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tire air/pressure

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tire air/pressure

Old 08-24-09, 05:03 PM
  #26  
j3ffr3y
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Originally Posted by Syscrush View Post
Is that equation correct? It will indicate a lower pressure for a narrower tire, and vice-versa.
whoops. Just did it for different weights, forgot to use different widths. Thanks for the catch
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Old 08-24-09, 05:25 PM
  #27  
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Code:
/* -*- Mode: C; indent-tabs-mode: t; c-basic-offset: 4; tab-width: 4 -*- */
/*
 * main.c
 * Copyright (C) Jeff Crowell 2009 <crowell@bu.edu>
 * 
 * main.c is free software: you can redistribute it and/or modify it
 * under the terms of the GNU General Public License as published by the
 * Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * main.c is distributed in the hope that it will be useful, but
 * WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License along
 * with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#include <stdio.h>
int main (void)
{
	int width, weight;
	
	printf ("what is your tire width (in mm) ?\n");
	// receives the tire width
	scanf ( "%i", &width);
	
	printf ("What is your weight (fully clothed, carrying all of your stuff you normally do while on your bike, including the weight of the bike)\n");

	//receives the rider's weight
	scanf ( "%i", &weight );
	
	float area, pressure;
	area=width/25.4;
	pressure=weight/area*.63;
	printf ("you should be running about %f psi", pressure);
	return 0;
}
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Old 08-24-09, 08:33 PM
  #28  
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Originally Posted by syscrush
If you're going on the assumption that your rear wheel is loaded more heavily (say a 55/45 split), then you'd want something like 120 psi for the front (101 lbs load) and 148 psi (123 lbs load) for the rear. Which would suggest that you may want to step up from the 23's to 25's so you could run 110 front / 136 rear
I never mentioned 23mm tires in this thread.

I ride with 25mm tires because I weigh too much for most 23mm tires.

I stand by my original statements on this subject, and I note that Sheldon Brown supports my posts.
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Old 08-24-09, 08:59 PM
  #29  
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Originally Posted by Ken Cox View Post
I never mentioned 23mm tires in this thread.

I ride with 25mm tires because I weigh too much for most 23mm tires.

I stand by my original statements on this subject, and I note that Sheldon Brown supports my posts.
I apologize for misreading and thinking you were on 23's, but the fact remains that the SB page does indicate a higher pressure than what you're running, even for 25's (which I did include in the #'s I extrapolated from SB's page on tires & pressure).

Anyhow, if you're happy with how your bike rides and you're not getting pinch flats, then enjoy it.
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Old 08-24-09, 09:00 PM
  #30  
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Originally Posted by j3ffr3y View Post
whoops. Just did it for different weights, forgot to use different widths. Thanks for the catch
Your original one did use different widths, it just had the width # in the numerator, instead of the denominator where it belongs.
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Old 08-25-09, 07:46 AM
  #31  
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Originally Posted by Syscrush
...the fact remains that the SB page does indicate a higher pressure than what you're running, even for 25's...
I suspect Syscrush has not actually read what Sheldon has to say on the subject.

And, no, the SB page does not "indicate" a higher pressure.

From SB's site:

"Pressure Recommendations

Most tires have a "maximum" pressure, or a recommended pressure range marked on the side of the tire. These pressure ratings are established by the tire manufacturers after consultation with the legal and marketing departments.

The legal department wants the number kept conservatively low, in case the tire gets mounted on a defective or otherwise loose fitting rim. They commonly shoot for half of the real blow-off pressure.

The marketing department wants the number high, because many tire purchasers make the (unreliable) assumption that the higher the pressure rating, the better the quality of the tire.

Newbies often take these arbitrary ratings as if they had some scientific basis. While you'll rarely get in trouble with this approach, you will usually not be getting the best possible performance with this rote approach.

Savvy cyclists experiment with different pressures, and often even vary the pressure for different surface conditions.

Optimal pressure for any given tire will depend on the load it is being asked to support. Thus, a heavier rider needs a higher pressure than a lighter rider, for identical tires.

Since most bicycles have substantially more weight on the rear wheel than on the front, the rear tire should almost always be inflated to a higher pressure than the front, typically by about 10%.

Rough surfaces generally call for a reduction in pressure to improve ride comfort and traction, but there is a risk of pinch flats if you go too far.

Rider skill also enters into this: more experienced cyclists learn to "get light" for a fraction of a second while going over rough patches; newbies tend to sit harder on the saddle, increasing the risk of pinch flats.

The table below is based on my experience and a certain amount of guesswork, and should only be used as a very rough guide to a starting point. Interpolate/extrapolate for your own weight/tire sizes.

Tire widths are in millimeters, pressure recommendations in pounds per square inch. (Divide by 15 if your gauge reads in bars/atmospheres.)
"

https://sheldonbrown.com/tires.html
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Old 08-25-09, 08:18 AM
  #32  
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Wow, just you people make a simple task as hard as possible.

Just inflate to max recommended PSI using a floor pump with a gauge. They cost a bit more, but they are well worth it as you will never pump them up enough without a gauge. I am always surprised how the gauge tells me MORE! MORE!!!

here is Sheldon's chart. I weigh 70kg, which means 35kg per tire, so I should put 100psi in. (oh shnizzle, i alreadly do that!)

Wheel load ____50mm ____ 37mm _____32mm ____28mm ____ 25mm ___23mm____ 20mm
100 lbs/50 kg __45________ 60 ________75 ______100 _______110 _____120 ______130
70 lbs/35 kg ___35 ________50 ________65 ______80 ________90 ______100 ______110

Last edited by the_don; 08-25-09 at 08:27 AM.
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Old 08-25-09, 08:31 AM
  #33  
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Read the side of your tires. Inflate accordingly. Re-check frequently.
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Old 08-25-09, 11:06 PM
  #34  
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anybody recommend a good pump with gauge for a reasonable price? i've went the cheap route twice now and regret it.
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Old 08-26-09, 11:30 AM
  #35  
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I picked up a Silca Pista at Bikeisland, so far so good, gauge works. Says it will hit 240 psi in the specs.

https://bikeisland.com/cgi-bin/BKTK_S...ils&ProdID=640
https://www.silcapompe.it/pista_en.htm
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Old 08-26-09, 01:14 PM
  #36  
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get the joe blow sport. the base joe blow sucks.
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Old 08-26-09, 01:25 PM
  #37  
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I check mine once per week or so, and I ride maybe 4 times per week or so. I run mine about 28-30 psi or so. Max is 33-34 or so psi, but there is this section of broken up pavement where I have to go to get on the trail, so I run a little low because of the bumps there.

Read the rec pressure on the side wall, and then experiment in that ballpark for what works for your ride. I find I vary the pressure right down to what road I'll be on regularly.
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Old 08-26-09, 02:04 PM
  #38  
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I use this pump from MEC - never seems to fail, and the gauge works out nicely.

I tend to feel when the tyres get low, so I go by that - depends how heavy I've hit weird things (sometimes I notice hard pothole bangs squeeze air out) or how far/often I've ridden - I ride to work everyday but only usually need a refill every fortnight or so.

I inflate to around 100-110psi, I've had some weird blowouts with Conti Ultrasports (max. rec. is 100psi I think) so I try to err on the side of caution with that tyre, my front is apparently able to deal with 120 (Vredestein Fortezza SE) but 100/110 seems to work well for me(tm).

It's not rocket science, and you ought to feel when the bike is sagging as low pressure can give you speed/effort differences if they're getting fairly low or feeling more bumps than it should.
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Old 08-26-09, 10:45 PM
  #39  
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Originally Posted by Fugazi Dave View Post
140 here. Definitely a plus.
Yes, I try to look at the bright side of being smaller in stature also!
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Old 08-27-09, 01:22 AM
  #40  
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don't worry about it and if you get a flat throw your bike out.
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Old 08-27-09, 03:27 AM
  #41  
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that weight/psi table is only applicable up to a certain weight. if you're more than ~175lbs the best you can do is inflate to max rated psi as stated by your tire manufacturer.

as for checking air pressure, i hate to do it so frequently, but it beats getting a pinch flat in BFE or the hoodz during a night ride. usually i pump up once a week but if there's an interesting group ride or whatever i'll top off just for that. kinda sucks because i rotate between 3 bikes (roadie, fixie, foldie) and i don't put enough saddle time across all 3 so i'm doing pressure checks/top offs more frequently.
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Old 08-27-09, 05:48 PM
  #42  
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thanks for the input, guys. i went with the serfas fp200 for $40 at the lbs. no problem getting the pressure high and the shop covers it with a lifetime warranty!
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Old 08-27-09, 05:52 PM
  #43  
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Originally Posted by the_don View Post
Just inflate to max recommended PSI...
no.

Originally Posted by Paratrooper View Post
Read the side of your tires. Inflate accordingly.
no.

Originally Posted by tFUnK View Post
if you're more than ~175lbs the best you can do is inflate to max rated psi as stated by your tire manufacturer.
no.

there is considerable evidence that lower pressures provide better wear, comfort, and grip. i am 175-180 and routinely run 100/110 f/r in 23c open corsas, 90/100 in 25c. the sidewall says 115-145. yes, i am under the minimum. i have never pinch flatted.

i used to be a pump-it-to-the-max kinda guy until somebody clued me in. the improvements are astonishing, and no, i am not any slower. as i understand it, the difference is essentially that the contact patch can give a bit to conform to the pavement rather than skitter over it.

i suggest you try it for a bit. what harm could it do? if you don't appreciate the difference, you can always jack 'em up again...
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Old 08-27-09, 06:06 PM
  #44  
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Originally Posted by dookie View Post
no..
Wrong.

100% of everyone will fare just fine by following the manufcturer's recommendations.

I did not offer incorrect advice.
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Old 08-27-09, 07:29 PM
  #45  
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Originally Posted by Paratrooper View Post
I did not offer incorrect advice.
i suppose not. only incomplete.

when it says only 'max pressure 130psi', what is 'accordingly'?

Last edited by dookie; 08-27-09 at 07:32 PM.
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Old 08-27-09, 07:40 PM
  #46  
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It isn't rocket surgery to read and follow directions. If you can't figure out what to do after you read the side of the tire then I cannot help you further. I'm not here to play the nit picking game and I don't appreciate being accused of telling an untruth to someone when I have not. If you want to offer advice on your supposed "evidence" based practices then offer it. Don't act like a child by telling everyone else they're wrong and only your input is sagacious.
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Old 08-27-09, 08:26 PM
  #47  
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i don't recall asking for your help and i absolutely did not claim my opinion as the sole truth. my point is only that the sidewall markings are hardly gospel and fail to take a lot of things into consideration. will they 'work' for everyone? sure. so does the abacus, but it's not optimal.

the research i mention is out there (if i had a link handy, i'd provide it) and convinced me to experiment and develop an opinion. my opinion is merely anecdotal evidence.

so what pressure do you ride? tire size? weight? road conditions? and most importantly, what does your sidewall say?

Last edited by dookie; 08-27-09 at 08:40 PM.
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Old 08-27-09, 08:32 PM
  #48  
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Tire Width=20: Pressure(psi) = (0.33 * Rider Weight in lbs) + 63.33
Tire Width=23: Pressure(psi) = (0.33 * Rider Weight in lbs) + 53.33
Tire Width=25: Pressure(psi) = (0.33 * Rider Weight in lbs) + 43.33
Tire Width=28: Pressure(psi) = (0.33 * Rider Weight in lbs) + 33.33
Tire Width=32: Pressure(psi) = (0.17 * Rider Weight in lbs) + 41.67
Tire Width=37: Pressure(psi) = (0.17 * Rider Weight in lbs) + 26.67


Example: You are 150lbs running 28's

Pressure (psi) = (0.33*150) +33.33 = 82.83psi (rear)
Front Pressure = .9*Rear Pressure = .9*82.83psi = 74.55psi front
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Old 08-27-09, 08:49 PM
  #49  
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Originally Posted by dookie View Post
i don't recall asking for your help...
But you sure didn't hesitate to knock everyone else while promoting your own opinion, didn't you?

Grow up.
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Old 08-27-09, 08:56 PM
  #50  
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Originally Posted by dookie View Post
my point is only that the sidewall markings are hardly gospel...
m'kay?
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