If you inflated your tires to anywhere near the pressure marked on the sidewall as "maximum" or "recommended," there's a pretty good chance your tires are over-inflated. This has two primary results:
1) your ride is harsher -- tires are great shock absorbers
2) rolling resistance is increased -- increased shock leads to more energy going into the vertical bouncing of the bike rider system.
To get the correct pressure, figure out of the weight of yourself, your stuff, and your bike. On a normal bike geometry, the weight is distributed about 45% in the front, and 55% in the back. Having a rack/panniers/whatever on your bike will affect this greatly (obviously). You want to shoot for 15% vertical deflection in the loaded tire. This should provided sufficient protection from pinch flats, and optimize the contact patch (comfort, rolling resistance) of the tire at the same time. The pressure required for 15% deflection for a given width of tire at a given load is found IN THIS CONVENIENT CHART.
My wrist has been noticeably more comfortable since changing how I inflated my tires (and I overshoot the chart by ~5 to 7 psi). Enjoy.
I will gladly trade a little
I will gladly trade a little comfort to reduce the possiblity of a pinch flat.
my experience: lotta
my experience: lotta comfort, no pinch flats
yeah...
on the one hand, you got high pressures to counter the force of something poking in to your tire.
on the other hand, a lower pressure makes a more compliant tire that can deform around the object, rather than get punctured.
i don't know where to draw the line.
and as far as flats from smacking in to curbs - what do you expect? :)
ATL roads are bad enough. I
ATL roads are bad enough. I don't smack curbs but I do hit potholes, storm drains and train tracks- usually at night and usually slightly drunkened.
You're right about the curbs
You're right about the curbs probably asking for flats, but now that I'm on fixie with super skinny tires, those storm drains near Johnny's and the tracks on Dekalb near Krog scare the bejezus outta me every time.
I wouldn't worry about flats as much if you're running skinny tires with huge 100 to 140 psi, it's those people running at 45 to 60 that I see being more at risk to pinching and flats. Is this right?
it's those people running at
it's those people running at 45 to 60 that I see being more at risk to pinching and flats. Is this right?
depends on how much weight the tire has to support...hence the linear relationship shown in the chart.
yeah
those storm grates on the other side of sampson street (i can't remember what it's called at the moment) are terrible. i ate some pavement once getting my front wheel stuck in there.
i'd worry
less about a pinch than a rim dent. higher psi prevents against that, but i'm not sure at which point you begin to see diminishing returns.
I'll totally buy #1 but if I
I'll totally buy #1 but if I were you I wouldn't start argument #2 on here :)
My tires say 120 and I pump them to ~100 and don't pump them up as much as I should, so they're likely averaging out to about 86 which seems fine. hmm.
i believe i've made mention
i believe i've made mention of this before as a bullet in one of my longer posts on the board. it's also part of a devious trick that i use to sell nice bikes to people.
michelin has a graph on the back of their packaging that shows proper inflation of the tire contained within based on rider weight.
longer posts
this is why i never get to spend any quality time with you.
K, lemme make sure I get this right:
I weigh ~150 lbs
I'm guessing my bike weighs 20-25 lbs
And my bag prolly ranges 10-20 lbs, depending on how many bricks I'm carrying around.
So say all told, with generous rounding up, my wheels carry about 200 lbs. 45% of that would be 90 lbs. And 55% would be 110 lbs. So if I'm reading the chart correctly, I should be running roughly 95 psi in front and 117 psi in back for 23mm tires. Is that about right?
My tires are 700c x 23mm. They're rated for up to 145 psi and require a minimum pressure of 100 psi. Currently, based on trial and error, I run 120 in front and 130 in back. Although, I get lazy about inflating them, spoiled as I am by michelin airstop tubes. I'm tempted to just keep doing what I'm doing and allow them to deflate into the sweet spot over the course of a couple days. But perhaps I'll see what paying more exact attention to my tire pressure for a few days does for my own comfort.
Paul, how much were you overshooting your ideal pressure before you discovered this chart?
a lot
I used to pump my 32 mm tires up to 90 psi (back) and 85 psi in the front ("recommended" sidewall pressures). Now I'm somewhere around 40/50 for those tires.
I used to pump my 25 mm tires up to 110 and 105 psi...I should have used about 70 and 80 psi. I no longer ride those tires. Having bought a nice pair of Specialized 28 mm tires, I'll probably never ride anything smaller than that again. They're squishy and fast as hell.
are you going on tire
are you going on tire measurement or tire markings? most brands overmark their tires to make them appear lighter on paper. measure your tires and i doubt they will be 23mm across... unless they are Michelins (fatties).
i bought a set of Maxxis Maxxlite310s (26x1.95)... they measure 1.72 fully inflated. hand me 88% of a burger and charge me full price and see if i don't send it back... did i complain when my 60$ea tires were less then a paid for? not until now.
Hrm
Tire markings, I'm afraid. They're Vittoria Rubino Pros. I'll take a calliper to them at some point.
unless you (me) are
unless you (me) are bigger/heavier than the average cyclist, in which case the convenient chart recommends a psi greater than the max rating of the tires.
and if you're smaller...
I should be running 67 and 88 lbs.. I don't think so. Maybe I'm not understanding this. I don't get the rationale in general- riding with squishy tires doesn't feel good and I'm pretty sure it's not as fast (no matter what they say). I go with what feels right- about 100-110.
but
at 15% vertical deflection, the tires don't feel squishy. Instead they feel comfortable and smooth (YMMV).
I have to second Lisa. Plus
I have to second Lisa.
Plus "riding with squishy tires doesn't feel good", well not only that, but its a really good way to allow your tire to roll off during a fast turn, which causes road rash, so I'd prefer to project my sexy smooth cycling legs rather than my ...
...
once you go slack...
seriously though...try the chart out and see for yourself. also, can you roll a clincher? do you have tubbies? either way, 15% vertical deflection is no where near "squishy"
can you roll a clincher?
yep. been there, done that, dont wont to do it again.
I hear what you're saying Paul and I've opened the chart in another tab, but given the lack of precision in my pump, etc. I'm going to stick closer to at least 100lbs, which is what feels best to me.
if you ever use the term
if you ever use the term 'tubbies' again, i'm going to take you off my list of people i defend online.
:(
fair enough
What about tire life? I
What about tire life? I assume that all of this math is based on brand new tires. After a while you notice how much the actual actual rubber is compressing (not just the shape of the tire). You esp notice this if you burn your fixie tires down to near-bald and then swap to fresh tires.
at some point in life people
at some point in life people just prefer the lincoln town car style ride. not me
I don't buy this vertical
I don't buy this vertical deflection stuff. The only reasons you should be deflating is if your baby bum can't handle the bumps that life sends your way or if you're worried about grippage on turns.
Personally I don't even want to imagine my tire rolling off in a turn, and every time I've run softer tires I've gotten flats from hitting curbs. Slammed into many potholes and curbs with fully-inflated tires and I can't tell you the last time I had a flat.
If you've gotta deflate cause you're worried about a softer ride, you might as well get a cruiser bike with the spring-coiled seat and chopper handlebars.
Didn't Paul have surgery on
Didn't Paul have surgery on his wrist recently?
yeah, I did
and for god's sake people: the the main point of optimizing the contact patch of tire is to lower the ROLLING RESISTANCE i.e., to go faster.
as a result, the ride is more comfortable and still resistant to pinch flats.
ok dude
wow
and...
we're not talking about partial inflation...we're talking proper inflation.
the concept of a pressurized, deformable body supporting a load isn't a difficult one.
Some people can't comprehend
Some people can't comprehend or tolerate science... just let them be.
Ok, I'll chime in here with
Ok, I'll chime in here with some anecdotal evidence of my own... I used to run 1.4 width Ritchey Tom Slicks on my commuter... pumped them up to the max (like 90psi, this was the value on the sidewall). I pinch flatted ALL THE DAMN TIME. It got so annoying I pondered returning the tires, until I read about proper inflation. I lowered the amount of pressure in that set of tires to 75 per wheel... no more pinch flats, and a much more comfortable ride. My average speed (as indicated by my computer)... exactly the same.
Take that as you may... continue to ride like you're speed racer or whatever... but eventually if you last out (and cycling isn't just some three year fad for you) you'll start to care more about comfort (and maybe, how many days worth of groceries you can haul on your bike) than looking and feeling 'fast'.
(And for the record, I roll on 1.9 Michelins now... at even lower pressures, like around 55psi.)
Q for phil
so do you stick to the maximum pressure or go above?
Sheldon sez that most tire companies mark the sidewall with ~half of the actual blow off pressure, fyi
http://sheldonbrown.com/tires.html#pressure
The 15% vertical drop thing
The 15% vertical drop thing is simply one man's opinion about what makes his bike ride best for him. His name is Frank Berto and he made a graph, so a lot of people take his opinion as gospel. His opinion was written for a magazine, and we must bear in mind that magazines don't write articles reading "inflate your tires to what it says on the sidewall" because nobody would buy that.
Yeah, if you follow his chart, you will get 15% VD. Whether or not 15% VD is what you want is up to you. It's YOUR opinion. If you want a ride like Frank, decrease your tire pressure (or increase). Otherwise don't.
As it happens, I do run about 15% VD thru personal experience. It works for me.
Oh but anyway what's missing is the VD you get at other PSIs... If you are "overinflated" by 20 psi, do you go from 15% to 14.8% VD? How much of a difference does that make? I am guessing between very little and none. It's just a fluff story to sell magazines.
Actually, this isn't
Actually, this isn't opinion, it's science.
you can scientifically
you can scientifically determine the VD of a tire, but whether or not 15% is "ideal" is purely opinion
Jobst Brandt and many others
Jobst Brandt and many others have published rolling resistance data that show a minimum C_rr at a given pressure. Basic mechanics determines the resistance of a tire to pinch flats.
optimizing two curves ain't hard.
Nothing about the story is
Nothing about the story is incorrect. It's just that there's really nothing to the story.
Minor tire overinflation does not cause any statistically significant loss in performance (unless perhaps you are trying to win a TdF time trial)
What PSI does it take to get 14% VD and how much worse than "ideal" is 14% VD than 15% VD? DATA MISSING!!!!!!
That difference in performance between 14% VD and 15% VD, if significant, would be a story. Since that info is missing, it's pretty safe to assume they are just selling magazines with science laced scare tactics.
The article scientifically
The article scientifically lays out how 15% vd is the optimum measure. Too much VD and you get a larger contact patch (increasing rolling resistance)... too little and you start to bounce (decreasing efficency).
Like I said above, sometimes science is lost on people and they would much rather do what 'feels' right...
well i already said what
well i already said what feels right to me, and it lines up pretty close to what the "ideal" graph says.
my point here is that the decrease in VD caused by inflating beyond "ideal" has not been presented because it likely becomes insignificant very rapidly due to diminishing returns.
they lay out how 15% is optimum, but not how much worse 14% is or how much more PSI is required to get to 14%.
my "feeling" tells me it's an article about nothing.
then again, maybe the stuff that's missing was editorially deemed "too confusing"
but we weren't talking minor
but we weren't talking minor deviations.
we were talking 120 psi vs. 80 psi (33%).
that's the difference in
that's the difference in PSI, but what is the difference in VD? that critical piece info is "tardy"
.
I'll hook my LVDT up to my handleabrs next week.
here's the plan
-Take commuter bike and nice bike pump (Serfas FP200) to lab
-Pump tires to sidewall pressure and measure axle heights with LVDT, measure tire width with vernier calipers
-Deflate tires "completely" and measure axle heights with LVDT
-Difference between the two reading is 100% deflection
-Weigh myself holding bike on fancy lab scale
-Determine 'ideal' pressures for my weight and tires (based on actual tire width)
-Starting at about 15 psi on the front tire (and whatever in the back), measure axle heights (with me on the bike) in 5 psi increments
-Determine relative deflection for each trial
sound good?
oh paul..
you and your scientific approaches to things...
no, start at the "ideal" and
no, start at the "ideal" and then do increasing increments of PSI until your tires pop
(because obviously
(because obviously underinflation is very bad, but overinflation liekly has marginal if any effect)
.
I'll work my way up to about 20 psi over the rated pressure. I like these tires -- ain't gonna pop 'em
nice plan, but you're
nice plan, but you're missing a clearly stated hypothesis!
mine hypothesis would be that diminishing returns of PSI into a tire result in a decrease in VD approaching zero as PSI increases beyond the "max" sidewall rating tire pressure
and why? because the mfgr had to determine max sidewall pressure somehow, so why not put it where the return on PSI dips towards the toilet?
.
b/c typically max sidewall pressure = 0.5 * blowoff pressure.
no one likes lawsuits. just ask the quick release camp.
and
I don't understand how going from recommended pressure to above that is better than starting well below recommended pressure and going above RP.
seems like you want me to only collect 1/3 of the data.
you're testing overinflation
you're testing overinflation based on 15% VD being ideal, right? testing underinflation won't help with that.
however, i did say to start at the "ideal," not the recommended pressure (assuming you mean recommended on teh sidewall)
i see the confusion
I was going to test your diminishing returns hypothesis.
Pi = ideal pressure
Pr = sidewall pressure
VD_P = vert. deflection at a some pressure, P
Somewhere up thread there was an assertion that VD_Pi = 15%
That point was countered with VD_Pr should be only marginally lower than VD_Pi. Thus Pr would be the better inflation pressure b/c of reduced pinch flats (ignoring the increase in road shock and rolling resistance).
So I thought it was my task to get a curve of front and rear tire deflections at a good range of pressures so that we could see where the change in deflection really starts to approach zero, i.e., where dVD/dP ~= 0
I just got a flat.
I just got a flat. When you're fixing a tire with a hand pump and a patch kit, none of this stuff matters. Anything is better than 0 psi. Unless you're on a track or a Very Long Ride, I don't see this making much difference. Sorry to kill the techno-speak, that's just how I felt reading this thread. However I do like the idea of actual experiments to test these ideas. Yay science!
If you had proper
If you had proper inflation/good tires/tubes then you would have never flatted.
I have excellent tires/tubs tyvm:
I have excellent tires/tubs tyvm: CST CZAR 700x23c tires w/ MAXIS Presta tubes @ 115psi. I don't see how any kind of inflation is going to stop a shard of glass.
however...
a patch kit, a pump, and some decent tire levers (or a spoon) will get you home, no?
and the most awesomest bike
and the most awesomest bike mechanic ever doesn't help either. You can't fit in my bag though.
or maybe you can...
this is per my point earlier
there is a balance to be reached with tire inflation such that there's enough pressure in the tire to counter the force of the glass pushing in, but not too much pressure that the tire can't also conform around the glass shard rather than get punctured.
hmm... if you covered your
hmm... if you covered your tires with scotch tape, then if you DID get a puncture at least the air would leak out slowly instead of the tire popping and causing loss of control / rolloever
maybe?
it seems like whatever punctured your tire would puncture the tape as well.
something like stan's notubes pulls this off a little better, by having an air-curing latex sealant that oozes out and solidifies when you get a puncture.
i dunno. works for balloons.
i dunno. works for balloons. maybe packaging tape would work for tires.
0.5 * blowoff pressure is
0.5 * blowoff pressure is far too simplistic. that could never work in business. not for long, anyway.
if tire seller A and seller B put their own logos on tires from the same manufacturer (as so many do), how does one seller beat the other?
seller A sells a better product even though they are both selling exactly the same thing!!! how to do that? marketing. "our tires can run 150 psi while seller B only goes to 100psi. we will win you races!"
right or wrong, people want higher pressure tires. the market will thus drive max psi claims up towards the real max.
Please make sure your lab
Please make sure your lab report is type-written in the proper format. (Actually, I'm really excited to see your results. Should be interesting. Yay science!)
are we talking about tour riders or?
i guess this is such a tumultuous issue since these figures are likely meant for a certain person. while running the perfect pressure may "ensure you against pinch flats" either i'm too fat, or riding at 130 and going up and down curbs is the only way for me.
.
the chart is meant for general cycling. brevets, races, grocery runs, whatever. what the chart is best for, however is yet to be determined.
man this is
number one reason i get frustrated with people on the internet, i just want to rip them the fuck apart and then i realize it doesn't matter. because that is what alleycats are for, and the next time i roll by you or someone like you who rides on fucking 15 percent what the fuck ever(and i do realize that due to the other amazing aspect of the internet, that you could be anyone and therefore be much faster than me)
who has another pinch flat then they realize that the efficiency has just gone out the window.
disclaimer
1.my name is skip and i am an asshole, however i find some people to be worthwhile even after hating them on this stupid box, and on occasion they are even friendly to me.
2. i get the point about it being most efficient, but upon my personal experience i find that atlantas streets, and the daily ride i encounter(and the odd alleycat) demand something greater in pressure, on a winding country road or a pace line somewhere far far away i would agree. But I feel aggressive city riding on 23c tires is beyond the scope of what they set out to prove here, i do not for a second believe that they included the effects me jumping up and down half foot curbs, nor the current state of affairs on peachtree in the scope of their study. but shit what the fuck do i care, listen to the people in the lab, and i'll just keep selling tubes and replacing rims.
?
what leads you to believe that a tire inflate to a pressure that yields 15% vertical deflection when loaded will be prone to pinch flats?
here is a plan
we'll start a thread where i do it my way and you do it yours, and we'll see over the course of lets say six months who gets more pinch flats per mile, because like i said the science behind this doesn't seem to apply to the "Xtreme" conditions and style of riding in the subculture of cycling a few of us on this board are involved in.
?
what leads you to think i've *ever* had a pinch flat?
i'd ask you why you think 700C x 23 mm tires are a good choice for what you brand as Xtreme riding, but i doubt you'd respond that question as well.
observation
has anyone taken into consideration paul's weight, paul's one of the lightest guys around, he puts sand in his pockets so a bird doesn't fly off with him, of course he can ride low and be fine....different strokes for different folks
(/edit: oops, the chart does take into account weight)
.
i still laughed audibly
.
But the observation you could make about me is this:
I'm currently working towards my second engineering degree. what have i been taught this past 7 or years? How to optimize stuff.
There's an old saying: anyone can make a building stand up. It takes an engineer to make a building barely stand up.
Who here rides a 50 kg steel bike? OMG WHY NOT?!? Don't you realize how much stronger a 50 kg bike is than a 10 kg bike? Well, be careful around those bumps, your top tube might bend or buckle.
No...no. We all ride bikes in the 8 - 15 kg range (roughly) b/c that's the optimum weight/strength of our frames. It provides a reliable, stiff, and responsive frame that shouldn't fatigue any time soon.
So why are we all riding around on rock hard wheels? If you want to get rid of all of the wonderful properties of pneumatic tires and not get any flats at all, why not just get airless tires?
Oh yeah, those aren't "cool." Wouldn't want anything not cool (or color uncoordinated) on our bikes.
to be honest
even as an engineer of sorts... this whole thread bores me. I am too lazy about my tires so they range all the way from 120 to 60 depending on how long it's been since I put air in them. but, as someone with a hideously color coordinated bike, let me just say for the record that skip doesn't give a fuck about his bike being flashy or color coordinated. he just cares about being faster than (the collective) you. and almost all of the time he his, and that's pretty *ehem* COOL. if he builds the wheel, he can ride it however he wants. the end.
.
you (as usual) bring up excellent points.
-do what you want
-riding at pressures well below the sidewall pressure won't be a detriment to your riding.
kind of lost sight of all that.
fuck you paul. my bike is
fuck you paul. my bike is sub 6.75kg. FEAfoR3aLz
wimps
19.5 kg. get a life
sorry
i was a bit busy fucking your mom(and i don't care if she's dead or not, it won't make me feel bad about saying it), so we didn't have time to check up on here. I used to get pinch flats like i said, now i don't. So obviously we both BELIEVE(and i know i know, you happen to "KNOW" that you are right) we are perfect and we should just resort to childish insults as the one above. in other news, i haven't had pinch flats since i began "overinflating" my tires, so that really isn't going to resolve our problem either. and if i had just gotten on top of branding the term "xtreme" when i was about five years old i would have to shoot myself. but hey i'm just another hipster rolling around on an inefficient machine. one day i'll grow up and put 28cs on my touring frame with full racks and i'll finally get the validation i need from the enginners in their ivory tower.
i'll finally get the validation i need from the enginners
This post made LOL 4 realz.
I was gonna put a long post
I was gonna put a long post supporting everything you say, but then it read like I was just as much of an asshole with much cursing, rants, and ramblings. So I'll censor myself for once and simply say I agree with your point of view. Entirely.
Nerd box.
Nerd box.
Tidbit
Just found out that Tom Boonen, at 82 kg (180 lbs) won this year's Paris-Roubaix with his tires at 6 bar (87 psi) in the rear and between 5 and 5.5 bar (72.5 - 80 psi) on the front.
In the same pre-race interview, he also advised everyday riders to not be afraid to let some air out of their tires, since you'll get less flats, be more comfortable.
Note that in that same interview he said "6 kilos" when referring to his tire pressure. 6 kg/m^2 is less than 0.01 psi and 6 kPa is < 1 psi, so I assume he meant 6 bar. Also note that he was riding tubulars, probably 23 - 26 mm wide, so they can handle lower pressures than clinchers. But then also remember that he's 180 lbs, and rides way faster over cobble stones than lots of folks would dream of riding on the flats (1600 W in a full sprint).
Or even 28 mm.
Or even 28 mm. http://www.cyclingnews.com/road/2008/apr08/roubaix08/tech.php?id=/photos...
yeah
I was reading a pre-race interview with George Hincapie's mechanics. When the dude said that they'd be running wider tires, the interviewer asked, "Like a 23 mm, or what?"
"Uhhh no, about 28 mm, actually."
28 mm = 95 psi
Sidewall pressure on my 28 mm Conti 4 Seasons is 95 psi, so Boonen's pressures would be in line with your formula.