Wheels, the inside story

What’s the most important part of your car? The one parts that connects everything else to the planet Earth? It’s those four circles at the corners. Tyres are the most critical part, but wheel come a very close second, upgrading them is one of the top modifications for most car enthusiasts, but the scary thing is that there are no regulations controlling the quality of the wheels on your car. Read on and be prepared to clench.

Anyone can make wheels, in the past unscrupulous thieves have set up shop melting down old coke cans and casting poor quality wheels for knock down prices. And whilst they looked ok when new, they were full of faults and broke up when cornering hard or at high speed.
Trading standards in this country has driven that sort of malarkey out, but now there is a fresh threat coming from far distant shores where the relaxed standards allow down right dangerous practices.
But its not just badly made rims that can ruin your day, we also face the threat of badly repaired or damaged rims, which can have faults hidden deep within the metal ready to pounce on the un-initiated.
So what ever you buy, look carefully at the details and ask the right questions. Here’s a brief outline of the key things to check out.

Lip damage.
Have a close look at the edge of the rim, this is where damage from bad parking near kerbs scuffs up the metal. Light scuffing will be taken in the wheels stride, but if there are chunks taken out then they can lead to cracks forming, which slowly eat away at the metal. Often this doesn’t result in a problem, but once in a while the cracks join up and a small chunk of the wheel comes away, this happens very suddenly which causes the tyre to deflate instantly. This is called a blow out, and unfortunately this failure is most likely when the wheel is working hardest; when the car is going fast or cornering hard, so most times it results in a catastrophic loss of control; crash, bang, dead.
If a rim edge is damaged on a kerb, get it sorted by a qualified engineer immediately, if its not too bad they can rework the metal until the edge is smooth again, which prevents cracks forming. But if its too far gone then its time for a new wheel.

Bead seat.
The lip edge covers and protects the part of the wheel that the tyre bead rests on, if this bead seat is bent then air can squeeze past and the tyre goes down, the bigger the dent the faster the tyre deflates.
Steel wheels can usually be bent back into shape, and very small dents in alloys can too, but big dents to the bead seat on alloy wheels spells the end of that wheel. Aluminium and Magnesium alloys are very strong, but when they are bent they crack microscopically inside, you cant see it but there will be a mass of tiny fractures making the metal much weaker. If the alloy is then bent back into shape then even more cracks turn up to the party, and just like the case above, when all these join together under high loads the whole chunk of wheel can move out and that spells game over.

Balancing act.
Badly made wheels will be out of balance, metal thickness has to be controlled very accurately during manufacture and this costs money. If the wheel is out of balance then you need lots of weights to get it right again, but even then it will still vibrate at some speeds. So beware of second hand wheels with tons of balance weights on.

Flutter.
The hub face has to be machined within a tiny fraction of a mm to make sure the wheel runs true. If its out then the tyre will get uneven wear and vibrate at certain speeds. Because the tyre is being constantly scrubbed in and out there is less traction available to the car, so your handling suffers. So if buying second hand wheels try to see them with the old tyres on, if alternate inside and outside edges are worn then walk away.

Bubbles.
When alloy wheels are cast, any moisture in the mould will turn to gas bubbles as the molten metal flows in. Wheels are cast with the outside face pointing downwards, so any bubbles will start floating up towards the inside face. So check very carefully this inside face for any uneven or pock marked metal, it should be as clean as the outside face but if there is any evidence that the inside looks like an Aero bar then just walk away, because a porous metal wheel will be as soft and crumbly as that well known chocolate bar. Poor manufacturing methods can also cause uneven cooling and build up internal stresses. All this means that a bad wheel can fall apart at the worst possible moment.
If the wheel has not got a tyre on then hold it loosely in the middle and give it a gentle tap with a solid object, it should ring like a bell, but if it makes a dull thud then it has cracks inside. But even this test is not fool proof, that’s why the top race teams get there wheels X-Rayed to ensure they are in perfect condition.

Markings.
Any quality wheel will have the manufacturers mark and the date when it was made either stamped or cast in. If it doesnt have any markings then ask yourself this; why would any proud manufacturer not put their name to their product, what are they trying to hide? The mark is usually in the hub area or on the inside of one of the spokes. No mark = no sale.
There are some voluntary tests manufacturers can put their wheels through. There is a Japanese test that all OEM wheels go through, the mark looks a bit like ‘JWL’ but is in fact a Japanese character. This is a good sign to have on any wheel. E marking is also good.
The other markings it should have are the size and rim style. Taking 7j17 as an example, the 7 means that the wheel is 7 inches wide between the tyre bead seats, the ‘j’ refers to the shape of the bead seat, and the 17 means the wheel is 17 inches in diameter at the bead seat. All these dimensions must match the tyre’s requirements, including the letter because a ‘j’ type tyre will not go on a ‘k’ type wheel.

Fakes.
Unfortunately some ‘very bad people’ have caught on to the fact that having a good brand name means they can get more money for their home made wheels, so there are now fake wheels that are copies of well know manufacturers top models and even carry the right markings. These are almost impossible to tell apart from the real deal, but as they are made on the cheap the problems of flutter, bubbles and balance are ever present. The only way to be sure is to buy from a trusted source, and if buying on the internet buy from somewhere that has a real shop too, that way if there are any problems you know where they live and can get compensation.
Second hand wheels are a whole lot more tricky, try to find evidence of where they were bought from originally, receipts are good but even these can be faked.

Nuts.
There is more to nut design than meets the eye. Steel wheels usually use nuts or bolts with a tapered shoulder, so that they naturally tend to centralise in the hole. Most, but not all, alloy wheels use sleeve nuts or bolts that have flat shoulders and a cylindrical part that fits inside the hole. The two are not interchangeable and put their force into the wheel at very different angles, using the wrong ones may crack the wheel, so make sure you get the right nuts for your new wheels.

Centre of control.
Obviously the wheel has to sit centrally on the hub, otherwise we would have a very bumpy ride. There are two main ways that car manufacturers do this. One is to use the hub centre to locate the middle of the wheel, this is ‘Hub Centric’. This means that the hole in the middle of the wheel has to be precisely machined to fit over a circular ridge in the hub centre. Many after market wheels have general purpose centres and you have to fit a plastic spacer to make it fit your car properly, it is absolutely essential to use the right ones, and if buying second hand make sure you have a full set and that they are not damaged.
The other location method is to use accurately machined wheel nuts and matching holes, this is ‘Bolt Centric’ and the centre of the hub does not touch the middle hole of the wheel.
Each method uses different designs in the hub, bolts and wheels, so the two systems are not interchangeable. This means you need to get the right type of wheels for your car.

PCD.
The distance between each of the wheel bolts/nuts/studs is critical, a fraction of a mm out and you will have problems. Hopefully it goes without saying that you need to get wheels that match the spacing on your car.
It stands for Pitch Circle Diameter, if you draw a circle round all the bolts this is its diameter. There are a few sizes that crop up time and again and can even be shared between manufacturers. For instance classic Jaguars have teh same 5 bolt PCD as Chevrolet, but modern ones are the same as BMW. This means that you can swap wheels between marques in some cases, which can be a cheaper way to upgrade.
Have a look at wheel web sites which have a size database such as this one:
http://www.alloywheels.com/fitmentguide.asp

Inset outset.
The wheel can be designed to put the tyre more inboard or outboard from the car, if the wheels stick out a long way this is called outset, if they are tucked well inside the wheel arch then its inset, either way its called offset and has a big effect on handling. Having wheels poking out of the arches looks good, and in the past some people have used standard wheels fitted with thick spacers on the hub, but this completely changes the way the tyre pulls on the suspension. And it’s the same if you fit massively outset wheels; imagine hitting a pothole with your left front tyre, if its sticking out a long way from the car then the force pushing the tyre backwards, caused by hitting the hole, makes the wheel turn left. In bad cases this can rip the steering wheel out of your hand and cause the car to swerve off the road.
Even changing the offset by a few mm makes a difference to how the steering fights you, so it is crucial to get wheels with the right offset for your suspension set up.
Rear axles are usually more tolerant of a bit more outset as it has less effect on steering, but there is still some sensitivity.
The other problem with spacing wheels out is that it increases the distance between the tyre contact patch and the suspension inner pivot points. This means that it reduces the effective spring rate at the wheel, making the suspension softer. The severity of this effect depends on how long the links are and where the spring is, for instance the front springs on a Jaguar XJ-S are almost half way along the lower wishbone so outset has a noticeable effect, but cars with struts have the spring pushing down close to the wheel and the effect is less noticeable.
Some wheels have the offset marked on them, for instance BMW wheels may have ‘ET45’ on them which indicates an inset of 45mm, so you can check to see if a new set of wheels is suitable.
Bolt on wheel spacers have questionable safety, if done properly so that there is full thread engagement on both the hub and wheel bolts/nuts. The spacer must also use the same method of wheel location, hub or nut centric, on both the hub and wheel. If it is strong enough and fitted properly then it can function well, but there are a lot of cheap units about that can distort and work loose, or snap the bolts, with disastrous results.

Girth.
With wheels the width must match the requirements of the tyre. All tyres can be fitted to a range of widths, but this makes a difference to handling; a wide tyre on a narrow wheel allows the tyre to wobble sideways when cornering which makes the car feel like jelly, stretching a narrow tyre onto a wide wheel holds the tyre in place but risks the bead being pulled off its seat. So always get tyres that match the wheels, and as every tyre design is different check with the tyre manufacturer to be sure.

Scabs.
Wheels are painted and then lacquered to keep them looking good, but over time this coating starts to come off. Loose paint under the tyres bead will cause a very slow loss in tyre pressure which is very dangerous, so if you buy second hand wheels inspect the bead seat area when the old tyre comes off and clean it up. In bad cases the wheel will need to be refurbished, where all the paint is taken off and the seat area made smooth again before re-painting and lacquering.

Well, I hope that was wheely useful!

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