Aluminium alloys...

Something for you to talk about. Enjoi.

Material | Mechanical Properties | Ultra 6 | 6061 | 7005
Tensile | Kg/mm | 42.0 | 31.5 | 35.0
Yield Strength | Kg/mm | 36.5 | 28.0 | 30.5
Elongation % | in 2 inches specimen | 12.0 | 12.0 | 11.0

Just to help people know a bit more about the properties of each aluminium alloy.

KH frames are 7005, and XTP and Domina frames are 6061. Ultra 6 alloy (aka 6066) is not used in unicycling yet as it’s currently a biketrials-specific alloy, but is the most reliable of all when manufactured correctly.

Kris was thinking of switching to U6, but 7005 seemed OK enough to him and figured that changing the design of the seattube-crown join was more important.

7000 series alu is generally harder, stiffer, more brittle than 6000 series alu. That’s the general conscensus. But that’s not meant to be a criticism to either. Kris chooses 7000 series for his muni frames because he wants a stiff brake response.

6000 series alu is good for frames to an extent, in that it is more flexible so it can absorb impact quite well. But is super soft and is therefore a bad alloy series to use for threaded parts or solid components like bolts, cranks, seatclamps, pedal bodies, hub bodies etc… as it gets eroded easily.

7000 series is better for those parts mentioned above. Thankfully, the majority of parts available like alu cranks and seatclamps are made of 7075. :slight_smile:

I’ve had bad experience with biketrials cranks made of 6061… thread strip is something that’s not uncommon.

How do you know what alloys the different frames are made of?

At least you got 4 good rides out of them :slight_smile:

It’s also important to note that some manufacturers hide the type of alloy they use by using marketing catch-phrases like “Ultra-C” or “Optim-AL” that make it hard to tell what alloy they are using. In probably 100% of those cases there is no special metallurgy being done by the bike manufacturer- they just want to brand their product as unique and that’s hard to do when numeric alloy specifications are published and well-known.

Aside from the basic specs it’s also important to consider reliability of performance in the manufacturing process. Uncommon alloys (like Ultra-6) have good strength characteristics but can be difficult to use with consistent high quality. A strong frame material is no good if a higher percentage of those frames fail due to manufacturer defects, due to construction challenges.

Consequently, I feel that 7005 Aluminum is an ideal material for uni frames. It’s much lighter than titanium (by volume) and is stiffer than most titanium tubing of comparable weight. It’s twice as expensive as steel but has much better strength-weight characteristics. That is why most DH bike frames are 7005 Al.

Although our frames do get bashed around, the loads they are forced to take are not at all comparable to a bike frame because (unlike bikes) loads off drops on a unicycle aren’t taken directly by the frame. Consequently, opinions on this newsgroup notwithstanding, I don’t feel that material fatigue is a big issue, or at least it is something that can certainly be taken into account with proper design.

Kris

I should add that that’s not a criticism of titanium or steel frames- both are good frame materials too. In any case, good design trumps material choice for most things.

K.

“It’s also important to note that some manufacturers hide the type of alloy they use by using marketing catch-phrases like “Ultra-C” or “Optim-AL” that make it hard to tell what alloy they are using. In probably 100% of those cases there is no special metallurgy being done by the bike manufacturer- they just want to brand their product as unique and that’s hard to do when numeric alloy specifications are published and well-known.”

In an aside to this comment by Kris it may be very hard for low end manufacturers to continue to make claims like this in North America, as I am being lead to believe by the cycle community that much tougher restrictions are in the works in order for bikes and their components to meet what may soon be ISO standards.

Among many other restrictions, frame materials and welding quality may come under the gun and substandard frames will not be allowed (too many cheeper bike frames are breaking and causing injury even in general riding conditions)

Competitors in different catagories i.e. trials, bmx etc type competitions may soon have to prove that they are riding on certified frames.

Time will tell.

What do you experts think of carbon fiber?

I watched a movie recently, “the John Britten story”, about a garage craftsman who constructed an entire race motorcycle himself, and won some super bike races in the early 90’s. The frame, hubs and rim were made of carbon graphite.
The entire bike weighed 303 lbs.

Presumably a motorcycle rim has a tougher job, and needs a bigger safety margin then a unicycle. It’s been over a decade, why isn’t anyone selling carbon uni frames ? Is it purely a cost-market size thing ? Or is there something about carbon frames that makes them inferior to metal ?

They have. Roger Davies handmade the original high-end uni frames from carbon in the 90’s and still rides one. But it’s expensive to produce and cost-effective designs usually have a machined aluminum crown and bearing housings, so the weight savings isn’t necessarily all that much.

Kris

That guy was truely amazing! I was a big fan of race bikes back then and I remember when his bike came out. Not only did he design and finance that bike himself, he had no engineering background (he was an architect). His bikes were compettitive w/ $300,000 factory Superbikes.

He died of cancer at only 45 in 1995:( The motorcycling world truely misses him.

A carbon uni frame and wheel rim would be cool, but pricey. I think the uni market would have to grow quite a bit.

I think a good full suspension uni would be better, but CF could help keep the inevitably higher weight down.

…also more development into multiple gears.

Sorry for the threadjack.
Back on topic.

Edit: wikipedia says he was a mechanical engineer, maybee I’m remembering that wrong.

I think Kris’s choice of frame material is a good one. There is no need to increase price for a mild, if negligible improvement in performance.

I do however believe fatigue life is an important part of frame stats, especially in relation to welded areas. It is much harder to measure, and usually harder to find for the more obscure alloys.

Another (less important) note is corrosion resistance. If there is a way for water or something worse to get into a frame tube and eat out weld joints from the inside, a strong, but corrosive alloy is a bad choice. Yes, water is corrosive, by the way. Look up carbonic acid if you want to know more.

I’m sorry, kg/mm? Kilograms are a unit of mass, and tensile strength is provided in units of force/distance, such as pound/inch or kN/mm (kiloNewtons/meter).

Where did you get this information? I am a little bit skeptical of any source that doesn’t bother to check its units. Which is not a criticism of you. I don’t have my machinery’s handbook with me in college (it includes this information for nearly all major alloys of aluminum, steel, titanium, and bronze/brass), so i can’t check those figures.

whoops, kiloNewtons/MILLImeter

carbon fibre is in some ways a fragile material. The kind of damage that dropping the frame on to rocks would cause would soon initiate cracking and ultimately cause failure. Roger can get away with it because he never falls off. He always wraps his frame up when it’s in transit because just banging around in the back of the car may well be enough to cause damage that leads to cracking. I believe a batch of 30 of the Roger Davies mk 5 were made, last time I saw they were selling at £800 each.

As for a rim, the problem would be even worse, hard muni would put chips in it that would lead to premature failure. Of course a motorcycle rim takes more force when riding, but when taking a uni off a six foot drop i’m not so sure. A carbon coker rim would be brilliant, lower risk of damage and the weight saving would be more worthwhile on a rim that big.

[quote=gerblefranklin]

I’m sorry, kg/mm? Kilograms are a unit of mass, and tensile strength is provided in units of force/distance, such as pound/inch or kN/mm (kiloNewtons/meter).

QUOTE]

Glass houses and stones!!
Ultimate Tensile Stress should be quoted in units of force/area, usually for this type of material that would be N/mm2.
Yield? I can’t be certain for these alloys but in my experience aluminium design stresses are normally quoted as 2% proof stresses not yields. Aluminium alloys don’t have an effective yield point as steel does, they basically just snap without any warning which is why the 2% figure is used. (I believe that this is defined as the stress that the material is under when it has reached a permanant or plastic elongation of 2%)