Bike Lessons For Rookies: Chain Tensioners

from Rookie’s keyboard,

Hello, friends

I decided that this Saturday is as good a time as any to make another technical post. I’m going to present you the basics of chain tensioners – a mysterious component that some people haven’t even heard of…yet. 

If you haven’t seen a chain tensioner, here’s a classic one.

As the name suggests, a chain tensioner…tensions the chain. lol. No surprises here. In the case of bicycles with external gears (which make like 90% of the bikes out there), the rear derailleur plays the role of a chain tensioner too. 

When the user shifts from one gear to the next, the spring of the rear derailleur maintains the tension by taking up the slack of the chain. 

When shifting to a larger cog (lower gear), the derailleur’s spring is pulled. When shifting to a smaller cog (higher gear), the derailleur’s spring is allowed to compress. 

Without that spring, the derailleur cannot function as the chain tension will never be correct and the chain will fall off. 

This is how a derailleur spring looks

Well, I guess it’s obvious that if you have a multi-speed bike, you don’t need a chain tensioner as you already have one. 

So, why do dedicated chain tensioners exist then? 

Their main function is to make it easier to tension the chain of single-speed bikes. They can work on any frame but are needed the most when the frame has vertical dropouts. 

Notice that you can’t move the axle forward or back in the dropout. Hence the name “vertical dropouts”

Vertical dropouts make it impossible to tension the chain by moving the wheel backward, and if you want to run a single-speed transmission on such a frame, a chain tensioner is a logical choice. 

I will admit that chain tensioners are not always aesthetically pleasing and do add some “complexity” to a single-speed setup that hurts the simplistic silhouette of the machine, but their functionality makes up for it.

Technically, you can tension the chain by using half-chain links, but that method is super slow and frustrating. Also, as the chain wears off, its length increases too, and very quickly you will have to perform the procedure again. It’s simply not worth the effort.

I don’t know about you, but I don’t enjoy playing with a dirty chain and would rather have a chain tensioner on my bike and ride more than worry too much about aesthetics. 

Some technical details

Chain tensioners are mounted to the derailleur hanger and function as a t-pulley stretching the chain.

There are two versions depending on the spring – push-down or push-up. In the push-down case, the pulley of the tensioner is “inside the chain” and pushes it down like a regular rear derailleur.

The push-up option goes below the chain and pushes it upward. 

The push-down version is the go-to choice as it fits a greater number of drivetrains and bicycles.

But the push-up model has a distinctive advantage – it reinforces the “chain wrap” over the rear cog and prevents teeth skipping. 

The good news is that most tensioners can be switched from a push-down to a push-up mode and vice versa.

+ Bonus(A tip for broke rookies who need a chain tensioner)

Chain tensioners are overpriced because the demand for them is low. And whenever that’s the case manufacturers always boost the cost as the margins are tight. It’s also possible that your LBS doesn’t have one.

If you don’t mind the look, the solution is to use your existing derailleur or buy the cheapest you can get and turn it into a chain tensioner. 

There are two options.

Option A: Limit Screws + Chain Shortening

This method is fast and simple but requires you to play with the length of the chain. 

Step 1: Remove the gear cable completely.

Step 2: Use the limit screws on the rear derailleur to align the pulleys with the rear cog that you’re going to use. Make sure that the chain line is as straight as possible. Crossed chains increase the wear of the drivetrain and hurt the pedaling efficiency.

Step 3: Shorten the chain until the t-pulley is positioned forward as shown in the next image. 

Option B:  Limit Screws + A Piece of Gear Cable

This method mimics the previous one but adds one complication that will allow you to make fine adjustments to the derailleur’s position in relation to the cassette/cog.

Repeat the steps from Option A, but also slide a cut piece of gear cable through the derailleur. Use a stopper on the gear cable as an anchor. Without the stopper, the method doesn’t work.

Run the piece of gear cable with the anchor pointing backward. When done correctly, you will be able to use the barrel adjuster knob to move the derailleur closer or further away from cassette/cog.

The images speak the whole story. 

Position А of Тhe barrel adjuster: The pulleys of the derailleur are maximally close to the outer side.
Position B of The Barrel Adjuster: The pulleys are pushed maximally to the inner side.

This method allows work as a ghetto multi-speed bike. In other words, you can use more than once cog and use the barrel adjuster to switch between the two.

Of course, to do that, you will have to get off the bike and spin the pedals with your arm. It’s super “ghetto” but some people might like it.

A Note On Horizontal Dropouts

If you have horizontal (or semi-horizontal) dropouts (HD), you can get away without a chain tensioner as pulling the wheel back will tension the chain. The process is annoying (I’ve done it too), but it works. 

Hence many people who want their single-speed monster to be as clean as possible convert frames with HD. 

If you plan on going this way, and you still haven’t found a frame that fits the bill, get one with longer dropouts as it will give you more room for adjustments. 

Horizontal Dropouts (HD)
Semi-horizontal dropouts

Loss of chain tension during pedaling

During a spring or a climb, the chain transmits a lot of force to the rear axle. If the wheel isn’t tight, it will move forward over time. And when that happens, you will lose chain tension.

To avoid this outcome, check the area frequently and tighten the axle nuts if necessary.

Single-speed bikes rely on axle track nuts with serrated built-in washers to ensure greater friction between the nut and the dropout. 

Don’t use quick-release skewers

QR skewers can work but are not ideal and very likely to cause frequent loss of chain tension when coupled with horizontal dropouts. They simply can’t offer the clamping force of standard track nuts. 

Important Note: Do Not Use А Chain Tensioner For Fixed Gear Projects

This is VERY important for all the rookies out there to understand. 

Both chain tensioners and rear derailleur aren’t suitable for fixed-gear conversions because the stress on the entire transmission is too great when stopping by resisting the pedaling motion.

If you do use one of those on a fixed-gear drivetrain, the chain will break, and the wheel will jam. I will leave the possible outcome to your imagination.

Precisely for that reason, frames with vertical dropouts aren’t suitable for fixed-gear conversions.

Rear-facing dropouts or “Track Ends” for greater accuracy

If you want to build a fixie, the ideal is to get a frame with rear-facing horizontal dropouts. The second less ideal but still not unheard of option is a frame with long forward-facing horizontal dropouts.

There you have it, friends. All you need to know about chain tensioners. 

Until next time,

– Rookie






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