How do you test wet braking?: Wet braking… This is one of the most critical performance where an outstanding tire is separated from a medium grade tires. As the braking distance is much longer compare to dry braking, wet braking has become a safety issue. Based on the press test in the comfort segment, the difference between the worse and best wet braking test were up to 7 meters. How much is exactly 7 meters? Just to put into perspective, we could almost fit 2 Myvi in this distance. Hence it is absolutely critical to know how well your prospective tire performs in wet braking.
Table of Contents
EU wet braking label
As the wet braking performance is deem extremely critical to road safety, the European union decided back in 2012 to have it included in the label. The current wet label is index is shown in the table below:
| Wet grip index, G | EU | Korea | Japan |
| 1,55 ≤ G | A | 1 | a |
| 1,40 ≤ G ≤ 1,54 | B | 2 | b |
| 1,25 ≤ G ≤ 1,39 | C | 3 | c |
| 1,10 ≤ G ≤ 1,24 | D | 4 | d |
| G ≤ 1,09 | E | 5 |
One might ask what is actually the wet grip index? The wet grip index is defined as
G, wet grip index = (µcandidate tire/ µSRTT )* 1.25 + C, correction factor.
In plain words, there is a reference tire which everyone is testing their product against it. Once the difference between this reference tire is known, you will be able to determine the wet grip index via a the calculation above. The reference is called standard reference test tire, SRTT which is currently produced by Michelin. Picture of SRTT is as below:
The correction factor C, is needed to compensate temperature & track effects in the effort to standardize the test.
Testing procedure
Evaluation speed: 80-5 km/h
One might wonder why the evaluation is not done till 0 km/h or when the car comes to a halt. The reason is fairly simple. We want the test to have as little influence as possible from external factors. With the combination of ABS(auto braking system) and EBD (Electronic Brakeforce Distribution) system, it was found that by evaluating till 5 km/h it reduces the error and it has been adopted for most of the magazine test.
In regards to the testing itself, the car is fitted with an accelerometer sensor and all the driver needs to do it to slam on the brakes and collect the corresponding data.
Surface: 1.5 mm water depth
In order to simulate real life wet situation, a water depth of 1.5 mm is implemented to ensure the tires are actually sliding on a reasonably wet surface. The water depth should be consistent though out the track.
What is the difference?
So what does it actually means when you have A label tire compare to a E label tire? Based on the illustration below, a E label tire will brake 18 meters later compare to an A label wet braking tire. Based on our previous example, this would fit close to 5 Myvi’s in between. Hence the benefit of the wet grip is clearly illustrated in the picture below.
Summary
Overall, wet braking is a simple test procedure where it involves the measurement of the braking distance at 80km/h to 5 km/h. The wet label index is calculated by comparing the tire results against a universal reference, SRTT. The benefits of having a good wet braking tire can be clearly seen as an “A” label tire can out break a “E” label tire by 18 meters.