Why do so many Americans hate European trucks?

No MX here

 

My logic is the same as yours. You'd be suprised at how much a frame flexes. Even the frame on my short W900A has a fair amount of flex. Granted I don't have the engine installed (which acts as a crossmember at the rear mounts) or the ramps bolted on the back yet.

 

I measured a few truck that I have here:
Isuzu single axle rigid, 5 meter wheelbase, 11.5 ton drive axle 7.5 tonne front axle: 260x85x10 mm
MAN 6x4 tractor, 3.8 meter WB, 26 tonne tandem 7 tonne front axles: 270x85x8

And for comparison:
FL Columbia 6x4 tractor, 6 meter WB, 40k lbs tandem 12k front axle: 10 3/16 x 3.5 x 11/32 (259x90x8.7mm)

All single frames.

So the heaviest spec truck has the lightest frame.
If I put the FL at 100%, the Isuzu frame is about 12% heavier per meter/ft, the MAN is 8% lighter.
But teh MAN is also the shortest truck.

An explanantion for this could be that the bending moment (both horizontal or vertical) increases with length (longer lever). A longer wheelbase needs a heavier frame to handle the higher bending moment.


I would think that the torsional stiffness of the frame is determined by the crossmembers much more than by the main frame rails itself. The main rail is very flexible when a torsional torque/moment is applied to it.

Then there are different philosopies about torsional stiffness, depending on the application.
In some applications you want the frame to be torsionally stiff and let the suspension travel compensate for uneven terrain for the most part, frame only for small part.
In other applications the choice may be to have the frame flex more, and not rely on suspension travel so much.
I'm sure @98989 knows a lot more about this than I do.

On uneven ground, for example if one of the front wheels is 1 ft higher than the others, the combined frame flex and suspension travel have to compensate for that.
Stiff suspension (and heavy anti-roll bars) combined with a stiff frame may lead to the other front wheel to be up in the air. Unless there is enough load on the truck to keep all wheel on the ground, but then it will lead to high tensions in the frame due to the torsional flex.

If the frame is flexible, there is a much better chance of the wheels to stay in contact with the ground, at lower tension in the frame.
A longer wheelbase/frame will allow more flex than a short frame/wheelbase, or same flex at reduced tension in the frame.


On the other hand, the combined torque of the engine and ratio in the transmission will lead to torsional flex in the frame. If the frames have the same torsional stiffness (per meter/ft) the longer wheelbase/frame (point where the engine mounts transfer torque to the frame) will flex more than the shorter frame.
The flex per meter.ft will be the same (if torsional stiffness per meter/ft is the same), but there is more length. For example: if the torsional flex per meter is 1 degree, the longer frame will have more total flex.


One of the main reasons, in my opinion, that you see much less flex in the european frames is the much shorter distance between rear engine mounts (where torque is transferred between engine and frame) and rear suspension (where torque is transferred between axle and frame)

 

That's very true of course, however.......
In the US the 6 x 4 tractor is the standard tractor, even though the weight of a US semi isn't very high.
In Europe the 4 x 2 tractor is the standard, and European semis can be a bit heavier than US semis.
The European 6 x 4 tractor is a heavy duty truck, used for pulling heavy trailers etc.

That is the reason that I used specifications of the 4 x 2 tractor.

 

I have noticed that if you put a fixed fith wheel right over the tandem,frames can be much lighter.
Offcourse with limited axle weights.

 

I would say that this is because all the weight on the fifth wheel is absorbed by the tandem, and does not put additional (vertical) load through the frame to the front axle.

Depending on the type of suspension, all of it stays within the area between front spring hangers (or airbags) of the front tandem axle and the air bags of the rear tandem axle.
Or in case of suspension like camelback or walking beam, all of the weight goes directly into the trunnion.
No additional bending moment and/or forces in the rest of the frame.


If a larger part of the load imposed on the fifth is to be transferred to the front axle, the bending moment and therefor also the stresses in the frame will be higher.

Of course there are additional forces when accelerating or braking, but these occur anyway, no matter the horizontal position of the 5th wheel.

Also, during cornering there will be lateral forces on the frame, front axle forcing the steer around the corner, but the lateral stiffness of the frame (main rails and crossmembers combined) is much much larger than the vertical/ (longitudinal?) stiffness, due to the distance between the frame members.

 

What Chassis rail Centres do the Euro & Jap trucks use??

US Trucks are all 34" (840mm) Centres. I've also noticed that Euro cross members seem to be heavier construction than NA ones.
Or at least to My Eyes..

 

Seems to be 790 mm.

 

This may also depend on application and what kind of suspension is used, some use narrower frame, others wider.
Few examples: outside - outside of main rail

MB Actros: 846mm, but pretty sure I've seen narrower MB frames as well. ( @98989 ? )


Iveco Stralis:



Scania: 770


Isuzu: 846 mm


MAN: 761/762mm