Optimum tyre pressure

Ngalbrai
Ngalbrai Posts: 279
edited February 2008 in Workshop
Just bought some new 700*23 tyres. have always had 700 *28 but wanted to try out 23s. Was talking to guy in LBS about pressures and he said that given i weight about 65kg I would find that about 105psi is optimum, rather than inflating to the recommended 130psi. I would logically assume that the harder the better for speed but he told me thats a common misconception, particularly if you arent very heavy, as if you run them at a slightly lower pressure (105) the deform very slightly when hitting bumps i the road etc and actually roll over them more easily than if fully inflated as the hardness of the tyre causes it to impact more harshly and so slow you down more - it made sense to me - thoughts/comments?

Comments

  • azzerb
    azzerb Posts: 208
    I weigh about 5kg more than you, and tend to inflate 700x23 to about 115-120 as a personal preference.

    I suppose what he's saying is correct, but just try different pressures to see which you prefer riding at. Higher the pressure, the less chance of getting a puncture, so i keep my pressure up. Basic tyres which came with my first bike (fairly cheap so cheap tires), didn't puncture once in about 1800 miles.
    New Dolan Volare (not quite complete)
  • on the road
    on the road Posts: 5,631
    I suppose he's right, but that's not the reason you pump them up hard for, the reason you pump them up hard is that they have low rolling resistance in normal road conditions, ie, on smooth roads.
  • alfablue
    alfablue Posts: 8,497
    Sheldon Brown's view of this is similar to the shop, you should be aiming for optimum pressure based on weight, overinflation serves no purpose and as he says, defeats the purpose of pneumatic tyres. There should be some deflection of the tyre to accomodate road irregularities (that is the purpose of the pneumatic tyre), but the reduction of rolling resistance at inflation beyond the optimum is negligable and has other disadvantages, see here. Interestingly, Sheldon reckons the rear tyre should be inflated about 10% higher than the front as it takes more weight.

    For 35kg per wheel he recommends 100psi for a 23, you are a bit less than that so maybe 95psi front and 105psi rear.

    Puncture resistance is important, but at the optimum pressure, this should be okay.

    I think the slightly smoother ride of a correctly inflated tyre should be faster than one pumped up rock hard that skips over road irregularities more.
  • on the road
    on the road Posts: 5,631
    I think you've read those statistics wrong. The 'wheel load' weight is, I assume, the weight of the rider, and maybe plus the weight of the bike. 35kg or 70bs = 5 stone.

    So for 100lbs (7st 2) rider and a 23mm wheel the recommended pressure is 120psi.

    So in theory, because my weight is 9st, my 23mm tyres should really be 140psi, I normally aim for between 110 and 118.
  • on the road
    on the road Posts: 5,631
    Ngalbrai wrote:
    Just bought some new 700*23 tyres. have always had 700 *28 but wanted to try out 23s. Was talking to guy in LBS about pressures and he said that given i weight about 65kg I would find that about 105psi is optimum, rather than inflating to the recommended 130psi. I would logically assume that the harder the better for speed but he told me thats a common misconception, particularly if you arent very heavy, as if you run them at a slightly lower pressure (105) the deform very slightly when hitting bumps i the road etc and actually roll over them more easily than if fully inflated as the hardness of the tyre causes it to impact more harshly and so slow you down more - it made sense to me - thoughts/comments?
    According to Sheldon Brown, he's wrong.
  • alfablue
    alfablue Posts: 8,497
    edited February 2008
    Yes, you're right, weight of rider (/2) in this case, 32.5kg, weight of bike (most likely 10kg or less/2) = 5 kg, per wheel that is 37.5kg, so 120psi (for 50kg), - 100psi (for 35kg) = 20 psi. 37.5kg therefore is 2.5kg above 35kg, and is 1/6 the difference between 35 and 50, so it is 1/6 of the sifference in psi of 20 so requires an addition of 3.2psi for the lower figure, so 103.2 psi.

    In your case, I reckon 28kg per wheel (surely weight is divided between the two wheels, and the table is based on loading per wheel), plus 5kg for the bike, so 33kg, so pretty close to the recommendation for 35kg of 100psi.

    Maybe 95 front, 105 rear if you take Sheldon's reccomendation.

    Of course I could be way out on the issue of dividing total weight by 2 to reflect the load on an individual wheel (assuming it is not a unicycle) - someone help me out here!
  • gundersen
    gundersen Posts: 586
    ever thought that sheldon brown was wrong?
  • on the road
    on the road Posts: 5,631
    alfablue wrote:
    Yes, you're right, weight of rider (/2) in this case, 32.5kg, weight of bike (most likely 10kg or less/2) = 5 kg, per wheel that is 37.5kg.

    In your case, I reckon 28kg per wheel (surely weight is divided between the two wheels, and the table is based on loading per wheel), plus 5kg for the bike, so 33kg, so pretty close to the recommendation for 35kg of 100psi.

    Maybe 95 front, 105 rear if you take Sheldon's reccomendation.

    Of course I could be way out on the issue of dividing total weight by 2 to reflect the load on an individual wheel (assuming it is not a unicycle) - someone help me out here!
    But most of the weight is on the back.
  • alfablue
    alfablue Posts: 8,497
    yes, Sheldon Brown is undoubtedly wrong some times. He says this is only a rough guide also, and I think individual experience should determine pressures, but as a starting point these values are probably as rational as any.
  • on the road
    on the road Posts: 5,631
    gundersen wrote:
    ever thought that sheldon brown was wrong?
    The thought has crossed my mind plenty times, that's why I always go by my own instincts.
  • alfablue
    alfablue Posts: 8,497
    on the road wrote:
    alfablue wrote:
    Yes, you're right, weight of rider (/2) in this case, 32.5kg, weight of bike (most likely 10kg or less/2) = 5 kg, per wheel that is 37.5kg.

    In your case, I reckon 28kg per wheel (surely weight is divided between the two wheels, and the table is based on loading per wheel), plus 5kg for the bike, so 33kg, so pretty close to the recommendation for 35kg of 100psi.

    Maybe 95 front, 105 rear if you take Sheldon's reccomendation.

    Of course I could be way out on the issue of dividing total weight by 2 to reflect the load on an individual wheel (assuming it is not a unicycle) - someone help me out here!
    But most of the weight is on the back.

    Yes, Sheldon says this accounts for the variation of 10% between front and back pressures. Working out realistically how much is split between the wheels is a puzzle I am ill equipped to tackle. Suppse 70kg total bike and rider weight, I wonder how you would split these proportionately. Or is it intended that we just assume total weight is on the wheel for the purposes of this calculation?
  • alfablue
    alfablue Posts: 8,497
    Okay, another approach, from the CTC

    Tyrepres.gif

    They are suggesting that 70% of bike and rider weight is on the back wheel.
  • alfablue
    alfablue Posts: 8,497
    Given the above, for your bike and rider weight on the road, that gives approx 46kg on the rear wheel, so for a 23 tyre (reading proportionally lower from the 22 in the graph), that would suggest a minimum of nearer 90psi, but these are minimum rather than optimum. They suggest adding a factor of 25% weight for poor roads, so then its 58kg, and around 100psi for the rear.

    In any event, I am sure your own instinct is the best determinant.
  • on the road
    on the road Posts: 5,631
    Conidering that I've only known about Sheldon Brown for about the last 2 years out of 30 years as a rodie :)
  • alfablue
    alfablue Posts: 8,497
    on the road wrote:
    Conidering that I've only known about Sheldon Brown for about the last 2 years out of 30 years as a rodie :)
    you probably got a few things right then :wink:
  • Steve I
    Steve I Posts: 428
    I weigh about the same as you Ngalbrai. I use 90 psi front and back on my road bike with 23mm tyres. Anything more and I find the ride far too harsh. I used to use 80 psi on a very rigid Ribble winter frame. I haven't experienced any problems with snakebite punctures even at 80 psi. Why beat yourself up with excessively high tyre pressures? ok, I'm maybe 1% (or less) slower due to extra drag from the tyres but I'm 25% more comfortable. BTW this isn't meant as a sly dig at others who like higher pressures, it's whatever you're happy with.
  • Monty Dog
    Monty Dog Posts: 20,614
    I used to be in the more is better camp many years ago - pumping my tyres right up to the max. In more recent years, based on experience, I use wider tyres at a lower pressure. Many of the roads I ride are rough country lanes and can be quite greasy. This morning, prior to my ride, I pumped my tyres to 95 on the front and 100 on the rear - and grip was all over the place on any incline. I weigh about 60kg and would say that 80psi is about optimum for comfort and grip on anything but the smoothest tarmac. Too much pressure, not only makes the ride harsh, but gives too much rebound meaning you lose grip too.
    Make mine an Italian, with Campagnolo on the side..
  • wildmoustache
    wildmoustache Posts: 4,010
    experiment is the best advice on this one I think, but an obvservation I'd make is that overinflating is one of the most common beginners' mistakes ... meaning an uncomfortable slower ride with loss of grip and sketchy handling. in wet weather it's positively dangerous, so too in long moutain descents where you're far more likely to blow.

    I weigh 73kg, so a little over 80kg including bike and kit. I ride pro2Race at 105 rear and 100 front. only on very smooth dry tarmac would it be better to go say 5psi higher.

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