Summary: Comparison of the technical differences between the
AH1500 Rapid Reload (Gen 2) and AH1300 Bump (Gen 1) heads.
DISCLAIMER: Ego's official position is that the AH1500 is NOT recommended for use on the older Gen 1 15" trimmers (ST1500, ST1500-S)
Purpose: Determine the technical rationale for Ego's position on non-interchangeability
1) The first and largest difference is that the RR head is 4 ounces heavier than the bump head; so right off the bat the motor has more inertia to overcome. The diameter is also larger by 1/2”, which means the mass is carried further out from the axis of rotation (the shaft), which contributes to rotational inertia. What this means in simple terms is that it will take more initial torque to get the head spinning. As this is brushless motor, there is a complex motor controller, which is good news because you can program the motor to start up in a very specific way. As it is generally a bad idea to apply full power to anything that is stopped, the engineers designed it to have a smooth ramp up speed. To digress to power tools for a minute, a table saw that does not have a “soft start” feature may jump a bit when the power is applied. Similarly, a hand drill that jumps will likely be disconcerting to the operator. The down side is that if you error on the low side, specifically start the motor too slowly, you could have a NON-START scenario if you increase the weight of the trimmer head.
2) The next important difference is that the height at which the string exits the head is 1/2" higher on the RR head than the bump head. This could cause issues if the string hits the cut off blade too low, as it will miss it completely, or the blade could bend over time. In initial testing, I have found that string cuts off normally, and no abnormal bending of the blade has occurred.
On a few occasions, I did notice that the RR head would fail to start. When this condition happened the head will "twitch", but then does not start. This is likely due to a safety feature which prevents the head from starting when it detects the trimmer head is bound up (tangled in vines would be an example). It is important to note that I had the modified spacer block installed (see below) when this happened, and since removing the spacer block, the non-start condition has not happened again. When this condition did occur, I was able to get the head turning by rubbing it on the ground, and once it was moving a little, applying power allowed it to start up. With the spacer block removed, I cut for about 20 minutes, and the condition did not recur, so it was likely the spacer block caused that issue.
My first attempt to add a “rapid reload” head for my 15” trimmer was to adapter the Echo Speed Feed 400 Universal Head (see link in related threads below). The Echo head sat lower, and it required a spacer block to move the guard and blade lower by 1”. This also inadvertently increased the string length from 15” to 16” (cutting swath). After initially installing the RR head, I left the spacer block; however, I have since removed it.
The likely basis for Ego’s position is that the motor controller ramp up curve is tuned to the lighter weight head. The RR head is 44% heavier (13oz vs. 9oz), with a corresponding increase in the inertia required to start it spinning. From an engineering perspective, 44% is too much of an increase to recommend that the average consumer use this configuration. However, this setup has worked for me (and others have reported success as well).
Echo 400 Speed Feed head MODIFICATION
15” Rapid Reload Head
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