Minimum Tyre Pressure

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Rhodrich
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Minimum Tyre Pressure

Postby Rhodrich » 16 Jan 2015, 11:44am

The tyres I'm running on my bike are Continental SportContact II, in 700x32c flavour, which have a recommended inflation pressure of 70psi, as shown in the chart here:

http://www.conti-tyres.co.uk/conticycle ... tact.shtml

This is also shown as the minimum pressure on the label that was attached to the tyre.

I think that this pressure is actually too high for me - I weigh 75kg, the bike about 15kg loaded (so 90kg combined, which is 200lbs), and this page suggests I should be running at 52psi front, 64psi rear, based on a 45% front, 55% rear weight distribution

http://www.dorkypantsr.us/bike-tire-pre ... lator.html

I'm currently riding at the minimum recommended pressure of 70psi, both front and rear, and the ride on the bike is a bit bouncy, which also suggests that this is too high.

What would be the consequences of reducing the pressures to 52psi front, 64psi rear, which are below the minimum recommended pressures as stated by Continental? I don't want to destroy my tyres, or get pinch flats......

Thanks

profpointy
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Re: Minimum Tyre Pressure

Postby profpointy » 16 Jan 2015, 12:05pm

Well I'm similar weight with somewhat lighter bike and run 25mm tyres at around 110psi front & rear.
The wider tyres would run a bit lower. On my 26" tourer / hybrid I had 1-1/2" width tyres, which is 38.1 in new money. They're labelled 40-85 psi - I think I ran them at 60 to 70 - but my memory could be faulty as said bike has been awaiting repair for a shamefully long time.

Anyhow, given your tyres are a bit narrower than the 26" , I'd have thought at least 70, maybe a good bit more, especially as mine were smaller wheels. But that said, there's nowt wrong with dropping them a bit and see if it's harder work and / or you get punctures. Harder than you'd think is still a good guide to tyre pressures all the same.

reohn2
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Re: Minimum Tyre Pressure

Postby reohn2 » 16 Jan 2015, 12:10pm

Rhodrich wrote:

What would be the consequences of reducing the pressures to 52psi front, 64psi rear,

You'd be more comfy and the tyres would grip better :)
which are below the minimum recommended pressures as stated by Continental? I don't want to destroy my tyres, or get pinch flats......

Thanks

With 90kgs on board there's little chance of pinch flat unless you should hit a really sharp corner at speed,say a deep pothole edge on the exit or sharp edged curb.
You won't destroy the tyres :wink:

I run 37mm at 40f 60r with an all up weight(bike,rider,luggage)of 100kg,we only run the tandem with 32mm Gatorskins(recommended TP is 85psi,which is bonkers for solos) @ 85f 90r(weight distribution is more equal front and rear on tandems) with an all up weight of 180kg.
I've never experienced pinch flats or tyre problems.
Last edited by reohn2 on 16 Jan 2015, 12:26pm, edited 3 times in total.
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Vantage
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Re: Minimum Tyre Pressure

Postby Vantage » 16 Jan 2015, 12:18pm

First thing I'd look at is how tight the tyres are on your rims. If they're a loose fit, there's a (ludicrously small) chance of them blowing off the rim. If they're a nice 'secure' fit (in that you can't just peel them off without any great force) then I'd say run them at whatever pressure suits you as long as you don't go over the maximum too much.
This http://www.bikequarterly.com/images/TireDrop.pdf gives an idea of what pressures to run although I find some of their recommended pressures way too low on some tyres.
Bill


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Rhodrich
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Location: Thames Ditton, Surrey

Re: Minimum Tyre Pressure

Postby Rhodrich » 16 Jan 2015, 12:34pm

Thanks for the input all. Just to confirm, I'm looking for the consequences of running the tyres at below their minimum recommended pressure, not above their maximum. The calculator I linked to above is based on a 15% tyre drop as promoted by Bike Quarterly.

My biggest concern is that I've seen lots of pictures of tyres with cracked sidwalls, which people say are as a result of running too low a pressure or overloading.....

Bicycler
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Re: Minimum Tyre Pressure

Postby Bicycler » 16 Jan 2015, 12:56pm

Rhodrich wrote:My biggest concern is that I've seen lots of pictures of tyres with cracked sidwalls, which people say are as a result of running too low a pressure or overloading.....

What is meant is too low pressure for the load. Low pressure doesn't do anything by itself. I'd go by what feels right. If using the BQ chart I'd measure the tyre width as it is on your rims, not as written on the tyre sidewall. Tyres can come up quite significantly narrower or wider than marked.

Brucey
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Re: Minimum Tyre Pressure

Postby Brucey » 16 Jan 2015, 1:53pm

Rhodrich wrote: this page suggests I should be running at 52psi front, 64psi rear, based on a 45% front, 55% rear weight distribution

http://www.dorkypantsr.us/bike-tire-pressure-calculator.html



well it would be more accurate to suggest that one of the several tyre pressure calculators suggests those pressures, but that the others on that page don't. Essentially the world is divided into two camps. You have the Berto/Heine camp who would suggest 15% drop, every time, and then you have the other camp with Brandt, a lot of racers, and a lot of tyre makers, who think it isn't that simple.

Another link on that page directs you to a discussion with Jobst Brandt who suggests that for heavy loads, you should use wider tyres (and presumably therefore the converse by implication), and also that if you ride a wider tyre at a low pressure you can experience other handing troubles. He (quite rightly) points out that if you use the brakes hard you have almost 100% of your weight on the front tyre. I also agree with him that that the handling of soft wide tyres is, in many cases not very good at all in hard downhill cornering. I also think it varies enormously with the width of the rim that is used.

So if you run your tyres soft/too soft you may find that;

- you get more pinch flats

- you gain a little comfort when riding normally and when on poor surfaces

- you have to work a little harder on good roads, because you have increased your smooth surface Crr (NB this always happens, though it may not be much)

- you may not have to work noticeably harder on poorer surfaces (because your suspension losses are reduced)

- you may get vague (or even dangerous) steering when cornering and/or braking hard; (this will be worse on narrower rims)

- the tyre may start to flex excessively at the rim edge, and may be more likely to fail there (worse on rims with a sharper radius on the hook bead; it varies)

- if you use very low gears/low pressures the carcass may start to 'wrinkle' in the sidewall near the contact patch; when this happens it means that the tyre is overloaded and its life may be shortened; [this situation is somewhat analogous to a wheel where some of the spokes go completely slack under heavy loads, which is similarly undesirable.]

- the tyre starts to move around on the rim. This is more likely to happen if the tyre bead/rim surface is contaminated (talc, release compound, grease) inherently slippery tyres (eg many conti models where there is an external chafer on the tyre bead, with little/no rubber facing) and on rims that don't have a pronounced hook bead. The first clue is often that the valve gets cocked over on the rear wheel. Rarely, the front tyre can slip round, apparently wholesale (and without any warning) during braking, which normally pulls the valve out and results in a nasty crash.

- that the wire bead on the tyre can fail. This appears a likely outcome with some recently manufactured Schwalbe tyres, because they have made them so that they rely on simple friction in the bead wires, rather than bonding them into the tyre structure with rubber as they used to (should?) do. But if any tyre moves often, even slightly, you may not notice it happening but it offers another failure mode.

Personally I don't think that the pressures you propose are completely unreasonable but I'd be mindful of the above. If you have narrow rims and ride aggressively, I think that you will soon find that 52 psi in the front is nowhere near enough.

cheers
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Rhodrich
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Re: Minimum Tyre Pressure

Postby Rhodrich » 16 Jan 2015, 2:34pm

Interesting - thanks Brucey. I've certainly had that wibbly wobbly feeling in the past from too low a pressure when I've had a puncture. I guess you'd never want to go that low.

For what it's worth, I'm using rims with an inner rim width of 19mm (http://www.rosebikes.co.uk/article/xtre ... 8-atb-rim/), and the tyre width, measured with calipers is pretty much spot on at 32mm.

The Michelin pressure chart recommends 58psi (based on a 100lbs wheel load), which is also below Continental's minimum recommended pressure. I just don't really understand why Contintental would have a minimum inflation pressure that is above what other manufacturers recommend for such a tyre size. This is not a touring tyre, and would not expect to be used with especially heavy loads.

I'll reduce the pressures down to a compromise 55psi front, 65psi rear, and see how that feels. For me, comfort is much more important than speed, but safety is of course an overriding issue.

Bikefayre
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Re: Minimum Tyre Pressure

Postby Bikefayre » 16 Jan 2015, 3:06pm

Run mine at 4 BAR about 60 psi with no problem as have Bronx tyres which run at a max of 75 psi.

Brucey
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Re: Minimum Tyre Pressure

Postby Brucey » 16 Jan 2015, 5:56pm

Rhodrich wrote: I just don't really understand why Contintental would have a minimum inflation pressure that is above what other manufacturers recommend for such a tyre size...


possibly because (as I noted above) Contis often have slippier beads than other tyres, because of the external chafer that is seen in many of their designs. If so, they will need more pressure so as not to move around.

cheers
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531colin
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Re: Minimum Tyre Pressure

Postby 531colin » 16 Jan 2015, 8:30pm

Is there any logic in giving a minimum tyre pressure without some indication of the rider weight they were thinking of at the time?
Is the "minimum pressure" for an eight stone rider or an eighteen stone rider?
I'm 11 stone, so a bit lighter than the OP. At the moment, I'm running Conti winter 2 tyres, that actually measure 32mm on appropriate rims, although they are labelled 37mm from memory. I run them at 60psi front, and 70 rear, but that's on a bike that's deliberately rather lightly built by to-days standards. On some modern (stiff) bikes I can easily get into a bind where to get a bit of comfort I have to run the tyres so soft as to risk pinch flats and tyre creep cornering.
Purely by chance I am within the limits on that table from Conti, that I have never seen before to-day.....I have no idea whats written on the tyre, I haven't looked.
My advice would be to run whatever pressures work for you. Personally, I think Contis quoted pressure range of 70 to 87 psi is illogical....if the OP at just over 11 stone is in danger of breaking the tyre at less than 70 psi, then an 18 stone man needs much more than 87psi.

Rhodrich
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Re: Minimum Tyre Pressure

Postby Rhodrich » 17 Jan 2015, 8:15pm

Well before riding home last night, I let some air out of my tyres, and I have to say that the ride was fantastic. The road surface disappeared, and it was like riding on freshly laid super smooth tarmac, with no road buzz. Proper magic carpet feeling. I didn't feel any slower either.

All good, but when I checked my tyre pressures when I got home, the front was at 34 psi, and the rear at 40! Clearly too low to be safe from snakebite punctures, so I've had to pump them back up (to 55 and 65psi).

The more I think about it, the more I can see the point of a high volume, low pressure 'balloon' tyre. Such a shame that the largest tyres I can fit in my 1980's Galaxy frame are 32s. 35's may possibly go in there, but I'm seriously tempted to get a 650b Randonneur style bike, with 40mm or wider low pressure supple tyres.....

niggle
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Re: Minimum Tyre Pressure

Postby niggle » 19 Jan 2015, 4:19pm

On my touring bike with 35mm tyres (very supple and slick Vittoria Randonneur Hypers) with an all up weight of 85kg I use 38psi front, 58psi rear based on 40/60% weight distribution*. The front feels totally planted at speed downhill, even hard on the brakes, possibly the best handling tyres I have ever had. I have ridden off road a fair bit with these pressures on rocky tracks and never had a pinch puncture either.

*BTW how did you get to the 45/55% weight distribution figure?

reohn2
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Re: Minimum Tyre Pressure

Postby reohn2 » 19 Jan 2015, 8:33pm

niggle wrote:On my touring bike with 35mm tyres (very supple and slick Vittoria Randonneur Hypers) with an all up weight of 85kg I use 38psi front, 58psi rear based on 40/60% weight distribution*. The front feels totally planted at speed downhill, even hard on the brakes, possibly the best handling tyres I have ever had. I have ridden off road a fair bit with these pressures on rocky tracks and never had a pinch puncture either.

*BTW how did you get to the 45/55% weight distribution figure?


Ditto (with 7psi more in the tyres for my all up weight of 100kg) :D
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reohn2
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Re: Minimum Tyre Pressure

Postby reohn2 » 19 Jan 2015, 8:35pm

Rhodrich wrote:Well before riding home last night, I let some air out of my tyres, and I have to say that the ride was fantastic. The road surface disappeared, and it was like riding on freshly laid super smooth tarmac, with no road buzz. Proper magic carpet feeling. I didn't feel any slower either.

All good, but when I checked my tyre pressures when I got home, the front was at 34 psi, and the rear at 40! Clearly too low to be safe from snakebite punctures, so I've had to pump them back up (to 55 and 65psi).

The more I think about it, the more I can see the point of a high volume, low pressure 'balloon' tyre. Such a shame that the largest tyres I can fit in my 1980's Galaxy frame are 32s. 35's may possibly go in there, but I'm seriously tempted to get a 650b Randonneur style bike, with 40mm or wider low pressure supple tyres.....


Well there's always one of these:- http://salsacycles.com/bikes/vaya/2015_vaya_2/overview/ :D
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