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Motorwoonwa Klub Van Suid-Afrika

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MOTORHOMES AND TYRES

(Written by Richard Clark)

INTRODUCTION

Most common road vehicles comply with a factory specification, and thus have predictable tare and gross permissible masses that can be found in tables available from tyre retailers, or from the vehicle manufacturer. Choice of correct tyres for such vehicle should therefore not be a problem. Equally important is the provision of correct inflation pressures, front and back, for the loads that such tyres will be expected to bear on that particular vehicle.

Motorhomes, on the other hand, differ in that they are extensively and invariably modified and customised after manufacture, to the extent that it has been said that there is virtually no such thing as a "standard motorhome". Tyre loadings for any individual motorhome can therefore only be known by measurement on a weighbridge or load-palette and recording these readings. Front-end and rear-end measurements (there will be errors, if the vehicle is not level) as well as total vehicle mass, loaded as if the motorhome would be prepared for a tour or holiday, should be recorded. It is illegal for any vehicle to have a left-to-right-side mass that differs in excess of 10%. It is also illegal for any motorhome to be on the road with a loaded-up total mass that exceeds the plated GVM, or either of the plated permissible maximum "axle masses", front or back.

IMPORTANT CONSIDERATIONS

A tyre should have a compatible fit with the rim to be used. Complete tyre tables provide a list of the acceptable rims for each particular tyre.

Beware of the following, when contemplating the use of after-market rims.

  • The mount offset should be the same as the manufacturer's original rim
  • Do not use wheel spacers or the like to increase the width of the vehicle track. There will be adverse effects on steering geometry, that will lead to abnormal wear, prejudice safe control of the vehicle (especially with a blow-out, or heavy braking) besides damaging wheel-bearings because the tyre footprint is no longer vertically below the wheel bearing. Tyre footprint will also be wrongly aligned with respect to the king pin rotional axis on the front wheels.

Check wheel alignment, and look for evidence of suspension wear, if there is abnormal tyre wear, or poor steering characteristics.

When buying new tyres, check the manufacture date (digits 1 and 2 = week of manufacture; digits 3 and 4 = year of manufacture) as your guarantee expires the day a tyre reaches 5 years of age. You might be taking old stock from a supplier.

Always renew valves when renewing tyres.

Suburban tyre retailers often do not have equipment that can balance a truck tyre. On bigger vehicles travelling under 100 km/h, balancing is hardly necessary, unless you have the misfortune of having bought a tyre that is seriously out of balance.

Be aware of the risk of being sold tyres that are intended for light-vehicle use (cars, bakkies and 4 x 4s). This can often arise through the ignorance or indifference of the retailer, especially if wheel-load data from recently determined weighbridge readings of the motorhome are not available at the time. With rare exception, only commercially-rated bus/truck-type tyres, cross or radial type, steel or textile, are adequate for motorhomes. Using under-rated tyres puts life and property at risk, can lead to prosecution and prejudice your case in the event of an insurance claim. You could also be held liable by third party claimants after an accident.

Rotation of tyre positions, at intervals, will equalise tread wear. However, with twin wheels, always keep these pairs together, as it is essential that they retain equal circumferences, to avoid stresses associated with road "creep".

Check and adjust tyre pressures to the correct values before starting a long trip. As tyres heat during long-distance running, it is in order for the pressure to rise, provided that this does not exceed 20%. Do not bleed air from a hot tyre to "correct" the pressure. Slow leaks should always be investigated. A tyre should hold pressure for a month, without losing more than 20 kPa.

Overinflation of a tyre gives an uncomfortable ride, contributes to suspension wear, and compromises adhesion with the road, as the patch in contact with the road is reduced in area. The tread will also wear more rapidly down the centre-line. Over-inflation increases the possibility of tyre damage by impact with sharp obstacles in the road.

Underinflation compromises fuel economy, over-flexes the tyre, leading to overheating, damage of the tyre structure, and ultimate failure, often through catastrophic blow-out. The thicker the tyre, the greater the risk, hence the common occurrence of shredded lorry tyre rubber on the highway. The practice of softening tyres for a more comfortable ride on a rough or corrugated dirt road is most dangerous one. In addition, sharp stones can "bite" the folded side-wall of an underinflated tyre, through upward contact against the rim edge. Damage may not be immediately obvious, but can be instrumental to a disaster later on down the road.

Some motorhomers experience recurrent problems with tyres that shed their treads. This is invariably from overflexure, reasons for which have already described, leading to overheating within the tyre structure ("thermal run-away") which results in delamination (separation) of the tread from the tyre body. The tyre may appear perfect in exterior view, but may create strange road noises, especially at higher speed. By the time that exterior splits on the sidewall are visible, tyre structural degradation has reached the point where it is likely to disintegrate. This type of tyre failure arises from product abuse, though it is common for motorhomers to explain their misfortune as a consequence of being sold an inferior quality tyre.

Do not mix tyre types (radial and cross-ply, or textile with steel) particularly on the same axle. New tyres should be fitted on both sides, and if only to one axle, it should be to the rear. This is because worn tyres at the back will lead to vehicle spin in the event of severe braking.

Remember that it the tyres that stop a vehicle; not the brakes. Brakes only apply resistance to wheel rotation. Worn treads on wet roads have extremely poor adhesion, especially at speed. Tread depth of less than one millimetre on any part of a tread is illegal, and can invalidate an insurance claim in the event of an accident.

Cultivate the habit of checking all tyres visually at each stop on a long journey. Overheating is evident from excessive rubber odour, confirmed by cautious touching with the hand.

Punctures can only be repaired if the damage is in the tread area. If the problem is on the tyre wall, the tyre should be scrapped. Externally fitted quick-fix puncture plugs and liquid sealants offer assistance in an emergency, but are not regarded as safe permanent remedies. At the earliest opportunity, a punctured tyre should be removed from its rim and inspected carefully, inside and outside, for possible structural damage, before being repaired, if judged as repairable, using the proper equipment, methods and materials recognised by the industry.

TYRE LOADING TABLES

As has been implied in earlier comment, a tyre is in balanced equilibrium between the downward force of the vehicle mass on that wheel, and the upward force of the air under pressure within the tyre. The tyre is carefully designed to operate with a specific degree of deflection on the road, and for that reason, tyre inflation pressure and the measured load on the wheel must match one another in accordance with the data given by the manufacturer's tyre loading tables.

Data are extensive, making it possible to list only the more popular wheel sizes and mass loadings for motorhomes in the table below. More information should always be available from a tyre dealer, or by phone from the technical information department of any tyre manufacturer, if you need it. Don't buy tyres on "blind spec". Have your measured wheel loads at hand, as without this, you are relying on guesswork.

If a wheel is loaded to 90% of the maximum permissible load for a given tyre, although "legal", for reliability, it would be better to select a tyre of greater load rating. A particular "stress" for motorhome tyres is intermittent use, with extended periods of standing, during which time some of the pliability of the rubber is lost through the simple process of static ageing.

The table below is an extract from Table 24, SABS 1550-3:1992 (amended 1997) for single-mounted commercial radial ply tyres. Double-mounted tyres of the same type should be weight-derated by about 6% per tyre. Load in kg is printed in normal type for 6-ply tyres, and in bold type for 8-ply tyres. Column 1 gives tyre sizes. Row 1 gives tyre pressures. The rest of the table lists the correct single tyre load in kg for a particular tyre size and given inflation pressure.

 

250kPa

275kPa

300kPa

325kPa

350kPa

375kPa

400kPa

425kPa

450kPa

145R13C

400

425

445

465

487

505

525

540

560

155R13C

425

445

470

495

515

       

165R13C

485

515

540

565

590

615

630

650

670

175R13C

530

560

590

615

645

670

690

710

730

185R13C

575

610

640

670

700

730

750

775

800

155R14C

460

485

510

535

560

       

165R14C

515

540

570

600

625

650

680

705

730

175R14C

560

590

625

655

680

710

730

750

775

185R14C

610

645

680

715

745

775

800

825

850

195R14C

670

710

745

780

815

850

885

920

950

205R14C

730

770

810

850

890

925

960

995

1030

215R14C

790

835

880

920

960

1000

1045

1085

1120

145R15C

435

460

485

510

530

555

575

595

615

165R15C

530

560

590

615

645

670

685

705

730

185R15C

630

665

700

735

770

800

820

845

875

215R15C

745

805

860

920

975

1030

     

175R16C

590

625

660

690

720

750

775

800

825

185R16C

650

690

725

760

790

825

845

870

900

195R16C

710

750

790

830

865

900

925

950

975

205R16C

750

790

835

875

910

950

990

1025

1060

215R16C

835

885

930

975

1020

1060

1085

1115

1150

 

250kPa

275kPa

300kPa

325kPa

350kPa

375kPa

400kPa

425kPa

450kPa



While every attempt is made to provide reliable information, errors may inadvertently be present. It remains the responsibility of the reader to verify the correctness of any part of this document by consulting the original source of data quoted.

 

 

 

Name: Albert Theron

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Hallo motorhome vriende. Ek beplan 'n toer na Etosha en Botswana Mei maand. Graag verneem ek van julle wat onlangs in Botswana was van moets en moenies en goeie kampplekke wat darem redelik bekostigbaar is. Die eerste stop vanaf Namibie sal Ngepi kamp naby Divundu wees. Wat om daar te sien en doen? Daarvandaan na Kongola en dan regs af en om na Kasane. Na Kasane ens. af na Nata en dan Maun .Ek sal enige aanbevelings baie waardeer aangesien dit my eerste besoek aan Botswana sal wees. Motorhome groete

Name: David Shreeve

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Message:

We are planning a trip down the Wild Coast from Port St Johns to East London in our Huyndai 2.5l Diesel Motorhome. The plan is to visit as many of the resorts/sites/beauty-spots as possible. I would appreciate any hints and advice. Has anyone done this trip recently? Will our motorhome rattle to pieces? What about security in the camps?

Name: Peter Lyne 

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Message: 
Does anyone know where I can buy an A Frame to tow a Jimmy? I live in Durban.

Thank you, Peter