Bike tire PSI question...

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Bike tire PSI question...

Postby anonymousmoose » Mon Nov 26, 2012 11:24 pm

newbie question again.

I pump by hybrid tiers up to 60psi from the service stations, a week later or so its back down to about 30 PSI. bike shop says its not unusual in the very hot weather.

Are they having me on?

PS
My tires take a max of 75PSI
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by BNA » Tue Nov 27, 2012 4:50 am

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Re: Bike tire PSI question...

Postby greyhoundtom » Tue Nov 27, 2012 4:50 am

They are quite correct, tyres do loose pressure as air molecules filter through the rubber of the inner tube.

The higher the pressure the faster the air loss.

On a road bike with tyres running 110 psi it’s not unusual to loose 10 psi in two or three days.
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Re: Bike tire PSI question...

Postby bychosis » Tue Nov 27, 2012 5:14 am

Normal, but not sure about the hot weather reason, happens in winter too.
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Re: Bike tire PSI question...

Postby Nobody » Tue Nov 27, 2012 6:00 am

I run latex tubes on the CX/road bike and I can lose ~20psi a day. I pump them before every ride. I have them on the MTB too and have to pump them up every couple of weeks just so I can move the bike around the garage.

I've got normal tubes with Presta valves on the utility bike and they are only at 35psi front and 45psi rear and they still lose ~10psi or more a week. The instructions that come with the tube say pump every 30 days, but I think that's highly optimistic.

I've read that Schrader valves lose more air than Presta. It's mainly going to be the losses through the rubber, but Schrader valves wouldn't be helping either.

I would say get a good track pump and use it regularly if you want to have consistent pressures.
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Re: Bike tire PSI question...

Postby Mike Ayling » Tue Nov 27, 2012 1:33 pm

Nobody wrote:I've got normal tubes with Presta valves on the utility bike and they are only at 35psi front and 45psi rear and they still lose ~10psi or more a week.



That seems a lot to me at those pressures. How old are those tubes?

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Re: Bike tire PSI question...

Postby Nobody » Tue Nov 27, 2012 2:13 pm

They would be at least 2 years old, but most of that was sitting in a box since I bought them. Geax MTB tubes. Maybe I let too much pressure out when I loosened the valve? Don't know. I can try again in 6 days if I remember as I topped them up yesterday.
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Re: Bike tire PSI question...

Postby bychosis » Tue Nov 27, 2012 2:28 pm

Nobody wrote:I would say get a good track pump and use it regularly if you want to have consistent pressures.


Also pays to get used to a quick check with your fingers. I.e. the squeeze test, that way you can determine if pressure is changing without hooking up the pump or guage.
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Re: Bike tire PSI question...

Postby gorilla monsoon » Tue Nov 27, 2012 3:58 pm

I don't disagree on the squeeze test but we chabnged a flat the other day, pumped it up with a mini-pump (no pressure gauge), it felt nice and tight and I rode it home. When I got home I put Monster Pump (with gauge) on it and laughed when I saw we'd managed to get less than 40psi into it. So finger-checking has its limitations.
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Re: Bike tire PSI question...

Postby il padrone » Tue Nov 27, 2012 4:01 pm

My tyres hold 60-70psi for two weeks with no worries. They are nice wide 26 x 1.75 too.

Many years ago I ran schraeder valves, thinking the convenience of access to service station air hoses was a plus. However I never used the servo air hose :roll: . I got new rims and decided to get presta (what I'd been using for years in the 80s). Instantly I discovered two things:

1. the presta valves were a lot easier to pump up than schraeder valves;
2. presta valve tubes held the tyres at pressure for a good deal longer - I had been topping up the schraeder-valve tubes every few days, now I cold go for well over a week and the tyres were still at pressure.
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Re: Bike tire PSI question...

Postby Mike Ayling » Tue Nov 27, 2012 5:56 pm

il padrone wrote:My tyres hold 60-70psi for two weeks with no worries. They are nice wide 26 x 1.75 too.

Many years ago I ran schraeder valves, thinking the convenience of access to service station air hoses was a plus. However I never used the servo air hose :roll: . I got new rims and decided to get presta (what I'd been using for years in the 80s). Instantly I discovered two things:

1. the presta valves were a lot easier to pump up than schraeder valves;
2. presta valve tubes held the tyres at pressure for a good deal longer - I had been topping up the schraeder-valve tubes every few days, now I could go for well over a week and the tyres were still at pressure.


1. the presta valves were a lot easier to pump up than schraeder valves;

Don't disagree.

2 I had been topping up the schraeder-valve tubes every few days, now I cold go for well over a week and the tyres were still at pressure.

My experience with K Mart 26 X 1.75 schraeder valve tubes on our old tandem is that it takes about three week for the pressure to drop from 60psi to about 45psi so I only check every three weeks.
Maybe the lighter more expensive tubes lose air a bit quicker.

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Re: Bike tire PSI question...

Postby anonymousmoose » Tue Nov 27, 2012 6:22 pm

Thanks for the reassurance buys.

One other thing, the bike shop told me that I should just put in 60psi, although my bike tires do state 'MAX 75psi'. Would they be correct? Also the local service station all have pumps going to a max of 60psi anyways so if they are right, it seems like I'm in luck :D

Its current very hot in Perth WA if that makes a difference.
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Re: Bike tire PSI question...

Postby Nobody » Tue Nov 27, 2012 6:30 pm

Just run 60psi then. More pressure will roll a bit better but have less shock absorption.
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Re: Bike tire PSI question...

Postby anonymousmoose » Tue Nov 27, 2012 6:34 pm

Nobody wrote:Just run 60psi then. More pressure will roll a bit better but have less shock absorption.


Good point, on my exercise rout there is a bike path that is not very smooth. I got shockies at the front but have no need for using them. Didnt want shockies but they can with the bike.

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Re: Bike tire PSI question...

Postby sturmey archer » Tue Nov 27, 2012 7:56 pm

anonymousmoose wrote:Thanks for the reassurance buys.

One other thing, the bike shop told me that I should just put in 60psi, although my bike tires do state 'MAX 75psi'. Would they be correct? .

Check out:
http://www.bccclub.org/documents/Tireinflation.pdf
1.370" x 24 tpi - what sort of stupid standard is that?
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Re: Bike tire PSI question...

Postby il padrone » Tue Nov 27, 2012 10:35 pm

I've looked at these graphs based on tyre drop of 15% before, and I really don't find them too relevant. Berto's graphs seem even more extreme. The tyre pressures suggested for me are so low as to be ridiculous.

Eg. I weigh 60kgs. My commuter roadster weighs ~16kgs = total of 76kgs. The tyres are 700C x 37mm. By Berto's graph the optimum tyre pressure for me is ~35psi. I would never run my tyres so low and I would expect regular pinch flats at this level, not to mention steering control issues.

My MTB tourer when in Central Australia had 26 x 2.3 tyres. I had up to 60kgs of gear. By Berto's graphs I should have been running at something below 25psi. I did do this on the sandy tracks in places, but definitely not on rocky tracks.
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Re: Bike tire PSI question...

Postby frb_bike » Wed Nov 28, 2012 6:19 am



Those inflations seem incredibly low (looking at the Road tyre graph)... Basically saying I should fill to 70-80psi for my weight for a 700x23 tyre.
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Re: Bike tire PSI question...

Postby rkelsen » Wed Nov 28, 2012 9:44 am

The Frank Berto system is flawed in many ways. He tries too hard, and complicates it to the point that it yields stupid results for some riders.

Tyre pressures can be a pretty simple thing. For example, I try to stick by these rules:

1. You're not Cadel Evans. You don't need his tyre pressures.
2. 23mm tyres = 100psi
3. 25mm tyres = 90psi
4. 28mm tyres = 75psi
5. 32 to 37mm tyres = 60psi
6. Bigger than that = <50psi (depending on size and application)
7. If any of these seem too low, refer to rule 1.

These would be suitable for most people for riding mainly on roads & paths.
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Re: Bike tire PSI question...

Postby twizzle » Wed Nov 28, 2012 10:08 am

Frank's system is O.K., if you treat the chart as a guide and work on getting the required tyre drop.

I use tyres (Panaracer) that require a much higher inflation pressure because of the tyre construction - if set too low the tyre flexes too much and the rubber detaches from the carcass when then get worn... but it's a race tyre, so you put up with that kind of crap.

For the Mich Pro 3 race tyres, they provide an inflation table, and for my old weight I was off the end of the chart, so I just always inflated them to 120psi, the max rating. But I know people who inflate well past the max numbers for their tyres because to them rolling resistance is more important than safety (don't try this with full carbon clinchers!!).

At the end of the day... if the ride is too harsh, use less pressure. If pinch flats become a problem, use a bigger tyre.
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Re: Bike tire PSI question...

Postby VRE » Wed Nov 28, 2012 11:45 am

I use 28mm tyres on my commuter (a touring bike), and generally have the front at 95psi and rear at 90psi, and once every weekend when giving the bike a quick (or sometimes not so quick) clean, I pump them up again. I prefer the higher pressure as they roll better and are less susceptible to punctures, although I do understand that some like lower pressures because it's more comfortable as you feel the bumps a bit less.
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Re: Bike tire PSI question...

Postby stanzarallyman » Wed Nov 28, 2012 12:36 pm

I have 32mm commuting tyres on the bike.

I run 45 to 50psi in the front and 75 to 80psi in the rear, based on that graph. I was sceptical at first, but it certainly worked for me.

Much more comfortable to ride, better grip, especially on the front and next to no increase in rolling resistence. Have not had a puncture in over 8,000km and I ride on all kinds of roads/trails/fire access tracks. I also moved, not at the same time but several months later, to a wider rim. This also helped as well.

Give it a try, you can always put the pressure back up.
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Re: Bike tire PSI question...

Postby Nobody » Wed Nov 28, 2012 1:16 pm

I'd say to anyone who wants to use the graphs to first actually measure your tyre width and not go by the numbers on the sidewall, since small increments can make a big difference in smaller tyres. For example I have:
50mm Marathon which measures 46 at the pressures I run.
23mm Rubino Pro which measures 24.
23mm Gatorskins which were wider than the Rubino Pro above.
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Re: Bike tire PSI question...

Postby Byke » Wed Nov 28, 2012 10:38 pm

twizzle wrote:But I know people who inflate well past the max numbers for their tyres because to them rolling resistance is more important than safety


…and because perception is more important to them than reality???

Obviously there are a lot of variables at play, but if you're not riding on an ultra-smooth surface (like an indoor wooden velodrome) then after a certain point - say around the maximum pressure advised by the manufacturer - then increasing pressure increases the rolling resistance.

As Nick Legan (ex TDF mechanic) explained beautifully in one of his great columns on Velonews:

"Imagine hitting a seam in the road. Would you say that hitting that seam slowed you down? Absolutely. Well, when your tire is over-inflated, every tiny bump on the road becomes a road seam. With lower pressure the casing of your tire can absorb that seam instead of bouncing (backwards) off of it."
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Re: Bike tire PSI question...

Postby twizzle » Thu Nov 29, 2012 12:35 pm

Byke wrote:
twizzle wrote:But I know people who inflate well past the max numbers for their tyres because to them rolling resistance is more important than safety


…and because perception is more important to them than reality???

Obviously there are a lot of variables at play, but if you're not riding on an ultra-smooth surface (like an indoor wooden velodrome) then after a certain point - say around the maximum pressure advised by the manufacturer - then increasing pressure increases the rolling resistance.

As Nick Legan (ex TDF mechanic) explained beautifully in one of his great columns on Velonews:

"Imagine hitting a seam in the road. Would you say that hitting that seam slowed you down? Absolutely. Well, when your tire is over-inflated, every tiny bump on the road becomes a road seam. With lower pressure the casing of your tire can absorb that seam instead of bouncing (backwards) off of it."


Well... in this case, it's a dedicated track using tarmac laid to the same surface standards as a runway. Does that qualify?

And the bike doesn't bound "backwards off it" and lose energy, the energy is transferred to the large fluid damper sitting on top of the frame and released as heat.
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Re: Bike tire PSI question...

Postby sblack » Thu Nov 29, 2012 9:36 pm

Ok, I should start by saying I am running tubeless so can get away with lower pressures without them causing issues but I have to say I am continually surprised by how low pressures can go without any noticeable drop in speed. For me total bike,rider and gear weight is somewhere around 90kg and I run 23mm tyres although on 19mm internal width rims so they behave closer to 25mm tyres. I honestly couldn't remember when I last pumped them up but it had been several weeks and at that time I'd set them at 75 front and 90 rear. I was thinking earlier this week they where feeling a little on the low side but had been slack and not got around to pumping them up. Despite that during this week I've had my two fastest rides home as well as setting some segement PRs on Strava. I've noticed many times after having good rides by my standards that the tyre pressure was well below what most would consider normal for road tyres. This afternoon, after setting another PR on the way home I got around to pumping them up, they where down to 50 front and 60 rear.
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Re: Bike tire PSI question...

Postby toolonglegs » Thu Nov 29, 2012 10:09 pm

^^^I am not the tyre pressure nazi I used to be :roll: ... OK I get a bit of side ways movement in my 35c's at 28psi but they still roll pretty fast ( some races have good stretches of tar seal ) and the steering can be a bit heavy but the only pinch flats I get are from really really hard hits... but in reality I have only had one pinch flat at those pressures from hitting a rock hard, I have had countless other hard hits with no problems at all.
So these days with in 20 psi on the road bike is good enough for me 70-100 psi on 25c, 90-110 on 23c, 130-150 psi on 19c's ( but they really are pretty tiny so I expect them to be a bit fragile) . Around 100kg rider and bike weight.
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