficiency. How much? Who knows. Not me.
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Old 09-28-08, 04:10 PM   #9
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I would be surprised if it made that much difference, unless they were just half flat the first time. Anyway, if you're comparing speeds, it's good practice to check them each time just to eliminate a variable (and to make sure they're not half flat).

Riding the same course the same way on two different days, my times or speeds are seldom the same. One thing I notice is the wind, or lack thereof. Another is the heat, and I'll do noticeably better when it's more pleasant- I can work harder, and don't have to carry as much water.
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Old 09-28-08, 04:14 PM   #10
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Basically the more air in your tire = less deformation = greater efficiency. How much? Who knows. Not me.
Except that article only considers one mechanism for rolling resistance. There is also energy loss from a tire bouncing on rough surfaces (as all real roads are). Above a certain pressure (usually around 120 psi) this grow faster than energy loss in the the rubber decreases. As a result further increases in pressure cause rolling resistance to rise.

And this has nothing to do with efficiency since efficiency makes no distinction for where energy is dissipated - heating air, heating rubber, or working against gravity.
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Old 09-28-08, 04:17 PM   #11
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I can tell the difference between 80 psi and 100 psi in my tires

The newer mid to high level road tires and rims can easily handle 120 psi.

Another way to make your wheels faster is to pump them up with nitrogen.


On the other hand, if mountain biking on dirt trails; climbing can be made easier with lower pressure in the tires.
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Old 09-28-08, 04:19 PM   #12
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Another way to make your wheels faster is to pump them up with nitrogen.
Don't you mean helium?
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Old 09-28-08, 04:20 PM   #13
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Except that article only considers one mechanism for rolling resistance. There is also energy loss from a tire bouncing on rough surfaces (as all real roads are). Above a certain pressure (usually around 120 psi) this grow faster than energy loss in the the rubber decreases. As a result further increases in pressure cause rolling resistance to rise.

And this has nothing to do with efficiency since efficiency makes no distinction for where energy is dissipated - heating air, heating rubber, or working against gravity.
Thank you for saving me from typing it all out.

Basically: Rolling resistance should decrease as pressure increases - only if you ride on glass roads. In the real world, rolling resistance is at a minimum at some pressure then only increases thereafter because the surface of the road is not perfectly smooth.
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Old 09-28-08, 04:21 PM   #14
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harder tires do make the bike roll easier, but there's a reason that there is a maximum pressure stated on the tire. i've over filled a time or two by accident and had belts showing on the sides after a few rides. you should be able to find some real good tires that take pressures in the 100's if you don't already.
Correlation does not imply causation. What you are seeing is a coincidence.
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Old 09-28-08, 04:21 PM   #15
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Another way to make your wheels faster is to pump them up with nitrogen.
If this were true, all the pro teams would be doing it. I've never _ever_ heard of this argument for putting nitrogen into bicycle tires before.
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Old 09-28-08, 04:23 PM   #16
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Except that article only considers one mechanism for rolling resistance. There is also energy loss from a tire bouncing on rough surfaces (as all real roads are). Above a certain pressure (usually around 120 psi) this grow faster than energy loss in the the rubber decreases. As a result further increases in pressure cause rolling resistance to rise.

And this has nothing to do with efficiency since efficiency makes no distinction for where energy is dissipated - heating air, heating rubber, or working against gravity.
You are right I should have stated that my assertion was operating under the assumption that this was all taking place on a perfectly flat surface in a vacuum. *Farts*

It does have to do with efficiency when you are talking about differences in energy losses. Seems pretty simple to me.
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Old 09-28-08, 04:24 PM   #17
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If this were true, all the pro teams would be doing it. I've never _ever_ heard of this argument for putting nitrogen into bicycle tires before.
I have; usually around places like this. Of course, just because it keeps getting repeated doesn't make it valid. The argument usually goes something like race cars do it so we should do it for our bike tires, too. Sadly the rationale for the former has no bearing on the latter.
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Old 09-28-08, 04:26 PM   #18
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It does have to do with efficiency when you are talking about differences in energy losses. Seems pretty simple to me.
Not if you use the word efficiency correctly.
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Old 09-28-08, 04:46 PM   #19
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If this were true, all the pro teams would be doing it. I've never _ever_ heard of this argument for putting nitrogen into bicycle tires before.
True, except the pro teams are prohibited from using nitrogen. Same as many of the other restrictions on bicycles and weight that they can ride with. After all, we do not see any of the pros using faired recumbents on any of the flat stages, do we. Therefore no nitrogen in their tires.
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Old 09-28-08, 04:48 PM   #20
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Therefore no nitrogen in their tires.
So what, pray tell, do you perceive to be the advantage of nitrogen?

What is the UCI reg. that prohibits nitrogen? I couldn't find any.

Last edited by asgelle; 09-28-08 at 04:52 PM.
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Old 09-28-08, 04:48 PM   #21
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don't you mean helium?
No
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Old 09-28-08, 04:57 PM   #22
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So what, pray tell, do you perceive to be the advantage of nitrogen?

What is the UCI reg. that prohibits nitrogen? I couldn't find any.
The advantage of a tire filled with an inert gas versus atmospheric composition is that it is less susceptible to pressure changes due to expansion/compression of the gas inside the tire/tube due to temperature changes.

I believe the Space Shuttle has its tires filled with an inert (I believe it is N2) due to the extreme temperature changes from space to earth.

100psi is 100psi, no matter if is air, nitrogen, oxygen, etc. But, each of the gases (or compounds as in the case of the atmosphere) is subject to different characteristics at different temperatures and/or pressures.
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Old 09-28-08, 05:06 PM   #23
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100psi is 100psi, no matter if is air, nitrogen, oxygen, etc. But, each of the gases (or compounds as in the case of the atmosphere) is subject to different characteristics at different temperatures and/or pressures.
And here I spent 30 years using the ideal gas law. Who knew?

I wonder how you explain NO, N2O, NH3, and the ever popular N2H4.
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Old 09-28-08, 06:00 PM   #24
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And here I spent 30 years using the ideal gas law. Who knew?

I wonder how you explain NO, N2O, NH3, and the ever popular N2H4.
I was referring to a tire's basic characteristics at a given pressure, not the properties of a given gas at any particular pressure or the relationship of molecules in differring state variables in relationship to kinetic theory.

Perhaps you were overthinking my response?
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Old 09-28-08, 06:09 PM   #25
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I was referring to a tire's basic characteristics at a given pressure, not the properties of a given gas at any particular pressure or the relationship of molecules in differring state variables in relationship to kinetic theory.
I have no idea what this means. Are you saying that a tire behaves differently at a given temperature and pressure based on the composition of the non-reactive (not inert) gas inside?
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