Why do tractor trailers have a different design in Europe?

I’ve noticed that European tractor trailers have a rather distinctive style, at least compared to the US ones. It’s enough that you can easily tell what continent the vehicle operates on just by looking at it. For instance, look at the pictures on this page: http://www.tip.ge.com/web/SilverStream/Pages/pgTIPTrailerTypes.html

One obvious difference: all the trucks there have 2 sets of axles on the tractor and 3 on the trailer, compared to the US ones, which usually have 3 axles on the tractor and 2 on the trailer. Anyone have any theories why?

Another difference I noticed is that European and Japanese tractors all have their engines underneath the cab, while many American tractors have them in front. As I understand it, the reason for the cab-above-engine design (what’s the official term for this?) is to make the tractor shorter, taking up less space and also allowing a shorter turning radius. This is an important concer on the narrow roads of Europe and Japan, but not as important in the US.

So I wonder if the same reason applies to the axle arrangement? Anyone know if having two fixed axles so close to the front wheels (as on American tractors) will affect turning radius?

US Tractor trailers (UK English, articulated lorries) differ because of environment and function.

US vehicles are designed to haul over long straight roads which are wide; European vehicles are designed to haul over generally shorter distances on less straight roads which tend to be narrower. This is particularly important at source and destination where the roads are often very narrow.

Additionally, of course, there is a fashion statement- all that chromium is not that necessary!

      • I noticed that in the linked photos, all the trailer axles are single-tire, I.E. not dual-tire as US trailers are. Single tires are easier to change (if that’s a factor), and also by using three axles they can use a single type of axle: if the trailer needs 6 tires to support the weight, they can either use three single-tire axles or one dual-tire and one single tire axle.
  • Considering that, it might be that most common Euro trucks are designed for hauling less weight: compare the tire counts. The Euro trucks I have seen in the US do seem somewhat smaller, as do the Japanese makes. A US-built cab has 2x4x4 tires [that is, axles front to back, counting the tires per axle] and usually 4x4 tires on the trailer, for a total of 18 tires (hence, an 18-wheeler). A Euro cab like the ones pictured has only 2x4 tires, and the trailer has only 2x2x2 tires, for a total of 12 tires/wheels.
  • Or maybe not: seems like I read that most Euro highways are built thicker than US highways, and so trucks aren’t required to have as many wheels for the same load that US trucks would. So we really need to know how much weight each can typically carry, and then you’ve got that bothersome kilopascals-to-meganewtons conversion. Or something. - MC

Just an optical illusion- usually all axles are four wheeled except the steering unit at the front. Tractors are either 2-4-4 or 2-4; trailers may be -4, -4-4, or even -4-4-4, depending on the weight to be carried. For instance you will see specialised furniture delivery trailers with on on efour wheel axle, but steel stock and concrete tractors will have 3 four wheeled axles. Specialised tractors for very heavy loads may have multiple (up to ten) four wheeled axles.

Maximum weight varies from 38 tonnes (38000kg) to 45 tonnes (45000kg).

I’d disagree about the optical illusion. I live near a factory and get up close & personal with the lorries when I’m cycling to work every day. All the trailers I’ve seen are 2 wheels per axle. Going from memory from when I lived in the States, the tyres on the trailers in the UK seem about 50% wider than the US trailer tires. (Notice the regionally accurate spelling :slight_smile: ) There is something about some of the tractors that I’ve always been curious about. On a three axle tractor, sometimes the middle one isn’t actually in contact with the road. The tyres seem to be sucked up into the wheel wells. This is always on an unladen truck. My question is does having a load force the wheels down, or are they mechanically wound down when needed?

Those axles are raised and lowered from inside the cab, by air pressure, the same system that applies the brakes. The axles are lowered when the load is too heavy for the two axles at the rear. The reason they’re not fixed like the rear axles is that they have to be raised to turn corners, and can be raised when the load is light, to save wear on the tires.

      • [Pjen] -In the linked photos, all the examples appear to use single-wheel axles on the trailers. Compare the trailer wheels with the rear axles of all the cabs (which all appear to be dual-wheels).
  • In the US we have trucks with “intermittent” axles too. I wonder how much they help though. Usually it’s on a truck that already has 4x4 wheels in back, the intermittent axle only has two more wheels on it, those wheels/tires are narrower than the rest, and they are undriven besides. I suspect that it might be a technicality to circumvent load limit laws, which here is generally rated as “so-many-tons per axle”. Seems like most of the time I see the “extra” axles retracted into the raised position. - MC

Judging by the photos, I would say the main design concern of these trucks was the ability to operate in tight spaces. Notice that there is almost no wasted space between the front of trailer and the back of the tractor. Also moving the trailer axles forward allows the truck to turn with less off-track*, but also puts a larger portion of the weight on the trailer axles, hence the three trailer axles and only one drive axle. The trailer’s kingpin (the thing that attaches to the tractor) also seems to be further back on the trailer (compared to US trailers) which would also help operation in tight spaces.
I don’t think these trucks would be very comfortable or stable for long straight highway trips.

The Cab-Over Engine design (COE or Cabover) allows for a shorter wheel base and therefore making tighter turns with the tractor, but does not affect off-track. The European tractor seems to have a slightly longer wheel base to accommodate the further forward kingpin mentioned above.

I must disagree with MC’s first two statements—
[li]The single tire (or tyre for you brits) design that replaces duel tires (AKA Super-singles) uses one larger tire with the same load capacity as two standard tires.[/li][li]Several mechanics I have talked to have complained that changing Super–singles is a pain due to their greater weight, almost twice that of standard tires, which weigh over 100 lbs. with the rim.[/li][li]With standard duel tires, if you have a blowout you can still drive a short distance at reduced speed to get it repaired.[/li][li]Super singles do see limited use in the US, particularly on spread axle trailers, but I am not sure what their advantages are.[/li][li]I don’t get to use the list command very often ;).[/li][/ul]
MC is somewhat correct on his third statement, US trucks must be designed to comply with the ‘bridge formula’ that sets limits on how close axles can be to each other. Also US trucks are limited (in most states) to 38,000 Lbs. per set of tandems (two axles), many states do not allow more weight for using a third axle (Michigan is one notable exception). I suspect the European design truck would not comply in many US states due to the axles being to close together and to much weight being carried on the trailer axles.

*Off-track – When turning a vehicle with a trailer, the trailer will follow a tighter radius than the tractor. This is why trucks must swing into the left lane before making a right turn.

Quick disclaimer - I do not claim to have knowledge of the weight per axle, or any other truck regulations in each US state. I am pretty familiar with the national laws, which are also the state minimums.

National limits-
Single tandem - 20,000 lbs
Duel tandems - 38,000 lbs
Spread axle - 20,000 + 20,000 or 40,000
No allowance for a third axle.

Total weight - 80,000 lbs

There’s another difference on the European trailers. They all have some sort of barrier underneath them (between the tractor/drive wheels and the trailer wheels). I imagine it’s to prevent cars from being wedged underneath in an accident, or even run over by the trailer wheels. U.S. trailers have plenty of room, and could probably swallow a Mini whole.

Which could also explain why you don’t see many Minis here in the States.

The barrier is a bit flimsy to stop a car, even a Mini - although it can’t hurt. It’s primarily put in place to protect bicyclists from the classical “being caught under a right-turning truck” scenario.

S. Norman

This topic brings up two questions I had about axle layouts:

In England, I noticed that a number of the heavy hauler non-articulated trucks [Sand trucks, Dump Trucks, Dumpster (or Bin) trucks, flat-beds, etc.] had a 4x4 layout [2 axles in front, 2 in the rear]. I don’t every remember seeing this layout in the US [or Canada], which generally uses 2x4 or 2x6 [1 axle in front, 3 axles (1 adjustable) in the rear]. Any thoughts on why this is so?
Also, and this may have been asked in the SDMB before, any reason why truck trailer combination [short truck-short trailer e.g. a short bed tanker towing a short tanker trailer] are popular in the West but seemingly not in the East [and I did see the tanker truck/trailers filling up gas stations in the Bay area, an area not known for it’s generous clearances].

The cab-over design was quite common in the US, specially as trailers got longer (remember the CW McColl song ‘Convoy’: “Cab-over Pete [Peterbilt]with a reefer on/and a Jimmy [GMC] haulin’ hogs”). States regulated the length of tractor-trailer combos, and by using cabovers they were able to haul longer trailers. Alot of these regulations have gone by the wayside, so when you add that to the higher fuel prices and emission standards the new ‘conventionals’ (which are more aerodynamic) have been the vogue. But the cabovers haven’t died - some fleets still prefer them.

To add to lawoots statement, the maximum length limits were not dropped, they were changed from total length of vehicle to length of trailer. In addition to being more aerodynamic, a conventional will also ride better due to longer wheel base and the driver not sitting directly over an axle. They also are usually cheaper to repair in an accident, any front end accident with a cab-over will usually damage the parts necessary to raise the cab for accessing the engine.

Also a couple of corrections, a tandem axle is limited to 34,000 lbs (not 38,000 that I stated above) in most states and the term tandem means a group of two axles. A single axle with four tires is simply called a single axle.

From What I’ve read and seen this Combination of Truck and trailer (Sometimes called a Rocky Mountain Tandem) is seen primarily in the west because of the load restrictions (or possibly lack of) on the Highways of the western states i.e. CA, CO, WY, OR, NM, NV, etc. Oddly enough there was, at the time of my research (3 years ago), only one other state that would allow such things and that was Massachusetts.

But to return to an international flavor, in Austrailia they have the “Road Trains” that cris-cross the interior desert and are usually one tractor (A big one) with up to SIX 53’ trailers in tow.

Not exactly something I’d like to get in the way of.

*Originally posted by Yardstick *

Yikes! I was impressed by the 3-trailer road trains I saw when I lived near Spokane. Six goes right past “wow” to “jaw-hanging-open amazement.”


[One of the guys where I work is a certified to test people for their CDL (Commercial Driver’s License). I think he’d like to read this thread.]

I’ve seen those myself, and I tell ya, I don’t envy the people who drive those things! ONE trailer is bad enough… The tractors down under seem to be designed more along the lines of the US Army’s heavy hauler tractors that they pull tanks around with, except with bigger tires. Scary.

There is another thread on this board (http://boards.straightdope.com/sdmb/showthread.php?threadid=59072) that asks about why some European model cars aren’t available in the US. Some posters pointed out facts about fuel economy and space considerations also styling issues. These are also probably concerns is relation to the Tractor portion of the Truck. The Differences in trailers are probably based upon Load/Axel limits and maneuverability. One other issue that was brought up about these cars and is extremely relevant to the trucks is safety standards.

In fact it was highlighted in the news today with a story about the lack of open access to Mexican Trucks in the US despite of NAFTA. The Trucks and Drivers from Mexico are not held to the same rigorous standards as American Trucks and Drivers. The Canadians have put in place the necessary training and safety measures to allow them to have open Access.

Under the current situation the Mexican truckers are only able to travel in a 20-mile buffer zone and are required to transload cargo to American Trucks for the rest of the journey. The Americans are held to the same arrangement due to reciprocity with one small stipulation, Mexico will not allow Trailer lengths over 48 feet on it’s roads even in the buffer zone, this leaves a significant percentage of America’s fleet of trailers, the 53’ box, in the lurch.