Downhill Speed skater needs help with foam core longboard

[jestah]

Hi

For over the past 8 years I have been teaching my self composite construction so I can make a better longboard skateboard. I started with simple ply wood boards and with the help of local boat builders and the internet I have recently finished my latest foam core downhill speedboard in preparation for my first world cup race later this year.



Wheel base: 800mm (inner truck holes)
Concave: W (0-8-3-8-0mm ) that fades into straight 8mm flat bottom concave.
Width: 245mm tapering down to 215mm
Nose wedged +10
Tail dewedged -10
Weight: 1314g
Designed for Landy Smokey 40degree trucks and Gumball wheels








From the deck down it has:
Carbon double bias 400g
Carbon uni 400g
10mm Airex 80kg Foam core with a Kevlar cloth and carbon dlb bias U beam
Carbon uni 400g
Carbon double bias 400g
0.6mm wood veneer
Foam spine that has 3 solid 5*5mm carbon uni I beams
Twill weave carbon cloth to finish.


The above photo shows the core and how I have “woven” the Kevlar and carbon double bias under the middle section to form the U beam.

This is the first board I have really put effort into finishing and can now see that the hours of sanding, polishing and waxing really pays off… MMmm shiny!



I am very happy with this board and it has proven that the Kevlar can help dampen the “twang” that carbon boards have that can kick you off your board at speed (speed wobbles). The shape and angels work well.

While the first attempt worked it was only after I fixed a few mistakes. I had guestimated that a 10mm core was going to be thick enough to make a board that had as little flex or movement as possible under my 60kgs. The board had 20-25mm flex at the first test so I vac a 5mm thick spine down the board and then cut 3 channels 5mm wide and suffed the gaps with 1000g carbon uni with a csm backing (off cuts that I get at a good price). This made the board very stiff with 5-10mm flex even with 80kg. . I also cut the U beam at 45degree angles so the carbon did not get pulled around right angles. After the fixes the board displays good stiffness longitudinally which is very important but could be a touch stiffer torsionaly which is also extremely important.

The other big problem was I made it far to complicate as I did not know you could use heat to bend the core. I had used used 3 layers of 5mm foam so it would be able to pull around my 40mm radii. My board had 15 different core pieces when it could have only had 9 that were pre formed and would not try to pop back out of the mold at any chance they could.

I feel confendent I have the skills now to make a plug, take a mould and use it to vac bag a board but still need some advice to calculating the amount of cf I need to make the board strong enough to do what I want as I feel that this last board has too much carbon making up for its thin core. My next board is going to be around a 15mm core using balsa wood (stronger compression and shere) for the flatter platform and airex heat formed around all the radii to make the “necks” because I don’t think I can bend balsa aaround my curves

I think that next time I should use carbon uni in the U channel and make it 90degrees as I think the angled u beam was flexing inside the core. I am also considering making some solid 5mm thick carbon or glass stringers into the board rather than “weaving” a single thickness of fabric.

I as to improving it resistance to twisting i have considered changing the shape of the necks so they go out at a wider (45 degree) angle outward to the riding platform and adding more double bias. This will ad a bit of weight and will also make the join in the neck a very complicated compound curve which I am trying to avoid. Why do I need to use double bias in the top and bottom of the laminate? I have been told/read often about balanced laminates are important but I don’t know why.

Lastly I would love to try to infuse this board as infusion has been a long term goal of mine for a long time but because the only way to learn is to do a $1000 training seminar I have been holding off. Could some one help me work out how it can be done?

Thanks
Jestah

Ps http://www.youtube.com/watch?v=T7XxbpJO2OI a sort clip of the first few test rides on this board.

[hansp77]

Nice Vid Jestah,
and very nice deck.
That was pretty wild through the traffic. What speeds are you reaching on that run, and what speeds are expected on the world cup course?
As I know nothing about what you are asking, being a humble wooden boat owner, this is pretty much a bump in the hope that you can get some help here. There surely are members skilled in composite materials and design around, its just a matter of getting their attention.

Three arthroscopy's later and much pain and suffering, I have have long left my skateboarding (and more sadly snowboarding and skiing) days behind me.

I'm gonna check out some of your other vids now. Watching that last one brought back the memories of the dodgey downhills we used to try in my coastal small home town- very rough country bitumen roads, death-wobbles and lots of missing skin
One of the most memorable was when I was twelve, on a coffin ride down way-too-steep hill above my house that ended at a T-intersection. I lost control half way down, death wobbles, couldn't slow down, straight across the road (luckily no cars) into the grass curb and flew off my board feet first down a 2 meter embankment and straight through a waiting barbwire fence Still got a few scars from that one.

Anyway, good luck with the world cup.
Hope you get some help here.

Hans.

[JRL]

The shape of your board would be perfect for resin infusion. I know RI is a sore subject with a lot of boat builders, but, on small projects its pretty hard to beat.

There are balsa cores meant for infusion that will conform well to vaious shapes.

Have you looked into using aluminum tubes as stringers? There wont be a reaction with carbon as long as there is no air/salt coming in contact with either.

If you try resin infusion and run into trouble send me an IM. Its amazing what minor detail can cause air to stay in the part. I cant stress enough that you should practice using bare glass first. As in no gel coat, no nothing. Just glass and resin. Youll learn a lot.

I know a lot of people who are making carbon parts with RI and have no clue that their parts are full of air (because you cant see it like you can with bare glass). They think that just because there vacuum guage is staying steady at "X" amount of vacuum that means the infusion will be successful. So wrong.

[longliner45]

how does it keep from turning over? I dont see a keel!