Binding Mounting: 3.5 vs 4.1mm depending on core material?

[petemorgan(pmoskico)]

Hello ski builders community!

I was speaking with a shop tech recently about when to use a 3.5 versus a 4.1 mm wide bit when drilling skis.

The conclusion was that a 4.1mm bit is best for a hardwood core material like rock maple or bamboo so you have a wider hole so you dont crack the wood with the screw.

and then the 3.5mm bit is for a softer wood core like poplar, since you will get better purchase into the wood without as much risk of cracking the core.


Is everyone in agreement on that?



side note:

i have posted a lot about binding retention over the years, I have always used hardwood under the bindings, and none of my skis have had a problem. It is just a big safety concern of mine, in 2008 or so I was skiing off a volcano in amazing conditions, and after making some fast arcing turns of the summit, my binding ripped out of my Volkl Gotama, and i had a serious long slide until i was able to self arrest with my pole, if i had lost my poles, i am not sure what the outcome would have been. apparently that year of the gotoma was notorious for bindings ripping out. anyways, it is just a concern of mine, as it should be for you too. we never found the ski, the binding was still attached to my boot. i ended up skiing over 5,000 ft on one ski, thankfully conditions were so good that that was possible!

[vinman]

3.5 for wood core. 4.1 and tapped holes for skis with a metal layer. tapped holes even in a wood only core makes for a cleaner process with less volcanoing of the topsheet.

[pmg]

Well,

there are screws with different diameter depending on the binding, but all bindings' screws for grown ups I have seen so far have an inner width (just the solid material without the thread) of about 4.1mm (always a bit hard to measure).

So if a 4.1mm hole is drilled, the solid material will fill up the hole completely, and the thread will cut in the surrounding material. Pretty nice that way if the surrounding material is solid enough.

With the 3.5mm hole, some material is compressed by the solid inner material of the screw. Good if the material is not sooo solid, like soft wood. Bad if the material is solid, creating too much counter force when driving the screw in.

Some points that are imo important when mounting bindings:

- use some glue in the screw hole! Otherwise water will slowly get in and the wood around the screw will rot. I always use epoxy as it does not shrink.

- Do not tighten the screws too hard or you will damage the material surrounding the screw. This is quite hard to do, luckily modern cordless screwdrivers have a kind of "momentun break". So just give it a try in the wood your core will be made of, and then you will get a good idea how much momentum is right for making the screws really tight but not too tight.

- Countersink the top of the hole a bit. Otherwise some material will be pulled above the top level of the ski. this might cause the binding to not sit 100% properly on the ski. Really just a bit, otherwise you loose "flesh" for the screw.

[vinman]

All binding screws are standard diameter, 5.5mm. All Adult screws are made to penetrate the core 9.5mm , youth screws 7 or 7.5.

The only difference is if the ski had a metal layer or not.

If you tap your holes you don't need to worry about compressing core material. But if you talk to most techs tapping is not required in a wood core only ski.

Epoxy is not required in most mounts except DPS brand skis. Mounts should have waterproof grade wood glue/ Analine resin glue.

Per ASTM/ISO cores need to hold a minimum of 5 N-m of torque, binding specs are for screws to be torqued to 4 N-M.

[petemorgan(pmoskico)]

Thanks for the replies!

Vinman, great info on the torque specs! I agree on avoiding epoxy and sticking with wood glue.

@PMG, i am not aware of screws being different diameter. sounds like children bindings do, but other than that I am not aware of different sizes. Also @PMG, i would avoid using an electric drill to screw in the binding as vinman mentioned 4 N-M is not a lot of screw force. hand tight was the method taught to me.


also, agree on countersinking after drilling the holes.

thanks again

[pmg]

Hi,

I mounted a lot of ski bindings in a ski shop, thats why I am used to the cordless screwdriver method. But I only did this on factory skis with pre-drilled holes or holes drilled with a proper drilling template.

For the bindings I put on my self-built skis I also used the hand method - much more feeling in there. But actually easy to "overdrill" a screw because the force needed to damage some hardwood hole isn't as much as one might think.

In my case, it was mostly Dynafit bindings so far - they are quite a PITA, because the holes in the Bindings are about 5mm wide - meaning that the 5.5mm wide screw also cuts in there. Never understood what this should be good for - just causing trouble.

And the screws with a different diameter I mentioned: I encountered them on some strange rather old children binding.

[pmg]

Why would you avoid epoxy? Any specific reason for that?

My reason using epoxy actually is:
If you overdrive a screw a bit and the surrounding wood gets damaged, the epoxy will help creating a strong surrounding again once its cured.

[petemorgan(pmoskico)]

I do not use epoxy because it makes it more challenging to remove the screws. can even make it impossible to remove the binding without damage to the skis or bindings.


I have not noticed there being any play in the screw holes for the dynafits. The toe piece has a thin metal sheet underneath, are you missing that?

[pmg]

Not play, the opposite - the holes being a bit too small, so the screw is cutting into the plastic too.

[falls]

If you are having trouble getting an epoxied screw out you can touch a soldering iron to the screw head. This heats the screw and warm/softens the epoxy and then they usually come it easily.

[pmg]

Well, I prefer having a binding that holds damn well and is hard to remove, so I will stick with the epoxy. Not too much of course, just enough to wet the walls and the bottom of the hole before putting the screw in.