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Alloy Wheel Sizing

When buying a set of alloy wheels there are some important details you need to look at to see whether the wheels you are interested in will fit your car. The first is the PCD or Pitch Circle Diameter then there is the spigot size and the other is the offset also referred to as the ET which is from the German word ‘Einpresstiefe’ translated as ‘insertion depth’.

So what is PCD Pitch Circle Diameter?

Consider the wheel on your car and in particular the holes where the bolts hold the wheel to the car. Now imagine someone has taken a compass and from the centre of the wheel has drawn a circle that passes through the centre of the bolt holes. The diameter of this circle is the Pitch Circle Diameter and is measured in millimetres.

Although the PCD strictly only refers to the diameter of this circle the PCD specification for a set of alloy wheels will also tell you the number of bolt holes which will usually be either 4 or 5.

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

This does not however mean that the wheels from one car that have a PCD of 4×100 will necessarily fit another car whose tyres have a PCD of 4×100. This is because they may have a different spigot size.

So what is the spigot size

The spigot is located in the centre of the wheel hub and is what holds the wheel in place while you locate the bolt holes.

On most aftermarket alloy wheels the spigot hole will be bigger than the spigot on the wheel hub. To get around this you will need to use a spigot ring this fits over the spigot on the wheel hub to increase the size to fit correctly within the spigot hole on the inside of the alloy wheel.

It would be possible to attach a wheel to a wheel hub without a correctly fitting spigot although it would be tricky to hold in place while screwing in the bolts. This would not be advisable for another reason though. The other purpose of the spigot is to carry most of the weight of the vehicle. If it did not fit correctly all the weight of the car would carried by the bolts which are not designed to do this. They would likely sheer off if you drove the car too hard or braked too heavily which is obviously not good!

You may also find that the wheels go in and out of balance.

So what is the offset ET?

The offset is the measurement in millimetres between the centre of the wheel not side on but turned to face you and the bolt face this is not the same as the outer most edge.

There are two different types of offset. There is a positive offset where the bolt face is closer to the outside edge of the wheel than the centre of the wheel this is most common on normal road cars. And there is a negative where the bolt face is closer to the inside edge of the wheel than the centre of the wheel.

Therefore if you put a wheel on your car that has a bigger positive offset than the previous one the wheel will sit further in towards the car. And alternatively if the positive offset is smaller or even changed from a positive to a negative offset the wheel will sit further out from the car.

For performance purposes you can improve grip and traction by going for wider wheels and tyres but in order to do this you will need to go for wheels with a smaller offset to allow for the extra width in the wheel arch. If you went for wider wheels that had too high an offset the tyres would probably make contact with the inside of the wheel arch causing damage to both the tyre and the arch.

Wheel Size

When buying a new set of alloy wheels it is easiest to select a size that is equal to that of the current wheels so you would replace 13 steel wheels with 13 alloy wheels.

But you may want to change the width and profile of the tyres for better performance. Again it is best to try and approximately match the overall diameter of your current wheels and tyres. To work out what you can go for you can use the specifications found on the side of your tyres. This would look something like 165/80/R13.

We can work out the overall width of wheel and tyre as follows:

The first number is the width of the tyre and the second is the profile height expressed as a percentage of the width so the profile height is 80 of 165mm. 165×80/100 = 132mm. This is the height of the profile of the tyre this needs to be multiplied by two for the height on either side of the wheel. 132×2 = 264mm. This figure can be converted into inches by multiplying by 25.4. 264/25.4 = 10.39. We then add the last number 13 which represents the diameter of the wheel. 10.3913 = 23.39. So the total width of wheel and tyre is 23.

If we looked at a wider 15 tyre with the specification of 245/50/14 and did the same calculation:

245×50/100 = 122.5mm. 122.5×2 = 245mm. 245/25.4 = 9.63. 9.6314 = 23.65.

So although the new wheel is wider the profile of the tyre is lower making the overall width within a few fractions of an inch of the old wheel and tyre.

Conclusion

Ultimately unless you are planning on buying some secondhand wheels and tyres or having them handed down to you you will probably be buying them from a professional store either in person or online. The best advice is simply to ask the experts they will know exactly what PCDs spigots offsets and wheel/tyres sizes will be appropriate for your car.

About the writer:  Adrian Ripp runs RTec a company that sells aftermarket modification parts and customises cars at a supermarketsized specialist garage in St Albans. RTec also have an online store at http://www.rtecshop.com that sells a wide range of accessories including alloy wheels car tyres suspension kits and much more.

Alfa Romeo — All Car History Volume 15

Alfa Romeo is a name synonymous with performance and style. The companys cuore sportivo or sporting heart lives on to the present day though the company has been a part of the Fiat Group since 1986. Alfas are thoroughbred cars with a racing heritage.

The fundamentals of sports car handling performance and safety are carried over from competition to road cars. The current Alfa Romeo models include the Alfa 147 159 Alfa GT Alfa Brera and Alfa Spider.

ALFA ROMEO HISTORY

Founded in 1910 in Milan Italy Alfa came under the control of Nicola Romeo in 1915 and the company was renamed Alfa Romeo five years later. In 1920 the Torpedo 2030 HP became the first car produced with the Alfa Romeo badge. Alfa Romeo enjoyed racing success between the wars but still made solid road cars.

After WWII Alfa changed to smaller massproduced vehicles introducing the Giulietta series and classic twin cam engine. Models produced since the Fiat takeover combine Alfa’s style and sporting panache with mass production.

ALFA ROMEO FACTS

• Type: a subsidiary of Fiat
• Founded: 1910
• Headquarters: Turin Italy
• Key people: di Montezemolo President Luca De Meo CEO
• Industry: Automotive
• Products: Cars

FEATURED MODELS

• ALFA ROMEO 147 The 147 is a small family car produced with three or five door versions.
• ALFA ROMEO 156 A compact executive car introduced in 1997 and produced until 2005
• ALFA ROMEO GT A Bertonedesigned coup. Engine options include two petrol and one diesel versions. It shares its he interior with the 147
• ALFA ROMEO GTV The GTV is a sports car produced by Alfa Romeo from 1995 to 2006. A 22 coup it was the successor to the longdiscontinued Alfetta GTV coup

DID YOU KNOW?

• Legend has it that Henry Ford doffed his cap when an Alfa went by
• The first time he managed to beat the Alfas Enzo Ferrari cried like a baby. Enzo was a racing driver and team manager for Alfa in the 1920s before founding Ferrari
• There are many myths surrounding the origins of the first Alfa Romeo badge but it is thought that it was inspired by Milano’s coat of arms the city where the company originated and still is
• From Edward Fox in The Day Of The Jackal to Dustin Hoffman in The Graduate Alfa Romeos have always had a place in popular culture. They are often considered as fiery temperamental Italian cars driven by likeable interesting people less concerned with cold efficiency and much more interested in being interested

About the writer:nbsp;nbsp;Auto Trader World of Cars
Main Fact Finder Index

Which Is A Better Buy Roll Up Truck Bed Covers Or Folding Truck Bed Covers?

Tonneau covers seem to be all the rage these days especially out here in Southern California. A lot of guys buy them because they make their trucks look better not because they really need them. Of course I like the way they make my truck look too but mainly I like tonneau covers because they keep people from stealing my stuff while protecting my possessions from the elements.

I like the lightweight rollup tonneau covers. They are simple to install and they open up the truck bed completely as opposed to the hard top covers which tend to offer more limited access. There are different types of roll up tonneau covers: there are the vinyl fabric types and the hinged hard type that roll up on a spool. On my last truck I had the Lund Genesis seal and peel tonneau. It had velcro side and could roll up like a sheet of heavyduty vinyl.

Lately I have noticed some trucks that use the foldingtype covers that seem more substantial then the roll up type. So I thought I would do a little comparison shopping. My main criteria was a lightweight but tough tonneau that gives easy and full access to my truck bed. I did a search online for both types of truck bed covers and read the descriptions as well as the customer reviews. One thing I noticed is that there seems to be a smaller selection of folding tonneau covers then roll up tonneaus. The roll up covers were made by manufacturers like Extang Truxedo and Lund with price ranges from 200 400.

The folding type truck bed covers were more expensive with price ranges from 500 800. One of the brands that caught my attention was BAK they make the Bakflip folding tonneau cover and it was on the lowend of the price spectrum. It comes in four foldable sections that can be locked in various positions including one that allows you to protect your rear window when you are hauling freight. You can fold it up completely or in sections.

After some searching a reading product description I narrowed it down to the Bakflip folding tonneau cover and the Truxedo Truxport roll up tonneau. Both have cool innovative features and can be locked for security. In the end I went with the Bakflip even though it was nearly twice the price. I really wanted a tonneau that was strong enough to stand on and that locked securely. And I really like the fact that you can open it in sections and even lock it down in different configurations. Very cool.

About the writer:  If you are in the market for folding truck bed covers or a roll up truck bed cover I recommend doing your homework. Pay special attention to customer reviews they really helped me decide. Tim Saunier