Wheels and Tyres - Your Options

Details

Written by B S Fine

Other than for serious competition use, Magnette owners tend to stick with the original wheels, so there is not as much discussion about alternatives as there is with models like MGA or MGB. However, there are certainly examples of cars fitted with wire wheels, which give the car a sporty look.

The original wheels are not without their problems. As is revealed by Confidential Service Bulletin MG218, the factory modified the design from car no. 25759 because of cracking problems and even the modified design is not immune from this problem in heavy or competition use. It is worth carrying out a routine examination when the wheels are off. The area prone to cracking is in the central flat zone between and round the stud holes.

 After Magnette production had ceased, the design was changed again for the MGA 1600, probably to ensure that there was adequate space to accommodate the disc brake calipers introduced with that model and to allow cooling air to circulate. It is possible that these wheels may have found their way onto Magnettes. The visible difference is that the older wheel has flat areas between the ventilation slots inside the rim. The 1600 wheel has no flats and is dished right up to the inside of the rim, with narrower ventilation slots.

The wheels are nominal 15” diameter and this is measured across the bottom of the bead, not the edge of the rim. The rim width is 4” measured between the insides of the rim. The PCD or Pitch Circle Diameter is the technical specification for the spacing of the wheel studs. In the case of the Magnette and a lot of other cars it is 4½”. (With four studs it is expressed as 4 x 4½”)  The stud thread is ½ “UNF and the wheel nuts have a conical inner face to locate the wheel.  This configuration was common to cars of different manufacture, including Triumph TR series and  Saabs up to about 1993, both of which have provided wheels for Magnettes, especially in competition use.

A further important dimension is the “offset”, especially if an owner is looking at fitting alternative wheels. Offset defines the position of the centre-line of the rim cross-section in relation to the fitting face at the wheel centre. If you look at a Magnette wheel, the centre section carrying the stud holes is visibly closer to the outside of the rim than the inside. It is, in fact, about 35mm closer and this is the offset. This is quite a high figure for a car with rear-wheel drive. Front-wheel drive cars need more offset because the drive shaft and hubs take up more space in the wheel-arch, so to get the wheel running centrally in the arch it has to wrap round the hub rather than sit flat on the outside of it. It is a common error, when choosing different wheels, to ignore the offset and simply concentrate on the radius and the PCD. This means that wheels with minimal offset will protrude beyond the wheel-arch and thus be illegal as well as ugly.

The original tyre size was the cross-ply 5.60 x 15 but 5.90 was also a contemporary  option. Tyre widths (i.e. 5.60) are normally an inch or more wider  than the rim size because the tyre shape extends outwards from the rim.  Modern radial tyre sizes are a bit confusing because they include metric and imperial dimensions. They also include a dimension called the “aspect ratio”. This expresses the relationship between the height of the tyre (its size measured from the wheel rim to the edge of the tread). So a tyre defined as 165/R15 is effectively the same as 165/80 R15. It is 165mm wide and the height is 132mm because the “80” shows that the height is 80% of the width. The R15 shows that the radius, and therefore the intended wheel size, is 15”. Cross-ply tyres generally had a 100% aspect ratio because their cross section was effectively circular. Now, of course, there is a wide range of tyres including low profile tyres that have a very low aspect ratio. Modern aspect ratios do not generally exceed 80% and where no aspect ratio is given in the size, this is what it can generally be assumed to be.

Because old steel wheels are not necessarily air-tight, it is common practice now to fit tyres with an inner tube. Because modern tyres are not really designed to take tubes, check that there is nothing on the inside of the tyre that would chafe the tube. The tube must be correctly sized for the tyre. A tube that is too big and not fully stretched is a risk.  

Modern tyre sizes also include a speed rating to show the maximum speed at which the tyre will safely operate. This is a letter ranging from N to Z, with H also confusingly used. N represents 87mph and Z is 150+mph. You need to ensure that your proposed tyres are specified for the speed at which they are likely to be used. Most tyre manufacturers’ and multi-branch retailers’ websites carry an explanation. In practice, tyre ranges do not seek to cover all sizes and specifications because the range is generally designed to suit a specific model or car type, so the necessary speed ratings are a known quantity.

The 165/R15 is used as the example above because it is the size most often used on the Magnette now and because it was used on the VW Beetle range is generally obtainable, though often only to order. However, other sizes are possible, albeit limited by the narrow wheel size originally fitted to the car.

Owners who like to preserve the appearance of period tyres are finding it harder and harder to source tyres with older-style tread patterns. Price is also an issue so an internet search is often the best answer. Judging by threads on the MGA bulletin board, Kumho and Vredestein ranges are yielding reasonably-priced rubber that does not look too out of place.

When choosing an alternative set-up, it is important to retain as much as possible the original over-all radius of wheel and tyre, so that performance and speedo reading are not adversely affected. If you stay with 15” wheels, wider tyres and different aspect ratios will affect the radius. There is an excellent website at www.carbibles.com/tyre_bible.html that sets all the necessary information out in great detail and includes calculators that will work out how the proposed tyre size will compare with the original. Speedo readings are never totally accurate so a variation of, say, 5% is probably not worth worrying about but it is better that it should overstate your speed than understate it. Take care that, when selecting wider tyres, they will operate within the wheel arch without rubbing on the bodywork at full lock or with body roll on cornering.

A known alternative wheel that does not too adversely affect the appearance of the car is the steel wheel used on the Triumph TR3 – TR4A. It is available in a 4½” width that will accommodate wider tyres more comfortably but it is similar in appearance to the Magnette original, being perforated with ventilation holes like the Magnette’s. Certain early Saab models also carried steel wheels that are similar enough to the Magnette’s to make them a stronger alternative for competition use where the risk of stress cracking is heightened. If you choose to fit a wire-wheel conversion (as sold for the 1500cc MGA) this will open up other options. With wire wheels you need to ensure that the spline centre of the wheel matches the hub adaptor. Different numbers of splines are used. Take care, also if buying second-hand wire wheels because splines wear with use and you may find that the wheels are a loose fit on the stub-axle.

Alloy wheels are a less popular choice for Magnettes because, with their origins well into the 60s, they are not in keeping with the age of the model and look out of place. However, for specialist use, alloys suitable for the MGA will generally fit the Z Magnette. Pre-1993, Saab in particular used a range of alloy wheels that will fit, including Minilite replicas.