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Car Tyre Pressures Towing


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#21 MauriceH

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Posted 03 October 2009 - 05:50 PM

All this talk about upping tyre pressures to the "Fully Laden" pressure when towing. Why? Most of us are only hitching a caravan up, typically 75Kg noseweight on the towbar. Fully Laden means 3 passengers in the back and luggage in the boot, according to our handbook, a possible 250 - 300Kg. Therefore we leave our pressures standard solo and towing.

Never thought of it like that -- thanks --

Maurice

#22 RogerL

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Posted 03 October 2009 - 06:02 PM

All this talk about upping tyre pressures to the "Fully Laden" pressure when towing. Why? Most of us are only hitching a caravan up, typically 75Kg noseweight on the towbar. Fully Laden means 3 passengers in the back and luggage in the boot, according to our handbook, a possible 250 - 300Kg. Therefore we leave our pressures standard solo and towing.

But many, perhaps most, manufacturers RECOMMEND using the full load pressures when towing even if the car has no greater vertical load.

Increasing the rear tyre pressures will generally increase the lateral grip of the rear tyres - which is a very useful aid to towing stability !

#23 BrianI

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Posted 03 October 2009 - 08:58 PM

You have to remember that 75kg on the towball means around 100kg on the rear axle and could be more with a long rear overhang. Simple moments - multiply tow ball load by distance of tow from front axle and then divide by wheelbase distance to calculate extra load on rear axle.
As Roger says increasing pressure stiffens tyre sidewall and reduces flexing of tyre sidewalls and hence sideways movement of vehicle. I have noticed this when I forgot to pump up my tyres. This effect may be more noticeable on high profile tyres that are typically fitted to 4x4's than low profile tyres common on modern cars.
Brian

#24 Gordon

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Posted 04 October 2009 - 08:01 AM

Most of us are only hitching a caravan up, typically 75Kg noseweight on the towbar.

The rear of the towing vehicle does not have to support just the static noseweight of the caravan. I agree that at rest the additional load on the tyres is very small, but when the outfit moving, the stability of the caravan is in part reliant upon the stability of the towing vehicle.
By increasing the towcar tyre pressure in accordance with the manufacturer's guidelines, the side walls of the tyres are stiffened up, thus minimising their deformation under lateral loads and providing a more stable platform. Also under braking, the caravan will rotate about the axle(s) line so the load on the towball will be significantly higher than the static value, and the rear tyres of the towcar have to remain firm under this condition if stability is to be maintained. If sharp steering and braking are required at the same time, as in an emergency manoeuvre, if the rear tyres do not perform well the result can be disastrous, even fatal.
It should be noted however that not all vehicles require the tyre pressures to be increased for towing, as some (predominantly the heavier vehicles) allow for this variation of load within the design.


#25 matelodave

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Posted 04 October 2009 - 09:49 AM

I would have thought that this was a question that should either be answered in the handbook or by the car manufacturer if they haven't included it in the handbook, hopefully they should know how their vehicles should perform when towing. Unfortunately a lot of them suggest that you consult the dealer who, in a lot of cases, hasn't a clue what you are talking about

#26 MauriceH

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Posted 04 October 2009 - 11:56 AM

Getting even more confused now !!

*** I agree that it should be shown by the manufactures in their information books
-- says all about roof loads
-- boot loading
-- extra people in the back
-- WHY not __ When towing you do this .. :o

they must know..

Maurice

#27 Gordon

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Posted 04 October 2009 - 01:03 PM

WHY not _ When towing, you do this?

Probably because their product is primarily designed to carry people and luggage, not to tow a load, otherwise there would be a towing hitch fitted as standard wouldn't there?

#28 RogerL

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Posted 05 October 2009 - 09:17 AM

Getting even more confused now !!

*** I agree that it should be shown by the manufactures in their information books
-- says all about roof loads
-- boot loading
-- extra people in the back
-- WHY not __ When towing you do this .. :o

they must know..

Maurice

I've never had a car where the vehicle owner's manual DIDN'T give towing information.

Knowing that Maurice has a Volvo, which used to be the towcar of choice, I find it strange that they, of all people, omit towing information from the owner's manual - Ford include it so it can't be a group policy.

#29 MauriceH

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Posted 05 October 2009 - 10:43 AM

I've never had a car where the vehicle owner's manual DIDN'T give towing information.

Knowing that Maurice has a Volvo, which used to be the towcar of choice, I find it strange that they, of all people, omit towing information from the owner's manual - Ford include it so it can't be a group policy.

They [Volvo] don't give information about tyre pressures when a caravan is connected -- they do give information about weights that you can tow etc..

just checked the handbook to make sure -- that just refers me to the plate near the fuel inlet --nothing about a caravan

Maurice

#30 BrianI

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Posted 05 October 2009 - 10:50 AM

Maurice
Just had a look in my Volvo handbook. There is a whole section under "towing with a trailer" if you look in index. Covers things like tyre pressures (says use full load pressures), gradients, towball load, auto boxes, etc.
Brian




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