Tyre Pressure - Berto's 15%

[reohn2]

I have had a snakebite puncture, but I hit a sharp pothole on a steep descent, the clank was sickening and I'm still rather surprised it only punctured the tube instead of at least wrecking the wheel

Potholes mosly avoidable those that aren't are either bunny hopped or under water

I knew nothing about it until after I hit it! I suspect it had been part-filled with chippings, which has become increasingly common since then.

You can't avoid what you cant see

[Samuel D]

I weigh only 66 kg and got a pinch flat a few months ago with a 23 mm rear tyre at 100 PSI. I know it was about 100 PSI because I pumped it up that morning.

That surprised me, and all the more because the hole that did it was not very sharp-edged (the edge was curved about like a Coke can). I hit it hard without warning when riding behind someone who jumped over it.

It made me worry about using 23 mm tyres far from home. A puncture is one thing but rim damage is another.

[MikeDee]

What happens if you run a "min 100psi" tyre too low?

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.

[Brucey]

both max and min ratings are there for a reason. Not BS.

cheers

[RickH]

both max and min ratings are there for a reason. Not BS.

cheers

And the reason for 38mm Voyager Hypers to have a minimum pressure of 60psi (90 max) is...?

A minimum of 60 on tyres that size seems baloney (a mistake or bottom covering) to me.

[simonhill]

Is the 15% thingy and associated pressures the same on small tyres.

I'm thinking my Brompton?

[kylecycler]

What happens if you run a "min 100psi" tyre too low?

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.

both max and min ratings are there for a reason. Not BS.

cheers

And the reason for 38mm Voyager Hypers to have a minimum pressure of 60psi (90 max) is...?

A minimum of 60 on tyres that size seems baloney (a mistake or bottom covering) to me.

There was an interview with Schwalbe's chief tyre engineer not so long ago (sorry, didn't bookmark it so can't post the link) where he sniffily remarked that the pressures on Schwalbe tyres' sidewalls were the work of his company's insurance department... In the context of the interview, the inference was that the minimum pressures on the sidewalls were unneccessarily high (maybe unless it's a seriously hefty rider with the bicycle heavily laden?).

What I find it hard to reconcile is that Berto's tyre pressures are 'static' - when you're just sitting on the bicycle, or at least 'JRA' - 'just riding along'. 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.

[MikeDee]

My 700x47 Schwalbe Marathons have a minimum pressure of 45 psi. I run the front at 34, according to Berto's recommendation. I see no detrimental effects on the tire. Thus I call BS on low pressure ratings.

[Mick F]

I weigh only 66 kg and got a pinch flat a few months ago with a 23 mm rear tyre at 100 PSI. I know it was about 100 PSI because I pumped it up that morning.

That surprised me, and all the more because the hole that did it was not very sharp-edged (the edge was curved about like a Coke can). I hit it hard without warning when riding behind someone who jumped over it.

It made me worry about using 23 mm tyres far from home. A puncture is one thing but rim damage is another.

I had a few pinch punctures running 20mm tyres, so changed to 23mm.

Never had one since ......................... and I swapped to 23mm in April 2009.
120psi rear and 90psi front.

[MikeDee]

What I find it hard to reconcile is that Berto's tyre pressures are 'static' - when you're just sitting on the bicycle, or at least 'JRA' - 'just riding along'. 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.

Jan Heine says the same thing, so does Brandt. However, I get few pinch flats and would rather have a better ride than run an over inflated front tire providing a jarring ride. On descending and braking, I found no detrimental effects of having the tire pressure on the front lower than the rear, following Berto's pressure recommendations.

[kylecycler]

Is the 15% thingy and associated pressures the same on small tyres.

I'm thinking my Brompton?

I suppose 'deformation' - the tyre becoming 'less round', which is what hurts rolling resistance - is greater the smaller the tyre - 15% drop might cause a small tyre to become more 'less round' than a large tyre, if that makes sense. Can't quite get my head around it, tbh, but it's a good question.

[reohn2]

both max and min ratings are there for a reason. Not BS.

cheers

And the reason for 38mm Voyager Hypers to have a minimum pressure of 60psi (90 max) is...?

A minimum of 60 on tyres that size seems baloney (a mistake or bottom covering) to me.

Quite right.

[reohn2]

What happens if you run a "min 100psi" tyre too low?

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.

[reohn2]

......... 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.

[reohn2]

Is the 15% thingy and associated pressures the same on small tyres.

I'm thinking my Brompton?

I suppose 'deformation' - the tyre becoming 'less round', which is what hurts rolling resistance - is greater the smaller the tyre - 15% drop might cause a small tyre to become more 'less round' than a large tyre, if that makes sense. Can't quite get my head around it, tbh, but it's a good question.

IMO you're right my Tern folder and Circe tandem run on 406x50mm Big Apples,both bikes need a fair bit more air in than a 700x50mm tyre would for the same load to get them rolling well.