Mechanical linked axles dynos, limits and disadvantages

The evolution of chassis dynamometers. The 90s, the evolution of vehicle electronics and the development of dynamometers with linked axles

A bit of history:
In the mid-90s, manufacturers started integrating electronic traction control systems in vehicles, which slowly continued to develop in the following years (TCR, ASR, ESP, etc).
These systems, which have different purposes and methods, are used to ‘cut’ the power delivered by the vehicle, thus favouring trim and stability during driving. These systems are the same as those that make it difficult to test a 4wd vehicle on unlinked axle dynamometers, without first providing any kind of function dedicated to disabling traction controls (with the risk of carrying out false tests due to the lack of availability of all engine power).

To meet these needs, the first dynamometers with linked axles were produced in the 90s. The idea was to offer a product capable of simulating road conditions perfectly. According to this principle, it SHOULD be able to correctly and rapidly test any vehicle (no longer needing to worry about traction control).

However, these products failed to meet the expectations by forfeiting precision in test measurement and repeatability in favour of assumed ease of use.

POOR measurement precision of dynamometers with linked axles

Each linked dynamometer must provide for additional inertia determined by the presence of transmission parts and their relative friction, which will inevitably affect the ability to achieve an accurate measurement. Apart from evident problems of perfect axle synchronisation, in general, we must keep in mind that more inertia always means less measurement precision. Just like less control of clearance friction (transmission parts) means poor test repeatability.

Many technical solutions on the market are not able to guarantee tests with errors under +/- 10%, making the said instruments almost unusable (many users look for solutions such as repeated tests, which result in waste of time and energy) and calculations by means of ‘weighted averages’.

Dynamometers with simple linked axles (with no electrical traction systems) can test any type of vehicle. However, doing so does not guarantee the desired precision since
they are unable to reproduce road conditions perfectly, leaving the electronics free to intervene on the engine’s power (thus distorting the data collected during testing)

SIMPLY PUT: by ‘feeling’ the front and rear axles rotate synchronously, the vehicle’s electronics should not intervene, precisely because they reproduce the vehicle’s operating conditions on the road (in theory), thus not seeing any anomalies that could compromise its stability.

Expectations created by the product come up against the inevitable technical problems in its application.
The constantly increasing sensitivity of electronic control systems is able to perceive even the most infinitesimal deviations, intervening by cutting engine power and consequently invalidating the conducted test.

Guaranteeing perfect synchronisation whilst maintaining measurement precision is technically impossible, unless we produce and offer much more complex and expensive technologies and solutions than the ones currently available on the market with the same intended use (such as test benches dedicated to the world of certifications – powered by electric motors – with very high running costs, starting with installation and maintenance in order to reach significant upgrades of the electrical mains, with minimum requirements of 60 kWh – compared to 7 kWh of our test benches).

A simple test with a vehicle and ESP system installed is the simplest way to demonstrate the above. During the test, you will easily see the relative gauge light up on the control panel, resulting in cutting engine power. The result is an invalid test that does not represent the actual engine power.

Moreover, even if the electronic system does not intervene, this type of test bench could not guarantee precision and repeatability of the tests.

Recent development has seen the introduction of new control sensors (such as gyroscopes) inside the cabin. As we will soon see others (moreover unforeseeable), this additional technological advance will make the ‘linked’ dynamometer solution more and more ineffective, thus inevitably leading to the search for ‘electronic’ solutions, which are useful for excluding said sensors during bench testing (a trend that is already underway and also due to increasingly stringent anti-pollution regulations).

Further developments in the electronics industry (also with regard to active and passive vehicle safety) are paving the way for greater integration in vehicle control units. Manufacturers are activating useful protocols for more control and integration among the various systems. For example, a common goal is to prevent delivery of full engine power if there is no signal from one of the ‘peripheral’ units.
This would be read as an error by the computer body, resulting in a power cut (making it more difficult to test vehicles on test benches without using systems that exclude electronic controls)

This is why Bapro has never offered and does not offer, unless specifically requested, solutions with ‘mechanically linked axles’.

Bapro has developed and patented ELECTRONIC AXLE SYNCHRONISATION, which enables high frequency control, thus guaranteeing the typical precision of our chassis dynamometer benches.

Bapro’s philosophy is to offer test benches that are characterised by precision, test repeatability and reliability, which are features that this technical solution cannot offer.

Bapro has chosen the development and implementation of software systems called DYNO MODE; able to exclude said control sensors, they therefore offer effective and reliable crankshaft power testing, regardless of current and future developments in vehicle electronic control systems.


critical analysis of the PROS AND CONS of various implemented technical solutions:

Hydraulic link between the axles:

Low inertial load, low/average price range

High routine and extraordinary maintenance costs; self-assembly not possible; synchronous delay between the axles due to the technical limits of hydraulic transmission, which is sufficient to make it impossible to carry out tests on vehicles with particularly sensitive electronic controls (highly probable electronic intervention) resulting in lack of repeatability of the tests.
The sensitivity and calibration of the test bench can be ruined by the different operating conditions and oil viscosity in the hydraulic system, especially during the coast-down phase (calculation of dissipated power), which leads to measurements with errors exceeding 20% (repeatability test)
Self-assembly not possible due to the complex nature of the product.

Mechanical link to driveshaft:

Good synchronisation, average price range.

High costs, self-assembly not possible; intervention of electronic systems highly likely due to the high inertial load; impossible to accurately determine the dissipated frictional power between transmission parts, resulting in measurement errors of more than +/- 15% and lack of repeatability of the tests.
Particularly unsuitable on low-power vehicles, which compromises them due to this increase in inertial load, thus further aggravating the above defects.
Self-assembly not possible due to the complex nature of the product and large dimensions due to the transmission system.

Mechanical link to belt:

modest synchronisation, low overall inertial load, and average/low price range.

Impossible to accurately determine the dissipated frictional power between transmission parts and hysteresis of the belt (especially in the coast-down phase), resulting in measurement errors of more than +/- 15%. Lack of test repeatability.
Requires frequent maintenance (machine downtime) for belt re-tensioning and replacement, with expensive parts in order to guarantee efficiency and prevent belt/pulley slippage. (Slippage occurs on most test benches on the market. Frequent traction electronic control interventions resulting in lack of repeatability of the tests.
Self-assembly not possible due to the complex nature of the product.

Synchronised test bench with electric motors

modest synchronisation, but not sufficient; ability to test almost all vehicles up to max 250 CV.

High purchase and operating costs (electricity mains upgrade to minimum 60 kWh!);
moreover, synchronisation is insufficient for testing vehicles higher than 250 CV, due to the limited power of electric motors and maximum acceleration values below 2 m/sec2.
Self-assembly not possible due to the complex nature of the product; high energy costs resulting in system upgrade costs (minimum 60 kWh).

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