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tire pressure

Old 01-22-09, 10:47 AM
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rumrunn6
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tire pressure

How big a difference has anyone seen in changing tires to a higher pressure. Will this so an obvious improvement in speed?
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Old 01-22-09, 10:51 AM
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The ride will be rougher but you get used to it. And yes you do notice a difference.
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Old 01-22-09, 11:05 AM
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I notice a difference, the ride is rougher and I mostly feel it in my hands and arms, my butt not so much. However, as was said above, I have gotten used to it, especially since it also comes with a little bit quicker feel. Low pressure tires, while a bit more comfortable, seem a little sluggish to me.
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Old 01-22-09, 11:18 AM
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I have a friend who rides an electric bike to work. He installed a pretty fancy motor control computer on it that reads out how many watts of power the motor is drawing. He says just letting the tires get even 10 pounds down can run his power consumption up by 20 or 30 percent.
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Old 01-22-09, 11:21 AM
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On the other hand, at some point its a diminishing rate of return. There have been studies showing that absolutely rock hard isn't always the best when not on a perfectly smooth surface. Most people run too soft anyway so it's probably not an issue, but depending on the tire pumping it all the way to max pressure might be counter-productive.
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Old 01-22-09, 04:23 PM
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My 26x1.50" street tires seemed to give me an extra .5 MPH for the same effort when going from 5 PSI under max (65) to 5 PSI over.
I "cruise" about 13 MPH, so that's about 4%.
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Old 01-22-09, 04:49 PM
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Keeping that pressure up to max will probably reduce chances of getting flats I think ..

EDIT -

It is unrelated to OP question because OP intends to find its impact on speed, but just my 2 cents ..
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Old 01-22-09, 04:58 PM
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Do you mean taking your current tire and pumping it to a higher pressure? Or putting on a new tire rated for a higher pressure? Width, tread, weight, and pressure will be factors when changing to different tires, and they all can effect speed.
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Old 01-22-09, 05:11 PM
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I keep my tires at 100 psi. Faster and fewer flats. I can definitely tell when the tires drop down to 80 psi.

Many of the new road tires can handle 120 psi.
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Old 01-22-09, 08:55 PM
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I do notice that I ride faster and with less effort when I have the tires pumped up. For me the proof is in my HRM. Over the same distance, if the tires are low I can spend up to 25% more energy than if they're up to or over pressure.

I also think I corner better when the tires are up to pressure.
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Old 01-22-09, 09:04 PM
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The more you weigh the harder your tires should be. The rubber should be slightly deformed where it meets the pavement (contact patch), but definitely not flattened.

The rear tire on the bike in this photo is too soft:

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Old 01-22-09, 09:28 PM
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Originally Posted by CB HI View Post
Many of the new road tires can handle 120 psi.
They do, i keep 120-140 psi on both tires of my road bike.
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Old 01-23-09, 04:36 AM
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EMPIRICAL EVIDENCE! Thanks guys!

BTW: I checked the tires in question, they are 26x2.0 with max pressure 45-65 lbs

Anybody know a big fat slick that pumps to 90 lbs?
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Old 01-23-09, 07:25 AM
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I recently changed from 100+ psi tires to 80 psi max tires. They're the same size (700x28) however, I've lost 4 to 5 mph off my commuting average. There are other factors: tread, weight, rubber compound, etc but, I feel, the biggest factor is the pressure difference. Even at max. pressure they're: spongy and slow, I really struggle to maintain a 16 mph avg. on my daily commute, and my thoughts were exactly this: I'm just not getting a good enough return on the energy I'm putting into it. I was recently told that I could, safely, increase the pressure 5 to 10 pounds, however, I'm reluctant to do it. I usually run all my road tires at max. or, slightly above. I think that ride quality is improved too as I feel less squishiness or tendency for the sidewall to fold in hard cornering.
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Old 01-23-09, 07:29 AM
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One question that I forgot to ask is: why isn't "hard" "hard"? That is, whether a tire has a max. pressure of 65 or 110 psi., won't max. pressure yield a similar "hardness" or fullness? I thought that the pressure would compensate for diff. tire volumes, material, design, etc, but would yield an equally full tire. Why would two diff. tires from the same manufacturer, in the same size, have vastly different max. capacities? Is it for ride quality and tires that are designed for diff. riding situations/styles? I guess I always assumed that whether max. was 65 or 100 I would get similar rolling resistance from both wheels, I've recently found that's not the case.
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Old 01-23-09, 07:41 AM
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I recently changed from 100+ psi tires to 80 psi max tires. They're the same size (700x28) however, I've lost 4 to 5 mph off my commuting average. There are other factors: tread, weight, rubber compound, etc but, I feel, the biggest factor is the pressure difference. Even at max. pressure they're: spongy and slow, I really struggle to maintain a 16 mph avg. on my daily commute, and my thoughts were exactly this: I'm just not getting a good enough return on the energy I'm putting into it. I was recently told that I could, safely, increase the pressure 5 to 10 pounds, however, I'm reluctant to do it. I usually run all my road tires at max. or, slightly above. I think that ride quality is improved too as I feel less squishiness or tendency for the sidewall to fold in hard cornering.
Sorry, going from 100 psi to 80 psi will not take 4-5 mph off your speed, I doubt it would even take 1. I normally start my tires at 100 and by the time I reinflate they're usually down to 70, but I barely notice a difference the next ride. I also don't think even the worst tires would have such a high rolling resistance as to drop your speed like that (21 mph takes twice as many watts as 16 mph), me thinks this is more of a perceived speed difference than actual speed difference.
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Old 01-23-09, 07:52 AM
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Bezier, I'm surprised as well, however, my actual speed is measured by my computer, not guesswork. Unless it became suddenly un-calibrated when I switched tires, same size mind you, it's an accurate estimate. I also judge speed using the clock: it's taking me longer to get to work/home, other riders, and perceived feel of input and output of energy. I'm definitly slower however, it's not entirely related to tire pressure. I know the other factors are: tire weight, compound, tread, punture resistant layers, etc. It's just that the tires are incredibly squishy and poor handling, the bike is slower rolling even when coasting downhill or under consistent pedalling on flat terrain, the hills are especially tough. I've used many types of tires, however, these have the lowest max. pressure of any I've used, thus my conclusion is that tire pressure is probably the biggest culprit in this case.
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Old 01-23-09, 08:27 AM
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Still ... I think changing my hybrid tires to a similar width but high pressure slick may make that thing ridable again ...?
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Old 01-23-09, 08:38 AM
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I notice a difference between 100psi (which is where I like to keep it) and 85-90psi on my 700x38c Specialized Nimbi (plural of Nimbus). The lower tire pressure makes me feel like I'm slogging compared to the higher pressure.

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Old 01-23-09, 08:43 AM
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All things being equal (size, tread, compound,etc), higher pressure will yield higher speed. That said, there are ride quality, wear, and handling issues. It's the first thing everyone does when converting a hybrid/mountain bike to a road or commuter machine is install higher pressure slicks. You'll likely have a slightly harsher ride but the return is improved speed and handling. There are times you'll want to run lower pressure: snow, off road, etc, you may only have to drop the pressure a few pounds to notice the difference. Mountain biking demands more attention to tire pressure and I'm always adjusting for the conditions, road/commuting, I nearly always run at max. pressure.
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Old 01-23-09, 09:10 AM
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The thing that kinda confuses me though, is that right now I'm riding a hybrid with 700X38's with a max of 65 psi. I keep my tires at 62-65 psi (I check them weekly), and they barely sag, but I still feel like I'm slogging. Is this because the tire is wider, or because it has a lower pressure, even though the tire holds it's "roundness?"
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Old 01-23-09, 09:39 AM
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Originally Posted by Bezier View Post
Sorry, going from 100 psi to 80 psi will not take 4-5 mph off your speed, I doubt it would even take 1.
+1 When riding my 28lb touring bike with 700x32 TT2000s inflated to 80 psi, my average speeds are maybe 1-2mph slower, tops, than when riding my 20lb road bike with 700x23 GP4000s's inflated to 120psi.
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Old 01-23-09, 11:11 AM
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If you put more air in the same tire, you make the diameter larger. Will this make your computer show a higher speed or a lower speed?
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Old 01-23-09, 11:14 AM
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Originally Posted by chipcom View Post
+1 When riding my 28lb touring bike with 700x32 TT2000s inflated to 80 psi, my average speeds are maybe 1-2mph slower, tops, than when riding my 20lb road bike with 700x23 GP4000s's inflated to 120psi.
I think this is about right.

The same holds on a mountain bike. When I change from a knobby at 45 psi to a slick at 65 psi, I only pick up a couple mph--on the same bike.
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Old 01-23-09, 11:18 AM
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I keep my front tire at 100psi and back at 120psi (different tires, same size). I can definitely notice a difference if I forget to top them off one day (5-10psi or so), especially in the back. Not so much in speed, but quality of the ride in general. That slight bounce going over small bumps when the tires are a little under inflated is really annoying.
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