Wednesday, March 7, 2012

The grip...it's all about the grip - Understanding tires Part 4

This is the 4th of 5 posts about tires.
1 is about how important it is to understand tires and what to consider when buying them.
2 is about the tire measures and proportions one can see in the tire walls.
3 is about tread design
4 (this) is about compound and structure
5 is about maintenance and repair

Tires are supposed to create drag. That's it's main purpose because by doing so they will allow you to grip the road. That's one of the things I've never really understood when people buy budget tires (plastic model copy or a well known tread design) or when they buy energy "efficient" tires that reduce rolling resistance. These are 2 examples of what's not to do. The plastic compound doesn't grip and the budget tire with low rolling resistance lacks structure and will bend all over during cornering, loosing shape and ultimately not griping.
For some reason people love to see their cars are equipped with competition sourced technology, but then go the opposite way when it comes to tires.

This is a post about the tire compound and the tire's structure.

Starting with the compound.
As the name states, a compound is a new material composed by several different raw materials in order to archive together what none of them would be able to alone.
Evidently, talking about tires, one always talk about a rubber compound. However the ingredients and cure involving these compounds are mainly composed by rubber, several others are added in different parts of the process and quantities, allowing for a product more suited to the usage its going to be subject to.
Most tire rubber compound mixes include:
  • Rubber or polyisoprene (sometimes cut with Styrene-butadiene co-polymer for low budget tires)
  • Polybutadiene (to control temperature build-up)
  • Halobutyl, Chlorobutyl or Bromobutyl  rubber (user for air insulating the inner side of the tire, being Chlorobutyl the most expensive option... see those tires that loose pressure too often? either they were cheap or they have a puncture)
  • Halogen (used in chemicals bonding process)
  • Carbon (increases hardness, resistance and longevity)
  • Silica (used in High performance tires together with carbon to increase durability by lowering heat build up)
  • Sulphur (not for the evil smell but rather to allow rubber molecules to link with each other during the eat and pressure of the vulcanization process...and thank Charles Goodyear for that process)
  • Zinc Oxide to catalyze vulcanization process, together with antioxidants and antiozonants that protect the rubber against oxygen and ozone that tend to make cracks all over the rubber during the vulcanization process.
Together with the rubber compound, a tire is also made out of steel and fabric, but we'll be covering that later.

There is no one single mix recipe. Different manufacturers would use their own ingredients choice and percentages according to brand and product line...but a simple example would be something like:
  • 60 to 65% Rubber (SB rubber would be natural rubber cut with Styrene-butadiene making it cheaper and ultimately worse)
  • 28 to 35% of Filler compound made form Carbon and Silica
  • 1.5% Catalytic ingredient like Zinc Oxide
  • around 1% Acid
  • around 1% Chemical Accelerators
  • 2 to 3% Oil 
  • around 1.5 to 3% "special ingredients" (silica mostly but depends on the tire brand and tire product line and the amount of silica already present on the filler compound)
So now we have a recipe of ingredients and proportions... and like the cakes one can cook at home, the ingredients have and order and time to get mixed in the blender. It's the same with composite materials, so rubber compounds are no different. Much like the cake, this recipe will have it's ingredients mixed together in a huge blender machine under some counter rotating mix-blades. The big difference here is that unlike sugar, flower and eggs , these ingredients are denser and heavier. As a consequence, the mix eats up lots of energy, generating loads of heat. If this was your cake, it would start cooking while under mixing and that's not desirable, so the tires mixing process is cooled to control temperature and avoid pre-vulcanisation (the cooking part)...don't get me wrong, temperature is good and will allow better mixing, it is however difficult to keep uniformly controlled. Better compounds require an expensive and energy consuming process that will process smaller amounts at a time in order to ease the process and produce better compounds...but in smaller quantities with an expensive process, making this part of the reason why better tires are more expensive.
Back to the cake, after mixing, you normally tender the mix into a mould powdered with flower...and then again, the tires are also tendered and extruded several times into a mould powdered in talc.

And again, just like those cakes your grandmother used to do that would take forever to mix and cook are far better than the ones you buy in the supermarket, rubber compound mixed without rush and using the best of the best available tools with perfect temperature control and movement control to assure the best mix will produce the better, more uniform compound. Like most things that are built, taking time to do it right will produce better results.

I'll be continuing the rubber compound process story later on in the "structure" section.

Now just how much of a difference can tire compounds or "recipe ingredients" do when one plays with the proportions?
First comes the original rubber compound. We can start a rubber compound with 4 different rubber materials:

  • Natural Rubber - This is an expensive raw material and is also very soft so it's essentially used as a part mix to BR (Polybutadiene Rubber) very common in heavy load tires (truck tires).
  • SBR (Styrene-Butadiene Rubber) - This is the most common used rubber base used for tires. The Styrene/Butadiene mix varies a lot. For instance, the most common mix is 20% to 25% Styren on 80% to 75% Butadiene. If you want an excellent gripping tire that will work in cold and wet conditions, all you have to do is UP the Styrene on the mix to say something like 40%. But the recipe is not just to UP the Styrene on the mix as a tire needs to maintain structure and be able to endure more than 4 corners...so some hardness is essential. This behaviour is a result of the compound mechanics that make it generate more heat while being deformed. 
  • BR (Polybutadiene Rubber) - This is the primary bi-product of an oil refinery. It's a much "plastic like" rubber that is very hard and generated a lot less heat while being deformed. Now one might think that this is the rubber used in those budget tires that wear little and grip even less...and be partially wring about it. Ever seen a F1 race be interrupted by a pace car and make all the F1 cars start swinging behind it? They do so to stress the tire compound enough to generate heat and maintain tire grip levels...and they do so on hard compound tires that are very good enduring abuse, but the high Polybutadien compound they use can't get too cold or it will not grip.
  • Butyl Rubber and Halogenated Butyl Rubber - This is a soft rubber that bounds in an very tight and elastic way. Essentially it's very soft and a very good sealing rubber. It's used inside every tire to ensure a tight air seal. 

 Then comes the filler compounds, and these are essentially 2:

  • Carbon black - As it's expectable, carbon adds hardness and friction resistance.
  • Silica - Silica is a recent material in tire manufacturing (being used since the 90's). It enables the tires to endure more while decreasing roiling resistance and increase grip.

These days Carbon black + silica are widely used to produce tires that are able to generate grip without drawing so much energy while maintaining it's endurance and friction resistance characteristics.

The Mixing of all these materials, as already explained is mechanically and temperature controlled. The initial mixing is done around 150 degrees Celsius, mixing rubber and filler together. The rest of the ingredients is mixed in later and cured at a lower (100 degrees) temperature.

The structure
These following tire cut drawing (Property of toyo tires http://toyotires.com) 
The tire structure starts with the tire bead components. These are marked in pink and are a steel cord wrapped in rubber. The tire bead will ensure a perfect fit between the tire and the rim. Its steel cored for the simple reason that while extremely endurable and stuff, steel also has elasticity. 
While inserting the tire in the rim, the machine will stretch the tire bed trough the rim lip (witch is a good 1,5cm higher that original rim and tire bed diameter). The steel bed will allow for this to happen with elastic deformation and return immediately to it's original size after passing the rim lip. This is one of the reasons one should not fit a 17inch tire in a 17,5 inch wheel. The bead is built to specification and takes the elastic deformation onto account...over that and you create uneven plastic deformation and that will not return to it's original form.
The grey part on the cut drawing in the inner liner. It's purpose is to insulate gas inside the tire under pressure. It's built from soft Butil Rubber pressed against another layer of steel wire mesh reinforced rubber.
The blue part is something not all tires have. It's a reinforced side-wall. This makes the tire very hard and resistant, but also less deformable and also less comfortable. It's a very important part of a performance tire because the stiffer it is, the less flex and bending of the tire structure (and tread area in consequence) under heavy cornering.
With the inner liner formed, the tire will gradually be layered.
The tire wall you see in blue will be the LAST part of the tire assembly process.
This type of construction is called radial construction as the tire is laid out in layers that will each have its own radius on top of the earlier one.

This other cut drawing shows a bit more on the tread bed and radial layers.
With the tire bed already laid out, the body ply (red layer) is layered on top of it.
This layer will add thickness and hardness in the tire structure.
The next 2 layers (green and orange) are made from smaller steel wires pressure-sandwiched in rubber and criss-crossed (each layer has a crossed orientation from the one below).
The Yellow layer is a nylon fabric tissue layer.
Last but not least comes the real tire tread compound layer. This can be made as one single layer or a mix of 2 different layers (a harder and a softer compound):

  • The best tires are made from the same kind of soft rubber compound into one single tread layer.
  • Good tires can be made from an inner softer compound and an outer harder compound. This will make the tire grip well in the wet while new (because the groves are deep and the soft inner compound allow more flexing of the tire tread pattern), and when they start wearing the loss of grip due to texture will be replaced by better rubber compound that will not allow so much water through but will generate more friction, compensating on the actual contact grip side.
  • Finally the "honey-moon" tire as I normally call it. Its an Inner hard compound tire wrapped in soft compound. You will have lot's of grip that will go away with the soft compound leaving a harder more durable compound that will grip less but will "live forever".
There are some exceptions to this rule and understanding how a tire compound can be mixed together, one can realize that there are several types of soft and several types of hard compound. A Budget hard compound will be hard because of excessive carbon-black... this will endure and not grip even when hot. A competition Hard compound will be made from high Polybutadiene Rubber compound, making it grip with high temperatures and endure abuse. Following this line of thought:
  • You can have a soft(outside)-hard(core) compound tire made to grip a lot without needing to warm-up and then wear to the hard compound made to grip while hot. This would make an excellent wet-to-dry competition semi-slick tire. The outside softer compound with groves would run colder because of water and channel water out of the tire's way ensuring grip; as the track would warm the softer compound would wear faster generating more heat and gradually let in the harder compound already up to heat spec.
Finally
As in our cake, after mixing, tendering and pressing against a mould, is time to cook.
Now cooking a tire requires a lot more than heat.
After all the layers are put together (already under heavy mechanical pressure) the rubber in the tire is still ductile. Like most composite materials, an high energy reaction is needed to bound everything together at the molecular level.
Remember the Halogen, Sulphur and Zinc Oxide? They all come to life here when the tire is pressured cook inside a huge doughnut shaped oven. This is called the curing process (as in most composite materials).
The tire is placed into the lower mould bead seat, a rubber bladder is inserted into the tire. While the mould its self is closed, the bladder inflates, creating pressure. A recirculating steam gas or water will then add heat to the process.
The tire will cook under around 350PSI at 180 degree Celsius for anything from 20minutes to over 20 hours, depending on the compounds, size and thickness of the tire.
Some tires require a cool down process using a cold inflater to allow the tire to maintain form.