Wheels & Tyres

Bike wheels buying guide

Category: Wheels & Tyres

Bike wheels buying guide

Your wheels are some of the most important components on your bike, having a huge effect on speed, weight, handling and overall performance.

Upgrading to a lighter wheelset will lower the rotational weight, improving acceleration and giving you a speed advantage – but some riding disciplines will require tough and burly wheels that can take plenty of punishment and still roll true. The ideal wheelset for you will offer the correct balance of light weight, strength and durability for your preferred riding discipline, at a price that suits your pocket.

All wheels share characteristics in common, consisting of hubs, spokes and a rim, while rear wheels are also a key component in your bike’s drivetrain and will in many cases be required to deal with more stress than the front.

Whether you need a tough set of BMX wheels to handle the stunts and jumps of the skate park, or are looking for a set of light and aerodynamic road bike wheels to give you a race day advantage, our guide will help you to choose the wheels that are right for you.

If you don’t want to read our in-depth guide on wheels, you can go directly to the relevant product pages via the links below:

Shop our MTB wheels now

Shop our road bike wheels

Shop our BMX wheels

Shop our cyclo-cross wheels

Find out more about…

MTB wheels
Road wheels
BMX wheels
Cyclo-X wheels


Bike wheels buying guide

Your MTB’s wheelset is one of the most important component choices on your bike, having a significant influence on how your bike performs – there are few parts of a bike where it is more essential to strike the balance between light weight, strength and performance.

When upgrading or replacing a wheel or wheelset there are a number of factors to take into consideration, including not only the size of wheel your bike is designed for, but also characteristics such as weight, rim width, spoke number and hub type, etc.

For example, lighter wheels mean less rotational weight, which translates into improved acceleration and easier speed – perfect for fast cross-country riding and racing. However wheels for certain MTB disciplines – particularly downhill racing (DH), Enduro riding Freeride (FR) require burly wheels with wider rims, which can take sustained punishment without folding like a taco (not good).


Which mountain bike wheels are right for you?


What kind of rider are you? The type of wheelset you choose must be matched to the type of riding you do – a lightweight set of XC racing hoops simply can’t withstand the beating dished out by a boulder-strewn descent, while a super-strong pair of DH wheels is just too heavy to pedal for any length of time.

Also, whether you are happy to choose an off-the-shelf set of factory spec. wheels or prefer to have a custom pair built to your own preference, you will first need to ensure that your new hoops are the right size for your frame and forks.

MTB wheel buying guide


MTB wheels: in-depth


Wheel diameter

The three most common MTB wheel diameters are 26”, 27.5” (sometimes referred to as 650b) and 29”.

Why are there three different wheels standards? Because different riders ride different disciplines and so demand unique characteristics from their components, and also because MTB is an evolving industry and the ‘three wheel sizes’ situation represents change in progress.

For many years 26” was the standard MTB wheel size, having evolved from the balloon-tyred cruiser bikes that were the forerunner of the MTB and offering riders a balance of light weight, stiffness and snappy acceleration that combined to deliver pin-sharp handling in twisty singletrack and technical terrain. After some manufacturers began to experiment with the larger 29” size – equivalent to the 700c standard size found on road bikes – the larger hoops caught on, especially with marathon racers and XC riders who found that the big wheels rolled easily over trail obstacles and held their speed better than ‘26ers’.

However ‘29ers’ still had their detractors, with some riders slow to switch to the bigger wheels because of disadvantages including an increased weight penalty and less stiffness.

Eventually, the “26er vs. 29er” debate led to the emergence of the new 27.5” wheel size as the ideal compromise, offering (its adherents claim) the best of both worlds – retaining the handling responses, stiffness and acceleration of the smaller wheel while improving on traction, braking and straight-line speed.

Today many major manufacturers have thrown their considerable weight behind the 27.5” standard, although many others still fly the 26” flag and more still offer a 29” option, especially with MTB models intended as mile-munchers rather than gravity sleds.

Whichever wheel size you opt for will depend on the type of riding you intend to do, the type of characteristics you value the most, and also your personal preference – some smaller riders, for example, have found it hard to get on with 29” wheels in XS or S-sized frames. Meanwhile in the hard-impact disciplines of DH and FR, the strength and stiffness of smaller-diameter hoops means they are still specced as standard on most gravity-oriented rigs.

Finally, some specialised disciplines – Dirt Jumping (DJ) and Street – opt to use smaller, 24” wheels.

Rim width

As well as wheel diameter, you will also need to consider your rim width. Narrow, lightweight rims are used for XC, marathon and general off-road riding, and tough, wider rims for more gravity-orientated adventure.

In recent years 23mm has become accepted as a standard rim width for XC and trail riding, usually matched with tyres up to 2.1” in width. More extreme AM or Enduro riders who aim to tackle rock gardens and technical terrain lean towards rims of 28mm in width, with the ability to comfortably take large-volume 2.25” to 2.4” tyres. Meanwhile DH and FR riders who put their wheels through serious punishment in the air and on the race course may choose rims of 36-40mm or even more, with the capability of using heavy, reinforced 2.5”-2.7” DH tyres.

NOTE: Remember that if you are choosing new wheels for a bike equipped with non-disc brakes you must make sure that the rims have a braking surface for the pads to make contact with. Rims without this should be marked as ‘disc’ or ‘disc only’.

Spokes and spoke patterns

The more spokes a wheel has, the more the load is spread and the stronger the wheel should be, while less spokes means a lighter wheel.

For general trail riding 32 spokes has become the accepted standard, with more lightweight race wheels featuring 28- or 24-hole drilling. More extreme riding styles call for more strength so 36 spokes are common in AM, Enduro, DH and FR wheelsets, while the most demanding jump and street riders may opt for anything up to 48 spokes in order to handle the impacts dished out by tarmac and concrete.

Spokes can be plain-gauge, meaning they are the same thickness for their entire length, or the more expensive butted models which are thinner in the middle to reduce weight without reducing strength.

Standard wheels are built using j-spokes, with a bend in one end where the spoke fits into the flange of the wheel’s hub. Because some regard this bend as a weak point in the build of the wheel, some manufacturers offer wheels with straight-pull spokes set into specially-designed hubs.

Most wheels are built with a three-cross (3x) spoke pattern, laced in such a way that each spoke passes either under or over three others between the hub and the rim. The other common lacing pattern (more often found on road bike wheels) is radial or zero-cross (0x), where the spoke goes directly from hub to rim without crossing any others.

NOTE: Building, lacing and truing (adjusting spoke tension so the wheel runs perfectly straight) wheels is an art in itself that demands experience, time and patience – something best left to the professionals if you are not confident. A new set of wheels may naturally go ‘out of true’ after a couple of weeks of riding and need to be adjusted – there are plenty of resources in print or online that will demonstrate how to do this, or your local bike mechanic can easily do it for you.

Wheel hubs

The hub is the engine room of the wheel, usually consisting of an alloy housing containing two sets of bearings through which the axle is threaded. Rear hubs also feature a freehub mechanism onto which the cassette fits, and which incorporates the ratchet into the hub body.

There are a number of important factors to consider with hubs:

Bearing type: Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Older or less expensive hubs may use cup and cone bearings, which contain two rings of loose ball bearings that sit in ‘cups’ inside the hub body, and are secured in place by conical nuts known as ‘cones’. The balls can rotate freely between the cup and cone with each unit making up one bearing. Cup and cone bearings have the advantage of being easily serviced at home – new bearings and a dollop of grease once in a while are all you need to keep them running smoothly – but they can be tricky to adjust perfectly. More expensive hubs may use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing, or replaced once worn out. Many riders prefer the convenience of cartridge bearings – no more tedious micro-adjusting or hunting for stray ball bearings mid-service – but they are more expensive to replace.

Brake rotor compatibility: Bikes running disc brake systems need hubs to which the disc rotor can attach. There are two main standards for rotor mounting, six-bolt and Centrelock. As the name suggests, six-bolt systems have hubs with six evenly-spaced holes to bolt on a matching rotor. Rotor bolts generally use Torx heads so you will need to make sure you have a Torx tool to tighten or loosen them – don’t try to fudge the job with an Allen key or you will risk stripping the bolts.

The Centrelock system used by Shimano foregoes rotor bolts in favour of a splined hub-rotor interface secured with a locking ring (special tool needed). This is claimed to reduce installation time but it does put limits on mixing and matching hubs and brakes as most other manufacturers stick with the six-bolt system. Again however, adaptors are available to mount six-bolt rotors to Centrelock hubs, and vice-versa. Before investing in new hubs check which disc interface you are currently using and buy accordingly.

Axle standard: Finally you must also ensure that your hubs are compatible with your fork axle. Most shorter-travel XC and trail forks attach to the hubs via quick-release (QR) skewers, but many forks for heavier applications now clamp on via bolt-through axles for extra stiffness and security, While 20mm has long been the industry standard size for bolt-through axles of this type, lighter-weight 15mm models have also been introduced.

If you intend to upgrade your wheels make sure the hubs match whichever axle standard you are using, or alternatively consider hubs that can switch between QR and bolt-through with the insertion of a simple adaptor to enable the wheels to be used with different fork types.

Shop our MTB wheels now


Bike wheels buying guide

A new wheelset can be one of the best performance upgrades for your road bike.

Lightweight, strong and stiff wheels reduce rotational weight offering increased speed for less effort and improving a bike’s climbing, acceleration and handling abilities.

Meanwhile investing in a wheelset with quality hubs and constituent parts will offer increased durability, longer life and better resistance to the elements.


Which road bike wheel is right for you?


Road bike wheels differ in many ways, from size (diameter and rim width) and construction (materials used, spoke pattern) to intended use (all-weather training or race day performance).

The choice ranges from good-quality budget and mid-range offerings built in the traditional way to the latest generation of carbon fibre aerodynamic wonders. The right wheels for you will depend not only on your budget but also on the type of riding you enjoy.

Sportive and leisure riders, for example, may be served best by an aluminium alloy ‘all-rounder’ wheelset offering a balance of light weight, durability and aerodynamic performance, while competitive riders may choose to invest in carbon fibre ‘race day only’ wheels which deliver stiffness, light weight and superior aerodynamics – but at a price.

Road wheel buying guide


Road bike wheels: in-depth


Wheel basics – construction

A typical road bike wheel consists of a hub in the centre containing the axle and bearings (which allow the wheel to freely rotate). The hub is laced to the wheel rim with spokes, the number, pattern and type of which depends on the design and intended use of the wheel. Meanwhile rims may be designed to take the more common clincher tyres or the tubular type (‘tubs’).

Rims also vary in construction and materials – the traditional ‘box-section’ rim is still found on wheels at the lower end of the price scale and is ideal for leisure riding, commuting or touring, but most mid-range wheelsets and up now use deep-section aero rims for added stiffness, strength and aerodynamic performance.

Meanwhile a breed of speciality wheels – including the disc- and baton-type wheels that don’t use traditional spokes – caters for specific disciplines including track racing, time trialling and triathlon, under conditions where aerodynamics and straight-line speed take priority over comfort or handling.

Wheel sizes

Road bike wheels come in a variety of sizes with the most common diameter being 700c (for most adult road bikes) and 650c (smaller racing bikes for juniors and ladies, some high-performance time trial and triathlon bikes).

Rim diameters also vary, with narrow rims – for narrow tyres that can be pumped to a high pressure for more speed – the standard for high-performance bikes and commuters/tourers requiring wider rims.

Rim depth is also to be considered. The new generation of aero rims and wheelsets feature deep-section rims for improved speed and stiffness but as they affect handling, ultra-deep section rims may not perform well under specific conditions (i.e. when it’s windy).

NOTE: Sizing for tubular wheels is different to clincher wheels with manufacturers still using imperial sizes (26” equates to 650c and 28” to 700c). The large variety of wheel sizes and measuring methods has always given rise to confusion, leading to the development of a universal sizing system by the ISO (International Organisation for Standardisation). This system was formerly known as the ETRTO system, developed by the European Tyre and Rim Technical Organisation.

Manufacturers now also provide an ISO/ETRTO number which gives a rim’s width (measured between the flanges) and diameter (of the bead seat) in millimetres. For example, a 700c wheel has a diameter of 622mm, so the ISO/ETRTO number of a 700c wheel with a 15mm rim would equate to 622×15.

So if you are unsure of the exact sizing of a wheel rim or tyre to match, check for the ISO/ETRO number to be certain.

Spokes and spoke patterns

The more spokes a wheel has, the more the load is spread and the stronger the wheel should be, an essential consideration for touring and training wheels that need to be durable and long-lasting. However less spokes means a lighter wheel, so manufacturers of performance wheels especially have worked hard to develop spoke designs and spoke patterns that cut down on the number of spokes required, without compromising strength or lateral stiffness – the evolution of stiff, deep-section aero rims having played a big part in this.

Traditionally wheels were built with a three-cross (3x) spoke pattern – laced in such a way that each spoke passes either under or over three others between the hub and the rim – with the standard number of spokes for road bike wheels being 24.

Most performance road wheelsets are however now laced with a radial or zero-cross (0x) pattern where the spoke goes directly from hub to rim without crossing any others. Wheels of this type are typically built with deep-section rims using less spokes – 18 or less on the front wheel and 20 on the rear (to handle the extra forces generated by pedalling).

Traditional spokes are made from steel wire – either plain-gauge, meaning they are the same thickness for their entire length, or the more expensive butted models which are thinner in the middle to reduce weight without reducing strength. Carbon fibre spokes are now also common on high-end wheels.

Standard wheels are built using j-spokes, with a bend in one end where the spoke fits into the flange of the wheel’s hub. Because some regard this bend as a weak point in the build of the wheel, many manufacturers (especially on premium road bike wheelsets) offer wheels with straight-pull spokes set into specially-designed hubs. It is also increasingly common to used spokes with a bladed profile to increase aerodynamic performance.

NOTE: Building, lacing and truing (adjusting spoke tension so the wheel runs perfectly straight) wheels is an art in itself that demands experience, time and patience – something best left to the professionals if you are not confident. A new set of wheels may naturally go ‘out of true’ after a couple of weeks of riding and need to be adjusted – there are plenty of resources in print or online that will demonstrate how to do this, or your local bike mechanic can easily do it for you.

Hubs

The hub is the engine room of the wheel, usually consisting of an alloy housing containing two sets of bearings through which the axle is threaded (a quick-release skewer runs through the axle to secure the wheel to the frame or fork). Rear hubs also feature a freehub mechanism onto which the cassette fits, and which incorporates the ratchet into the hub body.

Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Older or less expensive hubs may use cup and cone bearings, which contain two rings of loose ball bearings that sit in ‘cups’ inside the hub body, and are secured in place by conical nuts known as ‘cones’. The balls can rotate freely between the cup and cone with each unit making up one bearing. Cup and cone bearings have the advantage of being easily serviced at home – new bearings and a dollop of grease once in a while are all you need to keep then running smoothly – but they can be tricky to adjust perfectly. Many newer hubs use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing, or replaced once worn out. Many riders prefer the convenience of cartridge bearings – no more tedious micro-adjusting or hunting for stray ball bearings mid-service – but they are more expensive to replace.

Manufacturers of high-end wheelsets aimed at competitive riders have also in recent years taken advantage of developments in materials technology to offer carbon fibre hubs, superlight axles and lighter-weight, long-lasting ceramic bearings in place of steel balls. These improvements offer weight savings and corresponding performance advantages under the right rider, but come at a price.

Rims – ‘traditional’ vs. ‘aero’

There are two basic designs of rims, named after their cross-sections – the more traditional ‘box-section’ (roughly square or rectangular) rims and the ‘aero’ shape deep-section rim (roughly triangular).

The deep-section rim has a number of advantages – it is stronger and laterally stiffer, meaning less spokes can be used to build the wheel, and its aerodynamic profile provides a speed advantage by reducing drag forces. Deep-section rims can also be built using carbon fibre for a reduction in weight and improvement in stiffness, further boosting acceleration and speed.

However the major disadvantage of deep-section rims is that the steering and handling can be impaired while riding in a cross-wind; the deeper the rim, the worse the effect (taken to extreme by disc wheels which are generally only used indoors or in flat calm conditions).

Therefore many riders may opt to choose wheels with different rims or rim depths for different conditions or race situations.

Leisure riders and tourers may appreciate the comfort and simplicity of traditional box-section rims, while aero rims with a lower profile may be ideal for sportive events, climbing or all-round race riding as they provide a balance of improved stiffness and reduced weight without the trade-off of poor handling.

Meanwhile for time trials in calm, flat conditions, track racing and even triathlon stages a set of proper deep-section aero wheels (or even disc or baton wheels) may come into their own.

Tubular vs. clincher wheels

Finally road bike wheels are also available with tubular or clincher rims to fit two different types of tyres.

Clincher wheels and tyres (used with inner tubes) are by far the most common type. This is where the rim of the wheel features a ‘hook’ into which sits the bead of a clincher tyre.

Tubular tyres are sealed units with an inner tube sewn into the casing of the tyre. Rims for ‘tubs’ do not have the hook for the bead (and so are lighter), the tyre being glued in placed with tubular cement or secured using special tape.

Tubular tyres and wheels have long been the choice of the racing professional who values their performance advantages – lighter overall weight, better rolling resistance due to higher inflation limit, round profile aids in cornering – but for leisure riders clinchers are generally considered the better option due to their convenience, availability, affordability and ease of repair.

Shop our road bike wheels


Bike wheels buying guide

Mostly built for strength and durability rather than light weight, BMX wheels are tough little mothers designed to absorb the hard impacts of street, skate park, race and flatland riding and to still roll true afterwards.

As with all other bike wheels, BMX wheels come in a variety of incarnations with the main considerations being size (diameter and rim width), spoke count (more spokes for a tougher – but heavier – wheel), rim and hub type.

Also – they have to look cool, so take your pick from snazzy anodized rims, neon colour schemes, urban camouflage or graffiti-inspired decals.


Which BMX wheels are right for you?


As with most bike parts, the answer to this will depend on your preferred type of riding, but with BMX components it’s fair to say that the range is narrower than with, say, MTB or road bikes.

BMX racers on Olympic-style tracks, for example, may choose to focus on a lightweight wheelset with fewers spokes to shave precious grams at the expense of strength, and narrower rims to take slim, speedy racing tires.

Stunt, street and flatland riders meanwhile will opt for burly 36-spoke wheels with rims wide enough to take chunky tyres, while big-air jump riders might even find themselves looking for 48-spoke hoops,

However most riders will be satisfied with a durable ‘all-rounder’ wheelset – e.g. 36 spokes with rims suited for tyres of between 2” and 2.2” – that will serve them well on the street in the skate park, but equally be able to race or jump on in a pinch.

BMX wheel buying guide


BMX wheels: in-depth


Size

Unlike MTB wheels, the vast majority of BMX hoops come in the standard size of 20” diameter, although smaller 18” wheels are available, as well as 24” models for ‘cruiser’-type BMX bikes.

In the vast majority of cases you will be looking for 20” wheels to fit your frame and forks – for freestyle bikes and racing rigs alike – but if you are in doubt, check the sidewall of your tyres for your wheel diameter.

As to rim width, multiple sizes are available with 32mm being the standard ‘all-rounder’ size. Smaller riders (riding mini- or junior-sized bikes) or racers looking for a speed and weight advantage may opt for narrower rims (30mm) to mount slimmer tyres and reduce rotational weight, while hard-hitting street and stunt riders may look for a wider rim (36mm) as part of a burlier wheel package.

And no, we don’t know why manufacturers would measure wheel diameter in inches and rim width in millimetres. It can be confusing…

Spoke count and spoke pattern

BMX wheels can be laced using different numbers of spokes to influence their strength and weight – the more spokes used, the stronger the wheel is, but the bigger the weight penalty.

Again, a good ‘all-rounder’ figure settles at 36 spokes – tough enough for most applications but not so heavy as to be a misery to ride. Race riders looking for a lighter wheelset and less rotational weight will likely choose a 32- or even 28-hole rim, although these sometimes have a rider weight limit. Meanwhile BMXers who really put their wheels through their paces – and have destroyed multiple ‘standard’ hoops – may opt for burly 48-spoke models.

The term “spoke lacing” describes the pattern used to install spokes into a wheel. Most BMX wheels are built with a traditional three-cross (3x) spoke pattern – laced in such a way that each spoke passes either under or over three others between the hub and the rim. Burlier 46-spoke wheels may use a 4x lace, while lighter race hoops may use a 2x, 1x or even radial (0x) lacing pattern to save weight by using shorter spokes.

Traditional spokes are made from steel wire – either plain-gauge, meaning they are the same thickness for their entire length, or the more expensive butted models which are thinner in the middle to reduce weight without reducing strength.

Rim type

BMX rims can be pinned or welded, which describes how each end of the aluminium hoop is joined to the other to create a circle. Welded joints are the stronger of the two options, but pinned rims – where a small piece of metal joins the rim ends together – can still be found on lower-end and factory-spec wheelsets. If you’ve found yourself busting a pinned rim (or more than one), it might be time to step up to welded seams.

Rims also differ in their internal wall structure, with multiple-walled ‘box-section’ rims featuring hollow internal chambers, as opposed to single-wall rims which don’t. As you might expect, double- or triple-walled rims are stronger than single-wall (or ‘single-skin’) rims owing to the extra structural support, but the extra metal involved in their construction means an accompanying weight penalty.

Double-walled rims are regarded by many as the best ‘all-round’ option, giving the best strength to weight ratio for most riders’ needs. Single-walled rims may be used by race riders while triple-walled could be best for big hitters.

Hub type

BMX hubs consist of an alloy housing containing two or more sets of bearings through which the axle is threaded. Good-quality bearings, sealed and protected from dirt and water, are the key to long and healthy hubs. Most high-end BMX hubs use cartridge bearings, where the steel balls or needles rotate in a single cartridge unit that can be easily removed for servicing, or replaced once worn out.

The standard BMX axle size is 14mm (especially for rear axles), although some race wheelsets or front wheels may use a 3/8” (10mm) axle that is lighter, but more prone to bending. Your axle choice will be determined by the size of the dropouts in your frame and forks, so check before you buy or upgrade.

An important element of your rear BMX hub is the driver or freewheel, which are turned by the chain and translate your pedal stroke into movement.

BMX hubs can be further divided into three types depending on the driver/freewheel type used: cassette, freewheel and freecoaster.

Cassette hubs – These use an internal, independent driver that presses into the hub shell, and can run sprockets as small as 8 teeth, offering a weight saving owing to less materials
Freewheel hubs – The use an external freewheel that threads on to the outside of the hub shell, requiring the use of larger-circumference bearings and freewheel. Because of this, the smallest gear size available with a freewheel is 13t, necessitating a larger front chainwheel in order to achieve optimum gearing, and adding weight.
Freecoaster hubs – These use an internal clutch system to enable a bike to roll backwards without the need to back pedal at the same time.

Other BMX hub variations include the option of switching between left-hand and right-hand drive (as per rider preference), or ‘flip-flop’ hubs which carry different-sized freewheels on each side, and can be ‘flipped’ to offer a choice of gear ratios (e.g. 15t on one side, 16t on the other).

Shop our BMX wheels


Bike wheels buying guide

Standard 700c road wheels are in many cases suitable for the rough-and-tumble of cyclocross racing, but many riders also opt for the extra features of Cyclo-X-specific hoops.

As with road wheels, Cyclo-X wheel are available in different incarnations to accomodate clincher and tubular tyres.

While many hardened racers would have traditionally espoused the advantages of ‘tubs’ – chiefly a more supple feel and the ability to run tyres at lower pressure for improved traction – the latest generation of clinchers has narrowed the gap to the point where their convenience, availability, affordability and ease of repair makes them hard to best.

Cyclocross wheel buying guide

Again, as with road bike wheels, Cyclo-X hoops are generally made from aluminium, with top-end versions being constructed from carbon fibre for lower weight and increased stiffness. However deep-section aero wheels for ‘cross are less prevalent/sought-after because of the stop-start nature of the sport, which often involves riders shouldering the bike around the course.

Cyclo-X specific wheels may feature double-sealed hubs to prevent ingress of water and mud and help prolong bearing life, as well as a wider rim profile to support bigger Cyclo-X tyres (up to 42mm in width).

As many of the latest generation of Cyclo-X bikes come equipped with disc brakes as standard, this is another factor to bear in mind when replacing or upgrading wheels. Frames with cantilever rim brakes need wheels with a milled braking surface on the rim, while disc-equipped bikes need disc-ready hubs on which the brake rotor can be mounted.

Shop our cyclo-cross wheels

Shop at
Chain Reaction Cycles

CRC Logo

Got a question?
Contact our editor