Truss rod vs Shims

UnclePsychosis

Hi guys,

My jazz bass has quite high action, and its worse on the higher frets. The truss rod definitely needs a tweak, there's too much relief. 

That said, I'm also aware that loads of people end up shimming their Fender basses. How do you know when this is necessary? Is it if the action is still too high after correctly setting the relief? 

John_P

Yes - I use a shim when the action is too high after setting relief.
I hold down the string at each end of the neck and adjust the truss rod until it's almost flat - so the string has just a tiny bit of movement in the middle,   then look at the action and if the bridge won't go low enough to get the action where you want I'll add a shim at the body end of the beck socket to increase the neck angle.  

Reggaebass

I play with a really low action and I’ve shimmed a few of my jazzes and precision’s, as John P says, if you’ve set your relief and the saddles are as low as they will go then adding a shim will lower it, stewmac sell full shims but they are expensive, usually a piece of business card at the heel will make a significant difference, take your time and cut it snug 

ICBM

Yes, if you’ve set the relief - and nut height, which makes a surprisingly big difference to the action - correctly and the action is still too high without bottoming out the saddles - in my opinion they need to be up a bit, the height screws should be under a good amount of load - then it needs a shim.

Don’t bother with those fancy wedge ones, a simple card or wood veneer piece across the end of the pocket is easier and better. I prefer to make it big enough that the two end screws go through it, which prevents any possibility of the (I think largely mythical anyway) risk of a ‘turn up’ at the very end of the neck.

prowla

The truss-rod and shims adjust different things.

• The truss-rod adjusts how bent the neck is and only operates from the nut to the end of of the neck. To check the truss-rod, you should press on the first and top fret and be just able to put a business card in the gap between the string and the 12th fret. As such, it's either right or wrong and not really the way to adjust the action.
  
• The action concerns the full length of the string from nut to bridge. To adjust the height or action you raise or lower the bridge saddles; if they are maxed out then you can put in a shim to adjust the tilt of the neck, which effectively provides a step change to increment the bridge adjustment range.
  

There is a further adjustment available which is the nut height; if you find that pressing on the first fret is harder and has more travel than others, even to the point of the notes sounding out of tune, then the nut may need filing down to the appropriate height.

UnclePsychosis

Thanks guys.

That's confirmed the understanding that I thought I had - the problem was that I had done some reading on bass setups and very quickly people start talking about shims being "required" for Fenders etc etc - it's hard to differentiate the bullshit from reality sometimes. 

Reggaebass

You will always get the comments that if an instrument was built properly it shouldn’t need a shim bla bla , but if it allows you to get the action you want and let’s you get on with playing, I don’t see a problem with it 

BillDL

Take a look at a les paul type guitar.  The glued-in neck is on a slight back angle.  This is required because the tun-o-matic bridge sits higher than a fender style.  To get a decent string alignment angle down the neck from the raised bridge to the nut that allows a low enough upward and downward movement on the bridge for a decent action the neck angle needs to be correctly set.  The backwards neck angle is also found on an archtop hollowbody and semi-hollow electric because of the bridge height.  A problem that is sometimes encountered is where the bridge is decked to the body but the strings are still too far away from the fretboard because the neck hasn't been angled back enough.

To reset the neck angle on a glued-in neck requires radical surgery.  While building acoustic guitars or performing neck resets on older ones where the bridge has moved position due to deforming of the soundboard, luthiers will usually set a long straight-edge on the frets and try to get the neck angle where the bottom corner of the straight-edge just lines up with the top of the bridge.  The height of the saddle in the slot on the bridge then creates the additional height for a good action.

On a Fender guitar with the very low bridge bridge and saddles, sometimes the saddle height screws protrude above the saddles when the action is set perfectly and these can cut or scrape your hand.  In these instances, if new shorter screws don't take care of the problem, people add a thin shim at the closed end of the neck pocket to create a very slight back angle on the neck, and the saddles can then be raised higher than they would normally be so that the screw ends are hidden in the threads of the saddles.

In actual fact the neck angle of Gibson style guitars is pretty small, and on Fender style guitars that need a neck shim to correct a badly cut neck pocket or the sharp saddle screw issue, the neck angle is also very small and therefore only a very thin shim is needed.  An electric bass follows the same principles as a guitar except for the fact that the much thicker strings vibrate in a greater arc and the neck relief (upwards curve in the middle) needs to be quite a bit more than on a 6-string guitar so that the strings don't rattle.  Otherwise the shimming is much the same for a bass as it is for a guitar.  Experimentation is usually needed to find the correct thickness of shim.  If it is too thick and creates too much back angle on the neck, the strings will rattle on the highest frets.  I use various materials, including rough fabric-backed emery tape, wood veneer, cardboard business cards, plastic bank cards, thin maple or mahogany offcuts sometimes sanded into a wedge if they need to be thicker, and even bits of an aluminium drink can cut with scissors for thin shims.  I have a bag of such materials with the thicknesses (using a dial vernier gauge) written on them.

UnclePsychosis

Loosened strings, loosened neck, tightened truss, set saddle heights, re-intonated, I'm now at the "play the shit out of it for a bit and decide if I'm happy" stage. 

:-)