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Towing may require differing regenerative braking profile.

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Any reason why you can't start slowing down early enough to use regen rather than braking?
Sure, to some extent, but nobody is likely to want to drive that conservatively. Heavy trailers are required to have brakes for good reason - they are needed and get used. Any situation where the trailer brakes are used is going to burn energy in the trailer brakes instead of regen.

As a rough estimate, consider a combination of the truck and trailer at 18,000 lbs. But the down force on the truck, even with weight distribution hitch is only about 8000 lbs of that. Less than half the total weight you are trying to stop. In any situation where braking is limited by traction, you are going to have somewhat less than half the stopping ability or use 4x the stopping distance you had without the trailer.

As an aside, the numbers get really bad on wet pavement. The coefficient of friction on wet pavement is about 40%. So 40% * 8/18 = .18. Your max deceleration is going to be 0.18 G's before the wheels just skid(or anti-lock braking kicks in). Once the tow vehicle loses traction, loss of control and a crash or jack knife is a real possibility. So that is why you need to use the brakes on the trailer.
 
Sure, to some extent, but nobody is likely to want to drive that conservatively. Heavy trailers are required to have brakes for good reason - they are needed and get used. Any situation where the trailer brakes are used is going to burn energy in the trailer brakes instead of regen.

As a rough estimate, consider a combination of the truck and trailer at 18,000 lbs. But the down force on the truck, even with weight distribution hitch is only about 8000 lbs of that. Less than half the total weight you are trying to stop. In any situation where braking is limited by traction, you are going to have somewhat less than half the stopping ability or use 4x the stopping distance you had without the trailer.

As an aside, the numbers get really bad on wet pavement. The coefficient of friction on wet pavement is about 40%. So 40% * 8/18 = .18. Your max deceleration is going to be 0.18 G's before the wheels just skid(or anti-lock braking kicks in). Once the tow vehicle loses traction, loss of control and a crash or jack knife is a real possibility. So that is why you need to use the brakes on the trailer.

Of course you need trailer brakes to come to a stop quickly, but they can be avoided by controlling the stop. From 60 mph, regen can stop the truck / trailer combination (18000 lbs) in 23 secs and 430 ft. How does this compare to normal stops using brakes? This assumes the tri-motor will have a 200kwh battery so that regen power could go as high as 130kw. On the 100kwh packs it goes up to about 65 kw.