Hot cars, warped necks and maxed out truss rods

Discussion in 'Hardware, Setup & Repair [BG]' started by AndyPanda, Jun 19, 2020.

  1. Hey everyone! I wanted to share some of my experiences with necks and a theory about basses left in a hot car.

    And if you don't want to read all my babbling nonsense - TLDR is that I think basses left in a parked car for even 15-20 minutes is enough for the neck to bow under the string tension.

    I was drawn to guitars and basses (and violins and accordions and cuckoo clocks etc.) when I was really young and always had a keen interest in how to get the best setup etc. My first experience with a "truss rod that wouldn't" -- I was young and dumb and bought a '67 Candy Apple Red Jazz Bass with matching headstock at a pawn shop. The truss rod wouldn't take the forward bow out of the neck and, being young and dumb, I kept turning and snapped the truss rod. How I wish I hadn't done that and wish that I still had that bass.

    Anyway ... in the many years since that broken truss rod, I've bought many a used bass that came with super light gauge strings, too much relief and a truss rod that was cranked. And I've had exceptional luck clamping the neck into a backbow and heating it to get the neck back to where you have full range of truss rod adjustment. I've straightened several necks and had them hold up for years without any problem. And I've been able to buy several really high end basses for low money because the seller was frustrated with only being able to use super light gauges (it has rarely been disclosed to me that the truss rod was maxed out - but I assume that's why they sold it - the price was low and I knew I could fix it so I haven't complained)

    And I've read lots of threads about heat straightening necks pros and cons. In my experiences, however, I've found I don't have to heat it nearly as hot as many say --- nor do I have to keep it at that temp for nearly as long as many say. I've been able to straighten necks after holding them at 150F for only 10-15 minutes and leaving them clamped to cool down.

    And this leads to something that has been nagging at me for years. I suspect that when you leave a bass in your car with the windows rolled up on a hot summer day that is enough to cause this problem. I read a lot of posts here on TB about maxed out truss rods. Anyway ... that's what I'm thinking. Curious what you guys have to say about it. Just getting to a daytime gig early and leaving the bass in the car while you get lunch seems like it could be the cause of many of these posts I read about maxed truss rods.
    Last edited: Jun 19, 2020
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  2. Killed_by_Death

    Killed_by_Death Snaggletooth Inactive

    Some believe that the wood won't be affected by heat, but IME it certainly is, why else would necks be heat-treated to straighten them?
    Even basic Physics would suggest that HOT wood is going to bend easier than wood at room-temperature.

    and to make it more interesting, how MOIST the wood is plays a large factor, as well
    96tbird, Vinny_G, RSBBass and 2 others like this.
  3. I’m from Texas. In the summer here, it can get WAY hotter than 150F degrees in a car with the windows up. So, I don’t doubt you can mess one up. I wouldn’t try to ‘fix’ one by putting it in a car. I can tell you horror stories about some arch top, hollow bodies that twisted and cracked.
  4. And not just the wood. I'm thinking it's the glue between the fingerboard and neck and between the layers in a laminated neck. Not sure really but very surprised by how little time it takes to get a permanent bend when it's under pressure while hot and remains under that pressure to cool.
    Vinny_G, vid1900 and Killed_by_Death like this.
  5. Samatza


    Apr 15, 2019
    Temperature affects wood but mostly moisture content. The trunk of a car can get extremely hot and presents a rapid change so it's not good to leave your instrument in the trunk if you can avoid it.

    On straightening necks I've had two done that had gone in opposite directions. After more than 30 years my Tune fretless had a back bow and my Tune 5 string too much relief with the truss rod maxxed.

    I took it to a reliable luthier and he heated the necks, reset them to the correct position and I think we're good for another 30 years. These basses have travelled all over the place by truck, bus, car and plane so they have been exposed to some extremes, to be honest I'm surprised that after 30+ years they play and look as good as they do.
    AndyPanda likes this.
  6. Well it shouldn't even be a question whether or not heat can let you bend wood and have it hold that shape after it cools down. I mean just watch any violin making video and the hot iron they use to bend the ribs to shape

    Last edited: Jun 19, 2020
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  7. Killing Floor

    Killing Floor Supporting Member

    Feb 7, 2020
    Austin, TX
    Yep. I try to never leave anything I care a lot about in my car for longer than it takes to fill up with gas. Especially in summer, which is March through November here. Not only can the extreme heat do some hoodoo on your neck, it can ruin your awesome finish. Granted that a hardshell case is a pretty good insulator so that could arguably improve your luck for a little while longer.
    AndyPanda and Guild B301 like this.
  8. Big B.

    Big B.

    Dec 31, 2007
    Austin, TX
    The ambient heat in a car will definitely cause glue to soften and wood to move but in my experience direct sunlight is an even worse scenario. Let's say you have a bass in a gig bag/case in your car and the ambient heat is high. If there is direct sunlight shining on the case you'll be heating one side of the instrument significantly more than the other which can have particularly bad results. I've gigged a lot in Texas and and Alabama and I've learned to bring a fairly heavy blanket and arrange it over all my instrument cases to absorb the direct sunlight.
    AndyPanda likes this.
  9. I'm in southern Louisiana where it's more of a low pressure steam than actual weather 8 months out if the year. Stopping for a bite to eat I'm not concerned too much with leaving the bass in the car. If I take it to work I bring it in the shop with me, I don't have AC at work, but there is air circulating to keep it cooler. My biggest concern is the humidity, it seems to create more havoc than the actual heat.
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  10. Fender4Me

    Fender4Me The Undertaker

    I agree with you about the glue softening between the FB and the neck. I think that is what causes the most havoc and also the reason that heating the neck while it is fixed in the opposite direction, fixes the issue. To move a thick chunk of maple, you really need steam, not just heat but, if you are releasing the glue bond between the fingerboard and neck while it is straight or back bowed a little, and allowed to slowly cool you stand a good chance of having it hold just fine. It doesn't take a lot of heat to soften hide glue. That's why hot cars have destroyed a lot of instruments through the years.
    AndyPanda likes this.
  11. Fender4Me

    Fender4Me The Undertaker

    This pieces of wood have been soaking in water to soften them enough to bend without breaking. When heat is introduced, its the steam that allows the fibers in the wood to hold their shape once they cool down and dry out.
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  12. Big B.

    Big B.

    Dec 31, 2007
    Austin, TX
    While wet lumber is more pliable than dry lumber it's not really a useful practice in guitar making/fine woodwork. Completely soaking a piece of lumber might be ok for bending a wagon wheel rim but all the moisture will have to escape once the piece has been bent and shrinking and warping will likely be a problem. Not to mention the time you'll have to wait for the moisture levels to stabilize back to a workable percentage. If you watch most luthiers bend guitar sides they mist the wood with a spray bottle to keep it from scorching and blackening but it's the heat itself that loosens the chemical bonds between the wood cells. Conversely, when the wood cools those bonds are reset and the bent board retains (hopefully) its shape. An example would be fixing a warped neck with a heat lamp.
    AndyPanda and Fender4Me like this.
  13. This is my understanding too. I have straightened many a warped neck and I don't use any water or steam at all.
  14. JuanB


    Feb 20, 2010
    Slightly old thread!

    @AndyPanda, what have you used to heat your necks when you've done this? Thanks!
  15. lark_z

    lark_z Gold Supporting Member

    Feb 26, 2020
    Georgetown, TX
    A heating pad on high worked perfectly for me.
  16. mandolins and acoustic guitars are assembled using hot hide glue. (At least the good ones are) So what happens to that HHG when you expose the instrument to high heat after it’s been assembled? It liquifies again and the instrument comes apart at the seams.
  17. Bruce Johnson

    Bruce Johnson Gold Supporting Member Commercial User

    Feb 4, 2011
    Fillmore, CA
    Professional Luthier
    To add some more complexity to the discussion, there are three main ways that leaving a bass neck in a car can result in it bending or warping. They are different in how and why it bends, and the repair/treatment for them is different too.

    Internal Stresses. When the neck was built at the factory, the maple board that the neck was cut from may have had internal stresses. That is, within the maple board itself, some layers are pushing and pulling against each other and trying to make it bend. This board, over time, is going to warp. What we call long term warpage, that happens over some years. At the factory when it was built into a neck, the board looked reasonably straight, and they glued it tightly to a fairly stiff fingerboard. The fingerboard braces the neck and minimizes the bending....for a while.

    Then you leave it in a hot car. The glue joint between the fingerboard and neck softens a bit and creeps slightly.....And the maple finally can bend a bit, like it's been trying to do. The bending of the maple isn't really caused by the heat. It's from the internal stresses that were in the maple from when it was a log. But the heat releases the restraint that was holding it back.

    Differential expansion. The maple neck is properly sealed up with a good finish. But the fingerboard is a soft porous wood, and isn't sealed up well. They are living together happily until you leave it in a hot car....In New Orleans. With 100% humidity. The fingerboard soaks up moisture like a sponge and swells up, increasing in length. The maple doesn't absorb the moisture, and stays the same length. The neck bends backward. Or, you leave it in a hot car in Phoenix and the opposite happens. The fingerboard shrinks and the neck bends forward. Because of the heat, the glue joint may soften and creep, and the neck may stay in that bent condition, even after you get it back into a normal humidity.

    12th fret Kink, the dreaded Ski-Jump. In this case, the neck has a weak spot structurally. A design problem. The back of the neck behind the 12th fret is under high tension, near the structural limit of the maple. The continuous pull of the strings causes the wood at the very back, right around the 12th, to slowly stretch. Over time, the neck develops a kink at that spot, the result is the ski jump at the heel. The neck is slowly kinking by itself but, add some heat in the back of a hot car, and it speeds up. Again, the heat didn't cause it, but just made it happen faster.
    A neck that is bent after spending some time in a hot car can be from one of these three conditions... or a combination of them. And the technique for correcting each of them is different.

    Internal stresses: The whole main length of the maple needs to be heated evenly and thoroughly, down deep, while bending it overall with clamps. You want the maple to slightly slip within its own layers, relieving all the internal stress. Hopefully without creating any new stresses when it cools. And allowing the glue joint to creep with it, as it changes shape. Heating, clamping, cooling over a careful schedule. This has to be done carefully.

    Differential expansion: Get the fingerboard back to its normal humidity level. Apply heat only to the glue joint to get it to creep back to its normal position. Then seal up the fingerboard so it doesn't happen again.

    12th fret kink: Apply heat mostly at the kink spot, on the back behind the 12th. Apply clamping to bend the kink back to straight, compressing the stretched wood. Without unintentionally back-bending the whole neck.
    This is what Eric at is an expert at. Diagnosing what's actually wrong and applying the best repair technique.

    Last edited: Oct 31, 2022
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  18. JuanB


    Feb 20, 2010
    Thanks, @Bruce Johnson. What do you think about a carbon fiber neck with phenolic paper X grade fingerboard? The fingerboard on my bass has shrunk a bit giving it too much relief. The truss rod is at its max. I was thinking about clamping and heating the fingerboard gently with a heat gun. Might need a long treatment though so maybe a silicone heat tape would be a better option.
  19. JuanB


    Feb 20, 2010
    Thanks, Bruce. You have experience with synthetic fingerboards, I believe. What would you do with a shrunk synthetic board causing too much relief? Short of a replane/refret situation.
  20. SkylineFiver

    SkylineFiver Gold Supporting Member

    Jan 5, 2008
    Placer County CA
    To the original post…Wouldn’t this be a problem for all guitars shipped via UPS and Fed Ex GROUND?