just pressed my first full pair, skis 2 and 3, the only problem had was that the core shifted toward the tip by slightly over 12mm but i am not too concerned with that. but i am very confused on how i got rockered skis from a mold that is supposed to be completely camber. i have 9mm of camber and the first ski i pressed, although it had many problems turned out with the camber i wanted. does anyone have any ideas on how this happens????
i am pressing without heat blanket, at about 91deg, 85psi for 10hrs
on a side note how far back do you guys set your core from the tip of your ski. right now i am at 12mm on both sides, but that is starting to seem like not enough.
Got rocker from camber mold
Moderators: Head Monkey, kelvin, bigKam, skidesmond, chrismp
-
ggardner90
- Posts: 146
- Joined: Mon Jun 13, 2011 9:02 am
- Location: Salt Lake
- Contact:
-
doughboyshredder
- Posts: 1354
- Joined: Mon Sep 17, 2007 7:37 pm
Without heat, I am a little bit stumped.
A couple ideas though. What was your fabric layout? Did you have the same fabrics on the top as on the bottom? Did you orient the fabrics opposite of each other (minor detail often overlooked). Was there tension in your core before layup? Did you allow the ski to fully cure before removing from the press?
A couple ideas though. What was your fabric layout? Did you have the same fabrics on the top as on the bottom? Did you orient the fabrics opposite of each other (minor detail often overlooked). Was there tension in your core before layup? Did you allow the ski to fully cure before removing from the press?
-
ggardner90
- Posts: 146
- Joined: Mon Jun 13, 2011 9:02 am
- Location: Salt Lake
- Contact:
-
Sherpa Burns
- Posts: 73
- Joined: Mon Feb 07, 2011 8:39 pm
Camber can be a weird thing. I have found that if your glass orientation is not the same above and below the core, you can have unintended effects. For example, if you are using triax, and below the core, you orient the 90 degree fibers facing the core and the then orient the 90 degree fibers away from the core on top, you will have differing results than facing both 90 degree toward the core top and bottom. Make sure when you have your materials laid out that you have every piece consistent from ski to ski and find an easily repeatible process.
If you change anything at all from laminate to laminate you can expect different results. Early on, it is very useful to keep a journal so you can compare and dial in the results you are looking for.
If you change anything at all from laminate to laminate you can expect different results. Early on, it is very useful to keep a journal so you can compare and dial in the results you are looking for.
-
Sherpa Burns
- Posts: 73
- Joined: Mon Feb 07, 2011 8:39 pm
In regards to your core shifting, there are a few methods to prevent this. I use two 1/4" dowels that I superglue to the bases. These line up with holes that I recess into the core. . I have never had a core shift with this method. Some will say this affects the integrity of the core, but I cant tell a difference. With all the glass and uni carbon, I dont think there is any more affect than a binding hole.
I have also seen some builders superglue dowels outside the laminate and to the cassettes. The dowels sit tight next to the laminate to keep anything from moving.
I only have 10mm all the way around my core. Very little spacer material which is kinda cool. I calculated my tip/tail spacer costs at just over $1/pair. I prefer the core run as far through the tip and tail as possible. It makes for a much livlier ride.
I have also seen some builders superglue dowels outside the laminate and to the cassettes. The dowels sit tight next to the laminate to keep anything from moving.
I only have 10mm all the way around my core. Very little spacer material which is kinda cool. I calculated my tip/tail spacer costs at just over $1/pair. I prefer the core run as far through the tip and tail as possible. It makes for a much livlier ride.
-
twizzstyle
- Posts: 2207
- Joined: Tue Mar 07, 2006 8:25 pm
- Location: Kenmore, Wa USA
A layer of carbon fiber - only on one side? (i.e. top or bottom), or both sides?ggardner90 wrote:i left it for 12hours so i cant imagine it needed more time than that. i changed the layup a little from the first ski to these 2, i am using a layer of carbon on this one could the be the issue????
could i be using too much preasure?
As doughboy said, you need to pay attention to orientation of your fiber layers. If you are using triax, it has a side with diagonal fibers, and a side with longitudinal fibers. You should either have the top and bottom layers both have the longitudinal fibers facing the core, or facing away. If both layers have the longitudinal layer on top (or vice versa), you can get camber changes.
-
ggardner90
- Posts: 146
- Joined: Mon Jun 13, 2011 9:02 am
- Location: Salt Lake
- Contact:
i got the same result from skis 2 and 3 but different from 1 so should i try the same material or change it again for the next pair?
also since i am pressing in my garage i have a BIG propane heater 200,000btu that i put next to the press. it generally keeps the layup somewhere between 86-121 (the highs and lows i have witnessed) i think it is better than pressing at 60deg could the heat fluctuation cause problems
also since i am pressing in my garage i have a BIG propane heater 200,000btu that i put next to the press. it generally keeps the layup somewhere between 86-121 (the highs and lows i have witnessed) i think it is better than pressing at 60deg could the heat fluctuation cause problems
-
ggardner90
- Posts: 146
- Joined: Mon Jun 13, 2011 9:02 am
- Location: Salt Lake
- Contact:
-
twizzstyle
- Posts: 2207
- Joined: Tue Mar 07, 2006 8:25 pm
- Location: Kenmore, Wa USA
Well seems to me you've found your answer.
Skis 2 and 3 have the same layup, had the same result. Ski 1 had a different layup, had a different result. The only difference was the added single layer of carbon fiber? There's your issue.
Try carbon on top and bottom, that won't make the ski feel much stiffer on the mountain (your turning stiffness comes mostly from the ski flexing upwards, when the bottom layer is in tension), but you should get no camber changes. Ideally you want the top and bottom of the ski to be the same, if you're not compensating some other way (heat or mold shape)
Side fun fact: Fiberglass expands with heat, contracts when it cools. Carbon fiber does the opposite, it contracts as it heats and expands as it cools. You're not using heat so it's not a big deal, but something interesting to keep in your head anyways.
Skis 2 and 3 have the same layup, had the same result. Ski 1 had a different layup, had a different result. The only difference was the added single layer of carbon fiber? There's your issue.
Try carbon on top and bottom, that won't make the ski feel much stiffer on the mountain (your turning stiffness comes mostly from the ski flexing upwards, when the bottom layer is in tension), but you should get no camber changes. Ideally you want the top and bottom of the ski to be the same, if you're not compensating some other way (heat or mold shape)
Side fun fact: Fiberglass expands with heat, contracts when it cools. Carbon fiber does the opposite, it contracts as it heats and expands as it cools. You're not using heat so it's not a big deal, but something interesting to keep in your head anyways.
Hey Twizz, this is an interesting statement. Do you have data to back this up? You're referring to temps in the ski press, not post cured, right? What are their relative coefficients of thermal expansion; similar to each other or is one much different that the other (absolute value)?twizzstyle wrote:...Side fun fact: Fiberglass expands with heat, contracts when it cools. Carbon fiber does the opposite, it contracts as it heats and expands as it cools. You're not using heat so it's not a big deal, but something interesting to keep in your head anyways.
Just curious,
-S
A mix up in fibre orientation, as described by Sherpa, tends to be cause twisting - as reported by other users.
Carbon on the bottom, may adjust the camber slightly, but not greatly. You can get zero camber from using the same materials and camber from a flat mould. Generally, this issue is controlled by heat. I don't know what material your cat-track is made from - but if it's steel or aluminium, is it possible that the position of the heater allowed too much heat to be applied from above and give you rocker.
The only other thing is how the epoxy has been mixed ... may be the bottom mix was slight less reactive.
Movement of the core can be tricky to track down - take time out to check how the hose inflates, it should give you a good idea of what's going on. Do you put tape around the top and bottom cassette before loading? Plastic ties is another option. An alternative to either dowel method is the use of matchsticks, just depends what you have to hand. If you don't find anything you like, you could look at how you hold your tipspacers in place, but this starts to place limits your cassette.
Good luck!
Carbon on the bottom, may adjust the camber slightly, but not greatly. You can get zero camber from using the same materials and camber from a flat mould. Generally, this issue is controlled by heat. I don't know what material your cat-track is made from - but if it's steel or aluminium, is it possible that the position of the heater allowed too much heat to be applied from above and give you rocker.
The only other thing is how the epoxy has been mixed ... may be the bottom mix was slight less reactive.
Movement of the core can be tricky to track down - take time out to check how the hose inflates, it should give you a good idea of what's going on. Do you put tape around the top and bottom cassette before loading? Plastic ties is another option. An alternative to either dowel method is the use of matchsticks, just depends what you have to hand. If you don't find anything you like, you could look at how you hold your tipspacers in place, but this starts to place limits your cassette.
Good luck!
-
twizzstyle
- Posts: 2207
- Joined: Tue Mar 07, 2006 8:25 pm
- Location: Kenmore, Wa USA
I don't know the magnitudes off the top of my head, but I'm sure a quick internets search would find those numbers. I just know that graphite has a negative thermal expansion coefficient, from what I remember in school. (it's been a while)SHIF wrote: Hey Twizz, this is an interesting statement. Do you have data to back this up? You're referring to temps in the ski press, not post cured, right? What are their relative coefficients of thermal expansion; similar to each other or is one much different that the other (absolute value)?
Twizz,
You're right! I never knew this. Some easy Googling around the internet and I've learned that certain aerospace grade carbon fiber materials can in fact have a negative CTE. However most commercial grade materials do not.
I'm not so sure this CTE would cause positive rocker from a cambered ski form. Especially when not using elevated curing tempreatures and also having a bunch of E-glass in the mix. I think having an asymmetrical composite schedule below and above the core would have a more dramatic effect on the ski shape.
-S
You're right! I never knew this. Some easy Googling around the internet and I've learned that certain aerospace grade carbon fiber materials can in fact have a negative CTE. However most commercial grade materials do not.
I'm not so sure this CTE would cause positive rocker from a cambered ski form. Especially when not using elevated curing tempreatures and also having a bunch of E-glass in the mix. I think having an asymmetrical composite schedule below and above the core would have a more dramatic effect on the ski shape.
-S
-
ggardner90
- Posts: 146
- Joined: Mon Jun 13, 2011 9:02 am
- Location: Salt Lake
- Contact:
in terms of stiffness i am more concerned with the stiffness of tip and tail as it is a park ski and i want it to be really playful. i have ordered a heat blanket from MEI and as soon as it arrives i will be using it in the layup. will i need to change anything that i am doing to counter the effects of the heat.
i am using an AL cat track which theory do u think i should test in the next pair, adding another layer of carbon on top of core, orientation of triax, or heater modification, assuming the MEI blanket is not here in time
i am using an AL cat track which theory do u think i should test in the next pair, adding another layer of carbon on top of core, orientation of triax, or heater modification, assuming the MEI blanket is not here in time
-
doughboyshredder
- Posts: 1354
- Joined: Mon Sep 17, 2007 7:37 pm
Yep. The carbon is the culprit.ggardner90 wrote:i left it for 12hours so i cant imagine it needed more time than that. i changed the layup a little from the first ski to these 2, i am using a layer of carbon on this one could the be the issue????
could i be using too much preasure?
If you don't have the same carbon on top and bottom you will get camber changes. Usually, it's not as pronounced when not using heat.
A certain indy ski manufacturer had a run of skis that were built with an extra sheet of carbon on one side and had an extra centimeter of camber because of it.