Unbelievable Pressure

[Ajax Bay]

. . . the biggest giveaway that it is time to top up the pressures comes from my cycle computer. My commute measures 21.20 miles on freshly inflated tyres but as the pressure slowly drops over 3-4 weeks that measured distance creeps up to over 21.25, no need for fancy gauges or regular checks

So if you have a day with a headwind & your time is 45.25 what pressure do you pump your tires up to?

In a headwind with that time, Skolly just looks to enter the next available 25 mile TT and looks to go sub-50, with an extra 20psi front and rear, and to hell with the comfort.

 

[Sniper68]

Personally I can't see how 5 psi would make any noticeable difference whatsoever, are you honestly saying you can physically notice the difference? Taking into account the pump tolerance, road conditions & climate conditions, I don't believe it!
If you're using a indoor track that may be different perhaps?

Yes.
The first time I used 25mm up from 23mm was in France.I initially pumped them to 90F/95R and on the hot French Tarmac it felt like I was on 2.5" MTB tyres.I ended up with 115F/120R after much faffing.Same when I'm in Mallorca my UK pressures feel far too draggy so I up them by 10psi.
In my MTB days I used to run 22F/25R and I could feel a difference in grip if I went 25F/28R
You just know when it's not right!

 

[Ajax Bay]

The first time I used 25mm up from 23mm was in France.I initially pumped them to 90F/95R and on the hot French Tarmac it felt like I was on 2.5" MTB tyres.I ended up with 115F/120R after much faffing.Same when I'm in Mallorca my UK pressures feel far too draggy so I up them by 10psi.

The pressure inside a cycle tyre goes up by about 1psi for every 6 degrees Celsius increase. How hot was that French or Balearic temperature? How light was the rider plus bike load?
"Felt far too draggy." Mmmm.

 

[nickyboy]

Optimal tyre pressures are an art. In theory you can take your and your bike's weight and determine it.

But this only works on smooth surfaces. There is a trade-off between lower pressures increasing hysteresis which makes you slower (that's rolling resistance to you and me) and lower pressure reducing "micro bounce" on rough roads which speeds you up.

If you are on a v smooth road then 100psi may be optimal but that could reduce to 75psi if the road is particularly "chattery". It's really impossible to calculate optimal as it depends on road surface. Best is to try a variety of different pressures on your normal roads and see which is fastest

 

[Sniper68]

The pressure inside a cycle tyre goes up by about 1psi for every 6 degrees Celsius increase. How hot was that French or Balearic temperature? How light was the rider plus bike load?
"Felt far too draggy." Mmmm.

The temp was the high 20s I was riding a Carbon Canyon Ultimate SL CF with 2 bottles and a mini-wedge.At that time I was probably a bit heavier than I am now so maybe 88kg.I've ridden the same bike in Mallorca every year(apart from 2020) since and I always upped the pressure once the bike unpacked and built.
I'm still not 100% sold on 25mm tyres as I see no actual benefit.I have them on my current Titanium bike but will more than likely go back to 23mm once they've worn out.
One of my mates is a very fit 50 year old,about 60kg wet-through and he runs 100psi+ on all his road bikes.He always says my tyres look underinflated
I suppose it's what you get used to.My CX bike with 32s on feels as draggy as *&$£ at the pressures I run it at,90 psi/R-80 psi/F but it does have semi-knobblies on and any higher and it's useless off road

 

[Foghat]

GP5000 x 28mm here (75kgs)

Summer: 70r and 65f
Winter: 65r and 60f

'28mm' GP5000 tyres, in true inconsistent Continental style, are considerably narrower than '28mm' GP4000Sii tyres.

I use the same pressures as your winter values in the GP5000s, which measure 26.5mm wide on my 14mm rims - whereas the GP4000Sii tyres are 29mm wide on the same rims....at lower pressures (50F/55R).

Interestingly, the '32mm' GP5000 tyres are only about 1mm wider than the '28mm' GP4000Sii. And they are noticeably narrower than '32mm' GP4Season tyres.

 

[Ming the Merciless]

For every 100m vertical the temp drops 1C. You are in the Tour de France with sections of mountainous and flat. What pressure do you initially set?

 

[Ajax Bay]

"1psi for every 6 degrees Celsius increase "Is this right? My workings suggest not (I think it goes up a bit quicker), but my workings are probably wrong

You are right, it does go up a bit quicker. I was carelessly using a car tyre (pressure) example (they run at 40psi say).
It's just over 1psi for every 4 degrees Celsius increase (if the tyres start at 75psi) and about 1psi for every 3 degrees Celsius increase (if the tyres start at 100psi).
Detail:
The tyre's excess* air pressure should very closely approximate the ideal gas law, PV=nRT (* pressure above normal atmospheric pressure)
Since [n] and [R] are (effectively) constants [n is the amount of air compressed into the tyre (moles of N2/O2), and R is the ideal gas constant] and assume that the volume of the tyre [V] does not change (so no stretching or deformation of the tyre beyond what it was 'to start with'), then P [pressure] increase is directly proportional to change in temperature [T] (T measured in Kelvin)

 

[Ajax Bay]

'28mm' GP5000 tyres, in true inconsistent Continental style, are considerably narrower than '28mm' GP4000Sii tyres.
I use the same pressures as your winter values in the GP5000s, which measure 26.5mm wide on my 14mm rims - whereas the GP4000Sii tyres are 29mm wide on the same rims....at lower pressures (50F/55R).
Interestingly, the '32mm' GP5000 tyres are only about 1mm wider than the '28mm' GP4000Sii. And they are noticeably narrower than '32mm' GP4Season tyres.

In the interests of corroboration, my experience of these three models of Conti tyre (in 28-622, all on the same 15mm rim at 90psi) wrt spec v actual is exactly the same. 4000s come up wide by maybe 2mm, 4Ss come up narrow by 1.5mm, and 5000s come up 1mm narrow.
The latter is a bit of a bummer as I bought them for my new build which will allow 30s, assuming, wrongly, that they would come up wide like the model they replaced (the 4000s).

 

[Foghat]

4Ss come up narrow by 1.5mm

What age tyres is that based on?

All the '28mm' GP4Season tyres that I bought prior to about 2016 (quite a few) were indeed undersize, measuring only 26mm wide on 14mm rims.

However, I've bought 8 '28mm' GP4Seasons since 2018, and all of those are, much to my approval, the same size as '28mm' GP4000Sii - i.e. slightly oversize at 29mm wide on 14mm rims. The most recently purchased were in 2019, though, so Continental maybe had a relapse.

I do believe Continental had a much-needed rationalisation of SOME of its inconsistent tyre-size mis-labelling around the time of that change. For instance, its most egregiously mis-labelled tyre, the '32mm' Top Contact I, which only measured 27mm wide, was replaced by the '32mm' Top Contact II, which thankfully measures a true 32mm on the same rims.

Now they need to address the woefully mis-labelled but otherwise excellent '37mm' Top Contact II Winter Premium, which is actually a 32mm tyre (cf. the 42mm version that is a true 42mm tyre). There is no actual 37mm version, although Continental would do well to manufacture one, and a 28mm version wouldn't go amiss either.

Bummer, as you say, re the undersized '28mm' GP5000 tyres. I knew these are undersized when I bought them, as I was hoping to fit them to my No. 1 dry-weather fast bike, which only has clearance for 26-26.5 mm at the rear. Whilst the fork does have room, the tyre is just very slightly too large to fit at the rear as the profile is slightly tall compared to other similar-width tyres, so I run an oversize '25mm' Vittoria Corsa CX or oversize '25mm' GP4000Sii on the back wheel of that bike. Funnily enough, the older '28mm' GP4Season does just fit!

 

[GetFatty]

So I bought a small pump with a pressure gauge from Amazon a while back, pretty quickly I became convinced the gauge was faulty as it was showing 45-50psi and the tyres felt like they were at bursting point.

Today, whilst buying oil and what have you I bought another pump, a foot pump with a gauge. I pumped the tyres to their maximum 115psi, holy sh*t, I don't think the first pump was faulty anymore!

The tyres are now completely unyielding, does anyone actually cycle at that psi? Must be properly uncomfortable right? And won't they just puncture immediately?

It's a road bike by the way, Continental GP 4000ii 28s.

Yep on 25s all the time on Max psi

 

[Ajax Bay]

Those actual v spec width were for tyres made before 2018.
I had to go back to find my post from 4+ years ago: it was this one:

It maybe irrelevant to you, @I like Skol , but besides your (perfectly reasonable) parochial interest the knowledge (of variable and non-assured quality) these threads contain may be useful to others. Spurred on by your thought that "I don't think road bike sized tyres really stretch much between 60-120Psi."
I decided to test this.
I've measured the width of the new Continental GP 4 Seasons 28-622 I've just mounted on 622-15 Mavic MA3 rims (NB 15mm inside bead, butyl inner) at pressures from 40psi to 120psi (NB max rating is 115psi).
psi width (mm)
40 25.2
50 25.3
60 25.45
70 25.6
80 25.8
90 25.95
100 26.1
110 26.2
120 26.4

Comment: Fairly straight line, perhaps steepening after max specified pressure.
Note that @andrew_s 's measurement on 13mm rims @ 100psi was 25.7mm.
Experimental method and constraints:
Uncalibrated track pump (but the intervals were 10psi + or - 0.5 psi, so can assume equalish intervals). Done over short period indoors so temperature and atmospheric pressure would not have significantly changed.
7 width readings per pressure level taken by digital calipers; two outliers discarded and remaining 5 averaged, avoid measuring adjacent to valve.
Digital calipers calibration checked against steel rule (accurate to within 0.1%).

Skolly reacted with concern about my employment status.
Subsequent GP4S tyres (28s) I've fitted have always been 'narrow' but I have not quantified that.
Managed to buy 4 x 4000 28s last autumn before the supply ran out so that should do me till the 2022 spring.

 

[Foghat]

Those actual v spec width were for tyres made before 2018.

Subsequent GP4S tyres (28s) I've fitted have always been 'narrow' but I have not quantified that.

Hmmm......I really hope Continental hasn't re-rationalised the GP4Seasons, down to match the smaller GP5000s, sizewise!

Managed to buy 4 x 4000 28s last autumn before the supply ran out so that should do me till the 2022 spring.

Good move - I similarly acquired a (larger) stash of these tyres, which are sitting in a dark semi-sealed moderate-temperature environment for maximum longevity before needing to be deployed.

While my GP4000Sii stocks last, which will be a good few years, I view the GP4000Sii and GP5000 suites of tyres as one continuous size range of the (effectively, for my purposes) same tyre:

1. '32mm' GP5000, being 1mm wider than;
2. '28mm' GP4000Sii, being 2.5mm wider than;
3. '28mm' GP5000, being 1mm wider than;
4. '25mm' GP4000Sii, being 1mm wider than;
5. '25mm' GP5000, although these are too narrow for me so I don't own any; and
6. I will never ride 23mm tyres again in the UK! Anyone want to buy some well-matured, never-used tan-wall 23mm Vittoria Open Corsa CX tyres, still in their boxes? I've got a few left.....should get them onto eBay.....

 

[Grant Fondo]

I did notice the difference in GP5000 28c compared to 28c Duranos. Wish I had gone 32c now and had some 'actual' 29c's