Road Cycling - Tire pressures

I have the bontrager race lite 700x25 tires. The maximum pressure allowed is 120psi. My weight is 54kg.

Will any of that make a difference? Will a wider tire be better off with less pressure and will my weight affect the pressure that I should pump? Please recommend the appropriate pressures and should that be a difference between the front and the rear tires?

what sort of riding are you doing? What is the road made of? How much gravel/glass/misc crap is there?

How many punctures do you get? How many pinch flats do you get?

Brendon
:beer:

I'm not having problems with the tires as yet, hopefully none. This is my first road bike and so far so good. There's little glass whatsoever on the roads here. I'm not sure what the road is made of, asphalt?

Just stick to the recomended tyre pressure, regardless of road conditions, your weight, whatever, that way you will gain the maximum performance and longevity....

at your weight, go with the minimum allowable pressure +10%
I weigh 70Kg, and run less than 100psiin fornt wheel and 100ish psi on the rear. Nver had a pinch flat and wheels roll fast over any road.

They roll faster with max psi.

No they don't.....

Oh,ok.You are so confused but you run lower psi and work harder and coast slower.Lower psi will give you a softer ride but not a faster ride.The only time it might be better if the roads are really,really bad then a little lower might be better but if there were that bad,i would'nt run 700X23's.

There has been a running discussion in Uncle Al's Rant (http://www.roadbikerider.com/ua4.htm#Are%20Narrower,%20Harder%20Tires%20Better) on RoadBikeReview on the subject of tire pressure. He advocates lower pressure than is typical and says that many of the pros are running 100-105 lbs. Poke around the web site and you will find several references to this topic.

Shokhead, let me explain,

High tyre pressure = smaller contact patch right? Yes
Friction is INDEPENDANT (that means NOT AFFECTED) by surface area.

SO two tyres of the same weight & compound of rubber, one inflated to the minimum psi and the other to the maximum PSI will roll equally fast over a perfectly smooth surface area. Does this make sense. It's high school science!
(In reality the tyre with the lower pressure will roll faster because it will not be bouncing up and down over every bump)

Perfectly smooth roads do not exist. the closet you can get to it is a wooden tracked velodrome. So why is a high pressure tyre slower??

If you're cornering all your weight and the bikes weight acts downward through the contact patch. The bigger the teh contact patch is, the LOWER the shear stress in the rubber compound the more grip you have, the faster you can corner. But the centrifugal forces wanting to throw you off the bike have to be resisted so you want a rigid tyre to resist these forces.

The ideal tyre is one with a infinately supple or flexible contact patch with infinately stiff or rigid sidewalls.

This tyre does not exist. The closest you can get to it is to run your std tyre closer to minimum recommended pressure to have the flexible tread (low pressure) and still have a sufficiently stiff side wall (high pressure) to resist cornering forces.

Hence the reason low pressure = lower rolling resistance. The tyre is supple enough to absorb the irregularities in the road.

I really hope you can understand, it really is'nt a difficult concept to grasp.

Friction is INDEPENDANT (that means NOT AFFECTED) by surface area.

SO two tyres of the same weight & compound of rubber, one inflated to the minimum psi and the other to the maximum PSI will roll equally fast over a perfectly smooth surface area. Does this make sense. It's high school science!

It almost sounds like the drag racers are wasting there money on fancy super wide slicks and wheels.

Just to point out that Drag racers run very low PSI in there tires for traction and it doesn't seem to stop the top fuels hitting over 300 mph.

Yes but according to this they do not need to run such big tires at such low pressures.:confused:

At 300 if you watch,those tires grow to about twice the size.Well we all have a different say on how we ride and i will stick to max.

Wait a minute... I thought 'stones' were the official BF unit of weight. 54kg = 8.5 stone... that's better. ;)

Personally, I'd pump front & rear to 120psi. I've never noticed a minimum tire pressure (never looked!), but at your weight, you aren't at any risk running 100psi f/r, if you want to try the lower pressure.

The '25s are a good all-purpose road tire width. Your weight has no bearing on max pressure, but I think it is a factor in minimum pressure.

Quite a lot of the rolling resistance comes from casing flex which is reduced with high pressures test (http://www.terrymorse.com/bike/rrdiscuss.html) However I run lowish pressures otherwise my teeth rattle loose.

Well,somebody out there must be thinking these tests are wrong.A flat tire rolls faster then a pumped up tire.I didnt learn that in high school.LOL.Dont really matter,use the psi that suits you.

Originally posted by TimB
No they don't.....

Yes they do and I do not care what other people say, not only that they hold the road better as well.

let's take the extreme of a totally rigid tire. a totally rigid tire will most definitely have lower rolling resistance than an air-filled tire.... but what the rigid tire will do when hitting bumps in the road is bounce off of them and fly through the air for split seconds at a time, while the softer air-filled tire will better conform to the bumps in the road - acting as a "suspension" that keeps the tires glued to the road. as was stated recently in another thread, every little "jump" that a bike makes is vertical motion that subtracts from forward momentum of the bike. basically there is a point of diminishing returns where your tires start slipping as you go over rough spots in the road, or at worst you're actually catching air because the tires are so hard. Basically, a portion of the power you're applying at the crank is being wasted either spinning the rear tire in place or jumping the bike into the air.

if you've never experienced this, then you're probably heavy enough of a rider, or your air pressure is sufficiently low, that this isn't an issue. but you can be sure that pro racers would NOT be running 300psi in their tires, even if it was possible!

Shokhead, theres a vast difference between a flat tyre and one with RELATIVELY low pressure. If you choose to try to discredit my post with stupidity then thats your choice.

So opinion is divided. I leave it at that. Those rolling resistance tests were preformed on a steel drum i.e. a perfect running surface. Those tests have no bearing on reality and only serve to indicate which tyre has lower frictional resistance than another. also it's over 10yrs old.

I stick with my poin ot view and it seems to be supported by the knowledgable people in the industry.

Good day gents and ladies and enjoy your Max psi

Originally posted by deliriou5
let's take the extreme of a totally rigid tire. a totally rigid tire will most definitely have lower rolling resistance than an air-filled tire.... but what the rigid tire will do when hitting bumps in the road is bounce off of them and fly through the air for split seconds at a time, while the softer air-filled tire will better conform to the bumps in the road - acting as a "suspension" that keeps the tires glued to the road. as was stated recently in another thread, every little "jump" that a bike makes is vertical motion that subtracts from forward momentum of the bike. basically there is a point of diminishing returns where your tires start slipping as you go over rough spots in the road, or at worst you're actually catching air because the tires are so hard. Basically, a portion of the power you're applying at the crank is being wasted either spinning the rear tire in place or jumping the bike into the air.

if you've never experienced this, then you're probably heavy enough of a rider, or your air pressure is sufficiently low, that this isn't an issue. but you can be sure that pro racers would NOT be running 300psi in their tires, even if it was possible!

You guys are way over thinking this mater. First I will let my computer tell me I can always tell when my tires need filling cause it takes more work and there is a drop in my speed. And no since I don't know if there is any tire out there with a 300psi there won't be any racers out there with that amount of pressure as any tire person can tell you it is as dangerous to over fill as under fill (We are taking road here and not MTB since the original post mentioned 120psi I am presuming he is riding on a 700x25 or some such road tire) and I will bet that any rider that wants to get that extra speed is going to keep his tire up to the recommended pressure. Lower the air if you like it won't hurt you but you will slide more in the curves and since there is more tire in contact with the road there is more friction which means more resistance which means you all are going slower (Physics 101). All I can suggest for all of you ridding over those pebbles and going airborne STOP IT! you can go around them.

Originally posted by ngateguy
. Lower the air if you like it won't hurt you but you will slide more in the curves and since there is more tire in contact with the road there is more friction which means more resistance which means you all are going slower (Physics 101).

LOL PHysics 101! You obviously never attended lectures then

FRICTION IS INDEPENDANT OF SURFACE AREA
Hard tyres (in terms on inflation) will slide more than soft tyres because the area in shear is smaller than it is for softer tyres.
Obviously the tyre side walls need to be supported so the pressure can't be too low ( so no stupid comments about flat tyres are needed)

This is Uncle Al's take on the matter:



DEAR UNCLE AL: It's obvious that low rolling resistance is
a good thing. Yet trying to find solid information about the
rolling resistance of different tires is a problem. Do you
have a cluestick to point the way to some clarity on this
issue? -- Michael R.

UNCLE AL FIRES BACK: When I opened your e-mail,
I cringed at the thought of the negative reactions I might
get after answering your question.

I know some of you won't agree with me, and I pretty much
know your reasons. You don't need to send long e-mails to
dispute the way I look at this issue. Hey, it was Mike who
asked, not you!

Rolling resistance is one of the most misunderstood elements
of the bike. There will be those who insist that reducing it
is all about running super-high tire pressure. That is simply
not the case.

Rolling resistance is dictated by several factors, one
being tire construction. A top-quality clincher tire will
have a threads-per-inch count ranging from 125 tpi to an
astounding 440 tpi. Most companies use nylon for casing
threads, going by names such as Aramid or Polyamide.

There will be 3-5 overlapping casing layers to give the tire
strength, puncture resistance and good handling
characteristics. The suppleness of the casing material is
key to performance and reducing rolling resistance.

Another factor is the tread material. Most top-end clinchers
are now using some form of silica compound, which lowers
rolling resistance while increasing cornering traction.
Sounds like magic, doesn't it?

Understand that the designers and engineers behind quality
tires are some of the brightest lights involved with the
bike industry. We are spoiled riding tires that are as good
as the stuff used in auto and motorcycle racing, yet we can
buy them for just $50.

Remember this: The best tires cost what they do because of
the technology and materials that go into making them. We
are the lucky beneficiaries of this high science.

The last factor affecting rolling resistance, obviously, is
tire pressure. If a tire is flat, it has a whole bunch of
resistance and really doesn't handle very well.

A properly inflated road tire (700x23C) at 6bar/87 psi has
about the same rolling resistance as the same tire at
8bar/115 psi. But at the higher pressure, the rider gives up
some of the cornering traction and comfort so necessary to
cycling.

There's a perception that running tires at 115-140 psi
somehow makes a rider faster. What those maximum pressures
really do is accelerate wear, compromise handling and give an
extremely harsh and skittish ride.

Super-high pressure also compromises the integrity of your
wheels and, in the case of a high-speed blowout, can assure
you of a visit with the pavement. Heavy braking while
descending will heat the rims and increase pressures well
beyond the danger zone.

I've talked to pros about tire pressure. They universally
agree that inflation to 85-100 psi is plenty, depending on
road conditions. Pressures up to 110-115 psi are fine on a
smooth time trial course without many corners.

Most riders who run super-high pressure will remain
unconvinced about the benefits of lower pressure. I think
it's their placebo.

I'll take his wordover yours any day. It makes it easier when you actually run low pressure in all your wheels

Originally posted by TimB
LOL PHysics 101! You obviously never attended lectures then

FRICTION IS INDEPENDANT OF SURFACE AREA
Hard tyres (in terms on inflation) will slide more than soft tyres because the area in shear is smaller than it is for softer tyres.
Obviously the tyre side walls need to be supported so the pressure can't be too low ( so no stupid comments about flat tyres are needed)

This is Uncle Al's take on the matter:



I'll take his wordover yours any day. It makes it easier when you actually run low pressure in all your wheels

okay maybe I misssed that lecture all I know is what my speedometer and legs tell me I really don't know who Uncle Al is but I will take the word of Rivendell and all the other folks over the years who have told me about tire pressure again I really think ypou guys are over thinking this!

ngateguy: your point about avoiding pebbles is good for illustration.... why do you think track bikes in velodromes have their tires inflated to 200 psi? because they can TRUST that the surface of the track is perfectly smooth, and they don't need to worry about bouncing around from ruts and cracks and debris on the road. as road riders, we must accept those as a fact of life, and should lower our tire pressures accordingly.... ESPECIALLY for tight cornering maneuvers.

now let's think about a bouncy ball. inflate it to really high PSI, and bouncing the ball on the ground, it will fly really high in the air. take a little air out, and it doesn't bounce as high.

we notice this when we ride too.... going over a pebble at 150 psi will feel akin to going over a big rock at 80 psi. and the last thing you want to happen to you in the middle of a tight turn is losing traction. but running too high a pressure leaves you very vulnerable to this....

the shock absorption benefits of a less inflated tire can compensate for the slight increase in rolling friction afforded by running a lower pressure... think of it as a trade-off between two equally important aspects of confident bike handling feel, rather than as an all or none thing.

and i don't think we are overthinking this.... we are presenting very legitimate reasons why not to overinflate! In fact you said so yourself that it's not good to overinflate!

oh one final thing... actually you are much more prone to slide in the corners at too HIGH a pressure than at too low a pressure.... that's why uncle al qualifies his 110-115 psi reccomendation by saying its for a "smooth time trial course WIHTOUT MANY CORNERS". I have noticed this myself too, in my car.... when i cranked up the pressures in hopes of greater fuel efficiency, i found that my tires would start squealing much sooner than before in tight turns.

i found that my tires would start squealing much sooner than before in tight turns.

Is that because the contact patch (surface area?) is smaller so there is less friction to counter the cornering forces?

There are numerous reasons why delirious5 tyre's started squealing earlier at higher pressures.

1) smaller contact patch means theres less rubber area in shear so the tyre tread starts to break away from the casing. the cornering energy goes into breaking the tread compound. The friction is the same but the ability of the rubber to adhere to the surface is dimished because the stress in the rubber is too high. With a bigger contact patch, the stress in the rubber is less ( Force = Pressure x area or Shear stressx area or tesile stress x area etc) so the rubber does not break away from the tread. therefore the adhesion is maximised.

Over inflating your tyres does not reduce the friction, it jut makes the contact patch smaller, over stressing the rubber. The car manufacturer gives you a recommended tyre inflation pressure for a given tyre size so ensure that you have the correct size contact patch to maximise mechanical grip and have the best possible fuel efficiency on the road.

The same is true for bicycles and motorbikes. The friction is only due to the tread compound not the contact patch size. The contact patch size determines the ability of the tyre to distribute the cornerng and rolling stresses through the contact pacth without causing overstress and the subsequent slide.

Well TimB,are you calling some of us[like me] stupit because we dont agree with you?I think you are way to smart to handle us dumb guys.LOL.I'll stay stupit and keep my 23's at 120psi,thinks.I do need to add that a few friends i know,one races and the other works at bike shops and rides dirt and on the road,keep there tires at max so it seems to be divided.

I did'nt call anyone stupid. I refered to stupid comments like


.....A flat tire rolls faster then a pumped up tire.I didnt learn that in high school.LOL....

I will also turn your attention to offroad tyres. UST alows lower pressure so lower rolling resistance. See th Mavic website for references. If your'friends' are running max pressure on MTB tyres then I'd like to see them ride their bikes over varying terrain. Ill bet they walk more than ride. LOL!

Like Uncle Al says, keep your tyre at 120psi, thats your placibo. I've tried to open your mind to an different way of thinking but clearly you're not interested. Enjoy life at 120psi mate :-0

They ride road and yes,lower psi for dirt.

they should be lower for dirt, the volume is bigger.
What psi do they run on a 26x2.00 tyre?

Originally posted by deliriou5
ngateguy: your point about avoiding pebbles is good for illustration.... why do you think track bikes in velodromes have their tires inflated to 200 psi? because they can TRUST that the surface of the track is perfectly smooth, and they don't need to worry about bouncing around from ruts and cracks and debris on the road. as road riders, we must accept those as a fact of life, and should lower our tire pressures accordingly.... ESPECIALLY for tight cornering maneuvers.

now let's think about a bouncy ball. inflate it to really high PSI, and bouncing the ball on the ground, it will fly really high in the air. take a little air out, and it doesn't bounce as high.

we notice this when we ride too.... going over a pebble at 150 psi will feel akin to going over a big rock at 80 psi. and the last thing you want to happen to you in the middle of a tight turn is losing traction. but running too high a pressure leaves you very vulnerable to this....

the shock absorption benefits of a less inflated tire can compensate for the slight increase in rolling friction afforded by running a lower pressure... think of it as a trade-off between two equally important aspects of confident bike handling feel, rather than as an all or none thing.

and i don't think we are overthinking this.... we are presenting very legitimate reasons why not to overinflate! In fact you said so yourself that it's not good to overinflate!

Over-thinking the problem, As a seasoned bike commuter with 14 years under his belt I know just what and what I cannot expect from my bike. I am so in tuned with it that just a little variance in normal behavior is noticed. In the 14 years of riding I have become faster. So as for debris on the road I tend not to drive over it and for that nano of a second my bike may be airborne I will enjoy the sensation of flying :) I will also listen to my manufacture because varying the air pressure from the recommended amounts will shorten the life time of your tires. If I was lance and I had to worry about those nano seconds but I am not so I choose, as a road cyclists not to worry about saving time and work on enjoying mysel;f as I ride. If you are a MTB'er yes you want to deflate when you are off road. And you have been pressing the issue of under-inflating which is what I am pointing out as wrong by every thing I have read from legitimate sources.

Say TimB,what size tires do you use and what psi do you use?

ngateguy I am not advocating under-inflation. I am advocating 'low air pressure' i.e. running a tyre close to it's minimum RECOMMENDED pressure.
On the casing of all my tyres are 2 pressures. A minimum and a maximum. All I am saying is that if the tyre is designed to be used in a range of pressures then the best pressure for a tyre is the lowest that allows you to ride without pinch flating. I am not asking anyone to run their tyres without air nor am I asking anyone to run their tyres underflated. All I am postulating is that rolling resistance is not improved by running a tyre at it's ultimate maximum recommended pressure. The riders weight has to be taken into account. If you're heavy and for your weight the manufacturer recommends maximum pressure then that is probably what you should be using. However if you a re lightweight rider then running the tyre with lower pressure, but still within the recommended range has the benefit of lower rolling resistance for that rider.

Headshok the tyres are Michelin Wildgripper CompS UST 26 x 2.00. You guess the pressure

Call me crazy, but isn't rolling resistance, or friction, induced by the bearings in the hub? Unless, of course, you are trying to slide the tire across the road. . .

Myself, through experience, have found that inflating to 110 (under the 120 max of my tires) keeps me from getting pinch flats. And that's where I leave it so that my teeth don't get rattled any more than necessary.

Call me crazy, but isn't rolling resistance, or friction, induced by the bearings in the hub? Unless, of course, you are trying to slide the tire across the road. . .

Dear Crazy,

Yes indeed there is friction at the bearings of the hub but there is also friction between the tyre and the road. It is this friction that allows transfer of the power in your legs to the road via the drivetrain and tyre contact patch.
It is not even this friction that is the subject of debate.

Rolling friction is the cohesive forces between the molecules of the tyre tread and the road.

I can't believe the boys and girls are squabbling over this "tire pressure thing."

I already tried to answer it once but you ignored me so here I go again, listen carefully.

hys•ter•e•sis

Pronunciation: (his"tu-rE'sis), [key]
—n. Physics.
1. the lag in response exhibited by a body in reacting to changes in the forces, esp. magnetic forces, affecting it. Cf. magnetic hysteresis.
2. the phenomenon exhibited by a system, often a ferromagnetic or imperfectly elastic material, in which the reaction of the system to changes is dependent upon its past reactions

The guy who talked about "compliance" and tire rubber "hardness or "softness" was close to right. The part he left out was about the interference of the tube/rim matrix WITH the tire casing. Now the answer: once again is: PUMP THE TIRE UP TO A PRESSURE THAT LIMITS EXCESSIVE "FLEXING".

The surfaces to be ridden and the riders weight determine this adjustment.

Another real "Richard Cranium" posting.......

Whoa... I'm lost, Richard Cranium.

TimB, I can see your point, but I think that those are idealized tests. Sure, if your tire was perfectly compliant (0 energy to flex) then what you are saying would be right. however, I thought that rolling resistance came from the flexing of the tire as your weight was put on it. The more flex needed,(on the same tire) the more rolling resistance.

I think that this is why tubulars are so popular; you can run lower pressures because they are more compliant, and therefore require less flexing and energy loss.

On a different type of issue, do you think that you would experience "bob" on a lowered pressure tire?

OH SNAP! I JUST FIGURED OUT WHAT YOU WERE SAYING, RICHARD!

TimB,i'm not a good guesser so u tell me.You do know,right?

Oh no please i insist you first :-)

Originally posted by Richard Cranium
I can't believe the boys and girls are squabbling over this "tire pressure thing."

I already tried to answer it once but you ignored me so here I go again, listen carefully.

hys•ter•e•sis

The guy who talked about "compliance" and tire rubber "hardness or "softness" was close to right. The part he left out was about the interference of the tube/rim matrix WITH the tire casing. Now the answer: once again is: PUMP THE TIRE UP TO A PRESSURE THAT LIMITS EXCESSIVE "FLEXING".

The surfaces to be ridden and the riders weight determine this adjustment.

Another real "Richard Cranium" posting.......

Isn't there a tradeoff between fleing of the tire and flexing of the rim. When the tire flexes it tends to use significant energy, the hysteresis thing, slowing the rider. When the rim flexes excessively it tends to provide an opportunity for spokes to become loose. loose spokes can lead to an out of true wheel, leading to brake rubbing, which will slow a rider down more than hysteresis of the tire. There is very little hysteresis of the rim, spoke hub portion of the wheel, so bending them has little direct impact on the rider.

The real tradeoff is between bending the rim for short durations leading to spoke problems and bending the tire which causes loss of energy through hysteresis. as in all tradeoffs, there are values. You need to arrive at your values yourself, but I would suggest that for tires in the 700x23 range and riders in the 180 pound or less range, that 95 psi would be enough.

In my case I am heavier, by quite a bit, so I would probably get a slightly larger tire and keep the pressure. By the same token, if you know you are going to inflate to higher pressures for some reason, like competition, I would try to make sure that the spokes were well secured against loosening, perhaps using loctite or spoke prep.

Another issue which I don't think is properaly analyzed is the microskip problem where a tire is so inflated that it bounces over many imperfections rather than conforming to them. The friction on the tire during such skips approaches zero, but the forward force provided by the driving rear tire also approaches zero while the tire skips. Again if this were to become an issue the answer would be to find a way to reduce tire pressure.

Rich, fog, you guys are genuises...

Dont have or have ever rode a dirt bicycle so i dont have a clue about tire psi or shocks or anything.I do know when i rode dirtmotocycles i always ran pretty low in the rear if it was sandy or real soft dirt.I am going to try for a week a little lower psi and see what happens.

Ok I'll let youoff the hook. I run 28psi in the front tyre and 32-35psi in the rear.
It's an F-series Cannondale Hardtail. She roll fast and has loads of traction.
High tyres pressure off road just exaggerate the problem of the non ideal road even further. skippy front wheel means less control and more effort to keep you going forward..

Oh,well i dont care to much about dirt.My daughter's boyfriend is getting a giant,VT 1 i think it is,whatever that is.

it's the new full suspension bike from Giant that uses a stable platform valve equiped Shock to isolate rider nduced forces from the trail induceed forces. Nic bike.
Just reading my new copy of MBA and they have a test on it.
Really nice bike.
If you really want to p*ss him off ask him if he engine is still at the dealer or if he could'nt afford it. Full Suspension MTB riders hate that analogy.

This thread got me wondering, so I decided to see what Jobst Brandt had to say on the topic. Some interesting points.
http://yarchive.net/bike/tire_pressure.html

And now for today's trivia... his name is pronounced yobst, with a long o, as in "Check out my new wheels, yo!"

I read the Jobst article, and I have also read Uncle Al's rants. I think the issue of spokes loosening is not covered in the Jobst Article, yet remains a valid concern when determining tire pressure. Nontheless, what all this shows is there are lots of well-informed opinions, and they don't reach the same conclusions.

Re: Contact patch - my understanding is that different tire pressures will not change the size of the contact patch, but the shape.