Raising front tyre pressure on your towing vehicle ?

[Paulpro]

I have been towing a caravan for over 30 years now (of course not me personally !) and have always increased my towcars tyre pressures in line with the values suggested in the handbook. My current car is an Audi A6 Avant, the rear tyres are increased from 34 psi up to 46 psi and the front tyres from 34 to 42 psi. This all results in a pretty hard ride and the outfit is very stable. However, I recently went on a fairly short trip away and decided to adjust the rear pressures only and left the front ones as they were at 34psi. The result was that I noticed no drop in stability but did notice a much smoother ride by having softer tyres at the front.
Has anyone else tried this successfully or do you consider that this is a complete no no. I can't help feeling that as all the extra weight is more or less over the rear wheels then it is more logical to just adjust those tyre pressures. I can see the relevance in raising the front pressures when the car is fully loaded with passengers and their luggage etc. But when it's just myself and the wife, the front of the car must act as if the car was still solo. Any thoughts on this ?

Paul

 

[John_374564913]

Paul
I cannot comment on the viability of not raising your fron tyres or not but can quote the following facts

I check my caravan tyres before leaving and they are set at 29 psi then I fit my tyrepal system it reads 29 psi
After about thirty minutes tyre pressures rise to around 32.5 and this is normal

Therefore one can pressume the car is the same so even though you have not raised the pressure in the fron it will still be higher than you though after about thirty minutes.

I am sure somebody will have a definitive answer for you soon

 

[Lutz]

Although, loadwise, you are correct in saying that all the extra load is on the rear axle when towing, always assuming that the car isn't fully laden, too, by increasing the rear tyre pressure without doing the same at the front will make the car tend to understeer more. Under normal driving conditions this difference may not be noticeable, but you could run into problems if you ever have to take evasive action in an emergency in order to avoid a hazard. The steering will be less responsive, i.e. the car will tend to want to continue to go in a straight line.

 

[Paulpro]

Many thanks for considering this and I can see why towing will create slightly less downforce on the front wheels and therefore loss of traction for steering purposes etc. However, with a car being occupied by two people plus a normal amount of bits & pieces in the back, I cannot see how increasing the front tyre pressure will improve the roadhandling. Could it be that more psi will make the tyres slightly larger and thus making better contact with the road or, is it more complicated than that ? Have I been pumping up my front tyres unnecessarily for all those years and after all they just end up wearing out in the middle when over inflated !

Paul

 

[Surfer]

I would have assumed that by increasing the PSI you would have less contact with the road. I have never bothered with changing the pressure on either the font or the rear on my Mondeo and neither do I bother on my current vehicl which is a 4 x 4. Nearest garage is about 5 miles away so too much hassle. May consider it if travelling on a very long trip.

 

[GeorgeandAde]

My Honda CRV doorplate recommends increased pressure for the rear tyres from 29psi-32psi.
The front tyres recommends 30psi solo and 30psi towing.
Adrian

 

[Gareth111278]

I'd never considered changing the front tyre prssures on ay car when towing. TBH I don't do the rears but I'm only carrying 2 kids & a dog in the back of an eight seater so the extra 70kgs nose weight doesn't even represent one adult passenger.

Having said that I do find that if I enter a corner a little faster than I should have it all feels very 'understeery'!
I'll up my pessures 5 psi & see how it feels. [My car is a 'Grey Import' so i only have solo pressures on the plate, don't get all silly now & say the words Hand-book, there isn't one!]

Gareth

 

[Lutz]

Many thanks for considering this and I can see why towing will create slightly less downforce on the front wheels and therefore loss of traction for steering purposes etc. However, with a car being occupied by two people plus a normal amount of bits & pieces in the back, I cannot see how increasing the front tyre pressure will improve the roadhandling. Could it be that more psi will make the tyres slightly larger and thus making better contact with the road or, is it more complicated than that ? Have I been pumping up my front tyres unnecessarily for all those years and after all they just end up wearing out in the middle when over inflated !

Paul

Because the front axle is so much further forward from the towball compared to the rear axle, the reduction in front axle load is fairly small compared to the increase at the rear. But with less air in the tyres, they are more compliant, i.e. less stiff, and don't hold the road as well. So, if the front tyres are underinflated or if the rear tyres overinflated, the car will tend to understeer more (go more in a straight line as it enters a bend). The opposite is, of course, true if there is too much pressure in the front tyres or too little in the rears. Then the car will tend to oversteer, which, in the extreme, can cause it to swerve. It is therefore important that a proper balance between the front and rear tyre pressures is maintained.

 

[ProfJohnL]

This thread has set me thinking -

Car manufactures will probably work with tyre manufactures to work out what the optimum tyre pressure for each car will be. But clearly when they advise you should adjust the pressure depending on load and or the type of journey they must have certain concerns in mind.

For loads its fairly easy to understand the reasoning. By design the manufactures will aim to have a certain area of tyre in contact with the road. If you double the load then twice the area of tyre will be pushed to the road, so you need to double the pressure in the tyre to reduce the contact area back to optimum,

But it puzzles me why the type of journey (e.g. motorway speeds) may need higher pressures. Could it be that with long high speed journeys the flexing of tyre generates more heat, and by increasing the pressure the tyre flexes less thus reducing the heating effect? any answers?

Surfer asks "I would have assumed that by increasing the PSI you would have less contact with the road." Well it depends on what you mean by contact. For any given load the contact 'area' will reduce as you raise the pressure, but the 'force per unit area' increases.

 

[JohnGriffiths]

I always check and change tyre pressures when loaded and/or towing. Like the original poster, my last two cars have required quite different pressures when loaded. Manufacturers wouldn't bother quoting them if they weren't important. Plus you are probably wasting fuel running under pressure tyres. It is a bit of a pain but a cheap footpump or little electric compressor is all it needs.
Prof John, I think you're right about more heat being generated on Mways.

 

[Paulpro]

I still feel that when a vehicle is solo but not towing, I can always see the relevance of increasing the tyre pressures all round (in line with the manufacturers recommendations) as the extra weight will be shares out more or less equally, front and back and generally if my wife and I go on a longish journey solo and without a load I would not increase the pressures at all. However, it just seems odd that putting a few extra items in the boot plus the reasonable noseweight of a caravan should necessitate the increase in pressure at the front of the towing vehicle. If the front tyres could talk (LOL), I am sure that they would not notice any difference when the vehicle they support is towing a caravan but they would notice a difference with a fully laden (solo) vehicle - hence the manufacturers general recommendation for an increase when loaded. After all. most towing vehicles will always drop somewhat at the rear and leave the front as it was. With some cars in my earlier days, it almost felt as though the front was being lifted off the ground - not very good with f.w.d. ! I would also suspect that a towing vehicles front tyres should not really heat up any more because there hardly any extra downforce being exerted on them ie. they still feel that they operating on an unladen, solo vehicle. Perhaps I will have to go and ask my tyres how they see it !

Paul

 

[Lutz]

As I have tried to explain, if you change the tyre pressure for whatever reason, unless you do so on all four wheels, you will change the way in which the car handles. This applies regardless of whether the front axle is subjected to a higher load or not.
As the Prof pointed out, the car manufacturer, together with the tyre supplier, will carry out tests to determine by how much the inflation pressures need to be adjusted in order to ensure that the car handles roughly the same when laden or unladen.

But it puzzles me why the type of journey (e.g. motorway speeds) may need higher pressures. Could it be that with long high speed journeys the flexing of tyre generates more heat, and by increasing the pressure the tyre flexes less thus reducing the heating effect? any answers?

The short answer to your question is yes, your assumption is correct.

 

[Paulpro]

What I am therefore wondering is, is the towing vehicle classed as being "laden" when towing as whilst doing so, the front half of the vehicle is still effectively "unladen", if not more so. Perhaps I ought to recheck my handbook to see if the values recommended relate to a laden vehicle or one that is used for towing.

 

[Paulpro]

What I am therefore wondering is, is the towing vehicle classed as being "laden" when towing as whilst doing so, the front half of the vehicle is still effectively "unladen", if not more so. Perhaps I ought to recheck my handbook to see if the values recommended relate to a laden vehicle or one that is used for towing.

 

[Lutz]

As I said, if you increase the pressure at the rear because of the higher axle load, the pressure at the front must normally also be increased even if the load at the front isn't or else the car will handle differently. I say 'normally' because it depends a bit on whether the car inherently over- or understeers and other variables, for example, suspension geometry changes due to the back end of the car sitting lower down under the added load. One should therefore always rely on the car manufacturer's recommendations, whatever they specify.

 

[Beachball]

We have a Citroen Grand Picasso and the front tyre has to be 49 psi and the back are 46psi and the caravan is 42psi has anyone come across this and why because i always thought the back tyres were higher than the front .

 

[Royston]

My vectra estate hand book indicates that the rear tyres should be increased by 6 psi when towing a trailer , allways applied this rule, never had any problem when towing at 95%. Caravan always remains steady even going downhills.

 

[ProfJohnL]

Hello Beachball,

There is no truth in your assumption that rear tyres must always be higher pressure than front. The actual tyre pressure recommendations by the manufacture must be adhered to as they will have dome all the calculations and testing required to confirm the best combinations of pressures vs loads and usage.

Hello Royston,
What may be correct for your Vectra, may not be correct for other makes or models. You should always follow your car manufacturers recommendations for tyre pressures.