Which dyno to buy? How to choose a right one.
How to compare dynamometers available on the market?
Every chassis dynamometer consists of three components: hardware, software and the measurement technology used. These three elements must work perfectly together. Furthermore, a lack of hardware capability cannot be "fixed" by software or technology and vice versa. All components must be perfect.
What to compare in hardware?
The most critical problem with modern cars (and soon all cars) is that they cannot be measured without the front and rear rollers being linked physically. This is critical even if the engine drives only one axle of the car. The dynamometer must provide a perfect simulation of road conditions - the front and rear wheels must rotate strictly at the same speed - otherwise this will be detected by the ABS system and the car will not run, limit power or even throw errors. In theory, this can be solved by disconnecting the ABS plug or pulling the fuse, but not on all cars. There are ineffective alternatives. Some may limit power or cause errors in communication with the ABS and CAN bus, and you may need a professional OBD scan tool or factory scanner to reactivate them. Your customer may therefore be nervous.
An example would be an auxiliary electric motor to "synchronise" (drive) the non-driven axle. At higher speeds it is insufficient and cannot provide perfect synchronisation. Another known attempt is a hydraulic connection (but the energy loss in the hydraulic oil is non-linear and not measured at all). For today - the lack of accurate mechanical synchronisation means that the dynamometer will only work for old cars.
A simple advantage: our AWD dynamometers have mechanically synchronised rollers (linking the front and rear axles by a belt) - this can be switched on and off from the control panel or remote control. No modern car (Mercedes, BMW, etc.) can be measured without full synchronisation, even if the vehicle is single-axle driven. Over more than ten years we have produced dozens of mechanically synchronised dynamometers - all of which work flawlessly.
There are three solutions for rolling roads:
- One roller per wheel.
- Two rollers per wheel (like ours).
- One retarder per wheel is directly coupled to the wheel hub (the wheel is removed, and a retarder is connected instead).
All these solutions have advantages and disadvantages, and it is important to decide right at the beginning to avoid problems with usage in the future.
One big roller (US style)
The simple advantage is that such a roller can simulate a flat road surface better than two rolls. The rolling resistance will be almost the same as on a real road. The roller needs to be 900mm or more so that the top is flat enough to provide traction (otherwise you may need to use some rubber glue to maintain the traction). It also has significant natural inertia. This is critical for accuracy (more giant rolls = more inertia for the same roll weight).
There are two disadvantages:
- the contact area between tyre and roller is smaller than on the road and is about 1/3 of that found in a dynamometer with two rollers per wheel. This is because there is only one area of contact at the top of the roller (which is not flat like on a real road, as this is impossible - and this reduces the surface area even further). This limitation is a consequence of simple physics and cannot be overcome by any trick. Simple: you are much more likely to have the car will slip rather than provide energy for measurement. Less power can be measured. Less surface area = less traction = less maximum power measured.
- If you look on YouTube and search for the phrase "dynamometer accidents", - no surprise, most dangerous accidents happen on single-roller dynos. It is simple – the car is in an unstable position on the top of the roller and any breakage of the retaining strap or even insufficient tension causes the vehicle to fall off the dyno with the accelerated wheels touching the surface as if shot from a slingshot. In theory, if all the straps are used correctly, there is no risk – but people are inconsistent. With two rollers per wheel, this is much less possible (because the car is in a 'cradle' between the two rollers and can only move left and right, and will not fall out on its own).
Two rollers per wheel (EU style)
Simple advantage: such dyno needs much less effort to install the car on it. It is also much safer in everyday usage than a single-roller dyno. It also requires much less height in the dynamometer room, as it is about 1/3 the size of a single-roller dynamometer.
To achieve similar inertial capabilities, the mass of the rollers must be huge, as inertia increases with diameter - and two-roller dynamometers have a smaller roller diameter (around 320 mm). Our rollers are 320 mm and 140 kg in weight each - this gives them (a set of 4) similar inertia as one large roller (large rollers have thin walls, so they weigh less than a set of our four rollers).
The downside: such a dynamometer has less linear surface resistance. Therefore, although it provides much better tractive force transfer than a single large drum, it provides less linear road simulation (rolling resistance will be greater than natural road resistance at higher speeds). For normal use, such a difference is not significant (because a full road simulation must include a simulation of air resistance, which is also non-linear). It may even be helpful - you could say that the dynamometer simulates "naturally" some air resistance at higher speeds and less use of retarders is needed.
Retarder connected to the wheel hub
Advantage: no traction problems (mechanical connection to a wheel hub)
Disadvantage: a lot of effort to install a car on the dyno (need adapters for various types of mountings, wheel removal etc.).
There is no real 100% mechanical connection (1:1) between the front and rear. Also, mechanical connection between the front and back can be provided only by hydraulic transmission of energy, which is not perfect. It is also impossible to measure the energy loss in the hydraulic fluid.
This solution is ideal for racing cars with a single axle drive system. In other cases, the disadvantages outweigh the advantages.
What to compare in software?
Modern software must be able to control all dynamometer parameters and switches, including from a remote desktop. So you can manage the dynamometer from inside the car using a notebook or smartphone. The dynamometer software should allow the mechanical axle synchronization to be switched on and off, set any load, operate fans, exhaust extraction system and even activate the car's roller lock/lift from within the software. It simplifies and speeds up the work.
Today's cars are equipped with OBD ports and share much of the data available there. Modern software measures power and torque and additional parameters such as air/fuel ratio (lambda), exhaust temperature, and boost - not only from the sensors supplied with the dynamometer but also from the car's OBD2/CAN port. Why create OBD logs with programs like Ross-Tech - let the dynamometer create the logs in the form of a chart synchronized with the measurements. Isn't it a great idea?
The dynamometer software must allow easy comparison of graphs and parameters because a professional tuner needs accurate data and precise results. At least two full measurements with all measured parameters must be available for comparison (in our dynamometer - up to 4 measurements). The tuner can compare gains and results and find problems and areas where parameters need to be optimized. This part of the software is crucial because clear graphs and comparisons of results are fundamental for all tuners of cars and trucks.
What to compare in technology?
How many points per second (torque and power results) does the dynamometer provide? Are they linear independent (not interpolated, generated, etc.)?
The dynamometer's accuracy and response time to power changes are critical and depend directly on the accuracy of the speed sensor. There are three types of speed sensors: optical, inductive and Hall sensor.
Although they provide up to 360 pulses per revolution, precision optical sensors are susceptible to vibration. The signal in a device such as a dynamometer can easily be disrupted.
Due to their design and method of operation, inductive sensors have limited processing speed. They are also prone to failure.
Hall sensors are much faster than inductive sensors, durable and resistant to interference from, e.g. vibration.
In our test bench, we use a sampling rate of around 100,000 times per second when reading the signal from the speed sensor, and combined with our advanced signal analysis method, which we have called TrueForce, we achieve an accuracy of 0.1%. The fast signal processing and lack of averaging when using TrueForce allows even a single misfire to be recorded.
High speed and accuracy are features of our product.
Are rollers knurled?
Using any paint, glue with sand etc., to increase the friction between tire and roller lasts for maybe six months. Later all will be worn, and the bare steel surface will be your working surface. Slippy and ugly looking.
Our solution is different – we extrude a unique tread (knurling with high mechanical pressure). This tread is 3D and CAM-optimized and may be described as teeth-shaped lines embossed in the roller (see picture). Each "line" has two "tops" across the roller (instead of a standard method of cutting them and thus – having only one peak per line) to double the number of contacts to tire. Then, between both "peaks", there are micro-cuts (at a right angle to the teeth-shaped line) to stop the tire from flattening the tread.
With knurled rolls, the tire temperature is noticeably lower, so there is less risk of overheating (damage) the tire during the measurement. The noise generated during the run is considerably lower than on dyno with milled rollers (not to mention the plain rolls).
Finally, rollers are covered with a unique double-layer chromium cover to protect them from being worn. Your dyno will always attract the eye of customers. Even ten years of use will not kill their parameters and brilliant aesthetics.