Does hard braking while towing cause chassis bending?
QUESTION
Hi John,
In relation to your excellent video on bent 4WD twin cab utes chassis >>, I believe I have another factor when towing.
The static towbar weight you included, but then in the dynamic situation, when braking or hitting a wash-out there, there’s a dynamic downward force component on the (highly leveraged) towbar tongue. It stems from the trailer's centre of mass being higher than the slowing force.
The two mechanisms that come to mind:
1. Braking where the Centre of Mass is well above the deaccelerating force from the trailer tyres;
2. Breaking force from the ute via the tow bar. For a caravan, the Centre of Mass will still be above the tow bar coupling.
As a retired electronics engineer/grey nomad, I try to take this topic seriously and avoid safety mistakes like these. What do you think? Hopefully I’ve failed to be a pain in your arse by asking you this.
Sincerely,
Rod
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ANSWER
Hey Rod,
Thank you for watching, and yes - you have failed dismally to be a pain in the arse mate.
That bit in the video where I said ‘inertial loads’: That’s what you’re talking about below. Dynamic loads from the trailer resolve themselves into forces/loads at the towball. (The purpose of the ball is to eliminate the transmission of couples/torque/moments, right?)
Thus the inertial loads at the ball in any particular accelerating frame of reference can be resolved into three mutually orthogonal force components at the ball (lateral, vertical and longitudinal). The vertical one contributes to bending of the chassis…
In 1. you’re really just talking about the trailer pitching nose-down under brakes generally. It’s not really the mechanism that bends the chassis catastrophically. To do that, you need a severe vertical shunt caused by a combination of inappropriate speed, high static loads and an isolated geometric defect in the road, such as a washaway. That’s always how it happens.
Brakes can only cause about 1G of longitudinal acceleration, so they lack the capacity to amplify the load on the towball dramatically. Sure, the trailer pitches forward, but not enough to bend the chassis.
Sincerely,
JC
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