supcom

Fingers are far less accurate than a pressure gauge and pretty useless for typical road bike tire pressures. If you are inflating a tire on the side of the road, it may be all you have, but at home, a good floor pump with a gauge is the best way to optimize your tire pressure.

Retro Grouch 

Yeah, it does.

Pressure is measured in psi - pounds per square inch. A bigger tire has more square inches so the absolute force against the rim wall is greater.

vitualis

You're all describing it wrong.

Firstly, ideal gas law: https://en.wikipedia.org/wiki/Ideal_gas_law

Pressure is inversely related to volume. So in a simple system like a tube in a tyre, you can't say that something is related only to "pressure" and not "volume" as both are linked by a simple mathematical relationship.

The reason that wide diameter rims support smaller pressure is that the wall strain substantially increases as a function of diameter. A small balloon of 10 cm diameter inflated to 20 psi will have exactly the same "pressure/force" on the wall as a larger balloon of 20 cm diameter inflated to 20 psi. How much? 20 psi.

However, the strain on the rubber of the larger balloon will be much larger. I can't remember the formula or it's name but the increase in strain is not linear, but rather goes up by some power. That is, strains goes up very rapidly with increasing diameter size.

This is why arterial aneurysms are so much more likely to burst than ordinary sized arteries. The blood pressure is the same, but the strain on the vessel wall in the dilated part of the aneurysm is much greater.

Regards.

Metzinger

Virtualis is right about the inverse relationship of pressure to volume.
I was skeptical at first, but did a small experiment.
Took a balloon, fully inflated, and reduced the pressure by letting some air out.
I couldn't believe it- it got bigger!
Did it again, bigger still. But eventually I couldn't tie the knot properly and it's slowly leaking. It's getting massive.

I'm getting worried as it's taking over my house. My wife is gonna have an aneurism when she sees it.
Should I try to reduce its volume by blowing it up a bit?
What do you guys think?

well biked

I like the way I described it better, so there-

Garfield Cat

Are you saying that when a tire bounces, it goes up and then it goes down and that downward pressure due to weight will cause rolling resistance?

Are you saying that a tire that's overinflated merely "tends to bounce" but does not necessarily bounce?

If you ride a road say black asphalt and some concrete, and its for the most part, smooth without big cracks, then there shouldn't be too many bounce type areas. I imagine that a mountain bike rider going fast and getting airborne is an exaggerated bouncer. This hardly happens in road biking.

I can see where one is in a road race like a time trial, it would make a difference. But just for a recreational ride, or even a group ride, its not a big deal. That said, what the heck, inflate to 120 psi front and rear.

Retro Grouch 

You've got it backwards:

When your tire bounces over a pebble or something like that the energy that is required to lift the bike ultimately has to come from you. It's also energy that's being wasted because it isn't moving you down the road. Rock hard tires bounce more than softer ones so that element of rolling resistance is being increased.

Another element of rolling resistance is tire deformation at the contact patch. If you take a bare tire off of the rim and try to deform it with your fingers, it resists. As you roll down the road, that resistant patch is constantly changing. Soft tires deform more than hard tires so that element of rolling resistance is increased with softer tires.

The optimum air pressure will differ depending on tire size and construction, load, and road surface.A mountain bike on a rocky downhill will do better relatively soft tires because they will encapsulate many of the rocks and won't bounce so much. A TT bike on a glass smooth surface would benefit from very hard tires because the tire deformation at contact patch will less.

Retro Grouch 

Virtualis is goofy.

well biked

+1

I'd like him to point out exactly what he thinks is wrong with what I said or Retro Grouch said. Virtualis is just reciting a bunch of boring, scientific mumbo jumbo.

Anyone who's picked up on the fact that fatter tires generally have lower max. pressure ratings than skinnier tires probably understands the principles involved here, no need to get overly complicated in explaining it-

Moose

Uhhmm...what?

waldowales

I'm going to pop some popcorn, I'll be right back.

DannoXYZ 

Well, he kinda got it right in results, but didn't explain the causes so well. The reason larger tyres put more stress on the beads due to pressure is due the larger surface area of the casing between the beads. If you take a 1" long stretch of bead, the number of square-inches between bead determine how much force is stretching the bead at that point. A 30mm tyre has about twice as much casing between the beads as a 18mm and at the same pressure, will end up pulling on the bead with twice the force. Just calculate the number of square-inches of surface-area in the casing and multiply it by the pressure to get the total force, simple.

vitualis

I think you should actually learn what is basic high school science and think though the scenario properly before pretending that you're clever.

Pressure and volume is clearly inverse related in a closed system (i.e., same amount of gas... which should be obvious if you actually bothered to read the pretty short Wikipedia article). Get a pump and close off the valve and push down on the plunger. Same amount of gas, the volume of the gas decreases and the pressure of the gas goes up.

A bike tyre is a closed system unless you've got a puncture which was my point. It doesn't make sense saying something is "due to the pressure" only or "due to the volume" only because both are directly related to the other.

BTW: the problem here may be with the use of the term "volume". Put more air in a tube in a tyre does not "increase the volume" by any appreciable amount. The tyre will only expand to a certain size (i.e., the volume). Increase the amount of gas in the tube subsequently increases the pressure inside the tube. Again, look at the ideal gas equation. Assuming everything else is equal, if the volume is kept constant then the number of moles of gas (i.e., the amount of gas) is directly proportional to pressure.

Regards.

Metzinger

[QUOTE=vitualis;7928334]A bike tyre is a closed system unless you've got a puncture which was my point. It doesn't make sense saying something is "due to the pressure" only or "due to the volume" only because both are directly related to the other.


Wasn't trying to be malicious, Virt, just pointing out the obvious.
When discussing modifications to air pressure in a fixed volume tire, it's actually an open system. Air's being put in or let out.

The whole ínverse p/v relationship doesn't apply here.

I just thought it was funny that you put it in.

The other concept for which you were grasping is elasticity. For a membrane, the amount of strain varies with the amount of stress placed upon it. Different for every material. Graphs.

As a physics prof, I'm sure you know all this. Just clarifying for the others.

Sometimes adding more info, like p/v, muddies the waters when people are trying to solve a problem.

Peace.

bikinfool

Yep, brain fart, was only speaking of my mountain bike tires with the finger thing...I use a gauge on my road bike tires...

Garfield Cat

Sounds like I am more like that TT bike rather than the mountain bike example. So I'll stick to 120 psi. These small pebbles and the loss of energy going forward as opposed to up and down, it seems, are real small issues, great for scientific discussion but are all part of riding the real roads. Life is like that.

DMF 

Yes, afaict. My little friend has to ignore a "cast in rubber" minimum too.

I've asked around but have never understood why there is a stated minimum on some tires. Thus the recommendation to ask the manufacturer.

meb

Back to the original question-More comfort, better traction, and in SOFT surfaces (gravel, sand, off-road)-lower rolling resistance since you don't sink as much.

And just because you aren't pinch flatting, doesn't mean sometime you won't hit a big hole/curb.

My experience is self sealing tires are pretty effective on sealing low pressure tires, but with high pressure tires-the Slime fails to seal.

itsajustme

Does anyone else have any insight on this? I understand that the maximum pressure ratings on tires are due to the fact that the carcass may not be able to take the strain of higher pressures without bursting.

But why do tires have minimum pressure ratings?

Pocko

^ I'll give it a go...

Get a MTB and a Road bike next to each other and just use the front tires for the experiment. Release all the air and pump them both back up to 40psi. Then press the tires with your fingers. You'll notice that the MTB tire (2.2"wide) has already achieved full firmness or full inflation... the road tire (3/4" wide) on the otherhand is soft and under-inflated even at the same 40psi pressure.

Put your weight over the handle bars... 40psi on a fat MTB tire will hold your weight up... 40psi on a skinny Road tire won't be enough hold your weight up.

Did you also notice that you stroked that pump a lot more to get the MTB tire to 40psi, and you pumped a whole lot less to get the Road tire to 40psi.

Think about it for a while (have a coffee or something, relax)... the answer you're looking for should become apparent. I wouldn't even try to sift through all the information in this thread, if you can't get past this point and arrive at a sensible resolution in your mind.

Once you figure this one out, all the other bits will fall into place.


.

DannoXYZ 

Which tyres are you seeing with minimum pressures? If you've got big tyres with a large-tolerance fit on straight-side rims, like a 26x2.125" cruiser tyre, not enough pressure can cause it flop around and distort the contact patch while cornering leading to crashes.

itsajustme

Every tire made by schwalbe.

The issue is that just because you need a certain pressure to avoid squirm, pinch flat, poor handling, etc, etc it doesn't mean any particular tire can be used in that capacity. For example, although in theory a heavy rider shouldn't have any problems pumping up to a recommended pressure (such as enough pressure for a 15% drop), he may need to avoid certain tires in order to not exceed the rated maximums.

So the question is since the optimal pressure for a light rider like myself tends to be rather low, does that mean I need to select tires whose minimum pressure ratings are below my optimal pressures for their widths? Clearly a heavy rider risks carcass blowout by pumping a tire above rated maximums, even if the pressures are optimal for that rider and that width. So what would I be risking, if anything at all, by pumping to optimal pressures which are below rated minimums?

Pocko

In practical terms you can go as low as you like as long as the result does not impede you or becomes mechanically self-destructive. I know of several downhill mountain bike racers who frequently go way below recommended pressures, 12 to 17 psi for the front tire (2.5" Maxxis tire rated at 35-65psi) but use between 30 to 35 psi for the rear. Two of them hold multiple National Championship titles. I can't argue with their logic judging by what's in their trophy cabinets. I'm sure if higher pressures would've made them faster they would've done it.

At the opposite end of the scale, it would be pointless to go over the maximum air-pressure recommended by the manufacturer for any particular tire. Tires are always constructed to be at it's lightest weight possible. So their plys (kind of fiber, type and density of weave) are only designed to handle air-pressures to a certain point.


.

ascend

My best guess would be that the minimum rating is set high enough to avoid rim damage for normal weight ranges in normal riding conditions. Depending on your/your bike's/your cargo's actual weight you may still be at risk for pinch flats at minimum pressure, but probably won't damage your rim while riding.

What constitutes a "normal weight range" or for that matter a "normal riding condition" is probably one for the lawyers.

DannoXYZ 

Anyone remember the Ford Explorer fiasco? Firestone recommended 32psi minimum on their tyres. But Ford, to make up for a crappy suspension that had excessive vibration and uncomfortable ride, chose 26psi. A tyre that's underinflated flexes A LOT. Large amounts of flexing causes the casing cords to rub against each other more and generates more heat. This can lead to delaminations of the plies and catastrophic failure.

Also as previously mentioned, you may never know when you'll hit that unexpected rock or pothole. Always give yourself an extra margin of safety with 10-20 more psi than the absolute minimum to avoid pinched flats.