How Does Trimmer Line Feed Work?

Trimmer lines gradually wear down with use as they are made of composite or plastic material. Instead of manually pulling out a new line, trimmer heads have feed systems that effortlessly advance the line and make yard cleanup easier. So, how do these trimmer line feed systems work? 

The different trimmer line feed systems release the line into the trimmer head to cut grass and weed. 

  • Bump feed works by tapping the trimmer head on the ground to release the line.
  • The automatic feed mechanism provides a new line as its length in the cutting head gets shorter.
  • A fixed line feed uses pre-cut lines that need to be changed when they break down. 

Knowing how different trimmer line feeds work is essential if you want to pick the right one for yourself. I’ve covered all three types, how they work, and their advantages below. 

What Types of String Trimmer Heads Are There?

Trimmer heads do the actual cutting work in a trimmer. So, you must opt for the right one, as these can make or break your yard cleanup experience. 

They are divided into three categories based on their feed systems;

  • Bump Feed
  • Auto-Feed
  • Fixed Feed

All three trimmer head types are discussed below, along with a list of their pros and cons. 

What is a Bump Feed Trimmer Head?

Bump feed trimmer heads feed out line when they are gently tapped on the ground. They are the most commonly used trimmer heads. 

How Does a String Trimmer Bump Head Work?

Bump feed trimmer heads use a spring system to advance new lines. Pressing the head against the ground squeezes a spring inside it. This action releases the line from the spool using centrifugal force. 

So, each time you want to feed out line while trimming your yard, you only need to tap the head of the trimmer on the ground!

Pros  Cons
The bump feed system is a straightforward method of advancing the line. So you can quickly repair and fix it if it malfunctions.  You must be cautious while bumping the trimmer head as you can damage the spring or spool within it. 
You have complete control over the length of the line used.  You must be more attentive while trimming your yard, as you must know when to advance the line. 

What is an Auto Feed Trimmer Head?

Automatic feed trimmer heads are extremely popular among gardeners because of their ease of use. They automatically feed out the line without any manual effort. 

You don’t need to tap them on the ground or perform any other action to advance the line. 

How Does the Auto Feed Work on a String Trimmer?

Auto feed trimmer heads use an advanced internal sensor that keeps track of the line length in the cutting head. As the line is about to run out, the sensor senses it and automatically dispenses a new line into the cutting head. Also, there are models out there that just gradually dispense more line as the machine is running regardless of its current length, which may not be the best option for everyone. 

Just like the bump feed system, the automatic feed system uses the centrifugal force generated by the fast-spinning action of the equipment to pull the line from the spool.

Pros  Cons
Since the head automatically advances the line without manual intervention, you can focus on adequately trimming your yard instead of worrying about the line running out. The head uses a line-spooling motor to make the line-feeding process automatic. This motor can malfunction, and fixing it will take time and effort (or money).
It gets the job done faster as you do not have to tap it constantly on the ground. It is more expensive. 

What is a Fixed Line Trimmer Head?

Fixed line trimmer heads remove the hassle of winding in a line. They work with pre-cut lines that are placed inside them. They are mostly used in commercial jobs that require heavy-duty work, as they can hold extremely thick lines! 

How Does a Fixed-Line Trimmer Head Work?

The pre-cut lines are loaded or threaded into the trimmer head one piece at a time as required. 

Once the line in the head wears out, you must replace it with a new piece. 

Fortunately, most fixed-line trimmer heads use three to six lines at once. So, you can continue your trimming work even if one of them wears out. 

Moreover, this trimmer head gives you complete control over your line, as you can even choose the length of the line based on your work. The other two types of trimmer heads have guards with blades that maintain the line length according to the manufacturer’s recommendation. The blades cut off any excess lines, so you can’t use an overly long line. 

Pros  Cons
It can hold thick-diameter lines.  You will need to stop using the trimmer each time you want to replace the line.
Your line won’t break often. So, you won’t have to take frequent breaks to replace it and can complete the task faster.  N/A
You won’t have to go through the trouble of winding a new line into the spool each time the old one runs out.  N/A

Here is a summary of all three trimmer line feed systems:

Feed System Mechanism Pros Cons
Bump Feed Tap the trimmer head on a flat surface to advance the new line. More control.


Easy repairs.

Frequent repairs if you bump the head too hard. 

Time-consuming as you need to pause to bump the head. 

Automatic Feed  Automatically feeds out a new line when the current one becomes short.  User-friendly.

Saves time.

Expensive repairs. 

Uses expensive pre-wound spools.  

Fixed Line Feed Uses pre-cut lines that must be replaced once they wear out.  Holds lines with thick diameters. 

Less chance of line breaking. 


Time-consuming as you will need to stop trimming to reload the line after the old one wears out.

Bump Feed vs. Automatic Feed vs. Fixed Line Feed

I have compared the features of all three feed systems below so you can get a better idea of their differences and choose one that best fits your needs. 

Line Feeding 

You can advance the line with bump feed whenever you want. However, you can’t feed out more of it until it reaches a certain shortness in automatic feed heads. On the other hand, there is no method of line advancement in fixed-line feed systems as they use pre-cut lines. 

Moreover, bump feed, and fixed line feed trimmer heads use fewer lines as you have more control. On the other hand, an automatic feed system runs more quickly through a spool of the line as you have less control over the feed. 


Automatic feed systems are generally more user-friendly than bump and fixed-line feed systems. The line feeds out automatically without the need for any manual input. 


Trimmer heads with automatic or fixed line feeds are usually more durable. 

In bump feed systems, you must tap the trimmer head on the ground, which can damage it if you aren’t careful. 

Line Replacement 

Automatic feed systems tend to use pre-wound spools, which are more expensive. However, since you won’t have to wind the spool before using it, you will save a lot of time. 

Bump feed systems allow you to rewind the line when it runs out. This method is cheaper but more time-consuming.

Fixed line systems are both; affordable and time-efficient. They use pre-cut lines that do not need to be wound into a spool. 

I recommend using the bump feed or fixed line system for large trimming tasks where lots of line is required. But, if your yard is small and you do not have much time, you can opt for an automatic feed trimmer. 


Automatic feed systems use complex motors that can malfunction. Repairing them is expensive and time-consuming. 

Bump feed systems are straightforward, and fixing them is easy and cheap. However, they require repairs more frequently due to the constant bumping action. It wears down the spring and prevents new lines from feeding out until it is replaced. 

Like bump feed systems, fixed line systems are also easy to repair as they do not use complex mechanisms to advance new lines. Moreover, since you do not have to bump them on the ground, these trimmer heads don’t require frequent repairs! 


Automatic feed systems generally cost more as they use expensive lines, and repairing them can be expensive. 

Bump feed systems can help you save a couple of hundred dollars as they use cheap bulk rolls of line. 

Fixed line systems are also budget-friendly as they use more affordable lines. 

How To Choose The Right Feed System For Your Needs? 

Choosing the right feed system depends on your personal preference. Automatic feed trimmer heads are easier to use and more durable but expensive in the long run. Since they use expensive pre-wound spools, they are best for small lawns. 

Bump feed trimmer heads require you to be more attentive, but they generally cost less. They are recommended for gardeners with medium to huge trimming tasks. 

Fixed-line trimmer heads are mostly used in commercial business as they can hold extremely thick lines for heavy-duty work. They also remove the hassle of winding a new line into the spool, which is ideal for professionals who are short on time. 


How Frequently Do You Have To Feed New Line In A Fixed Line Trimmer Head?

Fixed-line trimmer heads simultaneously use three to six lines for trimming yards. You need to feed in new lines after at least two wear down.

Most users only have to stop once or twice to feed in new lines during a full weed-eating job. 

Final Thoughts

Each feed system works differently and comes with its own pros and cons. With so many options available on the market, you can pick one that suits your needs best. You can gradually work your way through your lawn with a bump feed trimmer head or quickly get your trimming tasks done with an automatic or fixed line trimmer head. 

Get the right trimmer head and turn yard cleanup from a boring and tiring task into an effortless and fun one. 

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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