Bicycle Materials

[Eng2012]

Hello guys,
I am new to materials engineering and am looking at making a BMX trick bike which is lighter than a steel one and just as strong. Please recommend me materials.
I have been looking at CNT-PPX, and something like polypropelyne.
What would be lighter and as strong or stronger than steel?
I have been mainly looking at carbon nanotubes with PPX.
Preferably just a light plastic like PPX or ABS.
Please help me so I can revolutionize the BMX industry with light bikes.
Thanks

 

[RPstress]

Current carbon nanotubes won't give a composite more useful than steel or aluminum. I suggest you research continuous carbon fiber reinforced polymer matrix composites which will give a mass advantage over a good metallic. This will be lighter than steel but very much more expensive. A thermoplastic matrix as you suggest will be more expensive than a thermoset matrix and will dictate very different possibilities for the manufacturing method, although its better toughness may make the composite needed lighter. The method of manufacture will be absolutely fundamental to any design. Research manufacturing methods for thermoplastic matrix carbon fiber reinforced composites.

 

[MikeHalloran]

My friend Irving rides a bike faster than most people drive a car on suburban streets.
He won some kind of competition, in which his prize was a very, very, expensive bike, made of magnesium, which made it very light.
He hated it, and stopped riding it almost immediately, because it was too flexible.

It was probably a lot stiffer than The Original Plastic Bike, which would have used vast quantities of Lexan, if it had been rideable. The Generous Eclectic company, then a big supplier of Lexan resin, basically swindled some people into tooling up a bicycle made almost entirely of Lexan and/or Lexan foam, which was a bit lighter than a common steel bike of the same era. Unfortunately, they made the frame geometry pretty much a copy of a generic steel bike, subbing foam for tubing, but using the same diameter, which made the bike very flexible. ... i.e., ignoring the advice in the GE plastic design manuals, to adjust geometries in proportion to the difference in elastic moduli.

At least the Itera Plastic Bike made some geometric accommodation for plastic's, er, weaker properties, but the designers didn't go far enough, and it was still too flexible.


I suggest you get started real soon, laying up carbon fiber prepreg on carved styrofoam disposable cores, and try to evolve as quickly as possible toward a bike that is about the weight you want, but first, even just barely stiff enough to ride.





Mike Halloran
Pembroke Pines, FL, USA

 

[RPstress]

The lexan (polycarbonate or 'PC') bike may have used short fiber reinforcement (injection molding possible) or may just have been unreinforced PC. Either way it would have been difficult to make stiff enough. If a very high percentage (about 40 or even 50% by volume should be achievable) of short carbon is used it may be possible but it would be very difficult (note difference between material modulus and stiffness of a structure made of that material). The basic material modulus of a short fiber-reinforced material would likely be even lower than mag alloy. A radically different design from a familiar diamond frame would be needed. Continuous fiber reinforcement (probably HS or just maybe IM carbon (price of IM was a bit high last I checked about a year ago)) is almost certainly the only practical way to make an improvement over metal structures. I'm surprised that Mike's friend's mag alloy bike was no good. An adequately stiff and slightly lighter design than steel is certainly possible in good aluminum or magnesium (but again may well need a variation over a diamond frame of round constant-section tubes; actually I think Reynolds steel tubes may vary in thickness with length).

P.S. probably avoid ABS or Nylon as a matrix; rather weak, very flexible and made worse by high ambient temp. A more expensive engineering thermoplastic is probably worth it. (Nylon might just be possible, but bad in the wet.)

I like Mike's advice. You could gain valuable experience by starting out with woven carbon prepreg or dry carbon fabric wet layup before changing the design to suit a more cost-effective method of manufacture.

 

[MikeHalloran]

I used to work in a shop that did wet layup of phenolic resin on high temperature cloth, including mixing the resin with carbon black and several kinds of magic goo. The floor was always a black sticky mess.
Switching to prepreg tape moved the mess to someone else's shop and kept our floor clean. The only capital cost was a big refrigerator in which to store the prepreg.
It was like night and day.


If the carbon prepreg gives you sticker shock, start with glass prepreg and use that while you're working out your process and learning how to bond the metal inserts you'll need for the steering head, the crank axle, and the rear wheel chain adjuster whatchamacallits.






Mike Halloran
Pembroke Pines, FL, USA

 

[Demon3]

Very light bicycles are made and sold for racing. You can buy them and they are made of carbon fiber composite material. So, in your shoes, I would use that already validated material as it is the best and proven in your application.

Chris DeArmitt PhD FRSC
President

Plastic materials consultant to the Fortune 100

http://www.phantomplastics.com

 

[moon161]

I used to know a guy. As an engineering student, he made a CF mountain bike, back when CF bikes, and mountain bikes were new. It was an x-frame, two intersecting tubes. He made the tubes by wrapping a wooden core with carbon fiber. The back end might have been a more conventional triangle. It's hard to get away from triangles in shapes where you have tubes, and want rigidity.

 

[RPstress]

Moon: Can you post a sketch of the 'X-frame'? Sounds curious. (Not quite sure how it would wotk.)