Aerodynamic optimization for towing 2.53 m width and 4 m height obejcts

Crissa

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Not really true. They know it will flex and they design it to accomodate that. By distributing the flexing more evenly, and avoiding "hinge points", the metal fatigue can be minimized.
But that's because they know it's not capable of more rigidity. Saying 'it's designed to flex' is misleading. Every structure has to accept flexing at some level. The ladder frame is incapable of more rigidity and strength, so it has to tolerate more flex.

Even so, that means it will fail sooner rather than later. And is capable of less. It's a compromise.

-Crissa

 

JBee

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But that's because they know it's not capable of more rigidity. Saying 'it's designed to flex' is misleading. Every structure has to accept flexing at some level. The ladder frame is incapable of more rigidity and strength, so it has to tolerate more flex.

Even so, that means it will fail sooner rather than later. And is capable of less. It's a compromise.

-Crissa
A ladder frame can easily be just as rigid as the CT structure, it's just that it's probably going to weigh more.

Look up metal yeild strength.
 

HaulingAss

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But that's because they know it's not capable of more rigidity. Saying 'it's designed to flex' is misleading. Every structure has to accept flexing at some level. The ladder frame is incapable of more rigidity and strength, so it has to tolerate more flex.

Even so, that means it will fail sooner rather than later. And is capable of less. It's a compromise.

-Crissa
It's not misleading. They could make it stiffer, with less flex (and at the same overall weight), but then it wouldn't have the load capacity. It is literally designed to flex in a manner that maximizes load capacity while minimizing weight and amount of steel required. I'm not playing word games here, they make conscious design choices to allow it to flex more uniformly so it doesn't fatigue as easily. This flexing also allows it to maintain it's full load capacity on ground that is more uneven.
 

Crissa

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A ladder frame can easily be just as rigid as the CT structure, it's just that it's probably going to weigh more.

Look up metal yeild strength.
No, rigidity also includes the external box. There is literally no way to make the ladder frame as rigid as you can make a larger box, There's a limit to how rigid a cross section of steel is, and one of those constraints is the cross section.

This is literally engineering 101, which I figuratively did not fail in university.

-Crissa
 


JBee

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No, rigidity also includes the external box. There is literally no way to make the ladder frame as rigid as you can make a larger box, There's a limit to how rigid a cross section of steel is, and one of those constraints is the cross section.

This is literally engineering 101, which I figuratively did not fail in university.

-Crissa
You can see it in the terminology and discourse if the appeal to authority has merit.

As you point out, "cross section of steel". So make it thicker.
But don't forget the primary structure of the CT bed is cast aluminum which has a much lower yield strength than steel. (24GPa vs 78GPa)

Are you trying to persuade me that a 40ton truck has less torsional rigidity than a CT?

Cool. I want a CT to pull my road train then. 🤪
 

JBee

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It's not misleading. They could make it stiffer, with less flex (and at the same overall weight), but then it wouldn't have the load capacity. It is literally designed to flex in a manner that maximizes load capacity while minimizing weight and amount of steel required. I'm not playing word games here, they make conscious design choices to allow it to flex more uniformly so it doesn't fatigue as easily. This flexing also allows it to maintain it's full load capacity on ground that is more uneven.
Otherwise known to engineers as "deflection". :ROFLMAO:
 

JBee

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"Springy" is a highly technical term for the opposite of "rigid".

Sheesh! ;)

A ladder frame is a traditional truck frame. They are designed to flex rather than crack and break. If you have taken these off-road you are aware of how much the frame can twist and deflect just from static loads on uneven ground. Dynamic towing loads are much higher. When a heavy trailer is attached to the rear-most point of a ladder frame it is usually a significant distance behind the rear axle and the side forces applied to this point from the normal motion of the trailer being towed are high enough to deflect the chassis laterally. This energy builds up in the ladder frame and then is released (for every action there is an equal and opposite reaction). A ladder frame does not dampen the energy much and every frame has a natural frequency or resonance that it responds to. This is the source of that most dangerous towing condition, often called "trailer sway".

While a ladder frame can be made as rigid as one would like, that would add a lot of weight that is not compatible with light duty trucks like the F-150 and the Silverado. The Cybertruck is not only light, but since the structural part of the frame is wider, outboard of the wheels rather than inboard, it will be naturally more rigid. It's simple geometry and leverage. Additionally, the material choice, cold-rolled steel, is naturally more rigid than mild steel. And the shorter lever arm that you mention (shorter rear overhang), will be an additional advantage.

The suspension plays into it as well because the suspension connects the frame to the wheels through the axles and the side loads are not always well controlled which allows the rear to go left while the front goes right. Leaf springs are incredible at handling huge loads but, on 1/2 ton trucks are often rather long (fore to aft) for ride comfort and suspension travel and this is less than ideal for towing stability because the springs allow a certain amount of twist and that is not damped by the suspension dampers. The Cybertruck will have beefy rods in the independent suspension controlling lateral loads and these will not store significant energy, unlike leaf springs.

A poor suspension that springs up and down with poor damping is not nearly as bad for towing as you think, except to the extent that they (poor suspensions) also tend to not control side loads very well. And of course, no one wants up/down oscillations while towing, if for no other reason, it's uncomfortable. But that motion is combatible with towing stability as long as side loads are well controlled, but they typically are not.
In this case "elastic" deformation would be the correct term.

As pointed out in previous other posts above, there are a lot of factors that make the CT different, and also better from a structural perspective.

The critical component is understanding what loads are transferred where into the structure and where they ultimately end up.

All loads from the trailer are transferred via the tow hitch. Those forces are then transferred to the wheels of the CT. A ball joint hitch, like most hitches on trucks this size, has no way to transfer a torsional load via the ball to the chassis of the tow vehicle, unless the trailer itself is about to tip over. The only forces it can transfer, intentionally, is horizontally and vertically, and are limited by the amount they can do so by the "fulcrum" of the system being the tyre contact patch adhesion. A ball hitch has 3 rotational axis freedom of movement.

So in the order of magnitude, of trailer induced elasticity, that would be:

1) Tyres - Traction co-efficient, wall thickness, rubber compound, air pressure etc
2) Suspension - Airbag pressure, suspension arm geometry, roll/sway bar (which I hope CT does not have to be off-road-able and has an active system like Kinetic on Landcruisers)
3) Hitch Leverage
4) Tow bar leverage
5) Weight distribution

Torsional rigidity of the frame, or it's "springyness" did not make the list because it is borderline irrelevant to the trailer hitch as those loads can't be transferred through it. But... the dynamics of the tow vehicle are relevant for pulling that point in space, the tow hitch around, in that a more stable tow platform will result in more stable trailer performance.

This leads the conversation to the next part, the tow vehicle dynamics.

There are various methods to optimise the tow vehicle. We have NO data to suggest the CT will be more or less torsionaly rigid than any other vehicle. Most of the forces from the bed and towing will be transmitted to the rear wheels via the rear casting, and the "exoskeleton", whatever is left of it, will transfer some off that to the cabin roll cage and structural pack to the front cast and wheels.

You can achieve this with a rigid and a not so rigid platform. Effectively, you could also just use the single rear axle of the CT itself (with a different hitch) to pull the trailer too. But like the Unimog, you could also design the ladder frame to intentionally twist, instead of having lots of suspension travel. Or you can have it as I would expect the case to be with a cast, have it rigid and have more suspension travel. Either will work, but obviously the cast optimization is superior in cost, weight , manufacturing, assembly time etc.
 
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JBee

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Do you know how I-beams work?

The limit to their rigidity is their size, the cross section.

-Crissa
So what is the point if your yield strength is 60% lower because of the material you used?

Your statement was that you can't make it as rigid, because a ladder frame is not as wide as the CT box can be. It doesn't have to be if it's made out of a material that has 3x the strength. Further most ladder frames are only a few mm thick, try using 10-20mm box profile to calculate the section modulus and get back to me with that.
 


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Will my two lab dogs change the aerodynamic if they are standing up in the bed of the Cybertruck going down the road?
 

Crissa

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Will my two lab dogs change the aerodynamic if they are standing up in the bed of the Cybertruck going down the road?
Yes, but less than the amount having the cover open will, and probably also less than the ticket for having unsecured animals.

-Crissa
 

 
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