Tyre Pressure - Berto's 15%

[fausto99]

I been reading about the 15% deflection recommendation and using bathroom scales to determine the front back distribution of your body + bike total weight.
This is probably fine in theory, but I suspect that the front/back weight distribution goes out of the window when you hit a deep pothole and that's when you're most likely to get snakebite punctures. Would it not be more prudent to assume that 90% of the total weight may have to be taken by just one wheel/tyre and up the pressure accordingly?

[The utility cyclist]

Nope, you might as well use a solid rubber tyre with virtually no cushioning whatsoever and it would be incredibly uncomfortable on all but the smoothest of surfaces (as in a track) and probably horrible to corner/make sharp turns on on an ordinary road.
I'm not of the belief that lower pressures are faster in most scenarios but using your method would give you a ridiculously high tyre pressure and one that most if not all tyres would not be recommended as the maximum for use (even with a lighter rider) never mind being utterly impractical.

For instance, myself on my CF racing bike, I normally use 27mm rear/25mm front (tubs) this works out roughly using the Berto calculation (there's an excel spreadsheet kicking around on the net to adjust as you want) to 195psi for the front with 90% of the weight (235lb total). I would normally be around 105/95.
For a heavier bike and with wider tyres, let's say 255lb and 32mm tyres this works out to 141 psi for the front, I have some decent 32mm tyres and the highest recommended they are marked at is 100psi, sure you can exceed that by a bit as manufacturers are somewhat understandably cautious, but according to the 55/45 ratio I would need a total all up weight of 300lb to need that 100psi and just on the rear.
I've never had a pinch flat, EVER, hit a pothole with a 195psi inflated tyre even if you could get it to that number and that's simply a recipe for disaster

[Brucey]

I been reading about the 15% deflection recommendation and using bathroom scales to determine the front back distribution of your body + bike total weight.
This is probably fine in theory, but I suspect that the front/back weight distribution goes out of the window when you hit a deep pothole and that's when you're most likely to get snakebite punctures. Would it not be more prudent to assume that 90% of the total weight may have to be taken by just one wheel/tyre and up the pressure accordingly?

In resisting snakebites you will get one if the tyre cannot deflect or deform so that the load is taken by enough area to support the instantaneous load (which can be, as you say, briefly much higher than normal).

To resist snakebites you need some combination of

- a smaller load (even unweighting the bars and saddle can help here, i.e. don't sit in the saddle like a sack of spuds)
- springier frame/fork/wheel
- higher tyre pressure
- less abrupt edge to the pothole
- fatter tyre

Your suggested approach would presumably make the tyre pressure about x2 which would indeed be far too high.

cheers

[fausto99]

I'm not suggesting anything - just trying to understand what people have written.

Looking at this http://road.cc/content/feature/180830-h ... t-and-grip and assuming 75kg rider+bike weight using 23mm tyres and a 45/55% front/back distribution I see recommended pressures of just above and below 5bar. Am I interpreting the chart correctly? Does this not seem a tad low for avoiding snakebites on current UK potholed roads?

[Brucey]

I'd go a bit more than that, but then I'm a bit heavier and most 23mm tyres are not the size they say they are either....

I'd take any such chart with a pinch of salt, or as a starting point.

Note that the tyre pressures will probably be a bit lower than you set them, by the time you get round to pumping them up again, too.

cheers

[thirdcrank]

I'm not suggesting anything - just trying to understand what people have written. ...

Your link suggests to me that this is not what might be called a mature area of knowledge. Apart from a certain amount of opinion rather than reference to known physical rules, there are also some big variables with regard to things like comfort.

I'm a bit bemused by the concern over snake bite punctures. There must come a point where if you achieve - by whatever means - total invincibility to innertube damage then there's a risk that hitting a kerb or a bad pothole will either damage something else and/or dump you on the floor.

I noticed that when I followed your link there was a feature about tyre pressure gauges. Advertorial?

[MikeDee]

I been reading about the 15% deflection recommendation and using bathroom scales to determine the front back distribution of your body + bike total weight.
This is probably fine in theory, but I suspect that the front/back weight distribution goes out of the window when you hit a deep pothole and that's when you're most likely to get snakebite punctures. Would it not be more prudent to assume that 90% of the total weight may have to be taken by just one wheel/tyre and up the pressure accordingly?

No because in your paranoia about getting a pinch flat (which in my case, is a rare occurrence) you are riding around with over inflated tires which give a jarring ride.

[Cugel]

Theories are useful, even concerning tyre pressures for bicycles. However, they only provide a starting point from which to derive suitable pressures for your riding, via your experience.

Having read all sorts of stuff about tyre pressures for bikes, over the past 2 or 3 years, I made experiment with mine. First was the acquisition of fatter tyres (28mm) prompted by the various mass media recommendations but mostly by my experience of riding a new Cyclocross bike (on the road) that came with 32mm tyres. What a difference to my old racing bike with 22mm tyres! Comfortable, grippy and surprisingly fast.

The 28mms (Schwalbe Ultremos) seemed to roll very fast, be very sticky in the bends and also comfortable, even at 90psi (I'm 80Kg). Over time I reduced the pressures to the Berto theoretical numbers and below. I now have 75psi in the front and 80psi in the back. At least I do after the track pump has just been in use. The pressures probably deflate 5-8psi before I next apply the pump.

I had one snakebite during all this experimental experience - but the front tyre was at 65psi at the time - the lowest I tried. It was bitten by a hard 90 degree edge of a circular sawn channel across a road, made for a cable but hidden by a big puddle. I ride roads in northern England that are as potholed and scabby as you've seen anywhere, yet I never seem to come near a rim-hit or snakebite. Of course, I do look where I'm going.

I find my rides are comfortable, very grippy and at least as "fast" for the same effort or gravity-effects as those 100psi 23mm tyres on the bikes of my club mates. A bit faster than most, in fact. And my nether isn't battered after the 80 milers.

Cugel

[fausto99]

Update:
I've been mostly riding my "new" winter bike with 26" hub brake wheels. Tyres are 47 x 599 Continental Contact pumpped up to 3bar/45psi. I've been very happy with the grip, comfort and downhill speed. All was well until last week when I came off on a corner. It was not raining. The surface was not icy, no gravel, no wet leaves, no potholes. I was not going fast nor, according to the motorist behind me who kindly stopped to inquire if I needed help, leaning very much. The surface was good but just a bit wet. The instant I started the turn, my front wheel skidded in the other direction and I went down. I got away with some cuts and bruises but nothing broken but am now wondering are my tyre pressures too high? I'm about 65kg, the bike is maybe another 15kg. Is 45psi too high. Would lowering to 25-30 give more grip?

[RickH]

Update:
I've been mostly riding my "new" winter bike with 26" hub brake wheels. Tyres are 47 x 599 Continental Contact pumpped up to 3bar/45psi. I've been very happy with the grip, comfort and downhill speed. All was well until last week when I came off on a corner. It was not raining. The surface was not icy, no gravel, no wet leaves, no potholes. I was not going fast nor, according to the motorist behind me who kindly stopped to inquire if I needed help, leaning very much. The surface was good but just a bit wet. The instant I started the turn, my front wheel skidded in the other direction and I went down. I got away with some cuts and bruises but nothing broken but am now wondering are my tyre pressures too high? I'm about 65kg, the bike is maybe another 15kg. Is 45psi too high. Would lowering to 25-30 give more grip?

I'm around 15kg heavier than you & have never run 47s at more than 30psi. I have ~40s on my main bike & only run those at 35F/40R which is probably a little high but allows for the fact I don't always check the pressure very regularly. If I hit something bigger than usual once they've drifted down to around 30F/35R that should still be a "safe" pressure for me.

So, yes, I reckon 25-30psi should be enough for you.

[reohn2]

I'm not suggesting anything - just trying to understand what people have written.

Looking at this http://road.cc/content/feature/180830-h ... t-and-grip and assuming 75kg rider+bike weight using 23mm tyres and a 45/55% front/back distribution I see recommended pressures of just above and below 5bar. Am I interpreting the chart correctly? Does this not seem a tad low for avoiding snakebites on current UK potholed roads?

For those figures the chart says around 7bar rear and 5bar front
EDIT,FWIW,I'm a low tyre pressure freak and believe most roadies are riding with way to much air in their tyres in the belief that it makes them faster,it doesnt if they have the correct tyre pressures for load.Ive ridden tyres from 23mm to 65mm and have never ever experienced a snakebite puncture and I ride on some really bad tarmac,bridleways,towpaths and singletrack.

[reohn2]

Update:
I've been mostly riding my "new" winter bike with 26" hub brake wheels. Tyres are 47 x 599 Continental Contact pumpped up to 3bar/45psi. I've been very happy with the grip, comfort and downhill speed.

All was well until last week when I came off on a corner

. It was not raining. The surface was not icy, no gravel, no wet leaves, no potholes. I was not going fast nor, according to the motorist behind me who kindly stopped to inquire if I needed help, leaning very much. The surface was good but just a bit wet. The instant I started the turn, my front wheel skidded in the other direction and I went down. I got away with some cuts and bruises but nothing broken but am now wondering are my tyre pressures too high? I'm about 65kg, the bike is maybe another 15kg. Is 45psi too high. Would lowering to 25-30 give more grip?

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.

[mjr]

EDIT,FWIW,I'm a low tyre pressure freak and believe most roadies are riding with way to much air in their tyres in the belief that it makes them faster,it doesnt if they have the correct tyre pressures for load.Ive ridden tyres from 23mm to 65mm and have never ever experienced a snakebite puncture and I ride on some really bad tarmac,bridleways,towpaths and singletrack.

I'd agree with that except that some road tyres have pretty high minimum pressures printed on their sidewalls and people almost never go below the minimum - and I'm not sure whether they should or not. What happens if you run a "min 100psi" tyre too low?

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!

[reohn2]

EDIT,FWIW,I'm a low tyre pressure freak and believe most roadies are riding with way to much air in their tyres in the belief that it makes them faster,it doesnt if they have the correct tyre pressures for load.Ive ridden tyres from 23mm to 65mm and have never ever experienced a snakebite puncture and I ride on some really bad tarmac,bridleways,towpaths and singletrack.

I'd agree with that except that some road tyres have pretty high minimum pressures printed on their sidewalls and people almost never go below the minimum - and I'm not sure whether they should or not. What happens if you run a "min 100psi" tyre too low?

Tyre pressures all depend on load per wheel,minimum pressures on the sidewall are to over all eventualities ie; fat lads .

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

[mjr]

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.