Kerbweight lower than Max Tow

Oct 25, 2017
14
1
10,515
Visit site
Something that puzzles me.
How can a cars details have a higher braked tow limit of say 2000kg when it's kerbweight is 1770KG giving a 85% of 1338KG or thereabouts.

I understand if the tow limit is lower than its kerbweight you must adhere to the lower limit. But surely this isn't the case in the above example otherwise why quote the higher number
 
Apr 6, 2014
6
0
0
Visit site
The vast majority of vehicles have higher towing limits than their kerb weights. The towing limits are based on what the car can safely handle before putting to much stress on components such as suspension, engine etc effectively to prevent damage to vehicles and to ensure the vehicle has adequate power.
The kerb weight of a vehicle also varies by defintion, but generally is the vehicle plus all necessary fluids and a driver at 75kg. Generally when towing this weight is increased with extra passengers, luggage etc, which then helps the towing ratio.
As for the 85% guideline has no bearing at all, that was created by the caravan associations many years ago to help novices tow safely. It is not taken into consideration by car manufacturers and has no legal standing.
 
Nov 11, 2009
20,847
6,513
50,935
Visit site
85% is guidance for caravans which aren’t the easiest trailer to tow and can be affected by loading and noseweight etc. Other trailers can be more stable and go above kerbweight if the makers specifications allow. Also the max tow limit relates to the cars ability to restart a number of times on a defined slope with that trailer weight but again this is a very slow procedure and doesn’t relate to the real world driving with a trailer at up to 60 mph in proximity to HGVs, buses and very fast vans. So 85% gives a good starting point especially for the less experienced.
 
Jun 26, 2017
445
17
10,685
Visit site
volts said:
Something that puzzles me.
How can a cars details have a higher braked tow limit of say 2000kg when it's kerbweight is 1770KG giving a 85% of 1338KG or thereabouts.

I understand if the tow limit is lower than its kerbweight you must adhere to the lower limit. But surely this isn't the case in the above example otherwise why quote the higher number

“a 85%” has no relevance whatsoever with regards to a manufacturers official stipulation of the maximum weight that a vehicle can tow.

Likewise, it has absolutely nothing to do with the kerbweight of the vehicle, but to comply with the law, you must not tow a trailer which exceeds the weight limit as specified by the manufacturer.. There are of course other factors to consider such as noseweight, but I have no intention to digress here ...

You ask “Why quote the higher number”... Assuming that the two numbers you are referring to are 1) The maximum braked trailer mass as stipulated by the manufacturer, and 2) 85% of the vehicles kerbweight, legally, the latter is absolutely irrelevant.

The 85% value to which you are referring is a round figure which is far too often quoted as a recommendation to protect those who are wanting to tow a caravan but have neither the intelligence or interest to fully understand the physics involved, and is based on the tow vehicle being a caravan.
 
Mar 14, 2005
17,860
3,230
50,935
Visit site
padmiester said:
The vast majority of vehicles have higher towing limits than their kerb weights. The towing limits are based on what the car can safely handle before putting to much stress on components such as suspension, engine etc effectively to prevent damage to vehicles and to ensure the vehicle has adequate power.
The kerb weight of a vehicle also varies by defintion, but generally is the vehicle plus all necessary fluids and a driver at 75kg. Generally when towing this weight is increased with extra passengers, luggage etc, which then helps the towing ratio.
As for the 85% guideline has no bearing at all, that was created by the caravan associations many years ago to help novices tow safely. It is not taken into consideration by car manufacturers and has no legal standing.

Hello padmiester.
There are some technical inaccuracies in your posting.

The towed weight limit is specified by the vehicle manufacturer, and as you suggest is set so that towing such a mass should not damage the vehicle. It does not infer safety which is more in the hands of the driver.

I agree there are several different definitions of kerbweight, but once the kerbweight for a vehicle has been established, the value does not change, even when you load the vehicle with passengers or luggage.

I agree the 85% guideline has no legal basis, but it is foolish to ignore the concept it supports in relation to towing a caravan for the reasons that Otherclive has added. I do believe the guidance is outdated, and it should be revised to take into account the greater depth of published technical data about vehicles, and the advances made in safety systems built into modern vehicles. Whilst I hold that view, I do not assume the percentage should necessarily be increased.
 
Nov 11, 2009
20,847
6,513
50,935
Visit site
Icaru5 said:
* Content deleted on reflection due to undue sarcasm :p

Cannot see what has been deleted. Your first post still contains content that is totally unwarranted and ignores the fact that 85% plus other guidance can be especially useful as a starting point for those who are starting out on the hobby of caravanning. I am a Chartered Mechanical and Marine Engineer and cannot recall my years of academic and practicing experience covering the physics of towing :)
 
Oct 12, 2013
3,037
4
0
Visit site
otherclive said:
Icaru5 said:
* Content deleted on reflection due to undue sarcasm :p

Cannot see what has been deleted. Your first post still contains content that is totally unwarranted and ignores the fact that 85% plus other guidance can be especially useful as a starting point for those who are starting out on the hobby of caravanning. I am a Chartered Mechanical and Marine Engineer and cannot recall my years of academic and practicing experience covering the physics of towing :)

I think he means he deleted his own post or edited his own post before it was given a chance to be read by others !
 
Nov 11, 2009
20,847
6,513
50,935
Visit site
Craigyoung said:
otherclive said:
Icaru5 said:
* Content deleted on reflection due to undue sarcasm :p

Cannot see what has been deleted. Your first post still contains content that is totally unwarranted and ignores the fact that 85% plus other guidance can be especially useful as a starting point for those who are starting out on the hobby of caravanning. I am a Chartered Mechanical and Marine Engineer and cannot recall my years of academic and practicing experience covering the physics of towing :)

I think he means he deleted his own post or edited his own post before it was given a chance to be read by others !

Craig thank you I thought he was referring to the one where the final paragraph mentions “intelligence, interest and physics”. So my comments still stand.
 
Apr 6, 2014
6
0
0
Visit site
Hi Prof,

I wasn't meaning that an increase in weight of the car would increase the kerb weight, my point was as the vehicle becomes heavier the towing ratio improves.

Also as you correctly state safety is indeed in the hands of the driver ultimately, but I would argue that the max towing weight also helps in terms of trying to prevent overloading which is dangerous to all road users.
 
Jun 26, 2017
445
17
10,685
Visit site
Damian-Moderator said:
Craigyoung said:
otherclive said:
Icaru5 said:
* Content deleted on reflection due to undue sarcasm :p[/quote

I think he means he deleted his own post or edited his own post before it was given a chance to be read by others !

The post referred to was deleted by me ,,,,,,,due to undue sarcasm.

You didn’t delete the post being referred to Damian. The post is still there, but on reflection, I deleted it’s content and replaced it with a explanation as to why.
 
May 7, 2012
8,598
1,816
30,935
Visit site
The maximum towed weight is the limit the car can restart five times on a 12% incline and is more a limit for the driveline rather than a safety limit.
In practice you could tow say a broken down car round town without jepodising safety where speeds are low and side winds are not a problem.
Caravans have large flat sides which can be caught by side winds and the bow wave from large vehicles and so you need to be more cautious. Basically the car has to have enough weight to control the caravan when the forces apply.
The 85% figure was the result of work carried out by Bath University for the Caravan Club but it is many years old. Things have moved on a bit since then and it is possibly now outdated but it still remains a reasonable starting point for beginners. What is certain is that towing a caravan which weighs more than the cars kerb weight is risky and although some outfits might manage it only the foolish do not heed this advice.
In practice different outfits safe limits will vary, but there is no way you as a buyer can establish yours before buying, so you should err on the side of caution.
Personally I would not go over 95% and for beginners 90% might be a top figure.
 
Mar 14, 2005
17,860
3,230
50,935
Visit site
Raywood said:
The 85% figure was the result of work carried out by Bath University for the Caravan Club but it is many years old. .

Hello Ray,

I have made extensive searches through contacts I have in the industry over the years and I have found no evidence that Bath University or any other university was involved with establishing the 85% figure. If you have any evidence to support your statement I would be very pleased to know more about it. can you enlighten me please?
 
Jun 20, 2005
17,633
3,721
50,935
Visit site
ProfJohnL said:
Raywood said:
The 85% figure was the result of work carried out by Bath University for the Caravan Club but it is many years old. .

Hello Ray,

I have made extensive searches through contacts I have in the industry over the years and I have found no evidence that Bath University or any other university was involved with establishing the 85% figure. If you have any evidence to support your statement I would be very pleased to know more about it. can you enlighten me please?
Dr Jocelyn Darling from Bath University has worked with Bailey Caravans for years on various aspects of caravan stability using different types of stabilisers. No where Have I ever seen any reference to the magic number 85. One of his students J Lewis did a Thesis testing various types of caravan suspension systems but again no mention of 85. As an aside he concluded shock absorbers didn’t really improve stability on the current chassis available.
My own recollection is back in the late 70s the Caravan Club recommended not exceeding 85% of the cars kerb weight. Not a rule, no scientific evidence in support. Just a cautionary suggestion.
However there is probably an element of common sense and an effort to avoid the tail wagging the dog. Caravans are hardly aerodynamic are they :woohoo:
This Forum has done the 85. To death over the years. It truly is a mystical magical caravan myth :evil: :whistle:
 
Nov 11, 2009
20,847
6,513
50,935
Visit site
Dustydog said:
ProfJohnL said:
Raywood said:
The 85% figure was the result of work carried out by Bath University for the Caravan Club but it is many years old. .

Hello Ray,

I have made extensive searches through contacts I have in the industry over the years and I have found no evidence that Bath University or any other university was involved with establishing the 85% figure. If you have any evidence to support your statement I would be very pleased to know more about it. can you enlighten me please?
Dr Jocelyn Darling from Bath University has worked with Bailey Caravans for years on various aspects of caravan stability using different types of stabilisers. No where Have I ever seen any reference to the magic number 85. One of his students J Lewis did a Thesis testing various types of caravan suspension systems but again no mention of 85. As an aside he concluded shock absorbers didn’t really improve stability on the current chassis available.
My own recollection is back in the late 70s the Caravan Club recommended not exceeding 85% of the cars kerb weight. Not a rule, no scientific evidence in support. Just a cautionary suggestion.
However there is probably an element of common sense and an effort to avoid the tail wagging the dog. Caravans are hardly aerodynamic are they :woohoo:
This Forum has done the 85. To death over the years. It truly is a mystical magical caravan myth :evil: :whistle:

Whilst I agree that the birth of 85% seems lost to the passage of time, it is the only guidance figure available on which to start ones search for an outfit if you are new to the hobby. Whilst modern cars are far ahead of their predecessors both in basic road holding and suspension and electronic aids, and many caravans now have ATC its neigh on impossible for a newbie to try (an outfit) before buy. Matching websites and owner reviews can also provide some guidance but again a newbie needs to understand the basic principles. So in the absence of any other quantitative guidance 85%, or thereabouts, plus attending a Club course is probably still a good starting point for most newbies. I shall now flood main ballast to hide from those who started at 97.5% or higher.
 
Sep 29, 2016
1,807
216
19,935
Visit site
If somebody (or nobody) came up with the 85% guidance with or without valid research is for me neither here nor there.

I am happy to accept the 85% figure as a good guidance reference start point for anyone.

Anseo.
 
Mar 14, 2005
9,824
716
30,935
lutzschelisch.wix.com
ProfJohnL said:
I agree there are several different definitions of kerbweight, but once the kerbweight for a vehicle has been established, the value does not change, even when you load the vehicle with passengers or luggage.

There is only one definition of kerbweight and that is the one that can be found in UK legislation. Anything else is not kerbweight, but mass in service (also known as mass in running order), unladen weight, or actual weight, each of which have their own definition.
 
May 7, 2012
8,598
1,816
30,935
Visit site
Possibly the guide reference to kerb weight needs to be updated to mass in service, as that is basically the figure you should use.
 
Mar 14, 2005
9,824
716
30,935
lutzschelisch.wix.com
Raywood said:
Possibly the guide reference to kerb weight needs to be updated to mass in service, as that is basically the figure you should use.
It's poor practice to change a definition because you never know whether someone is talking about kerbweight according to the old definition or according to the new one.
Mass is service isn't a particularly good choice if you want to be really accurate, either. It will nearly always result in a conservative weight ratio figure because mass in service is the minimum that the car can weigh to make it saleable. It wouldn't include any uplevel factory fitted options that the car may have. Bearing in mind that manufacturers are under no obligation to publish kerbweight figures, the closest one will get to kerbweight, as defined in UK legislation, is the actual weight figure that will only be found in the Certificate of Conformity issued with the car. The only difference between that figure and kerbweight is that it allows 75kg for the driver, which kerbweight doesn't, and only a 90% full fuel tank instead of a 100% full one as per the kerbweight definition.
 

TRENDING THREADS

Latest posts