Wheels & Tyres
15 inch diameter rim disc with 5 stud fixings or optional 15 inch centre lock 72 spoke wire wheels (painted or chrome) with 165 – 15in, HR rated, Dunlop SP41 radial ply tyres (with inner tubes) and two-eared-winged knock-ons (5J x 15).
For the 1969 models, Dunlop SP68 radial ply(tubed) tyres were used (from Car No. 4236 Roadster and Car No. 4266 GT). Whilst rostyle wheels were an option according to BMC literature, no customer seems to have taken that option. Chrome wire wheels are marginally heavier than painted.
The disc wheels are different to those used on the MGB both in terms of size as well as having 10 rectangular ventilation holes, hubcaps with a characteristic beveled edge and five stud fixing. Wire wheels were far more common than disc wheels and again bigger than the MGBs wires came with 72 spokes.
Both types (disc and wire) were painted in a BMC colour called Aluminium with an option for chrome plating. The knock on ears and octagonal nuts for the wire wheels are the same as those used on the MGB.
Many suppliers offer wire wheels that fit the MGC including 5 x 15”, 5.5 x 15 and 5.5J x 15 (the J referring to a deeper rim distance to the edge of the spokes). Tyre options include many different manufacturers in sizes such as 185/15 or 195/15. Personally I prefer the look of 185s.
Knock on Ears and Octagonal Nuts
Wire wheels are retained by two chrome plated knock-On’s with the words “Right Side” and “Left side” and the word “Undo”, together with an arrow. Although available currently, the originals never had the MG logo on them.
Octagonal nuts were used where wing nut Knock-On’s were illegal. This included Germany and Switzerland, and all states in the US from Feb 1967.
Threads per inch (TPI)
In 1964, all wire wheels in the MG range changed (from 12TPI) to 8TPI.
8 TPI with a 42mm fitment, right hand and left hand are obviously different parts (due to opposite turning threads on the knock-on’s). And they are also different from hubs used for wire wheels on the MGB.
First thing to say is that if the splines are worn, both the wheel hub and the hub adaptor must be replaced. If only one of them is replaced, the old splines will quickly wear down the new ones. So new wire wheels will be ruined by the worn splines on an old hub and vice versa.
How to tell if your splines are worn? The sketch below is the traditional way to assess worn splines visually. More info on the Moss website here and below.
Wire wheels require periodic maintenance, so checking the hubs and wheels should be a standard procedure. The easiest parts to inspect are the spokes.
· Removing wheels – Sometimes the wheels are very difficult to remove from the splines. Most of the time, this is due to the grease having dried to a hard binding consistency which ‘freezes’ the wheel’s splines to the splines on the hub. I would not recommend using heat, nor hammers nor driving with loose knock-ons. One recommendation is to try a spray carburettor cleaner to dissolve the hardened grease (jacking car and spraying onto the ends of the splines, leaving for a few minutes and moving the wheel by hand to try to release the splines). You might want to protect the rubber on your tyres when trying this.
· Visual check — First look, then feel each wheel for broken or loose spokes. Spokes usually break at the nipple or at the hub, but rarely in the middle. The break is often hard to see if it’s behind the hub or within the nipple. Nonetheless, the spoke will be loose if broken.
· Bad Spoke? — Spokes need to be checked for looseness or being over tightened or broken. To help locate a bad spoke, take a small block of wood or even a hammer and lightly tap each spoke. If it makes a “tinging” sound, it is tight. If not, it is loose. Hold off your temptation to tighten the loose spokes until you have finished your inspection.
· Loose / poorly tightened spoke - A dull thud indicates looseness
· Over tightened - a twangy sound, higher pitched than the others, denotes an over tightened one.
· Trueness — Next, check the wheel for trueness. As the spokes stretch and get tightened, they can throw the wheel out of true, so the trick is to tighten them without messing up the true. This is best done by a professional or with the wheel mounted on a truing stand. How often to true? Some experts say that a new set of wheels should be tightened and re-trued at about 500 miles, then again every 20,000 miles. So wheels must be trued to eliminate wobble which is also known as run out. To check this:
· Jack up the car and secure it on stands.
· Find a suitable tool item such as another stand and hold a screwdriver or other pointed object right such as a pencil right next to the edge of the rim while slowly turning the wheel. It is also possible to use a dial test indicator (if you have one)
· If the wheel isn’t true, the gap between the tool and rim will change as the wheel is spun.
· If the wheel moves side to side, then lateral run out is present.
· If the rim moves up and down, then it is suffering from radial run out.
· If the wheel stays pretty true—1/8 inch or so of deviation—it’s probably going to be okay with a tune up but if more than this considerable work will be needed or replacement.
· Splines — Now comes the most important inspection, the splines. Remove the wheel from the car and clean away the grease from the splines to allow a good visual inspection. The splines should be slightly rounded at their peaks, not sharp. They should also be symmetrical.
· Splines should look like? — The inner 1/4 inch of the splines does not (or at least should not) contact the hub, so you can compare that portion to the rest of the spline surface during your inspection. Clean the hubs and inspect them the same way you inspect the wheel. Remember, as I say above, that worn splines are dangerous, so this is an important inspection.
· Worn splines — If the splines on either the wheel or axle hub are worn, the only solution is replacement. Further, at the risk of repeating myself, if you put a new wheel on a worn hub, or vice versa, the new component will quickly wear out. So the best thing to do with worn splines is new wheels and hubs.
· Grease applied to the splines — Both on the splines on each wheel and those on the hub. Various greases are mentioned including high silicon content greases or marine waterproof type. The reason for the grease is to prevent water getting in as well as dirt and the amount of grease used should reflect that. When it comes to mounting the wheels on the car, it is often helpful to use silver or copper-colored anti-seize compound on the splines. The silver-colored anti-seize is the better choice because it matches the wheel color pretty well if it does manage to get flung out. Grease will work just fine, but is more likely to fling itself out through the nipple holes and get the wheel dirty. Putting silicone sealer on the inner nipples is a bad idea, as it will trap water and condensation inside the hub, which can cause big problems.
· Balancing—see box.
History of wire wheels
As a boy, I can recall the evocative sight of knock on hubs on classic wire wheeled British sports cars. Usually with 48, 60 or 72 spokes, wire wheels were not only visually appealing but because they were lighter than the steel disc wheels were at the time and were changeable via a single knock-off, they were used by racers. The extra cost and maintenance required for wire wheels is accepted by many owners as the price to pay for that pedigree.
It is of course possible to buy either painted or chrome wire wheels. Painted is more traditional but chrome is visually very attractive. One that is simply down to personal choice. It has been said that in the past, some wire wheels did have chromed steel spokes which lose strength over time due to the effect of hydrogen embrittlement caused by the plating process (not verified).
How a wire wheel takes the load
When a radial load is applied at the hub to a well tensioned wheel, it causes the wheel to flatten slightly near the ground contact area. The rest of the wheel remains approximately circular. The tension of all the spokes does not increase significantly. Instead, only the spokes directly under the hub have decreased tension.
Some authors conclude that the hub "stands" on those spokes immediately below it that experience a reduction in tension. Other authors conclude that the hub "hangs" from those spokes above it that have higher tension than the ones below it. My view is that it “hangs”; the weight of the car being transferred along the centre line of the hub, acting downwards from that point meaning that the top spokes are in tension; the car “hangs” from those top spokes.
Parts of a wire wheel
The single largest issue with wire wheels is the condition of the splined hub. The splines in the hub wear especially if the wheel is not tight enough. The wear on the wheel’s hub will wear the axle’s hub, which will further wear the wheel’s hub, creating a downward circle that eventually allows the axle’s hub to spin within the wheel. With the torque and power thru the rear wheels, it is important not to check just the front splines and conclude everything is fine.
The consequences of worn splines include the hub spinning the knock-on loose, the wheel coming off or even the wheel spinning independent of the hub which, given that the brake discs are attached to the axle hub means no brakes. As I mentioned elsewhere, damaged hubs should be replaced as there is no cost-effective way to repair them.
Apart from the fact that on a 72 spoke wheel, you have in excess of 140 parts (72 spokes, 72
nipples) the following are the main parts of a wire wheel:
· Parts of the wire wheel - The wire wheel itself is made up of:
· An outer rim (outer rims lose their trueness and get bent from being curbed or hit),
· the inner hub (inner hubs, just like axle hubs, are susceptible to spline wear),
· nipples (these are the small pieces at the outside of each spoke used to tighten the spokes to the wheel) and
· the spokes
· Axle Hub – this holds the wheel to the car. The hub is splined to accept the wheel hub as well as being threaded at the far (wheel) end to accept the knock-on. So the centre of the wheel mates with the splines on the axle hub and is held in place by a large nut, commonly called a knock-on.
· Different hubs, thread pattern and side of car - Each hub is handed / threaded for its particular side of the car and the front hubs are different from back hubs. It is very important to put the correct hub in the correct place:
· The left-side (near-side) hubs have a right-hand thread pattern
· The right-side hubs (off-side) use the reverse, left-hand thread pattern.
· Why different threads for each side of the car? – It would of course have been much cheaper from a manufacturer’s point of view to have had just one type of hub at the front (and one at the back). But there is a very good reason why the hubs are different on each side of the car; the hubs are handed and have different threads. This is to ensure that the knock-offs won’t loosen themselves whilst moving forward.
· It is all about the Hub - Everything revolves around the hub which has three basic components:
· the tapered surfaces that locates the wheel,
· the splines and
· the threads for the knock-on.
Rudge-Whitworth pattern of locking hub
Originally titled Principles of the Rudge-Whitworth Centre-Lock Wire Wheel, original printing in Issue #31 (1981) of The Vintage Triumph by F. Wilson McComb, submitted by Bill Sohl. Additional information added in Issue #51 (1992) of The Vintage Triumph by George Osmandson “What is equally widely misunderstood however, is the all important bit in the middle of the wheel. Let us take a closer look at this assembly, referring to the central portion of the wheel as the "wheel centre", which is fitted to the "hub" and fixed in place with a 'locking cap'. The first thing to be appreciated is that the wheel centre does not come into contact with the brake drum. There is, in fact, a clearance of about 1/8" when the wheel is fully home. It is the inner taper of the wheel which comes into contact with the back taper of the hub. Notice the taper which is formed on the outer surface of the wheel centre. This engages with yet another tapered surface formed on the inside of the locking cap. When the wheel is fitted to the hub, and the locking cap screwed on, it is therefore centralised and held between two pairs of tapers.
The only other contact between hub and wheel centre is provided by the splines, which carry the driving and braking forces. The locking thread, on the hub and cap, is right- handed on the left (near) side of the car and left-handed on the right (off) side. One of the endearing mysteries of the wire wheel is that the spokes are not, and indeed never can be, in compression. The weight of the stationary car is suspended from those spokes which are uppermost in the wheel when the wheel and locking cap are loosely fitted, therefore, the upper portion of the outer taper is pulled firmly into contact with that of the locking cap taper, and the lower portion of the locking cap thread is in contact with that of the hub.
A slight clearance then exists between the tapers at the bottom, and also between the threads at the top. As the car moves forward, a different portion of the wheel rim takes the weight, and relative movement occurs between wheel centre, locking cap and hub. The effect of this is to tighten the locking cap, and the locking action continues until there is firm contact between the tapers all round when it ceases.
The clearances involved are, of course, minute, but the locking action is nevertheless completely positive and entirely automatic. There are people who deny the very existence of the locking action, and presumably attribute the left and right hand threads to sheer cussedness on the part of the manufacturer. They are, no doubt, the people who bash their locking caps with heavy hammers.
The earliest instructions that I have been able to trace advise leaving the locking cap finger-tight, and no more. A later recommendation is to hammer the locking cap tight, check for slackness after twenty miles, and tighten again if necessary. "Hammer them tight" means the application of a lead, copper or hide mallet, and a little common sense, with the wheel locked up. Not a murderous attack with a blunt instrument when the wheel is on the jack. The tapers and splines must be kept scrupulously clean. As for checking the tightness occasionally, this is obviously a good idea. Most pre-war instruction manuals advise putting some oil in the groove of the locking cap. Opinions differ as to the advisability of oiling the back taper on the hub, but in my experience this gets oily anyway if the splines are lubricated. And lubricated the splines must be, for it they rust, the wheel can become quite literally immovable, which is awfully embarrassing when a puncture occurs.
What is the effect of over-tightening? We have seen that the wheel is held in place between two pairs of tapers, and does not touch the brake drum. Excessive tightening of the locking cap will, therefore, force the wheel centre farther on to the back taper of tie hub, expanding it and thus making it, eventually, a sloppy fit on the hub, The outer taper tends to be compressed, and the locking cap itself will actually expand to a small extent. This may cause the locking cap to contract the outer spokes or "bottom out" in the hub, in either case preventing proper tightening. An incidental calamity is that the inner spokes tend to be slackened and the outer ones over tensioned, thus pulling the wheel rim out of shape as well.
A sloppy wheel centre soon starts to "fret" on the hub. The splines wear rapidly, even the back taper begins to wear, and eventually the whole assembly - wheel, hub and locking cap - is fit for the scrap-yard. In advanced cases, the wheel may turn on the hub by half-a-spline, jamming behind the unworn portion of the splines, and becoming completely impossible to remove.”