Tuesday, January 3, 2012

Design serves function and that's the ways of engineering - Understanding tires Part 3

This is the 3rd 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 (this) is about tread design
4 is about compound and structure
5 is about maintenance and repair

Humans are a race seduced by design. We love curves and straights combined, sometimes in a concordant manner and other times edging all over the place. One of the things I've learned however is that design follows function and not the opposite. On some things you can have bad functional design, however when it comes to the choice of the little strips of rubber allow you to walk with your life out of a simple insignificant supermarket drive, because they manager to grip the 2 tones of moving metal, plastic, people and groceries each and every corner of the way without exception or failure, then the FUNCTION over design perspective starts to shine.
The design on the tread of your tires should have a purpose and that purpose should be understood when buying and using our tires.
Tires are mainly about the grip they produce.
Grip is created by 2 factors:
 1 - the compound (something to talk about in the next post)
 2 - contact area
In practice, it sums up to drag. The more there is, the better it will grip. This is pure physics and is something that is easy to spot in the racing world. Racing cars run on wide slick tires made from a soft rubber compound so soft it will not endure a full race, and even these need to be warmed up to grip enough to suit competition levels. In sum, the wide tires provide a big area of contact with the ground, the soft compound will ensure grip and warmed up will transform into something that looks more like glue than rubber.

So why don't we run on racing slicks? Well some actually do so. I'm one of them. The truth is that not every one out there has 2 sets of wheels and a garage to switch the wheels on the car according to weather.
Even watching an F1 race where the weather switches from sunny to wet and then to pouring rain? Immediately when the sun goes down and the rain starts to drop, the racing speed decreases. The reason for this is tire temperature drop and lack of drainage power from the tires. Then each car runs to the pit-stop and changes into either semi-slick or rain tires.
Why these 2 choices? Semi-slicks are slick tires made from an even softer compound (made do grip with lower temperatures) and with some grooves on the laterals of the contact area to the side of the tire to allow some water drainage; while the full rain tire was deep grooves that run from the center of the tire to the side and allow huge water drainage.
Now grooves allow water through but break contact with the ground and this reduces grip. At the same time, if the weather clears and the sun comes up, the lack of continuity on the tire surface, together with the excessively soft compound (that allowed grip on colder temperatures) the car will not be able to grip as good as with slicks (the groves make the tire wobbly and they flex) and the compound will literally be ripped apart between it's soft nature and the flexing happening all over.
So teams that wave weather reports saying the rain will not get worse choose the semi-slicks while the one with reports telling the sky is falling down for good will choose the rain tires.

Road tires are different. They have to drive under ALL circumstances since you can't pull over and switch tires every-time it starts raining, then switch back when the sun shines. It's also meant to be affordable and durable as you can't run you car on a racing budget nor have pit-stops every 1000km to switch your wear-down rubber.
That (just like anything that's main-streamed)  means compromise. So the tires you buy for your car don't grip as much as they should in the wet, nor they do in the dry. They don't drain as much water as they should but they don't allow the full grip it's contact area should. And they surely can't endure abuse as well as they should, in the same way they can endure huge mileage with no sweat.

Now compromise does have flexibility. There are lots of different cars, road conditions, weather and, evidently, so many different tires and designs.
Now I'm not a compromise guy. I rather have 2 sets of wheels (a 5zigen rims set running on ToyoR888 semi-slicks and the standard Honda rims running a set of Falken FK452 with V shaped grooving to channel water out of my way) than running on compromise each and every time I drive the car.
But I've not always been like this. These was a time when I hardly afforded the car alone so tires where an expensive choice to do only when strictly necessary and as little as possible.
Portugal is a 3rd world country. I know we classify ourselves as under-development country, but that's only truth if one can maintain the evolution instead of keep building new and let that new rotten way while we build newer things.
So we have plenty of brand new roads that will be poorly maintained and become the tire and suspension nightmare in a matter of years, while newer roads get built.
That means that in out country I can go in a full day from the northern region (wet and cold), through the mountains (ice ans snow) all the way to the south into the sunny warm beaches; and travel through just about every possible roads and conditions possible. Tire choice is a difficult job here.
Tire tread design is very important as most of you roads will have progressive radius and that means a huge load on the sides of the tire and need for instant response, and when it rains, the water forms authentic rivers crossing the highway meaning that you'll aquaplane often.

So here goes the basics of tire tread design: Pattern Design Types.
The Symmetric, Non-Directional Tread Design - In this tread design the tire centre line dividing the inside and outside halves of the tire. The tread design is rotated 180ยบ so that you can run the tire in any direction, allowing for tire crossed rotation (front-left to rear-right and front-right to rear-left) without needing to demount the tire from the rim and re-mount, at a tire shop.

The Symmetric, Directional Tread Design - In this design, the tire center line mirrors the inside to the outside half of the tire. This means that the tire has only one possible rotation direction. This design is more efficient but does not allow crossed tire rotation without going to a tire-shop for demount and remounting the tires to the rims.

The Asymmetric, Directional Tread Design - This design means that the outside and inside halves of the tire have absolutely nothing to do with each other, meaning that the tires are sold in either Left or Right designs and Cross rotation can only happen front-to-rear or rear-to-front, but always on the same side. It's the ultimate tire design built for the absolute best performance desirable.

Now for the more complex parts of the tire design: The tread design parts and considerations.

Shoulder Blocks - These blocks create road holding while cornering, when you rush into a corner, the suspension compresses, the camber changes and the tire will twist. Depending on the suspension setup, the turning radius, the speed, the weight of the car and the tire's wall structure, the pressure on these shoulder walls will be greater or smaller. You see all that moving mass created inertia and inertia has a simple law witch is conserving motion. If a car goes fast in a straight line it will generate kinetic energy. This energy will have inertia as big as the moving mass you have and when the corner comes, inertia will try to make the car continue in a straight line. When you turn, the tires will be the ones countering that inertia and keeping you on the road and they will flex under pressure transferring all the load to the shoulder of the tires.
From right to left you have 4 different tires from the same make.
  • A tire is a TrackDay Semi-Slick Racing tire - It's shoulder is square to allow immediate steering response, and it's blocks are very big and large. They are this way because of the extreme load they are subject to in racing environments. These big block however suffer less compression and generate a lower frequency vibration. Now the thing about lower frequency vibration is that they propagate very well and make themselves noticeable (remember the neighbour's stereo bothering your sleep? The bass sounds is felt far more loud that the treble sounds).  So this tire is hard and loud making it very uncomfortable of a luxury saloon, for instance.
  • B tire is a Ultra-High-Performance Sports tire - It's shoulder is still square, but the blocks are much smaller. This means that the tire blocks will not endure the track so well and flex a lot more, however the frequency they flex on is higher than that of Tire A, making it less noticeable that tire A, but still quite noisy and vibrant.
  • C tire is a High Performance tire - The shoulder is rounder making the ride smoother, but the steering less responsive. The Shoulder blocks are slimmer and cut through to flex more creating more comfort and much higher pitch vibration, meaning less vibration propagation and a sound wave that's not as audible as A or even B.
  • D tire is an all-weather SUV tire - Now this kind of car isn't made for cornering, hard acceleration or braking, so the tire doesn't need beefy shoulder blocks, However it does need to run across several different terrain types and that means the tire shoulder need to have a deep BITE, so the same design in the C tire is amplified and grown higher to allow muddy and icy terrain grip (much like the pythons in the under side of a rugby player's shoes).
On Winter (snow) tires, these shoulder blocks (like most tire treads) is cut with very small "w" shaped textures. It's effect is comparable to our fingerprints. They increase traction and grip on slipper surfaces like ice. Some of them are even Spiked. These tires are all about cleaning out the surface and grabbing new ground, so tread is very off-road alike.

Circumferential Grooves - These grooves are the main water drainage channels. Then the tire encounters a water pool, the weight of the car forces the tire against the ground and that will create a drainage of water out of the tire's way. The main "bite" on the sheet of water existing between the car and the ground will be created by these channels allowing water out of the way.
From right to left you have 4 different tires from the same make.
  • A tire is a TrackDay Semi-Slick Racing tire - This tire's asymmetrical design bets all it's water drainage power on the inside channel (the on on the right of the tire) since it's supposed to be used in a track-day car and the cambers will be very aggressive, allowing for that region of the tire to experience more "pressure" while draining water out. Second comes a Zig-zag'd central DUAL channel that will "shake" water out of the central contact patch's way, allowing for better acceleration or braking.
  • B tire is a Ultra-High-Performance Sports tire - This is tire splits the water channelling all over it's surface with 4 independent circumferential groves. It's a tire for the street's and cambers that shouldn't be too aggressive. This tire will perform better in the wet if all it's surface is being used to pull water out of the way.
  • C tire is a High Performance tire - Not quite as good as B tire but still has 4 circumferential groves. Though smaller, they still are evenly distributed across the entire surface as comfortable cars don't have aggressive cambers.
  • D tire is an all-weather SUV tire - Now this kind of car isn't made for fast driving. However the driving through mud, water and Ice require deep groves. The 4 circumferential groves are present, all across the tire tread surface and not only are deep, they are also large and helped by a deep groove that almost cut it's way to becoming full circumferential.

NVH (Noise Vibration Harshness) - These small cuts reduce the NVH level produced by the car. NVH is a measure of vehicle habitability of both noise and feel. Remember that low frequency vibration created by the tire shoulder and how it resonates through the vehicle? It's the same principle takes to the tire's compound contact with the ground and the tread's aerodynamics. In truth it's a combined measure of vehicle noise level inside the cabin during driving together with the vibration transmitted through the steering components and car body-shell, together with the harshness (or lack of smoothness) felt while transiting from pavement types or conditions. In simple terms, making small cuts will allow the rubber blocks to flex more and generate less vibration form the contact with the ground, making cuts that counter the aerodynamics or go with the aerodynamics of air swifting through the tires's groves will emit different pitches of sound and vibration meaning different tipes of propagation in both audible or feel-able form.
This however will mean a tire will more wobbling effect and that's worse steering response and road feedback, something you don't want in a High Performance Sports car.
From right to left you have 4 different tires from the same make.
  • A tire is a TrackDay Semi-Slick Racing tire - Noise?!?! on the track?! All can hear is engines and brakes, who cares about quiet tires that have less that perfect feedback.
  • B tire is a Ultra-High-Performance Sports tire - 7 points of small flexing, being 5 of them very discrete. Performance is more important than comfort so why would one buy a Porsche over an Jaguar and than worry about noise and vibration comfort.
  • C tire is a High Performance tire - 10 points of flexing, being most of them very noticeable. It's a tire that is build for comfort.
  • D tire is an all-weather SUV tire - This is a case of necessity. High groves make a rattling sound so annoying that without these grooves you would go crazy in minutes.

Finally we hit the full design and it's usage: To do this I'm explaining some design comparisons and single designs for special purpose.

Track tires - From the Pure Full Racing slick on the left to the full rain racing tire on the right, here is a selection of several designs.
  • The Kumho S700 is a full slick. No grooves, just 100% contact.
  • The Kumho V710 is a full slick that has 2 "emergency" grooves that will allow for some aquaplaning resistance from very shallow pools.
  • The Toyo R888 is a semi-slick. It's a slick tire with a symmetrical directional groove design to allow wet racing (wet but not pouring rain).
  • The Yokohama Advan A048 is also a semi-slick with a better aquaplaning resistance than the Toyo but a rather less area of central contact patch.
  • The Falken Azentis RT-615k is a semi-slick approaching the road tire design. It's not built for water pools but it car resist a couple of them if not too deep or long.
  • Finaly the Kumho W710 created for racing under pouring rain. There is no full contact patch but rather grooves (very deep ones) all round.
Ultra High Performance Tires
  • Falken Fk452. This tire has 4 full contact patches centred in the tread design. It's perfect for extreme braking and acceleration holding. The groove design goes all the way through and cross the 4 evenly distributed circumferential grooves. The beauty on this design is that every time you have initial flowing points or channel junction points, the lateral channelling groves are wider for a short time. This is brilliant fluid dynamics design. fluids (water in this case) suffer inertia too, so initial flow will happen slower and so a larger "Channelling chamber" is needed.
  • The toyo T1r is close but with a different compromise. You see the Toyo has the full contact patches spread all over it's surface and is special points like the shoulders. That ensures PERFECT cornering grip. However that also means that while flowing water out, it will only flow so much until all the channelling and circumferential grooves are overwhelmed and enter aquaplaning really fast. It's a compromised designs that has a good behaviour in the rain but only in small or thin pools.
  • The good old Bridgestone S02 Potenza Pole Position maintains a generous central patch that allows good braking ans acceleration. The grooves are brutal and able to channel water like a true water pump. I've never experienced so much drainage power like with these tires. These blocks that compose the channels walls are wide too, allowing some support while cornering.
  • The Bridgestone S03 Potenza Pole Position is like a half breed between the S02 design, the Falken Fk452 and the Toyo T1R. The central patch is there and with sizes approaching the S02, then comes the circumferential grooves spread all across the tread like the Fk452, but the channelling groves are not exactly all the way through like the S02. Instead, they start with a full patch band and grow from there, creating a very decent curve holding wall. 
  • Bridgestone Potenza RE050A is more like Frankenstein. The inner side looks like the Toyo T1r meaning that it's supposed to be run on aggressive camber cars. Then the central patch like it's predecessors allowing braking and acceleration. The circumferential grooves are extremely generous and the channelling groves are small but in great numbers. Not a perfect aquaplaning resistant design not an extreme corner holder... but giving the right extreme cambers it's designer for, it will grip very very well and the let go (under that extreme cornering) will be progressive and soft as the outside of the tire has lots of small grooves what will wobble around and make it smoother. This is a very very fun tire to fool around with... in the Dry

Hope you've enjoyed; next time I'll talk about the compound.

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