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Tyres for Load & Grip
 
 

Michelin TyresJust black things with a hole in the middle?  Err no not exactly - if you're new to motorcaravanning or indeed commercial vehicles and load ratings then this page is essential reading! Tyres are one of those very under-rated products that we are heavily dependent on but largely ignore. Here you'll discover there's a lot more to motorhome tyres than just the size.

Weight: The most important difference between your motorhome tyres and those for your car is their weight carrying capacity.  A typical car tyre can accept a maximum load of 400-500 Kg while a 'light commercial' tyre can support over 800 Kg - a big difference!  This is an important difference too - your camper or motorhome is likely to have a 'gross vehicle weight' of between 2600 and 3850 Kg.  If we just divide these figures by the four tyres (taking no account of the likelihood of a bigger load on the rear), this means a load on each tyre of between 650 and 963 Kg - easily enough to burst those 'passenger' tyres at speed!  For just this reason virtually all tyres have their maximum load at maximum pressure information moulded into the sidewall - often as a code called the Load Index.

To safeguard against overloading the tyres, the UK tyre industry strongly recommend that when choosing tyres, the maximum technically permitted mass (MTPLM) of the vehicle's axles should not exceed 90% of the tyre load capacity as indicated by the tyre’s load index. Regrettably this isn't always achieved; for example many motorhomes are fitted with Michelin or Conti Tyres with 109 load indices when their 1900Kg rear axle should have 110 rated tyres under the 90% rule. We fitted slightly larger replacement tyres to our motorhome to achieve this safety margin - 225 section in place of the specified 215s.

Pressure: Car tyres are usually made from just two 'plies' whereas commercial tyres are made with six or eight or at least with technology that gives the strength of eight, i.e. 'eight ply rating'.  This difference in construction also limits tyre pressures, car tyres are typically limited to a maximum somewhere around 40 psi or less whereas van sized commercial tyres usually inflate up to 65 psi.  Incidentally 'commercial' tyres are called 'light truck' or 'LT' by US manufacturers.  The special Michelin 'Camping' tyres inflate to 80 psi.

Is there anything else to consider? Well yes, there are other factors that might concern motorcaravanners, low annual mileages, old age and wet grass for example!

The maximum pressures quoted above are likely to be quoted as the correct ones for your vehicle but others are possible. To assess whether you can use lower pressures for improved ride comfort and get-away grip you must load up the motorhome normally and then weigh each axle on a public weighbridge. These weights can then be used to look up the minimum pressure for that load. Do be aware that some of this is tyre specific - in particular be very careful about the differences between 65psi and 80psi tyre types.

Age: The very low annual mileages that most motorcaravanners do means that tyres might well die of old age before they wear out.  Rubber is particularly vulnerable to 'weathering', so tyres should be discarded once the sidewalls show signs of any cracking or deeper crazing even if there is plenty of tread.  Five years is considered the maximum life for a tyre by many people, others say seven, just a few say ten years is the maximum. Some manufacturers offer 'camping' tyres with a some extra resistance to weathering and 'set'. As ever though you don't get 'ought for nowt' so these gains may be offset by worse performance elsewhere.

Grip: Some of us are very keen on rural campsites and not necessarily in sunny Provence or Spain.  Anyone who has had to get off a field in typical UK weather on standard commercial tyres will have discovered that most tyres are definitely designed for tarmac!  By the way, a little tip for front wheel drive on any tyres when first starting off from grass is to reverse out of your overnight tyre indentations immediately before pulling off normally, this makes use of the initial weight transfer to give maximum grip to the driving wheels to get out of those little hollows under the wheels. 

Even on tarmac not all tyres are created equal, have a look at this tyre test report where under wet braking the worst tyre was still travelling at nearly 30 mph when the best tyre had stopped!  Winter tyres sometimes also known as M+S have chunkier treads and may help on grass but don't abuse them when starting off because if you cause wheelspin at the outset and fill the tread with mud they're then little better than any other!  They often have improved cold weather performance as well as chunkier treads.

Off-Road: Van tyres designed for off-road use are very rare indeed but you can try to choose those with a chunky tread pattern such as on winter tyres or possibly look at '4x4' tyres if you have a lighter campervan.  These are designated as for on-road or off-road or mixed use. Some manufacturers define the bias between the two usually as a percentage, e.g. "On-Off 60-40". True off-road tyres have very low speed ratings and make a lot of noise on tarmac, the best ones for campers are in the 80/20 to 50/50 range but do check their load carrying capacity against your individual axle loads. Few are available in 15", rather more in 16" sizes.

Speed: All tyres have their maximum sustained speed marked on the tyre with a letter code.  Some off road tyres are not intended for regular tarmac use and a few have very low maximum speed ratings.  Speed ratings are also an indication of how much power and punishment will be reliably handled by the tyre without excessive heat build up.  Rating 'Q' is probably as low as you'll want to go and should be able to handle the power from larger turbo diesels like Fiat's latest 2.8s, bearing in mind the fairly modest driving style of motorcaravanners. Rating 'R' would be better and isn't too hard to find.

Markings: All tyres are obviously marked with their section width, aspect ratio, type and rim diameter, so a 215/70R15C is a European tyre 215 mm wide, 70% as high in section as it is wide, of radial construction and fitting a rim 15" in diameter, it's also a Commercial grade tyre. This is a typical Boxer/new Ducato tyre. Tyres have a lot of other information on them as well - a numeric load index, the maximum allowable pressure, load carrying ability at that pressure, maximum speed rating and date of manufacture. Commercial tyres usually have a prominent 'C' or 'LT' in their description but some semi-commercial tyres might only have '6 ply rating' marked on them somewhere.  Be careful about reading the figures moulded into tyres, some are in imperial and some in metric - don't confuse the two!

This pdf from tyresafe.org is a very useful guide to 'reading' a tyre - click to open in a new window.

Practicalities: Have a look at these tables for more specific information...

 

Load Indices

 

Speed Indices

Li 

Kg 

Li 

Kg 

Code 

max kph 

max mph

90

600

105

925

N

140

87

91

615

106

950

P

150

93

92

630

107

975

Q

160

99

93

650

108

1000

R

170

106

94

670

109

1030

S

180

112

95

690

110

1060

T

190

118

96

710

111

1090

U

210

130

97

730

112

1120

V

240

149

98

750

113

1150

W

270

168

99

775

114

1180

Y

300

186

100

800

115

1215

 

101

825

116

1250

102

850

117

1285

103

875

118

1320

104

900

119

1360

 

 

 
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