Cedar Cores

[Jekul]

I got ambitious on Christmas day and decided to make a pair of ultralight back country skis for a friend. I didn't have any bamboo immediately available for the core, and I wanted something lighter than the poplar cores I had on hand, so I went to Home Depot and picked up some knot-free cedar boards and planed/routed them to the right dimensions.
Here's the layup:
Entropy CPM - cured @ 175F for 40 minutes - blanket turned off and allowed to cool under pressure until 120F achieved (about 1.5 hours)
Durasurf 2000 base
8oz triax carbon fiber
2.5-11-2.5 mm core made of Cedar
8oz CF binding reinforcement
8oz triax carbon fiber
Wood veneer topsheet
187cm long. 108mm waist, 141mm tip
After I cut them out one ski weighed 1660g and the other 1620g, acceptably light for that size ski.

I gave them to my friend and he took them to the Wasatch range in Salt Lake city for 5 days. He came back to Colorado and achieved the following after one day on the hill:




This is a first for me. I have a couple ideas of what went wrong:
1. Cedar doesn't bond well (first time I've used this construction)
2. The Carbon fiber didn't cure properly (unlikely given my experience with Entropy CPM - about 10 pairs w/o failure)
3. The CF wasn't strong enough (I'd guess this is unlikely)
Anyone have suggestions/ideas/experience with this?

Let 'em slide!

[Jekul]

Oh, if anyone wants bigger pictures PM me and I'll send the picture link (or tell me how to post larger pictures)

[bhenry]

Interesting, I'm about to lay up my first pair using a red cedar and western maple core with entropy. I've glued red cedar in the past, but never for uses like a ski. Red cedar does have oils that give it its decay resistant properties...
a wipe down with acetone prior to lay up might be the answer. Similar to working with teak
I can't tell from the pics, but was there a clean separation?

[Richuk]

I understand Cedar can be used, so saying Cedar is difficult to bond would be like saying plastics are difficult to bond, however the same logic and test can be applied i.e. testing to see what process improves how wettable the surface of the wood appears.

The pictures show a thin layer of wood fibre attached to the carbon fibre, with piece of core, here and there. It suggests that your epoxy and core material aren't working well together. You might resolve this problem with an additional process: increasing the porosity of the wood using a chemical wipe, wetting out the core a couple of times with warm epoxy prior to layup or a longer dwell time. Whether you are using a CNC or a planner may be a factor in this instance.

Changing your process and running a few samples through the press should quickly show you where the problem lies. You just need to peel back the fibre to see the results.

[skidesmond]

I read an article recently about gluing oily woods, wipe with turpentine before gluing. I've never had a bonding problem before so I've never actually tried this remedy. Although since I'm using Ipe which can be an oily wood I may incorporate the wipe down with turpentine before layup.

[Jekul]

Thanks for the responses guys. I'm tempted to peel the top off, wipe with turpentine/MEK/Acetone and try to laminate back together. I'll also be trying some other samples at the same time (wood types with the same CF and epoxy)

[Jekul]

UPDATE -

I peeled off the top layer of CF and found this:






To me it appeared that the screw retention in cedar was very poor. So...I re pressed the delaminated ski. Then I purchases some Helicoil (screw inserts) and remounted the bindings (dynafit vertical TLT's). I've had two days on them so far without issue, and plan to ski them a few more times this season!

[gav wa]

The way you were able to delam that whole ski and the only bit of timber that stayed with the carbon fibre was the small pieces around the inserts suggests to me there was no real bond between those layers.

I'm only new to this so I quite probably am totally off.

I have also considered cedar for a core so I am keen to hear how it all ended up for you.

[skidesmond]

How difficult was it to tear it apart? Cedar is a really soft wood. Helicoil is a good idea. A hardwood insert in the binding area wouldn't be a bad idea either.

[Jekul]

Desmond - It was not that difficult to peel apart, at least once it was started. For reference - I have no baseline to compare to. I've never had an issue with delam before.

In total I rode them about ten times in total this winter, and didn't have any issues afterwards. I've glued up some new cores for the next pair that have a hardwood (oak) piece under the binding area. Should be much better.

[skidesmond]

Do you recall if the grain ran vertical or horizontal when looking at the end of the core? This is a guess on my part, but the way the cedar tore apart makes me wonder if the grain was horizontal. Vertical end grain makes for a better core. I suppose it still would have delamed either way but I think the wood core would not have torn apart like that.

[Jekul]

When looking at the end of the core the grain ran from bottom to top, sloping slightly to the left (or right, depending on perspective). Probably at a 30deg angle from horizontal.

[MontuckyMadman]

looks like a better bond than any of the mass produced skis I have slap tested to delam. Just the ceder is soft and weak so it splinters apart.

[Jekul]

Yes, soft and weak, but cheap and light too...

Still searching for the perfect balance...

[MontuckyMadman]

Look at chestnut. Really its a combo of polar and ash or pop and maple or pop and bamboo or anither variety maybe even the palowlonia or whatever.