Julie and Jonathan raise the question of correct motorhome tyre pressures ............
Comment from Steve Brock...................
Having been a qualified accident investigator and vehicle examiner for several years I feel able to offer some guidance in this respect (my comments are purely personal and should not be taken as absolute).
Correct tyre pressures are more important than most people realise - as well as affecting ride quality, the subject has an impact on fuel consumption, braking and steering and has a significant impact on the handling characteristics of the vehicle. This is why there is a specific offence under the Road Vehicle (Construction & Use) Regulations of having incorrectly inflated tyres.
Most cars and some lighter vans have different tyre pressures for front and rear, and for fully and partly laden conditions. The overall 'mass' (weight) and it's distribution across front and rear axles in such a relatively 'light' vehicle is an important consideration when tyre pressures are determined, and that is why manufacturers go to such trouble in specifying correct tyre pressures for certain conditions.
However, manufacturers of heavier commercial vehicles (exceeding 3.5 tonnes) generally specify a single tyre pressure for a fully laden vehicle. This is the case with Fiat, as quoted on page 161 of their 'Ducato Owner Handbook'. You may wish to look at this information to help your understanding.
When deciding the correct tyre pressure, manufacturers take into account not only the vehicle weight but the chassis type, wheel rim size and tyre size. These are important.
Jonathan is correct, in that tyre pressures inside the cab door are given by Fiat and are consistent with the vehicle chassis type, rim and tyre sizes at the time of manufacture. You will appreciate, however, that the base vehicle is then passed to Knaus for the coach-built body to be fitted and the 'mass' of the end product will also have a bearing when determining the correct tyre pressures.
Paragraph 20 - 3 of the Knaus Motorhome Owners Manual sets out the information indicated above by Jonathan, but it also indicates wheel and tyre sizes.
There does appear to be a discrepancy between the tyre pressures quoted by Fiat for their 'base' vehicle and by Knaus for their end product, and I will endeavour to get comment from Knaus.
So, where does this leave you?
Before you do anything, check the wheel rim sizes and tyre sizes actually fitted to your vehicle (they should be the same as at the time of base vehicle manufacture, but check it anyway to be sure). Rim sizes are stamped on the outside facing wall of the wheel, usually close to the wheel nuts. Tyre sizes are marked on the outer edges of the tyres.
Now check your motorhome documentation to determine the chassis version (the Traveller 608 & 708 have a maxi chassis, but the 508 can be specified with a standard chassis, so be careful). I can not comment on 'A' class and Sun Traveller models as I do not have access to their specifications.
If the chassis version, tyre and wheel sizes all match the specifications given in Paragraph 20 - 3 of the Knaus owners handbook, then I would suggest that it is safe to adjust your tyre pressures to those indicated pressures.
For those still working in pounds shilling and pence, 1 bar pressure equals approximately 14.5lb. psi. Therefore:
4.5 bar = 65 psi, 5 bar = 72 psi 5.5 bar = 80psi
Remember, all quoted pressures are for a fully laden vehicle. Given the nature and construction of the motorhome, I would not advise adjusting these pressures if travelling partly laden - it is a commercial vehicle chassis and should be treated accordingly.
The law allows a variation of 10% in recommended tyre pressures before prosecution. However, insurers have a more rigid approach when settling claims.
The (literal) bottom line is that, based upon all the information you should make your own decision as to what is the correct tyre pressure for your vehicle. However, I can assure you that any reply from Knaus in respect of this matter will be posted onto the website as soon as I have it.
Steve has an answer from Knaus concerning tyre pressures....................
As a result of the recent interest in correct tyre pressures for Knaus motorhomes, I sent an Email to Knaus Tabbert briefly outlining the different advice in the Knaus and Fiat Ducato owners handbooks. I quoted the Traveller 608 model as an example (maxi chassis) and they have informed me that the correct inflation pressure is 4.5 bar (or 65lb. psi). It is apparent that Knaus are satisfied with the tyre pressures quoted in Paragraph 20 - 3 of their handbook, and as a result of this I would advise that you use the figures quoted in that publication. Check that the chassis type, tyre and wheel sizes match those given in the handbook, and bear in mind that only 'vans with a maxi chassis have a tyre pressure of 4.5 bar (65lb. psi). Other chassis versions have different tyre and wheel sizes fitted as standard, and the inflation pressure for these is quoted as 5.0 bar (72lb. psi).