Alloy Wheel Sizing

When shopping for a set of alloy wheels, there are some necessary particulars you have to take a look at to see wheter the wheels you have an interest in will suit your automobile. The primary is the PCD, or Pitch Circle Diameter, then there’s the spigot dimension and the opposite is the offset, additionally known as ET (which is from the German phrase 'Einpresstiefe' translated as 'insertion depth').

So what’s PCD (Pitch Circle Diameter)?

Take into account the wheel in your automobile and particularly the holes the place the bolts maintain the wheel to the automobile. Now think about somebody has taken a compass and from the middle of the wheel has drawn a circle that passes by means of the middle of the bolt holes. The diameter of this circle is the Pitch Circle Diameter and is measured in millimeters.

Though the PCD strictly solely discuss with the diameter of this circle, the PCD specification for a set of alloy wheels will even inform you the variety of bolt holes, which is able to often be both four or 5.

So if a wheel has a diameter of 100mm and has four bolt holes, the PCD specification will learn as 4×100.

This doesn’t even imply that the wheels from one automobile which have a PCD of 4×100 will needlessly match one other automobile whose tires have a PCD of 4×100. It is because they might have a special spigot dimension.

So what’s the spigot dimension?

The spigot is positioned within the heart of the wheel hub and is what holds the wheel in place whilst you find the bolt holes.

On most after-market alloy wheels, the spigot gap might be bigger than the spigot on the wheel hub. To get round this you will want to make use of a spigot ring, this suits over the spigot on the wheel hub to extend the dimensions to suit appropriately throughout the spigot gap on the within of the alloy wheel.

It might be attainable to connect a wheel to a wheel hub and not using a appropriately becoming spigot, though it will be tough to carry in place whereas screwing within the bolts. This is able to not be advisable for one more motive although. The opposite objective of the spigot is to hold many of the weight of the car. If it didn’t match appropriately all the burden of the automobile can be transported by the bolts, which aren’t designed to do that. They might most likely sheer off should you drive the automobile too laborious or braked too heavy, which is clearly not good!

You might also discover that the wheels go out and in of stability.

So what’s the offset (ET)?

The offset is the measurement, in millimeters, between the middle of the wheel (not facet on, however turned to face you) and the bolt face (this isn’t the identical because the outer most edge).

There are two various kinds of offset. There’s a constructive offset, the place the bolt face is nearer to the skin fringe of the wheel than the middle of the wheel, that is most typical on regular highway vehicles. And there’s a adverse, the place the bolt face is nearer to the within fringe of the wheel than the middle of the wheel.

Due to this fact, should you put a wheel in your automobile that has a much bigger constructive offset than the earlier one, the wheel will sit additional in in the direction of the automobile. And alternatively, if the constructive offset is smaller, and even modified from a constructive to a adverse offset, the wheel will sit additional out from the automobile.

For efficiency functions you may enhance grip and traction by going for wider wheels and tires, however in an effort to do that you will want to go for wheels with a smaller offset to permit for the additional width within the wheel arch. In the event you went for wider wheels that had too excessive an offset, the tires would most likely make contact with the within of the wheel arch, inflicting injury to each the ty and the arch.

Wheel Dimension

When shopping for a brand new set of alloy wheels it’s best to pick out a dimension that is the same as that of the present wheels, so you’ll substitute 13 "metal wheels with 13" alloy wheels.

However you might wish to change the width and profile of the tires for higher efficiency. Once more it’s best to attempt to roughly match the general diameter of your present wheels and tires. To work out what you may go for you should use the specs discovered on the facet of your tires. This is able to look one thing like 165/80 / R13.

We will work out the general width of wheel and tire as follows:

The primary quantity is the width of the ty and the second is the profile peak expressed as a share of the width, so the profile peak is 80% of 165mm. (165×80) / 100 = 132mm. That is the peak of the profile of the ty, this must be multiplied by two for the peak on both facet of the wheel. 132×2 = 264mm. This determine will be transformed into inches by multiplying by 25.four. 264 / 25.four = 10.39 "We then add the final quantity, 13, which represents the diameter of the wheel. 10.39 + 13 = 23.39". So the entire width of wheel and tire is 23 ".

If we checked out a wider 15 "tire with the specification of 245/50/14 and did the identical calculation:
(245×50) / 100 = 122.5mm. 122.5×2 = 245mm. 245 / 25.four = 9.63 ". 9.63 + 14 = 23.65".

So though the brand new wheel is wider, the profile of the ty is decrease, making the general width inside a couple of fractions of an inch of the previous wheel and tyre.


In the end, except you might be planning on shopping for some second-hand wheels and tires, or having them handed all the way down to you, you’ll most likely be shopping for them from an expert retailer, both in individual or on-line. One of the best recommendation is just to ask the consultants, they’ll know precisely what PCDs, spigots, offsets and wheel / tires sizes might be acceptable on your automobile.

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