Wheels

This page is... UNDER CONSTRUCTION

WARNING: I am not an expert on cars, automotive engineering, VW Beetles, beach buggies or anything else discussed in these pages. What I have written is my current understanding of the issues involved in building MY buggy. These opinions are based only on my own research in books and on the web. They, therefore,have no basis in fact, may well be wrong and potentially downright dangerous if taken to be gospel truth. If you intend to use any of this information for any purpose other than pure entertainment, then please get its validity confirmed by someone who knows what they're talking about! You have been warned...

This page contains the following Sections:

• Wheel Overview
• Wheel Bolt Pattern
• Tyre Construction Types
• Tyres & The Law
• Radial Tyre Construction
• Retread or Remould Tyres
• Tyre Tread Types
• Understanding Tyre Size Notation
• Tyre Speed Ratings
• Tyre Load Indices
• Alternative Tyre Size Calculator
• Tyre Size Calculator
• Rim Width
• Rim Width Calculator
• Wheel Offset
• Wheel Offset Calculator

Wheel Overview

The choice of wheels and tyres for an off-road beach buggy seems to be a fundamental decision which needs to be made as early in the design and build process as possible, which I find rather surprising, as I had imagined it would be one of the last things to buy, along with the seats. This is because the choice of wheels and tyres are tied in with wheel arch clearance (both vertical and horizontal), body lift kits, suspension design, ground clearance, gearbox and differential ratios and engine power requirements. A significant change in the wheel and tyre sizes could result in an almost complete redesign of the car!

One of the most fundamental wheel design choices is to decide what they should be made of, which in turn is defined by what I want the wheels to do. The primary requirements for wheels which are to be used off-road is that they should be both strong and light. As well as being strong, they should be malleable rather than brittle so that when they do fail they bend rather than break. This will enable them to be beaten back into shape to limp home if necessary. Light weight is required in order to keep the 'unsprung weight' of the car to an absolute minimum. Unsprung weight is made up of the tyre, wheel, brakes, hub, axle and suspension components. Increasing unsprung weight increases suspension momentum, which makes the shock absorbers work harder, makes it harder for the suspension to keep the wheel in contact with the ground and increases the shock loadings as the suspension recoils from each bump. These are all bad things! The requirements for strength, malleability and light weight narrow the choice down to either steel or wrought (rather than cast or forged) aluminium. Most 'alloy' wheels, whilst they are light, are either cast or forged, therefore brittle, and so unsuitable for use off-road.

Steel wheels are relatively cheap, extremely strong and malleable but are relatively heavy and can weigh up to 3 times as much as an aluminium wheel of the same size. Corrosion can also be a problem as it can be difficult to maintain the condition of a paint or chrome finish when the wheel is being abused off-road.

Wrought aluminium wheels were once very common and were widely used in the US desert and off-road racing scene. There now appear to be relatively few people making this type of wheel, however, they are still made by Centerline Wheels in the US and there are several places which import them into the UK. They are not as strong as steel wheels but are malleable and very light. As they are aluminium, corrosion is less of a problem than for steel wheels and is only really an issue if I want them to look nice, which I'm no too bothered about.

Since I'm going to be running wheels which are as big as possible to give me the maximum ground clearance and good traction, the weight saving gained by using aluminium wheels is more significant than it would be if I was planning to use smaller wheels. I will, therefore, use Centerline aluminium wheels, front and rear, on my buggy.

Centerline wheels are available in the sizes shown in the table below...

Note: Centerline specify the dimensions of their wheels in terms of 'diameter', 'width' and 'backspacing', using inches as the units. Diameter and width are fairly self explanatory and backspacing is the distance from the inner face of the wheel mounting flange to the inner rim flange, where the tyre bead sits. I have added a column for offset, which is the distance between the inner face of the wheel mounting flange and the centreline of the rim as this is a more commonly used term in Europe and makes more sense to me. Positive offset means that the wheel sits further inboard than its centreline and negative offset means that it sits further outboard than its centreline. I have also added a column for 'Frontspacing', which I don't think really exists as a standard (I made it up) but this is the measurement that I actually want to know, to work out whether the wheels will fit under my arches.

Offset = Backspacing - 1/2Width

'Frontspacing' = Width - Backspacing

Whilst discussing wheel offset, it is worth noting that for off-road use, excessive offset (especially negative offset) is undesirable as it increases the loading applied to the steering components, spindles, bearings and ball joints at the front and the stub axles, bearings and trailing arms at the rear. In an ideal world, the offset of all the wheels would be zero, in order to centre the wheel exactly over its mounting flange. However, none of the above offsets are more than a couple of inches and so any of them would be fine in this respect. Also, spacers and wheel adapters should be avoided for off-road use, as they increase unsprung weight, increase offset and reduce strength. All of which are bad!

Wheel Bolt Pattern

Before 1967, Beetle brake drums used 5 bolts in a 205mm pitch circle diameter (pcd) pattern to attach the wheels (above left), whereas, after 1967 they changed to a 4 bolt 130mm pcd pattern (above right).

The larger diameter 5 bolt pattern is much better than the 4 bolt pattern for off-road use, as it provides stronger mountings for the wheel, the wheel itself is stronger and the wheel is significantly lighter, giving improved unsprung weight.

Look at any racing Baja Bug or Rail Buggy and you will see the 5 bolt 205mm pcd pattern being used.

I will, therefore, use the 5 bolt 205mm pcd pattern on my buggy. Note: All of the above Centerline wheels are for the 5 bolt 205mm pcd pattern :-)

Tyre Construction Types

The pneumatic tyre is now well over a hundred years old, and has not really changed much from its original concept during that time.

John Boyd Dunlop registered the pneumatic tyre with the British Patent Office in 1888 and is therefore generally considered to be its inventor.

There are 3 types of tyre construction: CROSSPLY (denoted by a - in the tyre size marking and also known as BIAS PLY); BIAS BELTED (denoted by a B in the tyre size marking) and RADIAL (denoted by an R in the tyre size marking). The majority of modern vehicles use radial construction, however for certain vehicles, including off-road vehicles, crossply tyres may still be used. Bias belted tyres are seldom used on production cars, and are not widely available in the UK.

Crossply Tyres

Crossply tyres have a casing that is made up of several superimposed layers of textile cords running diagonally, at alternate angles from bead to bead. With this design the sidewalls and tread are not differentiated at all and this gives the tyre structure great overall rigidity. With a crossply tyre the section width and the sidewall height are approximately equal. This relationship is known as the aspect ratio and with crossply tyres is approximately 100%, giving the crossply tyre its unique 'tall' appearance.

Other strong points of the crossply tyre are its good self-cleaning tread pattern and stiffer sidewalls which, along with the high aspect ratio make them highly suitable for off-road use.

Bias Belted Tyres

Bias-belted tyres are basically a hybrid between crossply and radial tyres. A bias belted tyre uses a crossply construction casing overlaid with belts set at an angle to the casing (radial belts are perpendicular to the casing). Rarely found in the UK they can be ignored for all practical purposes.

Radial Tyres

In the early 1950s, a new concept of tyre design was developed, namely the 'radial', wherein the casing was made highly flexible by setting the cords at 90 degrees to the direction of rotation. In order to regain the lost strength, inextensible steel belts were placed on the top of the casing under the tread.

The beauty of the radial design is that it separates the functions of the sidewall and crown of the tyre, allowing greater vertical flexibility whilst ensuring that there is still as much surface in contact with the road as possible. In radial tyres the sidewall has only one or two layers of textile cord giving very good flexibility, and the tread is made very rigid by laying two (or more) layers of steel cord bracing plies over the casing. This gives a far stiffer tread portion, leading to higher tread life (mileage) and a much more comfortable ride due to the flexible sidewalls when compared to crossply tyres.

Comparison of Crossply vs Radial Tyre Performance

This table gives you some idea of the advantages and disadvantages of the two main types of tyre construction. It is easy to see from this why radial tyres are used on almost all cars now, including their resistance to tearing and cutting in the tread, as well as the better overall performance and fuel economy.

Tyres & The Law

It is illegal to use a tyre on the road if the grooves of the tread pattern have a depth of less than 1.6mm throughout a continuous band comprising the central three quarters of the breadth of the tread and around the entire outer circumference of the tyre. (Breadth of tread means the width of that part of the tyre which is in contact with the road surface under normal conditions).

The "Road Vehicles (Construction & Use) Regulations 1986", specifically Reg.26, specifies exactly what types of tyre construction can legally be fitted to a vehicle and how they can be arranged. This is shown in the table below:

All other combinations of tyre construction are illegal. It is also illegal to mix tyres of a different construction on the same axle.

The basis of this law is the different handling characteristics of each of the tyre constructions. The mixes of tyres permitted and listed above will all produce 'understeer' whereas all other combinations will produce 'oversteer'. (Oversteer refers to the car turning more tightly into a corner than it is steered, whereas understeer indicates that the vehicle turns less tightly than it is steered). Of the two conditions, understeer is generally accepted to be less dangerous and easier to control.

Radial Tyre Construction

1 - Tread strip

Material: Rubber compound

Function: The tread strip has to provide high wear resistance and good grip under all road conditions. In some instances the tread strip combines two different materials (cap and base); the base is there to minimise the tread temperature and the rolling resistance.

2 - Multi-ply steel belt

Material: Steel cords embedded in rubber compound

Function: Enhances driving stability, reduces rolling resistance and gives the tyre its long service life. Restricts casing growth and increases the tyre's structural strength.

3 - Fabric casing

Material: Advanced fabric mesh

Function: Gives the tyre its structural strength and its deflection characteristics; substantially determines driving comfort.

4 - Inner lining

Material: Rubber compound

Function Major factor in preventing diffusion of air and moisture in tubeless tyres.

5 - Sidewall

Material: Rubber compound

Function: Protects from lateral scuffing and the effects of the weather.

6 - Bead reinforcement

Material: Nylon, aramide, steel cord

Function: Securing the end of the steel cord ply on the bead core. Reinforcing the bead against high shear forces.

7 - Bead core

Material: Steel wire embedded in rubber compound

Function: Ensures the tyre sits firmly on the rim.

Retread or Remould Tyres

A retread tyre - also known as a remould - is made from a used tyre. The essential building block for a retread tyre is a used tyre whose tread is worn-out but whose carcass (basic structure) is sound. Retreading involves stripping away both the remaining tread and sidewall of the used tyre. The final part of the process moulds new rubber to the old carcass.

In the past, retread tyres have been manufactured in accordance with BS AU 44e. However, this standard did not specify a type approval test for retread tyres, which would guarantee a standard tyre quality.

On 1st January in 2004, ECE Regulations 108 and 109 came into effect, making it mandatory for retread tyres to be subject to a type approval test. This ensures that retread manufacturers must meet a specified basic standard in terms of the tyres' suitability for retreading prior to the process, and their performance after it. It is now illegal to sell retreaded tyres that that do not have the "e" mark.

This means that, although retread tyres have had a bad name in the past, they are now more than adequate for use on a buggy and they also have some distinct advantages over 'normal' tyres. The first, and most obvious, of these is the price. However, another advantage is that retread tyres tend to be made of a softer rubber compund than normal tyres and this gives improved grip, especially on a light car like a buggy.

Tyre Tread Types

There are several different types of tyre tread that you could buy for a buggy. What you choose depends on what you want to use your buggy for and how you want your buggy to handle in different conditions. The different general classifications of tyre type are as follows:

Performance Tyres

Performance tyres are designed for fast cars or for people who drive harder than average. They put performance and grip ahead of longevity by using a soft rubber compound. Tread design is biased towards grip rather than shifting water out of the way on a wet road. This type of tyre would be good on a roadster buggy that is used only in the summer for racing away from the lights and throwing round roundabouts but is not suitable for an off-road or long-distance buggy.

All-Season Tyres

These are the type of tyres that are fitted to most production cars. They're designed to be a compromise between grip, performance, longevity, noise and wet-weather handling. They're made with a harder rubber compound, which sacrifices grip for longevity. The tread design is a compromise between quiet running and shifting water. This type of tyre would be good on a roadster buggy, especially one used as a daily driver, but is not well suited for a long-distance off-road buggy.

Winter Tyres

Winter tyres come at the other end of the spectrum to performance tyres. They're designed to work well in wintery conditions with snow and ice or mud on the roads. Winter tyres have bigger, and thus noiser tread patterns and are made with a harder rubber compund. This type of tyre would be good on a roadster buggy that does a high road mileage, year round, with occasional off-road use.

All-terrain Tyres

All-terrain tyres are typically found on SUVs and street driven 4x4s. They are larger tyres with stiffer sidewalls and bigger tread block patterns. The larger tread block means the tyres are noisy on normal roads but grip loose sand and dirt well when you take them off-road. As well as the noise, the larger tread block pattern means that less tyre surface is in contact with the road, so they handle less well on dry roads. The rubber compound used is normally neither particularly soft nor hard. This type of tyre is well suited to an off-road buggy that is also used regularly on the road. I will probably use this type of tyre on my buggy most of the time.

Mud-Terrain Tyres

Mud-terrain tyres are designed for extreme off-road use and are usually fitted to 4x4s that are used in anger regularly. These have massive tread blocks to give good grip on loose mud and dirt. The down-side of this is that they are extremely noisy on the road and have poor grip on dry roads compared to the other tyre types. This type of tyre would be very good on an off-road buggy that doesn't spend a great deal of time on the roads. I will have a set of these tyres for my buggy but will only fit them when going off-road.

Note: Since buggys are two wheel drive, it is not uncommon to see different tyre types on their front and rear wheels, with a more aggressive tread being used on the rear wheels.

Understanding Tyre Size Notation

In order to select the right tyre sizes for my buggy, it has become necessary to gain some understanding of what the various hieroglyphics on tyres mean. This is often not as straightforward as you may think, particularly when considering tyres for an off road buggy, as several different tyre notations have been used historically as tyres have developed and many of these are still in use today. The sections below describe how to read and interpret most of the tyre size notations you are likely to come across in your search for the perfect tyre:

European (DIN) Notation

Tyre markings have the format: '235/75R15 91H'. Where:

• '235' = Maximum width of the tyre from sidewall to sidewall in mm. Known as the 'Section Width'.
• '75' = Height of the sidewall, expressed as a percentage of the width. Known as the 'Aspect Ratio'.
• 'R' = Denotes 'Radial' tyre construction.
• '15' = Diameter of the rim the tyre fits on, in inches.
• '91' = Load rating of the tyre (see section below).
• 'H' = Speed rating of the tyre (see section below).

This is the most common notation in use today and accounts for over 90% of modern tyres. Whilst it certainly is the most common standard in use, it's symptomatic of the crazy world that we live in, in that it uses a mixture of metric and imperial units and whilst it's fairly easy to see how wide the tyre is, it doesn't tell you the diameter of the tyre unless you know how to work it out.

In order to work out the diameter of the tyre;
(1) Choose your preferred units (either inches OR millimetres but not both) and convert the section width and rim diameter into the same units (Note: There are 25.4 mm in an inch);
(2) Multiply the section width by the aspect ratio and then divide by 100 to give the sidewall height;
(3) Double the sidewall height then add it to the rim diameter.
Then, and only then, will you know the diameter of the tyre. I have written several tyre size calculators which are given below on this page to help you out with this tedious task.

Note: If, like me, you're a bit of a sad muppet and like to know these things, you may like to know that 'DIN' = 'Deutsches Institut fuer Normung' and is the German institute for standardisation, so they're the ones to blame for this half-baked standard! ;-)

US Notation

Tyre markings have the format: '235/75HR15'. Where:

• '235' = Section width in mm.
• '75' = Aspect ratio percent.
• 'H' = Speed rating (see section below).
• 'R' = Radial construction.
• '15' = Rim diameter in inches.

Not to be outdone by the Europeans, the Americans have a very similar and equally stupid standard; the only difference being the different position of the speed rating and the omission of the load index. If you undertand the one above, then this one shouldn't pose any problems for you.

Light Truck (LT) Imperial Notation

Tyre markings have the format: '31x10.5R15'. Where:

• '31' = Diameter in inches.
• '10.5' = Section width in inches.
• 'R' = Radial construction.
• '15' = Rim diameter in inches.

Although designated 'Light Truck', this notation is most commonly used for 4x4 tyres. In many ways, it's the most sensible form of notation in use today, in that it uses only inches, rather than a mixture of inches and millimeters, and it also tells you everything you want to know about the tyre without having to use a calculator, i.e. it's 30 inches in diameter, 10.5 inches wide and fits on a 15 inch rim. What could be simpler, except perhaps using millimetres instead of inches?

Imperial Crossply Notation

Tyre markings have the format: '7.5x15'. Where:

• '7.5' = Section width in inches (decimal).
• '15' = Rim diameter in inches.

In this form of notation, the aspect ratio is not stated explicitly and has to be assumed. Different (lower) aspect ratios only appeared in the late 1960s. Prior to this, the aspect ratio wasn't stated on either radial or crossply tyres. For crossply tyres, the aspect ratio was assumed to be 100% , so the section width was equal to the sidewall height. Therefore, a '7.5x15' crossply tyre is equivalent to a '195/100-15' in DIN notation. Note: 'R' is missed out, as the tyre is not a radial.
Note: With crossplys only being used for specialist applications now, this notation is rarely seen and can be ignored for all practical purposes.

Classic Radial Notation

Tyre markings have the format: '195R15'. Where:

• '195' = Section width in mm.
• 'R' = Radial construction.
• '15' = Rim diameter in inches.

Like with crossplys, from the late 1960s onwards, but prior to the widespread use of the DIN notation, the aspect ratio wasn't stated explicitly for radials either. However, unlike crossplys, radials should be assumed to have an 80% aspect ratio, rather than 100%. In fact, it could actually be anywhere between 80% and 85%, as each manufacturer used a slightly different aspect ratio as their standard, with 82% probably being the most common. For all practical purposes, I'd recommend using 80% in any calculations for this type of tyre. Therefore, a '195R15' radial tyre is equivalent to a '195/80R15' in DIN notation.
Note: This form of notation is still commonly used for 4x4 tyres, with '195R15' and '215R15' still being widely available.

Land Rovers & Off-Road Notation

Tyre markings have the format: '700x15'. Where:

• '700' = Section width in inches (decimal multiplied by 100. Therefore 7.0" = 700).
• '15' = Rim diameter in inches.

This form of notation was used on early Land Rovers and other older off-road vehicles. Here, the section width is given in decimal inches multiplied by 100. It also has no explicitly stated aspect ratio but as the original tyres would have been crossply an aspect ratio of 100% should be assumed. Therefore, a '700x15' off-road tyre is equivalent to a '175/100-15' in DIN notation. Note: 'R' is missed out, as the tyre is not a radial.
Note: This notation is rarely used, except for '750x16' for old Land Rovers in the UK. However, '700x15' is very common in the US and is probably the most popular size for front tyres on VW Bajas and buggies. They are pretty difficult to find in the UK but most US VW parts suppliers that specialise in off-road equipment will have them in stock. Strangely, Yokohama actually make a radial tyre using this notation!! The Y742S: Click Here for details.

Tyre Speed Ratings

All modern tyres are rated with a speed letter. This indicates the maximum speed that the tyre can sustain for a ten minute period without failing.

'H' rated tyres are currently the most common for 'normal' cars and 'Q' is the most common for 4x4 mud terrain tyres. Anything around 'Q', or above, is suitable for use on a buggy, due to the light weight and fairly low speed of the car.

The most common speed ratings are given in the table below, along with their associated speeds in both kilometers and miles per hour:

Tyre Load Indices

The load index of a tyre is another code and it denotes the maximum load the tyre can carry at speeds under 210km/h (130mph).

The table below gives Load Index (LI) values and their associated loads in Kg/Tyre. For example, if a four wheel car weighs 1 ton (1000Kg), then each tyre will carry (approximately) 250Kg. This is equivalent to a load index of 60.

Again, due to the light weight and fairly low speed of a buggy, it is very unlikely that you'll find a tyre with a load index that is too low, unless it's designed for a moped!

Wheel Offset

Wheel offset is described by the term 'ET', which is from the German word 'Einpresstiefe', which translates literally into English as 'insertion depth' and is the distance in mm between the centre line of the wheel rim, and the mounting surface of the wheel. Offsets can be either negative (-), positive (+) or zero (0); for example ET-15, ET25 or ET0.

Negative offset is the when the wheel mounting surface is closer to the centre of the car than the wheel centerline. This is not common on 'normal' cars. Positive offset is the opposite, and is when the wheel mounting surface is further away from the centre of the car than the wheel centerline. This is far more common on 'normal' cars and is most often between 15 and 50mm. Zero offset is when the mounting surface is exactly centered in the wheel and refers to a wheel that has neither positive or negative offset. This is also not very common on 'normal' cars.

Offset, along with wheel width and diameter, determines whether or not a set of wheels will fit a car. Offset (in combination with the width) will determine whether the wheels will stick out too far - rubbing or protruding beyond the wings, whether they will be in too far and not fill the wheel arches adequately, or fit nicely, filling the wheel arch.

It seems somewhat counterintuitive to me but a wheel with positive offset 'tucks in' under the wheel arch, whereas a wheel with negative offset 'sticks out'.

If you get the combination of wheel width and offset badly wrong, the wheels may scrub against the bodywork, suspension or at worst not turn at all! Since offset also contributes to the self-centering effect of the steering, a big change in offset may make the steering either too heavy, making it difficult to turn the steering wheel, or too light, making it difficult to keep the car travelling in a straight line.

Occasionally, offsets are referred to as 'standard', 'reverse', and 'front wheel drive (FWD)' offset. Standard offset is a moderate positive offset, that will fit most rear wheel drive cars. Reverse offset, also known as 'deep dish' or 'minimum offset', is a negative offset, used on cars with very wide wheel arches. Front wheel drive (FWD) offset, is a high positive offset, used to accommodate the additional hub protrusion on most FWD vehicles.

Determining offset: If you look carefully at a wheel you should find a set of numbers/letters stamped or cast somewhere on the wheel. Once you've found it, the information is fairly easy to read but on many after-market wheels it's not easy to find, as they don't want to ruin the style of the outside of the wheel so it's more likely to be found inside the rim (so not visible unless you take the tyre off) or at the back, or on one of the inner mounting surfaces.

These markings have a variety of formats which may (or may not!) be similar to the following:

'6Jx15 205-5-25' or '6Jx15 H2 ET35' where...

• '6Jx15' = Width and Diameter of the wheels. 6" wide and 15" diameter.
• 'J' = Shape of the tyre bead profile (you don't need to worry about this).
• '205' = Pitch Circle Diameter (PCD). Diameter of a circle drawn through the centres of the wheel bolt holes in mm.
• '5' = Number of wheel bolt holes.
• '25' = Positive offset of 25mm (ET).
• 'H2' = Wheel rim is designed to take 'H' speed-rated tyres.
• 'ET35' = Positive offset of 35mm.

Wheel Offset Calculator

Use the calculator below to determine the effect of changing your rim width and offset on inner and outer wheel arch clearances. First enter the wheel width and offset for Wheels 1 and 2, then click the calculate button. It will then show the difference between Wheel 1 and 2, in terms of clearance between the inside of the wheel and the inner wing and how far the outside of the wheel edge will extend or retract. If you reduce the inner clearance too much or push the wheel out too far, the tire might rub or not fit at all.

- End -

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