Surging towing light trailer

[Ishkatan]

I experienced surging (back and forth pulsing) in my 2023 Lightning Lariat ER while towed a Harbor Freight 1750 lb rated trailer with two 10 Ft logs about 0-12 inches in diameter when going up 2-3 degree hills.

• I estimate each log was about 300+ lbs. so total loaded trailer weight was likely under 1000 lbs.
• The hitch was tight and there was very little play and no banging at stop lights or accelerating.
• Trailer tires were at 70-75 PSI - below the recommended 80 psi for a fully loaded (1750 lbs) trailer but did not show much sidewall flex.
• Trailer tongue weight was at the limit of what I could lift from a vertical cement block to the hitch ball. I am told I am strong and I estimate 100 lbs.
• Trailer was at a slight upward angle and a 1-2 Inch drop would make it level. I have used this same hitch setup on an Avalanche 1500 with no surging but a different load.
• The trailer tracked perfectly with no sway on straight sections and on turns. I had the trailer camera turned on for most of the trip,
• This trailer does not have brakes - 4 pin connector.
• The surging only happened going up hills. 1-3 degrees per the dash angle indicator, which seems to be reacting slowly.
• Flat and down hill sections had no surging. One road section was new asphalt so road was very smooth.
• *** The surging was much worse with Adaptive Cruise Control. Much less noticeable but still there with adaptive cruise controll turned off.
• I do NOT use one pedal driving.
• 2.1 m/K electric draw for the 28 mile trip.
• I had the truck set for towing. The Harbor Freight trailer settings gave a range estimate of 104 miles but changed to 164 after a short while towing. My conclusion is that I gain battery range if I tow with a lightning.
• Speed varied 40-55 mph, mostly under 50 mph.
• Truck is stock with 16,400 miles on it. Tires were rotated but not balanced at 10k miles.

Because the surging was much worse with adaptive cruise control than with manual driving I believe this is a software issue. Lane keeping was turned on throughout but I don't think it would have an effect. Turning off all the assistive features is a PIA.

Minor info note: The phone app is showing me milage estimates based on the trailer still being connected (192miles at 80% vs 246 normally). I plugged the truck in (Pro charger at 80 amps) with the trailer connected but now the trailer is disconnected. I have not started the truck since plugging in with the trailer connected.

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[bmwhitetx]

I am not a trailer guy and have no experience there. The physics guy says tongue weight is too light combined with trailer already leaning back from the hitch height. Going up hill the logs lean the trailer back even more removing tongue weight. Whether that causes surging, I have no idea!

This is my wild a$$ guess but others will hopefully give a correct answer.

 

[Duke_239]

Tounge weight too low. That's why it only happens on the uphill, when effective tounge weight goes down. It might feel like 100 on flat ground, but it's probably not. Also, turning off driving assist features on my 2023 involves hitting two buttons on the steering wheel with my left thumb.

 

[EliPNW]

As for the truck still showing low mileage on the GOM after disconnecting the trailer, you need to go into the Trailer app and select No Trailer. Even though you physically disconnect the trailer, it will continue to show the towing range until you change it in the app. (Note this is different than the driving modes (Regular, Sport, Towing/Hauling...)

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