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In the length of your towbar matters, here’s why, we explain the effect it has, along with wheelbase, on the dynamic performance of your tow vehicle.

The length of your towbar matters! Here’s why

In the first part of Understanding the dynamics of towing, we looked primarily at the effect towing a caravan or trailer has on the overall vehicle-trailer combination, now we will have a more detailed look at what affects the tow vehicle. One of the important things to consider in a tow vehicle is wheelbase and towbar lengths. In general, a longer wheelbase (axle-to-axle distance) and shorter towbar length (rear axle to towball distance) will make for a more stable tow vehicle. Let’s look at why that is.

NOTE: The towbar lengths quoted are to demonstrate the principle and are not related to any vehicles.
If we first look at the towbar length…

Understanding The Dynamics Of Towing Pt2 01

In the above drawing, we can see the two vehicles, one with a 500mm axle to towball distance, the second with a 750mm axle to towball distance. Now, using leverage calculations, we can calculate what the same side load or lateral load (red arrow) on the towball will have (blue arrow) on the rear wheels with the 750mm towbar length compared to a vehicle with a 500mm towbar length. Let’s assume that the caravan or trailer doesn’t impart a sideways load on the front axle. The front axle then effectively becomes the pivot point.
Oh no … here comes the maths! I’ll keep it simple. The acting forces are calculated (Second Class Lever) using:

Understanding The Dynamics Of Towing Pt2 Forces (1)

Now for the maths.

Understanding The Dynamics Of Towing Pt2 Calculated Forces

So it can be seen that the resultant force of a 100kg acting on a 500mm towbar is 120kg while the same force acting on a 750mm towbar is 10kg more.

We learned in part one that there is an effect called ‘dutch roll’, most often felt when on motorways where the inside lane has been worn down to two grooves by heavy vehicles and the caravan or trailer is ‘hunting’ between these grooves. This will set up a rolling motion in the caravan and will have the effect of trying to move the hitch from side to side. As the vehicle and caravan travel forward, the undulations will also set up a rocking backwards and forwards motion in the caravan, which has the effect of first lightening the load (nose weight) on the hitch, then increasing it. The combined effect of this is the caravan hitch will move in a circular motion first sideways then down, then sideways in the opposite direction and finally upwards again, repeating continuously. This is one source of lateral load on the towball.

The rear tyres only have so much grip. The grip the tyre has is a function of the friction of the road surface, the friction of the tyre compound, the tyre pressure and the axle load. (Formula 1 fans know it should include the temperature of the road surface and the tyre temperature as well.) This grip is not unlimited; this is easily displayed when trying to pull away rapidly and the tyres spin, likewise when trying to stop quickly the tyres’ lock-up’ and start to slide (until ABS takes over!).

If we were to put a rope through the rim of the rear wheel, it might only take a ‘pull’ of 300kg to drag the rear of vehicle sideways on the road. As you start to apply the force, the tyre wall will deflect and distort the tyre reducing the grip available (more about this later). A static wheel on a vehicle has the most grip, as soon as it starts to rotate, this grip reduces and will continue to reduce the faster the wheel rotates. This is one reason a car can go round a sharp corner at, say, 30km/h but at 50km/h, the dynamic side load has increased, and also the grip (friction) the tyre has on the road has also reduced, and the rear end will slide outwards resulting in a skid.

Short and long-wheelbase vehicles are affected differently with shorter or longer towbars

Short And Long Wheelbase Vehicles Have Their Own Dynamics

Does the length of wheelbase matter?

Let’s revisit our earlier calculations to see what happens:

Understanding The Dynamics Of Towing Pt2 02

Now we have two vehicles, one with the same 2500mm wheelbase and a 500mm towbar length. The second has a wheelbase of 2000mm and the same 500mm towbar length.
Using the same equation as before…

We now calculate for differing wheelbases:

Calculated Forces 2

Looking at the table above, we can see that the same 100kg sideways force on the towbar has a greater effect by 5kg on the short-wheelbase vehicle when compared to a longer wheelbase vehicle. There is also something else that happens when you compare a short- and long-wheelbase vehicle. If you apply a sideways force on the rear of the vehicle, it has the effect of ‘steering’ the back-end – i.e. pushing it to one side. If we compare the two vehicles, we can see what happens:

Understanding The Dynamics Of Towing Pt2 03

Given the same 100mm of sideways movement of the towball by the forces imparted on it, basic trigonometry demonstrates that a short-wheelbase (2000mm) vehicle is deflected 2.3 degrees off path, while the same 100mm of sideways movement only deflects the longer-wheelbase (2500mm) vehicle by 1.9 degrees. While this does not seem much, remember, you can usually feel though the steering wheel if the tracking is out by more than half a degree, so the lateral push/pull felt in a short-wheelbase (SWB) vehicle will be more noticeable than a long-wheelbase (LWB) vehicle. While it will be easier to ‘feel’ the deflection earlier in a short-wheelbase vehicle, the long-wheelbase vehicle will be more stable.

Why are semi-trailer tractors so short then?

With an articulated tractor unit, the pivot point (towball, in effect) is actually directly in line with or slightly in front of the rear axle, therefore any lateral load imparted on the pivot point is actually less at the rear axle as some of the force is also transmitted to the front axle. It’s a bit like having your towball just behind the front seats.

Low profile tyres on a Range Rover Sport

Image credit: Brett Hemmings

Tyres…

Earlier we were talking about the grip of the tyres on the road surface. There is something else to consider, though. We know that sidewall flexing has an effect on grip and as we have just seen, towing a caravan or trailer will put more lateral force on the rear tyre sidewalls. So what’s the solution?

“Put low-profile tyres on.” Okay, who said that?

That is one solution and, on the face of it, low-profile tyres would reduce the amount of sidewall flexing, therefore, be more resistant to the lateral forces imparted on it by the caravan or trailer. But before you rush out to change the tyres, let’s have a closer look at this.

Image: Brett Hemmings

With a ‘tall’ tyre (think: most of our 4X4 tow vehicles) there is quite a bit of ‘feel’ in the tyres, i.e. you can easily feel when something is pulling you from side to side well before the tyres lose grip. With low-profile tyres, the sidewalls still deflect but usually with little warning to the driver. The ‘feel’ is almost non-existent. The lateral load will increase without much ‘feel’ and the first you know of it … well, it’s usually too late as you have passed the limits of grip and the rear of the vehicle is being pulled side to side in the dreaded ‘snake’. Low-profile tyres are great if you are a racing driver and have super-sharp reflexes and know how much you can push the tyres to the limit holding back just before they break loose. If you don’t believe low-profile tyres deform, just watch the super slow-motion clips of Formula 1 cars clipping a kerb.

So what is the solution?

Well, normal-profile tyres actually. It’s the best of both scenarios, enough ‘feel’ to give you some clues all is not well and enough rigidity to help resist the lateral loads imparted by the caravan or trailer. However, you do need to check the tyre pressure guide in your owner’s manual for heavy loads and towing. For almost all vehicles, the manufacturer will recommend a higher running pressure in the rear tyres. Please check you own owner’s handbook for details, as there are exceptions for certain vehicles and wheel/tyre combinations.

So what does this all mean?

You are travelling along the road at 80km/h towing your caravan or trailer that weighs 1500kg. The caravan or trailer starts to sway as a truck passes you and a tiny amount of the mass of the caravan or trailer, just over 6.5% or 100kg of the weight, now starts acting sideways on the towbar of the tow vehicle, trying to pull the rear axle sideways. We know there is only so much grip available from the rear tyres and now the force acting on the rear towball is taking some of that grip. We feel the caravan or trailer starting to pull us from side to side, and the natural reaction is to brake. Wrong!

We now know there is only so much grip available at the rear tyres and some of that grip is being used to resist the sway of the caravan or trailer. If we brake, we also use some of this grip – possibly more than is now available which we don’t want to do. We also know (from part one, see below) that braking reduces the load on the rear axle as the centre of mass of the towing vehicle moves forward, therefore, reducing grip available still further. If we accelerate we also need some of this grip. Accelerating with 1500kg swinging about behind us will use a lot of the available grip, and we might just run out again, and we really don’t want to go faster at this point, do we?

What else have we learned?

Well, we have proved that a longer wheelbase is preferable to a shorter wheelbase given the same towing conditions and trailer. We have also proved with a bit of school maths that a short towbar to rear axle length is better than a long towbar as it imparts less sideways (lateral) force on the rear axle and tyres. We also now know why it is important to keep an eye on the tyre pressures and condition of the tyres on the tow vehicle, as they are now doing so much more work.

Is there a downside to having a short towbar?

Yes, there is. With a short towbar, the caravan or trailer will “cut the corner” more than a long towbar vehicle. So it requires a wider ‘sweep’ when going round corners. On the upside, when reversing the reduced length of the lever effect makes it slightly easier to manoeuvre as the steering effect on the trailer is slowed down, so it becomes less sensitive and gives you more time to react to a deviation off course, although you will have to put slightly more steering input to correct.

original source: https://rvdaily.com.au/the-length-of-your-towbar-matters-heres-why/

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Towing a trailer requires additional knowledge and skill. All trailers, including caravans, affect the performance of the towing vehicle.

They affect fuel consumption, acceleration, braking ability, general control and manoeuvrability. These effects worsen as the size and weight of the trailer increase relative to that of the towing vehicle. The extra length and width can be hard to manage, with wind, road roughness and passing vehicles having a greater effect than on the vehicle alone. This puts additional responsibilities on a driver.

The information on this page applies to vehicles not exceeding 4.5 tonne gross vehicle mass (GVM).

The information given in this section is a guide only and is subject to change at any time without notice.

This information is also available in the Road Users Handbook.

You can access the full text of the NSW Road Rules on the NSW Legislation website.

On this page

Rules for towing

  • Towing more than one trailer at a time is not allowed
  • Nobody is allowed to ride in trailers or caravans
  • When towing and driving on a road without street lights, drive at least 60 metres behind heavy vehicles or other vehicles towing trailers,unless overtaking
  • Learner drivers and learner and provisional motorcycle riders are not allowed to tow
  • P1 car licence holders can tow small trailers with up to 250 kg of unloaded weight.

The driver

Driving with a trailer takes practice. Remember:

  • Allow for the trailer’s tendency to ‘cut-in’ on corners and curves
  • Allow longer distances for braking,overtaking and joining a traffic stream
  • When reversing, it is advisable to have someone outside the vehicle giving directions
  • Avoid sudden lane changes and changes of direction
  • Look further ahead than normal so you can react to changes in traffic or road conditions
  • Use the accelerator, brakes and steering smoothly and gently at all times
  • Use a lower gear when travelling downhill to increase vehicle control and reduce strain on brakes
  • Slow down well before entering corners and curves
  • Trailers tend to jerk the back of the vehicle around and can cause sway (snaking).If a trailer starts to sway, the vehicle’s brakes should not be applied, except as an absolute last resort.If the trailer’s brakes can be operated by themselves they should be applied gently, otherwise a steady speed or slight acceleration should be held if possible until the sway stops
  • Take care not to hold up traffic unnecessarily
  • Plan more rest stops and shorter travelling days as towing is more stressful and tiring than normal driving
  • There is no specific speed restriction while towing a trailer. However, the posted speed limits must not be exceeded.Always drive to the road,traffic and weather conditions.

Before each trip, check:

  • Vehicle and trailer are roadworthy.
  • All tyres are properly inflated.
  • Trailer’s wheel-bearings, suspension and brakes work properly.
  • All lights work and safety chains are properly connected.
  • Oil,water,brake fluid,battery and other service checks on the vehicle.

At regular intervals during the trip, check:

  • Couplings, all doors, hatches, covers and any load or equipment are still properly secured.
  • Tyres are still properly inflated and not rubbing on suspension or body work.

If travelling to another State, check with the relevant roads authority whether there are different rules.

The towing vehicle

Vehicles must be suited to the trailer. Vehicle manufacturers usually indicate in the owner’s manuals the maximum weight and other features of trailers appropriate for the vehicle.These limits should not be exceeded.

Registration:

  • All vehicles must comply with all relevant standards for registration and be roadworthy at all times
  • Rear number plates and lights must not be obscured by the towbar when there is no trailer connected.

Towing vehicles must be properly equipped with:

  • Towbars and couplings of a suitable type and capacity
  • Electrical sockets for lighting
  • Brake connections if the trailer is fitted with power or electric brakes.

Additionally:

  • Extra mirrors may be needed for the towing vehicle if towing a large trailer
  • For vehicles with automatic transmission, an extra transmission oil cooler may be needed
  • Some vehicles need structural reinforcement and/or special suspension and transmission options and load-distributing devices to be able to tow heavier trailers.

Tow bar

A properly designed and fitted towbar is essential for towing. The rated capacity of the towbar and coupling should not be exceeded.

The towbar should be clearly and permanently marked with its:

  • Maximum rated capacity
  • Make and model of the vehicle it is intended for or the manufacturer’s part number
  • Manufacturer’s name or trade mark.

This is compulsory for vehicles built after 1 January 1992. The exception is where the towbar is a permanent part of the vehicle.

Towbars must not protrude dangerously when there is no trailer connected.

Load equalisers

Load equalisers can be used when towing large caravans. Load equalisers:

  • Help the vehicle retain normal suspension height and effective steering control
  • Transfer some of the weight from the towbar to the front and rear suspension of the vehicle.

As load equalisers may overload the towbar and its components, check with the towbar manufacturer for advice before use.

The trailer

Trailers must be a suitable size and type for their intended tasks. They must be built to meet the standards for registration. If a trailer is required to be registered it must be fitted with a rear number plate.

Towing ratio requirement

The loaded mass of the trailer must not exceed the lesser of:

  • Rated capacity of the towbar and tow coupling.
  • Maximum towing capacity of the vehicle.
  • Maximum carrying capacity of the trailer.
  • Maximum rated carrying capacity of the tyres.

If the vehicle manufacturer has not specified the maximum towing mass, the maximum towing mass is:

  • One and a half times the unladen mass of the towing vehicle, provided that the trailer is fitted with brakes which are connected and in working order, or
  • The unladen mass of the towing vehicle if the trailer does not require brakes.

Vehicles with a manufacturer’s gross combination mass (GCM) more than 4.5 tonne may tow in accordance with the above requirements.The GCM is the gross combination mass of the car and loaded trailer.

Braking system

The minimum braking system for a trailer depends on the type of trailer, its weight and the weight of the vehicle:

  • 0 – 750 kg loaded weight – no brakes required.
  • 751 – 2000 kg loaded weight – braking on both wheels on at least one axle.
  • 2001– 4500 kg loaded weight – braking on all wheels, and an automatic breakaway system in case the trailer becomes detached from the vehicle.

Brakes must be operable from the driver’s seating position.

Towing coupling

All couplings:

  • Must be strong enough to take the weight of a fully loaded trailer.
  • Should be marked with the manufacturer’s name or trade mark and rated capacity.
  • Must be equipped with a positive locking mechanism. The locking mechanism must be able to be released regardless of the angle of the trailer to the towing vehicle.

Safety chains

  • Must comply with Australian Standards.
  • Trailers less than 2500 kg when loaded must be fitted with at least one safety chain.
  • Trailers over 2500 kg when loaded must be fitted with two safety chains.

To prevent the front end of the drawbar from hitting the ground if the coupling is disconnected,safety chains must be:

  • As short as practicable and connected to the towing vehicle.
  • Crossed over if two chains are fitted.

Loading trailers

It is important that trailers are not overloaded and that loads are properly secured to or contained within the trailer:

  • A load must not project more than 150mm beyond the trailer’s width or be more than 2.5m overall width, whichever is less
  • Loads that project more than 1.2m behind a trailer must have a red flag attached to the end of the load. This flag must be at least 300 mm square and clearly visible. To avoid having an overhanging load, you should purchase a trailer that suitably contains the load
  • Between sunset and sunrise, or when there is insufficient daylight, a clear red light or at least two red reflectors must be fixed to the end of any projecting load
  • Overall length of the vehicle and trailer combination including its load must not be more than 19m
  • To reduce sway, heavy loads should be concentrated towards the centre of the trailer
  • Loads should be kept as low and as close as possible to the axle or axles with about 60 per cent of the total weight forward of the centre of the axle or axles. As a general rule, about 5-10 per cent of the total mass of the trailer plus load should be supported by the vehicle through the coupling. The trailer drawbar should be level or slightly ‘nose down’
  • Loads must be covered to secure and contain all materials within the vehicle and trailer. Fines apply for uncovered loads.

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We weight up the considerations in the age-old debate: Caravan or Camper Trailer?

If you’re looking for recreational vehicle to take camping, is a caravan or camper trailer better? Choosing between the two is a question of how close you want to get to the elements, how far you want to go and, ultimately, what lifestyle you want to lead while you’re on the road.

SET-UP TIME AND CONVENIENCE

Quick and convenient set-ups tend to boil down to the person(s) doing the setting up and what it is you’re doing. Some caravan owners might just chock the wheels, drop the jack stands and be cooking dinner within minutes, while others will have awnings, annexes, power, toilet cassettes and kids to organise before they can relax.

Likewise, camper trailer owners may just need to wind-up the roof or fold out the frame and they’re cooking with gas in no time, while more involved set-ups may require the clearing of the camp area, an annex to be assembled and pegs to be hammered into the ground.

WHY CHOOSE A CAMPER TRAILER?

  • Lower mass: a typical camper will weigh between 600-900 kilograms; much less than the average caravan. This ensures that they are generally easier to handle on the road and, of course, that they can offer better fuel-efficiency than caravans do.
  • Lower profile: further aiding fuel-efficiency, the lower profile of a camper trailer means they offer much less wind resistance.
  • Space-efficiency on the road: because of that lower profile, you’re able to stack a bit of gear on top of your camper. Some manufacturers even offer fold-out boat racks designed to carry and deploy small boats.
  • Space-efficiency in storage: this is a big issue for many people and one that some might not consider until it’s too late. Camper trailers offer a great balance between function and space-efficiency. Most campers will have a smaller footprint, not to mention a lower profile, than that of a typical caravan.
  • Access: all of these factors combined mean that the off-road camper can go places that even the best off-road caravans can’t go to.

WHY YOU’D STICK WITH A CARAVAN

  • Weather protection: assuming water-tightness, there’s no denying that a caravan is always going to offer superior weather protection than even the best camper trailer. This is particularly true with extremes in temperature.
  • Comfort and convenience: modern caravans have reached a point where they carry all the comforts of home, such as washing machines, home theatres and the trusty toilet
  • Security: no caravan is going to stop a determined thief; however, with four walls and a door, a sturdy caravan will deter the opportunist. On the other hand, a camper trailer offers little or no security while it’s erected.
  • Setup: we’ve already discussed that this is highly-dependent on the experience of the user and what they want to do, however, after a long drive, it’s a joy to be able to jump straight into a caravan’s cabin, especially if it’s raining.

CARAVAN VERSUS CAMPER TRAILER

In the end, deciding between a camper trailer and a caravan means choosing between the camper trailer’s versatility, flexibility and freedom and the caravan’s outright comfort. The choice is yours. Whether you choose a caravan or camper trailer, here’s how to tow it.