my frame is cracked, can it be repaired?

[EdwardNY]

I do not know how or when it cracked but I noticed I have had to tighten my shock bolt every ride as it came a bit loose. Today when I was riding I noticed a weird noise and tried flexing the rear tire and noticed play in the shock. Then I noticed a crack on both sides where the shock bolts to the frame.

IT seems to have cracked at a weak point. I do not think much forces get pushed on that crack and that is why I am able to use my bike with the crack.

Is there anyway this can be fixed or is the frame shot? I am looking for any ideas to try and reinforce this, I do not care if I have to scrape away paint or make an ugly fix. I do not think it is a very high stress point so hopefully something can be done.

I figure I will just use the bike if it can't be fixed as if the shock came loose I figure all that would happen is I would bottom out if the shock somehow got dislodged?

 

[biohazardman]

Looks like aluminum and if so although it can be welded it will not be as strong and may crack in the same place like mine did. Aluminum frames are tempered to strengthen them and heat such as welding destroys this. Yes, you can have it welded but will it last ??? I would ride it sensibly and keep and eye on it while I looked for another frame.

 

[Stevil_Knevil]

Bummer. Seems a bad design, looks like it cracked when/where the rear shock unloaded.

Warranty replacement?

 

[EdwardNY]

Yeah, no way I could warranty it.

I was wondering if I could weld something on that whole side to help support it, rather then just welding the little crack. Like building up the whole area by a 1/2 inch or something.

I was just thinking if I used a nord lock washer, do you think that could dig into the aluminum around the bolt and help to keep the bolt tight. The nord lock washer would dig into the bolt and the aluminum and possibly keep the stresses off the crack. It seems that if I tighten down the bolts tight it will hold for a bit but eventually come loose. The bolt is probably coming loose because of the small amount of play the crack causes. The problem I see with this is that the bolt has a rubber washer that goes into that hole and not sure if that is needed as I would have to remove that to use the nord lock washer.



But I was hoping to somehow weld something to the whole side.

 

[Stevil_Knevil]

I was wondering if I could weld something on that whole side to help support it, rather then just welding the little crack. Like building up the whole area by a 1/2 inch or something.

That is a solid plan 'b'.

Can you TIG weld?

 

[Samd]

There is largely three different kinds of aluminium.

until you know what you have, don't take advice. Not everything applies.

you may get lucky though!
http://forums.mtbr.com/bike-frame-discussion/6061vs7005vs7075-375489.html

 

[Stevil_Knevil]

..Bikey stuff tends to be 7000 series..

..but you are right, Samd .. everything never applies :wink:

 

[-dg]

IT seems to have cracked at a weak point. I do not think much forces get pushed on that crack and that is why I am able to use my bike with the crack.

Is there anyway this can be fixed or is the frame shot? I am looking for any ideas to try and reinforce this, I do not care if I have to scrape away paint or make an ugly fix. I do not think it is a very high stress point so hopefully something can be done.

"cracked at a weak point" : -> "not much forces" and "not a very high stress point".

Smells like wishful thinking. If there is little stress there, why would it crack there instead of somewhere with higher stress? Almost by definition the crack will be at a high stress point, because cracks are caused by stress. If it was at a low stress point there would be no crack.

Probably this frame was built the way most of them are: form and weld the aluminum in the soft condition, then heat treat the whole frame to make it strong. If so, then welding will kill the heat treat in the area around the weld, in this case probably weakening the seat tube where the suspension loads are fed in and which is also resisting the bending loads from the riders weight.

 

[dogman dan]

I'd be inclined to make a steel repair plate, then bolt it to the frame through small new holes above and below the crack. Then use a longer bolt to attach the shock. That would give you some more use, but not likely forever.

Buys you some time for looking for the new bike.

 

[mat h physics]

Take it to a professional TIG welder, keep your fingers crossed. Heat treating is not some voodoo magic, it's REPEATABLE.

Some people think aluminum is a strong metal, their false belief leads them to buy into the HOAX that commercial airliners hit the twin tower. COMPLETE BS. Aluminum can not shear structural steel at 500fps, or even 1000fps. Anyone who looks at witness statements of the July 28th 1945, Empire State building strike, knows what really happens when an aircraft strikes a skyscraper. WTC accounts made AFTER 01', don't match the laws of physics.

 

[MadRhino]

Tig welding and heat treatment is the way to repair an ALU frame, if it is worth the repair price of course.
ALU is not structurally solid as steel, but it is much stiffer and lighter, making better MTB frames.
Steel is out of the mountains for a decade, and that was a major improvement.
Yet, cheap ALU frames are not made strong enough, in both the design and the size.
If you are gonna go cheap, go for steel. A good ALU frame is always expansive.

 

[www.recumbents.com]

Stress crack because the frame was not designed for a 20lb rear wheel. Take it to a welder and have them build up over the crack with a thick bead of aluminum. Both sides!

 

[el_walto]

Buy some dp420 glue, glue some extra metal on it, with a slightly longer bolt?

http://www.youtube.com/watch?v=3NWzODqhSbw

 

[EdwardNY]

Buy some dp420 glue, glue some extra metal on it, with a slightly longer bolt?

http://www.youtube.com/watch?v=3NWzODqhSbw

Interesting, almost like mini doc torque arms to go around the bolt hole and expoxied to the aluminum. I guess that should work?

 

[dogman dan]

Yep. I'd glue and bolt them with small grade 8 allen bolts. Add some steel, then screw it, glue it and tattoo it.

Then look for another bike meanwhile.

 

[el_walto]

Buy some dp420 glue, glue some extra metal on it, with a slightly longer bolt?

http://www.youtube.com/watch?v=3NWzODqhSbw

Interesting, almost like mini doc torque arms to go around the bolt hole and expoxied to the aluminum. I guess that should work?

Maybe you can glue a thick washer on each side. Not really sure the config.

 

[Chalo]

You broke your frame; too bad. My experience with getting aluminum frames welded to repair them has led to quick successive failures, even when reinforced. At some point you have to attach to the existing structure, and the weld weakens it at that point.

While mat h physics is technically correct that you can get your frame heat treated after welding, in fact that process will cost you more than a secondhand replacement frame, and will require you to remove the paint first.

My advice is to get a proven rigid diamond frame with room enough for big fat tires. That's cheaper, lighter, stronger, more durable, and easier to maintain.

 

[John in CR]

My downtube was cracked near the shock mount. My permanent and unbreakable solution was to fix it with fiberglass epoxy and steel. I filled the lower section of the downtube most of the way up with a solid plug of fiberglass and epoxy with the plug ending at about a 45° to the plug so it wouldn't create a stress point where the AL tube changed from hollow to the solid plug. I made a new shock mount of carbon steel and shaped it to match the surface of the bike tube. I drilled 2 large bolt holes in the steel with matching holes through the downtube for mounting the new multi-hole shock mount. Then I slathered the mating surfaces and the hard steel bolts with metalized epoxy, and attached it to the bike with some hard rubber washers between the thin AL and the steel washers on the bottom side of the downtube.

I made the holes through the plug somewhat off parallel, so I doubt the nuts even needed, but the bolt ends make for a super strong mount for anything I might want to attached under the downtube. Before filling with epoxy/fiberglass I drilled several small holes in the downtube below the crack to make sure that the tube was well stuffed with fiberglass, and later that the epoxy flowed through the fiberglass I stuffed in the tube as well as give the air a way out as pressure from an air pump and gravity forced the liquid epoxy down through the fiberglass. As the epoxy reached a hole I taped it closed, and when it reached the bottom hole I turned off the pump. The epoxy for the plug was the slowest set type I could find down here, but my test cup put off plenty of heat, so I kept cold water running on the outside of the tube to keep it cool and slow the reaction as much as possible. It was a one shot deal and even one hour set epoxy was much faster in such a thick piece, so my helper and I rehearsed the process before mixing the epoxy.

A glass plug wouldn't reinforce the headset much at all, otherwise I would have filled the entire frame, but I don't go off of big drops and I avoid hitting big potholes, so the headset should be fine though I do inspect it regularly.

Chalo may think I just have a janky emoto, but other than the normal strength at the headset, my SuperV has probably the strongest bicycle frame on the planet, which goes pretty well with the fastest hubmotor.

John

 

[Chalo]

[...] other than the normal strength at the headset, my SuperV has probably the strongest bicycle frame on the planet [...]

That must be why Cannondale continued to manufacture that distinctive, visually branded, easy to produce design.

Oh wait, they didn't.

Cannondale did a lot of good engineering and design over the years, but like Trek's Y-frame, the Delta V and Super V were doomed by their mediocrity before they even got started. They might work adequately, but they don't do anything better than a triangle and they have to weigh more.

Here's another historical anomaly you might like. It's made out of plastic. It also failed to take the world by storm.

 

[John in CR]

Chalo,

LOL! I guess you missed that what was a decently strong frame made of 3" diameter aluminum now has a solid core of epoxy/fiberglass throughout it's stressed areas except the headset making any frame you have a wet noodle in comparison. I made the mod to repair and reinforce an otherwise impossible to repair crack in the AL frame.

 

[bowlofsalad]

I recently had an aluminum bike rack crack after a mere 500 miles in service. I had less than 30 pounds on it, it was rated for 110 pounds. I have studied metals online and in person. One detail many don't understand is the idea that pound for pound, aluminum isn't really any stronger than steel. While each metal has tremendously different potential applications in a lot of ways relating to ideas like corrosion resistance and conductivity, one critical difference concerning aluminum is a lack of fatigue limit. The summary is that instead of continuing to take a beating and, at worst, bending or tearing (tearing isn't cracking) such as steel will, instead aluminum will crack. The way my imagination plays it out is like comparing glass to cardboard, both can and are strong and useful in their own ways, both can hold weight, but cardboard will bend where glass will shatter. This cracking in aluminum can happen much sooner than tearing would in steel, this translates to longer life in steel parts in some applications.

Part of the trouble with repairing anything metal is how the repair process effects the regions around it, this relates to temper and concepts surrounding vanity like paint and so on. I have high doubts you'll (anyone) be able to perfectly build up an area that needs welding in a useful way, worst case scenario is it simply breaks again and you waste a lot of time and money. Most bicycles aren't designed with ebike use in mind. The speeds, weight, and placement of said weight often puts a lot of unimagined stress upon a bike. With these things in mind, my advice is to avoid aluminum when things like baring the weight of your body for plenty of applications with few exceptions. Look for a steel or cromoly frame.

 

[Chalo]

LOL! I guess you missed that what was a decently strong frame made of 3" diameter aluminum now has a solid core of epoxy/fiberglass throughout it's stressed areas except the headset making any frame you have a wet noodle in comparison. I made the mod to repair and reinforce an otherwise impossible to repair crack in the AL frame.

Yes, I did miss that. Looks like I have a little catching up to do.

One of my favorite frames was an '80s Cannondale I bought new and rode to death. It cracked in a few places, and I had it welded a few times, but the repairs never held out very long. I probably should have lashed them up with glass or carbon/Kevlar tape instead.

 

[dogman dan]

Sounds like that was a bombproof repair. Unfortunately in this case, he's got a fairly small surface area to work with. I still think some epoxied and screwed on steel plates will last for awhile.

Though it could be a permanent fix, I wouldn't want to have to fret about it. So I'd ride it while still hunting down that good deal on a new frame or new bike.

 

[John in CR]

It depends on if he can access a significant opening in that tube. If so then filling it with a solid plug of fiberglass is easy, as is bolting and glueing on a new steel shock mount after cutting off the 2 AL tabs one of which is broken.

Once I found the crack on mine, which had cracked before and welded by the previous owner who hid the new crack in the repair using paint, I knew with my weight multiplied by a longer swingarm and a 6" rearward seat extension that the thing was going to just crumple like a beer can. The solid plug not only is a permanent fix for the damaged area, but now I can drill holes in the plugged part of the frame and mount anything. Other than a few small hidden holes and the steel shock mount, the frame looks stock.

John

 

[EdwardNY]

Looks like I did a temporary fix (probably will be a permanent fix) using Nord Lock washers. I figured these would work as the nord lock washer digs into bolt and also digs into the bike frame holding everything in place.
It did take me a few re-torques after my first few rides for both of them to be 100% solid. The first few rides they did lose a small amount of tension probably because they had to wear away the bike paint to be able to grip into the aluminum frame. This seems the case elsewhere on the bike where I used Nord Lock washers.

Today I did two hard off-road rides and they didn't slip at all, still as tight as I left them. These nord lock washers really work wonders and also helped fix my axle from slipping down a small amount in the dropouts.

I think the nord lock washers should be enough to hold it as all the forces of the shock are pressed in the opposite direction of the crack. I can not see how any forces ever get pushed towards the crack or the rear of the bike as the air shock is always trying to push out.

What may have caused the crack was I let the suspension pivot bolt get loose which added a bit of play to the swingarm. This may have put side forces into shock causing it to break the frame at that point. When it was loose I grabbed the rear wheel and put side forces on it and this force was enough to move the shock cylinder side to side.