Tongue weight is the weight of the trailer that is supported by the vehicle not the trailers axles, or in other words the weight on the trailer that is on the hitch, stock 80 coils cannot handle much weight I was sagging badly in the back on a trip back from Florida with just a few cases of bottled water (200 lbs?) some camping supplies in the back and a small trailer with a tongue weight that I could pick up.
A heavy trailer will make a stock coil spring compress more than it would the leaf spring of a truck. the reason trucks are still made today with leaf springs thier ability to handle diffrent weights well
Not sure about the wheel base thing, a short wheel base (with matching turning radius) makes backing a trailer easier, also the longer the trailer (from hitch to the trailers axle) the easier it is to back up. A short trailer on a long wheelbase vehicle is hard to keep up with and jackknifes easily.
What weight a vehicle can tow is decided by a lot of factors you got most of them, breaking, engine power, engine and transmission cooling, and load capacity of the suspension are some I can think of.
There are some mods you can do to make the 80 a more capable at towing, some people here that regularly tow heavy trailers add air bags to the rear of their 80's, heavier springs would help also, with a heavy trailer brakes are highly recommended.,
How large of a trailer did he want to rent? From you original post I thought he wanted to tow a trailer more than 5K ponds ( a big trailer)? The 80 meets the requirements on U-Hauls web site, seams their trailers are all rated below 3500 pounds, there are some inconstancies though they say you can put a vehicle up to 4K ponds on their trailer but only require a 3500 pound hitch that is shy before you even count the weight of the trailer
Your towing vehicle:
• Must be equipped with a hard top if it is an SUV. U-Haul does not allow any type of sport-utility vehicle not equipped with a hard top to tow U-Haul equipment.
• Must weigh at least 3,500 lbs. (curb weight)
• Must have Class 2 tow hitch (3,500 lb. minimum weight-carrying rating)
• Must have 1-7/8", 2" or 2-1/8" hitch ball (3,500 lbs. minimum)
• Must not exceed maximum allowable hitch ball height 25"
• Must have external mirrors on both sides
• Must have fully operational lighting
• Trailer lighting must be operational at all times, day and night
• Maximum recommended speed is 45 MPH
And the OM
_ The total trailer weight (trailer weight plus its cargo
load) must not exceed the following. Exceeding
this weight is dangerous. If towing a trailer over 907
kg (2000 lb.), it is necessary to use a sway control
device with a sufficient capacity.
Without towing package 1587 kg (3500 lb.)
With towing package 2268 kg (5000 lb.)
_ The gross combination weight (sum of your
vehicle weight plus its load and the total trailer
weight) must not exceed the following:
Without towing package 4547 kg (10025 lb.)
With towing package 5228 kg (11525 lb.)
