Mountain bike frame questions

General Composites Discussions Composites Talk
#1

I will open by offering an apology as you’ll have to bear with my while I try and articulate my thoughts.

My mind has (unfortunately) been a bit active lately and got me thinking it would be possible to make a carbon mountain bike frame.

Firstly, if I use my own bike as the template, how would you suggest I go about marking out the exact center line of the frame in order to create a flange to make a couple of female molds? A bike frame isn’t the easiest shape to stick a flange to.

With 2 female molds made of each side, I then planned to temporarily line the inside of the female molds with some sheet wax - so I can get the desired wall thickness of the carbon frame.

Once the wax was fully laid out, I’d close both female molds then fill the cavity with foam. Once dry, I’d remove the foam and then lay the carbon over the foam. The carbon would be layered up to the thickness of the wax and either encased in the female molds, or maybe wrap the frame in peel ply (to regulate folds/bridges in the ply) then release film, then breather, then probably throw it in a silicone vacuum bag.

Now I know it won’t be the right term, but the bike in this picture - http://bimg2.mlstatic.com/quadro-scott-ransom-carbon-cr-1-usado-apenas-60km_MLB-F-202402640_5716.jpg - has what I call ‘inside out’ surfaces - i.e the outer surface being the one with the Scott graphics on and the inner surface being the ‘tunnel’ around the rear shock linkage.

I was wondering how you’d manage to, when using female molds, to create a perfectly spaced ‘inside’ surface yet also getting a perfect ‘outside’ surface (in that case the arms where the linkage is bolted to).

I hope that’s comprehensible!

Thank you kindly,

Kyle.

#2

Hey Kyle, Ive given a few frames a go, one turned out well, the other I over complicated and is a monument to failure at the moment!!
What type of foam are you planning on using for the ‘core’?

Silicone intensifiers seem to be the way to go for those inner surfaces if you have the time, possible made of RTV silicone or something along those lines. Now I havent actually made one yet, have the materials there but just too busy to do any one of the hundred things id like to! he he.

#3

In what way did you over complicate the other frame? Do you have any pictures of the frames you had a go at?

Regarding the foam for the core - One of my future questions was going to be which type people would recommend. I was thinking expanding foam in order to completely fill every crevice and also it’s very light weight, but the mold would have to be strong enough to counter the pressure exerted by the foam expansion.

Like you there are lots of things I’d love to make and not being busy is ideal - I just don’t have any materials or the space to make things.

#4

Well if you want to do it cheaply, its male tooling all the way. I havent used female tooling for my frames - yes its a pain in the ar*e with sanding, finishing and possible wrinkling and thus a heavier frame but to keep costs down, seems to be the best way unless you have easy access to machining centres - i do but Im all out of favours so have to pay for all that kinda stuff!

I over complicated things by trying to incorporate super thin alu tubing in the main frame for impact resistance which would have worked really well if I could actually bend it without kinking it! Also, instead of a rear pivot close to the rear axle for a four bar linkage design, I tried to build a low second moment of area section that would flex instead - i calculated it wrong and ended up screwing a lot up!
This is the first frame which was done with pretty pre-historic and simple methods!
http://www.facebook.com/media/set/?set=a.1103052233517.12976.1743082718&type=3
Same time, it was the one that worked and has been in XC races and DH races. Absolutely awesome for DH so it is! add me on fb there kyle and i can walk you through it sure and my experiences, no point you going through the same mistakes i made