The popularity of Xiaomi SU7 has broken through the automobile circle and become a social hot spot. We don't know how much of Mr. Lei's personal charm accounts for this, but combined with the tests of major media, the popularity of Xiaomi SU7 is by no means a waste of fame - in terms of battery life, control, lap speed, charging and other hard indicators, Xiaomi SU7 does not look like a product that was created in only three years.
With such a performance as an endorsement, it is no wonder that the CEO of Zhiji will question it first and then touch the porcelain.
However, the truth of the tree is sometimes more effective than the boomerang. Just when Xiaomi SU7 was hotly discussed by the whole network, a news that "Xiaomi SU7 rushed out of the track due to brake problems" went viral, causing widespread discussion in the industry.
At the beginning of April, a car blogger drove the Xiaomi SU7 on the Tianmashan Circuit in Shanghai, only to crash off the track at Turn 1 and crash into the tire wall after the driver made constant adjustments.
Fortunately, thanks to the car's kinetic energy recovery system and the cushioning of the sand, the driver was not injured, the damage to the vehicle was not very serious, the A-pillars were barely deformed, and the doors were able to open normally.
The appearance of this accident has puzzled many people, Xiaomi SU7 clearly emphasized the acceleration and braking performance at the press conference, so why do they still rush off the track? Is it the accident caused by the driver's technology, or the pot of the vehicle's hardware? In daily life, will this have an impact?
We will take a step-by-step look at the causes of the Xiaomi SU7 accident and the impact it may have in the future.
●Event review and analysis
The process of the accident was mentioned above, so let's look directly at the follow-up.
Judging from the live video, Turn 1 of the Tianma Mountain Circuit is a slow bend after a large straight line, and this bend is also a high-risk accident in previous tests of other models. Combined with the impact angle of Xiaomi SU7 this time, it can be basically determined that the vehicle is directly pushing its head off the track because the deceleration is not in place.
So, why didn't the deceleration fall into place? The driver responded in a follow-up video, in fact, the brake pads were completely worn out. As for the cause of brake pad wear, it needs to be analyzed from multiple angles.
First of all, although the Xiaomi SU7 has been emphasizing its strong performance before the launch, to be honest, this is just a fast family car, which is completely different from a track-level product. After the accident, Xiaomi officials quickly responded in "Xiaomi SU7 Answers Questions from Netizens (Episode 12)".
The official bluntly said that Xiaomi SU7 is a high-performance luxury technology sedan for urban use, please do not easily try extreme driving such as professional tracks. It mentions a very important concept, that is, the brake pads of Xiaomi SU7 are made of NAO material, its full name is None Asbestos Organic, also called non-asbestos organic material, characterized by the fact that the brakes are very quiet and suitable for civilian use.
This material is a better type of material in civilian cars, but if it is used to brush laps on the track, it obviously cannot cope with this working condition. According to the driver's later recollection, he basically braked with full force in every corner, and applied the brakes more than 40 times before and after, which led to rapid wear and tear of the brake pads.
Judging from the later video, the brake pads of Xiaomi SU7 are already as thin as a piece of paper, and no matter how hard the caliper is, it can't pull the brake pads to stop, so it is inevitable to rush out of the track.
Some people may question,Xiaomi SU7Since it has track-level acceleration ability,Why can't it be equipped with a track-level brake? Is it reduced? This is actually not the case。 You must know that the brakes used on the track and the civilian are two completely different directions, they are not good or bad, just look at the use scenario.
For civilian brake pads, comfort and quietness are the first priority. For racing brake pads, heat resistance and anti-manufacturing are the core purpose. There is no doubt that you can't have both.
We all know the principle of brake pads, in fact, many people don't drive too many miles, so until the vehicle is retired and even the brake pads have not been replaced, which is a relatively common phenomenon. But in fact, brake pads are one of the important consumables of the vehicle, if the civilian brake pads are opened normally, they can be replaced for 4-60,000 kilometers, and the brake disc should be replaced for about 80,000 kilometers.
And what about racing brake pads? If you usually drive it, you don't have to think about it at all. Even on the track, the limit of what it can resist is much higher than that of civilian brake pads.
One of the most important issues here is heat dissipation, on the track, the driver basically has to press the brakes to the end every time he turns a corner, although once or twice there is no problem, but if you don't stop pedaling, it will continue to overheat, which is the result of a large number of deceleration points on the track. We can take a look at the top view of the Tianma Mountain circuit, there are 13 corners in one lap, which requires higher heat dissipation.
Generally speaking, there is a critical value for the thermal attenuation of brakes, and when the temperature of the brake pads exceeds this value, the vehicle will lose braking force. The instantaneous temperature of civil brake pads is usually about 110 degrees Celsius, even if it is a sudden brake, it is 150 degrees Celsius, so the upper limit of the temperature of this kind of brake pads will be basically controlled at 300 degrees Celsius.
The racing brake pads will greatly increase its temperature ceiling, and can be kept at a high temperature of about 600 degrees Celsius for a long time, therefore, the material of the racing brake pads will be very different from the civilian brake pads, such as the carbon ceramic material we often hear, which is born for the track equipment, not only heat resistance, high strength, the overall quality will be lighter.
So can Xiaomi SU7 use carbon-ceramic brake pads?
First, the cost of carbon ceramic brake pads is indeed too high, in the era when cost performance is king, with a lot of money invested in a configuration that 99% of people can not feel, it is completely abandoned.
Second, carbon-ceramic brake pads make a screeching sound when braking, whether at low or high speeds, which is nothing for a racing car, but a family car suffers a big sin. And to be more serious, on ordinary roads, the braking effect of NAO material is not necessarily worse than that of carbon ceramic material.
This is because the NAO material takes care of the all-weather car scene, and the carbon ceramic will basically only work if the brake pads are already overheated, and if it is outside the track, the effect of the carbon ceramic may not be as good as the NAO material. So Xiaomi SU7 rushed out of the track, essentially has nothing to do with quality.
But judging from the results, one configuration of the Xiaomi SU7 is likely to be a good intention to do a bad thing, which is the Bosch DPB+ESP 10.0 fully decoupled brake-by-wire scheme it uses.
● What is the difference between the brakes of electric vehicles?
To put it bluntly, there is no mechanical connection between the brake pedal and the brake, and the brake pedal is only responsible for inputting the pressure signal and applying pressure to the brake by the electronic control system, while the braking system of most models (mainly fuel vehicles) is the traditional mechanical scheme of the brake pedal - vacuum booster pump - brake master cylinder - hydraulic push brake.
This configuration is a bit like steer-by-wire, all of which do away with hardwires and replace the mechanics with electronic sensors.
This decoupled structure does not have the unrealistic feel of racing peripherals from brands such as Logitech, and Bosch simulates real braking pressure in order to give the driver a clear sense of braking force. However, the results presented by the simulator are difficult to compete with the mechanical feedback.
According to the driver, his brake feel did not change during driving, but the car just couldn't brake. In other words, the brake pads of the Xiaomi SU7 are actually crumbling, but the main control computer does not know it, so the brake feel has not changed, which also prompts the driver to ignore the problem of thermal attenuation.
If it is mechanically coupled, experienced drivers can actually judge the degree of thermal attenuation by the feeling of braking feet, which is also one of the important reasons why Xiaomi SU7, or electric vehicles, are not suitable for the track.
So, why does Xiaomi SU7 adopt a fully decoupled brake-by-wire braking scheme? In fact, the fundamental reason is for battery life and automatic driving.
As we all know, electric vehicles weigh a lot, especially with large batteries, and you may seem to get a good range, but in fact, the vehicle's suspension system, braking system and other hardware are under great pressure. If the mechanical brake is completely used, firstly, all the recovered energy is wasted, and secondly, the life of the brake pads is much lower than that of fuel vehicles, which is more likely to cause thermal attenuation.
In order to solve this problem, Bosch simply fused the two together, that is, the deceleration of the vehicle will be responsible for a large part of the energy recovery, and if it continues to increase, it will let the mechanical structure intervene.
The matching of this system and intelligent driving can be described as a match made in heaven, because all the deceleration needs in intelligent driving can be handed over to energy recovery, if it is manual energy recovery, it is difficult for the vehicle to drive very evenly, and there will be a phenomenon of "one meal", which is an important reason why many people say that they will get motion sickness when riding an electric car.
The main control computer of the braking system is obviously smarter, it can adjust the deceleration force more accurately, and there is no need to use the counter-intuitive operation logic of "lifting the accelerator pedal is deceleration", so the combination of fully decoupled brake-by-wire scheme and automatic driving is completely logical.
In fact, friends who like to watch Formula E must know that in addition to the top braking system, braking heat dissipation efficiency, aerodynamic design and special tires, Formula E cars will also have a set of kinetic energy recovery system called ERS (Energy Recovery System), which is used in Formula 1 cars (KERS), Formula E cars, Le Mans and other races.
Taking Formula E racing cars as an example, at present, this kind of car has canceled the rear wheel braking system, only the front wheel brake is retained, and the braking power source of the rear wheel is completely decelerated by the ERS energy recovery device, and the kinetic energy recovery efficiency can reach 40% in the full power operation state of 250kW + after 350kW.
If this system can appear in family cars in a more accessible way in the future, it may no longer be difficult to drive an electric car off the track.
● Write at the end
The performance of electric vehicles is too strong, and whether it is an advantage or a disadvantage for today's consumers has actually become a metaphysical question. In a sense, getting to the top of your game too easily can lead to a lack of reverence for speed, which can lead to disaster at some point.
The speed of the electric car is only the innate characteristics brought by the physical structure, and whether it can get off the track depends more on the manufacturer's positioning of the vehicle itself. Why is it not a problem for the Taycan to run ten laps in a row on the track? Because Porsche's genes determine its upper limit in track conditions.
And the Xiaomi SU7, in the final analysis, is just a family electric car. Users can praise it, but please don't kill it.