Brake test bench

Roller dynamometer

A brake tester is a technical device that is mainly used to test the braking system of motor vehicles . Since checking the effectiveness of the brakes , as required, for example, in Section 29 StVZO (Austria: Section 57 KFG ), is hardly possible on the road, brake test stands are used. Roller brake test stands and plate brake test stands are used here.

Technical rules

For vehicles that are to be registered in EU countries, certain requirements apply to the maximum actuation forces on the pedal in accordance with the EC directives . In order for the vehicle to comply with the applicable technical guidelines, the difference in braking effect may only be a certain deviation from the maximum value.

Allowed differences in the braking effect of the wheels on an axle:

for the service brake a maximum of 25% of the higher value
for the parking brake , which can be operated while driving, and for trailers, a maximum of 50% of the higher value
for the parking brake that can not be applied while driving (primarily the new electronic parking brakes), a maximum of 95% of the higher value

The percentage deceleration is also prescribed and must reach certain minimum values.

Percentage deceleration using the example of a car :

Service brake at least 50% of the permissible total mass (GVW); For vehicles from 1 January 2012, it is 58% of the total gross vehicle weight with a pedal force of ≤ 500 Newtons
Parking brake at least 16% of the total gross vehicle weight; with a hand force of ≤ 400 Newtons or foot force of ≤ 500 Newtons

In order to achieve correct results, the tire pressure must be within the prescribed range.

Structure and function of the roller brake test bench

So that the two wheels of an axle can be tested simultaneously on the brake test bench, it has two identical sets of rollers. Each of the rollers drives a wheel during the test. The drive rollers are driven by an electric motor coupled by means of a gearbox. A feeler roller is used for blocking protection monitoring and for automatic switching on and off. In modern roller brake test stands, the braking forces are determined using the drive roller and the braking torque roller. The reaction force is transmitted from the braking torque roller to the display via a load cell. Both digital and analog instruments can be used to display the measured values. In addition, the measured values can be printed out on a connected printer for the purpose of logging. For vehicles that are equipped with permanent all-wheel drive, special test stands must be used. These test stands are equipped with two drive rollers that rotate in opposite directions.

The measurement

The braking forces are measured on the running surfaces of the wheels. As soon as the wheels are braked, a braking torque is generated which is opposite to the direction of roll rotation. As the pedal force increases, until the wheels lock, the course of the braking forces provides information about possible errors in the braking system. With the roller brake test stand, the following can be measured separately for each wheel:

Rolling resistance
Braking force
Variation in braking force
Occurrence of the tendency to block

In the event of incorrect braking values or functional errors, systematic troubleshooting must be carried out.

calculation

The minimum braking is calculated as follows: Sum of all braking forces ( ) in Newtons , divided by the total vehicle weight ( ) in Newtons, multiply the result by 100 percent. ${\ displaystyle F _ {\ text {ges}}}$ ${\ displaystyle G}$

The following applies to the minimum deceleration: ${\ displaystyle z}$

{\ displaystyle z = {\ frac {F _ {\ text {ges}}} {G}} \ cdot 100 \, \%}

For four-wheeled vehicles, the following applies to the sum of the braking forces of all wheels ( is the braking force at the front right): ${\ displaystyle F _ {\ text {VR}}}$

{\ displaystyle F _ {\ text {ges}} = F _ {\ text {VR}} + F _ {\ text {VL}} + F _ {\ text {HR}} + F _ {\ text {HL}}}

The total vehicle weight is the product of the vehicle mass and the acceleration due to gravity ${\ displaystyle G}$ ${\ displaystyle m}$ ${\ displaystyle g}$

{\ displaystyle G = m \ cdot g}

This results in the minimum deceleration : ${\ displaystyle z}$

{\ displaystyle z = {\ frac {F _ {\ text {VR}} + F _ {\ text {VL}} + F _ {\ text {HR}} + F _ {\ text {HL}}} {m \ cdot g }} \ cdot 100 \, \%}

The braking forces can be converted into corresponding deceleration values: The
following applies to the braking deceleration : ${\ displaystyle a}$

{\ displaystyle a = {\ frac {F _ {\ text {ges}} \ cdot 9 {,} 81 {\ frac {m} {s ^ {2}}}} {G}}}

This results in the deceleration in percent: ${\ displaystyle z}$

{\ displaystyle z = {\ frac {F _ {\ text {ges}}} {G}} \ cdot 100 \, \%}

literature

Peter Gerigk, Detlev Bruhn, Dietmar Danner: Automotive engineering. 3. Edition. Westermann Schulbuchverlag GmbH, Braunschweig 2000, ISBN 3-14-221500-X .
Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition. Verlag Europa-Lehrmittel, Haan-Gruiten 2001, ISBN 3-8085-2067-1 .
Kurt-Jürgen Berger, Michael Braunheim, Eckhard Brennecke: Technology automotive engineering. 1st edition. Verlag Gehlen, Bad Homburg vor der Höhe 2000, ISBN 3-441-92250-6 .
Traffic Gazette 9/2011. No. 107 Guideline for the application, condition and testing of brake testers (Brake Tester Guideline). Bonn, April 12, 2011 LA 20 / 7345.2 / 80-3

Individual evidence

↑ Guideline for testing the brake systems of vehicles during general inspections (HU) according to § 29 StVZO (HU brake guideline). BMVBS / LA 20/7345 / 22-3 dated May 24, 2012, VkBl p. 432 with changes dated September 3, 2014, VkBl 2014 p. 655

[1] Guideline for testing the brake systems of vehicles during general inspections (HU) according to § 29 StVZO (HU brake guideline). BMVBS / LA 20/7345 / 22-3 dated May 24, 2012, VkBl p. 432 with changes dated September 3, 2014, VkBl 2014 p. 655