Bump Feed vs Automatic Feed String Trimmer: Which Head to Choose

Bump feed vs automatic feed string trimmer heads differ in one thing: who lets the line out. On a bump feed head you tap the spinning head on firm ground, and on an automatic head the machine does it, so tapping can damage it. I trim the grass at my orchard strip edges and around the house lawn, and the feed system decides how often I stop in the middle of an edge. Below I explain how each head works, where each one fails, which yard suits which, and the safety steps that apply to both.

Bump Feed vs Automatic Feed String Trimmer: The Short Answer

A bump feed head releases more line when you tap it on the ground while it spins. An automatic feed head releases line without that tap, either by sensing that the line has worn down or through a trigger sequence the maker defines. The two have opposite rules, and the rule that counts is the one in your model’s manual. My guide on how a string trimmer works follows the head, the line and the drive step by step.

My pick follows the ground I am working over. If most of the trimming happens over grass with no bare soil to tap on, I want an automatic head. If the job runs along paths, a drive and bare soil, a bump head lets me decide when the line comes out. Skip bump feed if you tend to tap in tall grass, and skip a purely automatic head if you want to feed line on demand. A button or trigger sequence, as on the LSTE523 and the Greenworks 2101602, gives back some of that control.

How a Bump Feed Head Works

A bump feed head stores a spool of line and lets out more when you tap the running head on the ground. EGO’s manual for the ST1610T describes the head as one that stores the cutting line and releases it when the head is lightly tapped on firm ground during operation. Gas and battery models from several makers use this design, including the EGO ST1610T and the Husqvarna 130L, which I cover in my Husqvarna 130L gas trimmer review.

The Tap Rule

The two manuals give the same two conditions. EGO says to tap the head on bare ground or hard soil, and Husqvarna says to tap it on a hard, flat surface with the product at full speed. So the head needs two things at once: working speed and something firm to hit.

On my property the tilled strips under the trees are bare soil, so a bump head always has something to tap there. I never tap on loose gravel, because stones can be thrown, and the gravel yard edge is where I walk to bare soil first.

Where a Bump Head Struggles

EGO warns that attempting line release in tall grass can overheat the motor, and that release gets harder as the line gets shorter. The rule that follows is to keep the line fully extended and never let it wear down to stubs. When the line is already too short, EGO’s troubleshooting table has you remove the battery, pull the lines out by hand and alternately press down on and release the head. If you need a string trimmer for tall grass, I compare nine picks for overgrown ground.

EGO’s POWERLOAD button shows that line release and line loading are separate jobs. The ST1610T releases line with a tap, yet its spec list shows motor-driven line loading, and a button winds the new line onto the spool. Read the loading line in the spec list before you assume a bump head means winding by hand.

On the ST1610T the order is: switch off, wait for the head to stop, remove the battery and thread the line. Refit the battery only for the winding step the manual shows, then switch off, wait and remove the battery again before any other work on the head.

How an Automatic Feed Head Works

An auto feed trimmer head lets out line without a tap, but makers use that label for more than one mechanism. The three below come straight from the manuals of two trimmers.

Sensing Wear

The BLACK+DECKER LSTE523 manual says its advanced automatic line feeding mechanism senses when more cutting line is needed and feeds and trims the correct length. The same manual says not to bump the unit on the ground to feed line or for any other purpose. The line is 0.065 in. round nylon, and the manual says smaller and larger sizes affect how well the auto feed works.

Trigger Sequence

Greenworks does it with the trigger on the 2101602. You release the trigger, wait two seconds and press it again, and the manual says the line extends about 1/4 in. with each stop and start until it reaches the cut-off blade on the guard. It also says that bumping the head damages the trimmer and voids the warranty.

Take a gap of 2 in. between worn line and the cut-off blade. At 1/4 in. per cycle that is about eight stop-and-start cycles, with a two-second wait each time. The manual gives the 1/4 in. as approximate, so treat the count as a rough guide for that one model.

Button Feed on Top of Automatic

The LSTE523 adds an EASYFEED button on top of the automatic feed. Press the button while trimming to advance line, and the trimmer stops cutting while you hold it down. BLACK+DECKER says not to keep pressing once the maximum length is reached, because that overfeeds the head and uses up string quickly.

Walls, curbs and fences wear line fast, so along a fence I would rather correct the length with one press than stop and open the head. I treat the button as a correction, not a habit.

Where Each Feed System Fails

A bump head stops feeding when the line is too short, tangled or too heavy. An automatic head stops when the line is crisscrossed, the spool tracks are damaged or the line size is wrong.

Breaking line is the same on both, because the line meets the hard surface, not the feed system. EGO says the very tip of the line should do the cutting and warns that stone and brick walls, curbs and wood wear string rapidly. I check the head before each session instead of waiting for the line to stop coming out, and I keep a pre-cut length in the machinery shed.

Reloading Each Head According to the Manuals

Reloading is where the two systems look most different on paper, and the line size is the part people get wrong. Every manual below names the line for its own head, and a head takes only the diameter or range its manual allows. I explain how to choose the trimmer line size to use, starting with your model’s manual.

EGO says a full spool usually loads in about 12 seconds and asks you to avoid repeated winding in rapid succession. Husqvarna asks for the trimmer line to be wound tightly and evenly, and BLACK+DECKER says crisscrossed line on the spool is a cause of no feed. I wind snug and even for that reason, and I follow the arrows on the head instead of what I remember from an older trimmer. I compare single-line heads with dual line string trimmers in a separate guide.

Which Feed System Fits Your Yard

Match the head to the ground you trim over, then to how much you want to manage the line yourself.

Choose Bump Feed If

• Bare ground is close: you trim along paths, a drive or bare soil, where a tap on firm ground is easy.
• You want control: you decide when line comes out and how much.
• You accept the hand fix: you are comfortable stopping to pull line out by hand when it gets short.

Choose Automatic Feed If

• There is no ground to tap: most of your trimming is over grass with nothing firm underneath.
• You want no tap rule: you prefer a head that never asks for a tap.
• You will keep to one line size: you can buy the exact line or spool the manual names.

Check the Spec Sheet Before You Buy

Spec lists name the feed system, and the name decides your habit. Greenworks lists Feed Type as Auto Feed for the 2101602, and EGO lists the ST1610T as a Bump Head with motor-driven line loading. Read that line before you order, then open the manual to the feed section.

A button or trigger sequence changes the choice a little. The EASYFEED button and the Greenworks trigger sequence let you ask for line, so an automatic head can still suit a worker who wants to correct the length at a fence. I describe one cordless auto feed head in my review of the EGO ST1623T, including how it behaves in use.

Line Safety With Either Head

The feed system changes how line comes out, not how dangerous the line is when it leaves the head.

A review of US emergency room data counted an estimated 81,907 lawn trimmer injuries from 2000 to 2009, and the eye accounted for 42.5% of them (1). A series of 26 lawn trimmer open-globe injuries in Taiwan found a foreign body inside the eye in 57.7% of those cases, most often metal (2). Published eye trauma cases also include bystanders (3).

Before You Touch the Head

This order applies to the hand-pull fix on a bump head, the jam fix on an automatic head and every line change.

1. Release the trigger or switch the engine off.
2. Wait until the head has stopped completely, and on a gas model until the engine has cooled.
3. Remove the battery, pull the spark plug wire on a gas model, or unplug a corded one.
4. Only then clear the head, pull line by hand or change line, and wear gloves, because the cut-off blade on the guard is sharp.

Battery trimmers take only the battery and charger the maker approves for that exact model. Charge and store the pack within the temperature range in the manual, and never use a damaged or swollen pack. Refuel a gas trimmer outdoors with the engine off and cool, and never run one in a garage, shed or other closed space. A corded trimmer needs a GFCI outlet and a cord kept clear of the line. If you are still choosing a power type, my guide to battery and gas string trimmers compares the two for a typical yard.

Keep the Line the Right Length

The cut-off blade on the guard trims line to length, and EGO warns that excessive line length overheats the motor. If line still sits past the blade, stop the trimmer, wait for the head to stop, remove the battery (gas: pull the spark plug wire) and only then cut it back, using the order above. Length also changes how the tool feels in the hands.

In an experiment on one grass trimmer, thread length, engine speed and handle material all changed hand-arm vibration (4). In a laboratory comparison, grass trimmers gave higher hand vibration than backpack blowers, and gloves lowered the vibration on grounds equipment (5). I check the line length before each edge and keep my gloves on for it.

Choosing a Head and Learning Its Rule

Three things to remember. A bump head needs a light tap at working speed on bare ground, never in tall grass. An automatic head needs the right line size and no tapping. Both need the motor off, the head stopped and the battery out (gas: spark plug wire off) before a hand goes near the head. When you are ready to buy, the string trimmers I rate best are in my roundup.

My own tip is to write the feed rule from the manual on a strip of tape on the shaft of a new trimmer, so nobody who borrows it bumps an automatic head. Read your model’s feed section and its reload steps before the first cut. In the bump feed vs automatic feed string trimmer choice, the feed rule on your model’s manual page is the one that counts.

Frequently Asked Questions

How does string trimmer auto feed work?

String trimmer auto feed works by advancing line without a tap, either by sensing wear or through a trigger sequence. The BLACK+DECKER LSTE523 manual says its mechanism senses when more line is needed and feeds and trims it, while the Greenworks 2101602 releases about 1/4 in. each time you release the trigger, wait two seconds and press again. The method differs by model, so follow your manual.

How do you refill a bump feed trimmer?

You refill a bump feed trimmer by switching it off, waiting for the head to stop and removing the battery, or the spark plug wire on a gas model, before you touch the head. Then load new line the way the manual shows, using the line size it names. On the EGO ST1610T you thread the line with the battery out and refit it only for the button winding, and on the Husqvarna 130L you push about 22 ft of line through the holes and turn the head.

How to use a bump feed string trimmer?

To use a bump feed string trimmer, run it at working speed and lightly tap the head on bare ground or hard soil when the line gets short. Keep the line fully extended, and do not attempt the tap in tall grass, where EGO warns the motor may overheat. If the line is too short to release, switch off, wait for the head to stop, remove the battery (gas: pull the spark plug wire) and pull it out by hand.

Why won’t the string feed on my trimmer?

The string will not feed when the line is too short, tangled, too heavy for the head or blocked by debris on the spool. EGO and BLACK+DECKER both list these causes, and BLACK+DECKER adds damaged spool tracks and the wrong line size. Switch off, wait for the head to stop and remove the battery (gas: pull the spark plug wire) before you open or clear the head.

How does a bump feed work on a string trimmer?

A bump feed works on a string trimmer by storing line on a spool and releasing more when you tap the spinning head on firm ground. EGO describes the head this way, and Husqvarna adds that the product must run at full speed. The tap is the only trigger, so an automatic head works differently and should never be bumped.

References

1. Leinert J, Griffin R, Blackburn J, McGwin G. The epidemiology of lawn trimmer injuries in the United States: 2000-2009. J Safety Res. 2012;43(2):137-139.
2. Zhang JR, Hsieh TC, Chang FL, He MS. Lawn trimmer-related open-globe injuries in Taiwan. Retina. 2022;42(5):973-980.
3. Lubniewski AJ, Olk RJ, Grand MG. Ocular dangers in the garden. A new menace: nylon line lawn trimmers. Ophthalmology. 1988;95(7):906-910.
4. Mallick Z. Optimization of the operating parameters of a grass trimming machine. Appl Ergon. 2010;41(2):260-265.
5. Oh J, Chen N, Boyd C. Laboratory evaluation of occupational exposure to hand-arm vibration (HAV) during grounds maintenance equipment operations. Ann Agric Environ Med. 2023;30(2):384-389.

Safety Note: Read your manual first. A string trimmer or brush cutter throws debris with force, so clear stones, glass and wire from the area before you cut. Wear impact-rated eye protection with side coverage that meets ANSI Z87.1, hearing protection, gloves, heavy long pants, long sleeves and sturdy slip-resistant boots (steel-toe where your manual says so). Keep people and pets at least as far away as your manual says (50 feet in some manuals, 100 feet in others), and stop if anyone comes closer. Keep the correct guard fitted, use only the cutting attachments, handles and harness the maker approves for your exact tool, and never replace trimmer line with wire. A blade can kick sideways when it hits a stump or a thick stem, so fit the handlebar and harness your manual requires for blade work, hold both handles firmly and keep secure footing. Before you clear a jam, clean the tool or change line, a blade or an attachment, switch it off, wait for all movement to stop, then remove the battery, unplug the cord or disconnect the spark plug wire. Refuel a gas model outdoors with the engine off and cool, away from flames or sparks, and move away from the fueling spot before starting, as far as your manual says (often 10 to 30 feet); never run it in a garage, shed or other closed space. For a battery model, use only the maker’s battery and charger, charge it within the temperature range in your manual and never use a damaged or swollen pack. For a corded model, use a GFCI outlet, keep the cord clear of the cutting line and avoid wet conditions.