Which dyno to buy? How to choose a right one.
How to compare dynamometers available on the market?
Each chassis dynamometer consists of three components: hardware, software and technology of measurement used. Sounds trivial, but these three components must perfectly cooperate. Moreover, the lack of hardware capabilities cannot be “repaired” by software or technology and vice-versa. All the components must be perfect.
What to compare in hardware?
The most critical problem for modern cars (and soon – all the cars) is that they cannot be measured without the link of front and rear rollers. It is critical, even if an engine drives only one car axle. Dyno must provide a perfect simulation of road conditions – front and rear wheels must rotate strictly at the same speed – otherwise, it will be detected by ABS system, and the car won’t run, limit power or even show errors. Theoretically, this can be solved by unplugging ABS plug, or fuse, but not in all cars. Some may reduce power or catch ABS and CAN bus communication errors, and you may need a professional OBD scanner tool or factory scanner to re-activate them. So your customer may get angry). There are inefficient alternative solutions. An example is an auxiliary electric motor for “synchronizing” - rotating unpowered axis. For higher speeds, it is insufficient, and it cannot provide perfect synchronization. Another known attempt is a hydraulic connection (but energy losses of hydraulic oil are non-linear and not measured at all). For today – no accurate mechanical synchronization means that the dyno will work for old cars only.
Simple advantage: our AWD dynamometers have mechanically synchronized rolls (linked front to rear, via a belt or two in EV+ version) – it can be switched on or off from the control panel or remote. You cannot measure any modern car (Mercedes, BMW etc.) without full synchronization, even if the vehicle is one-axle driven. We have produced several dozens of mechanically synchronized dynos for more than ten years – they all work perfectly.
There are three solutions of rolling roads: with one roll per wheel, with two rolls per wheel (like ours) or with one retarder per wheel directly coupled to the wheel hub (the wheel is removed for that, 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 roll (US style)
The simple advantage is that such a roll can simulate a flat road surface better than two rolls. The rolling resistance will be almost the same as the rolling resistance on a real road. It must be 900mm or bigger to have enough “flat” surface at the top to provide traction (otherwise, you may need to use a kind of rubber glue to maintain traction). It also has big natural inertia. It is critical for accuracy (bigger rolls = more considerable inertia for same roll weight).
There are two disadvantages:
- surface contact between tyre and roll is smaller in size than on the road, and it is about 1/3 of the surface that exists in a 2-rolls-per-wheel dyno. It is caused by having only one area of contact at the top of the roll (that is not flat like a real road because it is impossible – and this reduces that surface further). Simple: it is much more probable that the car will spin tires instead of providing energy to rolls to measure it. So less power can be measured. This limitation comes from simple physics and cannot be overcome by any trick. Less surface = less traction = lower maximum power of dyno. Period.
- If you look into YouTube and search for “dyno accidents”, – NOT surprisingly, most of the dangerous accidents happen on single-roll dynos. It is simple – the car is in an unstable position on the top of the roll, and any mounting belt break or even not enough belt tension cause the vehicle to fall from the dyno, reach the surrounding surface and jump out. Theoretically, if all belts are properly used, there is no risk – but people are erratic. It is much less possible for two rolls per wheel dyno (as a car is in a “cradle” between the two rolls, and it can only move left and right and does not fall out by itself).
Two rolls per wheel (EU style)
Simple advantage: such dyno needs much less effort to install a car on. It is also much safer in everyday usage than single roll dyno. It also requires much less height of your dyno room, as it has about 1/3 of the measurement of a one-roll dyno. If one-roll dyno has a good roll size, like 900 mm – smaller single rolls are inefficient because of minimal surface contact to tyre, and thus – traction force and measured power limits.
To have similar inertia capabilities, the weight of rolls must be huge, as inertia arises with diameter – and two-rolls-per-wheel dyno has a smaller diameter of rolls (like 320 mm or so). Our rollers have 320 mm and 140 kg of weight each – this gives them (set of 4) similar inertia to one big roll (big rolls have thin walls, so they weigh less than a set of our four rolls).
Disadvantage: such dyno has a less linear surface drag. Thus, while giving much better traction transmission than one big drum, it provides less linear road simulation (rolling resistance will be higher than natural road resistance for higher speeds). For regular usage, such difference is not essential (as full road simulation must include simulation of air drag, which is non-linear too). It can be even helpful – as we may say that the dyno simulates “naturally” some drag of air for higher speeds, and less retarder usage is needed.
Retarder connected to 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, need for wheel removal etc.).
Also, mechanical connection between front and rear can be provided only by hydraulic transmission of energy, which is not perfect. There is no real 100% mechanical connection (1:1) of the front and rear. Also, energy losses in a hydraulic fluid cannot be measured.
Such a solution is perfect for race cars with a one-axle driven system. Otherwise – disadvantages overwhelm advantages.
What to compare in software?
Modern software must be able to control all dyno parameters and switches from it, including virtual desktop – so you can manage your dyno from the inside of the car, from a notebook or a smartphone. Dyno should synchronize and disconnect axis mechanically, set any load, start fans, exhaust gas extractors, even car liftoff from software. It simplifies and accelerates the work.
Modern software measures not only power and torque but additional parameters like air to fuel ratio (lambda), exhaust gas temperatures, boost – not only from sensors provided with dyno – but also from OBD2/CAN port of the car. Nowadays, cars have OBD ports and provide a lot of data there. Why create logs of OBD with programs like Ross-Tech – let the dyno produce a log itself, as a graph synchronized with measurement. Isn’t that a perfect idea?
Dyno software must easily compare graphs and parameters, as a professional tuner needs exact data and simply readable results. At least two complete measurements with all measured parameters must be available for comparison (our dyno – up to 4 total measurements), so the tuner can compare gains, results, find troubles and areas where parameters still need to be optimized. This part of the software is critical, as clear graphs and results comparison are primary for all car/truck performance tuners.
What to compare in technology?
How many points per second (torque and power results) does the dyno provide? Are they linear independent (non interpolated, generated etc.)?
Accuracy and speed of reaction of dyno for power change are critical, and it is directly dependent on the accuracy of the speed sensor. There are three types of speed sensors: optical, inductive and hall sensor.
While providing up to 360 impulses per revolution, Precisely made optical sensors are susceptible to vibration. A signal in a device such as a dynamometer can easily be disrupted.
Induction sensors, due to their construction and method of operation, are limited in processing speed. Also, they are prone to failure.
Hall sensors are much faster than inductive, durable, resistant to interference caused by, e.g. vibrations.
In our test bench, we use a sampling rate about 100,000 times per second when reading the signal from the speed sensor, and combined with our advanced method of signal analysis, which we have called TrueForce, we gain an accuracy of 0.1%. High-speed signal processing and a lack of averaging when using TrueForce allow registering even a single misfire.
High speed and accuracy are the features of our product.
Are rolls knurled?
Using any paint, glue with sand etc., to increase the friction between tire and roll 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 emboss 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 roll (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 rolls (not to mention the plain rolls).
Finally, rolls are covered with a unique double-layer chromium cover to protect them from being worn. Even ten years of use will not kill their parameters and brilliant aesthetics. Your dyno will always attract the eye of customers.
The tire runs silently, with perfect friction, even with wet tires!