Contact patch
I haven’t visited these boards for some time (my handle used to be ET and I’m using my friend’s handle since I’m having trouble logging in) and noticed the huge thread on tire width and contact area. Nice to see some things never change: the usual slugfest pitting theoreticians relying on basic principles against practical engineers, members of each group against each other and against well-intentioned laymen, disparaging comments about the other two groups from others, etc., all of which often resolves nothing,
Just a friendly suggestion: no need to re-invent the tire, er, wheel each time: there are already some quality, carefully researched books already out there covering such topics, so if interested, just get a couple of them and quote from the books. Here are two:
Performance Cycling, by Stuart Baird,
http://www.cyclepublishing.com/cyclingbooks/pc.html
and Bicycling Science, by David Wilson,
http://www.amazon.com/exec/obidos/tg/detail/-/0262731541/104-3965678-0863124?v=glance
In Baird, on page 105, in the section on cornering, it says:
“…The bottom of the tire, the part touching the road, deforms under the load it is carrying, flattening out into what is called a “contact patch.” How large is this contact patch? It should be easy to see that the size depends on the tire pressure and the load. No load, or rock-hard pressure, and the tire doesn’t deform at all: the contact patch is just a tiny point. Heavy loads and less pressure increase the size of the patch…Some bicycle tires, too, have stiff portions of tread…But it is still true that for tires of similar construction, two tires with the same load and pressure will have the same contact area. Notice that we have not mentioned tire width at all. The omission is intentional, because the area of the contact patch does not depend on the width of the tire. Inflated to 90 psi and fitted to a bike under the same rider, a super-narrow 700C x 19 clincher would have the same contact patch area as a medium-width 700C x 25 or a 700C x 32 loaded touring tire. Only the shape of their contact patches would be different.”
So that answers the question.
On page 132-133 of same, there is a discussion of the coefficient of rolling resistance (CsubR) as a function of contact patch area, the difficulties obtaining an accurate CsubR, and whether the relationship would in fact be close to linear or not. It mentions Wilson’s book.
In Wilson (third ed.), Chapter 6 (Rolling: tires and bearings), regarding measuring rolling resistance on a flat surface, it says,
“Slope is highly important. Nominally level indoor surfaces can easily slope 0.001 in places, altering the apparent value of CsubR by 10-50%. [So much for measuring in your house, let alone outdoors.] …The entire subject of rolling resistance has been treated primarily empirically from a variety of perspectives, and much further study is needed. For these reasons we simply summarize a wide range of published results.”
It then goes on to give numerous and oft-complicated empirically derived formulas for CsubR as a function of contact patch area and other parameters depending on such things as firmness of the road and wheel, tire diameter, and even speed. There is some debate between the two books as to the accuracy of the formulas used.
It is obvious, then, that it would be difficult to determine the gain or improvement of one slightly different tire over another.
Both books give formulas under certain simplifying scenarios. In general, Wilson is far more technically detailed, but Baird is better with explanations as to what’s going on, so they both have their place.
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Chili hed & old bike fixr
Joined Jan 22, 2002
1,126 Posts
#2 ·
Jan 19, 2005
As said before, there are those that have their mind made up, and refuse to be confused by the facts, for whatever reason( you fill in the reason!!!!)
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C
Registered
Joined Sep 14, 2004
491 Posts
#3 ·
Jan 19, 2005
Howard3 said:
The omission is intentional, because the area of the contact patch does not depend on the width of the tire. Inflated to 90 psi and fitted to a bike under the same rider, a super-narrow 700C x 19 clincher would have the same contact patch area as a medium-width 700C x 25 or a 700C x 32 loaded touring tire. Only the shape of their contact patches would be different.
Well, this is true, but neglects some important points. Compare for instance a car tire with a bike tire. If you were to put a car tire on a bike, the contact patch would NOT be the same. It is not the width, but rather the shape of the tread. In the picture attached, the tire to the right will have a larger contact patch due to its flatter tread. In the first and second tires, since both can be thought of as clocks, and make contact only at the 6 o'clock position with the road surface, it's true that the tire WIDTH does not make a difference, because the tire makes contact ONLY at the 6 o'clock position, which is essentialy just one point on a circle (called a tangent point).
It is for this reason that bike tires do no hydroplane and treads are not channeled as in cars, where the tread is like the one on the right. In that case, the water can build up and treads need channels to funnel the water out of the area between the contact patch and the road surface. Poor channeling yields hydroplaning.
Suppose you have a 75 kg rider, and the area of contact patch between the two tires combined is just one square inch. That means the load is 75 kg/square inch. Now, put that same 75 kg rider on the tire to the right, and you can imagine the 75 kgs are distributed over 3 square inches, or 25 kg/sq inch. That has significant implications for road grip.
Would it surprise you to find out that a needle on a record player supports more weight per square inch than a car tire? That's because the contact area of the needle on the record is so small that the weight:contact area ratio is greater than a 3,000 lb car on 20 square inches of road tire contact.
So, if the tire is of the type that has one (tangent) point of contact, it is true that tire WIDTH has no effect on contact patch. But that does not account for tread type. And tread type does differ between tires to a degree. The flatter the tread, the more area of contact, the less weight/square inch, creating different handling characteristics.
Theory is one thing; practice is another. And I know of no tread that makes contact at one tangent point. In fact, underinflate your tire, and you can see for yourself the contact patch grows rapidly as psi drops. Because this yields less force/square inch, underinflated tires tend to slip more in slick/wet conditions.
Let's take this just one step further. Ice skates are sharpened for a reason. It would seem to make sense that a wider skate surface would give you a better grip on ice. Just the opposite is true. The narrower the skate blade, the better. The principle of why a skate glides over a surface is as follows: the increased weight/square inch is converted to heat as the skate glides over the ice surface. This actually causes local ice melting, thus the skate glides on a very thin film of WATER over the ice. The water quickly refreezes as it is so thin. If you don't sharpen the skates, the weight/square inch falls, this causes less heat and less melting, thereby decreasing local melting. This slows a skater down and causes more slippage.
So, tire width per se has little effect. But do not be fooled to believing that tread SHAPE has little to do with tire handling. It has EVERYTHING to do with it.
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