Sooner or later, every cyclist will have need of a spoke wrench. Spoke wrenches, sometimes called spoke keys, are small tools that are used to install, remove or tighten the small attachment that holds each spoke in place where it meets the rim of the wheel.

Before we can really explain what a spoke wrench does, you have to understand the basics of how a spoked wheel functions.

How A Spoke Works

Spoked wheels are relatively simple, yet ingenious. Spokes provide strength while keeping the weight down and allow cyclists to replace damaged sections with ease.

The small mechanical piece that attaches each spoke to the rim is known as a spoke nipple. Spoke nipples consist of two parts: a plate that rests on the inside of the rim, and an internally threaded, square sided nipple that extends through the holes punched along the inside edge of the rim.

In order for the spokes to be installed, each spoke must first be aligned with it's matching spoke nipple. Each spoke is threaded on the end, and is inserted into the spoke nipple and tightened. This creates tension between the hub of the wheel and the rim, and it gives the wheel its strength and structural integrity. The plate mounted on the base of the nipple creates a firm attachment point, preventing the nipple from pulling out of the rim.

In this way, tension can be created between the hub and the rim by tightening each spoke nipple. The spoke wrench is the tool that is needed to turn a spoke nipple and therefore tighten or loosen the connection with the spoke.

Spoke Wrench Design

Almost all spoke wrenches look the same: small, square-edged tools with a looping handle and the ability to grip spoke nipples on three sides for turning in either direction.

Spoke wrenches vary slightly in size. Most American wheels have a 3.23mm spoke nipple diameter, while European-made wheels generally run at 3.30mm, and Asian wheels at 3.45mm. All these sizes can be encountered around the world, which is why some common spoke wrenches are circular or triangular in shape and incorporate all the common sizes.

Spoke wrenches are ultra-simple and pretty darn cheap, but keep an eye out for slipping. Some poorly made spoke wrenches will slip on the nipple and can strip the corners, making them unusable. This can be a serious issue -- you can totally ruin expensive wheels this way or at least be in for an expensive trip to the repair shop. Be careful when you tighten your spokes. Go slow, and keep an eye out for slipping.

Using A Spoke Wrench

When first assembling a wheel, a spoke wrench in the proper size is essential to attaching each spoke to the wheel. It's also a useful tool for maintenance, since spoked wheels need to be adjusted periodically.

Whichever is the case, there is two main goals to using a spoke wrench. The first is to ensure adequate tension to provide structural integrity. Spokes should feel firm when wiggled, shouldn't move around, and should be close to the same tension all the way around the wheel.

The second goal is to the keep the wheel true, which means as close to perfectly circular as possible (when viewed sideways) and as close to flat as possible when viewed from the top. When a wheel isn't true, it will wobble from side-to-side, which can severely compromise its strength.

The procedure for correcting these issues is called truing a wheel and is an important process for any serious cyclist to understand. The process consists of gradually tightening or loosening individual spokes or groups of adjacent spokes to correct any imbalances in the wheel's geometry.

Bike shops will use a special piece of equipment for this, called a wheel truing stand, but for emergency and at-home truing, you can use your front fork or rear frame triangle. Simply flip your bike upside down, and spin your wheel. Observe any wobbles from side to side, and correct them by tightening the spokes opposite the direction of the wobble. Continue this process (making sure not to over-tighten your spoke nipples) until the wheel is more or less true.

It's also possible that you could break a spoke while riding. Depending on how many spokes your wheel has (in general, the more spokes the stronger the wheel), you may be able to make it home without much worry. Many touring cyclists and long-distance riders carry extra spokes with them for emergency replacement, and a spoke wrench is a critical tool for these situations. Many bike multi-tools have a small, built-in spoke wrench, but these are often awkward to use. If you're doing anything more than emergency repairs, you'll probably want the real deal.

Most spoke wrenches are available for between $5 and $20.