Tech Talk: Caravan/Camper Trailer Wheel Size

CARAVAN/CAMPER TRAILER WHEEL SIZE: DOES IT MATTER?

Wheels and tyres do not necessarily have to be matched to the tow vehicle, and in fact would be illegal in some cases if they were. This is because there are certain load and speed rating requirements that must be adhered to.

Tyres are designed to take a certain load, expressed as a load rating in three digits, and are capable of driving at a certain maximum speed, and so have a speed rating expressed with a letter. These are identified on the sidewall of the tyre. So ‘110T’, for example, can be broken down into the load rating (110 equals 1060kg) and a speed rating (T, which equals 190km/h).

When replacing tyres, check that the load rating meets or exceeds the rating requirement of the car and van. Your vehicle must have a tyre placard (or sticker) showing what size wheels and tyres should be fitted, the tyres’ minimum speed and load ratings and appropriate pressures as an Australian Design Rules (ADR) requirement.

The ADRs do permit lower speed-rated tyres, though, as it states, “When fitted with winter tread tyres, this vehicle should not be driven at speeds in excess of the tyre speed category as moulded on the tyre, or in the absence thereof, at high speeds.”

While some States and Territories, such as the NT, are in accordance with the national ADRs, others are not, and it’s up to the States and Territories to administer vehicle standards. So check first with your State or Territory registration authority before varying tyre size or speed rating from those recommended on the placard. Don’t fit tyres with a lower load rating than recommended.

Where cars and caravans are concerned, they can carry drastically different loads, and their tyres are designed for those loads only. Generally speaking, ‘Passenger’ rated car tyres are more lightly constructed than the ‘Light Truck’ tyres on caravans and can sustain higher speeds but are not designed to carry heavy loads.

P-rated radial tyres fitted to cars are made with parallel fabric casing cords, which run from one side of the tyre to the other, so they’re more flexible when cornering and ensure the tread stays in contact with the road surface.

Radials also have layers of steel fabric running around the inner circumference of the tyre under the tread. These steel belts ensure less tread movement when braking/accelerating and the flexible sidewalls allow even tread contact during cornering.

Light Truck tyres have more application of steel cords and often a nylon cord, providing better stake resistance offroad and higher load-carrying capacity, but have a much lower speed rating.

When you’re talking about a large 4WD wagon or a ute, it’s a different scenario altogether. As a caravan wheel bears a similar load to the 4WD or ute wheel, load ratings are similar. Many 4WDs are fitted, or can be fitted, with LT tyres, and utes are fitted with them to suit their commercial load-carrying application.

In these instances, and especially where camper trailers or offroad caravans are towed to remote locations, it suddenly makes a whole lot of sense to match wheels and tyres with the whole rig so that in an emergency the spare wheels can be shared. Some caravan manufacturers and many camper trailer builders offer a variety of different stud patterns so you can match your trailer’s wheels to your vehicle’s.

If this is the case, check that the pitch circle diameter is correct, that the wheel fits snugly over the hub and that the wheels’ offsets are the same. You might find that the spare wheel from your van bolts up perfectly to your ute, but thanks to too much positive offset, the tyre rubs the guard. This measurement is usually stamped on the inner rim. If in doubt, see your nearest tyre retailer to check. – PL

The vehicle tyre pressures should be set according to the factory recommendation found on the tyre placard, usually located in the glovebox, door aperture or fuel filler flap. If the manufacturer specifies a higher, heavy-load pressure setting, use this when towing. Car manufacturers spend a lot of time making sure this crucial figure is correct, so stick by it.

A caravan manufacturer may or may not make a tyre pressure recommendation. If in doubt about the van pressures, ask your selling dealer.

A good way of double-checking that your pressures are correct for your application is the 4psi rule, recommended by a Pirelli tyre engineer as a good rule-of-thumb for accurate, appropriate tyre pressures.

Check tyres cold and then straight after a decent highway run you should see a 4psi increase. If it is less, you have set your cold tyre pressures too high; if it is more, then you have set them too low. Adjust the cold pressures accordingly and check the results when hot again and you should have a 4psi difference.

I have mentioned this before, but don’t let tyres get too old. It is common to see an older van with its original tyres on it, with plenty of tread left. This can be an accident waiting to happen. Tyres deteriorate over time, and once they are six or seven years old, they have reached the end of their service life, no matter how much tread they have left. The rubber goes hard, and when heated to operating temperature they can simply blow apart.

Since about 1990, tyre manufactures date-stamp their tyres. Look for a four digit figure like ‘3006’ stamped into the sidewall within an indented border (see illustration). The first two digits represent the week of the year and the last two digits the year itself. So in our example, the tyre was made in the 30th week of 2006.

If you have tyres that are seven years old or less and are showing signs of cracking or other damage, replace them immediately.

Look after the wheels and tyres on your vehicle and van and they’ll look after you! – PL

ROADSIDE ASSIST SIZE INCREASE

Thanks to Don Creed of Frenchs Forest, NSW, who wrote regarding inadequate breakdown coverage due to weight and length limitations (Tech Talk, CW July 2007). He has noted an article in the May/June edition of Open Road which states that the NRMA Premium Care package has increased the towing limit for vehicles from 2.5 tonnes to 3.5 tonnes and that they will now tow caravans of any length.

Regarding brake controllers (Tech Talk, July 2007), Ellen Chasemore of Lilydale, Vic, also had a major problem with an AU Falcon with lights coming on. She emailed Tekonsha (see www.tekonsha.com) and received a response within 24 hours explaining that they had a lot of problems. She was told to take the unit to a mechanic quoting the name and address, phone number, and that the unit would be exchanged and fitted no charge. She has had no problems since.

TEKONSHA BRAKE CONTROLLER UPDATE

WEIGHTS AND MEASURES

Later, under TBM (towball mass) you say that if you add the GTM to TBM, you should have the ATM (aggregate trailer mass). I think that this calculation is a little more complicated than that.

In Tom Olthoff’s articles published in CW in 2002 he says that the WDH throws around 27 per cent of the TBM on to the wheels of the trailer.

So if you arrive at, as I think most people would, the GTM over a weighbridge with the trailer attached to a vehicle and a WDH fitted, then the GTM includes 27 per cent of the TBM.

To proceed to add the GTM to the TBM to arrive at the ATM would then seem to be incorrect

Geoffrey Wilson

Quite right, Geoffrey, you should disconnect the WDH if you wish to get accurate static mass figures. – PL

HITCHES AND TOWING ALTERNATIVES

We recently purchased a Jayco Feather and we are towing it with a Camry Altise 3L V6 automatic. The van has a Tare weight of 1008kg, GTM of 1210kg, ATM of 1308kg, and an axle group loading of 1450kg.

On our first long trip, I visited the weighbridge fully loaded and with the tank full of water. I calculated that the ATM was 1300kg; the GTM 1190kg with a ball weight of 110kg, which is well within the Toyota specifications.

The Jayco dealer fitted a four-bar Camec weight-distribution hitch and a Tekonsha Voyager brake controller.

Is this hitch suitable for the above weights in the context that the Feather is the smallest Jayco pop-top?

My main query, though, relates to choosing an alternative towing vehicle. We want to access national parks and while the Camry is a great car, it is a bit low at the front (115mm) and so we are looking at alternative tow vehicles with more ground clearance so we can travel along dirt roads.

We have no desire to go offroad and so we have naturally favoured AWD vehicles such as the Nissan X-Trail and Toyota RAV4. These vehicles are rated to tow up to 1500kg and our van sits in this range.

The problem is that the advice I have had on the ground is that I need more power than these vehicles can provide for effective and safe towing. If this is the case, we need to spend more money on a considerably bigger vehicle such as a Ford Territory or Hyundai Santa Fe. While our general lifestyle favours a smaller vehicle, we are planning some longer trips and we obviously need a tow vehicle that can do the job.

I have decided on automatic transmission.

Both Toyota and Nissan dealers say that a transmission oil cooler is not necessary but the blokes I talk to say it is. Do you have a view on this?

John Oakley

As I have not towed a Camry and Jayco Feather, I can’t comment on that particular setup’s stability, but I would certainly continue using the weight distribution hitch. The Camec four-rod equaliser you have is designed for the 70kg to 125kg towball download range, so you have been supplied with the correct levelling device to use for your load.

I assume by the vehicles you mention that you have around $30k to $40k to spend on a small or medium wagon with good ground clearance that won’t struggle when faced with towing a 1300kg van. While I think that the RAV4 and, in particular, the X-Trail (which is actually rated to 2000kg braked) will do the job, there are a few other choices that may better suit you.

For a more nimble, car-like all-wheel drive than the X-Trail and RAV4, and one that has sufficient power, I would recommend a Subaru Outback 2.5i ($38,990), which has a 1500kg braked trailer capacity.

Two slightly bigger 4WDs, offering turbodiesel fuel economy with stronger mid-range performance (and each with 2000kg braked towing capacity) are the Hyundai Santa Fe SX CRDi ($40,490), or for slightly less money and with less torque and overall refinement, a Holden Captiva Diesel SX auto ($36,990).

One vehicle you should not ignore and just over your theoretical price range is the Hyundai Terracan at $40,590. It is a bargain-priced, medium-size genuine offroader with 2500kg tow capacity. It has excellent fuel economy, good performance and plenty of room while still being relatively nimble in the city.

While it will take you over rough terrain more easily than the others here, you may find that it’s too cumbersome for your liking, especially if you have come from a Camry and expect car-like levels of dynamics.

As for a transmission oil cooler, it does depend on the manufacturer’s recommendation. You don’t want to make a mistake with this – don’t rely on the memory of a car salesman who has to keep plenty of facts and figures in his or her head. Go straight to the manufacturer’s website, or phone direct, to see exactly what they recommend.

If the recommendation is for no cooler, then I would still keep a close eye on transmission fluid level, colour and smell. If it begins to discolour and smell burnt, it is an indication that an overheating problem may exist. – PL

ON THE LEVEL

I understand and accept that caravans need to be reasonably level when on site for any length of time for a variety of reasons. I do wonder how level they need to be, though. You would think the more level the better, but would within about 5° be adequate most of the time?

One set of levelling ramps and chocks I looked at raised the issue of how these things work with a double-axle rig as it appeared it would be difficult to fit a set of ramps in between the two wheels to be able to drive up on to them. There was another set of which you would have to buy at least four to span the distance between both wheels to work.

I got into quite a discussion with the young sales guy about various jacks that would take half the weight of a van for any extended period of time and he seemed concerned about the safety issue of having a van jacked up for more than the time it takes to change a wheel.

My question is this: is there a permanent support system capable of taking half the van’s weight while people move around inside, for days or weeks at a time? Aren’t the corner stays just for stability rather than long term weight bearing?

A simple wind-down locking mechanism from a reinforced point on the chassis near the wheels would surely be a simple thing for the manufacturers to provide, and would do away with the need for any complicated and potentially dangerous misuse of ramps, blocks, etc.

I can’t be the first bloke who’s run into this problem.

Joe Smith

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You seem to be finding a few problems that I didn’t know existed! We have never found levelling a van to be much of a problem as most sites are pretty level anyway. We have had a single-axle van for the past 12 years or so and raising one wheel using a ramp is no trouble.

I seem to remember that when we had a tandem van we just used a ramp under one wheel and the load-sharing suspension took care of the other wheel. It is possible to shorten one of the plastic ramps so it will fit between the wheels if the suspension is not load-sharing.

You are right about the corner steadies – they are not designed to take a huge weight. We have a hydraulic system and that’s much more robust.

If the site is so non-level that you have to jack up the van then I suggest you move to somewhere more level.

Quite often when on a daily moving schedule we don’t even bother putting the steadies down – a bit of movement isn’t a big deal.

A Trail-a-Mate jockey wheel/jack is a good investment and can in fact lift one side’s wheels off the ground to enable you to put your ramps or blocks under the wheels if this becomes necessary. Having the fridge level is important if you are running it on gas. – LM

LONG OVERHANG

We have a 7.5m (24ft 7in) caravan and are travelling in it full time. Our problem is that we can’t get in and out of many garages as the back of the caravan hits and drags. It even does it on some caravan parks and streets that have drain ditches across them.

Our van is two years old and has dual wheels and independent wishbone suspension. We have been told that we can lift it by 4in to make this problem go away. How does this, or would this, affect the towing of the van?

We pull it with a Toyota LandCruiser turbodiesel trayback. Also, would it affect our insurance cover? We were thinking of bigger wheels but there is not much room around the wheels now. 

John & Vi Oversby

This is a common problem: many of the longer vans have a considerable overhang, and there is no simple solution to changing the departure angle. A 4in lift may well improve it, but will it be enough? You should check with your State registration authority to confirm the current requirements regarding the lift’s legality.

Raising the caravan chassis will increase the centre of gravity, which may alter the van’s towing dynamics for the worse.

Increasing wheel diameter, particularly if you have little room in the wheel wells anyway, will be of negligible benefit. – PL

DIGITAL TV

At home we have found that an average picture can be made almost perfect by using a digital set-top box with a standard TV set, but these only work when a digital signal is available.

Do you have any experience with a digital set-top box in a caravan?

Is digital transmission generally available or is it restricted mainly to capital cities? These set-top boxes can be purchased for not much more than $50 now and could make life a lot easier.

Ken Hammond

I have not used the set-top box while travelling, but Dave Barnes from Barnes Caravans says that his customers report good results by using them in their vans. Digital reception is generally better than analogue – but its signal also drops off far more sharply than analogue, so in some areas you may find no digital reception but at least a grainy picture with analogue. The beauty of a set-top box on an analogue TV is that you can use both according to reception conditions. – PL

INSURANCE IN THE UK

My wife and I are going to the UK for six to 12 months and we are going to buy a motorhome. Two years ago we did the same and had to insure it with an insurer Down Under and we paid an exorbitant price.

Are you aware of any insurers in the UK that will insure us? Last time we ended up paying about four times what it would have cost a resident. We have British passports, British licences, have lived in the UK and have a good vehicle insurance history.

Simon Wilson

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Have you tried The Caravan Club in the UK? You would probably have to become a member but I believe they have very competitive rates for insurance for all types of RV including motorhomes. The web address is www.caravanclub.co.uk – LM

BEARINGS CHANGE

In the May issue of CW there was an article about the Durahub oil bath system for wheel bearings.

My question is: What happens if you have to remove your combined hubs or replace bearings?

Are you able to remove and replace the Durahub?

Mike Jones

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The simple answer is yes! According to Durahub’s Andy Chan, it’s a simple matter of lightly tapping the hub around the sides with a block of wood.

The Loctite 515 is more of a sealant than a glue and can be worked loose.

See the Durahub website www.durahub.com.au for more information. – MS