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post #1 of 5 (permalink) Old 02-06-2015, 14:19 Thread Starter
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From Michelin - tyre pressures and axle loadings.

FWIW, I emailed Michelin to ask about tyre pressures based on the max axle loadings for our 2008 Kontiki 669 tag. We're not loaded to the max yet but thinking that the max loads would apply to many other AL-KO tag owners, I waited to see what they said.
Based on max axle loading, ie. 2100kg front and 1500kg for each rear axle, fitted with Michelin Agilis CP 225/75-16CP 118Q. This is their reply but you have to ignore the sales blurb re. the Agilis CP tyre. Interesting to note that they are now saying as a standard (like Continental and others), that the rears should be run at max, ie 80psi.
"Thank you for your enquiry about pressures for your motorhome.
The Michelin Agilis Camping tyre is designed especially for motorhomes and would be the preferred fitment on a motorhome.
The Agilis Camping tyre boasts a long life by using the same technology as the Michelin Agilis van tyre range. It has a reinforced construction to enable the use of higher pressures, has 8 kerbing protectors built in to each sidewall to increase resistance of sidewall scuffing and is M+S (mud and snow) marked to give sufficient grip when occasionally travelling in difficult conditions such as unsurfaced roads, mud and snow.
The suggested tyre pressure for the rear of a motorhome running on the Agilis Camping is 80psi.
This is due to the construction of the tyre with 2 casing plies enabling the use of higher pressures. Its construction and the use of higher pressures is designed to cope with continual heavy loads sometimes found on Motorhomes and can help with wear pattern issues if lower pressures are used particularly on the rear axle.

The front pressures however can be adjusted according to accurately weighed axle loads for a more comfortable ride and optimum performance.This recommended pressure is 54psi.

Should you require any further assistance please contact us on 0845 366 1590 (UK & NI) or 0044 (0) 1782 401590 (ROI"

I'm now going to drop mine from 70 to 60 front and rear and see what the effect is. Hopefully with the fronts at 60psi, the ride might be less "crashy" over bumps. May also try 54psi on the fronts (they WILL look flat as have been there before) but with the rears at 60psi.

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Last edited by deefordog; 02-06-2015 at 14:22.
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post #2 of 5 (permalink) Old 02-06-2015, 14:42
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Michelin have changed their tune since I sent them axle weights a couple of years ago and they only answered 80psi all round and did not include

" he front pressures however can be adjusted according to accurately weighed axle loads for a more comfortable ride and optimum performance.This recommended pressure is 54psi."

However I think that info is suspect without knowing the axle weights.

I then asked VOSA, supplying tyre specs and axle weights, on the basis that they do roadside safety checks which I would have thought included the safe inflation of tyres for the load. They declined to answer and said 'ask the tyre manufacturer' Round in circles - and you know the result of that!

Since then I have relied on Tyresafe's figures.

Michelin in particular seem to be unhelpful, but other manufacturers have given more specific answers - not much help if you have Michelin fitted.

{ Sorry about italics and bold - could not change it]

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post #3 of 5 (permalink) Old 02-06-2015, 16:13
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The information from Michelin is in line with their website except it quotes 4.5bar for front and consistent with the telephone pressures/information they provide. There is a diagram that shows how the current tyre sits on the road camper to other tyres - there seems to be more tread contact. It's nice to see it all laid out in a post.

When I spoke to Michelin a few months ago, they did say I could reduce the rear axle pressure on my 1850kg loaded rear axle a little but that would mean the tyre operating below optimum performance. My polite response was that optimum performance should not include a rock solid ride on the rear and whilst tread wear might be an issue, most motorhomers' tyres will age before they wear out.

I've never known so much debate and confusion on tyre pressures created by Michelin in all the years I've been motorhoming. I don't see me buying their tyres when they wear out; I prefer something other than a bone asking ride.
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post #4 of 5 (permalink) Old 02-06-2015, 16:55 Thread Starter
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I guess Michelin mean that 54psi is the pressure based on the size of tyre I gave them and the front axle loading of 2100kg that I also asked them about?

We're currently at 4880kg gross (including wife, me, dog and fully laden). The recent weighbridge results are however somewhat dubious. Whilst I can't dispute the gross mass, the balance beam bridge came up with 2020kg front, 1680kg middle and only 1180kg on the rearmost axle - hmmmmmm.

As most of the boneshaking ride seems to come from the front, will have to try 60psi in the fronts.
It's a shame the Tyresafe doc doesn't include the 225/75-16 118 tyres but I've also followed a calculation that someone gave on here.

118 load index = 1320kg per tyre. Michelin stamp this tyre "@ 80psi". So 1kg = 0.061psi (80 divided by 1320). So taking the max loading on the front axle of 2100kg and halved to get load on each wheel, that's 1050 x .061 = 64psi, not 54psi as Michelin suggest. Now even more confused.
Now take the rear axle/s of 1500kg, apply the same logic and the rear tyre pressures equate to 46psi each.

How on earth do Michelin come up with the pressures, even if they assume that motorhomers overload their rear axles? 80psi on the rears would equate to 1320kg x two tyres = 2640kg for one axle!!

Last edited by deefordog; 02-06-2015 at 17:05.
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post #5 of 5 (permalink) Old 02-06-2015, 20:30
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You are assuming that the weight vs pressure relationship is linear. That's very unlikely to be the case.

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