Motorhome Stuff - Tyres

By Ian Stewart

Understanding the sidewall markings

Using the illustration above . . .

1. The first three digits [165] give the tread width (in millimeters).
2. The next two digits [70] show the profile.
   (This is the height of the sidewall shown as a percentage of the width).
3. The [R] indicates it's a Radial tyre.
4. The [13] is the diameter of the wheel rim (in inches) the tyre has been manufactured to fit.
5. Then there is a SPACE
6. The two digit Load Index number [79] determines what payload (in Kg) each tyre can carry.
   (The table below shows the equivalent load values from 76 to 126)
7. The letter [T] shows the maximum permissable road speed when operating at full load.
   (The table below shows the equivalent ratings from J to H)

Tubeless tyre valves

These are generally made from rubber with a central core of metal containing the components required for inflating / deflating the tyre and providing the air seal. The standard snap-in rubber valve has a maximum working pressure 4.5 bar (65 psi).

Where pressures exceeding this figure, a steel valve should be used.

A rubber tubeless valve will, over a period of time, deteriorate due to several factors.

1. Natural ageing and the effects of hydrocarbons
2. The effects of centrifugal force
3. Temperature changes (due to their close proximity to brake discs or drums)
4. Poor weather conditions and the effects of road salt

It is therefore usually recommended that whenever new tyres are fitted a new tubeless valve also be fitted. It is also recommended that, whenever a tyre is removed from the rim (puncture repair etc), that a new valve be fitted.

Tyre Pressures

In Regulation 27 (section b) of The Road Vehicles (Construction and Use) Regulations 1986, it states 'the tyre is not so inflated as to make it unfit for the use to which the motor vehicle or trailer is put'. However this statement is fairly ambiguous as there is no indication of at what point an under or over inflated tyre becomes unfit for the use to which it is being put.

Most motorhomes have a plate attached (often on the door pillar) indicating the recommended tyre pressures. However, and especially with the dedicated camper tyres produced by companies such as Michelin (XC Camping and Agilis 101) and Continental (Vanco and Vanco Four Seasons), these are often too high for normal usage and tend to give a harsh ride.

If stopped, the Police will almost certainly go by what's on the plate unless you can provide something different. So if you want to run with reduced pressures and get a more comfortable ride without a roadside argument with the Constabulary . . .

Take the vehicle (loaded normally) to a weighbridge to ascertain the front and rear axle loading. Contact the tyre manufacturer requesting their advice on the correct pressures.

Contact for Michelin	Contact for Continental	Contact for Hankook

Include as much relevent information as possible in your request.
What's shown below is probably the minimum required.

I have [Michelin / Continental / Hankook] tyres fitted on my [Year, Make and Model] motorhome which is plated at a GVW of [*,***] Kg.
Tyre Type . . . .
Tyre Size . . . .
My normal operating weights are :
Front Axle . . . . Kg
Rear Axle . . . . Kg.
Could you please advise of the correct tyre pressures for use under these conditions ?

Print off and carry the answer in the vehicle (along with the weighbridge certificate). At least you'll then be in the position of showing that, by contacting the tyre manufacturer, you have done your best to establish the optimum pressures for your vehicle.

If you've never heard the name, at this point you may be asking, who are Hankook ?
16th March 2010 : Hankook Radial RA14
Promobil magazine names Hankook light truck tyre as test winner.

'Load Index' Table

Load Index	Load in Kg	Load Index	Load in Kg	Load Index	Load in Kg	Load Index	Load in Kg
76	400	89	580	102	850	115	1215
77	400	90	600	103	875	116	1250
78	425	91	615	104	900	117	1285
79	437	92	630	105	925	118	1320
80	450	93	650	106	950	119	1360
81	462	94	670	107	975	120	1400
82	475	95	690	108	1000	121	1450
83	487	96	710	109	1030	122	1500
84	500	97	730	110	1060	123	1550
85	515	98	750	111	1090	124	1600
86	530	99	775	112	1120	125	1650
87	545	100	800	113	1150	126	1700
88	560	101	825	114	1180

'Speed Rating' Table

Truck	J	62 mph	K	68 mph	L	75 mph	M	80 mph
Van	N	87 mph	P	93 mph
Winter / Reinforced	Q	99 mph	R	105 mph
Car	S	112 mph	T	118 mph	U	124 mph	H	130 mph