Any tyre pressure experts out there??

[Pampam]

Would you say 60psi is correct for an autotrail arapaho on all the tyres ?? Thank you pampam

[Keithl]

Quick answer, No!

 

You almost certainly should have different pressures front to rear.

 

You will need to know tyre size and axle weights to determine them correctly.

 

For a start have a look at the Tyre Safe brochure linked here... Link

 

Keith.

 

PS Caveat - This is just a quick answer and there are more factors to take into account.

[Tracker]

Pampam - 2013-08-09 10:17 PM

 

Would you say 60psi is correct for an autotrail arapaho on all the tyres ?? Thank you pampam

 

You are probably not wanting to hear this but nobody can advise anybody on tyre pressures without knowing the fully laden axle weights and tyre make and sizes Pam - not even the tyre makers would dare to guess, so a trip to the weighbridge fully loaded up to get individual axle weights plus gross vehicle weight is your best course of action.

 

Unless your tyres are Michelin (used to be helpful but no longer seem to be so helpful) the tyre makers should then be able to advise on pressures when you contact them.

[Derek Uzzell]

A 2010 Auto-Trail Arapaho will have been fitted with 225/75 R16C tyres (almost certainly of the 'camping-car' variety.)

 

As this model has an Al-Ko chassis with tandem rear-axles, obtaining individual axle-loadings for the rear axles via a weighbridge will be tricky.

 

The first question to ask Pampam is "What tyre pressures are advised by Auto-Trail in their handbook for her motorhome?"

 

It wouldn't surprise me if 5.5bar (80psi) were advised by Auto-Trail for all three axles. If that's so, where has the 60psi Pampam mentions come from?

[Tracker]

Derek Uzzell - 2013-08-10 8:56 AM

It wouldn't surprise me if 5.5bar (80psi) were advised by Auto-Trail for all three axles. If that's so, where has the 60psi Pampam mentions come from?

 

When we had our Starlet on an Alko chassis the Alko handbook gave us lower tyre pressures for the chassis weight than the Autocruise handbook and on checking with the tyre makers (Continental) we found that their recommended pressures for the tyre size and axle weights were very similar to the Alko pressures. We ran very happily for four years on the lower tyre pressures which improved the ride no end without affecting the handling and body roll on bends or tyre wear and could therefore see no advantage in the overly high tyre pressures that Peugeot suggested.

 

But you do have to start with the known actual weight in fully loaded trim.

[Derek Uzzell]

Al-Ko's recommended tyre-pressures for 'triple axle' chassis can be found on Page 11 of this file:

 

http://www.al-ko.co.uk/edit/files/handbooks/amc-handbook.pdf

 

The advised rear-tyre pressure is 3.8bar (55psi), while the advised pressure for the front tyres of vehicles fitted with 225/75 R16CP 'camping-car' tyres is 5.5bar (80psi).

 

I don't know what Auto-Trail advise (though Pampam should), but it's been far from rare for motorhome manufacturers building on triple-axle Al-Ko chassis to recommend 5.5bar (80psi) 'all round'.

[Bulletguy]

Derek Uzzell - 2013-08-10 8:56 AM

 

The first question to ask Pampam is "What tyre pressures are advised by Auto-Trail in their handbook for her motorhome?"

 

 

Absolutely right Derek!

[George Collings]

Weighing a three axled Al-Ko based motorcaravan to abtian axle weights is only slightly more complicated than a two axled vehicle.

 

Just weigh the whole vehicle then the rear pair, subtract the rear from the total to give the front weight and divide the rear pair by two to give an average on each rear axle. Provided say the rear locker(garage) is no massivley overloaded the rear axles will be sharing the load reasonably evenly.

 

HGVs with twin or even triple rear axles have quite complicated arrangements to ensure axles bear an even weight but the relatively soft independent Al-Ko suspension inherently evens the load on each wheel.

Its the reason they ride so comfortably.

[Derek Uzzell]

The Auto-Trail website's FAQ section includes the following:

 

"Question: Please can you tell me what the tyre pressures are for my vehicle?

 

Answer: The tyre pressures for the Fiat chassis are listed on the left hand side door pillar just below the door latch. For Alko twin axle vehicles, please refer to your Alko handbook."

 

An Arapaho has the following weight-related maxima:

 

GVW - 5000kg

Front-axle - 2100kg

Rear-axle 1 - 1500kg

Rear axle 2 - 1500kg

 

Given a 225/75 R16C tyre-size, generic inflation pressures for those axle loadings would be around 4.0bar (58psi) for the front axle and 3.0bar (44psi) for the rear axles. But I'm tempted to ask "So what?"

 

If Auto-Trail advises owners of their 3-axle Al-Ko chassis motorhome models to obtain tyre-pressures from the Al-Ko handbook, and the Al-Ko handbook advises a rear-tyre pressure of 3.8bar (55psi) and a front-tyre pressure of 5.5bar (80psi) when the vehicle is fitted with 225/75 R16CP 'camping-car' tyres (or 3.8bar (rear)/4.5bar (front) if the 225/70 R15C tyres are 'white van' type) why should that advice not be followed?

[Tracker]

An interesting post Derek and these pressures are very much in line with those we used on our Starlet for four years at Alks's recommendation with no adverse effects on handling or tyre wear - which makes me firmly believe that all manufacturers and converters are trying to make all motorhomes run on tyre pressures that are way too high for comfort.

 

To me better ride and handling make for a more stable vehicle with less surprises on bends, rough roads and under heavy braking - but maybe I am out of date and there is a good reason to run on seemingly over inflated tyres?

 

Yes I know that Alko chassis differ from the standard chassis in that generally there is a lot less tail overhang with an Alko chassied van but the weights are broadly similar?

 

However I am not suggesting for one minute that anyone go against the tyre pressures specified by the base vehicle maker, the converter or the tyre manufacturer as the legal and insurance ramifications in the event of a serious accident could prove to be very expensive.

 

[George Collings]

I suspect the reason the Al-Ko recomendation for front tyre pressures being significantly higher than the tyre manufacturers is that the tyre makers is primarily axle load related while the chassis makers is to compensate for a strong tendecy for understeer ( the link between tyres abilty to carry a load and behaviour when cornering is quite complex ) and the natural tendency of two fixed axles to prefer to travel in a straight line rather than on a curved path and push the nose of the vehicle wide during cornering.

[Derek Uzzell]

George Collings - 2013-08-12 2:28 PM

 

I suspect the reason the Al-Ko recomendation for front tyre pressures being significantly higher than the tyre manufacturers is that the tyre makers is primarily axle load related while the chassis makers is to compensate for a strong tendecy for understeer ( the link between tyres abilty to carry a load and behaviour when cornering is quite complex ) and the natural tendency of two fixed axles to prefer to travel in a straight line rather than on a curved path and push the nose of the vehicle wide during cornering.

 

In the case of a 3-axle Al-Ko chassis with 225/75 R16C tyres (as Pampam's Arapaho has) Al-Ko's front-tyre pressure recommendation of 80psi ('camping-car" tyres) or 65psi ('white van' tyres) will be within those tyres' design-pressure 'window'.

 

I agree that a highish pressure is likely to be needed to counteract potential understeer - an Arapaho is, after all, 8.73 metres long with a 5000kg maximum overall weight.

 

If an Arapaho owner weighed his/her motorhome and obtained real-world, fully-laden axle weights, and then contacted the manufacturer of the vehicle's tyres for inflation-pressure advice, it would be interesting to know what that advice would be.

 

If the vehicle had Michelin 'camping-car' tyres, I suspect Michelin would advise 80psi for the rear tyres even if it was emphasised that the motorhome had a pair of rear axles with a 1500kg weight limit for each. Would the tyre manufacturer take the 3-axle Al-Ko chassis into account when recommending a front-tyre pressure? - I have my doubts. If the measured front-axle weight turned out to be, say, 2000kg, would the tyre manufacturer recommend the around 53psi appropriate to that static loading, or increase that figure to allow for the vehicle's chassis specification? - I suspect the former.

 

Returning to Pampam's original question "Would you say 60psi is correct for an autotrail arapaho on all the tyres ?" All the indications are that the answer should be No. This response is based on the information found in Auto-Trail's website FAQ section and in Al-Ko's handbook (which it would seem Pampam should have).

 

Whether 60psi for all the tyres would be suitable and safe (and might provide a more comfortable ride quality) is another matter, but commonsense in this instance would be to follow Al-Ko's advice and only consider different inflation pressures if there are good reasons to believe the Al-Ko advised pressures can safely be modified.