Braking Distance

[Lutz]

In their September issue, the German 'Caravaning' magazine published a report on how the caravan affects braking distance. The results are maybe surprising to some so I thought I'd share them here.

The test setup was a BMW 530xd Touring towing a caravan with varying weights up to 1800kg. For reference, the braking distance from 100km/h for the solo lightly laden BMW on a dry road was 39m and 42.5m when wet. The fully laden car towing at the manufacturer's towload limit of 1800kg needed only 4m more on a dry road surface, but a whopping 17.5m extra (i.e. a 41% increase) in the wet.

Summarising, the main conclusions were:

1. Under dry conditions the caravan, whether fully or only partially laden, increases braking distance only marginally.

2. Braking distance (in the dry) is more or less the same whether the same payload is distributed inside the caravan or in the car.

3. Wet conditions have a much greater effect on braking distance when towing compared with solo.

 

[RogerL]

Interesting!

Any theories why the braking distance goes up 41% in the wet but only 10% in the dry?

 

[Lutz]

I would imagine it's because caravans don't have ABS. If the wheels of the caravan lock up on a wet road surface there can be no further increase in the caravan's own braking performance. All further effort required to stop the caravan must then be provided by the car's brakes. Also, if the caravan isn't braking as hard as in the dry, it won't transfer as much noseload increase to the towbar. This, in turn, means that the dynamic rear axle load while the car is braking is lower, too. As vertical load is proportional to friction (so long as the outfit doesn't go into a skid), the braking performance of the car's rear brakes won't be as effective, either. At least, that's my theory.

 

[rob_jax]

Sounds very reasonable...

Be nice if some UK caravan magazines either performed tests like this, or obtained permission to reprint such test reports...

 

[RogerL]

I can't fault that theory Lutz.

 

[rob_jax]

Lutz,

Do you mind if I reproduce your posting on our Eriba forum - I think it would be useful safety advice.

Regards, Robert

 

[KeithJ]

Sounds like a very good theory, Lutz. Any idea why caravans don't have ABS - I am always very conscious about braking in the wet, as I know the van will easily "lock up".

Keith

 

[Lutz]

No problem, as it's not my intellectual property. Alternatively, you could contact the publishers directly and ask whether you may use the diagrams and graphics in the report (although the text itself is, of course, in German and therefore of limited use). Maybe the diagrams bring the message over even better than my post.

The postal address is:

CARAVANING

70162 Stuttgart

Germany

Or send them an email

 

[rob_jax]

Hi Keith,

"Any idea why caravans don't have ABS"

Take a look under the bonnet of your car - you should see the ABS block and it's high pressure fluid feed, and the computer ECU that provides the signal to modulate the brakes - and imagine transfering that to your caravan and the auxilliary systems that make it work.

These include the individual wheel sensors feeding signals to the car ECU, and whilst Ford made ABS work with drum brakes - most ABS systems use disc brakes.

So that's quite a lot of technology and cost to transfer ABS to a caravan:

1. An ECU to monitor caravan wheel sensors and detect lock-up - expense

2. Probable need for disc brakes

3. The current mechanical system would need to be replaced by a fail safe hydraulic or electrical "amplified" braking system

4. The above needs a reliable electrical supply

So I wouldn't hold my breath.

Robert

 

[Lutz]

Sounds like a very good theory, Lutz. Any idea why caravans don't have ABS - I am always very conscious about braking in the wet, as I know the van will easily "lock up".

Keith

Overrun brakes on caravans are purely mechanical and work on the left and right hand side wheels simultaneously. To work properly, ABS would require a split between left and right and that can really only be achieved with a hydraulic or electric braking system. This would make the braking system very sophisticated and costly. Also, ABS would require relatively complex electronic control and a method of comparing and matching actual road speed with the rotational speed of the wheels. The speed the wheels go round could be monitored electronically reasonably easily, but it would be relatively difficult to measure the actual road speed of the caravan. Caravans, of course, don't have a speedometer so the caravan would have to get a signal from the car. It is doubtful whether the cost and complexity of such an arrangement would really be worth it.

 

[rob_jax]

No problem, as it's not my intellectual property. Alternatively, you could contact the publishers directly and ask whether you may use the diagrams and graphics in the report (although the text itself is, of course, in German and therefore of limited use). Maybe the diagrams bring the message over even better than my post.

The postal address is:

CARAVANING

70162 Stuttgart

Germany

Or send them an email

Lutz - thank you - Robert

 

[KeithJ]

Thanks Rob, and Lutz, for going to the trouble of explaining. It was just a thought - would have been a great safety addition, but obviously not practical. (I quite fancy the idea of disc brakes on my 'van, though!)

 

[Lutz]

From memory, back in the late '70s, a manufacturer did actually fit disc brakes to caravan chassis, but the problem was that it was too difficult to achieve adequate pad pressure without a servo unit, and the product was withdrawn from the market very quickly. I tried to find out who the manufacturer was by 'Googling', but without success. I do know, however, that it wasn't one of the big 3, AlKo, BPW or Knott.