The MGA With An Attitude

Now I will play some of my favorite trailer games and calculations. The 500 lb trailer is only 18.5% of a 2700 lb gross tow combination. You can wind up the engine flywheel and pop the clutch in 1st gear to spin the tires on take-off. With sticky tires (1.0 COF) and 50% of the car weight (1100 lb) on the drive wheels, plus maybe 100 Lb of tongue load, you can generate 1200 Lb of thrust. Dividing that by gross weight of 2700 gives momentary acceleration of .444G. Multiplying that by the trailer weight (500) gives a hitch pull load of only 222 Lb. That amount of pull should be easily accommodated by the U-bolt over the round tube (prior page).

For stopping you may have 2200 lb car plus 100 lb tongue load and 1.0 COF for sticky tires, in which case locking up all four wheels would generate 2300 lb of deceleration thrust. Dividing that by gross weight (2700) gives .852g deceleration. Multiplying that by trailer weight (500) gives 426 lb thrust on the hitch. That is the load that must be held repeatedly by the thin frame tube with at least a 3:1 safety factor. That is, the yield point of the frame tube must be at least 1278 lbs applied in the center of the span.

My calculation says it should take about 2233 lbs to reach yield, but that would require a brand new steel tube with no flaws and a load distribution sleeve in the center to prevent crushing of the tube. Without the center sleeve I'm not sure it could even take the 1278 lb load without deforming the back side of the tube. If not, then you have to add spot loading and fatigue cycling in the life cycle calculation. It would help immensely to add a slip fit reinforcing sleeve a few inches long around the tube in the center where the U-bolt clamps down. Then I might consider towing something a little heavier.

Consider a 1000 lb trailer with 200 lb tongue load. 3200 lb gross with 2400 lb on the car tires. Locking up the tires could give 2400 lb deceleration thrust vs. 3200 lb gross weight, or .75g deceleration. That results in 750 lb push on the hitch from the 1000 lb trailer. Referring to prior calculation, 750/426 x 1278 = 2250 for required yield point of the thin frame tube. That is spot on at the upper limit of my calculation for a safe tow without damaging the thin frame tube (and 3:1 safety factor). For what it's worth, I have towed that sort of load a couple of times with my MGA. I once had two engines in my fiberglass box trailer for a 60 mile haul. Another time I hauled three gearboxes, an engine block, flywheel, cylinder head, two engine rear plates, two sets of carburetors, two propshafts, and some other smaller stuff.

Bear in mind that the above example reduced the braking deceleration from 1g to .75g. That increases stopping time by a third (to 4/3 of the original time), and increases stopping distance by at least 78%, (4/3)^2. In reality, it would nearly double stopping distance, because of a front to back imbalance in the tow vehicle brakes, reducing effectiveness of the rear brakes.

After that you get into the requirement for having brakes on the trailer. In Illinois there is a requirement for a 1000 lb gross weight trailer to have brakes on two wheels (even if it is a tandem axle trailer with four wheels). For a 3000 lb gross weight trailer (or more) it is required to have brakes on all trailer wheels. In more practical manner, it is a damn good idea to have brakes on all trailer wheels when the trailer weight might exceed 20% of the gross weight (or 25% of the tow vehicle weight), because of the increased stopping distance. This is often ignored by amateur drivers running a heavier trailer with no brakes.

Putting brakes on the trailer, properly balanced with the same sticky tires, allows the trailer to add considerable braking thrust. A 1000 lb trailer with 200 lb tongue load has 800 lb on the tires and may produce up to 800 lb of braking thrust. Now you get the full 3200 lb total thrust to go with the 3200 lb gross weight. But it's not quite a full wash on perfect deceleration because of the tongue load. 3200 lb thrust would appear to give 1g deceleration. That gives 1000 lb forward load for the trailer, 800 of which is taken up by trailer tire braking, and the other 200 lb thrust transferred through the hitch to the car to be taken on the car rear tires. That only works if there is a perfect balance for braking on all wheels so they would all lock up at the same time.

As MG brakes go, the added 200 lb load on the car's rear brakes is an overload, so the front wheels will lock up first while the rear brakes have not risen to the full requirement to lock up the rear wheels. This is likely to happen with any tow vehicle when you have a significant tongue load on the hitch ball. The only way to get near full braking force is to apply more hydraulic pressure until the rear wheels will also lock up, at which time you have an all-wheel skid, no steering control, and a possible jack knife action pending. To retain steering control you have to limit braking force NOT to allow lock up of the front wheels, and that increases stopping distance. In this case it will help to reduce the tongue load and increase trailer braking force. But you can never get to the optimum because of a requirement that the trailer tongue load should be a minimum of 10% of trailer gross weight (for good trailer handling behavior with no fishtailing). Modern vehicles with antilock braking system might be able to spoil or reduce braking force on the front wheels to prevent lockup, thereby retaining steering control. That would allow more hydraulic pressure to be applied to the rear brakes, but at some expense in slightly reduced braking effect up front.

When you are flat towing another car, or towing another car on a dolly, you have no tongue load (or very little), which is nice for not disturbing the front to rear brake balance on the tow vehicle. Unfortunately, most flat towing rigs will have no brakes at all on the towed load. If the towed car weighs as much as the tow car, you have double the gross weight with no increase of braking force. This quadruples the braking distance. If you thought your MG could stop in 150 feet from 60 mph, then your tandem tow rig would require 600 feet to stop from 60 MPH, nearly a full city block, and you are screwed if some vehicle within 450 feet in front of you was to stop suddenly. That's when you need to be attentive to allow a HUGE safety gap between you and the car in front, and also keep the speed down to reduce stopping distance as prudent. Flat towing with no brakes on the towed vehicle is always tricky business.

For a short story of interest, about ten years ago I was regularly towing a small stock car on a tandem axle flat bed trailer with a Ford Ranger pickup truck for the tow vehicle. The stock car weighed 2200 lb, the trailer was 1200 lb empty, and the Ranger truck was close to 3000 lb. That made the towed trailer load slightly more than the weight of the tow vehicle. Fortunately the trailer had adjustable electric brakes on all wheels. When the trailer brakes were cranked up to high force, just short of locking up the wheels, we could run well in traffic with stopping distance only slightly longer than the empty truck alone. It was kind of fun to run on the expressway without having to worry about running into some idiot who might pull in front and hit the brakes.

More recently I was using my 1990 Chevy Lumina Eurosport to pull a dolly with an MG on it (more than once). The MG and dolly combined weigh less than the Chevy, but not much less. Even with four wheel disk brakes on the Chevy we still had to allow for quite long stopping distance from any kind of speed. Jolly good fun though, as long as your nerves hold up.

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