20 questions about aluminum frames!

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spakbros
Participant
#

okay, not twenty but I just got an aluminum fuji road frame and I am going to attemp building a bike around it but it has a few problems:

1) it is hideous

2) there is a small dent in the top tube

So the questions are:

1) Where do I go to get this powdercoated or painted?

2) More importantly, is this small dent going to make me die if I hit a pothole at 40mph?

thanks all


erok
Keymaster
#

loook reaaaaaly reeaaally close and make sure there are no cracks in it. do you know how the dent got there?


spakbros
Participant
#

no, but it looks like maybe a repair stand crushed it a bit?


mark
Participant
#

dings in aluminum frames are much scarier than dings in steel. i don’t know that anyone can really say “oh ya thats fine” with a ding in an aluminum frame, but when you get it powder coated ask the guys there to look for cracks after they clean it.

other than thick there is this place, pulled the info from an older thread: Allegheny Metal Finishing. It is out in Imperial, PA, but they do a great job for a great price. I got my frame media blasted and powdercoated with a color they had in stock for $50; it is about $80 for a color that they order specifically for you.

724-695-3233, ask for Jason Cruise


Hayduke
Participant
#

I don’t think a ding/dent in the top tube is really that big a deal on any metal frame. Aluminum frames have a tendency to fail near the bottom bracket because of all the torque created from pedaling. If there are cracks that’s different.

I’ve been riding and racing my bike with a pretty significant dent in my top tube for well over a year, no problems.


BradQ
Participant
#

I’d take the paint off of the frame and check around that dent. If there are any creases, sharp ridges or lines extending from it the frame is toast. If it was crushed by a repair clamp I’d guess it has some sharp edges to the dent. Official word from any shop worth going to on dented aluminum is going to be rider beware.

Aluminum has a different fatigue and failure mechanism than steel… Where steel tends to fail incrementally, with a crack growing large enough to notice before failure, aluminum is a whole different story. Aluminum tends to fail catastrophically, with little warning. Cracks propagate quickly in aluminum, and can easily go from absolutely tiny to complete tube failure in a blink.

I’d be suspect of riding a significantly dented aluminum frame and would trash any frame that had creases or sharp ridges around a dent.


zflat
Participant
#

This sort of discussion is pretty interesting to me, but in the end, it is up to you to decide how risky it is to ride that bicycle.

We are throwing around a bunch of terms, and a few are mixed up.

Failure for a metal can mean different things. Bending and denting is failure since the metal will not return to its original shape; this kind of failure is called yielding. Breaking completely is also failure, and that is often referred to ultimate failure.

Metals first yield, and then fail ultimately. I would not say that Aluminum fails catastrophically because that would imply it does not yield, but fails ultimately right away. The mere fact that you see any dent or bend in Aluminum shows that it yields prior to ultimate failure, which means that it does not fail catastrophically.

There are also different causes for failure. Brad mentions fatigue, which is one cause for failure. It is likely that an aluminum frame will have a shorter fatigue life than steel based on material properties alone.

One of the characteristics of fatigue failure is described by Brad. There is a small crack that grows over time. An area with a crack has significantly less strength, of course, and it will only get worse over time. Eventually, the crack will grow enough for the area to become so weak that ordinary every day use could cause the frame to break. Since you don’t know when ultimate failure will happen (the straw that broke the camel’s back), it may appear to happen suddenly.

A dent or a crease could mean that there is a crack near the edge and you just can’t see it yet. It may also be a place where a crack is more likely to form.

This isn’t the whole picture, but if you want to learn more: http://materials.open.ac.uk/mem/mem_mf.htm


erok
Keymaster
#

thanks.

I’d jump across Free Ride screaming “No! NO! NO!” like a wildman when i’d see someone putting an aluminum frame in a work stand using the top tube.

I always liked the “but i don’t feel like changing my seat height just to put it in the stand” argument


BradQ
Participant
#

I stand by my statement, aluminum fails catastrophically. Cracks propagate quickly throughout it, going from tiny to complete failure quickly, and can lead to a failure with no warning.

I can talk stress/strain curves and grain structure too, but I’d rather not having left that part of my brain behind a few years ago.

Tons of busted parts here: http://pardo.net/bike/pic/fail-001/000.html


netviln
Participant
#

If done correctly, I dont see any problem using a repair stand on the top tube of an aluminum bike. What if you have a cf seatpost? You wouldnt want to clamp to the seatpost then. Obviously you cannot clam it down as hard as you would steel, but in almost everything I do on a bike, I dont need a ton of force on the frame, just hold the frame off the ground and keep it from wiggling too much.

As for the frame in question, I would say yes have a professional metal worker inspect it. If you are planning on having it painted tho, you would probably be wise to have it media blasted as well. After doing all this, you could have purchased a new frame for the same cost.


spakbros
Participant
#

wow. thanks for all the info everyone, I inspected the frame very closely last night and didn’t see any cracks anywhere. The dent is rounded all over ( no sharp creases) but I am still a little worried about it snapping like a toothpick. Does anyone know of a good place to get a frame around 58 or 59 cm?


netviln
Participant
#

Depends if you want a new or used frame but im actually in the process of getting stuff together to build a cx bike and am planning on getting Nashbar’s cx frame. It is 179, but they have an aluminum road frame for 99 I think.


erok
Keymaster
#

If done correctly, I dont see any problem using a repair stand on the top tube of an aluminum bike.

the qualifier, “if done correctly” is an important one. i’ve seen work stands crush aluminum tubes, esp if there are cable or housing guides on the tube. many people who come into free ride aren’t very experienced, which is fine, but you need to be extra aware of situations like that where a lack of experience can seriously destroy a frame.


netviln
Participant
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Agreed Erok. Or if you are like me, Crush your finger in the stand.


spakbros
Participant
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I’m kinda into that nashbar double butted aluminium frame you speak of. Is it possible to make that into a singlespeed? People talk about needing horizontal dropouts but I can’t figure out why for the life of me.


mark
Participant
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you need horizontal dropouts to move the rear wheel forward or backward to make the chain taut, if you have vertical dropouts you can use one of many fancy methods to have a single speed, but its more money and the extra tech and money ruins the simplicity and cheapness of single speed bikes, the reason that lots of people use them.

thisthread discusses some of these fancy methods: http://bike-pgh.org/bbpress/topic/long-time-rider-first-time-repairman-thinking-flip-flop


netviln
Participant
#

Horizontal dropouts are to adjust the tension on the chain like if you are in between links as far as getting the slack out. If you are building a single speed (as opposed to a fixie) then you can install a tensioner. Another issue is that single speed and fixie hubs are 120mm rather than 130mm of most road hubs, so you would need spacers. Not a big deal, but just one more thing.

Here is Sheldon’s Article on it.

http://www.sheldonbrown.com/singlespeed.html


spakbros
Participant
#

Thanks netviln

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