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. This is trivial, but these three components must perfectly cooperate. Moreover, the lack of capabilities of hardware 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 is that modern cars (and soon – all the cars) cannot be measured without mechanical connection of front and rear rolls (even if only one car axle is driven by engine). Dyno must provide a perfect simulation of road conditions – front and rear wheels must rotate exactly at 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 (a brutal ABS deactivation) but not in all cars (some will reduce power then, some other will catch ABS and CAN bus communication errors and you may need professional OBD scanner tool or factory scanner to re-activate them – so your customer may get angry).
There are inefficient alternative solutions, like additional electric motor for “synchronizing” - rotating unpowered axis (but for higher speeds it is insufficient and it cannot provide perfect synchronization) or hydraulic connection (but energy losses of hydraulic oil are non linear and not measured at all). For today – no real mechanical synchronization means that the dyno will work for old cars only.
Simple advantage: our AWD dynamometers have mechanically synchronized rolls (front to rear, via a belt) – it can be switched on or off from control panel or remote. You cannot measure any modern car (Mercedes, BMW etc) without full synchronization, even if the car is one-axle driven. We have produced several dozens of mechanically synchronized dynos for more than 10 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)
Simple advantage is that such roll can simulate a flat surface of road 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 in size 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 has also big natural inertia and this is critical for accuracy (bigger rolls = bigger 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 2-rolls-per-wheel dyno. This 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 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 car to fall down from the dyno, reach surrounding surface and jump out. Theoretically if all belts are properly used there is no risk – but people are erratic… This is much less possible for 2 rolls per wheel dyno (as 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 more safe in everyday usage than single roll dyno. It needs also much less height of your dyno room, as it has about 1/3 of height of one-roll dyno (if one-roll dyno has a good size of roll, like 900 mm – smaller single rolls are inefficient because of extremely small 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 smaller diameter of rolls (like 320 mm or so). Our rolls 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 4 rolls).
Disadvantage: such dyno has less linear surface drag and thus, while giving much better traction transmission comparing to one big drum – it provides less linear road simulation (rolling resistance will be higher than real road resistance for higher speeds). For normal usage, such difference is not important (as full road simulation must include simulation of air drag, and this one 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 wheel hub)
Disadvantage: a lot of effort to install a car on the dyno (need of adapters for various types of mountings, need of wheel removal etc).
Also, mechanical connection between front and rear can be provided only by hydraulic transmission of energy, and this is not perfect. There is no real 100% mechanical connection (1:1) of front and rear. Also energy losses in hydraulic fluid cannot be measured.
Such solution is perfect for race cars with 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 control your dyno from the inside of the car, from a notebook or a smartphone. Dyno should be able to mechanically synchronize and disconnect axis, set any load, start fans, exhaust gas extractors, even car liftoff from software. This 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 provide easy comparing of various graphs and parameters – as professional tuner needs exact data and simply readable results. At least 2 full measurements with all measured parameters must be available for comparison (our dyno – up to 4 full measurements), so tuner can compare gains, results, find troubles and areas where parameters still need to be optimized. This part of software is critical, as clear graphs and results comparison is a basic method of work of 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 is critical and it is directly dependent on the accuracy of speed sensor. There are three types of speed sensors: optical, inductive and hall sensor.
Presisely made optical sensors while providing up to 360 impulses per revolution are very sensitive 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, very durable, resistant to interference caused by eg. vibrations.
In our test bench we use sampling rate about 100,000 times per second when reading the signal from the speed sensor, and combined with our developed method of signal analysis which we have called TrueForce, we gain accuracy of 0.1%. High-speed signal processing and a lack of averaging when using TrueForce gives opportuninty to register even 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 6 months. Later all will be worn and bare steel surface will be your working surface. Slippy and ugly looking.
Our solution is different – we emboss a special tread (knurling with high mechanical pressure). This tread is 3D and CAM-optimized and may be described as a kind of teeth-shaped lines embossed in the roll (see picture). Each “line” has two “tops” across the roll (instead of common method of cutting them and thus – having only one top per line) to double amount of contacts to tire. Then, between both “tops” there are micro-cuts (at a right angle to the teeth-shaped line) to stop the tire from flattening of 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 special double layer chromium cover to protect them from being worn. Even 10 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 a perfect friction, even with wet tires!