# Regarding tire width and contact area



## Howard3 (Mar 30, 2004)

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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## curlybike (Jan 23, 2002)

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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## CoachRob (Sep 14, 2004)

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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## Anti-gravity (Jul 16, 2004)

Howard3 said:


> 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:
> 
> ...


I've always understood the whole lack of relation between tire width and contact patch, I don't see how that is obvious to more people. But...I think one can still justify narrower tires over wider ones because of the ability to inflate narrower tires to a higher pressure range (since the wider a tire gets, the lower its max pressure). So by having that option to increase tire pressure and make that contact patch smaller, that somewhat justifies the usage of narrower tires. However, the actual benefits of decreasing the contact patch and thus rolling resistance are pretty irrelevant from what I have heard (especially when compared to aerodynamics and other sources of inefficiencies). Unless you're comparing a 2" wide MTB slick to a 700x20 road tire, the differences between a 20 and 23 road tire at their respective max pressures is miniscule, from what I have heard.
-Ryan
-Ryan


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## symbo (Dec 7, 2004)

*Again?, we covered this!*

Here we go again. Coach Rob, sorry to say, but you are wrong. Even if the car tire magically weighed the same as the bike tire in your analogy, the contact patch is a function of load and pressure only. It's easy to get caught up in geometry, but think about it. Here is another analogy, lets say you are on a trike. The three wheels have a cumulative contact patch based on the load, you go around a corner sharp, lifting one off the ground, the other two contact patches increase to compensate.

If you looked at the interface of your car tire microscopically, you would see the actual surface contact decrease with decreased load. If the geometry was very flat as you noted, you would see just dots of contact on the highest ridges, but with the cumulative area the same.

Lets reduce the load to nearly zero, the contact surface gets smaller and smaller until it reaches zero - at that point your floating above the ground. If the tire, or asphalt for that matter, happens to be very gummy, your talking about surface tension, which is a different topic.

Take the opposite case. the area getts smaller and smaller until the bike is sitting on the head of a pin. The contact area is very very small, the load the same, and thus the pressure very high, probably more than the tire can handle.


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## rivercliff (Jun 2, 2004)

*So....*

OK, so taking in what I have read in other posts about a 700 X 25 tire inflated to 100 PSI being a more comfortable tire, AND less likely to flat... and put that post with this post, then perhaps the tire to ride is a treadless (race) 700 X 25 ?


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## Kerry Irons (Feb 25, 2002)

*Wrong on so many levels*

First, you are looking at the wrong dimension. For a car tire you have a rectangular contact patch of relatively fixed WIDTH, it is the LENGTH (front to back) of the patch that changes with either tire pressure or load. As the load per tire approaches zero, the contact patch aproaches a line of zero width. With a bike tire, the size of the oval patch shrinks in both dimensions as the load is reduced/pressure is increased. Both the car tire and the bike tire have some casing and rubber stiffness issues that cause deviations from ideal, but particularly in the bike tire situation, these deviations are small. Compared to the 100 psi pressures in the bike tire, casing stiffness and rubber properties are very minor.

Regarding hydroplaning, it is not really about the shape of the tire. There is a formula for determining whether a tire will hydroplane, and it factors in speed and tire pressure. A bike tire will not hydroplane because the speed is way to low and the pressure is way too high, not because of the shape of the tire.

Putting a 150 lb. load on a car tire will give you a 3 square inch contact patch if the tire pressure plus casing stiffness is 50 psi. In a car tire, casing stiffness would be significant at this loading, but it would be meaningless in a bike tire. Recognize that a typical car tire is carrying more like 750 to 1000 lb, and so has a contact patch of 25-30 square inches - 6 inches wide by 4-5 inches long. Again, car tires have a roughly fixed width of contact, whereas a bike tire contact patch changes in both length and width as the load or tire pressure changes. Talk about tangential contact is meaningless.

A phonograph stylus is made of an essentially incompressible substance, like diamond. There's no relevance to this discussion. Same with the ice skate analogy.

Reducing tire pressure increases contact patch and INCREASES traction in wet conditions. Just the opposite of your claim. Ice skates and ice are essentially non-deformable, and the fact that the skate is melting the ice to allow glide makes the comparison additionally meaningless. A bike tire becomes more deformable with reduced pressure, improving traction as the tire contacts the surface irregularities in the pavement.

If there were bike tires that had a non circular cross section, it would be a different situation. A car tire achieves its contact patch through adding relatively huge amounts of rubber to the shoulder. No light weight bike tire does this.

You've simply got it wrong. Any differences caused by bike tire casing stiffness, width, or tread are in the 3rd or 4th significant digit. Bike tire contact patch is the load divided by the tire pressure. Full stop.


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## lithiapark (Apr 6, 2003)

"If you looked at the interface of your car tire microscopically, you would see the actual surface contact decrease with decreased load. If the geometry was very flat as you noted, you would see just dots of contact on the highest ridges, but with the cumulative area the same."

I don't remember seeing this point being well made in the other long thread, but this is very important. The inner "contact patch" of the tire is quite uniform-a gas fills in all gaps and crevices on the inner surface of the tire. The tread contact with any surface, even if it is plate glass, will never be as uniform. If you look at the actual contact of the tread very microscopically, there are many gaps, and they all add up, so the macroscopically observed contact patch will be larger than calculated to make up for the "denser contact patch" between the gas and the inner surface of the tire. 

Tread stiffness does make a difference at the microscopic level. Not all points of contact of the tread will see the same "microscopic PSI" that the inner surface of the tire will, some will be slightlly lower and some slightly higher. Because of the increased filling of the gaps between contact points of a soft tread with the surface by greater conformity to it because of more deformable tread material, a soft compound tire should have a smaller "macroscopic contact patch" than a hard compound tire. I am not a tire chemist, but what I have read leads me to believe that greater microscopic contact of the tire tread with the road surface is one of the important elements increasing the relative coefficient of friction.

The laws of physics are the laws of physics in determing the contact patch area for a given pressure and load. We must be very careful, though, in how we are measuring the contact patch, macroscopically or microscopically. The microscopic contact is what we should be considering.


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## ETthat'sMe! (Aug 17, 2004)

Funny how no one took my suggestion to acquire the book(s); it would prevent needless debate, which was my point. CoachRob, the books go into detail comparing a car tire to a bike's.

Sincerely, ETthat'sMe!

BTW, I got a password and am now logged in under myself. Does anyone know how I can change my user handle? I tried to re-register under a new handle but it matched the email address to the old handle and wouldn't let me re-register, instead directing me to retrieve a new password. Thanks.


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