How to select a rim?

The most important characteristic of a rim is its bead seat diameter. It should match the intended wheel bead seat diameter of the bicycle, although in some cases for bicycles having disc brakes it may be possible to fit smaller than original wheels (but then the bottom bracket might be too low, causing pedals to hit the ground in corners). Most common bead seat diameters are 559mm (for most mountain bicycles) and 622mm (for most road bicycles), although there certainly are other sizes available as well.

After the bead seat diameter, the second most important characteristic is adequate spoke density. An inadequate spoke density in a bicycle having disc brakes may lead to the front wheel spokes to loosen if braking hard. Also, an inadequate spoke density may cause the spokes to loosen even if not braking hard. An inadequate spoke density in disc brake front wheels can be mitigated by using large-flange hubs (but it's heavier than a small-flange hub); and inadequate spoke density when not braking hard can be mitigated by using a stiff rim (but it's heavier than a less stiff rim). Quality bicycles have standardized on 36 spokes per wheel, which is what e.g. Jobst Brandt and Sheldon Brown recommended. In fact, Sheldon Brown called using less than 36 spokes "The Great Spoke Scam". For 559mm bead seat diameter wheels, 32 spokes gives similar spoke density than 36 spokes, and the spokes transmit disc brake torque as well as 36 spokes in a 622mm wheel. Too loose spoke density is a major headache and should be avoided, thus being the second most important characteristic of a rim.

Lower than 36 spokes per wheel may work if a number of conditions are met:

• The spoke count is not ridiculously low, i.e. something like 20 is clearly out of the question but 32 might be acceptable
• The rider is lightweight
• The rider never brakes hard with a front disc brake
• The front hub has large flange if it's a disc brake hub to cause as small spoke tension changes as possible when braking
• The rim is very stiff (i.e. its second moment of area is large)
• The spokes are flexible (i.e. as thin as possible and butted/swaged, not straight gauge)
• The spokes are evenly tensioned (which is only possible with an accurate-tolerance rim)
• The spoke tension is high (which in practice needs a rim strong around spoke holes, i.e. using double eyelets)
• The wheel size is small so the spoke density is not low despite low spoke count
• (Seriously, don't do this as it makes spoke adjustment hard:) The thread between spokes and nipples is secured by thread glue

Ideally, when selecting a rim one could compare their manufacturing tolerances because better-tolerance rims make it easier during wheelbuilding to achieve even and high spoke tension. However, in practice the only definitive thing about this that can be said is that quality rims from brand names have good tolerances. Usually you get what you pay for. Cheap no-name brands should be avoided.

The material of rims is usually aluminum, as steel is heavy and offers poor wet braking for rim brakes, and carbon fiber is prone to fail catastrophically after an incident that may have weakened it in an invisible manner. The catastrophic failure can happen "just riding along", if the rim has a history of such incidents. Aluminum rims are manufactured by extrusion.

A good rim has a double wall design, although for fatbikes such a rim would be heavy and single wall rims are used. The reason is that a double wall rim has a larger second moment of area, i.e. it is stiffer and thus distributes the load over a larger number of spokes. The single wall design for fatbikes is not a problem as fatbike tires provide lots of cushioning so they never see high momentary loads.

If the rim is intended for rim braked bicycle, it must have brake tracks. Rim brake rims can be used for disc brake bicycles but the converse is not true. Thus, if there's a possibility of ever needing rim brakes on any bicycle used, it makes sense to build wheels only with rims that have rim brake tracks. The brake tracks would ideally have wear indicators. Two common designs are groove and small holes. The groove has a problem that if the rim and wheel are very precisely build, it can wear an identical groove in the brake pad, and a groove in the brake track may need a tubular section behind it to strengthen the rim. Small holes that are visible when not worn and disappear when worn are stronger, thus not needing any additional strengthening material behind them, and cannot wear a groove in the brake pad, so this is the preferable wear indicator design.

Good rims withstand large spoke tension. The best way to achieve this is usually double eyelets / sockets made out of steel. Such double eyelets make wheelbuilding easier (no possibility to drop a nipple inside the rim channel accidentally), and make the rim stronger. If the rim does not have double eyelets, it should at least have single eyelets, although double eyelets are heavily preferable.

One major decision is whether the rim should be tubeless ready. Double eyelet rims are not tubeless ready, so one has to choose between easy wheelbuilding and strong rim able to withstand spoke tension, and the possibility to use tubeless tires. It may be harder in some cases to mount tires on tubeless ready rims than non-tubeless-ready rims, so if there are no tubeless plans, it may make sense to standardize on rims having double eyelets that are not tubeless ready.

Rims that require use of nipple washers should be avoided, as wheelbuilding with such rims takes more time.

Anodized, especially hard anodized, rims should be avoided as they have reduced fatigue life and thus can crack around the spoke holes. Preferable finishes for rims are polishing and powder coating. Black rims are not always anodized -- if the finish is smooth like paint, it is probably powder coated, whereas if it is rough, it is probably anodized.

Good rims are drilled for Presta valves. One can always enlarge the valve hole if needing to use Schrader valve. The opposite (reducing hole diameter) is not possible. A Presta valve hole is not much larger than spoke holes so it does not weaken the rim, whereas a Schrader valve hole is always a weak point in the rim. Besides, using Presta valves in rims drilled for Schrader valves is difficult, requiring the use of specially shaped valve locknuts and grommets.

Good rims have hooks at their sides, allowing the usage of high pressure clincher tires. Only such hooked rims should be used. They can be identified by "C" after their width, e.g. 622-19C means 622mm bead seat diameter, 19mm internal width, hook-type rim.

If selecting a rim to replace an existing rim, spoke reuse (which is the preferable way to rebuild an already spoked wheel) is possible only if the effective rim diameter (ERD) is similar. Millimeter or two millimeter difference may be acceptable, but over 5 millimeter differences should be avoided. Unfortunately, rims go out of fashion very quickly and thus it may be hard to find a rim having the same ERD as some existing rim in an already build wheel. Thus, it may make sense to keep a small stock of good rims so that rebuilding wheels with new rims is possible.

Rims are available in many widths. There is a chart showing which tire widths and rim widths are compatible. For example, 28mm tires can be used with 15mm, 17mm and 19mm rim internal widths. As it is usually not justified to use a tire size narrower than 28mm as narrow tires need exceptionally large inflation pressures, provide a harsh ride, and have worse rolling resistance even on smooth steel drums (and thus much, much worse rolling resistance on non-smooth roads), and sometimes one might want to use a much wider tire than 28mm, it may not make sense to use a rim narrower than 19mm internal width. A very wide tired bicycles which never use 28mm tires should obviously use a rim internal width of more than 19mm.

Lastly, one should consider rim weight as well. Usually manufacturers aim for the smallest possible weight, so if selecting a rim of a certain design from a brand name manufacturer, you can almost always know its weight in advance from the requirements. One should not obsess too much about weight as a little extra weight does not take the joy away from bicycling.

Simple question, long answer requiring a book to cover it fully. Here is my attempt at the introduction chapter.

The biggest concern for most people selecting a rim would have to be total cost of the rim built into a wheel. You have to consider the cost of spokes and wheel building. In all but the high cost product range it is often cheaper to buy an entire wheel then buy a rim and have it build up to a wheel. The new wheel does not have to match an existing one, apart from cosmetics, its OK to have different rims and spoke counts on the front and back of a bike.

Once it has been decided to buy a rim and build it into a wheel, you will need to select the correct size for the bike you are fitting it to. The diameter chosen will usually be the size the frame builder designed the bike for, however it is possible to go up and down under limited circumstances. If the brakes are disc, you can go smaller, but risk lowering the bottom bracket too much. Smaller rims may be compensated for by wider tires, some mountain bikes are sold that will take a 650B+ wheel and tire, or a 29" wheel+tire. Larger rims can some times be fitted to bikes not designed for it, but you run the risk of not having enough room depending on tire size, so should be done with caution. If the bike has V or canti brakes, changing rim sizes may still be possible, requiring new brakes, but is probably best avoided.

The tire width you want to run will determine the ideal rim wide, but there is quite a lot of room for wider and narrow tires and a lot of overlap with rim widths, so this will probably not limit choices.

If you run disk brakes, any rim will be good. if you are running rim brakes, you need a rim with a brake track.

Once you have sorted the rim size, you need to match this to the hub you will use. Pretty much all that has to match here is a hub that fits the bike and has the same number of holes as the rim. If you are buying a new hub as well as rim, you need to chose the spoke count. Some people get hung up on this, but always keep in mind a well build 24spoke wheel will be stronger than a poorly build 36 spoke. Choose more spokes (32 or 36) for heavier riders and hard duties like tourers, cargo and down hill mountain bikes. Choose fewer spokes if you prefer lighter wheels and the look, and you are riding smoother surfaces.

At the same price point you may need to decide between a lighter rim or a stronger rim. Look at things like double wall and eyelets. In all but the cheapest rims (when you would be buying a made up wheel) the rim will be build for a purpose, and be strong enough for the purpose. Double wall and eyelets are a given at a price point its worth buying a rim on its own, so you are looking at weight vs strength.

Finally you have a choice of material - I'll limit it to carbon vs alloy. Here you need to decide if the extra cost of carbon is worth the weight saving and performance benefits, While cheap carbon can be lower cost than brand named high quality alloy, I would caution buying cheap carbon on such a safety critical part of the bike. This is probably decided by the answering the cost question.

If you plan to reuse a hub and spokes (not recommended) you will need to match the Effective Rim Diameter (ERD) of new and old rims.

The new/replacement rim you need MUST match these items. Any rim that doesn't match is excluded from your shortlist:

• Spoke holes - same as what's in the hub
• Rim diameter - there's almost no way to put a bigger wheel in most bikes, and a smaller one is rarely a good idea. So same ETRTO size as the old wheel.
• Brake tech - if you have rim brakes, you need a rim with a brake track. Disk brake bikes can use a wheel with a rim track perfectly well.

Flexible options:

• Rim width - ideally your new rim will have the same internal width as the old one, but there's some leeway here. A wider rim will allow larger tyres while excluding narrower ones.

Completely optional:

• Profile - If your old rim was shallow, you can choose a deep section one, or vise versa. Be aware that your tube/valve may have to be longer, or the spokes replaced with longer/shorter ones if the new rim's ERD is different.
• Material - Rims don't have to match. You can choose to replace carbon fire with aluminium (cheaper) or the other way if you want an upgrade (not cheaper).

Do be mindful of changing too much - a wheel replacement should not grow to replacing transmission parts and shifters and bar-tape and then needing new grips and-and-and then you're broke.

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The above assumes you're reusing an existing hub and spokes. If you're replacing all of it, then the only things you have to match with your bike are

• Brake tech
• Wheel size
• Over-Locknut Dimension (ie the width of the axle between the dropouts
• Wheel retention system (Quick release vs through-axle vs wheel nuts)
• Cassette specs and format (if its a rear wheel)

And if this is your aim, a complete ready-made rear wheel is often a more cost-effective purchase.