A truss rod that is not a truss _rod_ ...

[SamIAm]

I've built myself a bass out of a 2x4. (Build diary) her name is Twiggy.

She started as a pure prototype with a bit of wood I had lying around, to try out some ideas, to have a play ... but as I've put more work into her I've fallen in love.

 

Her neck is asymmetrical, quite thin and ... is starting to develop a bit too much relief.  The idea of installing a truss rod simply did not occur to me at the start (She was just a prototype); now I rue this lack of foresight; there is not enough depth to accommodate a truss rod near the zero-fret end of the neck.

 

I got to thinking/pondering and wanted to throw my thoughts out here to benefit from the collective wisdom of BC.

 

Without any strings, my neck would probably remain fairly flat.  The strings apply a force that is pulling the neck into a bow shape providing relief, but this relief is too much and so my action is too high.

 

The extremely silly thought experiment of putting a set of strings on the rear of the neck to counter this got me thinking ... I was reminded of the rigging on sailing craft that needs to be light and strong.

 

Why not route a channel (or even two parallel channels)  into the back of the neck, say 5mm deep, lay in stainless steel cable of 2.5mm diameter and tension this up to apply a counter force to the 'playing cables' (strings!) then drop a 2mm strip of wood in to hide the cable (Skunk stripe style).  Any lateral forces would be applied towards the fingerboard so having such a thin piece of covering would not be a problem. 

 

The zero-fret end of the cable would have a fitting that would be anchored to the neck, perhaps something like this:

These are rated with a breaking point of 165kg and my total string tension is about 95kg.

 

At the body end I'd need some sort of screw to adjust the tension and an anchoring system, probably run the cable right down to the end of the body with an exposed adjustment hex bolt accessed from the end.

 

I've found a UK supplier of bits (https://www.tecni.uk/) and their costs are quite low, well under £10 for a complete assembly.

 

I could increase the depth of the cable slot at the ends, shallow in the middle to introduce an arc along the length of the cable which I think would impart some mechanical advantage (not sure of the proper technical term)

 

I've been unable to find much on google about this topic, one thread discussing it (), suggesting that they thought that Ken Parker might have investigated this approach with his Fly bass but ...

 

Pros

• Able to be installed in a thin neck
• Low cost
• Light weight
• Flexible

 

Cons

• Untried
• Others ???

 

Thoughts/views/experience welcome.

 

S'manth x

Edited July 18, 2022 by Smanth

[tauzero]

It might work.

 

Things that make it more complicated: there are four strings (I counted) at the front so a single string at the back would need to have four times the tension in it (I'm ignoring the basic strength of the neck). Also, the strings are offset from the face of the neck whereas the truss string is inset into the rear so the torque from the strings pulling forwards would be greater than the truss string pulling backwards assuming that the truss string tension = sum of string tensions.

 

Things making it easier: you're reinforcing a reasonably strong neck, not providing the entire countering tension. Probably.

 

Stupid idea #1: drill a couple of longitudinal holes and put metal pipes down them

 

Stupid idea #2: the same, but with carbon fibre rods

[3below]

A good bit of lateral thinking. If it does not work to your satisfaction, you would still be able to try carbon rods in the channel(s).

[Random Guitarist]

How much of your 2x4 cross section is left? A standard 2x4 can support support a 1000lbs without too much strain. That's a vertical straight 2x4. If you have removed half of the 2x4 area to make a playable neck then You could reasonably estimate 500lbs strength remains. Assume the new cable applies 50 of tension opposing the strings than you're up to around 150lbs total which should be fine. 

 

Another factor will be leverage, the more deeply recessed the cable is the closer it is to the effective centreline of the wood, and the more tension will be required.

On the other hand, the more shallow the recess is the closer the tensioned cable is to your hand if anything should go wrong.

 

 

 

[Acebassmusic]

As an engineer I really like seeing things like this which take convention and ask "can it be done differently". It will be interesting to see how it progresses 👍

[SamIAm]

@tauzero, thanks.

 

I'm unclear as to whether the truss cable would need to exert less/same/more tension than the sum of the string tensions (On Twiggy, with TruBass Flatwounds it is under 95kg. Rotosound quote 207.5lbs, but that is for long scale length and Twiggy, being a short scale, will have a lower tension); I'm guessing it will be less.  So I'm aiming for a truss cable that will allow up to 100kg of tension (or a pair of 50kg each).

 

Regarding insertion of pipes/carbon fibre supports (Also @3below) the issues I see here is twofold; 1) No adjustability; 2) Upon insertion and whilst glue dries I would need to have the neck at the exact relief I wanted and this might be problematic to do.

 

@Random Guitarist, very interesting perspective.  The neck cross section looks like this.

 The yellow box is 45mm by 15mm.

 

This is the extreme at the zero-nut end, it gets thicker as it approaches the body (abt 60mm by 20mm) ... neither offer a high proportion of the original 2x4 cross section.

 

A 2x4 I see is 38mm by 89mm -> 3382mm^2

 

My thinnest neck profile is perhaps 75% of 45mm by 15mm -> apx 500mm^2

 

If the load bearing is directly proportional to cross sectional area than it suggests it can cope with only 150lbs, less than I already have acting on the neck.  I wnoder what sort of 'safety factor' is in that 1,000 lbs.

 

S'manth x

[Happy Jack]

Sorted. No need to thank me.

 

https://www.ebay.co.uk/itm/265328011729?chn=ps&var=565176482583&norover=1&mkevt=1&mkrid=710-134428-41853-0&mkcid=2&itemid=565176482583_265328011729&targetid=293946777986&device=c&mktype=pla&googleloc=9045963&poi=&campaignid=17116423086&mkgroupid=134652533165&rlsatarget=pla-293946777986&abcId=9300856&merchantid=107659768&gclid=CjwKCAjwrNmWBhA4EiwAHbjEQD5mPk7bGR19w6NNFOlZAUmkBWH2KurrNC_V5grZd_bH39DiVHpt8BoCxSgQAvD_BwE

[Random Guitarist]

1 hour ago, Smanth said:

 

If the load bearing is directly proportional to cross sectional area than it suggests it can cope with only 150lbs, less than I already have acting on the neck.  I wnoder what sort of 'safety factor' is in that 1,000 lbs.

 

The thousand pounds thing is a commonly quoted number that is very safe. There will be a significant safety factor, lifting slings and suchlike are typically specified to break at something like five times their rated load. I'd be surprised if the breaking strain of a vertical timber is not similarly weighted. If I had to guess I'd go with a 3x factor.  

 

(Some extra googling indicates x3 seems pretty reasonable)

 

[tauzero]

4 hours ago, Random Guitarist said:

The thousand pounds thing is a commonly quoted number that is very safe. There will be a significant safety factor, lifting slings and suchlike are typically specified to break at something like five times their rated load. I'd be surprised if the breaking strain of a vertical timber is not similarly weighted. If I had to guess I'd go with a 3x factor.  

 

(Some extra googling indicates x3 seems pretty reasonable)

 

 

It also appears to be for an 8' length (or 244cm in proper money). I would expect the failure mode to involve some lateral flex, which would decrease on a shorter length of timber as there's less length to flex, so the 450kg limit would be increased. But that's just speculation on my part.

[Andyjr1515]

It's not an easy rout and back-fill at all - but, in terms of neck depth, you do have enough wood for a standard modern 2-way rod...especially in that you could also rout a couple of mm into the back of the fretboard too.

 

My rule of thumb in terms of ensuring I never get a trussrod bursting out of the back of the neck is to retain at least 4mm of wood underneath the rod channel at the nut end.

 

A typical Andyjr1515 calc would be:

 

Total depth at nut= 21mm

Fretboard thickness =  7mm, leaves 14mm

Rod channel               = 9mm, leaves 5mm

                                    = happy bunny 

 

So, even if you don't rout into the back of the fretboard, you still appear to have enough room:

Neck thickness, excluding fretboard=15mm

Rod channel                                        =9mm

Leaves                                                  =6mm of packer under the rod end

                                                              =plenty

 

This does assume the simple two way like they sell in Tonetech, not the ones with the enlarged, enclosed, adjuster end that are sometimes sold elsewhere

 

Tricky rout, mind...

[Adey]

Could you route out the back of the neck and install cabon fibre square / rectangular section rods x 2. I understand that they are extremely stiff, so maybe you could clamp the neck dead straight or with slight relief and glue it all up and not need a truss rod.

 

You can then trim off the bits of carbon outside the neck profile and refinish to give you a techy looking neck with dual carbon stiffeners (which it would be)

[SamIAm]

@Andyjr1515, those figures gave me hope!  Routing the back of the neck I can do with a CNC machine.

 

However, I measured incorrectly!

 

The distal end is indeed 15mm thick, however (either to allow extra space for the mounting screws or simply as I got too enthusiastic with my neck shaping) upon measuring at various points along the neck I've found that the minimum thickness drops to 13mm and runs for 14mm ... and that this is at the thickest section, the profile has a triangular shape and it drops off either side.

 

Having said that, it strikes me that when correcting for too much relief (My situation) the truss rod force acts towards the rear of the neck at the most distal and proximal ends and towards the front of the neck in the middle of the trussrod.

 

I'm thinking that I could either add a bigger section of wood at the distal end of the neck (A bit like the lump found at the end of Strandberg neck) or add a aluminium plate over that end.

 

What length of the neck does a trussrod run?  From zero-fret to joint with body?

 

S'manth x

[mario_buoninfante]

Quote

Why not route a channel (or even two parallel channels)  into the back of the neck, say 5mm deep, lay in stainless steel cable of 2.5mm diameter and tension this up to apply a counter force to the 'playing cables' (strings!) then drop a 2mm strip of wood in to hide the cable (Skunk stripe style).  Any lateral forces would be applied towards the fingerboard so having such a thin piece of covering would not be a problem. 

Just thinking out loud here, but I suspect that this way the "counterforce" is applied at the anchor points, while a truss rod with its rigidity, offers "support" to the whole neck, if that makes sense.
I suspect that with anything different from that, you might damage the neck in few specific points (i.e. where the cable is attached to the neck) with the passing of time.

 

Edited July 24, 2022 by mario_buoninfante

[alyctes]

Could you get away with three strings?  That should reduce the problem considerably - and maybe to a manageable level, at least for a while?

[alyctes]

Could you rout the front of the neck for a trussrod and add a fretboard?  With a bit of tactical hacksawing and routing of the back of the board you could probably add Happy Jack's rigid steel L-beam

You'd need to raise the bridge, but that should be reasonably easy.

Edited July 24, 2022 by alyctes

[Andyjr1515]

3 hours ago, Smanth said:

What length of the neck does a trussrod run?  From zero-fret to joint with body?

 

There is a reasonable leeway.  'From where the heel thickens to close to the nut' is a decent rule of thumb - but you can take liberties.  But the risk area is where the rod is going to be primarily pushing. which is up against the fretboard in the middle and down against the neck bottom at the nut and at the heel ends ('up and down' relative to being looked on from the side).  So, if you are going to suffer breakthrough, it will be generally under the 1st fret area, where the wood under that force is at its thinnest, and if the fretboard is going to pop off, it will be in the middle of the fretboard.

Edited July 24, 2022 by Andyjr1515

[SamIAm]

6 hours ago, alyctes said:

Could you rout the front of the neck for a trussrod and add a fretboard? ...

That had not even crossed my mind.  I shall explore it further, thank you!

[Baloney Balderdash]

On 19/07/2022 at 01:04, tauzero said:

It might work.

 

Things that make it more complicated: there are four strings (I counted) at the front so a single string at the back would need to have four times the tension in it (I'm ignoring the basic strength of the neck). Also, the strings are offset from the face of the neck whereas the truss string is inset into the rear so the torque from the strings pulling forwards would be greater than the truss string pulling backwards assuming that the truss string tension = sum of string tensions.

 

Things making it easier: you're reinforcing a reasonably strong neck, not providing the entire countering tension. Probably.

 

Stupid idea #1: drill a couple of longitudinal holes and put metal pipes down them

 

Stupid idea #2: the same, but with carbon fibre rods

Well, I am not an expert of any capacity, but to me it seems like all the potential issues you point out would be shared with the concept of a regular trussrod, and as we know that concept is very well proven and works brilliantly (as for the tension of the strings and strength of the experimental "trusscable" OP already accounted for that).

 

 

And to OP:

 

Super cool and interesting project, and I kind of really like how the bass looks too. 

 

Best of luck with it.

 

 

 

Edited July 25, 2022 by Baloney Balderdash

[tauzero]

On 25/07/2022 at 07:04, Baloney Balderdash said:

Well, I am not an expert of any capacity, but to me it seems like all the potential issues you point out would be shared with the concept of a regular trussrod, and as we know that concept is very well proven and works brilliantly (as for the tension of the strings and strength of the experimental "trusscable" OP already accounted for that).

 

It's a bit different to a truss rod as the truss rod acts along its whole length to provide a bending pressure (forwards or backwards for a dual-action one), whereas this is more akin to a bowstring, which only provides a bending tension at each end of its run.

[MoonBassAlpha]

Late to the party,  but how about glueing a thin fingerboard on whilst under string tension and clamping it at the exact  relief you require,  say,  using a metal straight edge,  g-clamps small and large,  and blocks . Difficult to explain without a picture,  and it's bedtime!

[itu]

As I have or have had basses without truss rods, I am not so afraid of the lack of one.

 

This wire thing is of interest. To adjust it I have already two ideas. Simplest is a screw and nut, another is a modified tuner. A tuner that can handle all five strings.

[3below]

37 minutes ago, itu said:

As I have or have had basses without truss rods, I am not so afraid of the lack of one.

 

This wire thing is of interest. To adjust it I have already two ideas. Simplest is a screw and nut, another is a modified tuner. A tuner that can handle all five strings.

This?

 

[pete.young]

I remember reading an article in the NME or similar about a guitarist called Hartley Cain, who had an 18-string electric guitar. It had 19 machine heads, the 19th one was a bass machine head attached to a motorcycle clutch cable which ran down the back of the guitar to the bottom strap button to stop the tension of the other strings folding it in half.

[Downunderwonder]

The strings are doing a number on twiggy by virtue of the nut and bridge introducing a bending torque as well as direct longitudinal compression. Curiously the twisting moment must be equal at both ends or twiggy would be spinning end over end faster and faster. Back in the day I treated these problems as free body potatoes to analyze reactions (buildings subjected to wind and earthquake loads transmit forces into the ground via floors and walls and columns). Perhaps someone will run with it, I have better things to do right now.

 

Suffice that the purchase of the strings via nut and bridge is smaller than what is available within the neck for the cable so the tension in the cable will be comparitively smaller than the combined strings to balance.

[tauzero]

On 26/10/2022 at 04:14, Downunderwonder said:

The strings are doing a number on twiggy by virtue of the nut and bridge introducing a bending torque as well as direct longitudinal compression. Curiously the twisting moment must be equal at both ends or twiggy would be spinning end over end faster and faster. Back in the day I treated these problems as free body potatoes to analyze reactions (buildings subjected to wind and earthquake loads transmit forces into the ground via floors and walls and columns). Perhaps someone will run with it, I have better things to do right now.

 

Suffice that the purchase of the strings via nut and bridge is smaller than what is available within the neck for the cable so the tension in the cable will be comparitively smaller than the combined strings to balance.

 

Surely it's the bending moment which is significant, and the strings are offset from the centre line of the neck by a greater amount than the truss cable, hence the tension in the truss cable would need to be higher than that in the strings in order to create an identical bending moment.