Monday, March 2, 2009

The Tire Contact Patch

Tires are a very interesting element as they have such a significant impact on the handling of your car. Humorously though, they are pretty misunderstood. How a tire keeps the car gripping the road is an extremely in depth and technical issue, so lets preface this discussion by saying we are going to only go in to the broader generalizations and some of the more easy to understand points. It gets very complex when you start trying to approximate contact patch deformation, sidewall spring constant, pressure distribution etc. There are a lot of great books out there, so definitely find them and read them, but the hope here is to get you thinking about a few simple items to begin with.

So far starters… wider is better right? There isn’t a straight answer to that. So lets discuss what happens with the tire first of all. Think of a balloon for starters. If you could put it on a piece of glass and look at the area that is “flat” where the balloon contacts the glass you’ll notice one thing quickly and simply – the more pressure you apply the bigger that area gets. This is a pretty simple concept. When you have a weight applied to an inflated item it has to respond with a force equivalent to the weight to be able to support it. This applies to your tires on your cars as well. As I mentioned before, though, this gets pretty complicated when you really consider how much support the sidewall gives etc, but for now lets ignore that. Most tires for people on high performance cars will run in the 35-45 psi range. So lets choose 40 psi as your inflation pressure. Let’s also use my BMW 318i as an example. It weighs about 2800lbs and is very close to 50/50 weight distribution. So we’ll assume the car is properly corner weighted (corner-weighting is when you adjust spring height and rate etc to distribute the car’s weight evenly across the diagonals and each side/edge of the car) so that the weight is evenly split between each of the 4 tires. This means that each tire is getting 700 lbs of weight on it. At 40 lbs per square inch (psi) that means we need 17.5 square inches of area to support that for my car. Sounds big right? Not really. My car during the summer runs a tire I can only get in a 195 width. So that means it is roughly 7.7 inches across at the point where it touches the pavement. If I’m 7.7 inches across, to get to 17.5 inches means I only have a rectangle that measures 2.3x7.7 inches. Not a lot of area to keep that car on the road is it! So now lets go to a wider tire and see what happens. If I keep the tire pressure the same but go to a 235 width tire my contact patch now measures 9.25x1.9 inches. It is still 17.5 square inches at the road, it’s just wider now. Yep, that’s right, you just went WAY up in tire size and you still have 17.5 square inches in contact with the road! What you have done is changed the shape of your contact patch. Now, this is somewhat misleading because with many of the larger tire sizes you can run a little less tire pressure which will increase your contact patch for real. However, as you reduce pressure you lose some of the structure effects in the tire and you may actually end up with strangely shaped or poorer contact during cornering as you roll on to your sidewalls or have other issues. This is where you need to work on properly balancing the pressure for your tire.

It is also very important to keep in mind tire compound. Often you can achieve much more grip with a smaller contact patch area due to the adhesion factors of certain tire compounds. Tires generate their “grip” in a few ways. One way is simple generic friction between the surfaces and the way they bond to one another. This is greatly affected by compound. A second way is mechanical grip that is generated by interference between the objects, for example, if the rubber complies to the shape of a gravely pavement, you will have some significant vertical walls the rubber is being pushed against and torn away from the tire by. This generates grip as well. The third major grip component is adhesion. Some tire compounds actually are truly “sticky” and will adhere to the surface they roll over. This allows coefficients of friction greater than 1 to be achieved! What this means is that for every 1 lb of force applied down on the tire, it can generate more than 1 lb of force perpendicular to that. This is HUGELY dependent on compound and is a significant factor in the advantage that race slicks will have over street tires (although that changes more and more every year with some of the crazier street tires).

You also need to consider what happens to the contact patch during maneuvers. A tire actually provides the MOST grip at a point when it is “sliding” a little relative to the angle of travel. This is referred to as slip angle and varied greatly depending on tire design. It is often in the range of 3 to 10 degrees though. This matters because as you make your tire contact patch wider you change the amount of difference between the arc radius the inner portion of the tire travels versus the outer portion. This can lead to those portions of the tires actually traveling at different slip angles from one another and it can lead to conditions such that one portion of the tire may be at optimal slip, while another is not. This is one relatively extreme case where wider would indeed NOT be better. Also, as tires get wider things like turn in response and their behaviors under braking or acceleration and the changes in the tread deformation introduces a lot of other variables and possibilities. Again, the topic is very complex so it is hard to argue it fully in a short essay. I’ll try to revisit the topic in the future and get into some of the more specific details for the above, but hopefully this got you thinking and drop at least a few new ideas out there for you.

Happy motoring!

Friday, January 30, 2009


I have a few more topics I want to cover, but if you have any suggestions or burning questions you'd like to see my take on, please do comment and let me know! It's no good if I'm not getting at the issues and areas that you are interested in.



Wednesday, January 7, 2009

Air Fuel Ratio - Basics

Now that the holidays are past and our year end accounting is nearly done I have a little time to get another post out for you.

We covered timing before, but as I said previously AFR (Air Fuel Ratio) matters a lot as to how the vehicle will perform. First a simple background on AFR. When it comes to standard gasoline stoichemetric (meaning chemically balanced for both sides of the equation for a chemical reaction, in this case combustion) is 14.7 parts of air to 1 part of fuel or 14.7:1. ANYTHING higher than this (say 15.2:1) is considered a lean mixture, and anything lower is considered a rich mixture. A lot of times people will say when a turbo car is running 13:1 in boost that is running to lean, but what they really mean is that the car is not running as rich as they think it should, because even at 13:1 the air fuel mixture is definitely a rich mixture.

So that all said, what do various AFR mixtures get you. Well obviously the leaner you go the less fuel is being used, but that does NOT mean the vehicle will use less fuel. In fact, as you go leaner than about 15.5 you typically lose enough in power output from the leaner mixture that the additional throttle input and resultant use of fuel to maintain that mixture uses more fuel than if you had simply run a mixture closer to 14.7. So in other words, don’t think that if you managed to safely tune your car to 16.8:1 that you are getting better gas mileage as this is often not the case! Additionally, when you go to mixtures leaner than 14.7 two major things happen: combustion occurs at a higher temp thus heating your cylinders and valves more significantly, and second some of the “bad” pollutant gases increase significantly leading to a more polluting or problematic emissions footprint from the car. This is why most manufacturers design cars to run at 14.7 AFR. In fact in the vast majority of vehicles out there, the only oxygen sensors in the vehicle are all what is referred to as a narrowband sensor which can ONLY tell you if you are rich or lean relative to 14.7 and provides essentially no useful information otherwise. Sorry folks on a budget, but you can not ever tune a vehicle accurately to some other AFR value with a narrowband sensor, you need to drop the money and get a wideband! At 14.7 AFR emissions are typically the most balanced and allows catalytic converters to do their job.

The fact that manufacturers tune most street vehicles to 14.7 AFR is exactly why chipping a vehicle can sometimes get you more horsepower with fuel only changes. 14.7 is the magic number for naturally aspirated vehicles trying to make emissions. However, that number changes whenever you are in boost for a forced induction vehicle, or if you are trying to make power on a naturally aspirated vehicle. The values are hotly contested, and as I alluded to in my last post on timing these values change quite a bit when you work to balance out your timing values with your AFR. But to speak in general rules as guidelines is relatively simple. Gasoline mixtures in MOST modern combustion chamber designs will provide peak power output at AFR’s between 12.3 and 12.8. The value is often narrowed down to be 12.5-12.6 but as I said these values are hotly contested. I personally consider 12.3 to 12.5 to be optimal for turbocharged vehicles and 12.6 to 12.9 to be optimal for naturally aspirated vehicles. So when I’m tuning an NA car I tune wide open throttle AFR’s to 12.8 to 13.0 and then maximize the timing. NA cars are much less sensitive for timing and air fuel balance so this typically works well and is quick and easy. Turbo cars it is much more complicated. Turbo cars I usually tune to about 11.8 and work out the timing. If I can advance the timing fully without detonation issues then I work to see if more can be gotten from leaning out the AFR. This rarely is the case so I settle in near that AFR range almost always.

There are lots of other purposes and uses for AFR though that can yield interesting improvements. Running very rich mixtures can sometimes help with turbo lag and help cool off the valves. The type of injector being used and it’s placement and the atomization of the fuel can also change what AFR is effective in a given vehicle. Sometimes, you need to use rich mixtures to help warm up a car or stabilize an idle, and sometimes a lean mixture will help with rough running on a vehicle. There are a host of options, but we’ll leave that for another post.

Thanks for reading and happy motoring!