The origin of PART I

The car has been using the original factory tire, in 2010 40 weeks production, rubber has been aging, partial wear. The figure below is as follows

Outside, depth 3.5mm

Inside, deep 3mm

There is a 0.5mm gap between the inside and outside, and the tire block to the ground has been ground into a bevel.

Change

PART II preparation

Start with RP76, use the front and back match, the front 225/50R17 (350rn / bar), the last 235 /fifty-fiveR17 (390 ocean / article) see here, some people will question: this rear wheel installed, the speedometer will not be accurate, will exceed the speed limit!
First of all, the measured results: not at all, the meter speed and the real speed are basically the same!
The speedometer of this car has to be renamed “real speedometer”..There is a certificate later.
The 245 is not used because the original wheel is only 8 inches wide and does not provide enough support for the 245 tire.In vernacular, it is:Can not give full play to the performance of the tire, and will obviously partial wear.
The best fit for an 8-inch wheel is a 225 tire.235 unknown.

Some basic parameters of this RP76 are shown below: wear stage, grip grade, temperature control grade, maximum load, maximum cold tire pressure, crown composition (2 layers of polymer + 2 layers of steel wire + 1 layer of nylon)), fetal wall composition (2-layer polymer))

Insert an irrelevant photo, which can be seen in this photo.Chinese radial tyresLayer 2The steel wire is X-shaped oblique crossing.
This photo was taken at the entrance of an inaccessible scenic spot. The overfired vehicle is a BYD F3.

A digital explicit radius gauge is obtained.

PART III, change!

Make an appointment with the shopkeeper and make it clear in advance that I’m going to do this.
Remove the old tire and use a clean water rag (next to the green bucket) to clean the joint between the wheel and the tire.
The brand of the wheel?

Then, use the radius gauge to find the “low point” of the four wheels one by one. Measuring point density: one measuring point every 2 centimeters.
Each measuring point has to be tested 3-5 times.
Every time you find a wheel, the master installs a tire and replaces four new nozzles.
Cold tire pressure: front 2.2, back 2.0.
It takes 1.8 hours to change a tire, but it’s not over yet.

Take a picture after loading the car.I think it’s a coincidence. As for what a coincidence, please look at the picture.The black mark on the wheel is the “low point” I think I found.

The store gives away a front wheel beam angle adjustment.
The connecting rod of the front anti-tilting bar was replaced at the SSSS store last year.Plum string)After this new tire, the front wheel bundle angle is: left + 0 °10 ‘right-0 °2’
The left and right inclination angle is not consistent. I can see with my eyes that the negative inclination angle on the right side is more obvious than that on the left side.I personally think that the installation of plum strings is too hasty and the inclination angle caused by the one-sided pursuit of completion speed is inconsistent.
However, the four-wheel locator in this store has been used for a long time, and now only the measurement data of the beam angle are accurate, and nothing else is correct. The beam angle can only be adjusted to + 0 °5′ on the left and right side.
It takes 2.5 hours to change the tire and adjust the beam angle.

PART IV road test

Before talking about the effect of the road test, it is very necessary to explain some basic parameters of the car.It is convenient for you to see and understand.
First, the parameters of the transmission system of this car:
1. Power taking box (gearbox)
Model: W5A330, one of the so-called “small five-speed” automatic gearbox. Due to the use of fifth gear in this road test, it is only marked with a fifth gear reduction ratio of 0.833333333.
two。 Main reducer (tail tooth)
Reduction ratio: 3.266666666
3. Fifth gear reduction ratio: 0.833333333X3.266666666 ≈ 2.7222222205777
4. Speed per hour
The original 245amp 45-17 tyre, perimeter rounded 2049mm, engine speed 2000rpm90.32kph per hour(Display 91kph), 2500rpm, speed per hour112.90kph(Table 113or 114kph)No one will say that the original factory is not allowed.
Now 235plus 55-17 tyre, perimeter trimmed 2168mm
Engine speed 2000rpm, speed calculation result is 95.56kph
Engine speed 2500rpm, speed calculation result is 119.46kph
Please look at the picture
Look at this picture first.This photo shows the GPS speed of the electric dog, the speed of the speedometer and the speed of the tachometer. The purpose of this photo is to show the extent to which the GPS speed of the electric dog is consistent with that of the speedometer.It’s useful later..
Start a constant speed cruise when you step on the accelerator and accelerate to 93kph.In order to keep the speed stable.. The following photos are deliberately selected to display the interface of the constant speed cruise speed setting on the instrument.It is convenient for you to read it intuitively..

Cruising at a constant speed to 95kph and holding, the first photo of the PART was taken to prove that the speed was basically the same as that of the GPS.At this time, the speed is 2000rpm.

Still cruise at a constant speed to accelerate to 119kph and maintainAt this point the speed is 2500rpm.
The car is not shaking at all, which shows that the force balance of the road is all right. I haven’t tried more than 130kph. I don’t know.
However, the left and right inclination angle of the goose is different, the returning force of the tires on both sides is different, and the direction is unstable.

The actual road test basically proves that after changing a larger tire, the speedometer will not be wrong. Practice has proved that my worries are superfluous.
As for the reason, in the following superficial understanding, wild guess, unconfirmed:
The speed meter of W212.048 has the function of self-correction according to the on-board GPS, which is based on the data given by the speed sensor.
The title of speedometer is not as appropriate as “real speedometer”.
I don’t know about other MB models.