CityCyclingEdinburgh Forum » Questions/Support/Help

Contact patches

(9 posts)
  • Started 10 years ago by Snowy
  • Latest reply from I were right about that saddle
  • This topic is not resolved

No tags yet.


  1. Snowy
    Member

    I read something this evening that intrigued me and I thought I'd run it past people with knowledge of such things.

    It went like this: the weight of a vehicle (including driver/rider) in pounds, can be be divided by the PSI of the tyres, to give the size of the vehicle's contact patch with the road. You would then divide by the number of tyres to get the size of the contact patch for each tyre.

    Apparently this gets you within 10% of the right value, due to factors like variation in sidewall stiffness.

    So if my bike plus myself weighs 220 pounds (cough) and I run my tyres at 110 PSI, I have a contact area of 2 square inches, or 1 square inch per tyre.

    This seems somewhat small. Can this be right? If so, I have a new found respect for the gripiness of my tyres...

    Posted 10 years ago #

  2. I were right about that saddle
    Member

    That formula is touted in the motorcycle world (where the contact patch is about the size of an adult male hand). I reckon it would be right for a latex tyre with no inherent stiffness, provided you measure the air pressure while sitting on the bike.

    I'm an empiricist though, and am now tempted to make the actual measurements. Is there anywhere you can ride a bike over a plate glass surface and observe it from underneath? Can't be bothered getting paint on my tyres....

    Posted 10 years ago #

  3. wingpig
    Member

    Film it from the side riding past a tape measure to get the maximal fore-aft length of the contact areas and ride through a small puddle then a dusting of flour to get the maximal width? May require assuming the contact area to be elliptical.

    "Is there anywhere you can ride a bike over a plate glass surface and observe it from underneath?"

    I have two expired glass shower screens and some bricks in my back garden which could, if properly arranged, support a bike on top with a camera underneath.

    Posted 10 years ago #

  4. I were right about that saddle
    Member

    @wingpig

    Game on. I've got a very accurate schraeder tyre pressure gauge and a presta track pump with a decent gauge built in. Bathroom scales...anyone got a digital set?

    Only the rear wheel needs to go on the glass - photograph the contact patch and let the tyre down in 5psi increments until the rim touches down - plot up the resulting 1/pressure vs area curve. Should be linear, with a slope equal to the weight on the wheel if the theory is correct....

    How best to get a millimeter grid onto the glass?

    Posted 10 years ago #

  5. wingpig
    Member

    "How best to get a millimeter grid onto the glass?"

    Print one out on acetate or just glue two perpendicular rulers on the underside and Photoshopically extend lines from each into a grid?

    Posted 10 years ago #

  6. acsimpson
    Member

    With two shower screens you could put the acetate between the layers.

    Posted 10 years ago #

  7. Dangerous
    Member

    There is a good reason that the formula is touted in the motorcycle world. It's called Physics.

    http://en.wikipedia.org/wiki/Pressure

    Pressure = Force / Area

    Posted 10 years ago #

  8. Arellcat
    Moderator

    Tulyar and I discussed this a while ago in relation to friction of surfaces (eg tarmac vs steel rails).

    From Continental:

    and possibly more than you wanted to know about contact patch shape, and how pressure influences the shape, and how shape influences rolling resistance (as does carcass ply direction and sidewall TPI).

    http://flocycling.blogspot.co.uk/2011/11/flo-cyling-contact-patch-why-wider-is.html

    Posted 10 years ago #

  9. I were right about that saddle
    Member

    @Arellcat

    Thanks - really interesting.

    You know you still want to join me, acsimpson and wingpig with a pile of bricks and shower cubicle doors though, don't you?

    Posted 10 years ago #

RSS feed for this topic

Reply

You must log in to post.


Video embedded using Easy Video Embed plugin