Tuesday, June 25, 2013

Homebrew Horn Strap-button Extender for the G&L ASAT Bass

This is not my typical blog post, but it's a problem I recently became interested in, and others might find it useful.

Being a lefty, it's difficult to purchase a bass guitar.  Most guitar stores, if they have any left-handed basses at all, will only have one or two, and these are typically entry level models.  So it can be a frustrating endeavor if you want to find specific models to try out prior to purchase.

It's almost enough to make one switch to playing right-handed!


I recently picked up (via the internet) a G&L ASAT bass.  The seller was too distant for me to try out the bass in person, so I took a gamble.  Overall it was a good purchase, but...the ASAT does have one flaw:  Because the horn is short, the strap button mounted on the horn has moved, horizontally, much closer to the guitar's center-of mass, compared to the strap button location on other basses (such as my Fender Jazz bass).  This shift in position of the horn strap button creates two problems:
  1. As this button moves horizontally closer to the guitar's center of mass, the guitar's weight distribution between the two ends of the strap (neck side and bridge side) shifts towards the neck-side end of the strap and away from the bridge-side strap.  That is, the neck side of the strap pulls down harder.
  2. The neck-side strap drops more vertically, thus, more of the strap is grabbing onto your shirt-front and pulling it down, resulting in shirt bunching.
The overall result, for me, is that the ASAT is not a very comfortable bass to play when standing up.

I did some Googling and discovered that this is a known problem:  http://www.bassesbyleo.com/forum/viewtopic.php?f=4&t=385

There's a clever design shown in the above link.  Here it is:

Unfortunately, this extender seems to no longer be available (also, it's designed for a three-hole mounting system that is not compatible with my ASAT, whose neck-mounting scheme has 6 holes in a rectangular pattern).

Well, I thought, if I can't purchase an extender like the one above, why not make an extender myself? 

I did, and here it is:

The Design:

As you can see, the design differs from the original one shown in the earlier link.  This was dictated by my design goals, which were:

I wanted to keep the amount of metal I would need to purchase to a minimum, which dictated that the holes to mount the extender to the guitar's body be somewhat in-line with the hole for the strap button.

I also wanted to play around with the location of the strap button.  On my Fender Jazz bass (and many other basses), the button is above the 12th fret.  The ASAT's button, by comparison, is above (roughly) the 17th fret.  Was there a better location for that ASAT than above the 12th fret?

I also didn't want the extender to interfere with my fretting hand as it slid down towards the body, so the extender needed to quickly bend up above the neck (but not so quickly that it looked awkward).

Given these constraints, I decided to use the two holes below (shown sans screws) for mounting the extender.

  1. The distance between the centers of the two mounting holes is 1.95 inches.
  2. The original "shoulder washers" will be left in their respective holes (as shown above), so that they'll be available if someone wants to remove the extender.
  3. The original screws are also used -- they have plenty of length to provide adequate grip.

I dropped by a local surplus-metals shop and had them cut me some 8 inch long pieces of 6061-T6511 aluminum flat bar stock (0.125" thick by 1.5" wide).  I drilled a hole in the lower corner of one of them, mounted it to the lower-middle neck mounting hole shown above, and rotated the bar about this mounting point until I had it in a position that seemed good.

Important Note:  Put something between the bar and the guitar body to keep burrs and other sharp bits of metal on the bar from scratching the body when rotating it.  Thick paper or cardboard should work fine (but use your own judgement) -- I cut a piece from a manila folder and put this between the bar and the body.

(I also purchased some 8" lengths of 3/16" thick flat bar stock, but it turns out the 1/8" thickness is fine).

Once the bar's position seemed good (I wanted to try mounting the button above the 12th fret and at the same height that the button was currently mounted), I then determined where I wanted to place the second mounting hole.  After drilling it, I also drilled some holes for experimentation with the strap-button placement (to give me a range of choices to play around with).

Here are my final dimensions:

(click on images to enlarge)

(The only important dimension is the 1.95" center-to-center distance between the two mounting holes.  You can play around with all of the other dimensions to suit your personal design esthetic).

Here's the bar mounted on the bass (for testing strap-button positions)

...and its view from the front (note that the strap-button is not in its final location). 

By the way, the strap button is held on with a 6-32 screw and nut with integral lock (star) washer.

I played around with the location of the strap button and decided that I liked it best near the 13th fret -- As the button is moved further out from the body towards the neck the guitar body will shift in the opposite direction.  I decided that I didn't like the way the tummy cutout was hitting me when the button was at its furthest location, and that a good compromise was with the button near the 13th fret.

If you like rectangles, you can stop now (although your fretting hand will probably run into the extender, however, as you slid it down the neck towards the body).  But I wanted something a bit more curvy, to go with the curves of the ASAT and to also put the extender out of the way of my fretting hand.

Here's the design I came up with:

Cut, File, Sand:

To make hacksawing easier, remove metal by drilling around the shape's outline.
You won't be able to hacksaw too far along the curves (at least, I couldn't).  Whenever you get stuck, just cut as far as you can go along the outline, then remove the hacksaw and make a new cut from the outside of the bar to the last hole that you cut through.  Or if the metal is still held to the body by a few thin channels of metal between the holes, you can use a pair of pliers to bend the metal piece back and forth until the metal at the holes fatigues and the piece breaks away.

Filed and sanded.  Ready for paint.

Paint and Backing:

The extender is painted flat black.  No primer was used because, well, the paint I used is supposed to contain both paint and primer -- it's Rust-Oleum Painter's Touch 2X Ultra Cover Paint+Primer (picked up at Home Depot).  (We'll see how well it holds up).

After the paint dried (I gave it 48 hours) I buffed it down with a soft cloth and then added a green felt backing to protect the bass body (felt is usually available at Arts and Crafts or Hobby stores and ought to be inexpensive -- a piece of 6 inch by 9 inch green felt only cost me 29 cents).

The felt is held in place with a spray adhesive (be sure to mask off the rest of the extender!).  I used a can of 3M Super 77 adhesive that I've had around the shop for a few years.  A very thin smear of Elmer's glue in lieu of spray adhesive would probably work fine, too.

After the adhesive dries, mount the extender onto the bass, attach your guitar strap and away you go!

Ahhhh...that's better!

A Bit of Physics (extra-credit reading):

So what's going on?

First, let's talk about the guitar's center of mass.  Where is it?  (Actually, we're really talking about the center-of-gravity, but in the case the two are essentially equivalent).

I found its approximate location by balancing the back of the bass on top of a pill bottle.  Doing this, I discovered that it runs along the center of the bass neck.  For a more accurate position, you can hang an object from a string, and a line drawn along that string must pass through the center of mass.

For example, the ASAT center of mass (the red dot):

For comparison, Fender Jazz bass center of mass:

It is the distance between these strap buttons and the guitar's center of mass which is important:  what happens when the position of the strap button is shifted horizontally with respect to the guitar's center of mass?

First, we need to recognize that, because the guitar is not moving (that is, it isn't falling towards the ground, nor is it rising towards the heavens), it is at equilibrium:  all of the forces acting upon it (gravity pulling down, my body pulling the straps up) sum to zero (this is one of Newton's laws of physics).

What are these forces?  For the sake of discussion, let's simplify them to be the following:
  1. There is the tension of the "neck" end of the strap pulling down (and, as shown below, it's also pulling horizontally to the right).
  2. There's the tension of the "bridge" end of the strap pulling down (and, as shown below, it's also pulling to the left).
  3. And then there's the force my neck is exerting, pulling UP, to keep everything from dropping to the ground.

 Here's a drawing showing how these vectors sum to zero:

(Click on image to enlarge)

What is important to me (and my neck and shoulders) is the vertical component of these forces.  The horizontal components cancel out without any effort on my part (they are equal and opposite), but I myself must counteract the weight of the guitar, transmitted to my neck via the "tension" on the two ends of the strap.  That is, the force I'm exerting pulling up equals the sum of the forces on the two sides of the strap (neck side and bridge side) pulling vertically down.

Of the two vertical tension components shown above (neck and bridge straps), it is the neck strap tension that we are affecting when we move its strap button left or right.  And, because the neck strap button is closer to the center of mass than the bridge strap button, it carries the majority of the guitar's weight.  What happens when we move its position?

Changing the position of the Neck strap button will change the angle "alpha" shown above.  As the neck button moves towards the center of mass, the angle alpha becomes smaller and smaller.  Assuming the angle of the Bridge strap doesn't change when we move the neck button (it does, but let's not worry about that), this means that the vertical component of "Tension, Neck Strap" increases (and therefore the vertical component of "Tension, Bridge Strap" must decrease, because they both must still add up to be the weight of the guitar, which itself is unchanging).  That is, as I move the neck button in towards the body of the guitar (and its center of mass), the "Tension, Neck Strap" vector's point moves down along the vector I've drawn for "Tension, Bridge Strap".

Here's an image showing this:

(Click on image to enlarge)

Note how the vertical component of the Tension, Neck Strap, has increased while the vertical component of the Tension, Bridge Strap has decreased by an equal amount.

On the other hand, as I move the neck button away from the center of mass and towards the fingerboard's nut, the force exerted upon the neck strap will decrease, and the bridge side of the strap takes up more of the weight.

And this is what we are doing when we move the strap button away from the body of the guitar:  we are lessening the weight felt on the neck-side of the strap and transferring that amount, instead, to the bridge-side of the strap.

Other stuff:

Link to post at TalkBass Forum:  http://www.talkbass.com/forum/f8/homebrew-g-l-asat-strap-button-extender-994333/

Links to other Bass posts of mine...

Sonic Blue Bass (part 1 of a 3 part series)

Mellow Yellow Bass

Short-scale Telecaster Bass

Bass Guitar Painting Jig

Repairing a G&L Butterscotch Blonde Paint Chip

G&L ASAT Bass Strap-button Extender