Aluminum and Hybrid guitars: Pros, Cons, Whys..
All-aluminum and hybrid wood-aluminum guitar construction has been around since the inception of the electric guitar. In fact, the first commercially viable guitar was all aluminum. The Electro Hawaiian Guitar Company (Rickenbacker) produced an all-aluminum body lap steel guitar The Frying Pan in ~1931.
- Rickenbacker history
- Walter L. Fuller finalized the design of Gibson’s first pickups for an all aluminum electric E150 lap steel guitars, introduced in 1935.
- A large aluminum and hybrid manuf. list, at JediStar and history at MugWumps and Appleton
The bottom line is a guitar’s sound can’t be categorized solely by what the body material is, it is a lot more complicated. Body shape and structure, stiffness, moment of inertia in all axes, alloy/material, alloy heat treatment, bolted on dampeners-masses (neck, pickup module, pickups), etc. etc.… All effect the body’s resonant nodes, anti-nodes, dampening, and feedback to the neck and the strings (via the bridge).
Aluminum doesn’t have a lot of dampening compared to wood. So, some frequencies that are selectively damped (vs. frequency) with different woods aren’t with aluminum. We use our wood necks and wood pickup modules for this flavoring.
“Tap Tuning” our body, with our constantly changing 3D curving surfaces, material, bolt-on choices, etc., gives a thunk rather than a single note bell ring.
Hybrids aren’t unusual in instruments. Check out the plate or frame, the “corner stone” of a Steinway grand piano, is 400lb of cast iron. Not just selected or shaped to fight string tension.
A small bodied wood guitar tends to flop around eating stringenergy (killing sustain and dropping volume on various resonant notes). The lack of mass and material on bending axises combined with woods poor stiffness means they’re rarley seen in studios. Aluminum and carbon fiber’s stiffness give them a special advantage here.
Young’s modulus is Stress/Strain, describing the stiffness of a material. For maple bending with-the-grain = 1.8×106 psi, whereas aluminum is at 10.5×106 psi; a ~6x higher stiffness.
Not very easy bending our body, to bend a note. This together with the body shape, help us go to a small body and smaller dimensions around the neck heel pocket, for better reach-around at high notes. Look at the wood left after the pickup cutouts are done on a Flying V body and try not to cringe.
Aluminum bodies have far less problems with cracks than wood, far higher unit strength, and not in just the grain direction. Yield stress is a mechanical property describing the limit after which a part doesn’t return to its original shape. For maple bending is 5 kpsi with the grain and .75 kpsi perpendicular to the grain; with our aluminum ~ 20 kpsi.
A 4X difference compared to with-the-grain, 25X stronger comparted to cross grain wood. Anywhere wood gets thin, loads in a non-grain orientation, and especially hi stress areas, it will crack; a few typical heel pocket cracks here. Wood Properties here.
Tuning Stability -> from Temperature changes, Hybrids rule!
Our hybrid’s high expansion rate of aluminum body combines with the low temp expansion of our wood neck to get close to the medium temperature expansion of string steel music wire; almost self-compensating for temperature!
Expansion coefficients for our example; maple at 2.1, aluminum at 11.9 and music wire at 6.7 uin/in*F. Our wood neck makes up 16.7in and Al body makesup 8.8in of a typical 25.5” scale length.
So, a 30degF (16.6C) change on a headless or string locked guitar:
- All wood body length difference = StringGrowth- WoodGrowth=
25.5” x (6.7-2.1uin/in*F) x 30F= +.0035”, 25 Cents Flat!
- All aluminum length difference = StringGrowth – AlBody Growth=
25.5” x (6.7-11.9) x 30F = -.0040”, 28 Cents Sharp!
- Hybrid, Wood neck and Aluminum body length difference = StringGrowth –(WoodGrowth+AlBodyGrowth)=
[(25.5”x6.7) – (16.7”x2.1 + 8.8”x11.9)] x 30F = +.0009”, 6 cents slightly flat
So, the Hybrid combination of a wood neck and an Al. body gave a ~4x reduction in differential expansion from a 30F temp change, for significantly more tuning stability; compared to all wood or all aluminum.
- A standard headstock guitar uses a longer total string (including overhung string) and body length; and thus would fare worse.
- Calcs assume even temp changes, like thru a case. Open air fast warmups and chill downs would depend on surface/volume ratio, masses, and conductivity. Generally, the steel strings would change temp fastest, aluminum body next, then wood the slowest.
- An all carbon fiber guitar’s low expansion coef. (close to wood) puts it going flat 25cents in our example.
Tuning Stability –Humidity changes, all Aluminum or (Carbon Fiber) wins
Steel strings and aluminum bodies don’t grow with increasing humidity. The all-aluminum guitar would fare best, and our hybrid wood/Al would be middle of the road while still far better than the all wood guitar as 35% of the length isn’t affected by humidity.
Wood grows with increasing humidity depending on direction to the grain, cut (quarter sawn or flat sawn), preparation, and coating. Although humidity dependency for wood in the grain direction is low, other grain direction dependences can fight the truss rod and change tuning from bending.
Our standard neck is roasted (torrefaction) maple. This heat-treating process gives the wood much less response to humidity changes, less weight, and increased long term stability.
- Sounds must be judged on a case by case basis, (like people), no generalizations are applicable based on appropriate materials. The design details typically rule. (yes, rubber wouldn’t be a fun starting point)
- All material choices have trade offs
Aluminum Body-Wood Neck Hybrid
o Bridge can be built into the body for a more direct string to body connection, less mud and cleaner separation of notes
o Lower dampening = more sustain
o Higher Stiffness, 6X
- Helps raise resonant frequencies
- Lets a small body do its job, like a bigger massive one
- More contouring can take place around the neck’s heel for better hand access ergonomics, without sacrificing flex
- Mass reduction in body can made up with stiffness
o Built-in electric shielding
o Greater durability from higher strength material, 4->25X
o Better tuning stability, 4X, temp sourced
o Aluminum is a great recycling material
o More expensive to make
- Large tooling costs don’t encourage design changes, whereas a wood body design is a far simpler CNC program change
o Some Al guitars are heavy
- Only in a non-optimum design. (our guitar is 1/2typical wood guitar wt.)
o Body contact
- Forearm feels hot/cold against guitar depending on temps, from the higher heat transfer coefficient. Designs can mitigate this with other materials and coatings in resting forearm location.