There is no doubt pure combustion-engined vehicles are doomed, but they still have a thing or two to teach EVs. Standards, for example. Most of them have the same technical solutions. For instance, you know that a car with a manual gearbox will have a clutch pedal whatever its OEM is. And that automatic vehicles have a neutral position that allows them to have all wheels disengaged regardless of which company made them. Towing an EV shows that this is something their manufacturers still have to figure out.
Gallery: Lack Of Standard Makes EVs Towing Difficulty Depend On Manufacturer
When we read a recent article from Motoring Research about EV towing, it said the British Transport Minister, Baroness Vere, was “astonished” to discover EVs pose hazards if they break down on highways. One of the main reasons would be that they are harder to tow. After our research, we can say there was no reason for astonishment: the problem with towing EVs is of a different order.
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Greg Carter, the technical specialist at AA, told us more about that.
“There are no issues in recovering broken down EVs on a flatbed truck, although EVs involved in road traffic collisions present different hazards. It is generally accepted that if the wheels roll, it is OK to flat tow EVs at walking speed a short distance to a place of safety.”
That depends on the manufacturer. We have looked at the Tesla Model 3 manual to see how the Tow Mode – officially called Transport Mode – works. The first warning Tesla gives is that the Model 3 rear motor generates power when the wheels spin. It is the permanent magnet synchronous reluctance motor Tesla developed for the Model 3 – PMSRM, for short.
In the towing instructions for the Model 3, Tesla says the wheels cannot spin unless this is done at under 3 mph (5 km/h) and for up to 30 ft (10 m). Among the forbidden towing modes is one of the front wheels suspended and the rear ones touching the ground. There is no mention of the rear ones suspended and the front ones on the ground, though. Could a Model 3 be towed that way?
In the towing instructions for the Model S, the company says the same, but shows an image of the sedan being flat towed with its front wheels on the ground and the rear ones suspended. It also adds a fire icon to show what it fears that may happen if the car is towed this way.
We have then checked the Porsche Taycan's instructions. The Porsche has synchronous motors on both axles. Porsche says it can only be towed on a flatbed truck. There is no word about flat towing, even at very low speeds. According to its manual, if the Taycan stops, you would have to put it on dollies to ensure none of its wheels spin.
From a technical point of view, it seems synchronous motors pose a bigger issue to towing because they have permanent magnets. The magnetic field they create is enough for any movement to be converted to power. Induction motors have to be excited to create a magnetic field and generate power.
In other words, it seems that having them permanently connected to the wheels may be great for energy efficiency, but bad for towing.
Carter mentions why having the wheels connected to the motors can be a problem.
“EVs tend to be permanently engaged drive systems so they can only be recovered by methods that lift the driven wheels. In some cases, that’s all four. The reason this is an issue – in this context – is that either front, rear or full lift options take specialist recovery equipment and have longer set up times than a simple flat tow.”
That is the major AA’s concern about electric cars: the time some may require to be rescued.
“The issue with flat towing from an AA perspective is largely based on manufacturers guidelines, which as a responsible business we would always adhere to. But there is also the question “what is the fault?”. EVs are extremely reliable so if one has broken down on a motorway and it’s not simply a case of it ran out of charge, we have to consider what has gone wrong and how that will impact the recovery method.
Flat towing has always been the quickest method for getting a vehicle moved from a dangerous location – such as a motorway live lane – which is why this recovery method has been focussed on.”
That can even affect your ability to push your EV to a safer place, although this is not framed as flat towing.
“Flat tow is usually done by attaching the casualty vehicle to the van via a rigid tow pole and the driver would be required to stay in their vehicle to steer and brake. We do not push vehicles for health and safety reasons and we would never ask anyone else to either, for the same reason.”
On the other side of the pond, the AAA does not recommend flat towing of EVs if all wheels are on the ground, especially the driven wheels.
“EVs should generally be transported on a flatbed and never be flat towed with all four wheels on the ground in an effort to use regenerative charging to try to charge the traction battery. Additionally, there may be some challenges with getting the EV in ‘neutral’ when the traction battery is completely depleted. Special procedures may be necessary in this case so the vehicle may be moved onto a flatbed without damage.”
Would this be another of the reasons for automakers to consider transmissions on EVs? The Porsche Taycan two-speed gearbox has two clutches that allow the rear wheels to decouple from the motor but not the front ones. As you have seen, it makes no difference in towing it. Perhaps it should. Maybe that is a proper solution for towing EVs: a neutral mode that allows them to be moved more quickly.
A recent Carwow video shows how each manufacturer has a procedure for pushing or towing their EVs. A standard for all of them in this regard would surely help rescue teams. Carter said AA is trying to make that happen.
“We are liaising with manufacturers to establish acceptable recovery methods for these vehicles.”
AAA also sees that as a good policy.
“It would be positive for all manufacturers to incorporate a standardized procedure for handling unique challenges technicians encounter with EVs.”
Would a proper neutral position for EVs make them unnecessarily more complex and less efficient? Would it affect energy regeneration? What about induction motors over synchronous motors? Would they make it easier to tow an EV? This is a fascinating discussion we intend to propose now and to discuss in further detail with more articles about how to tow an EV safely. Perhaps we can help EVs have a standard for that as soon as possible.
Source: Motoring Research