Frame: Steel vs. Aluminum?

[BarelyAWake]

That "80% strength loss" quote is most likely referring to a thick material multiple pass or TIG w/o filler - ie an absolute worst case scenario & not applicable.

Look into using 7005 instead of 6061 as it does not require heat treating & can be naturally aged. It's often used in bike frames & where heat treatment cannot be done after welding. Almost all aluminum bicycles are are designed utilizing a somewhat heavier gauge than the minimum possible, the joint design usually includes gusseting w/this in mind.

http://www.sapagroup.com/pages/493467/Answer Book/11_Welding.pdf
http://forums.mtbr.com/frame-building/aluminium-6061-7005-a-607488.html

If you don't like aluminum, don't use it - but it is commonplace regardless & readily utilized in bicycle manufacturing & fabrication, both heat treated and non-heat treated, artificially or naturally aged, or simply compensating for the material's characteristics.

It all boils down to common sense, if you start with an overbuilt platform (such as a mountain bike) of even average acceptable quality (wall thickness & factory welds) and are familiar with basic construction & engineering concepts - welding/modifying any material is simply a matter of considering "acceptable risk" and compensating as necessary...

...yet if you are not familiar with such basic construction & engineering concepts and/or utilize only the lowest quality/most inexpensive platforms to modify in any way (including simply motorizing something not designed for such) - no material is "safe" and it all entails some amount of risk. If you have even some fabrication & modification experience, using aluminum has no greater risk then steel - yet if you have little to no experience in fabrication and welding, the material alone won't make any difference.

Again, it's not so much about the material used - it's the design that material is used in that needs be considered, the care & quality in which it's constructed... and that's so variable with our hobby, there's no clear cut rules, no easy answers - for every post warning against aluminum there's an equal or greater number of reported steel frame failures. Every motorized bicycle on this forum has been modified well beyond it's initial operating parameters, every one a risk to some degree - simply focusing on the material used without considering it's design is the fundamental mistake here, most focusing on price and/or aesthetics alone.

 

[Ibedayank]

For what stresses we put these bikes through is there really such a thing as a factory produced frame that is over built? How many frames on this site have you seen hacked up to make something fit that probably should have not been used in the first place. You really think that some noob with a 110 volt welder that can barely weld mild steel will be able to correctly weld an aluminum frame? Come on back it up with facts not your opinion

a HAZ zone is a HAZ zone it does not matter the thickness
only diff between think and thin is the thicker it is the more HEAT you have to put in it and the slower it will cool down

 

[BarelyAWake]

...You really think that some noob with a 110 volt welder that can barely weld mild steel will be able to correctly weld an aluminum frame? Come on back it up with facts not your opinion

I'd not trust that "110 volt welder that can barely weld mild steel" regardless - whereas if you're even somewhat competent & w/a quality machine the transition to aluminum fabrication requires only a basic understanding of the materials involved.

"...yet if you are not familiar with such basic construction & engineering concepts and/or utilize only the lowest quality/most inexpensive platforms to modify in any way (including simply motorizing something not designed for such) - no material is "safe" and it all entails some amount of risk."

The facts both for & against are referenced via the included links and by common experience - the aluminum products & fabrications available everywhere.

 

[Saddletramp1200]

How you ride makes a difference also. I rip hard on my MTB but take it easy on my Cruiser.

 

[happyvalley]

The manufacture of bicycles with aluminum tubing did not come about because aluminum is a stronger alloy than steel, it was because it is a lighter one. The problem with aluminum is it suffers stress cycle fatigue.

This was a mid-range road bike from 8 years ago, about $2500, made with 7005 aluminum tubing that I helped my son build up for him racing the regional club circuit. The pics were after a couple seasons, perhaps 4000 miles or so.




He's a strong rider, rode it hard and did well with it, but it just became fatigued and developed cracks. He's had a lot better luck with carbon tubes as had the road bike industry in general.

Incidently, most of the tubing makers, Easton and Columbus among them, advise 7xxx series tubing should be heat treated after welding.

To quote Columbus:

The alloy 7000 used by Columbus for the tube sets is defined as self-tempering: it is air-hardened. This means that in the areas overheated by the welding the supersaturated structure is restored, which, owing to the natural aging (which takes place at ambient temperature), allows for a 75% recovery of the initial properties after a lapse of time of about 3 weeks. Nevertheless, Columbus advises to carry out a precipitation hardening treatment in the oven in order to give the structure a bigger homogeneity of the precipitates, with consequent improvement of the fatigue behavior of the frame.

 

[BarelyAWake]

...not only lighter, but stiffer as well, which also has advantages/disadvantages.

A cracked head tube & seat post clamp are... odd choices to exemplify "stress cycle fatigue" as I would think those would be more directly related to crash damage (head), potential maladjustment (over tightening/crush damage), even poor design (post clamp) *shrug*

In any case, it's well known "road bikes" embrace minimalist construction methods - w/failure not uncommon in the slightest, regardless of the material used - the primary concern is saving every last ounce, even sacrificing integrity to do so. That frame's tubing could be even 1/2 the wall thickness of an aluminum mountain bike, the "integrated" seat post clamp (just tabs welded on instead of an actual clamp) another example of a sacrifice made to conserve weight.

My LBS often has broken road bike frames of all materials & price ranges, it seems w/road bikes the more you pay the less you get.... even a lot more for even a little less lol

Conversely, here's a typical example of a inexpensive ($200) non-heat treated aluminum mountain bike, one that's seen well over 10,000 miles of "high speed" motorized use (two stroke) & abuse with all-season, all-terrain riding & the occasional crash such entails;





While every other component is worn, corroded (rusted steel parts) or just plain tired from such constant utilization - the frame itself shows no signs of damage, fatigue or cracking & has never needed any repair whatsoever. If it wasn't cheaper to just buy another - I'd happily replace/repair it's assorted components (wheelset, derailleurs, cables, brakes, shifters, etc.) & ride it for another 10,000 miles.

Here's another pic of mountain bike aluminum frame construction just because I think it a wonderful example of material consideration & stress diffusion (an old Haro of mine, prolly going to be an ebike ...someday);




edit: I'm not advocating a cheap bike over an expensive one BTW, rather pointing out the fact road bikes aren't a good example of material integrity (or lack thereof) due to their design perimeters (weight conservation), nor am I even trying to say aluminum is "better or worse" than another material - again, it's all about the design used w/a material & it's application. I'm sure there's other "examples" of broken/cracked aluminum bikes, then again there's plenty of examples of bent/broken/fractured & just plain busticated steel/chromoly/carbon/whatever bikes at every bike shop I've ever been to heh

I think the most common reported motorized bicycle frame failures just happen to be steel - both the Cranbrook & Micargi (prolly b'cause of their commonality/price), but I'd still not blame or recommend against that material itself, I'd suggest poor quality, design & construction.

 

[thegnu]

Yeah keep it a lil simple for the first build , as far as aluminum frames I personally will not use them its a soft metal a brittle metal , those two things in my opinion dont make it ideal for a motorized bike the motor adds significantly different loads an stresses to the frame . just my opinion.
Gary

 

[happyvalley]

A cracked head tube & seat post clamp are... odd choices to exemplify "stress cycle fatigue" as I would think those would be more directly related to crash damage (head), potential maladjustment (over tightening/crush damage), even poor design (post clamp) *shrug*

Nope, no crash nor maladjustment or any other errant assumption, simply as stated, the aluminum frame began to weaken and developed cracks, as shown, over time and under fairly strenuous but not uncommon road bicycle racing use. I try to post empirical info of what I've seen in actual experience and not the too common internet blather that sometimes poses as expertise. Apples and apples, in this case for example aluminum road bike vs steel road. We have and have had dozens of steel road bikes and never, once, had a steel frame crack.

As to the the use of weld-gusset-weld to beef up aluminum tube joinery, I think the inherent weaknesses in the material speak for themselves by the necessary reinforcement and compensation needed. Frankly I see no wonder in oversize welds and added plate, in fact I find it downright ascetically displeasing, but that's just me, lol. Compare that to the elegant utility of a lugged steel Bridgestone MB1 currently residing in my shop, something no one has yet been able to do with aluminum.

 

[BarelyAWake]

...as I have never, once, had any of my aluminum mountain frames crack & I maintain that ultra-light road bikes are a poor choice to exemplify or illustrate the potential in motorized frame use as again, sacrifices have been made for weight considerations - I'd hesitate recommending or using any material "racing road bike" as a motorization candidate for that reason.

It's interesting you've chosen to deride the "weld-gusset-weld to beef up aluminum tube joinery" for the "necessary reinforcement and compensation" needed as when aesthetic considerations are set aside - do not lugs & gusseted plates serve the same purpose, to strengthen the joinery? This need is seen as a liability w/aluminum but not with steel?

I agree lugged steel is often pleasing to the eye, but the purpose is exactly the same as the oversize welds and added plate found on reinforced aluminum... and I find that equally as pleasing when it's well designed & executed, but that's just me, lol.


Again, point being - I maintain that making generalizations for or against a material without consideration of the design used with that material is erroneous. Every material ever used in bicycle frame construction has examples of failure, every material has it's own unique benefits & liabilities - to present examples while disregarding design to substantiate a bias is a misrepresentation of the potential of that material.

"nor am I even trying to say aluminum is "better or worse" than another material - again, it's all about the design used w/a material & it's application"

For another example, it's well known that stretch choppers can suffer flex & integrity issues - yet would one blame the material for these shortcomings or the design?

 

[happyvalley]

.I'd hesitate recommending or using any material "racing road bike" as a motorization candidate for that reason.

Nor would or do I and thought that was apparent above but if not, let me disabuse of the notion. As stated, I was comparing road frames, aluminum to steel, for their intended purpose.

It's interesting you've chosen to deride the "weld-gusset-weld to beef up aluminum tube joinery" for the "necessary reinforcement and compensation" needed as when aesthetic considerations are set aside - do not lugs & gusseted plates serve the same purpose, to strengthen the joinery? This need is seen as a liability w/aluminum but not with steel?

Deride? heh, your word and too harsh a term, I don't care that much to de ride but I could also say I don't care to ride them either, lol.

BTW, I was talking about lugs, as in lugged frame bikes so where the mention of "lugs and gusseted plates" in the same sentence comes from I'm not sure. Lugs are fitment, most often hand assembled and brazed, they are the joinery, not slapped on after the tubes are joined to strengthen their union.

 

[Ibedayank]

Nor would or do I and thought that was apparent above but if not, let me disabuse of the notion. As stated, I was comparing road frames, aluminum to steel, for their intended purpose.



Deride? heh, your word and too harsh a term, I don't care that much to de ride but I could also say I don't care to ride them either, lol.

BTW, I was talking about lugs, as in lugged frame bikes so where the mention of "lugs and gusseted plates" in the same sentence comes from I'm not sure. Lugs are fitment, most often hand assembled and brazed, they are the joinery, not slapped on after the tubes are joined to strengthen their union.

a lugged frame can not be welded only brazed or silver soldered.
Silver solder have a strength of 70,000 psi wich as it happens to have more strenght then 6000 series arcwelding rods wich have 60,000 psi

now ask your self what is stronger a welded buttjoint frame in steel or aluminum or a lugged silver soldered frame

 

[Goat Herder]

To quote Columbus:

I just wanted to note that gussets would not have prevented these cracks where they occurred? Least that's my opinion of where these cracks are presently at.. quoted from above..

I did crack the trailing arms on this bike twice. Never ramped it or down hill-ed it. Never had a motor on all pure peddled. I ran the rear shock stiff enough not to bob when peddling and put triple tree forks on the front with BMX bars. I put about 15,000 miles on this bike replaced the rear triangle once and patched it in the end.

It had a custom tight rear ''road'' cassette with a wider bottom crank bracket. 180 cranks and 52 tooth front cog. this was the quickest street touring, geared full sus bike I ever owned. Sure was bummed when I had too retire it. It has been near impossible to reduplicate this bike. The leverage every where for peddling was perfect. I feel I never did.. Did some sixty mile bikes rides on many occasions. It still sits in my back yard. This one is a sample pict.

This bike weighed nothing too.. man it was light.

If it was made of steel might still be on it today? * pun


I know that when that first Morini motor of Easy Riders was put on a aluminum frame they gave it a good collage try and it was not enough? They did beat the bujeebers out of it tho off roading it. Must be why my Steel bike from them works so well!. lol http://motorbicycling.com/showthread.php?t=243&page=6

 

[David eulen]

Getting ready to throw down some dollars for a Felt. Common sense says that the steel frame is by far the more durable, however I see alot of you guys using aluminum frames. Why is that? Being a super noobie, I want to buy the most durable frame, engine, etc. Seeing as how I'm aiming for max power (considering a 9HP Morini), low maintenance. So, aluminum or steel?

I'd go for steel, it will flex, aluminium will crack from the vibration.
Ozy Dave

 

[LaLongueCarabine]

the diameter of the tube is not as important as the thickness of the walls and the strength of the welds.

and pre-weld and post-weld heat treatment.

 

[happycheapskate]

I'm using an aluminum frame because I got the bike for $50, and I'm using a Dax Friction drive with a Tanaka motor (pretty smooth). If I was going to put 9hp on a bicycle, I would get a frame built of thick steel, something seriously overbuilt and gussetted, because I'd be using the pedals mostly for a semblance of legality or for wheeling it through foot traffic. hahaha.

http://www.650motorcycles.com/images/BSC1.jpg

Getting ready to throw down some dollars for a Felt. Common sense says that the steel frame is by far the more durable, however I see alot of you guys using aluminum frames. Why is that? Being a super noobie, I want to buy the most durable frame, engine, etc. Seeing as how I'm aiming for max power (considering a 9HP Morini), low maintenance. So, aluminum or steel?