## Bike tire PSI question...

K2
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### Re: Bike tire PSI question...

bychosis wrote:Normal, but not sure about the hot weather reason, happens in winter too.

Given this...

greyhoundtom wrote: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.

Gases expand when heated = pressure is increased even more = faster loss due to above. Movement of the tube whilst cycling probably helps the process (where "helps" means quite the opposite from a rider's perspective).

Diffusion still occurs in winter, just not as quickly (where "quickly" = probably rather slowly in the grand scheme of things).

A leaky valve is just a bigger opening for inner and outer pressures to try and equalize through. Same principles should apply.

bychosis
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### Re: Bike tire PSI question...

Any science type out there care to calculate the difference in pressure between say 10 degrees in winter and 30 in summer? Surely it can't be that much. And if I pump my tyres regularly to a predetermined pressure, weekly to 90psi what is the difference in pressure going to be when the ambient temp probably changes a similar amount between day and night in winter and summer, ie 8-18 winter, 20-30 in summer. Maybe the rubber in the tube is softer in summer heat and allows more molecules out?
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Nobody
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### Re: Bike tire PSI question...

il padrone
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### Re: Bike tire PSI question...

If tyres are inflated in the morning at a low temperature and the daily maximum is say 20oC higher than the morning, then the tyre pressures would increase in the order of 28kPa (4psi) during the day and could exceed the tyre's maximum.

Oh No!! That sounds like sheer disaster

Not many days where the temperature difference is 20 degrees, and in that case a 4 psi presusre increase. That certainly will not compromise your tyre's safety
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toolonglegs
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### Re: Bike tire PSI question...

The biggest scare by far which I seem to see a bit too often is people pumping up their clincher tyres too hard when on holidays in places with proper big hills... It really can kill you on the descents and can't be stressed enough.

sblack
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### Re: Bike tire PSI question...

il padrone wrote:Not many days where the temperature difference is 20 degrees, and in that case a 4 psi presusre increase. That certainly will not compromise your tyre's safety

Interestingly my Garmin tells me that was exactly the temperature difference I encountered on today's ride (from 21 to 41). Can't say I personally noticed any change in tyre pressure.

twizzle
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### Re: Bike tire PSI question...

il padrone wrote:Not many days where the temperature difference is 20 degrees, and in that case a 4 psi presusre increase.

Yeah? Come to my area. 20C difference isn't that uncommon.

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Byke
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### Smooth vs Rough Surfaces

twizzle wrote:
Byke wrote: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.

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

Yeah, I reckon that qualifies!

I've ridden on (closed) freeways a couple of times, and the difference in the lack of vibration through the handlebars compared to riding on (Victoria's) standard coarse-chip tarmac is dramatic.

I haven't ridden much overseas (and not at all in Europe) but almost every car review by an Australian journalist of a new car driven in Europe or Japan pointedly reserves judgement on the car's suspension setup until they have driven it on Australian roads. Quite often manufacturers alter the suspension settings of imported cars specifically for Australian roads.

Bike riders can achieve the same thing by not going too hard with the pump.
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Nobody
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### Re: Bike tire PSI question...

Nobody wrote: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.
OK, so it has been a total of 8 days and both tyres lost 6psi (compared with 10psi in 7 days) as this time I tried to lose as little air as possible when freeing the Presta valves.

DavidS
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### Re: Bike tire PSI question...

Hmm, I run 700 x 28 tyres, I weigh about 100KG and carry a bag with clothes and the like. So I would guess all up the weight is at least 120KG. That chart quoted about 40psi which I reckon is rubbish. I run 85psi on the rears and 90psi on the front (Marathon Supreme tyres, ran slightly lower pressure with Marathon Plus). Occasionally I break a rear spoke but I like hard tyres and do feel they make a difference in rolling resistance. I suppose part of this is preference but I have always preferred well pumped up tyres. I find I need to check pressure about once a fortnight.

DS

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sblack
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### Re: Bike tire PSI question...

DavidS wrote:Hmm, I run 700 x 28 tyres, I weigh about 100KG and carry a bag with clothes and the like. So I would guess all up the weight is at least 120KG. That chart quoted about 40psi which I reckon is rubbish. I run 85psi on the rears and 90psi on the front (Marathon Supreme tyres, ran slightly lower pressure with Marathon Plus). Occasionally I break a rear spoke but I like hard tyres and do feel they make a difference in rolling resistance. I suppose part of this is preference but I have always preferred well pumped up tyres. I find I need to check pressure about once a fortnight.

DS

You might want to check the chart again, I think you're reading a weight of 120 lbs not kg. 120 kg on 28mm tyres looks to be around 100 psi to me.

human909
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### Re: Bike tire PSI question...

toolonglegs wrote:The biggest scare by far which I seem to see a bit too often is people pumping up their clincher tyres too hard when on holidays in places with proper big hills... It really can kill you on the descents and can't be stressed enough.

Can't be stressed enough? It sounds like you are already stressing it too much.

The rated tyre pressure already have a significant safety margin. Unless you are well already above and beyond the recommend pressure combined with crazy overheated tyres then I don't see any risk. Your tyre needs to be above boiling point for there to be much risk.

DavidS
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### Re: Bike tire PSI question...

sblack wrote:
DavidS wrote:Hmm, I run 700 x 28 tyres, I weigh about 100KG and carry a bag with clothes and the like. So I would guess all up the weight is at least 120KG. That chart quoted about 40psi which I reckon is rubbish. I run 85psi on the rears and 90psi on the front (Marathon Supreme tyres, ran slightly lower pressure with Marathon Plus). Occasionally I break a rear spoke but I like hard tyres and do feel they make a difference in rolling resistance. I suppose part of this is preference but I have always preferred well pumped up tyres. I find I need to check pressure about once a fortnight.

DS

You might want to check the chart again, I think you're reading a weight of 120 lbs not kg. 120 kg on 28mm tyres looks to be around 100 psi to me.

Haha, bloody imperial measurements, didn't notice that one. Looks to be about 80-90 so I'll retract my remark about it being rubbish

DS

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Howzat
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### Re: Bike tire PSI question...

We've looked at weight distribution, tyre diameter, valve type, seasonal and diurnal temperature variations, and pressure change over time, but there are several other factors to consider here -

1. Air pressure varies at altitude. So the pressure differential for a tyre inflated to 100psi is lower at sea level than at the summit of Mont Ventoux. This affects the rate of deflation.
2. Increases in air pressure increase the mass of air in your tyres, which will affect acceleration at a given force according to Newton's laws of motion. Also, this mass of air flows inside the tyre, producing complex fluid dynamic effects, only recently tractable with advances in computational technology.
3. When a gas permeates through the rubber and goes from a high pressure to a low pressure, a cooling effect is produced on the tyre due to Charles' law. Reducing the amount of deflation can change tyre temperature, which may affect handling.
4. Friction with the road increases tyre temperature, which in turn heats the air, which in turn increases effective tyre pressure as you ride, temporarily counteracting deflation by permeation.

Ignoring these effects is a common newbie mistake, which is why your tyres, whatever they are inflated to, are not at the right pressure.

twizzle
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### Re: Bike tire PSI question...

human909 wrote:
toolonglegs wrote:The biggest scare by far which I seem to see a bit too often is people pumping up their clincher tyres too hard when on holidays in places with proper big hills... It really can kill you on the descents and can't be stressed enough.

Can't be stressed enough? It sounds like you are already stressing it too much.

The rated tyre pressure already have a significant safety margin. Unless you are well already above and beyond the recommend pressure combined with crazy overheated tyres then I don't see any risk. Your tyre needs to be above boiling point for there to be much risk.

Go and find the post from TLL from spectating at the TDF this year when the guy cooked his brakes on a descent, blew the front tyre off and landed on his face. Water in the tube is another factor, if you had a buildup of water in there over time from inflation (trapped condensation), and it vapourises... a massive pressure increase is going to occur.
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InTheWoods
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### Re: Bike tire PSI question...

Howzat wrote:We've looked at weight distribution, tyre diameter, valve type, seasonal and diurnal temperature variations, and pressure change over time, but there are several other factors to consider here -

1. Air pressure varies at altitude. So the pressure differential for a tyre inflated to 100psi is lower at sea level than at the summit of Mont Ventoux. This affects the rate of deflation.
2. Increases in air pressure increase the mass of air in your tyres, which will affect acceleration at a given force according to Newton's laws of motion. Also, this mass of air flows inside the tyre, producing complex fluid dynamic effects, only recently tractable with advances in computational technology.
3. When a gas permeates through the rubber and goes from a high pressure to a low pressure, a cooling effect is produced on the tyre due to Charles' law. Reducing the amount of deflation can change tyre temperature, which may affect handling.
4. Friction with the road increases tyre temperature, which in turn heats the air, which in turn increases effective tyre pressure as you ride, temporarily counteracting deflation by permeation.

Ignoring these effects is a common newbie mistake, which is why your tyres, whatever they are inflated to, are not at the right pressure.

I'm not sure if you're being funny or not I'm hoping funny

The differences are small enough that there would be a range for the "right" pressure. Otherwise I would have to get off my bike every 5 minutes for the first 20 minutes of my ride to adjust the pressure, every time the garmin says the temp has changed (when climbing into cooler air this would suck), while factoring in that while I am adjusting the pressure I'm not moving so the tyres are cooling at the same time

toolonglegs
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### Re: Bike tire PSI question...

human909 wrote:
toolonglegs wrote:The biggest scare by far which I seem to see a bit too often is people pumping up their clincher tyres too hard when on holidays in places with proper big hills... It really can kill you on the descents and can't be stressed enough.

Can't be stressed enough? It sounds like you are already stressing it too much.

The rated tyre pressure already have a significant safety margin. Unless you are well already above and beyond the recommend pressure combined with crazy overheated tyres then I don't see any risk. Your tyre needs to be above boiling point for there to be much risk.

I would expect nothing less in a post from you.

ironhanglider
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### Re: Bike tire PSI question...

toolonglegs wrote:
human909 wrote:
toolonglegs wrote:The biggest scare by far which I seem to see a bit too often is people pumping up their clincher tyres too hard when on holidays in places with proper big hills... It really can kill you on the descents and can't be stressed enough.

Can't be stressed enough? It sounds like you are already stressing it too much.

The rated tyre pressure already have a significant safety margin. Unless you are well already above and beyond the recommend pressure combined with crazy overheated tyres then I don't see any risk. Your tyre needs to be above boiling point for there to be much risk.

I would expect nothing less in a post from you.

I am interpreting from this that TLL has witnessed tyre blow-offs on big descents and is attributing this to pressure. These would almost always be seriously scary at best.
H909 is suggesting that the pressure changes aren't all that great so there is not that big a risk.

I think that there is an area for common ground here. Hot clinchers do blow off, it has been demonstrated often enough, but I don't think it is solely due to pressure. I routinely run over-pressured tyres on my tandem (220kg in race mode) to the tune of 30% or more. It was touched on earlier that the universal gas equation has pressure varying in direct proportion to the temperature in Kelvin. Kelvin has the same unit dimensions as Celcius but room temperature is near enough to 300K. How hot do tyres get? If they got to 600K (seems unlikely) it would only result in doubling the pressure, even 450K would only be an additional 50%.

Cold tyres with excessive pressure are not a problem since tyres are routinely tested to double their nominated pressure without a blow-off, there is something else at play rather than just the pressure. I suspect that the temperature is the key factor here, maybe it affects the slipperyness of the sidewalls or relaxes the bead or both. Perhaps this is enough to lower the blow-off pressure from 250psi to something more attainable like 180psi.

In this scenario if someone had tyres at 120psi and then raised the temperature to 450K, the tyres get to the magic 180psi and Boom. If instead they had lowered their pressures to 100psi got to the same 450K then their tyres would only reach 150psi and they have a wonderful time.

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il padrone
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### Re: Bike tire PSI question...

This is an issue for tandem riders on long descents. Tandems have the twin conflicting requirements of high tyre pressures to avoid pinch flats, and heavy braking requirements on long descents due to the heavier load. Tyre pressures for normal single bikes are less of an issue. Tyre manufacturers set their pressure max at 50% of the pressure where the tyre blows off the rim (ie. a 100psi rated tyre will not blow off until 200psi). For single bikes on a long descent the rims will heat up, however as long as you pulse the brakes on and off every so often the heat will dissipate (especially with conductive alloy rims). Only sustained braking for longer time periods (a minute or more at >15kmh) would cause a tyre temperature rise that may be problematic for the tyre.

Tandems on the other hand may need longer braking periods with fewer releases of the brakes. This would be quite OK if you are traveling at >40kmh, where the rim will be cooled by the passing air, but this is a touch too fast for some steep winding descents. Failing this you would need to keep the speed below 15kmh when the rim will not be heated enough to be a problem. Between 15 and 40kmh is the problem zone where the rim under sustained braking will heat up to such an extent that the tyre pressure will soon heat up to a very high level, beyond twice the rated pressure. The solution for most tandems, especially those used for loaded touring is to run a rear drum brake in addition - a third brake that can take some of the braking load off the rims. These days many tandems are fitted with discs which prevents rim overheating but may still have serious problems on a fully loaded tandem if the discs heat up to the extent that the disc warps, or the disc pads lose their traction.
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sblack
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### Re: Bike tire PSI question...

ironhanglider wrote:Cold tyres with excessive pressure are not a problem since tyres are routinely tested to double their nominated pressure without a blow-off, there is something else at play rather than just the pressure. I suspect that the temperature is the key factor here, maybe it affects the slipperyness of the sidewalls or relaxes the bead or both. Perhaps this is enough to lower the blow-off pressure from 250psi to something more attainable like 180psi.

There is also more to consider in this equation. Is the problem the tyre blowing off the rim or is a sudden flat causing the blow off? I've only seen a couple of cases where it would appear heat has caused a flat tyre. One of those was a rider dragging the brake on a longish descent (nothing like the ones you're talking about TLL but with poor technique we still have long enough descents over here) and the other was a bicycle left sitting in the sun (and also after a longish descent). In both cases the initial point of failure appeared to be the edge of the reinforced section of tube around the valve. My thought was that the heat caused expansion of the rims, opening the valve hole up to larger than normal and allowing too much tube to push into the hole. As the section in contact with the edge of the valve hole reached the edge of the reinforced section of tube it cuts through causing an instant flat. Increased tyre pressure, both through higher initial setting and/or temperature generated increases, would also increase the possibility of this occurring but I don't believe the tyres in these cases where inflated to their maximum pressures to start with.

Of course the expansion due to heat could manifest itself in other ways as I'd guess it has the potential to affect he rim to tyre bead interface as well.

toolonglegs
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### Re: Bike tire PSI question...

I saw it happen 3 times in July,twice right in front of me. First was on the descent of the Tourmalet, only about 2 or 3 kms down from the top. I was sitting on about 70 when the guy about 100 meters in front of me had his front tyre explode rather loudly. It blew off the rim and he went down very quickly into a rocky gutter. The blood was flowing in rivers down the road and I was thinking the worst. He was very lucky though and was up and walking ( not very prettily ) the next day.
The other was a girl on the Port de Balès the next day... I was going up and she was coming down, coming into a hairpin her tyre exploded, she did very well to stay up right.

It is not just pressure, but a combination of everything I expect... over heating rims, tyres pumped up too hard, the heat build up in the rims probably softens the rubber and the bead and it comes off with a big bang. I don't think anyone can dispute that running lower tyre pressures would give you some leeway especially if you are heavy or heavy on the brakes... plus adding in the ambient temp and road temp. On the Tourmalet incident the guy probably had his bike sitting in the sun for 30 minutes or more on the summit ( like everybody else ). On the Port de Balès it was a horrible cold day though.

I stand by my original statement... you don't need to have your tyres pumped up to maximum in the big hills ( on clinchers )... you probably don't need your tyres pumped up to maximum anywhere but that's another matter. Run a good 10-20% below the maximum or buy a set of tubulars .

rkelsen
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### Re: Bike tire PSI question...

toolonglegs wrote:you probably don't need your tyres pumped up to maximum anywhere but that's another matter.

This man knows what he is talking about.

So many people rely on "conventional wisdom" for their car tyre pressures, without even knowing what the maximum pressure rating is (because Bob Jane said so), yet absolutely must run maximum pressure on their bicycle tyres. It defies common sense, especially when you consider the potential consequences.

I've found that running lower pressures is not detrimental to the feel or performance of the bike. In fact, it makes the bike more comfortable to ride. Conversely, running higher pressures is detrimental to the service life of equipment and gives less shock absorption, making the bike less comfortable to ride.

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### Re: Bike tire PSI question...

From a performance perspective, you don't want to under or over inflate tyres. Nice item by Tom Anhalt:

http://www.slowtwitch.com/Tech/What_s_i ... _1034.html

One quick “takeaway” from the above plot is that if one hasn't identified the "optimal" pressure for their particular setup, then it is far better to "err" on the side of too little pressure, than to "err" on the side of too much!

A good starting point for tire pressures is to follow the recommendations of the tire manufacturers. Most will give both a recommended pressure that's less than the maximum pressure, and some give fairly detailed recommendations based on rider weight and conditions.

Add to that there are certain circumstances where you don't want too high a pressure in order to improve handling, think crits, and also wet surfaces.

As for safety and heat factoring in blowouts, it's definitely a problem on road bikes on long descents with frequent heavy braking requirements. The sidewalls/beading becomes more easily removed from the rim. e.g. if you have a clincher tyre-rim combo that's hard to get on/remove on a cold day, then simply warming up the tyre can help make the job easier. And if your tyre-rim combo is pretty easy to remove by hand, then keep that in mind as pressures build.

Heat blowouts are even a problem for tubulars. Anyone who's raced at an outdoor track meet in an Australian summer will probably have heard that unmistakeable gunshot sound of someone's tyre blowing out while the bike is on the rack sitting in the sun.

Nobody
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### Re: Bike tire PSI question...

rkelsen wrote:I've found that running lower pressures is not detrimental to the feel or performance of the bike. In fact, it makes the bike more comfortable to ride. Conversely, running higher pressures is detrimental to the service life of equipment and gives less shock absorption, making the bike less comfortable to ride.
I also notice the grip improves a bit as the tyre pressure drops. Probably from less tyre bounce only, but it is still a noticeable effect IMO.

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