Track-tested: Bridgestone Potenza S007A
Tyre technology is improving every year, but it’s hard to tell because tyres still look black, round and boring. We track-test the all-new Bridgestone Potenza S007A.
WHEN WAS THE LAST TIME you had a puncture? For me, over the last five years I’ve only had two on-road punctures,and I drive a lot of kilometres every year.
Speaking to car-enthusiast friends, we all reckon we’re seeing fewer punctures now than ever before, and that has to be down to tyre development. In the 4×4 world there’s general acceptance that today’s moderate all-terrain tyre delivers performance as good as yesterday’s specialised mud-terrain tyre, and that view of tyre improvement is shared by the racing community which is enjoying better grip and longer life than ever before.
So, there’s definitely tyre development going on, and that’s why I was excited to spend a day at Phillip Island with Bridgestone and BMW Driver Training, exploring how the new Bridgestone Potenza S007A has been improved over its predecessor, the S001. Bridgestone say the S007A is better in every measure of a performance tyre, but you’d expect them to say that…
To back up the claim, Bridgestone provided an impressive statistic; their factory test driver lapped their test circuit in 1.40.0 seconds for the S001 and 1.37.6 seconds for the S007A. That’s a 2.4 second improvement over about 90 seconds, the sort of gap you expect to see between different classes of tyres such as road to track, or track to race, not in iterations of the same class of tyre.
But, so much for the claims, now let’s look at the reality. The comparative test on the day was braking distances, and you can see the results in this short video:
Aside from that demonstration which was performed by a BMW driver, the invited journalists were also given the opportunity to drive both the S007A and S001 in the same stopping-distance test, and in a similar swerve test too. While there were variations in exact timing and skill, over the exercise the S007A performed better as it stopped shorter, which means more grip. So, all that evidence supports Bridgestone’s claim of improved traction, and there was noticeably less fuss and tyre squeal from the S007As too.
But there’s another, equally important factor for a sportscar performance tyre, and that’s life.
BMW’s driver training unit now use Bridgestone S007As, and you can imagine how many tyres it gets through in a year with 75 track-day events and 3000 participants driving high-performance M cars. That’s plenty of data to form an opinion with, and it reckons tyre life is double the previous tyres, good news for those of us who enjoy race or track work. Your average weekend-warrior racer would much rather have a tyre that offers good, consistent grip over a long life than one which has extremely high grip levels for the first few sessions, but falls off quickly and so needs to be replaced often, which is both expensive and time consuming.
So how did Bridgestone make the S007A better than the S001? The answer begins with Bridgestone’s Ultimat Eye (yes, no “e”) technology. Tyre development needs solid test data, and that’s difficult to obtain because mostly you can only look at the effects, not the actions. For example, you can try a new tyre design and discover you’ve got better lateral grip, but why exactly does that design work? That’s where the Ultimat Eye comes in.
The Ulimat Eye system rolls a tyre and wheel against a drum, and that drum has sensors to precisely measure data such as the size, pressure and temperature of the tyre contact patch. Scenarios such as braking and turning can be designed and replayed, instantly and precisely, time and time again which not only cuts development time but opens up a whole new world of test data for tyre engineers.
As you’d expect, Bridgestone also makes extensive use of computer modelling, but it’s not giving up on real-world testing any time soon, so the Ultimat Eye is a useful halfway point between the pure theory of the computer and the reality of the test track. Here’s the Ultimat Eye view of the difference between the S001 and S007A:
The cornering force contact patch is the most interesting. The colourmap indicates contact pressure (not heat), and it shows the S007A has five distinct tread areas to the S001’s four, so there’s another section of tyre gripping the road. The deformation is also more consistent – when a vehicle corner and shifts its weight to the outside wheels, the tyre contact patch becomes a trapezoid, and that’s more clearly defined with the S007A. That should mean more grip, but also more consistency of handling and tyre wear. Incidentally, this deformation is why racecars tend to run significant negative camber, so the tyre on the outside of the corner is flat when the weight of the car is pushing on it. The fact the contact patch is deformed to a trapezoid shows the image is from is a roadcar which doesn’t have significant negative camber.
Another S007A improvement is the compound. A tyre’s grip is multi-dimensional; there is adhesion, which is the stickiness of the tyre to the road, and there is macro grip, which is the tread blocks grabbing large parts of the road, like a hiking boot gripping rough ground, which is why offroad tyres have deep, open tread patterns.
The other type of grip is micro grip, which is the same thing as macro but on a tiny scale. If you were an ant, the surface of road that looks smooth to us would seem to us like a rough 4WD track, and ideally you want your tyre to mesh into the tiny imperfections in a road surface. For that you want a soft tyre, like a racing slick tyre, which keys into the road surface. Slicks are perfect for dry grip, and indeed the only reason road tyres have tread is for use in wet weather to disperse water.
For a road tyre you ideally want some tread for wet-weather use, but a compound that’s soft enough to meld into the road surface. But there’s another problem. If you make your tread-blocks out of a soft compound then you’ll have tyre ‘squirm’, which is when the car moves on top of the tread-blocks relative to the road. This is not good for handling.
So, as ever with tyres, the compound is a tradeoff. You want it soft enough to micro-grip, but hard enough to wear and not suffer squirm. What Bridgestone has done with the S007A is create a compound called Nano ProTech, which it says provides good micro-grip and yet is hard enough to offer decent handling and tread life.
Another area of interest in the S007A is tyre stiffness and rolling resistance. When a car corners, the weight shifts to the outside of the vehicle; the tyre contact patch can be thought of as staying in one place, and the entire car is pushed to one side, deforming the tyre. This is far from ideal, as the car’s handling is compromised. This is why sports and racing cars have large wheels with thin, low-profile, wide tyres; so the wheel/tyre combination has little sideways flexibility. The downside is a smooth, comfortable ride which is why cars more oriented to comfort run narrower tyres with taller profiles.
Regardless of the type of car, all tyres need stiffness, and this is why the rubber compounds in tyres are reinforced. Often that reinforcement is via steel belts across the carcass, but in the case of the S007A there is something called the Kevlar Flipper and Hybrid Spiral. Using Kevlar instead of steel means the tyre is lighter, and Kevlar is better able to handle the repeated flexing of a tyre as it rotates with less energy loss. There’s also what Bridgestone says are “Multi-Round Blocks” and “Optimised Crown Shapes” which are tread blocks that “provide higher contact pressure for better grip during braking on wet roads, as well as uniform contact pressure which prolongs tyre life”.
The third S007A design feature is called 3D-M Shaped Sipes. A sipe is a tiny groove cut into a tyre, but not deep or wide enough to seperate the tread into blocks. The sipes help with mico-level grip, and helps break up the thin film of water left on the tread during wet weather.
One feature the S007A’s don’t have is directionality, and that’s good news. Many high-performance tyres are uni-directional, which means they are designed to work when rolling forwards only; tractor tyres are an extreme example. In a road car, directional tyres can be optimised for forward-movement grip as reverse-movement isn’t needed. However, the tyre then cannot be swapped left and right, which is a problem when you have a puncture and if your spare tyre isn’t on the correct side, and it makes rotating tyres from one side of the vehicle to the other impossible unless the tyres are demounted, flipped, and remounted. Bridgestone have managed to achieve the desired performance without resorting to directionality, so the S007A is more practical for road and track use than a directional tyre.
So after the test and the theory, what’s the verdict? From what we can see the Potenza S007A is definitely worth a look as a high-performance tyre for road use on sportscars, and moderate track use although they aren’t a track-specialist tyre.
BMW M2 vs M4
This isn’t a full-on comparo as it’s based on a handful of laps of each car at Phillip Island with all the aids turned on, but here’s a quick view anyway. Both cars are impressive handlers; quick, precise, powerful with gearboxes that even in auto manage the Island’s sweeping corners without gear-hunting, not something that can be said for even more expensive vehicles. The one real track-driving criticism is the seats, which don’t provide enough lateral support given the performance of the vehicles.
The specifications are broadly similar; both rear driver, 7-speed auto, front engine. The M2 has 272kW, 456Nm, weighs 1495kg and has a 0-100km/h time of 4.3 seconds, normally aspirated engine. The M4’s specs are 317kW, 550Nm, 1585kg and 0-100km/h in 4.1 seconds with a turbo.
The M4 is much quicker on the straights than the 0-100 times suggest, as being two wheel drive vehicles the 0-100 sprint times are traction-limited, and I would expect a bigger difference in the 100-200 sprint. Certainly the M4 feels, sounds and drives like more of a beast, and would be my strong preference as a track car. However, on the road I feel the M2 would offer way more performance than could sensibly be used, and while I didn’t do much low-speed work in the M2 I think it has slightly better low-rpm characteristics than the M4 as well as being smaller and slightly lighter. I’ve never driven an M-car I haven’t liked, and neither of these two changed my view.