Why are tri-axle trailers with single wheels more stable than double wheels?

Question

I'm revising for the UK LGV theory test. One of the questions goes something like this:

Which axle configuration is most effective at preventing petrol tankers from rolling over?

• A. Tandem axles with double wheels
• B. Tandem axles with air suspension
• C. Tri-axles with single wheels
• D. Tri-axles with double wheels

The correct answer is apparently C, with the following explanation:

The type of suspension fitted to a vehicle will influence its resistance to ‘roll-over’. Modern tri-axle semi-trailers fitted with single wheels on each side extend the tracking width available, making this the most stable configuration.

I have the following questions:

• Why are single wheels better than double wheels?
• Is this specific to a tri-axle setup, or does it also apply to twin-axles or single-axles?
• Is this specific to trailers, or does this apply to rigid vehicles as well?
• If single wheels are better, why do so many vehicles have double-wheels?

Answer 1

If the load is evenly distributed, then double wheels would be something like:

[Mg/4] [Mg/4]              [Mg/4] [Mg/4]
 \/     \/                   \/     \/
[tire] [tire]--------------[tire] [tire]
  |      |                   |      |
  |      |------(L_inner)----|      |
  |-------------(L_outer)-----------|

Where single wheels would be something like:

[Mg/2]                            [Mg/2]
 \/                                 \/
[tire]----------------------------[tire]
  |                                 |
  |-------------(L_outer)-----------|

There's a reaction force at the road, so the torque applied to the body is higher, because each side has a torque of $0.5L_{outer}\left(\frac{mg}{2}\right)$ instead of $(0.5L_{inner} +0.5L_{outer})\left(\frac{mg}{4}\right)$

I tend to agree with @ratchet freak about this feeling like a trick question/gotcha, though. I think there are aspects that are being ignored, like what happens to the suspension if the tanker actually starts to roll. If all of the load is put on one side, and there are only single tires, is the loading then sufficient to blow out the tires?

This is the crux of your other questions: by the logic above, there's more "available" restoring torque to correct a roll-over with single wheels, so then they'd be better in all applications - twin-axle, rigid vehicles, etc. However, you typically see heavy-duty vehicles with double wheels in order to distribute the load enough that it doesn't exceed the tire ratings.

Ultimately, I don't think the tire arrangement has much to do with tanker roll-over. I think a well-rested, qualified operator that understands the maximum safe speed is more important, along with a well-serviced brake system to allow the driver to slow the vehicle down in time.

If you're to the point that you're depending on the specific tire configuration to save you, then you've probably already screwed up.

Answer 2

Track width is one of the most important factors in stability and resistance against rollover in cars and trucks.

That's why we see in race cars they even extend the wheels outside of the car body, or all street legal fast sport cars have flared out fenders to accommodated a wide track width stance.

In a double wheel axle the width of the wheelbase is from the center of the left double wheel to the center of the right double wheel which is approximately 20% shorter than a single wheel width, even though it has the benefit of distributing the wight over larger area of the pavement, hence applying less pressure on the road.As has been mentioned in @Chuck's answer.

The other reason is when there is two wheel at each end when the truck is urged to roll as by a turn or a bump, the outside wheel's tire starts to compress into a more flattened shape but the the tire next to it winch is under much less torque because the fact that it is not far enough will bulge out a bit and exacerbate the situation on the outermost tire.

If you watch some videos of monster truck races which some have double-wheels you'll see this effect.