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towing capacity

Road Traffic Regulation 151 - towing capacities

Is a category O1 or O2 trailer overloaded?
Or is that trailer towed by the wrong light motor vehicle?

Quoting an expert in Road Traffic who said: "Our issues are not so much the overloading of heavy vehicles, but the light trailers which are in so many cases too heavy for the towing vehicle."

Of course will there be a trailer on its own overloaded at times. But that is more the exception than the rule.
It is far more likely that the towing vehicle, which is in most cases a light motor vehicle of category M1 or N1, is simply too light for the trailer in tow. 

In July 2019 we published the article 'Towing capacities of light motor vehicles'. This document has attached a listing of popular LCV's and some top-range (by mass) SUV's. It shows that in many cases the manufacturers of such vehicles allow a gross combination mass of up to 6000 kg and trailer towing capacities of 3500 kg, based on various design capabilities of the vehicle. (You can also see or download this list here.)

These limits are of course applicable to category O2 trailers, fitted with overrun brake systems and specifically to those with a gross vehicle mass in excess of around 2200 kg.
Why the approximate 2200 kg limit?
There is not a single light vehicle on the South African market whose tare is beyond this approximate figure. Most of the LCV's have a tare of between 1800 and 2200 kg, with some exceptions where some SUV's are plated with around 2400 kg.

Did not all of us often hear remarks like these or similar comments on forum pages:

  • "I don't have to worry, the manufacturer allows me to tow such a trailer".
  • "Your bakkie is strong enough with that 3 litre engine to tow a 3.5 ton trailer."
  • "With a 4x4 you can tow any trailer"
  • "The traffic inspectors only issue a fine when my trailer weight is over the limit on a weighbridge."
  • "The law is only applied for the actual weight of my trailer, not for what is stamped on the data plate."

I bet all of us have heard one of these comments 

The focus onto this "heavy" type of category O2 trailer does not mean that there are no problems with the "lighter" type of O2 trailers or the O1 trailers, but the severity of non-compliance for the heavier type is far more severe.

Regulation 151 of the National Road Traffic Regulations is very clear in this regard and the legality of a vehicle combination with a category O1 or O2 trailer is very clear.

  • A category O1 trailer (without brakes) can only be towed by a motor vehicle, if the tare thereof is at least twice the mass of the gross vehicle mass of the trailer.
    • If the trailer is plated with 750 kg, then the towing vehicle's tare mass must be at least 1500 kg.
  • And a Category O2 trailer (with overrun brakes) can only be towed by a motor vehicle, if the tare thereof is at least the same mass as the gross vehicle mass of the trailer.
    • If the trailer is plated with 3500 kg, then the towing vehicle's tare mass must be at least 3500 kg. 
    • There is only one problem - no light motor vehicle will have a tare that high; it would need a midsize goods vehicle or small bus in the 6-ton range to legally tow such an O2 trailer with inertia brakes.
  • For both trailer categories the state of load is totally irrelevant, Reg 151 always considers the gross vehicle mass of the trailer as relevant, not the tare or any state of load in between.
  • Reg 151 also states "gross vehicle mass", which is denoted by GVM, and not the "legal mass V", as would be appropriate and allowing the trailer to be matching with the towing vehicle.

An officer will only have to check and confirm the legality of such a combination by reading and comparing the Tare mass of the towing vehicle against the Gross Vehicle Mass GVM of the trailer.

If this weight ratio is exceeded, then the trailer must be equipped with overrun brakes if the trailer is of category O1 or service brakes if the trailer is of category O2.
The officer should also take note of the Legal Mass V of the trailer and apply this mass if it is lower than the GVM.

But is Reg 151 really correct by limiting overrun brakes fitted to a category O2 trailer to the limit of the Tare of the towing vehicle?

Let us just compare:

A double-cab LDV with a Tare of 2200 kg is towing an unbraked trailer of 750 kg GVM. When braking this trailer with a deceleration of 0,49 g then the thrust D on the coupling will be 367.5 daN. (0,49 g used as reference to braked trailer.)

Now the same LDV is towing a trailer with overrun brakes and a GVM of 3500 kg (with the brakes maintained and adjusted correctly). When braking this combination, then the thrust D on the coupling head may not exceed the maximum of 0,10*g*GVM = 343 daN.

D is the maximum allowable thrust at a deceleration of z=4,80 m/s2 or 0,49 g. and its value must be between 0,05 and 0,1.

This shows clearly that a trailer with correctly adjusted brakes applies at least 6.5% less thrust during braking onto the towing vehicle through the tow coupling (about 1,5% of the Tare of the towing vehicle. 

Based on this it can be said with confidence, that a category O2 trailer with overrun brakes and a GVM of 3500 kg, coupled to a towing vehicle, whose GCM allows the towing of such a trailer, poses no more of a risk of losing the control over the vehicle combination during braking, than yhe same vehicle towing an unbraked trailer with a GVM of 750 kg!

It should also be noted that such braked trailers with a high GVM rating are usually fitted with a tandem axle, increasing the stability even more during braking.

Observations made whilst engaging with this project:

  • Both, the towing motor vehicle and the trailer in isolation are considered to meet all the Comjpulsory standards and Road Traffic Regulations. However, when coupled together, Reg 151 can in many cases not be met for above mentioned reasons.
  • All stakeholders place the onus for compliance for the vehicle combination on the end user, who is ill equipped to be fully versed with all regulations.
  • Motor vehicle manufacturers often promote their high towing capacities with the full knowledge that the legal limits might be much lower
  • Trailer manufacturers are in some cases ill informed about this regulation and confirm the legality of towing a trailer in excess of the mass limits.
    • This happened in two cases to me where I was confirmed by e-message that I can tow a 3500 kg trailer with overrun brakes legally with my virtual LDV, having a tare of 1830 kg, saying "they (the trailer builder) do it all the time".
  • Trailer rental companies do in some cases provide category O1 trailers with a GVM of 600 or 650 kg, but by far the majority thereof is plated for GVM as 750 kg. It is usually the renter's choice what trailer he wishes to rent.
    • In all cases where I visited rental companies to enquire about renting a trailer did I have a free choice of the size of trailer and neither the assistant nor the documentation for the rental contract made any reference to legal limits. They are aware of the tare/GVM limits but consider them valid for the actual weight only.
    • The trailers observed by me all have an identical GVM and V rating stamped on the data plate.
  • Law enforcement appears to have compliance to Reg 151 not as priority.
    • I have witnessed in numerous cases where motor vehicle - trailer combinations were waved through at safety road-blocks where the combination clearly appeared to be over the mass limits.
  • Motorists downsize their type of motor vehicles for their own reason and unknowingly use their existing trailer with a too high GVM or V rating, which may have been balanced with their previous motor vehicle.
  • Some motoring magazines and forum groups do a splendid job in creating awareness for the limitations of Reg 151. But is it enough or is it deliberately ignored at all levels?
  • Could the introduction of a tolerance limit, allowing compliance over a limited spread , be helpful? It could certainly assist the "lighter" trailer category, braked and unbraked, because in many cases the limit might not be exceeded by more than a few percent. It is certainly impractical, if not impossible to re-plate a rental trailer to match the criteria for all the different towing vehicle. Similarly, if a motorist acquires a new vehicle with a slightly different tare, is it really necessary to re-plate such a trailer for a few kgs if it matched the previous vehicle? 
  • Do we know why an arbitrary limit for the GVM to tare relationship has been set? Would a series of scientific tests to evaluate the effect of a higher trailer GVM be of any help?
  • Are service brake systems available for these types of trailers, which comply in all respect with the Compulsory Vehicle Specifications? Or do we ask for a brake system which is at this stage not commercially available?
  • A 3500 kg trailer has a rightful place in the construction industry for mobile machinery or for other purposes when towed by at least an adequate category M2/N2 vehicle. (E.g. luggage trailers for buses mobile display units etc.) 

The Regulation

151. Brakes on trailers

(1) Subject to the provisions of subregulation (4) no person shall operate on a public road a trailer, if—

(a) the gross vehicle mass of such trailer does not exceed 750 kilograms and the gross vehicle mass—

(i) does not exceed half the tare of the drawing vehicle, unless such trailer is equipped with a parking brake or other device to keep such trailer stationary;

(ii) exceeds half the tare of the drawing vehicle but does not ex-ceed such tare, unless such trailer is equipped with a parking brake and either a service brake or an overrun brake; or

(iii) exceeds the tare of the drawing vehicle, unless such trailer is equipped with a parking brake and a service brake;

(b) the gross vehicle mass of such trailer exceeds 750 kilograms but does not exceed 3 500 kilograms and the gross vehicle mass—

(i) does not exceed the tare of the drawing vehicle, unless such trailer is equipped with a parking brake and either an overrun brake or a service brake; or

(ii) exceeds the tare of the drawing vehicle, unless the trailer is equipped with a parking brake and a service brake;

(c) the gross vehicle mass of the trailer exceeds 3 500 kilograms, unless such trailer is equipped with a parking brake and a service brake, and where more than one trailer is drawn by a drawing vehicle, the foregoing requirements shall apply in respect of each such trailer, and in such event the gross vehicle mass shall be construed as the total of the gross vehicle mass of all trailers so drawn.

[Sub-reg. (1) substituted by r. 42, GNR.1341 w.e.f. 25 September 2003.]

(2) The service brake of a trailer shall be capable of being operated by the driver of the drawing vehicle while such trailer and drawing vehicle are in motion.

(3) If the service or overrun brake of a trailer is capable of being used as a parking brake, a separate parking brake need not be fitted to such trailer.

(4) Notwithstanding subregulation (1) (c), if a trailer referred to in that subregulation is drawn by a tractor and such tractor is not designed for or capable of operation at a speed exceeding 40 kilometres per hour on a reasonably level road, such trailer may be equipped with an overrun brake in lieu of a service brake.

Bert Mauch

Bert Mauch

For more than 45 years have I made South Africa my country of choice. My professional career has been in the automotive field throughout all the years, having worked for great local and international companies and institutions in Vehicle Design, Production, Project Management and Digital Marketing. My passion is now to provide the Heavy Vehicle Automotive Industry a platform through the Technical Review Forum to share common interests.