Mozella - 22 April 2009 04:52 PM

GreyWolf74 - 08 January 2009 11:58 PM

The thing to keep in mind when comparing aluminium, steel and carbon/kevlar fiber for frames is/are the structural properties of each material.

Steel is the hardest and stiffest material of the bunch. Pound for pound, it is stronger than aluminium and more elastic -- that is, if you bend it, it is more likely to spring back to its original shape. And it is probably the easiest stuff to weld. If you're not worried about weight, it is still a good choice for a welded double-hoop or trellis frame.

Aluminium is softer and easier to bend permanently, so you have to use more of it for the same strength and rigidity. However, it is considerably lighter than steel per unit of volume. It is also easier to form into light extrusions and castings, which is why it is common in sport-bike frames.

Carbon fiber is a different thing altogether. As the name suggests, it is fibers of carbon in an epoxi matrix, either unidirectional fibers, or woven into a coarse cloth, something like burlap or canvas. It is more difficult to work with in production because each piece has to be precisely laid up by hand in several layers, then "cured" in an autoclave under heat and pressure.

The result is a material that resembles fiberglass, but is considerably siffer and stronger. While carbon/epoxi is very strong, it is still not as strong per unit of weight as steel, so like aluminium, you have to use more of it for the same strength.

The third thing against carbon is that you can't repair it. It is very strong and rigid up to a point. Once past that stress load, it cracks or shatters. Worse still, broken carbon fiber is not repairable. If you crack your new $23,000 carbon frame, it's done; you can't straighten or re-weld it like a metal one.

Thing four is safety. If a carbon frame has taken a knock, you may or may not be able to spot cracks in the layup. Thus what looks like a "perfectly good" frame could break on you while you're going 80 mph in traffic. That would ruin your day real quick, especilly if you've just passed a semi on the highway.

Don't look for carbon fiber frames in mass production any time soon.

I'm afraid you may be inadvertently misleading some readers here by posting incorrect information about various materials used in frames. Let me correct a few points.

Steel is not the "stiffest" material of the bunch. Depending on the alloy and manufacturing process, steel and carbon fiber are about equal in stiffness; however, when it comes to "specific" stiffness (where the weight is factored in, Carbon wins by a WIDE margin. Figure on carbon fiber tubes being roughly three times stiffer than steel for the same weight. Aluminium, although not as stiff as steel, is lighter and the stiffness-to-weight numbers are nearly identical.

When it comes to tensile strength, it's no contest. In a strength to weight ratio comparison, figure carbon fiber tubes to be roughly TWENTY times as strong as steel and about 25 times as strong as aluminium.

Then there are fabrication considerations but, depending on the situation, carbon fiber may have the advantage here too. No problem with metal if all you need a tube, but how about fabricating a complex swing arm? Forging, casting, and welding a part like this out of metal, if one is interested in optimizing the overall strength and stiffness properties, takes a good deal of sophisticated equipment while doing the same thing in carbon fiber may be less complicated. There is no clear winner here, at least at the moment. Plenty of people have welders and not too many have autoclaves, but that's changing.

True there are damage and repair issues, but it certainly isn't fair to say that carbon fiber can't be repaired. Talk to any F-1 team.

As for a production carbon fiber frame? I'm willing to bet that if Stoner or Hayden wins the championship with Ducati's new carbon fiber frame bike, dealers and customers will DEMAND a production version immediately.

Thanks for the update, Mozella. To respond to your points:

You're right, steel is not as strong or as stiff as I'd thought. I don't know what I was thinking when i wrote that part, but it wasn't about the mechanical properties of ferrous metals. I think thoughts of ice cream were involved.

Steel's one advangage in production motorcycles is that it is easy to work with and it's been around for a long time. Designers know what it can do, fabricators know how to bend, cut and weld it and so do repair people. Further, if steel is bent, it's the easiest to un-bend.

You're also right to say that carbon fiber is extremely strong and light in undamaged form.

The problem is, carbon, like fiberglass, is a composite material that depends on the resin to give it much of its strength. Once the resin is cracked and starts fracturing, there is no way to just un-bend it. Or re-weld it. And if the crack turns into a split under load, you are in serious trouble.

If you don't believe this, look at a Formula One car where a carbon fiber suspension component suddenly breaks. Or carbon wings that shatter instead of bending as the old aluminium ones did. More spectacularly, there was a New Zealand racing sail boat had a crack that suddenly split, breaking the boat in half and sending it to the bottom of the ocean. Catastropic failures like that happen with carbon.

Is carbon repairable? Yes and no. Like fiberglass, you can grind down the crack and cover it with resin and fiber. But it will not be as strong as the original part. Remember, carbon fiber is laid up in full-length strands or oriented strands. Once you break them, the repaired part is heavier and not as strong as before.

So, given carbon-epoxi's penchant for shattering and not being repariable to full strength, it's not a good choice for a street bike frame.

This same debate has been raging in the bicycle industry for 20-plus years, and the truth is that there are no absolutes when it comes to comparing frame materials.

"Aluminium is softer" than steel? Which aluminum alloy are we talking about? 6000- or 7000-series? Which heat treatment? The yield strength of 7075-T6 aluminum, for example, is higher than most mild steels. Or are we talking about 4130 chrome-moly steel? Or a more exotic alloy?

And that's just the tip of the iceberg. Beyond the mechanical properties of a given material, you have to consider design parameters such as wall thickness and tube diameter. It's not hard to design a chassis made from large-diameter aluminum tubes that is both stiffer and lighter than a comparable steel piece.

GreyWolf74 -

Steel's one advantage in production motorcycles is that it is easy to work with and it's been around for a long time. Designers know what it can do, fabricators know how to bend, cut and weld it and so do repair people.

You speak as if aluminum and carbon fiber are still somehow mysterious and scary from an engineering standpoint, when both have been used in high-performance aerospace applications for decades. If steel were truly easier to work with than aluminum, then why has every major Japanese OEM been building sportbikes out of the stuff for 20-plus years? Even Harley uses aluminum for the chassis on its fastest, most powerful bike ever, the V-Rod.

Even if you don't have the resources of an OEM, it's arguably easier and requires less technical expertise to build with composites than with steel. No, not everyone has an autoclave at their disposal, but neither does everyone have the skill and equipment to cut, bend and weld steel. And I would argue that carbon-fiber layup is far easier than mitering, jigging and welding tubes.

As for carbon's "penchant for shattering," I presume, then, that you won't be flying aboard either Boeing's 787 or the Airbus A350, both of which will use 100% composite airframes.

can't repair carbon fibre? please do not let the entire open wheel racing and sports car groups know this. They have been repairing carbon tubs for years.

As I said, carbon-epoxi layups are very strong and very stiff per unit of volume. But they do have a problem with cracking, as does fiberglass, only worse.

Is carbon-epoxi repairable? Yes, up to a point. As I said, you can grind the edges of a crack down and lay in more material, but it may or may not be as strong as the original layup.

Now about "the entire open wheel racing and sports car groups" repairing carbon fiber tubs. Maybe they have and maybe not. Carbon is not permitted for use as a chassis material in most ameture classes. As for the pros, they don't tell you this, but they keep an extra tub or two in the spare parts trailer in case the old one gets written off. The "repaired overnight" car you see may not have the same tub it did yesterday.

If a tub is repaired at all, it will be more for cosmetic damage than structural repair. If you try that, you basically have to put the tub back in the mould and start over.

Now about aluminium. There is nothing wrong with it as a structural material. Some alloys are indeed considerably higher in strength and harder than pure aluminum. It is also workable using standard metalworking techniques, like cutting, grinding and welding, if it's done properly. That's why you find so many sportbike frames made of the stuff. It's a familiar material, but it is also more expensive, which is why less expensive bikes still use steel.

The one thing I'm not sure about is whether or not aluminium can be heated and straightened out after a prang the way steel can. Peter Egan attended a "journalist's training" session at Ferrari in Italy. He said that if a Ferrari's aluminium frame is bent, they cut out and replace the damaged section because it can't be bent back into shape; something about a compromised structure. Is this true or not? I don't know, all I have is Egan's word on what Ferrari told him.

GreyWolf74 - 19 May 2009 11:57 PM

Is carbon-epoxi repairable? Yes, up to a point. As I said, you can grind the edges of a crack down and lay in more material, but it may or may not be as strong as the original layup.

Now about "the entire open wheel racing and sports car groups" repairing carbon fiber tubs. Maybe they have and maybe not. Carbon is not permitted for use as a chassis material in most ameture classes. As for the pros, they don't tell you this, but they keep an extra tub or two in the spare parts trailer in case the old one gets written off. The "repaired overnight" car you see may not have the same tub it did yesterday.

Hey Grey,
As to repairs on carbon, the way it works is; the strength is in the fibres. The loads are transmitted to the fibres in shear at the fibre / matrix (i.e. epoxy resin) interface. Correctly done with enough overlap, this can be repeated in a repair. The repair may have to be heavier, but it is certainly done. If you look carefully, you can see the seams on the side of open wheel chassis as they tend to be made in multiple parts. I have seen tubs repaired, but specialists in F1 and aerospace are on site with special crack detecting and other non-destructive testing (NDT) equipment.

So, I think it is down to complexity.
Formula Student/SAE likes steel roll bars because they don't think students can do proper ones in Ali or carbon, but I think also because the organizers wouldn't know a good one from a bad one either.

Certainly, when damaged Carbon Fibre Reinforced Composites (CFRC) have very severe damage modes (as said b4, they break big time) Not too often an F1 team repairs a front wing or wishbone. Often parts are left scattered all over the track. That would not please a bike owner.

Still, years ago they said bicycles would never be made from CFRC and Ali wasn't used often because it too is not repairable. Ali is usually in a heat treated form thus if welded the entire frame has to be retreated.

I think, honestly, as all the posts here point out. Learning new materials, repair and manufacturing must all be considered when choosing a material for any part. Mercedes and BMW make some car parts (and limited production cars) from composite i.e. bonnets. http://quadrantcomposites.com/English/idqpc100.asp
I suspect that chemical/petrol costs and environmental concerns may limit this in the future.

There are CFRC wheels. Benelli is experimenting (not sure where that project has gone) with a CFRC frame.

As to Duc, in addition to the marketing aspects, I suspect their production volumes have not reached the break even point to justify the cost of extruded ali/cast ali frame machinery, and certainly wouldn't exceed the lost differentiation with the japanese bikes. CFRC while an inexpensive material in low volumes, has some big limitations and is quite expensive and labour intensive in large volume. I suspect any CFRC version will be a limited run introduction.