Ski Builders Forum

I know Burton use finger joint cores on their boards. What do you guys think. Is this to increase torsional stiffnes or is it just cheaper? Anyone using this method?

Are you asking whether it is the correct method for creating the joint or a wider point about what is the purpose of the manufacture method?

I'm guessing your more interested in the purpose. Have a look at cores shown in the Never Summer Snowboard video - one post Falls kindly pulled together. Consider the purpose of stringers, dimensional stability, binding inserts, dampening, transfer of power into the edge set (more immediately relevant to snowboards) and enhanced bonding to composite materials.

Have you ever gotten sets of inch squared stringers together, different species, clamp them together in pairs, test their response to vibration?

Watch how certain species with there own harmonic frequencies, work with or against each other when placed together.

I think Kam has done a lot of work on this, so he's the turn too guy on this.

Thanks Richuk!
Well I dont say this is the best and easyiest way to make the cores but I think this actually could improve dampening and torsional stiffnes. It adds a little weight offcourse and that it self improves dampeing.

You need weight - the question to answer is where to put it within the ski.

I think there are a few pages on the web that show how skis cores were made back in the 60's - when a lot of skis were made in Northern Europe ; )

Hi Guys!

I use finger joints in my cores. In fact, I've glued stringers together without the fancy finger joints. For example, that a look at my old post when I made cores from an old pallet:

http://www.skibuilders.com/phpBB2/viewtopic.php?t=810

Those cores worked just fine and the two pairs of skis I made I absolutely loved and I put about 60 days on one pair!

Of course the finger joints provide better bond because of the increased surface area when gluing two pieces together. I've seen mass produced skis made with finger jointed cores and the joint was right at cord center!

But the bottom line is, the strength of the overall ski is more dependent on the type of composite layer you are using because that's the layer taking most of the load! The core material simply raises the composite material away from the neutral axis, providing stiffness. I'm not saying that using the lowest grade wood or 'fill' material is perfect -- it does have some effect, but it's not as important as one might think. In fact, major ski manufactures have used wood with knots, defects, etc., in the material, and it's not that big of deal. Now, what is important of course is binding screw pull-out strength. This is very much dependent on the type of wood you use, so don't use wood with poor pull-out strength where bindings will be mounted. Woods to avoid are soft stuff like pine and poplar. But, I do use pine in a lot of my nice skis (it's one of my favorite woods for skis) -- it's just I mix it with hardwoods such as ash or maple in the binding area, or I use metal inserts. Design the cores carefully, but you can mix/match woods and using finger joints is fine.

I know a lot of ski companies that use this method, mostly because it's cheaper and if you do it correctly, you can cut down on the waste by nearly 50%!!!! This is a big deal if you want to minimize waste and be green. But one would have to have some specialized equipment, not just to make the finger joints, but the part about minimizing waste. Bigger manufacturers in my opinion are way more efficient with how they use materials and minimize waste in making cores.

As far as adding more weight with the finger joint, it might be a little but it's not that noticeable. You might see more weight difference in the variation in density of wood from the same pile, of the same species. I've certainly noticed this.

Regarding adding damping by using finger or other type of joint -- this is rubbish, I think. I'm happy to eat a dirty rubber shoe if I see real data that shows this effect. I've been wrong, but let's see the data. The fact is, you're not going to add damping by simply adding a few finger joints. You need to do better than that, like some fancy tricks of mixing viscoelastic materials and metals together and putting the mixture in the right location. Speaking of damping, I need to make that simple video of some examples of damping for skis. Need to get on that...

The bottom line: finger joints are more cost effective. I wouldn't bet that they increase torsional stiffness, etc. Besides, there's no supporting data for this, but it would be interesting to see if anyone has it.

Hope this helps!

But the bottom line is, the strength of the overall ski is more dependent on the type of composite layer you are using because that's the layer taking most of the load!

... until the core fractures right by that big ol knot, so I'm not a fan.

Page 7-11 is helpful when considering the binding insert area.
http://www.woodweb.com/Resources/wood_e ... k/Ch07.pdf ... removed Ash (White Americian) from the binding area. Used on sidewalls last season and found it shrank - so concluded it was not idea for the binding area. Probably just unlucky with local source, as it has been used by many others as sidewall material without this issue being reported.

Not had the same issue with Maple or Steamed Beach. The mechanical properties of steamed beach are altered as a result of processing, but its dimensional stability is an advantage which can be exploited.

bigKam wrote:Hi Guys!

I use finger joints in my cores. In fact, I've glued stringers together without the fancy finger joints. For example, that a look at my old post when I made cores from an old pallet:

http://www.skibuilders.com/phpBB2/viewtopic.php?t=810

Those cores worked just fine and the two pairs of skis I made I absolutely loved and I put about 60 days on one pair!

Of course the finger joints provide better bond because of the increased surface area when gluing two pieces together. I've seen mass produced skis made with finger jointed cores and the joint was right at cord center!

But the bottom line is, the strength of the overall ski is more dependent on the type of composite layer you are using because that's the layer taking most of the load! The core material simply raises the composite material away from the neutral axis, providing stiffness. I'm not saying that using the lowest grade wood or 'fill' material is perfect -- it does have some effect, but it's not as important as one might think. In fact, major ski manufactures have used wood with knots, defects, etc., in the material, and it's not that big of deal. Now, what is important of course is binding screw pull-out strength. This is very much dependent on the type of wood you use, so don't use wood with poor pull-out strength where bindings will be mounted. Woods to avoid are soft stuff like pine and poplar. But, I do use pine in a lot of my nice skis (it's one of my favorite woods for skis) -- it's just I mix it with hardwoods such as ash or maple in the binding area, or I use metal inserts. Design the cores carefully, but you can mix/match woods and using finger joints is fine.

I know a lot of ski companies that use this method, mostly because it's cheaper and if you do it correctly, you can cut down on the waste by nearly 50%!!!! This is a big deal if you want to minimize waste and be green. But one would have to have some specialized equipment, not just to make the finger joints, but the part about minimizing waste. Bigger manufacturers in my opinion are way more efficient with how they use materials and minimize waste in making cores.

As far as adding more weight with the finger joint, it might be a little but it's not that noticeable. You might see more weight difference in the variation in density of wood from the same pile, of the same species. I've certainly noticed this.

Regarding adding damping by using finger or other type of joint -- this is rubbish, I think. I'm happy to eat a dirty rubber shoe if I see real data that shows this effect. I've been wrong, but let's see the data. The fact is, you're not going to add damping by simply adding a few finger joints. You need to do better than that, like some fancy tricks of mixing viscoelastic materials and metals together and putting the mixture in the right location. Speaking of damping, I need to make that simple video of some examples of damping for skis. Need to get on that...

The bottom line: finger joints are more cost effective. I wouldn't bet that they increase torsional stiffness, etc. Besides, there's no supporting data for this, but it would be interesting to see if anyone has it.

Hope this helps!

Thanks a lot Kam!
I am thinking the more finger joints you have the more torsional stiffnes. What do you think? I also think that you are absolutely right about not using to soft wood under the bindings. Inserts works really good combined with ash.

Looking forward to your video about dampening!