psi sensitivity

Most likely in your head, but by definition the placebo effect is as good the real-deal. a 5lb psi difference IS measurable in AeroLab, however, and well-worth figuring out what works best for you with your particular weight and setup. That said though I would be very surprised to find that any of my various floor pump gauges were accurate to withing 5lbs of each other....

That's not air coming out of your tube, that's air pressure being released from the hose of your floor pump.  If you ever tried using a mini hand pump (no hose), you'll notice that you don't get nearly as much of a sound of air pressure being released.

Where did you read this?  I'm interested in looking up the source.

Ideal tire pressure is going to vary based on the weight of the rider, the quality of the road surface, and the amount total air volume the tire and rim well can accommodate.  

The heavier the rider, the more tire pressure is going to be needed.  Someone who weighs 115 pounds is likely not going to enjoy the ride on 120 psi, especially when trying to corner as they don't have enough weight to properly displace the tire in such a way that it was designed to roll.

The poorer the road surface, the less pressure you're also going to want.  It's a balance between not pinch flatting, but I suspect the source you read suggesting there is no difference between 90-120 psi was making that comment assuming very nice roads.  Because that's generally true for your average sized rider (140-170 pounds).  If the road is so smooth that your tire does not need to absorb imperfections in the road, then having more or less pressure won't be a penalty.

The new wider rims and wider tires, can allow you to get a  pretty firm tire on lower psi.  Think of it like your car tires...do you air them up to 100 psi?  Despite carrying a couple thousand pounds of weight, they are still pretty firm even at say 30 psi right?  Same goes with mountain bike tires and such.  They hold more air volume, so higher psi isn't required.

But back to the original question...can *I* tell the difference.  Yes, I can.  Maybe not 5 psi, but certainly 10.  If I run my 23 mm GP4000s tires right at 90 psi, I'm usually a pretty happy camper on the roads I ride on.  If I pump them up to 100 psi...I can tell the bike tends to hop a bit more over rough surfaces.  80 psi and I'm getting a little close to pinch flat territory, but sometimes I'll risk it if it's wet and I know I'll be doing some cornering.  I've tried running at 1105-110 psi and it was the absolute worst experience ever on my typical roads.  When riding on silky smooth roads like the Queen K in Kona...perfectly fine though.

Yeah, my guess is that test was done on some type of stationary roller that is perfectly smooth.  So it's not surprising that 120 psi tested the fastest as far as rolling resistance given those assumed circumstances.  Unfortunately, we don't ride or race on rollers outdoors in the real world, so the optimal tire pressure for speed is going to vary based on the factors I listed earlier.  Also note that they were using a 17 mm wide rim, which is on the narrow side for today's standards.  So 120 psi for that tire on that rim will not feel as firm as the same tire on a 23-25 mm rim.  A 25 mm tire on a 25 mm rim, pumped up to 120 psi will be hard as a rock...and it will ride that way too as your wheels hop over every little pebble on the road.

I'm usually around 150-160, but again, I need to qualify that most of the roads I ride on are not that great in quality.  Those riding on really smooth and well swept asphalt likely will not see much difference between 90 and 110 psi...assuming 90 psi is okay for your weight.

I still remember one TT where I pumped up to 120 psi on roads that were somewhat rough.  My teeth were chattering almost the whole way and my front wheel was constantly bouncing.

I've put in a lot of miles on my bike (over 35k in the past 5.5 years)...so I've had plenty of time to experiment.  I will admit that when I first started, I would not pump up to anything less than 110 psi.  Then slowly I came down to 105...then 100...and now I sit at right around 90.  

To add to Jason's point - road conditions and your weight should always be key factors.  Like Jason I'm usually around 155 lbs, and a typical bike ride for me is around 95-100 psi.  Rough roads during a race and I've gone down to 85 to improve the rolling resistance and my speed.  On the trainer, I'm always at 120.

Aaaahhhh.... Maybe some day.    (not giving you a hard time, btw - it was great following your journey there!)

My tuppence.  I'm b/w 160 and 165 pounds and have been riding tubies on my tri bike and clinchers on my roadie.  Tri bike are GP4000s on race wheels (Sub-9 disc and 808 front) and Gatorskins on my daily wheels (808 rear, 404 front).  I can definitely tell the difference in about 10 pound increments.

When I first got the tri bike and tubies, I'd been riding my roadie at about 115-120, no matter the conditions (!), and "heard" that tubies could go to 130 no problem and that I "should" run them there and that it would be "better" to do so (faster, no pinch flats, etc.).  So much for tribal knowledge from that LBS...  I thought I was going to jar my molars loose for about 3 months!

Jason and a few others here (and elsewhere, but not that LBS!) were then kind enough to educate me around this, as the ride became less fun.  I now ride my tubies between 90 and 110, depending on road conditions.  Definitely feel the difference and am just as fast over the same courses at the same W as at 130 psi (probably faster, safer and more comfortable if I get the pressure right).

This is one of those little things in the sport that took me a while to tumble to, but one of the reasons why I love tri.  Getting to be a newbie for years at three different sports means there's lots to learn and improve on, season in and season out!

Matt

Ya know...it is a public road.  Anyone can ride there.  It's not something like Augusta National we're talking about.  

Fair enough!  In that case, I am officially giving you a hard time - for living in paradise. 

Follow on question for the group.  Should I think about tire pressure differently for my clinchers than I do for my tubies?

Matt

You can't pinch flat a tubular, so in that sense, you have the option to run a lot lower pressure if the conditions call for it.  Of course you don't want to run such a low pressure that you can bottom out the rim and cause damage there.

Here's an interesting article on tires (all tubulars) and tire pressures used for varying teams during cobbled races.  Most of these guys racing cobbled classics are not your rail thin climbing physique either.  They typically average 155-180 pounds...or similar to your average Joe age grouper.

There are some pro teams that will use clincher rims/tires for normal road stages or time trials, but you will see nothing but tubulars for rougher stages or classics.  Same goes for cyclocross at the highest end of competition.

Interesting indeed.  Even a bit lower than I expected for the pave.  Thanks.