Thanks for the explanation. I understand what added stiffness does, wasn't really asking that. As someone who traded in a 3rd gen to a 4th gen I understand the benefits, particularly to suspension setup.

But you also answered my main question, which was what benefit is gained over increasing the rigidity by enormous amounts.

 

...and don't forget my favorite part about a stiffer frame... the car tends to feel 'tighter' over bumps. In other words, when you hit a pot hole, speed bump, etc.. in a car with a stiffer chassis, the car seems to feel more 'unitized'.

 

Now that you understand the good things that added stiffness does, let's cover the possible problems:

1) You better be REALLY good at welding. Screw up a weld and bad things start to happen, VERY quickly. Riveting is far more tolerant of variation (it is designed to TAKE UP variation). When welding, the parts to be welded have to be located dimensionally very precisely. Welds DO NOT like being put in shear, and bad welds will make steel brittle.

2) You'd better understand fatigue and how it effects your structure. Over-constraining parts through perimeter welding and using closed sections can cause catastrophic fatigue failures that would not be immediatly obvious in the early design stages. Ask me how I know this...

3) Old cars and trucks, with lots of slop in their body and frame structures, are fairly insensitive to over-loading. Without slop, your margin of safety can decrease if the frame is forced to do something it doesn't want to do (like hit a tree).

 

Who says that's because of the frame? There's still a lot of pieces between the seat of your pants and the road. Maybe the body mounts aren't all that stiff, or maybe there's just soft bushings in them. Perhaps the firewall/front-bulkhead area isn't as stiff as it could be. The seat structure may flex. It's about a lot more than just frame stiffness, although frame stiffness is something that's easy to pin a number on.

And please don't get it stuck in your head that hydroformed frames are automatically stiffer. Hydroforming does typically reduce the number of welded joints and forces an engineer to use a tubular member; both good things, but they're not enough in and of themselves to promise a stiffer structure.

Chris's point about the real-world effect of stiffness improvements is excellent. If I took, say, the rear axle of my truck and made it 900% stiffer, it's unlikely to help anything. Likewise, it's possible to make magnitude-of-order improvements on any number of components, only to find out that some other component quickly jumps in to take its place as the critical component. I've been through this many times at work - I'll look at the output of an analysis, make a dramatic improvement to the biggest contributing factors, and find that I've made very marginal improvements to the end result. Of course, the difference there is that my customers tend to see past the BS, where as the OEM's customers get suckered in real easily.

Pacer's last three points are dead-on. I've had some experience building and using bleeding-edge structures, and they're often not all that they're cracked-up to be. His point #3 is going to be the one that gets a lot of truck users in trouble, especially if they want to modify their vehicle.

 

OK. Since I'm a contrarian, I need to throw another banana peel on the floor of reason. A couple of questions for the engineers:

Is it possible for a less-stiff frame to be the SUPERIOR frame, when looked at over the long term? If the frame has very little deflection, then isn't it more likely to overstress welds and other inherently weak points--perhaps causing welds to shear and cracks to develop? So your "rock solid" structure ends up having catastrophic failure over time, whereas something that gives a little will last longer.

And what about the effects of damping? I'd venture to guess an underdamped frame with a natural frequency of 35 Hz may be a heck of a lot less pleasant to drive than one that's overdamped and 15 Hz.

 

Well, generally speaking, the stiffer the structure, the better when it comes to fatigue. The problem comes when you increase the stiffness of all but a little bit of the structure, so that you now get most of your strain from one small area. This is why stiff structures have a bad reputation - it's really easy to half-*** the design process, overlook a small detail, and end up with a weaker product.

As Pacer stated over the course of a few posts, you also increase your sensitivity to defects and design errors if you try to push the limits of your material by trying to stiffen up the structure. As I've learned with some personal projects, it can be very difficult to design-in every scenario that you may encounter in the field, and the design becomes intolerant of minor fabrication problems.

I'm no chassis engineer, but I don't think you've got much damping inherent in a metallic frame. Now, if you look at two frames with the same resonant frequency but construct one from a non-metallic material (say, fiber and resin or a thermoplastic), then maybe you can start to see some damping effects. It seems as if the stiffer composite materials have little or no damping qualities, though. I've seen some 60% glass-filled nylon that sounds just like aluminum when dropped on a floor, and carbon-fiber with high fiber-to-resin ratios is similar.

 

Yes. Instead of the words "best" or "superior", "ideal" is usually a better word.

There is a balance in everything that gets engineered. You have to balance cost, mass, complexity, features, and plenty of other things.

The "ideal" design achieves the requirements with an "ideal" balance of those considerations.

If, for instance, it costs me $40 and 100 lbs. to make a frame 20% stiffer, but I can change my sway bar, spring and suspension design to achieve the same ride quality for $10 and no weight penalty, it becomes obvious as to which desing is "ideal".

The difficulty of the job is hard to overstate. Given enough time and effort I can improve on anything I designed or engineered before. But I don't usually have the time to do that, and back then I didn't have the experience. Even today there is SO MUCH that I DON'T know that it is pretty humbling.



Really, what you have to do is understand the differences between the two on a whole lot of levels. Fatigue (the condition you are describing) is a difficult issue. It is relatively difficult to calculate for, the conditions that cause it aren't always obvious.



The general trend lately has been to rely on the suspension for damping and make the structure supporting it as rigid as possible. Basically, trying to remove the structure as a consideration in the damping equation.

 

but its ok for you to use F-150 superduty (or ford's HD) when quoting numbers like towing capacity and payload ehh?

I got it now

Chevy 1500 < F250

I agree

 

Remember that Ford extended the cab of the Supercab & Supercrew by 6" , and extended the wheelbase an equivalent amount.

What would 6" worth of boxed hydroformed steel & double wall sheet metal add up to in LBs ??


FWIW, my opinion is that this new F-150 is a nice looking truck inside & out, with some neat features like the power rear windows for the supercab.

It also has plenty of fluff, like the moonroof option.

I am not a fan of the floor shifter in the Lariat, either , but hey - that's me.

The idea of the 5.4 reaching 80% of it's rated torque by 1000 RPM strikes me as great for it's intended purpose of pulling/carrying a load.

Oh, that tailgate assist is a great idea and it works !

Ultimately, though, I think if I was in the market for a new truck it would boil down to $'s for equivalent options.

It is fun , though, to keep informed of the latest offerings by torturing dealers and I highly recommend this as a cheap form of entertainment.


Britt