Safe Speed Forums

I have two bikes - both by Specialized. My main bike is a Globe Pro Hybrid. I have modified it slightly by replacing the 700x42c tyres with 38c tyres. The 42c tyres could take 85psi but the 38c tyres can be run at 100psi. No problem there - I've had a few punctures, but nothing serious.

The other bike is a Sirrus Carbon Ltd. flat barred road bike. It has 700x28c tyres which are supposed to run at 115-120psi.

I've had a few problems with this Sirrus - not punctures but inner tube failures - always on the rear tyre. I was riding along when suddenly - BANG! I thought I'd been shot, until I noticed my rear tyre about to fall off the rim. The inner tube was blown to ribbons for about one foot of its length. I replaced it and was fine for a while, but owing to a sticking valve (I hate Presta valves!) I decided to replace the tube. I was inflating the tyre with the wheel off the bike when same thing - BANG! - another tube blown. I replaced that and inflated it to 120psi. Out on the road, I'd only gone a mile when BANG! - you know the rest.

I'd really like to know what's causing these tube failures. I'm tempted to believe that my stirrup pump gauge underreads, and that I'm overinflating the tyre. Another thing is that although I'm slim & trim compared to what I was before I started cycling 100 miles a week, I'm still around 15st. Given that it's always the rear tyre tube that fails, could it be that my weight is compressing the tyre to such an extent that its pressure is pushed up beyond limits?

I've started inflating the tyres on the Sirrus to 100psi, and so far so good. I'm just about to try 105psi this afternoon, so if I'm late back you'll know why!

On the same topic, what sort of portable pumps are available that can reach pressures around the 120psi mark? I was out with a friend last year who has the same Globe Pro as I do, but with the original 42c tyres, and he had a puncture. The portable pump he had could not come anywhere near the 85psi he wanted, but could only reach about 40psi. For a road bike, this would not be much use at all.

i go to 100-110 psi on my clinchers but ive not heard of anything above that unless it's a tub.

Interesting problem DM, the only time I've experienced this kind of regular failure of innertubes is when I've had a very small cut in the tyre itself. The cut wasn't visible when the tyre was deflated, but when I put 80+ psi into it I could see the inner tube pushing through the tyre, after a couple of minutes of riding on it it would burst with a the typical gunshot sound!

I don't think 15st is excessive for those tyres (or any kind of cycling really!) so I would think it highly unlikely that your weight is causing the punctures.

The only other thing I can think of is that the tyre isn't sitting properly on the rim and possibly pinching the inner tube. But if you've replaced the inner tube several times, I can't imagine this fault repeating itself.

Try inflating it to 80psi and check the tyre for any faults, if you can't see any try another 10psi, and so on. I doubt whether the problem is caused by over inflation, most tyres are designed to cope with pressures quite a bit higher than they recommend.



I use a Specialized Road Air on my road bike, it gets upto 100psi, not really high enough for 700c x 23 tyres, but enough to get home with:

http://www.cyclestore.co.uk/productDeta ... uctID=6473

Hope that helps!

Hi Peyote!

Yes that was a help. Certainly the first couple of times changing the inner tube on this wheel, I was a bit of a klutz - the first time, the tube appeared between tyre and rim, and I was unable to deflate it quickly enough to avoid the explosion.

But I've been very careful, pinching all round the tyre at low pressure to make sure the tube is not trapped. I rode 30 miles on the Sirrus today, without incident.

I've just ordered a CO2 pump from Wiggle - one like this. It can easily inflate a road bike tyre to 120psi, if the website description is to be believed. Once I've got this, I'll be able to go out on the road and feel more confident at experimenting with higher pressures again.

I run with 120 on front and 130 on the back, no real problems and I do 20 miles a day, I'm also quite heavy (very tall and big boned).

I did have a tube break down at the seam opposite the valve other week, and air escaped throught the striations, but I think this was more to do with a manufacturing fault than the pressure I run it at.

I've always rolled my wheel with tyre on along the ground putting pressure on it before I pump it up to full pressure, don't know if this helps but it feels like it does ensure the beads are seated properly.

Ignoring the spam - wow I never realised you could put your tyre pressues that high - I was inflating mine to about 40-50. What's the right PSI for a generic mountain bike?

For fat nobbly type tyres: 40-50 is about right; any less risks 'snake eye' punctures when off-roading keenly or running over potholes. ~60 is generally the upper limit. You can afford lower pressures on wheels that are suspended.

The spam has been removed.

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Views expressed are personal opinions and are not necessarily shared by the Safe Speed campaign

Carefully check the inside of the tyre and the rim. I very much doubt you would blow a tube by simply over inflating it, there will be something causing the failures.

It usually tells you on the side of the tire.

40-50 is usual for off road work, 50 to 60 if you are doing most of your riding on road. The higher the pressure the easier it will be to pedal but the harder the ride.

Road bikes have narrower tires so have less square inches to carry the same weight of rider so need more pounds per square inch.

Which brings me to something I often wonder, what is 60 pounds a square inch travelling at 20mph going to do to dog flesh?

Hello Guys,

The principle is that a well inflated tyre has less area in contact with the ground, and hence less friction - so you will go faster. There is usually a slightly less comfortable ride with a highly inflated tyre but that is a price we are happy to pay!

Thanks,

(edited by PeterE as moderator to remove spam link: user also deleted)

As I understand it the main part of the rolling resistance actually comes from the effort required to flex the rubber at the point where it contacts the road surface (and having to bend it back afterwards).

That effort, of course, is proportional to the area of the contact patch

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

That's interesting weepej. I don't know if you have ever had a motorbike but I've had many and also had the experience of trying to push one when a tyre is low. It's like pushing it through mud! So you are 'right on the money' there bud. (Americanism).

I've recently bought a bike, a Trek , where for the first time in my cycling life the recommended pressure was a 100+ psi I'm with JtB, in that I've never had to go to those extremes before..

Even when I had road tyres for my new 1999 Stumpjumper the max, from memory, was ~50 PSI. Which brings me onto my experience of explosions DM..

I had the front tyre blow out on me because I pumped it to 60 PSI, (trying to get an edge/advantage ), but luckily I was going so slowly at the time it blew I was able to hold on to it and come to a safe halt - just! It was, as you said DM, like an explosion. I honestly thought someone had fired a shotgun or something!

I know this isn't helpful but thank god motorbike tyres aren't as flaky!!!

I have just bought a new, compact, pump which wasn’t cheap and if the gauge is representative of what it can actually do I would recommend it; but I’ll have to get back on that...

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The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

Curious, Tone. I have always been accustomed to blowing racing bike tyres up to about 150 psi. But that is for tubulars not wired on.

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown

Confession time.. I don't know what you mean by "wired on" dcb.

I've had racing bikes, albeit a long time ago, and I don't remember pumping them up into three figures. The last racing bike I had was something I bought second hand in circa 2000 and was an old British Raleigh and I don't remember pumping them up to the pressure of the vein in my head which is going to one day kill me if cycling doesn’t get me first! (I'm such a drama queen )

But this is about DM's question and I am trying to help. When I put a tyre on a rim I use talcum powder on the inner tube which helps stop the resistance of inner tube against tyre as they find common ground. In the old days, before motorbike tyres were fitted and you had to do them yourself with metal levers, blood, sweat and tears, it was common practice to use a solution of washing-up-liquid on the bead too, so that as you pumped them up it would seat itself properly on the rim and the tyre ran 'true'.

In fact what I used to do is pump it up WAY over pressure to seat it properly, simulating real road usage/pressure, and then let it down to normal.

Hope this is useful?

_________________
The views expressed in this post are personal opinions and do not necessarily represent the views of Safe Speed.
You will be branded a threat to society by going over a speed limit where it is safe to do so, and suffer the consequences of your actions in a way criminals do not, more so than someone who is a real threat to our society.

I think he's referring to the now ubiquitous clincher, i.e. tyres with a bead (often wire) that grip the inside of the rim as you pump up the tube in them, as opposed to tubular tyres.

http://www.sheldonbrown.com/tires.html

That is so. Clincher is an American word which I can't recall hearing in this country before the mid eighties. Modern racing tyres use kevlar rather than metal wires. For some reason, which I never really understood, you could pump tubulars to a higher pressure than clinchers without them loosing traction over rough bits.

As for tyre pressures this, from the CTC, is useful http://dl.dropbox.com/u/7839435/Recommended%20pressures.doc

From the chart you will see that a 60Kg rider on 18mm tyres is looking at 130psi minimum on his rear tyre

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When I see an adult on a bicycle, I do not despair for the future of the human race. H.G. Wells
When I see a youth in a motor car I do d.c.brown