SINGLE-ROLLER CHASSIS DYNAMOMETERS
Single-roller chassis dynamometers, as vehicle power increases,
have over time become the most popular solution among all the different manufacturers of chassis dynos.
As we can see from the analysis below, tyres find their
optimal operating conditions on the road (image A).
Looking at the simulations below (images B and C)
we can observe how at the larger diameter of the roller, corresponds to less 'indentation' and thus
less stress (image. B)
for tyres, also obtaining:
- Increased grip (larger contact area)
- Improved maintenance of constant operating conditions, during testing
TWIN-ROLLER DYNO TEST BENCHESE
Originally
born for diagnostic needs (at a time in history when vehicles had much lower powers than today)
offered good stability of the vehicle under test, but sacrificed tyre retention.
This design choice was at the time also justified by the
purposes for which the double-roller bench was originally designed; or its use
within diagnostic centres that they were merely doing
short and quick power tests.
HIGH STRESS CONDITIONS FOR TYRES
As can be seen from the figures below (images. D, E) the
car or motorbike test bench with double rollers (whether connected by a belt or not)
submit anyway
tyres under high stress conditions, also due to high indentation due to the
impossibility of using large-diameter rollers, implicit in these solutions.
INCREASINGLY POWERFUL OF MODERN VEHICLES
These stressful conditions
become increasingly evident as the power and torque of vehicles increase.
We therefore think of the
modern endothermic, hybrid or electric vehicles, the torque and power of which have grown disproportionately in recent years, reaching
more than double levels compared to the powers for which this type of bench double rollers
was initially put on the market (1980s).
Typically,
conducting medium/long-term tests, tyre warm-ups are likely to affect the results; due to:
- different grip in tyre/roller contact at different temperatures (cause of energy dissipation and variation of wheel power readings and the power dissipated.
- tyre diameter variations, which could occur due to their increased internal pressure with possible differences in engine speed readings.
DIFFERENT WHEEL AND ROLLER SPEEDS
Further
criticality of double roller dyno test benches is represented by the
different rotation speeds of tyres and rollers.
During the acceleration period,
tyres naturally tend to want to 'go up' on the front roller (image D)
, deforming even more due to displacement and different vehicle mass distribution
.
This also involves the
displacement of wheel and roller centres of rotation (and relative rolling diameter),
modifying rotation speeds accordingly of the individual components involved (front roller/rear roller/tyre).
For
understand this dynamic, one only has to look at Fig. D and E below; the distances from the axes of rotation from the centre of rotation of the tyre,
change as the position of the vehicle axle changes.
This creates two different possible operating conditions, depending on the
presence or not a mechanical constraint between front and rear rollers:
- When NOT bound together (Image D), the rear roller rotates at a higher rotational speed, condition that can cause slippage and adversely affect tests
- When bound together (image E), the rear roller carries a rubbing effect on the tyre surface, due to its lower rotational speed in this case; thus increasing tyre wear and operating temperature.
