Aluminum VS steel frame construction, a discussion

[johnrobholmes]

After running across an incident where an aluminum frame bike snapped in half, it has me pondering whether aluminum should be used at all for the majority of Ebikes/ mopeds, etc. With a finite fatigue life, it just doesn't seem to be a good material for an "investment" into a frame. Sure, for a racing frame aluminum has plenty of advantages, but they are typically replaced often.

What say you? Are there certain cases where you think aluminum is better suited to steel?

 

[john2364]

There are many different types of aluminum. Most high end mountain bikes have aluminum frames. Many of these frames are meant to take far more abuse than an electric motor will put it through. I am guessing that your frame was either poorly designed or made out of a weak aluminum alloy

 

[johnrobholmes]

While the frames will take plenty of abuse, the finite cycle life and lack of an endurance limit just make me uneasy about having aluminum frames on mopeds.

This is the latest incident. http://www.pinkbike.com/photo/5947707/

 

[mdd0127]

A well built and properly heat treated design shouldn't break. For example, I have a 1998 Haro MX3 monocoque frame that is overbuilt enough to last forever. I break stuff easily and frequently and I have no worries about this frame. The reason we're starting to see breakage is that for the last twenty years, the main selling point in the bike industry has been weight. Everybody's engineering everything to be as light as possible thinking people will trade up before the cracking starts. My bike may weigh 45 lbs but it rides like a Lexus, eats up ten foot hits like nothing and yes I XC ride it. We did the East Rock trail at Canyon of the Ancients yesterday. You should see the looks on the spandexers faces when a big guy on a 7 inch travel bike wearing combat boots and fatigues goes blasting by. It's pretty funny.

If they put another pound of aluminum in frames, we probably wouldn't be seeing problems.

 

[Hillhater]

This is the latest incident. http://www.pinkbike.com/photo/5947707/

Thats a good example of poor construction ...break is at the welds,.. so not a metal problem, more a workmanship issue !

PS : i have seen steel Cro-mo frames break, also usually at or near a joint..

 

[HAL9000v2.0]

"There is no bad construction, there are just bad users" ...somebody, sometimes.

This is one of the toughest frames in the market. GT DHI.

 

[johnrobholmes]

So maybe the issue is that factories cannot properly weld aluminum? It makes me wonder why there are relatively few public incidents of steel frames snapping. Maybe because the majority of bikes now are alloy?

 

[john2364]

The strongest alloys can not even be welded. 7075 for example has to be milled. I think that 6061 is the hardest aluminum that may be welded. Trust me, It is hard enough for planes, cars and most other machinery. I can guarntee that it will be ok for a moped.

 

[johnrobholmes]

What about building light frames? Sure, if we overbuild aluminum 4x we won't have a problem. What if I want light, and something that isn't designed to be replaced on a yearly basis?

Just trying to encourage discussion here. Seems we have a lot of aluminum fans so far.

 

[neptronix]

For this kind of duty, i'll take a metal that bends before it snaps / fractures any day.
At these speeds, our frames get to deal with a lot of vibration. Steel handles this way better than aluminum does.

There is a reason why i am using basically a department store steel bike frame rather than the other newer aluminum bikes that i own.

I have already seen some bad stuff on the pedaled bikes though. A 2008 Giant Rincon i had was able to get some 1/3 inch deep dents in the frame!! A 2001 Canondale h300 is starting to develop a creaking sound where the steer tube mounts.

To be honest, i have basically ~$500 bikes and not high end stuff. So maybe things improve as you go higher up the price scale.

But so far, my cheap steel bikes have been far better than any aluminum bike i've owned.

 

[mdd0127]

I was planning to use a headtube and bottom bracket from a high end giant with a dented top tube and the metal was absolute garbage. It's very soft and evaporated under 30Amps in two seconds. I ordered some real 6061 stock to machine down.

Most of the big manufacturers are now having their top end bikes produced in the same factories as the Walmart bikes and they're still trying to charge thousands for them. The only way I could justify spending that kind of money is if a skilled craftsman built the frame. Then, problems with bad stock or iffy welds are sorted out before it gets shipped out. GT is not "GT" anymore, it's Pacific Cycles if I remember right. Quality has come way down but the prices have remained high and people keep buying the crap. It's gone the same way with guitars. It's really sad.

As far as designing with aluminum goes, it's pretty easy to overbuild stuff sufficiently without being too heavy. The frame for my bike will be all 6061 1/8" wall 1/2" od tubing and 1/8 think plate on the sides of the monocoque with 1/4" plate internal bracing/battery tray. The head tube and bottom bracket will be machined from 1/2" wall 6061 tubing. I was messing around with some calculations(and I may be way off...rusty), and with my design, it would take a 44,000 lb rider to flex the frame 1mm between the head tube and bottom bracket once properly heat treated. I think that'll do. The reason I'm using the 1/8 plate on the sides is more for dent resistance. I know it's total overkill and the frame could be a little lighter but I really like the idea of an ebike you could dump at 40mph, get up, dust off, and continue riding and that's what I'm going for.

 

[Malcolm]

I think the choice comes down to your design skills/resources. If you build in steel you can take a sloppy design and usually get away with it, or at least get some warning before it fails. If you try the same with aluminium it's likely to bite you.

To make significant weight savings with aluminium you need to design to minimise fatigue and use stress analysis software to put metal where you need it most. If you're making a one-off frame I would choose chrome moly every time. It's a vast improvement over mild steel and you can get very close to the lightness of aluminium with good design.

 

[etard]

There are sooo many variables as to why a frame breaks. Only an X-Ray machine can see how good a weld is, and I doubt any of the frames we buy are ever put through a scope. Most of the sub $2000 bikes are made in China, $2000-$4000 are built in Taiwan and over that it better be handbuilt by the best as well as stress tested before handing it over. GT's are most likely built in China, even the "high end" models. It's very sad what has happened to this brand, having said that, I just picked one up a month ago and was flabberghasted by the heft of the frame. I suspect it is heavy cuz poorer quality alloy was used. I am not a weight weenie, so I'll just pedal harder right? Even gusseting the hell out of a frame can weaken it in other areas, so for our needs, I think CrMoly is the best way to go.

For motorcycles aluminum will save nearly 100 pounds, so here it makes sense. Although I have seen alot of wrecked bikes that snapped at the headstem weld. Perhaps this "give" is good and absorbs some impact.

Also, alot of guys running the super light 20 pound mt bikes only use those to race on, they know the life of that bike is finite and will probably sell that off within 2 years.

Steel seems to be making a big comeback right now, I think it is all the handbuilt framemakers that are fueling it, along with the fixie crowd and the popularity of singlespeeders. There's a great cover art for Dirt Rag magazine that highights this shift. I'll try to post a pic later.

 

[neptronix]

Steel seems to be making a big comeback right now, I think it is all the handbuilt framemakers that are fueling it, along with the fixie crowd and the popularity of singlespeeders. There's a great cover art for Dirt Rag magazine that highights this shift. I'll try to post a pic later.

I heard that trek has or is coming out with an 'eco' line of bikes that are made out of steel. This is a newer development.

Having owned basically only cromoly treks for 3/4 of my bicycling career, i must say that it is a fantastic material. Never a single issue with the frame; just bombproof all around. Those bikes were as light as many aluminum bikes coming out today. I just wonder why cromoly never really took off. Perhaps the 'look' of wider aluminum tubing is more popular in America, where bigger is better?

I hope steel makes a comeback. I would like to buy a newer mountain bike and be confident that it can handle eBike duty.

 

[johnrobholmes]

I do think the larger look of the alu tubes had something to do with it. Personally, I can't stand riding an aluminum frame if it is rigid. They are just too stiff and they transmit too many vibrations.

 

[The Mighty Volt]

Great thread- the timing couldn't be better.

Here is what the Alloy/Aluminium drops look like on my old Raleigh Chinhook frame

Myself and NovemberSierra have been having an on-off debate/discussion/idea swap about the reliability of these aluminium frames.

There is no, as I see it, trend towards aluminium, at least not directly by ES forumites.....the simple reality is that Aluminium frames are dirt cheap and are everywhere. Myself and NovemberSierra have been doing online searches together for steel frames and the proper one {20", MTB style} seem to be rare as hens teeth, especially in Europe, for a reasonable price.

so I posted the above pictures and you can all have a look and tell me what you think of my drops!!

I understand it is not the thickness of the drops per se, but rather the character of aluminium, it's refusal to snap rather than sway, which makes it such a potentially hazardous choice.

PS the hub motor in question is a 5304, 72v planned, 50-60 Amps.

Cheers.

 

[The Mighty Volt]

I do think the larger look of the alu tubes had something to do with it. Personally, I can't stand riding an aluminum frame if it is rigid. They are just too stiff and they transmit too many vibrations.

I've seen really brilliant cars get crap reviews because they had crap tyres on at the time of testing.

Sounds like a lot of rides are being trashed because folk haven't come around to using oversized tyres.

 

[johnrobholmes]

Big tires or not, I can still feel the high frequency vibrations. It is like a hum that steel just doesn't have. Maybe I have a super sensitive taint :lol:

 

[neptronix]

Are those aluminum shavings i see on your gears? ....
BTW, you still need a torque plate. And that bolt you have on there looks like a total disaster waiting to happen!!!

Those dropouts look thicker than most aluminum dropouts i've seen. I've looked at a LOT of aluminum bikes before i did my eBike build and gave up because i couldn't find anything that sturdy looking.

 

[The Mighty Volt]

Are those aluminum shavings i see on your gears? ....
BTW, you still need a torque plate. And that bolt you have on there looks like a total disaster waiting to happen!!!

Those dropouts look thicker than most aluminum dropouts i've seen. I've looked at a LOT of aluminum bikes before i did my eBike build and gave up because i couldn't find anything that sturdy looking.

No no :lol: those are just sundry pieces of dirt....you have to realize that photo is magnified many times. At high mag an atom of cotton can look like a shard of diamond....

Yeah they are very fat drops and it is pure coincidence I have them, its an early model Raleigh Chinhook frame.

PS the bolt wasn't secured, the pic was just taken with them in that state/condition. It happens to be the bolt which came as standard with the 5304. What's so "disasterous" looking about it?

What sort of torque plates do you recommend?

Thanks.

 

[The Mighty Volt]

Big tires or not, I can still feel the high frequency vibrations. It is like a hum that steel just doesn't have. Maybe I have a super sensitive taint :lol:

No......you are correct. Aluminium is tinny.

 

[etard]

Maybe the current trend toward steel has something to do with the tubes can't get any bigger because of the headstem is only 1 1/8", that is the limiting factor to the size of the tube if you think about it. I think that is why the manufacturers have come up with 1 1/2" head tubes now. I mean, yeah it's stronger, but has anybody ever snapped their steer tube? I do think there is some valor to the thin walled big tubing though, seems like I just read somewhere that for two tubes of the same material and weight, one being 55mm the other 50; the 55mm is going to be like 25% stronger. Correct me if I am wrong. Can somebody shed some light on the hydroformed aluminum tubing? I'm at a loss as to the advantages.

Here's that steel pic, I would love for frames to be made in the USA again, steel is the answer:

 

[neptronix]

on second glance, those bolts look a bit larger than i thought. I see the washer now too. A wider washer would be preferable, though. It also doesn't look like you have a washer on the inside ( just from the pictures )

Here's what my axle looks like. The axle looks wider. The bolt has a washer built into it, so it has a large clamping surface. This came on a kit that is rated at 250w.

Since you have those rear rack mounts like i do, you could design a torque plate that looks like such. eBike kit also sells a 'chinese quality' one that may work for you, but the axle looks quite short for your motor so you'd have to use a thinner nut.

Ok... back on topic!! :lol:

 

[The Mighty Volt]

Cheers neptronix!

Yes...back on topic!

Personally, I prefer steel.

Weight is a dead issue- you are going to be adding a heavy motor and heavy batteries.

If you are worried about 12lbs then just hit the gym.

Steel, for me, all day.

It can be welded to, painted easily, is better under stress. It's a no brainer.

 

[Hillhater]

I think that 6061 is the hardest aluminum that may be welded..

There are many 7xxx series alloy frame bikes (welded) to be seen
I myself ride a "Al Mg Zn 7005 hard tail that has many welds...no issues

...For this kind of duty, i'll take a metal that bends before it snaps / fractures any day.

Most Aluminium bends a long way before it snaps or fractures .. ?
Some steels snap before they bend !!

Its all down to the correct choice of metal for the application, the correct design and good fabrication.
Most all failures have a human root cause.