We’re in Normandy on a D-Day tour. On the road coming down here from Paris I noticed something different about the trucks. The tractors are all single axle and the trailers have three axles. Most “over the road” tractors in the U.S. have 2 axles as do the trailers.
Anyone have any insight as to why the configuration is different here in Europe?
I know this is GQ, but my initial thoughts were tighter turning radius on narrower roads.
perhaps axle loading regulations are different? The trailer is likely to be more than a third heavier than the tractor.
Whether they have one axle, two or three, the detail I find missing in the OP is that the semis often have them all together at the back, or one at the front and two at the back. Some older models which have three at the back are a 2+1 configuration: two normal axles plus one which can be pulled down if one of its brothers fails (the truck can go on with a flat tire). Those where there is no front axle have a pilon which can be pulled down to park the trailer in a standing position. Smaller trailers with two separate axles can stand on all wheels (no pilon) and they’re often intended to be used while in this standing position; camping trailers are one type which is still likely to have separate axles, and to only have two.
I am surprised that ‘all’ the tractor units were single axle. The number of axles determines the maximum gross weight of the combination and a five-axle combo is only allowed to go up to 40 tonnes, while the extra axle allows four more (97,000lbs), but with the weight penalty of the extra axle. Most tractor units are what you would call ‘cab-overs’; this is because there are strict limits on the overall length of vehicle combinations and operators want to maximise the load space. No huge bunk areas for drivers here - they get a 3’ shelf behind the seats.
A lot of this is EU wide legislation, but individual countries have their own rules as well. In the UK, six axles are the most common, because they have a tax advantage.
In general, the trend is for more axles to spread the load which is intended to be less damaging to roads and adjacent buildings. Many (most) of the three-axle units can lift one of the axles; either to reduce running costs when running empty or to enable tight manoeuvring, even when fully loaded at 44 tonnes
While that is a feature, I believe is a an unintentional feature. The reason has to do with economics and laws. Some areas will require 3 axles for a certain load, others will allow 2, depending on the load carried. Fuel economy is effected by this, also certain tolls depend on the number of axles.
If you want to have a play with the tractor units available in Europe you can download a free demo of Euro Truck Simulator 2 from Steam.
Yeah, my explanation was bad; as you say that’s one of the uses of that axle but not the main one.
WAG based on physics & geometry : American trailers have wheels near the rear end, which means a considerable amount of weight is resting on the tractor. Hence 2 axles at each. I think this is optimal for high speed stability but not great for turning radius. European trailers have the wheels further forward to improve turning radius, but this means more weight on the trailer wheels and less on the tractor. Hence 3 axles on the trailer and one on the tractor.
Having driven just about every configuration there is, I would say that six axles is the best from the driver’s POV. A three axle trailer takes a more ‘natural’ line on bends, both forward and reverse, while two axles tend to cut the corner more. A single axle trailer can be quite tricky. One place I worked for has several, with the axle right at the back of the trailer; I can tell you that both forward and reverse took practice.
With lift axles, there is no real consensus about which axle is best. A mid-axle lift tends to be more stable, but a rear axle lift makes the tractor unit shorter and more manoeuvrable.
I wouldn’t even say it’s one of the uses. If you have a flat on one of the main axles, you’re not allowed to run anyway. Dropping the lift axle down doesn’t do you any good.
I looked this up once and it was challenging to find the answer. A semi trailer has all it axles in the rear. A full trailer has axles in the front and back. With a full trailer, not much weight is added to the tractor, so the tractor could get by with only one drive axle. A semi trailer puts some of it’s weight on the 5th wheel, between the steer and the drive axles. By sliding the trailer axles fore or aft, you can adjust how much weight is on the drive and trailer (aka ‘tandems’) axles. You can slide the 5th wheel also which is a true pain in the ass unless you have help.
I drove box trailers. The Max Gross weight permitted was 80,000 lbs. 12,000 on the steers, 34,000 on the drives, and 34,000 on the trailer. A fully fueled tractor with driver weighed about 18,000 lbs. Empty box trailers weighed from 13,000 to 15,000.* I paid attention to the weight listed on the Bill of Lading.
*some had fiberglass roofs, some had metal roofs. Some had plywood covering all interior walls, some had bare metal.
Another difference between US and EU practice. Sliding axles are not generally used here; if the axle load needs adjusting the 5th wheel is fairly easy to slide (apply the trailer brake, pull the pin and drive the unit forward or back as required. The objective is to reduce the space between the unit and the trailer to the minimum as that improves airflow and therefore mpg.
https://motortransport.co.uk/wp-content/uploads/2018/01/Asda-Scania-and-Cartwright-trailer-3.jpg
Haven’t been to Europe for almost a decade, but my impression was that with a lot less of the limited access highways and a lot of tighter roads both country and in town, they tended to have a lot fewer of the giant tractor-trailer units that are common in the USA. Most trucks were the larger single-piece ones. (Also they tend to have a lot more of the “toy” cars like Smart cars, and fewer giant gas guzzlers. Gee, I wonder why fewer gas guzzlers?)
Tractor-trailer units are limited by law to 16.5 metres total length. Trailers are usually 13.5 metres long to accommodate 26 standard pallets which I think is longer than a typical US trailer, so the unit is limited to 3 metres. To operate an American style tractor unit here means using shorter trailers and that means less room for cargo.
With petrol costing North of £1.20 a gallon which I think would be around $6 per US gallon, you can see why we tend not to go for “giant gas guzzlers”. ‘Toy’ cars are often the choice for those who do a lot of city driving as parking is hard to find and expensive. Why would you need room for six people when all you are doing is carting your own fat arse to work and back.