Gentlemen, I asked the "big tire" question way back in engineering school, and the explanation was that when a tire slips on concrete it is not a matter of exceeding a friction factor, it is actually tearing the material.  Since larger tires have more surface contact area, there is more material to tear which takes higher forces and results in more traction.  This is not the classical "friction" scenario. There is another situation with big tires.  If you are on wet concrete, then big tires may afford less traction because the water cannot squeeze out from under the tire.  (hydroplaning) This is a lubricated interface, unlike the one above. Best regards to all, Steve Fujan <div> On 4/17/07, Porritt, David <<a href="mailto:dporritt@mail.smu.edu" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">dporritt@mail.smu.edu</a>> wrote:<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"> Well, I don't want to argue with you.  Traction is a phenomenon related to friction and large tires have more traction than small tires.  While the friction coefficient of the materials don't change the increased area provides more traction. Also I don't have any soft cast iron (at least not soft enough that I want to bang my head on it) but the cast iron I see in pianos is soft enough for strings to groove it as I see on the capo bars when the strings are removed. dp David M. Porritt <a href="mailto:dporritt@smu.edu" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">dporritt@smu.edu</a> -----Original Message----- From: <a href="mailto:caut-bounces@ptg.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> caut-bounces@ptg.org</a> [mailto:<a href="mailto:caut-bounces@ptg.org" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"> caut-bounces@ptg.org</a>] On Behalf Of John Delacour Sent: Tuesday, April 17, 2007 2:27 AM To: College and University Technicians Subject: Re: [CAUT] Friction (was restrung D) At 6:11 pm -0500 16/4/07, David Porritt wrote: >You stated "Friction <<a href="http://en.wikipedia.org/wiki/Friction" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">http://en.wikipedia.org/wiki/Friction</a>> is proportional >to (a) the coefficient of friction of the materials and (b) the normal force >between the surfaces." > >It is also proportional to amount of surface contact involved. I wonder why this  belief of yours has never made it into the physics text books during all the time friction and printing have been around! >That's why racers have big tires.  It's why pushing a large sanding >block on wood is harder than pushing a small block.  That's why big >trucks have big brake pads. Big trucks achieve greater stopping power by the application of greater _force_, using compressed air.  It is the increased normal force that results in greater friction and the parts are made big in order to distribute wear and spread the heat produced (which causes a reduction in the coefficient of friction, and thus brake fade) over a larger area. >Strings burried into soft iron will have more surface contact and >result in more friction than a string touching only at one point. Just as a matter of interest, where do you get cast-iron that is soft?!  Why, after all this time, don't they make agraffes with square holes or at least the bearing surface straight and not rounded?  In fact you will occasionally come across agraffes with a steel pin inserted across the top to reduce contact area.  Apart from the expense, the reason they did not catch on is that they are designed on the basis of the false principle you are putting forward. I suggest you do some reading and get your facts straight before you contradict the laws of nature. JD </blockquote></div>