What all does what for stability? Seems like longer wheelbase would obviously be more stable. How about location of the hitch ball? Would a shorter overhang (hitch ball closer to rear wheels) result in greater stability while towing?
I'm not sure what you're getting at with wheel sizes and how they contribute to stability.
You're on the right track, as i see it. Got 15 minutes to read?
I want to illustrate my reasoning with two scenarios. In both we think of the tow vehicle as a long rod. Har Har. Undoubtedly ive already lost someone, lol.
First, imagine the rod with a ball hitch at one end of it where force is imparted, a pivot point on which the rod can rotate which is the contact patch of the rear tires, and the front bumper at the other end of the rod. Consider that in a typical tow vehicle and especially in a diesel truck that most of the mass of the vehicle is located at the front end, closer to the front bumper than the pivot point of the rear tires.
A few things become obvious immediately. Force is imparted at the hitch ball, and the distances on either side of the pivot point determine how much of an effect a certain amount of force at the hitch ball will have on the rest of the vehicle. Simple leverage. A longer wheelbase vehicle will probably have proportionally more of the vehicle forward of the pivot, which would increase its resistance to force imparted at the hitch ball, but in some vehicles a long wheelbase version will also put more distance between the hitch ball and the pivot point, which is detrimental.
So yes, a longer wheelbase and a shorter rear overhang (or distance from hitch ball to rear tires) would be best.
For the next scenario, imagine that now the front bumper end of the rod is a fixed pivot point, the other end is still the hitch ball where force is imparted, and the contact patch of the rear tires, instead of fixed, is able to move laterally but is kept centered by the elastic force of some springs. Same basic implications. The longer the distance between the hitch and the springs, the more lateral movement will be produced by a given force. The more distance between the springs and the front bumper, the less lateral movement will be produced at the springs by a given movement at the hitch ball. Also, increasing the strength of the springs will reduce movement generated by a given force. The springs are undamped, so another thing to note is that there will be some oscillation when the force at the hitch ball is removed.
This simulates the lateral elasticity of the vehicle between the body and the contact patch of the tires. We can break this down into two basic areas, the suspension design, and the wheels and tires.
Regarding the wheels and tires, a shorter, stiffer sidewall such as what you'd find on a 20" rim and tire will allow MUCH less lateral deflection than the typical 15" truck tire like a 235/75r15 assuming the same overall diameter, which is what i meant with the wheel size reference. The amount to which the sidewall of a truck tire can 'roll over' is almost sickening to my autocrossing mind. Remember, lateral movement between the rim and the contact patch is not damped by anything either and could contribute to an oscillation effect.
On rear suspensions, im sure that independent rear suspensions allow the least amount of lateral deflection, but most tow vehicles have solid rear axles. Among solid rear axle suspensions, some allow much more deflection than others. A caprice or roadmaster, for example, benefits from this wonderful invention called a track bar. However, your typical leaf spring style rear suspension on a truck depends on the leaf springs themselves for lateral location of the rear axle relative to the body, and there is a LOT of deflection possible there. If you have seen the rear end of a leaf-sprung truck sliding sideways while NOT spinning the tires, you can imagine what i mean. They can be so bad that they will store up energy with huge amounts of deflection in the springs and then release it all at once when the energy overwhelms the grip of the tires, and essentially 'hop' sideways over and over. Additionally, the arch of the springs and whether the axle rides on top or bottom of the springs can contribute to the lateral deflection. Of course, the potential for undamped oscillation here is huge. Anyone ever experienced the 'death wobble' in the steering of a solid axle leaf sprung front end 4wd truck on big tires? Scary.
So, everybody knows about the death wobble of trailer towing. Even scarier than death wobble in the steering. Its easy to blame it all on the trailer or how its loaded, yadda yadda yadda, but in my uneducated opinion its better to look at the tow vehicle itself. The distance from the hitch ball to the rear contact patch is a force multiplier, and the undamped lateral elasticity of the rear suspension and tire sidewalls can store and release energy and set up a scary, destabilizing lateral oscillation.
So, some vehicles may have a surprising stability advantage over the standard issue half ton truck. A vehicle with a short rear overhang, a good track bar setup, and stiff sidewall tires will have a big stability advantage over a vehicle with a long overhang, no track bar, soft leaf springs, and massive sidewalls on the tires, like a stock longbed chevy 1500 for example. But rarely is a tow vehicle going to have it all.. Despite its long overhang, a caprice wagon on 20s with a factory track bar might be pretty dang stable. A 1 ton dually diesel truck, despite its long overhang and tall sidewalls may be pretty dang stable because its sidewalls are relatively stiff compared to lighter trucks, the leaf springs are SUPER stiff and dont allow much deflection compared to 1/2 ton springs, and it has a long wheelbase and a ton of mass at the front end. And i imagine it would be horribly unstable to tow with a 15 passenger van with a super long overhang, soft sidewalls, and no track bar.
Ive never towed with a fifth wheel setup but im guessing they almost never feel unstable, because the location of the hitch ball over the axle removes the leverage effect of having it on the back bumper.
/ too long of a dang post.
