Optimum tire pressures

littledove44
littledove44 Posts: 871
edited February 2014 in Road general
Very interesting article.

http://www.bikequarterly.com/images/TireDrop.pdf

I normally run (25mm tires) 110 rear, 100 front. Bike and rider total weight 95 kg. This is based on the Psimet calculation Tire Width=25: Pressure(psi) = (0.33 * Rider Weight in lbs) + 43.33

According to the linked article I should, for optimum performance be running quite different pressures.
Having done the thing with the scales I am about 60kg rear, 35kg front which indicates a pressure more like 120 rear 70 front.

So who knows best?

Perhaps I just need to lean forward more!

Comments

  • What happens when you pedal out of the saddle?

    This transfers weight forwards.

    Maybe stop beforehand and readjust pressures?!
  • BrandonA
    BrandonA Posts: 553
    70 on the front would be too low for me, I'd feel the tyre bobbing up and down when I stand to get up a steep hill.

    I don't know what the optimal is but I like to run 125 in both wheels. Not had any issues doing this so far.
  • ai_1
    ai_1 Posts: 3,060
    I've a similar rolling weight at about 98kg (I blame the bike for some of it: Me = 88kg, Bike = 10kg).
    I also use "25mm" tyres. They're Continental 4 Seasons which are perhaps a little narrow compared to others.

    I typically run about 85psi front and 105psi back. I've run lower pressures down to about 70psi at the front but while that's fine cruising in the saddle and provides good vibration damping, it's too mushy when I get out of the saddle to climb and especially unnerving for sprints. If I know I'll be riding on smooth and dry roads and likely to be sprinting I'll sometimes put 10 to 15psi extra in each.
  • 76KG here, bike approx 8.5KG, I tend to run 110psi front and rear on 23mm Conti GP as I hate that soft feeling when you get out the saddle. Not sure whether this is optimum in terms of comfort and grip but it feels right to me.
  • ai_1
    ai_1 Posts: 3,060
    76KG here, bike approx 8.5KG, I tend to run 110psi front and rear on 23mm Conti GP as I hate that soft feeling when you get out the saddle. Not sure whether this is optimum in terms of comfort and grip but it feels right to me.
    The area of the contact patch is dictated by pressure and load regardless of the tyre size, rim width, etc.

    contact area = load/pressure

    So for example, if you've got 50lbs resting on a wheel and a tyre pressure of 90psi:

    50/90 = 0.56 square inches (not much is it :wink: )

    If you increased the pressure to 120psi:

    50/120 = 0.42 square inches (contact area is inversely proportional to pressure

    Since the front/back weight distribution is probably between 40%/60% and 45%/55% the contact patch at the front will be noticeably smaller. That means the tyre will compress less to provide that patch size and therefore you'll get less flex. So assuming there's an optimal amount of flex that provides the optimum balance of comfort, control and performance I would expect a lower front pressure to be best. Also I'd rather maintain lots of grip at the front so I don't want to use more pressure than I need as that will reduce grip. However, there are lots of variables that mean the front and back are not directly equivalent so you could debate all day what balance of pressures is best without reaching a definite conclusion. I don't believe the common perception that a higher pressure always provides lower rolling resistance is accurate.

    I like the feel of my bike with the front about 15% softer than the back and since I ride on poor roads and wet conditions fairly regularly I like to avoid unnecessarily high pressures. I used to use higher pressures but having backed off a bit I still never get pinch punctures, the bikes more comfortable and just as fast as far as I can tell. Only slight mush in the sprint stops me dropping the pressure a little lower.

    These are just my musings so take them or leave them. I doubt you'll get a "correct" answer so I'd say go with whatever feels best to you. If you've always done the same then maybe experiment a bit and see if you end up back where you started or somewhere else!


    P.S.
    For the area calcs above I'm assuming consistent pressure across the contact patch. This is probably pretty accurate for light supple racing tyres but might vary a bit for thicker, tougher tyres.
  • ednino
    ednino Posts: 684
    Been running 80psi front & 90psi rear. No punctures in the last 2,500 miles

    Im 64kg + 7kg bike
  • Moonbiker
    Moonbiker Posts: 1,706
    Im 78kg 5 10" ( plusbike weight 12kg) so 90kg total & my rubinos 23mm are & both aprox @ 120psi :o
  • Interesting replies, but if you read the article they are purporting to scientifically calculate the optimum pressure based on tyre deformation.

    As far as standing up goes, I tried that and I don't really load the tyre a lot more than when sitting.

    I realise of course a lot of this is personal preference, but it's the science I am interested in. When two supposedly academically correct sources come up with different answers you know that someone has missed the point.
  • ai_1
    ai_1 Posts: 3,060
    Interesting replies, but if you read the article they are purporting to scientifically calculate the optimum pressure based on tyre deformation.

    As far as standing up goes, I tried that and I don't really load the tyre a lot more than when sitting.

    I realise of course a lot of this is personal preference, but it's the science I am interested in. When two supposedly academically correct sources come up with different answers you know that someone has missed the point.
    As per my previous post, no-one can give a "correct" answer.
    There's no reason two academic studies wouldn't get different answers unless they are trying to answer the exact same question with the exact same criteria. The question "What's the best tyre pressure?" is wide open to interpretation. You need to decide if you want the best for comfort, best for cruising efficiency, best for sprinting, best for climbing, best for cornering, best traction, best for maintaining road contact on bumpy surfaces, etc, etc.... and even these terms are too vague. Best for sprinting could mean best rear wheel traction for straight line acceleration or best handling for manoeuvering, it may depend on how far out you start your sprint or what surface you're doing it on. To give a correct answer you need to knnow how to measure it. How do you measure "best"?

    Before answering a vague question you have to specify exactly what it means and what assumptions you'll make.
    The marketing world likes to gloss over this and give confident definitive answers to fluffy questions - pointless!
    In reality, that's not how it works.
  • marcusjb
    marcusjb Posts: 2,412
    ^Pretty much what he says.

    Jan Heine comes from the randonneurring direction, and, as such, will tip the balance in favour of comfort over out and out performance. I believe he is largely correct, but I myself am a randonneur. But his thoughts won't be for everyone.
  • imposter2.0
    imposter2.0 Posts: 12,028
    Interesting replies, but if you read the article they are purporting to scientifically calculate the optimum pressure based on tyre deformation.

    As far as standing up goes, I tried that and I don't really load the tyre a lot more than when sitting.

    I realise of course a lot of this is personal preference, but it's the science I am interested in. When two supposedly academically correct sources come up with different answers you know that someone has missed the point.

    As mentioned - science plays a role in many aspects of cycling, but unless scientists are testing your tyres on your roads with you riding, then the science doesn't actually count for very much in this case. Just try some different pressures and go with what you prefer.
  • Ai_1 wrote:
    Interesting replies, but if you read the article they are purporting to scientifically calculate the optimum pressure based on tyre deformation.

    As far as standing up goes, I tried that and I don't really load the tyre a lot more than when sitting.

    I realise of course a lot of this is personal preference, but it's the science I am interested in. When two supposedly academically correct sources come up with different answers you know that someone has missed the point.
    As per my previous post, no-one can give a "correct" answer.
    There's no reason two academic studies wouldn't get different answers unless they are trying to answer the exact same question with the exact same criteria. The question "What's the best tyre pressure?" is wide open to interpretation. You need to decide if you want the best for comfort, best for cruising efficiency, best for sprinting, best for climbing, best for cornering, best traction, best for maintaining road contact on bumpy surfaces, etc, etc.... and even these terms are too vague. Best for sprinting could mean best rear wheel traction for straight line acceleration or best handling for manoeuvering, it may depend on how far out you start your sprint or what surface you're doing it on. To give a correct answer you need to knnow how to measure it. How do you measure "best"?

    Before answering a vague question you have to specify exactly what it means and what assumptions you'll make.
    The marketing world likes to gloss over this and give confident definitive answers to fluffy questions - pointless!
    In reality, that's not how it works.

    Sorry. Best to me means fastest in a straight line uphill or downhill for the energy utilised.
    Don't care about cornering. I don't race.
    Don't care about comfort. If I wanted to be comfortable I would sit in the car.

    If you read the report (which it seems you did not do) you will see it is not vague. It focuses on the pressure at which an increase will no longer significantly improve speed.
  • imposter2.0
    imposter2.0 Posts: 12,028

    If you read the report (which it seems you did not do) you will see it is not vague. It focuses on the pressure at which an increase will no longer significantly improve speed.

    And if you read the report (in particular, the 'conclusion') it says that optimum pressure will vary according to road surface. Which is what lots of people have already said.
  • Imposter wrote:

    If you read the report (which it seems you did not do) you will see it is not vague. It focuses on the pressure at which an increase will no longer significantly improve speed.

    And if you read the report (in particular, the 'conclusion') it says that optimum pressure will vary according to road surface. Which is what lots of people have already said.
    Oh dear. Have you never put together a scientific report?

    The report is based on average road conditions. We all know that pressures need to be reduced if it is rough. Give us a little credit here FFS.

    The point is that based on average road conditions, the suggestion in the referenced report is that tire pressures need to be a lot lower than what is commonly accepted as "normal" for average road conditions.

    When someone questions common practice it is the sport of the narrow-minded to try to perpetuate the norm. Are you in the "passive smoking isn't harmful camp" as well, or just the "it's a new bit of research so I will disregard it until I am in the majority" camp?

    Personally, I like to consider new information, question it through the views of others (even anonymous, potentially ignorant strangers (no offence intended :D ) ) and form my own opinion.

    In this case I am prepared to consider that the sweet spot being referred to may be lower than is commonly considered correct. I am not a sufficiently talented cyclist to make my own determination based on personal feel (which I know depends greatly on whether I drank Tempranillo or Pinot noir the night before riding) . However, I would be interested to know if there are further studies (not random opinions) on this topic in order to better inform my attempt at an informed opinion.

    Formula one car racing has a huge bank of knowledge on tire pressures and their effect on performance. It somewhat amazes me that cyclists seem to be happy to go with, as recommended above, "what feels best for you".
  • imposter2.0
    imposter2.0 Posts: 12,028
    Oh dear. Have you never put together a scientific report?

    Nope - although it's worth pointing out that the article you are linking to is not a 'scientific report'. Anyway, I'll leave you to it.
  • rolf_f
    rolf_f Posts: 16,015
    Imposter wrote:

    If you read the report (which it seems you did not do) you will see it is not vague. It focuses on the pressure at which an increase will no longer significantly improve speed.

    And if you read the report (in particular, the 'conclusion') it says that optimum pressure will vary according to road surface. Which is what lots of people have already said.
    Oh dear. Have you never put together a scientific report?

    It's not a scientific report. It's an article that tells you how to set your bike up to tie in with the conclusion of a study or studies which are not actually published in the report. There's no data provided to indicate that increasing tyre pressure doesn't increase speed (straightforward) but that comfort drops off if you do (a bit trickier). There's nowt wrong with that but scientific it isn't.

    It does at least relate rider weight to appropriate tyre pressure (albeit only in passing) which is useful. All too often this isn't accounted for yet it is as much a variable as tyre width (which does seem to be a trendy topic).

    Mind you, I struggle with the conclusion. My wheel load is going to be about 34kg and I ride 23mm tyres - that gives a pressure of only about 75psi (or less than 65 if I were on 25s) - that sounds very squishy and very pinchpuncturetastic to me. Would be interesting to hear from anyone who has tried this out.
    Faster than a tent.......
  • Im too heavy for my tyres according to that graph. I must need something like 200psi!!!
  • ai_1
    ai_1 Posts: 3,060
    ....Oh dear. Have you never put together a scientific report?
    Well yes I have actually. But it says a lot about you that you actually asked that quetion in that tone and context.
    The reports you are referencing are not, however, scientific reports. They're quasi-scientific technical discussions. Have you never read a scientific report?
    The report is based on average road conditions. We all know that pressures need to be reduced if it is rough. Give us a little credit here FFS.
    You're not giving much credit, not sure you've earned any. You say you're looking for new ideas and disparaging others for accepting popular wisdom and then you say this?
    .....Formula one car racing has a huge bank of knowledge on tire pressures and their effect on performance. It somewhat amazes me that cyclists seem to be happy to go with, as recommended above, "what feels best for you".
    That appears to be because you misunderstand the different variables at play in each sport. Formula one is about grip and durability on consistent surfaces and fixed wheel geometry, with easily predicted loadings and power delivery. Neither is easy but cycling is far more difficult to analyse in a useful way and there's much less reason for anyone to bother trying. Cycling is more about psychology and physiology than it is tyre pressure. It matters but it's rarely a critical factor.

    If you just want to know the criteria for minimum rolling resistance on a smooth surface while pedaling smoothly in the saddle - well that can probably be done relatively accurately and will be somewhat relevant for time trialists. The other opinions you've received and disregarded in this thread are far more relevant for most cyclists. i'm not saying that because I don't understand the difference between "science" and feelings but because on the bike: feelings = perfomance, most of the time.

    In case you're not aware of this, a well educated mind that understands the physics, context and available criteria regularly outperform insufficiently detailed mathematical/engineering models. Simple physical systems are easily modelled and analysed but when modelling complex systems, the assumptions of the person designing the test can have more of a bearing on the conclusion than anything else. You're often dealing with guesses either way. The important thing is to realise that.

    For example: My background is aerodynamics. One common tool in aerodynamics is Computational Fluid Dynamics or CFD. This is where you define a mathematical computer model of a fluid dynamics problem and use it to predict the real behaviour of the system. The physics governing this are well understood as all of the parameters are accounted for by the Navier-Stokes equations. However these are indeterminate so you are forced to make many assumptions and simplifications. Even if you've got a supercomputer network available to run the calculations you need serious computing time to run even very simple simulations without making major assumptions. So, those assumptions end up having more bearing on the result than anything else. That's why we still use wind tunnels. However you'll see people wheel out "scientific" findings heavily based on CFD modelling which are in reality utter nonsense - mostly for the purposes of marketing. Why? Because CFD gives pretty pictures - I'm sure you've seen them - multi-coloured renderings of vortices swirling off a formula one car, aeroplane or building?
    So, what's my point? Well, CFD gives detailed data that looks great and has lots of solid numbers to many decimal places. It appears very concrete and precise and it's very tempting to say "That's the right answer" now we know what to do. The reality is that the data means nothing unless you understand how it was created and how to interpret it. A good aerodynamicist could create a more accurate picture with some crayons and a sheet of paper than the most advanced CFD package with a slightly flawed problem definition. CFD is a great tool but don't start thinking these tools make opinion/judgement irrelevant.

    I do go on don't I.............. :wink:
  • I am 95kg and run 112 front and back.

    Had no issues yet but guess its what you get used to and what type of ride you are looking for
  • mugensi
    mugensi Posts: 559
    I'm 76kg and the bike with saddle pack and water bottle weigh approx 9kg and i pump my tyres to 110psi front and 115psi rear, anything lower and i find the bike sluggish especially on rougher surfaced roads.