How to approximate the maximum towing capacity of a School Bus

Question

I have a 1991 rear engine Thomas SAF-T-LINER school bus conversion with

1. engine - CAT 3116 (MUI, turbocharged w/ unknown torque (between 495 and 735ft lbs))
2. transmission - MT643 - (~70K lbs weight limit I believe)
3. 40 ft body with frame access at the rear for a welded hitch (not installed)
4. GVWR 36,200
5. Approximately 9 ft between the rear axle and the rear of the bus.
6. The four rear tires have a weight rating of 6005 lbs each (24,020 lb rear axle)
7. GAWR Rear 23,000 lbs (likely limited by the factory tires, which have been changed)
8. GAWR Front 13,200 lbs
9. I don't know how much weight is on the rear axle currently. (will def check before doing anything). However, I've removed all the seats (~40lbs each) and left it pretty light inside.
10. Air brakes

Thomas obviously doesn't say anything about towing with their buses (not a common occurrence I imagine). However, I have seen this being done before and would like to know what to consider when determining the safe towing capacity of this vehicle. If necessary, I can purchase a very old work truck, but I would prefer to avoid this as it is rather pricey for only occasional towing. I need to tow upto 15,000 lbs.

Update:

9. I took the bus to a weigh station and the weight on the axles is 23480lbs. The rear axle is 14,940 lbs and the drive axle in 8540 lbs

Answer 1

It can tow the same weight as the kids it would hold, usually about 12,000 lbs. for an 88 kid bus

The whole thing is the weight of the vehicle, versus the weight of the trailer, the vehicle needs to weigh twice as much as the trailer, so a 6000lb vehicle can safely pull 3000lbs, no sweat at all.

6000lbs and the truck feel sluggish, and 10,000 lbs and the truck feels really wrong.

Answer 2

Non weight distributed hitch

After I weighed the bus, I was able to do some simple math to determine the most limiting factor in towing the bus.

It is, in fact, the weight that can be put on the rear axle. With a GAWR rear of 23,000 lbs and an actual weight on the rear axle of 14,940 lbs, I'm left with an extra 8,060 lbs I can add to the axle. If I were to tow a 15000lb trailer, the appropriate tongue weight of 1500 lbs would add 1500*9 (13500lbs) to the rear axle. The moment added is far in excess of what the rear tires can support and likely in excess of the suspension's limit too.

Weight distributed hitch

With a modest weight distribution system at 2000lb tension and a 30 inch bar length and a trailer that is 15,000lbs and an axle 200 inches from the hitch one can determine (with the use of an online calculator :P):

I entered the following information:

The results are as follows:

According to this calculator (bottom, middle image) 15,001 lbs is indeed that max weight with this weight distribution system.

But wait!

In California, the maximum trailer + vehicle length is 65 ft. The bus is 40ft....That leaves 25 ft for the trailer and the hitch. While I'm just within weight requirements I think the length requirements will mean that I need to look into a different tow vehicle...maybe a shorter bus with similar specs?

The calculator I used was https://www.ajdesigner.com/apptrailertow/weightdistributionhitch.php

Hopefully the above will help someone stay safe while towing.