Custom Build · Behind the Build
You have probably seen the headline number: 159.9 dB, recorded at a formal SPL competition, on a 2017 Road Glide Special that Alex still rides every day. The full system breakdown lives in the main article, Hertz Road Glide Audio Monster. This one is about how it was actually built: the teardown, the fabrication, the wiring and the decisions made along the way, in photos straight from the workshop.
A build like this doesn't start with speakers. It starts with a plan. If you've bought the parts before the plan, you've already failed. Get the plan right, and it becomes a bare bike, a bench full of boxes and a clear order of operations.
The Starting Point
Every component was chosen before a single panel came off. Three Hertz amplifiers, the S8 DSP, eight full-range speakers, dual 10-inch subwoofers and a dedicated auxiliary power system. The shopping list is in the main article. What the spec sheet doesn't show is how much of the build is not audio gear at all: cable, connectors, fusing, mounting hardware and fabrication.
Teardown
Before anything goes in, a lot comes off. The fairing is stripped to its frame, the factory radio comes out, and the rear of the bike is opened up for the saddlebag and Tour-Pak work.
By the end of teardown, very little remains on the bike, but nothing is left lying around. Panels are protected, components are labelled, fasteners are organised and everything is stored safely until it goes back together. It all has to go back on, unmarked, over the top of a finished audio system.
Fabrication: Lids and Enclosures
The saddlebag stage is where the fabrication hours went. The 6×9 positions and the two 10-inch subwoofers needed openings cut and sealed enclosures built. We never cut a lid without a full template: sometimes MDF, sometimes polycarbonate, sometimes supplied by the manufacturer. Measure twice, cut once is cheap advice when the part under the blade is a painted saddlebag. The sealed enclosures do two jobs: give the subs the air volume they need, and isolate the 6×9s from subwoofer backpressure.
The Amp Rack and the Brain
Up front, both SP4.900 amplifiers live on an XEL Customs 2015-2023 Road Glide amp rack, the same rack we run on our shop demo bike. The S8 DSP is mounted beneath the head unit on a custom 3D-printed plate. Anything vulnerable stays masked and protected while the surrounding work happens.
Power and Protection
Three amplifiers means serious current, and serious current means proper distribution and fusing, not a single inline fuse and hope. The power and ground runs are KnuKonceptz Kolossus Flex tinned OFC: 1/0 AWG for the main feeds, stepping down to 4 AWG for the individual runs to each amp. Karma SS tinned copper speaker wire in 12 and 16 gauge serves the subwoofers and full-range speakers respectively, and Krystal twisted-pair RCA interconnects (with Krux right-angle adapters where space is tight) carry the signal. Tinned copper throughout is a deliberate choice on a motorcycle: it resists the corrosion that plain copper suffers in the weather.
The main 1/0 AWG feed runs into a Bassik fused distribution block. One 120 A branch supplies the two front-mounted SP4.900 amplifiers, with each 4 AWG amplifier run protected by its own 60 A Mini-ANL fuse, and the separate 100 A branch supplies the SP1.900 mono amplifier. The bike's main frame earth connection was also upgraded. On a system drawing this much current, the ground path matters as much as the power feed.
The Wiring
By this point there are hundreds of individual terminations, labels and cable runs. None of it will be visible once the fairing goes back together, but it is the part of the build that determines whether the system will still be reliable years down the track. Everything is loomed, labelled and left with service slack, and quick-disconnect connectors sit at every point where a panel or speaker pod needs to come off again.
The Auxiliary Battery System
The build's quiet hero is the power system: a 100Ah LiFePO4 auxiliary battery in the Tour-Pak runs all three amplifiers and the DSP. The auxiliary battery supplies the amplifiers and DSP directly, while the current-limited DC-DC charger places a controlled charging load on the bike's electrical system. This isolates the factory starter battery and charging circuit from the system's high transient audio demand. A BMS display keeps the state of charge visible at a glance.
Finished Details
The finish work is what makes a system this size look like it belongs on the bike rather than bolted to it.
From Bare Fairing to 159.9 dB
The number gets the attention, but the number is a consequence. It comes from the parts of the build nobody sees once the panels go back on: the sealed enclosures, the fused distribution, the upgraded earth, the DSP processing every channel, and a power system engineered for the load. That is the difference between a collection of upgraded parts and an engineered system.
For the full component breakdown, the DSP configuration, the power figures and the story of the bike night where the stereo won the loudest-bike competition with the engine off, read the main article: Hertz Road Glide Audio Monster. The build is also on video: watch it on our YouTube channel.
Frequently Asked Questions
Do all Harley audio builds involve this much fabrication?
No. This is the top of the scale. Most systems we supply are plug-and-play kits and direct-fit speakers that need no cutting at all, and purpose-built lid kits exist for riders who want rear speakers without permanent modification. Fabrication at this level is for competition-grade builds where the enclosure design is part of the performance.
Why do the subwoofers need sealed enclosures?
Two reasons: the subs need a controlled air volume and structural rigidity to perform, and sealing them isolates the 6×9 rear speakers from subwoofer backpressure. Without it, the drivers fight each other inside the bag at high output.
Why not run everything off the bike's electrical system?
Three amplifiers at high output draw more current than a factory Harley charging system is designed to supply continuously. The auxiliary LiFePO4 battery carries the audio load, recharged through a current-limited DC-DC connection while riding, so the starter battery and factory charging circuit are not exposed directly to the amplifiers' high transient current demand. The detail is in the main article.
Thinking About a Build Like This?
Start with the planning, not the parts. Most riders don't need triple amps and a second battery, but every good system begins the same way. Tell us your bike, how you ride and how far you want to take it, and we'll map the right build.
Get in Touch →Plan Your Own Build
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- Stage 1 to 3: How to Think About a Harley Audio Upgrade
- DSP Tuning for Harley Baggers (Explained Simply)
- Hertz Road Glide Audio Monster: the full system breakdown
- More customer builds
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