General Discussion Triathlon Talk » tire pressure...i'm kind of a moron Rss Feed  
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2014-04-21 9:52 PM

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Subject: tire pressure...i'm kind of a moron
I've always hand pumped my tires to a feel, never measured. Well...I got a pump this weekend, pumped to what the sidewall says and just rode. It was jerkier, but wow. What the heck have I been missing? Faster, hills were easier, all in all kind of ridiculous of me. You ever do something in this sport that is quite a "duh"?


2014-04-22 3:25 AM
in reply to: gatorcali21

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Subject: RE: tire pressure...i'm kind of a moron
By the same token most people overinflate their tires at first.

2014-04-22 7:05 AM
in reply to: gatorcali21

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Subject: RE: tire pressure...i'm kind of a moron

I like the graph at the below link a little better. It shows a the difference between front and rear tire inflation.

If you pumped to the max pressure on the tire, you are most likely pumped up too much now.

http://www.bikequarterly.com/images/BQTireDrop.pdf
2014-04-22 7:08 AM
in reply to: gatorcali21

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Subject: RE: tire pressure...i'm kind of a moron

"It was jerkier"

That's due to forcing the stiffer/harder tire to move more vertically which actually results in slower overall riding (depending on the road surfaces of course).  The max tire pressures are ideal for pancake flat smooth such as velodromes (indoor track racing).  I'm 180lbs and ride ~95-100 psi and my max tire pressures (GP4000S) are 120psi.  If I know I'll be riding some old rough roads in the country I'll go down to 90 psi.

Your tire is the only "active" suspension part your bike has.  The more compliant the tire can be the better it can roll over small road imperfections so less is energy you put into forward motion is lost to vertical motion.  The ideal combination for any tire that uses an inner tube is to use a latex tube.  This helps lower rolling resistance even more for any tire.  The ideal clincher tire/tube combo often mentioned here on BT for racing is Continental GP4000S and latex tubes.  Heck I get lazy and leave them in after races.

2014-04-22 12:43 PM
in reply to: mike761

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by mike761I like the graph at the below link a little better. It shows a the difference between front and rear tire inflation.If you pumped to the max pressure on the tire, you are most likely pumped up too much now. http://www.bikequarterly.com/images/BQTireDrop.pdf

FWIWthat generally applies to road bikes. For TT bikes front and rear should be the same (as long as you are spending the majority of time in aero), at least according to zipp: http://www.zipp.com/accessories/detail.php?ID=51

2014-04-22 1:06 PM
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Subject: RE: tire pressure...i'm kind of a moron

Keep in mind that those tire pressure charts are likely in reference to optimal pressure when each tire is paired to a standard 19 mm rim.  With more and more rims going wider, pressure drops even more due to the increased air volume that can be held within the rim bead.

I'm running 25mm wide HED Belgium + rims for training.  I pair them up with 23mm Gatorskins and run about 80 psi if roads are dry and smooth.  Mid 70's if they are rough/wet.  I'm 158 pounds.  

I've experimented running them as low as 60 psi and they were fine...but due to the shape of the 23 mm tire being slightly flattened with the wider rim, I was scared of hitting a big pot hole and damaging the rim.  If I went with 25 or 28 mm tires, I'd probably run around 60 psi since there would be more vertical bulge with the tire.  Not as aero, but certainly more comfortable.



Edited by Jason N 2014-04-22 1:11 PM


2014-04-22 1:17 PM
in reply to: TankBoy

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by TankBoy

Originally posted by mike761I like the graph at the below link a little better. It shows a the difference between front and rear tire inflation.If you pumped to the max pressure on the tire, you are most likely pumped up too much now. http://www.bikequarterly.com/images/BQTireDrop.pdf

FWIWthat generally applies to road bikes. For TT bikes front and rear should be the same (as long as you are spending the majority of time in aero), at least according to zipp: http://www.zipp.com/accessories/detail.php?ID=51



The article had tire pressure based on tire width and load on the tire. It also tells you how to determine the load on each tire as every person and bike setup therefore weight distribution will be different. I have not measured a bunch of people on a tri bike so I can't tell you if most people have 50/50 distribution or not. The zipp article give tire pressures based only on a 21mm wide tire, which most people do not ride.

Short of individual testing nothing is perfect. I do ride lower pressure in my front tire 90 psi vs 100 psi in the rear, I never pinch flat and it rides well.
2014-04-22 1:28 PM
in reply to: mike761

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Subject: RE: tire pressure...i'm kind of a moron
I found when I increased my tired pressure I got less flats. I am heavier and I was just doing 100psi and I got flats all the time, I went to 120psi on the rear and 110psi on the front and I get 1/4 the flats I used too.
2014-04-22 2:53 PM
in reply to: mike761

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by mike761


I like the graph at the below link a little better. It shows a the difference between front and rear tire inflation.

If you pumped to the max pressure on the tire, you are most likely pumped up too much now.

http://www.bikequarterly.com/images/BQTireDrop.pdf


I'm 250. I pump them up to 120 and call it good no matter what chart is used
2014-04-22 5:29 PM
in reply to: DanielG

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Subject: RE: tire pressure...i'm kind of a moron

Originally posted by DanielG By the same token most people overinflate their tires at first.

 

Kind of a newbie question here, but what would be the disadvantage of pumping up the tire to the max pressure if a lighter weight?  I mean, I understand why you wouldn't want to not have it underpumped if a heavier person, but what's the big deal if they're pumped to max psi and you're light?

2014-04-22 5:53 PM
in reply to: C_Hassard

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Subject: RE: tire pressure...i'm kind of a moron

When you roll over small imperfections in the road (rocks, cracks, pebbles, sticks, etc), a properly inflated tire will be able to absorb those imperfections better while maintaining forward momentum.  A tire that is pumped up too high will bounce causing some of your forward energy to be pushed upward.  Thus it slows you down...not to mention makes the ride extremely harsh.  This could also impact your ability to corner well...but that's not usually a big deal in triathlons.



2014-04-23 7:26 AM
in reply to: C_Hassard

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Subject: RE: tire pressure...i'm kind of a moron

Originally posted by C_Hassard

Originally posted by DanielG By the same token most people overinflate their tires at first.

 Kind of a newbie question here, but what would be the disadvantage of pumping up the tire to the max pressure if a lighter weight?  I mean, I understand why you wouldn't want to not have it underpumped if a heavier person, but what's the big deal if they're pumped to max psi and you're light?

Take a look at this n=1 test performed on the road by a very analytical person.  It showed (blue diamonds) that increasing PSI to a point resulted in a very measurable shift, for the worst, in Crr (rolling resistance).  Now does this always happen with all tires, I don't know, but am not willing to find out as ride quality/comfort suffers to a point I don't want to use max high PSI on the road.

2014-04-23 8:27 PM
in reply to: Donto

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Subject: RE: tire pressure...i'm kind of a moron
Whoah!!! I was advised by my shop to run on 110psi front and rear, 700x23, for both my 48kg wife and myself 68kg!!!
2014-04-23 8:59 PM
in reply to: DanielG

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by DanielG

Originally posted by mike761


I like the graph at the below link a little better. It shows a the difference between front and rear tire inflation.

If you pumped to the max pressure on the tire, you are most likely pumped up too much now.

http://www.bikequarterly.com/images/BQTireDrop.pdf


I'm 250. I pump them up to 120 and call it good no matter what chart is used


cosign
2014-04-24 8:36 AM
in reply to: knuta99

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by knuta99

Whoah!!! I was advised by my shop to run on 110psi front and rear, 700x23, for both my 48kg wife and myself 68kg!!!


Lots of people including shop employees will tell you to pump up your tires, much higher then you need to. I'm about 77KG and ride at 85psi front and 100psi rear, no pinch flats and its a much smoother ride than in the past when I pumped up to 110psi front and rear.
2014-04-24 8:39 AM
in reply to: gatorcali21


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Subject: RE: tire pressure...i'm kind of a moron
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2014-04-24 8:45 AM
in reply to: chatterswalliam

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Subject: RE: tire pressure...i'm kind of a moron
are you selling air from india specifically for 120 psi?
2014-04-24 8:49 AM
in reply to: ajusf16

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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by ajusf16

are you selling air from india specifically for 120 psi?


Nigerian air scam!
2014-04-24 9:38 AM
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Subject: RE: tire pressure...i'm kind of a moron
Originally posted by DanielG
I'm 250. I pump them up to 120 and call it good no matter what chart is used


LOL...I'm 195 & looked at these charts & immediately felt like a big fat fatty.

Edited by Meathead 2014-04-24 9:39 AM
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