Pressure in tyres 700 x 25

mikeyj28
mikeyj28 Posts: 754
edited February 2015 in Workshop
Hi all

Simple one for advice. What pressure should I be running in my700 x 25c wheels (tubes)? I am 11st 9lbs.
I realise there isn't a definitive pressure but a guideline would be great.

Thanks.
Constantly trying to upgrade my parts.It is a long road ahead as things are so expensive for little gain. n+1 is always the principle in my mind.
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Comments

  • martino53
    martino53 Posts: 29
    Not sure what the stones & pounds works out to, but I'm 68-70kg and ride at 85lbs FW - 90lbs RW, on modern wide rims it could even go lower. I should also mention that I enjoy the luxury of nice roads.
  • arlowood
    arlowood Posts: 2,561
    martino53 wrote:
    Not sure what the stones & pounds works out to.

    74kg near as dammit for the metric uninitiated.

    I'm way heavier than that at closer to 90kg and run my 700x25's at 85psi front and 95psi rear. I'm sure the OP could get away with lower pressures.
  • ai_1
    ai_1 Posts: 3,060
    I'm about 85kg and use approx 80-85psi front and 95-105psi rear. I could go 10psi lower and sometimes do. You don't want to go so high as to have poor comfort and poor grip on bad surfaces or low enough to suffer pinch punctures when you roll over a drain or pothole. After that it's really down to preference. I like a reasonably comfortable ride but I rarely go much lower than the ranges above since I find that feels a little mushy when climbing out of the saddle or sprinting. However I'm 10kg heavier than you so you might well go lower.
  • keef66
    keef66 Posts: 13,123
    I've been running 25mm tyres for coming up to 8 years. Trial and error has led me to running them at 70 psi front / 80 psi rear. Weight is 11 st.

    Lower than that and I start to get pinch flats, higher and it's not as comfy. As always, YMMV.
  • lostboysaint
    lostboysaint Posts: 4,250
    arlowood wrote:
    martino53 wrote:
    Not sure what the stones & pounds works out to.

    74kg near as dammit for the metric uninitiated.

    I'm way heavier than that at closer to 90kg and run my 700x25's at 85psi front and 95psi rear. I'm sure the OP could get away with lower pressures.

    That. I'm closer to 100kg and run at the same pressures on my Pro4 Endurance (which admittedly size up quite big for a 25mm). Great comfort, roll and no pinch flats.
    Trail fun - Transition Bandit
    Road - Wilier Izoard Centaur/Cube Agree C62 Disc
    Allround - Cotic Solaris
  • mikeyj28
    mikeyj28 Posts: 754
    Thanks guys for all of your responses. It is much appreciated!!
    Constantly trying to upgrade my parts.It is a long road ahead as things are so expensive for little gain. n+1 is always the principle in my mind.
  • de_sisti
    de_sisti Posts: 1,283
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
  • andy_wrx
    andy_wrx Posts: 3,396
    De Sisti wrote:
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
    ...which has no bearing at all on the pressure you should run your tyres at !

    Sheldon Brown : http://sheldonbrown.com/tires.html
    Most tires have a "maximum" pressure, or a recommended pressure range marked on the side of the tire. These pressure ratings are established by the tire manufacturers after consultation with the legal and marketing departments.

    The lawyers want the number kept conservatively low, in case the tire gets mounted on a defective or otherwise loose fitting rim. They commonly shoot for half of the real blow-off pressure.

    The marketing department wants the number high, because many tire purchasers make the (unreliable) assumption that the higher the pressure rating, the better the quality of the tire.

    Newbies often take these arbitrary ratings as if they had some scientific basis. While you'll rarely get in trouble with this rote approach, you will usually not be getting the best possible performance.

    Savvy cyclists experiment with different pressures, and often even vary the pressure for different surface conditions.

    Optimal pressure for any given tire will depend on the load it is being asked to support. Thus, a heavier rider needs a higher pressure than a lighter rider, for identical tires.

    Since most bicycles have substantially more weight on the rear wheel than on the front, the rear tire should almost always be inflated to a higher pressure than the front, typically by about 10%.

    Rough surfaces generally call for a reduction in pressure to improve ride comfort and traction, but there is a risk of pinch flats if you go too far. Even at the lower appropriate pressure, wider tires, because they also are deeper, are more immune to pinch flats.

    Rider skill also enters into this: more experienced cyclists learn to "get light" for a fraction of a second while going over rough patches; newbies tend to sit harder on the saddle, increasing the risk of pinch flats.

    The table below is based on my experience and a certain amount of guesswork, and should only be used as a very rough guide to a starting point. Interpolate/extrapolate for your own weight/tire sizes.

    Tire widths are in millimeters, pressure recommendations in pounds per square inch. (Divide by 15 if your gauge reads in bars/atmospheres.)
    Tire width in mm
    Wheel load 50 mm 37 mm 32 mm 28 mm 25 mm 23 mm 20 mm
    100 lbs/50 kg 45 60 75 100 110 120 130
    70 lbs/35 kg 35 50 65 80 90 100 110

    Note that these recommendations are based on the actual tire width. Many tires are marked wider than they actually are.
  • de_sisti
    de_sisti Posts: 1,283
    andy_wrx wrote:
    De Sisti wrote:
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
    ...which has no bearing at all on the pressure you should run your tyres at !
    Yes it does, as it's a guide, not set in stone.
  • ai_1
    ai_1 Posts: 3,060
    De Sisti wrote:
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
    Max and min values on the sidewall are the safety design limits. You should ideally operate within those limits but they don't tell you what pressure is best for any or all riders or take into account any of the many variables in terrain, weather, personal preferences, rim sizes, etc.... :wink: :roll:
  • mikpem
    mikpem Posts: 139
    This is handy to see, I've not long started riding on proper road tyres (700x25) and I've had both front and rear up at 100psi at 75-80kg.
    I think I'll let a little air out and see if it's a bit comfier!!
  • andy_wrx
    andy_wrx Posts: 3,396
    De Sisti wrote:
    andy_wrx wrote:
    De Sisti wrote:
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
    ...which has no bearing at all on the pressure you should run your tyres at !
    Yes it does, as it's a guide, not set in stone.
    No, you've still not got it

    Read Sheldon
  • proto
    proto Posts: 1,483
    700 - 25 Gatorskins pumped up to 100-110psi, front and back.

    I run my 700 - 23 @ 120psi

    I weigh 75kg.
  • meanredspider
    meanredspider Posts: 12,337
    proto wrote:
    700 - 25 Gatorskins pumped up to 100-110psi, front and back.

    I run my 700 - 23 @ 120psi

    I weigh 75kg.

    There's plenty of opportunity for a more comfortable ride there if you want it. Comfier (and every bit as puncture resistant) tyres and less pressure - especially at the front. But you may well be happy.
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • ai_1
    ai_1 Posts: 3,060
    proto wrote:
    700 - 25 Gatorskins pumped up to 100-110psi, front and back.

    I run my 700 - 23 @ 120psi

    I weigh 75kg.
    Since you have less weight on the front wheel there's generally no need to have it the same pressure as the back. You could drop it 15psi or more and should have much more comfort through the bars, and better steering stability on rough roads too.
  • rafletcher
    rafletcher Posts: 1,235
    andy_wrx wrote:
    De Sisti wrote:
    andy_wrx wrote:
    De Sisti wrote:
    Perhaps you should look at the sidewall of the tyres. There you'd find a suggested max and min pressure guide. :wink:
    ...which has no bearing at all on the pressure you should run your tyres at !
    Yes it does, as it's a guide, not set in stone.
    No, you've still not got it

    Read Sheldon

    Is he absolutely right all the time? Or are they just his prejudices in print :D

    But even so, his range of pressures is really quite close to what everyone recommends AND within the limits on typical road tyres don't you think?
  • ElectronShepherd
    ElectronShepherd Posts: 227
    rafletcher wrote:
    Is he absolutely right all the time?

    Bit of a thread hijack, but I've always thought the advice

    "The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground"

    to be deeply suspect (see http://www.sheldonbrown.com/brakturn.html)
  • meanredspider
    meanredspider Posts: 12,337
    ElectronShepherd wrote:
    rafletcher wrote:
    Is he absolutely right all the time?

    Bit of a thread hijack, but I've always thought the advice

    "The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground"

    to be deeply suspect (see http://www.sheldonbrown.com/brakturn.html)

    Why do you think that?

    Watching MotoGP, he'd appear to be right - those guys appear to do exactly that
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • ElectronShepherd
    ElectronShepherd Posts: 227
    The idea behind it seems to be predicated on the fact that the rear wheel is unloaded, and that doesn't seem reasonable in all cases. To take a pathological example, if I put a one tonne weight on a cycle rack behind the rear wheel (leaving aside the practicalities- this is a thought experiment), do you think that minimum stopping distance would be achieved using only the front brake? I don't - I think that in that situation the rear brake would definitely help, since the rear wheel would remain in contact with the ground.

    The above example is clearly ridiculous in the real world, and only serves as an extreme example. But, imagine a 120kg rider sitting back on the saddle and applying the brakes. Some of his weight is taken by the front wheel, but the majority of the load is on the rear. Do you really think that the grip from a 23mm tyre on road tarmac would be enough to lift the rear wheel (with, say, 90Kg of load on it) before the tyre itself started to skid? If not, then we are, essentially, back to the example at the top of the post, and using the rear brake will help.
  • proto
    proto Posts: 1,483
    The reason the back brake is much oess effective is that is is very easy to lock up the wheel and then the tyre just slides greatly increasing stopping distance.
    Grabbing a fistfull of front brake loads up the tyre greatly increasing the grip. The downside though is that if you lock the front then you crash.

    I can recall an ex Met Police motorcycle instructor telling me that he never used the back brake on his bike.
  • ai_1
    ai_1 Posts: 3,060
    ElectronShepherd wrote:
    The idea behind it seems to be predicated on the fact that the rear wheel is unloaded, and that doesn't seem reasonable in all cases. To take a pathological example, if I put a one tonne weight on a cycle rack behind the rear wheel (leaving aside the practicalities- this is a thought experiment), do you think that minimum stopping distance would be achieved using only the front brake? I don't - I think that in that situation the rear brake would definitely help, since the rear wheel would remain in contact with the ground.

    The above example is clearly ridiculous in the real world, and only serves as an extreme example. But, imagine a 120kg rider sitting back on the saddle and applying the brakes. Some of his weight is taken by the front wheel, but the majority of the load is on the rear. Do you really think that the grip from a 23mm tyre on road tarmac would be enough to lift the rear wheel (with, say, 90Kg of load on it) before the tyre itself started to skid? If not, then we are, essentially, back to the example at the top of the post, and using the rear brake will help.
    I read that piece at one time but not recently. I'm pretty sure he said that the case for front wheel doing all the braking was predicated on the assumption that the front wheel has enough grip that it will not skid at the maximum deceleration the brakes can provide. Doesn't he also say that in slippery conditions front wheel only braking may not be the fastest which he would surely only say if he were assuming sufficient grip for the dry case.
    Any bike has a centre of mass above ground level and therefore applying brakes will impart a moment about the contact points tending to rotate the rear wheel off the ground. Therefore maximum braking is dependent on grip and centre of mass. If grip does not run out first then the rear wheel will fully unload.
  • meanredspider
    meanredspider Posts: 12,337
    ElectronShepherd wrote:
    The idea behind it seems to be predicated on the fact that the rear wheel is unloaded, and that doesn't seem reasonable in all cases. To take a pathological example, if I put a one tonne weight on a cycle rack behind the rear wheel (leaving aside the practicalities- this is a thought experiment), do you think that minimum stopping distance would be achieved using only the front brake? I don't - I think that in that situation the rear brake would definitely help, since the rear wheel would remain in contact with the ground.

    The above example is clearly ridiculous in the real world, and only serves as an extreme example. But, imagine a 120kg rider sitting back on the saddle and applying the brakes. Some of his weight is taken by the front wheel, but the majority of the load is on the rear. Do you really think that the grip from a 23mm tyre on road tarmac would be enough to lift the rear wheel (with, say, 90Kg of load on it) before the tyre itself started to skid? If not, then we are, essentially, back to the example at the top of the post, and using the rear brake will help.

    What you're missing is the dynamic shift in load and the fact that a bicycle is a very stiff system. The weight transfer on hard braking will shift the weight very quickly over the front wheel, loading up that tyre and giving it extra traction. Because that weight has to come from somewhere, the rear tyre unloads very quickly - losing traction as it does. Any braking going through the rear wheels will almost instanteously overcome the level of traction the rear wheel has, causing it to lock and lose directional stability as it does. This weight transfer occurs whichever brake you apply.

    Up to a limit, the size of the front tyre doesn't matter on secure surfaces because, with a smaller footprint, the local pressure on the road goes up. You can certainly lift a rear wheel through braking - I've done it plenty of times on my disc-braked Volagi. I weigh 90+kg. Of course you can minimise the effect of this by putting more and more weight over the rear wheel but, ultimately (assuming you don't end up with the centre of gravity behind both wheels - effectively giving two front wheels and no rear wheels - or at ground level) weight transfer leads to the same conclusion. I've never managed to lock the front wheel on a firm dry surface.

    All of this leads to real world observations: larger disc rotors on the front of many bikes than the rear, massively different brakes on the front of motorbikes (and that MotoGP observation), same with race cars (mine has 6-pots on the front with huge discs and little twin pots on the rear despite having a very low CofG)
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • ai_1
    ai_1 Posts: 3,060
    meanredspider wrote:
    Of course you can minimise the effect of this by putting more and more weight over the rear wheel but, ultimately (assuming you don't end up with the centre of gravity behind both wheels - effectively giving two front wheels and no rear wheels - or at ground level)
    Even the "two front wheels" example doesn't change things, except that when not braking your bike would fall over backwards. If the centre of mass is above ground level then there will still be a moment around the contact point of the wheels which will lift the rearmost (of the two fronts ;)) off the ground given enough deceleration.
  • lostboysaint
    lostboysaint Posts: 4,250
    I'm glad you lot got the replies in first. (Shakes head in disbelief!)

    The bit about the ton weight and the lack of understanding that the most efficient braking would STILL be to have the front brake powered to the point of lifting the back wheel is just astounding.

    A further point on MotoGP bikes is that most of the riders have their rear brakes adjusted so far OUT that if they applied it the bike would never stop, it would simply roll and roll, slowing very gradually. This is done deliberately so that the minimal pressure that they will put on the rear brake (IF they use it, most now are so embedded in the "dangle the leg to help turn in" idea) will not lock the rear brake given the lack of grip that the tyre has when it's an inch or two up in the air!
    Trail fun - Transition Bandit
    Road - Wilier Izoard Centaur/Cube Agree C62 Disc
    Allround - Cotic Solaris
  • meanredspider
    meanredspider Posts: 12,337
    Ai_1 wrote:
    meanredspider wrote:
    Of course you can minimise the effect of this by putting more and more weight over the rear wheel but, ultimately (assuming you don't end up with the centre of gravity behind both wheels - effectively giving two front wheels and no rear wheels - or at ground level)
    Even the "two front wheels" example doesn't change things, except that when not braking your bike would fall over backwards. If the centre of mass is above ground level then there will still be a moment around the contact point of the wheels which will lift the rearmost (of the two fronts ;)) off the ground given enough deceleration.

    That's true - I was stuck with the mental picture of two front wheels :D
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • ElectronShepherd
    ElectronShepherd Posts: 227
    lostboysaint wrote:
    The bit about the ton weight and the lack of understanding that the most efficient braking would STILL be to have the front brake powered to the point of lifting the back wheel is just astounding.

    I would say that the idea that the front wheel will have sufficient traction to lift the rear wheel at all (in the hypothetical case of the 1 tonne weight) is just astounding. :D

    I stand by what I said: In cases where the weight on the back wheel (be it a mythical 1 tonne weight or just a big rider who likes his pies and is sat firmly on the saddle) is too much to be lifted by the available traction from the front wheel, the rear wheel will be in contact with the ground regardless of the braking from the front wheel, and in those cases, braking the rear wheel too will reduce the stopping time.

    I agree that in some limited circumstances (e.g. lightweight rider, dry conditions, grippy tyres and tarmac), you can get minimum stopping distance just using the front brake. However, it simply isn't the case in all circumstances, even in the dry, which is what Sheldon Brown is saying.
  • lostboysaint
    lostboysaint Posts: 4,250
    You are wrong then!
    Trail fun - Transition Bandit
    Road - Wilier Izoard Centaur/Cube Agree C62 Disc
    Allround - Cotic Solaris
  • ElectronShepherd
    ElectronShepherd Posts: 227
    lostboysaint wrote:
    You are wrong then!

    Which bit? About the lifting the rear wheel?
  • meanredspider
    meanredspider Posts: 12,337
    Yup - at over 90kg, I'd consider myself at the larger end of road bike riders and I can lift the rear wheel
    ROAD < Scott Foil HMX Di2, Volagi Liscio Di2, Jamis Renegade Elite Di2, Cube Reaction Race > ROUGH
  • lostboysaint
    lostboysaint Posts: 4,250
    ElectronShepherd wrote:
    lostboysaint wrote:
    You are wrong then!

    Which bit? About the lifting the rear wheel?

    About it all. Why not go away and read up on the subject?
    Trail fun - Transition Bandit
    Road - Wilier Izoard Centaur/Cube Agree C62 Disc
    Allround - Cotic Solaris

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