Tyre Pressure - Berto's 15%

[MikeDee]

Nothing because minimum pressure ratings are BS, unlike maximum ratings, which are have some relevance related to blow off pressure. Most mountain bikers are running tire pressures in the 20-30 psi range, which violates most tire manufacturers' low pressure ratings, and they work fine.

You've misquoted me there Mike.

Sorry about that. Tapatalk isn't the best for editing a quote.

[reohn2]

Nothing because minimum pressure ratings are BS, unlike maximum ratings, which are have some relevance related to blow off pressure. Most mountain bikers are running tire pressures in the 20-30 psi range, which violates most tire manufacturers' low pressure ratings, and they work fine.

You've misquoted me there Mike.

Sorry about that. Tapatalk isn't the best for editing a quote.

I'll forgive you

[Brucey]

it may interest folk to know that a tyre (any tyre pretty much) only has its full load rating if it is also inflated to its maximum pressure rating. The consequence of this is that at the lowest recommended pressure, the load rating is somewhat reduced. If you run a tyre below its recommended minimum pressure, you need to be very careful with the loading. Decades of MTBing have lead me to expect rear tyres that are inflated to low pressures to suffer carcass failures prematurely. The same thing happens on loaded touring bikes, tandems etc, often at tyre pressures that don't (on the face of it) seem particularly low.

However that is not the only concern; at low pressures tyres tend to move around on the rims and this may tear the valve out of the tube; (offroad motorcycles have their tyres clamped to the rims to avoid this). If the tyre moves around on the rim then the bead itself may start to come off the rim and/or suffer a premature failure; it isn't designed to flex in service. If you go round corners 'enthusiastically' on tyres that don't have much pressure in them, you can have the tyre deform in unwanted ways that affect the steering and the grip in an adverse fashion. This varies with the width of the rim to which the tyre is fitted; narrow rims render the same tyre at the same pressure laterally less stable than on wider rims. I have a very low tolerance for front tyres that have low pressures inside them; the handling is in many cases ruddy dangerous. Running at low pressures can cause tyre carcasses to come apart prematurely too, through delamination.

Some tyres may blow off the rim (varying with the rim) at high pressure. There is also an issue with some tyres whereby the local loads on the carcass become excessive when the pressure is too high; in extremis running over a loose chipping may pose a serious risk of overstressing the carcass and initiating a failure. Tyre carcasses are typically designed so that if one cord fails, the tyre doesn't immediately unzip itself. However at very high pressures it may do exactly that.

So a scheme that only considers the vertical compliance of the tyre to determine the 'correct pressure' is bound to have limitations. These limitations will be different with different tyres and different rims. To blithely ignore the manufacturer's recommendations, assuming they are B.S. etc is something that shouldn't be done, not without due consideration, anyway; there are many factors that could be limiting in some way.

cheers

[MikeDee]

it may interest folk to know that a tyre (any tyre pretty much) only has its full load rating if it is also inflated to its maximum pressure rating. The consequence of this is that at the lowest recommended pressure, the load rating is somewhat reduced. If you run a tyre below its recommended minimum pressure, you need to be very careful with the loading. Decades of MTBing have lead me to expect rear tyres that are inflated to low pressures to suffer carcass failures prematurely. The same thing happens on loaded touring bikes, tandems etc, often at tyre pressures that don't (on the face of it) seem particularly low.

However that is not the only concern; at low pressures tyres tend to move around on the rims and this may tear the valve out of the tube; (offroad motorcycles have their tyres clamped to the rims to avoid this). If the tyre moves around on the rim then the bead itself may start to come off the rim and/or suffer a premature failure; it isn't designed to flex in service. If you go round corners 'enthusiastically' on tyres that don't have much pressure in them, you can have the tyre deform in unwanted ways that affect the steering and the grip in an adverse fashion. This varies with the width of the rim to which the tyre is fitted; narrow rims render the same tyre at the same pressure laterally less stable than on wider rims. I have a very low tolerance for front tyres that have low pressures inside them; the handling is in many cases ruddy dangerous. Running at low pressures can cause tyre carcasses to come apart prematurely too, through delamination.

Some tyres may blow off the rim (varying with the rim) at high pressure. There is also an issue with some tyres whereby the local loads on the carcass become excessive when the pressure is too high; in extremis running over a loose chipping may pose a serious risk of overstressing the carcass and initiating a failure. Tyre carcasses are typically designed so that if one cord fails, the tyre doesn't immediately unzip itself. However at very high pressures it may do exactly that.

So a scheme that only considers the vertical compliance of the tyre to determine the 'correct pressure' is bound to have limitations. These limitations will be different with different tyres and different rims. To blithely ignore the manufacturer's recommendations, assuming they are B.S. etc is something that shouldn't be done, not without due consideration, anyway; there are many factors that could be limiting in some way.

cheers

Berto's pressures are based on a 15% tire drop for a given tire width and the actual weight of the rider + bike on the tire. The 15% value came from the tire manufacturers themselves, so I don't know what you are going on about. I'd venture to say that if you follow Berto's recommendation, you will be under the manufacturer's minimum pressure recommendation for the front tire. I think if you follow Berto's pressure recommendations, you should be OK, and the manufacturers low pressure recommendations are too conservative.

I don't know any mountain biker that follows those low pressure recommendations. Everyone uses pressures under that.

Even at the high end, Continental says that their max pressure is half what it takes to blow the tire off a hook beaded rim. That's pretty conservative. Now you have moron rim makers (like Mavic even) offering hookless rims that flies in the face of all of that.

[Brucey]

part of the trouble is that under certain circumstances a nominally 'lightly loaded' front wheel actually has to bear the full weight of the bike, the rider and all the luggage. At which point it is deflecting far more than 15%. If you try tens tests to measure the pressure at which a tyre blows off various different rims, you will get ten different results, and those may not represent the point at which the tyre becomes unstable (and is in danger of parting company with the rim) in practice.

As I mentioned, MTB tyres fall apart on a regular basis, often because they are run at excessively low pressures.

Ignore the manufacturer's recommendations at your peril.

cheers

[kylecycler]

......... But under heavy braking, like on a fast descent or in an emergency stop, virtually all the weight is on the front tyre - 'active' as opposed to 'static' - and I don't get how Berto's front pressures, at least, would be sufficient in these circumstances.

Fast decents don't put all the weight on the front wheel they may even out the weight distribution at most,plus the bike's front wheel is he most important for keeping things black side down especially in wet weather and so needs more grip
Heavy braking is a rare occurance or at least it should be.
For the most part weight distribution is as per static measurement or a little either side.

I meant heavy braking on a fast descent, braking late and hard for corners being the fastest way down a mountainside (athough if you're not careful that could mean something else entirely! )

Makes you wonder, though, even if or when you do brake hard, if the front tyre is at a relatively low pressure - up to a point - would that help increase its contact patch and make it grip better? It wouldn't help you to turn, but you shouldn't be braking hard when turning, or at least you shouldn't still be slowing down.

Suppose it's a case of finding the balance, as ever - too low a pressure could cause the tyre to 'roll' on the rim when cornering and that's dangerous.

One could argue, of course, that if any of that ^ matters, we're not riding responsibly.

[fausto99]

There could be other reasons for that but you've far too much air in those tyres which would make them slide out PDQ and without warning,eg; 700x50's(measuring 47mm actual) I'm riding presently with a 100kg all up load and a 40%f 60%r distribution 25psi front 48psi rear stable fast(mid teens on a flat road with an old unfit engine) and very grippy off road on stoney tracks.
Continental Contact arent a particularly supple tyre either so need even less air in them to support the load than the Marathon Supremes I mention.

Well I rode in the dry today with the front at 25psi and the rear at 30psi. Seemed OK so far re grip, comfort and rolling resistance. It was a bit disconcerting to be able to feel so much give when squeezing the tyre sidewalls . I thought I detected a bit of squirming in slow manoeuvres sometimes. I'll keep an eye on the valve stems to see if there is any creeping with use.

Have to disagree about Continental supplesse vs Marathon. I've always found Schwalbe tyres very stiff and agricultural in most incarnations.

[reohn2]

I meant heavy braking on a fast descent, braking late and hard for corners being the fastest way down a mountainside (athough if you're not careful that could mean something else entirely! )

Makes you wonder, though, even if or when you do brake hard, if the front tyre is at a relatively low pressure - up to a point - would that help increase its contact patch and make it grip better? It wouldn't help you to turn, but you shouldn't be braking hard when turning, or at least you shouldn't still be slowing down.

Suppose it's a case of finding the balance, as ever - too low a pressure could cause the tyre to 'roll' on the rim when cornering and that's dangerous.

One could argue, of course, that if any of that ^ matters, we're not riding responsibly.

I couldn't have put it better myself .
FWIW,since dispensing with narrow high pressure tyres to big 40mm supple tyres at much lower tyre pressures I never descended as quick ,more so on bad bumpy unecen tarmac.

[reohn2]

There could be other reasons for that but you've far too much air in those tyres which would make them slide out PDQ and without warning,eg; 700x50's(measuring 47mm actual) I'm riding presently with a 100kg all up load and a 40%f 60%r distribution 25psi front 48psi rear stable fast(mid teens on a flat road with an old unfit engine) and very grippy off road on stoney tracks.
Continental Contact arent a particularly supple tyre either so need even less air in them to support the load than the Marathon Supremes I mention.

Well I rode in the dry today with the front at 25psi and the rear at 30psi. Seemed OK so far re grip, comfort and rolling resistance. It was a bit disconcerting to be able to feel so much give when squeezing the tyre sidewalls . I thought I detected a bit of squirming in slow manoeuvres sometimes. I'll keep an eye on the valve stems to see if there is any creeping with use.

You could try 5psi extra and see how it feels.

Have to disagree about Continental supplesse vs Marathon. I've always found Schwalbe tyres very stiff and agricultural in most incarnations.

Marathon Supremes are a very supple slick tyre,nothing like any other Marathon in the Schwalbe touring range.

[MikeDee]

part of the trouble is that under certain circumstances a nominally 'lightly loaded' front wheel actually has to bear the full weight of the bike, the rider and all the luggage. At which point it is deflecting far more than 15%. If you try tens tests to measure the pressure at which a tyre blows off various different rims, you will get ten different results, and those may not represent the point at which the tyre becomes unstable (and is in danger of parting company with the rim) in practice.

As I mentioned, MTB tyres fall apart on a regular basis, often because they are run at excessively low pressures.

Ignore the manufacturer's recommendations at your peril.

cheers

In all of this discussion, you haven't described how a tire maker arrives at their low pressure rating. An earlier post basically said it's a WAG, that being from a Schwalbe engineer.

So what if the tire defects more than 15%? You might get a pinch flat if the tire bottoms against the rim. That's a rare event for me, so I must be doing something right. Casing damage? Not seen it. If I follow your recommendation, I should pump my tires to the maximum to guard against pinch flatting and any and all foreseeable off normal occurrances. We used to do that, but nobody does that anymore. People realized that comfort, handling, and traction were improved at pressures less than maximum rating, and rolling resistance did not suffer due to the suspension effect. What about heat related blowoffs? Max pressure doesn't help that.

We are not talking about running stupid low pressures like some people (mountain bikers in particular) do. My mountain bike tires are rated at 30 psi minimum. I'm running 25/28. I see no degradation of the casing at these pressures, and I've worn out a number of tires. There's no evidence of tire rotation WRT the rim.

Many tires don't even have a low pressure rating.

Just my experience. If you have further info as to how the tire manufacturers establish their low pressure ratings, I'd be interested to hear it.

[Brucey]

So what if the tire defects more than 15%? ......

it inevitably will do this (almost regardless of the pressure used) but the more often it deflects, the greater the chances are of the tyre failing

Casing damage? Not seen it.....

so what? (*)

....If I follow your recommendation...

er, which 'recommendation' was that? You have inferred things into what I have said that are simply not there.

My advice -in case you missed it- is that you do not blithely assume that the tyre makers are writing things on their tyres that are "just B.S". I have explained why already. Have another read of it.

(*) what carcass failure rate would be 'acceptable'? One in ten thousand? Or one in ten? Would you expect to see it? IME if you run your tyres at very low pressures (and actually ride your bike very far/hard) then carcass failure is made very much more likely.

Almost every rear tyre I have used (at low pressures) for more than a few hundred miles on my MTBs has either shown clear signs of carcass damage (and has had to be retired because of it) or has actually failed. I've also seen many of the other issues I have described previously when riding in groups.

Tyres are in any event one of the parts of any vehicle that require a lot of routine maintenance and can be a source of unreliability; running them outside of the manufacturer's recommendations (concerning load, speed, pressure, rim fitment) is a good way of making them less reliable than they otherwise might be.

cheers

[reohn2]

.

Tyres are in any event one of the parts of any vehicle that require a lot of routine maintenance and can be a source of unreliability; running them outside of the manufacturer's recommendations (concerning load, speed, pressure, rim fitment) is a good way of making them less reliable than they otherwise might be.

cheers

I must be doing something right then,I've never had a Hyper run at low pressure(40 or 45 front,60 or 65 rear) fail and I'm in the tens of thousands of miles area with them,in fact I've only wornout out two in that maileage.
My 29er x2.4inch(700 x 65) Conti X Kings I run at 15 front 25 rear and will go as low a 10 or 12 in the front is it's really rough,TBF I've only done a couple of thousn and miles on those but never had a problem though it has to be said I'm no tearaway.
It's way to early to tell yet as it's new but the new Vagabond on 700 x 50 Supremes I'm riding at 25 f 45r.
All three bikes roll well,handle great and I rarely puncture(never had a snakebite puncture),with no carcass problems.
I'll trot it out again,IMO many riders,especially roadies,ride with way to much air in their tyres.

[meic]

I'll trot it out again,IMO many riders,especially roadies,ride with way to much air in their tyres.

They (I) wouldnt agree because we are making our choice on different criteria to you.

On the other hand, to show the power of anecdotal science over statistical science, I have had plenty of tyre failures with my well inflated tyres.

[Cugel]

.

Tyres are in any event one of the parts of any vehicle that require a lot of routine maintenance and can be a source of unreliability; running them outside of the manufacturer's recommendations (concerning load, speed, pressure, rim fitment) is a good way of making them less reliable than they otherwise might be.

cheers

I must be doing something right then,I've never had a Hyper run at low pressure(40 or 45 front,60 or 65 rear) fail and I'm in the tens of thousands of miles area with them,in fact I've only wornout out two in that maileage.
My 29er x2.4inch(700 x 65) Conti X Kings I run at 15 front 25 rear and will go as low a 10 or 12 in the front is it's really rough,TBF I've only done a couple of thousn and miles on those but never had a problem though it has to be said I'm no tearaway.
It's way to early to tell yet as it's new but the new Vagabond on 700 x 50 Supremes I'm riding at 25 f 45r.
All three bikes roll well,handle great and I rarely puncture(never had a snakebite puncture),with no carcass problems.
I'll trot it out again,IMO many riders,especially roadies,ride with way to much air in their tyres.

My experience echoes yours. I ride mostly 28-30mm tyres, both tubed and tubeless, at around 70-80psi. Typically they're at 80 or a little under when I inflate them and go down to 70 or just above by the time I get 'round to putting the lost air back, usually 10 - 14 days between inflations. I put the same in the front as the back, these days, as braking down descents does momentarily put weight forward, so that should determine the tyre pressure rather than normal running.

The only time I had a snakebite was when I was experimenting with pressures and had the front at 65psi when it hit the 90 degree edge of a flooded channel cut in the road by builders laying pipe to a new housing estate. They'd just filled the channel with gravel, which sank below the cut edges in the road. Yet I ride over some seriously scabby roads and do hit the odd potential snakebiter from time to time. Just 70+psi under my 13st 0llbs + bike seems to be snakebite proof.

I'm also good at getting loadsa miles out of tyres. I don't really know why I seem to get more miles than others, although I never do skids as some intemperate lads do now and then. I have a feeling that part of the rapid tyre wear cause is over-inflation, as the tyre is then riding on a smaller patch. If it's also 23mm rather than 28mm this seems to wear the central part of the tyre very rapidly, especially on the driving back tyre, as the contact patch is long and thin.

I'm careful to examine the sidewalls for damage or deterioration. I've never had any, despite some Schwalbe Ultremos having been on a wheelset for 7 years and done quite a few thousand miles. Perhaps tyres made with more supple sidewalls not only ride better and more efficiently but also flex without causing the sidewalls to crack or split, since this is what they're designed to do with their high thread count et al?

Cugel

[reohn2]

My experience echoes yours. I ride mostly 28-30mm tyres, both tubed and tubeless, at around 70-80psi. Typically they're at 80 or a little under when I inflate them and go down to 70 or just above by the time I get 'round to putting the lost air back, usually 10 - 14 days between inflations. I put the same in the front as the back, these days, as braking down descents does momentarily put weight forward, so that should determine the tyre pressure rather than normal running.

The only time I had a snakebite was when I was experimenting with pressures and had the front at 65psi when it hit the 90 degree edge of a flooded channel cut in the road by builders laying pipe to a new housing estate. They'd just filled the channel with gravel, which sank below the cut edges in the road. Yet I ride over some seriously scabby roads and do hit the odd potential snakebiter from time to time. Just 70+psi under my 13st 0llbs + bike seems to be snakebite proof.

I'm also good at getting loadsa miles out of tyres. I don't really know why I seem to get more miles than others, although I never do skids as some intemperate lads do now and then. I have a feeling that part of the rapid tyre wear cause is over-inflation, as the tyre is then riding on a smaller patch. If it's also 23mm rather than 28mm this seems to wear the central part of the tyre very rapidly, especially on the driving back tyre, as the contact patch is long and thin.

I'm careful to examine the sidewalls for damage or deterioration. I've never had any, despite some Schwalbe Ultremos having been on a wheelset for 7 years and done quite a few thousand miles. Perhaps tyres made with more supple sidewalls not only ride better and more efficiently but also flex without causing the sidewalls to crack or split, since this is what they're designed to do with their high thread count et al?

Cugel

I couldnt agree more and I think you're spot on about less air in the tyre in relation to the contact patch making it last longer.
High thread count is as you say a sidewall saver too.

What surprises me is how many time looking around £2k bikes outside a cafe is how so many ride cheap tyres