Tyre pressures

[Jakob]

Mike's reference of minimising rolling resistance under hard braking

Is this not a rather daft thing to want to do?
If you are braking hard, extra rolling resistance is surely an advantage?

If you are braking hard, the influence of rolling resistance is minuscule and irrelevant.

[Morat]

My thumbs are very innacurate
I spent a couple of years puffing around at the back until I finally bought a track pump and realised I'd been using MTB slicks at 30PSI on the road. 60PSI made everything a lot easier!
Then after I'd read some threads on here I dropped the 28mm tyres on my CX bike to 80/90 instead of MAX PRESHARRR! and started to enjoy riding it instead of enduring it.

Some of my MAMILy mates are still riding 23mm tyres at 130PSI on truly agricultural roads. I'd tell them but I wouldn't want to come accross all preachy, would I?

[Highlander]

Mike's reference of minimising rolling resistance under hard braking

Is this not a rather daft thing to want to do?
If you are braking hard, extra rolling resistance is surely an advantage?

If you are braking hard, the influence of rolling resistance is minuscule and irrelevant.

This is one of those things I wish I understood better. 

I can sort of see how rolling resistance per se might be irrelevant, but surely a (non suspension) bicycle tyre's ability to grip in the corners and under braking is strongly related to how supple it is, which is related to rolling resistance.  The typical road is a rough bumpy surface, so it's not just about friction is it? 

The reason why the pros generally still use tubs is they don't just roll better, they grip in the corners better (than clinchers). 

I can see a certain logic now in running the front tyre higher then the rear if it helps to maintain the tyre characteristics when braking hard, and on steep corners, ie where a significant increase in the front axle weight might be experienced.  Whether it's sufficient justification considering most of the time the front tyre would be running inefficiently, I doubt.

     

[Nelson Longflap]

So I invested in a reliable gauge, did a bit of trial and error, and arrived at pressures that seem to work well for each bike.

An example to us all ... however results are bound to be bike specific (a bit) and especially rider specific (especially rider weight). Hopefully we might discover some general principles (a formula) that can be applied to any particular bike and rider.

In the meantime I'll stick to what works for me with f:r pressures in the ratio 40:60. I run my 50mm Big Apples much softer than my 28mm Schwalbe Ones, but the f:r pressure ratio is the same.

This thread has introduced me to a new worry of attempting to corner under maximum front braking. That sounds like something likely to end badly regardless of tyre pressures; I'll try to stay a bit further inside the safety envelope with the rest of the chicken astute riders.

[Kim]

So I invested in a reliable gauge, did a bit of trial and error, and arrived at pressures that seem to work well for each bike.

An example to us all ... however results are bound to be bike specific (a bit) and especially rider specific (especially rider weight). Hopefully we might discover some general principles (a formula) that can be applied to any particular bike and rider.

I think what I'm hinting at is that it's really not very critical.  High enough to avoid pinch flats with the given load (as you say, bike, rider and luggage specific).  Low enough to reduce vibration and skitteryness on loose stuff to an acceptable level.  Preferably low enough that nothing explodes, too.  Within that, anything's basically fine.

The trick is optimising for rolling resistance, which is a can of worms that's hard for a cyclist without lab equipment to be empirical about.

[Ningishzidda]

Those were the days.
Criterium racing round tight tarmac circuits on tubs at 150 psi.
Stuck to the road like sh*t to a blanket.
Always used tub glue, not tape.

Nowadays, I timetrial on 20mm clincher slicks at 145 psi.
Use the same tyres on my newer road bike.

[Samuel D]

Highlander: I didn’t mean to suggest that Jobst Brandt recommended a higher pressure at the front than the rear, just a higher front pressure than might be expected from its typical load (which is much less than the rear).

Racers almost universally run higher pressures at the front than the back per unit of static load.

For example, the link andrew_s gave says Cancellara used 81 PSI at the front and 88 PSI at the rear, which is clearly not in proportion to the static load on the tyres.

Why might that be? Of course no-one cares about rolling resistance when braking, but they might care about avoiding pinch flats on rough surfaces. Or avoiding the tyre rolling off the rim when cornering hard.

I used Heine and Berto’s chart directly for a while but ultimately ended up preferring slightly higher pressure at the front. There is otherwise a steering vagueness I just don’t like, plus I don’t like seeing huge tyre deflection every time I gingerly roll over a bump (thankfully I can’t easily see what the rear is doing). I didn’t actually get a front pinch flat at Berto’s recommended pressure, but it felt like I had to baby the front tyre a bit too much.

Michelin has a chart somewhere that makes no distinction between front and rear pressure.

Kim’s comment about pressure gauges brings up another concern. What’s a trustworthy gauge? Maybe something like the Wonder Eurodainu?

[Ningishzidda]

Highlander: I didn’t mean to suggest that Jobst Brandt recommended a higher pressure at the front than the rear, just a higher front pressure than might be expected from its typical load (which is much less than the rear).

Racers almost universally run higher pressures at the front than the back per unit of static load.

For example, the link andrew_s gave says Cancellara used 81 PSI at the front and 88 PSI at the rear, which is clearly not in proportion to the static load on the tyres.

Why might that be? Of course no-one cares about rolling resistance when braking, but they might care about avoiding pinch flats on rough surfaces. Or avoiding the tyre rolling off the rim when cornering hard.

I used Heine and Berto’s chart directly for a while but ultimately ended up preferring slightly higher pressure at the front. There is otherwise a steering vagueness I just don’t like, plus I don’t like seeing huge tyre deflection every time I gingerly roll over a bump (thankfully I can’t easily see what the rear is doing). I didn’t actually get a front pinch flat at Berto’s recommended pressure, but it felt like I had to baby the front tyre a bit too much.

Michelin has a chart somewhere that makes no distinction between front and rear pressure.

Kim’s comment about pressure gauges brings up another concern. What’s a trustworthy gauge? Maybe something like the Wonder Eurodainu?

A Fluke-Ruska P3000.

In the days of my career in Engine testing, my company had a J size bottle of purified air with a certified set of 0 – 10 bar gauges. Primary gauge showed contents of bottle and secondary showed output pressure. On the end of a length of hose I had a Presta connector.
Make sure regulator was wound fully back. Connect presta connector to inner tube valve.
Open bottle.
Adjust regulator to 90 psi. Switch off bottle and disconnect Presta connector from inner tube valve.
Repeat with front tyre at 80 psi.
Sit on bike and look at tyre sidewall flare. Memorise.

[Highlander]

Highlander: I didn’t mean to suggest that Jobst Brandt recommended a higher pressure at the front than the rear, just a higher front pressure than might be expected from its typical load (which is much less than the rear).

Racers almost universally run higher pressures at the front than the back per unit of static load.

For example, the link andrew_s gave says Cancellara used 81 PSI at the front and 88 PSI at the rear, which is clearly not in proportion to the static load on the tyres.

Why might that be? Of course no-one cares about rolling resistance when braking, but they might care about avoiding pinch flats on rough surfaces. Or avoiding the tyre rolling off the rim when cornering hard.

I used Heine and Berto’s chart directly for a while but ultimately ended up preferring slightly higher pressure at the front. There is otherwise a steering vagueness I just don’t like, plus I don’t like seeing huge tyre deflection every time I gingerly roll over a bump (thankfully I can’t easily see what the rear is doing). I didn’t actually get a front pinch flat at Berto’s recommended pressure, but it felt like I had to baby the front tyre a bit too much.

Michelin has a chart somewhere that makes no distinction between front and rear pressure.

Kim’s comment about pressure gauges brings up another concern. What’s a trustworthy gauge? Maybe something like the Wonder Eurodainu?

Samuel - yes my apologies I misinterpreted your post but it did provoke an interesting discussion and as a result I'm inclined to rethink my own approach to inflation pressures.  Though I sometimes wonder if I should do less thinking and more riding...

Regarding guages I have one of those old "Accugauges" that I don't think are available in the uk any more. When I bought it goodness knows how long ago, they were well regarded but I wonder if there are better modern alternatives.
 

[P Walsh]

On the subject of pressure guages, I'm not sure it matters too much how accurate yours is. What matters is that it gives the same reading for a given pressure each time (e.g. if it reads actual 100psi as 110psi, it does so every time) and you use that same guage every time for consistency. By experimentation you can arrive at your ideal pressures (front and rear) and it doesn't matter if you think you are on 85 / 95 and you are actually on 90 / 100. So long as it performs well for a rider of your weight and it doesn't take you outside the tyre's safe range.

[Ningishzidda]

It is still written in bicycle maintenance manuals tyre pressures should be checked and rectified every four weeks.
I personally check and correct them every Monday morning on my commute bike and before every ride on my Sports bike.
Just how much pressure is lost during the week’s riding is unknown, because before I connect the Presta adaptor, I knock the presta pin in to release it from its seat and some air escapes from the tyre. No more than 10 psi, IME.

[Kim]

It is still written in bicycle maintenance manuals tyre pressures should be checked and rectified every four weeks.
I personally check and correct them every Monday morning on my commute bike and before every ride on my Sports bike.

Depends greatly on the volume and pressure of the tyre.  I find a mountain bike tyre will maintain its pressure for absolutely ages.  A wide touring tyre for several weeks.  Road bike tyres are worth checking every week or two, and a Brompton tyre seems to lose pressure over a few days.

And of course if there's CO2 in the tyre, it'll go down much quicker.  Best to replace it with air at the next convenient opportunity.

[Biggsy]

Also depends on inner tube thickness.

I leave a few weeks between tyre inflations with my what-I-call utility bike, as precise optimum pressure is hardly important for my little local trips.  I'm much more fussy with my what-I-call racer and top up as often as every other day, and the ultra-light tubes in Open Corsa tyres don't air very well.

[Kim]

Good point.  I don't have any need for waffer-thin racing tubes.

[Samuel D]

On the subject of pressure guages, I'm not sure it matters too much how accurate yours is.

I know what you mean and agree for comfort, grip, etc., the optimum pressure for which can easily be found by experimentation and feel.

But for rolling resistance, most of us are entirely at the mercy of academic work and testing websites like Bicycle Rolling Resistance. You can’t feel or sensibly road-test the difference in rolling resistance between 80 and 90 PSI, but that doesn’t necessarily mean you don’t care about that difference. So for optimising rolling resistance, I think an accurate gauge would be useful.

[P Walsh]

On the subject of pressure guages, I'm not sure it matters too much how accurate yours is.

I know what you mean and agree for comfort, grip, etc., the optimum pressure for which can easily be found by experimentation and feel.

But for rolling resistance, most of us are entirely at the mercy of academic work and testing websites like Bicycle Rolling Resistance. You can’t feel or sensibly road-test the difference in rolling resistance between 80 and 90 PSI, but that doesn’t necessarily mean you don’t care about that difference. So for optimising rolling resistance, I think an accurate gauge would be useful.

But wouldn't that rolling resistance vary with rider weight? Surely my great carcass would need higher tyre pressures than that of a little light person to get the optimum rolling resistance. And if nobody is providing a table of weights and pressures for my chosen tyre I am back to experimentation and guestimation.

[sojournermike]

On the subject of pressure guages, I'm not sure it matters too much how accurate yours is.

I know what you mean and agree for comfort, grip, etc., the optimum pressure for which can easily be found by experimentation and feel.

But for rolling resistance, most of us are entirely at the mercy of academic work and testing websites like Bicycle Rolling Resistance. You can’t feel or sensibly road-test the difference in rolling resistance between 80 and 90 PSI, but that doesn’t necessarily mean you don’t care about that difference. So for optimising rolling resistance, I think an accurate gauge would be useful.

But wouldn't that rolling resistance vary with rider weight? Surely my great carcass would need higher tyre pressures than that of a little light person to get the optimum rolling resistance. And if nobody is providing a table of weights and pressures for my chosen tyre I am back to experimentation and guestimation.

The Bicycle Quarterly article and, related, Berto tyre pressure app for your smart phone address this. You have to accept that optimum pressure is similar across tyres.of.similar actual size rather than carcass specific. Probably an approximation, but it provides a good starting point.

Mike

[Kim]

Surely it's worth knowing the real pressures for the odd occasion you use an unfamiliar pump (hopefully with a reliable gauge).

[Mr Larrington]

Surely it's worth knowing the real pressures for the odd occasion you use an unfamiliar pump (hopefully with a reliable gauge).

"Blimey", I thought, "this is riding funny", shortly after fixing a flat in a 559 Hutchinson Top Slick.  Applied the track pump to it when I got home.  145 psi.  That'd be why, then.  Proof of the efficacy of the Topeak Road Morph (an old-skool one without a gauge).

[andyoxon]

Having had same on F&R, I've dropped the front pressure, so now using ~75:95 psi (F:R), on my 28mm Paselas.  Definite improvement.

[Frank9755]

the weight distribution i have is 38/62 (just me on the bike, hands on the hoods). i normally put 10psi more in the rear tyre. this weight distribution explains the rear tyre wear, which is normally twice as fast as the front (and rear tyre also wears more due to the torque when riding out of saddle).

This is probably a good average over a ride, but weight distribution is not fixed.  It varies all the way from 100% on the rear (on a very steep climb where the front wheel starts to lift up) to 100% on the front (braking sharply when the rear wheel starts to lift up). 

I generally have a bit more pressure in the rear, but not more than 5-10psi difference.

Greater rear tyre wear is due to it driving the bike, not because it is bearing more weight.

[P Walsh]

I'm not sure that rear tyre wear is greater because the rear drives the bike. I think it is the extra weight. I have noticed that my touring bike has more pronounced rear tyre wear when I tour with heavy rear panniers.

[ElyDave]

With 23mm GP4000s I use the old rule of thumb: 1/10 the weight of the cyclist at the rear and half a bar less at the front.  Roughly 7.5 rear and 7 front.  With 25s I use pretty much the same.  Used to run 23s on 8/8.  Dunno why I changed, really.

Don't get this, or am I being a bit thick?

I'm currently 70kg ish 1/10 of that would be 7kg or 14.5lb ish.  can't see the connection to anything in bars or psis that remotely resembles a tyre pressure.

FWIW, I tend to go same front and rear, typically 8 bar or so on road tyres, a bit more when I was time trialling. 

[Biggsy]

I'm currently 70kg ish 1/10 of that would be 7kg or 14.5lb ish.  can't see the connection to anything in bars or psis that remotely resembles a tyre pressure.

7 bar resembles a popular tyre pressure: 100 psi.  It's lower than you and I use (for 23mm), but personal preference varies!

I weigh about the same and prefer 130 psi for rear 700x23, but that is a lot higher than necessary.

[Biggsy]

I'm not sure that rear tyre wear is greater because the rear drives the bike. I think it is the extra weight. I have noticed that my touring bike has more pronounced rear tyre wear when I tour with heavy rear panniers.

It can be both.  Driving causes some scrubbing motion, and weight increases contact area (when you don't use higher pressure to compensate).