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"Cold Forging" (aka Bending) an Alloy Rear Carrier to Fit?

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"Cold Forging" (aka Bending) an Alloy Rear Carrier to Fit?

Old 03-15-13, 04:39 PM
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PJMC
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"Cold Forging" (aka Bending) an Alloy Rear Carrier to Fit?

Sheldon tells us that you can't "cold forge" (aka bend) an alloy rear triangle, which will have 100 lbs on it, to fit a larger hub. So what about "cold forging" the struts of alloy rear racks (that will have 50 lbs on them) to fit onto that alloy rear triangle?

The outside eyelets measurement on my standard 135mm O.L.D. rear dropouts is about 145mm.

I'm finding that almost no racks specify the attachment point width. I just got an Avenir "Universal" (he utters sneeringly) rack that is 124mm between the attachment points.

This Planet Bike model says "spacing between lower attachments is 5 inches" (= 127mm):

http://ecom1.planetbike.com/4001.html

We hear about "cheap alloy racks" breaking all the time -- and this could be deadly in the wrong circumstances. But is it the alloy, or the alloy that's been riddled with micro-fractures by attempted "cold forging"?

I know yours hasn't broken -- but your jacket and iPod are not all that heavy.
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Old 03-15-13, 04:40 PM
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Rack under load:

http://news.bbcimg.co.uk/media/image...kenyareu10.jpg
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Old 03-15-13, 04:51 PM
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There is a bit of a difference between a rear triangle, which is rigid (welded and held together by the rear hub, and a rack which has relatively flexible legs. (sure someone can give the engineering terminology for this)

Being realistic, there isn't much need to ever need to bend a alloy rear triangle, as most will be 130mm, which has been the standard for road hubs for many years if a road bike, and MTB's are a constant 135mm (any which will take a rack), and for a rack, unless you are taking it off every day, it is only being 'bent' once in it's life.

From personal experience with alloy racks, have found that they will bend a bit, but that's when carrying max or over weight loads repeatedly and they will bend a lot when you are hit by a car, writing them off, even after being hit by a car, it didn't break, just was totally bent out of shape.
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Old 03-15-13, 04:59 PM
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Hi,

Just opening up the bottom of that alloy rack by pulling
apart the stays at the bottom is not a good idea.
Carefully bending the stays out though I'm sure would
work, in fact the front looks like it would be alright, its
not stressing the joint at the rear that needs avoiding.

rgds, sreten.
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Old 03-15-13, 05:01 PM
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bending the rack for installation is normal practice, you'll be fine.
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Old 03-15-13, 05:26 PM
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You have your terms confused. You are talking about cold setting. Cold forging is a completely different thing. In other words, cold forging is definitely not aka bending. Google it.
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Old 03-15-13, 07:28 PM
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Originally Posted by xenologer View Post
bending the rack for installation is normal practice, you'll be fine.
OK so how do I actually bend this:

http://ecx.images-amazon.com/images/I/41msIufGzjL.jpg

It has to expand by 21mm, leaving the attachment points at a serious angle vs the bike eyelets.

The rack is tubing almost all the way down to the attachment point. There's only 6mm of flattened out tube just above the rack attachment points, and I'm seeing that snapping if I try to bend it in a vice.
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Old 03-15-13, 07:28 PM
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Originally Posted by Grand Bois View Post
You have your terms confused. You are talking about cold setting.
Thank you.
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Old 03-15-13, 09:16 PM
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Originally Posted by xenologer View Post
bending the rack for installation is normal practice, you'll be fine.
+1
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Old 03-15-13, 10:44 PM
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Yes you can bend it 21mm but try to also bend the lower stays of the rack to be flush with the mounting eyelets. Do not bend at the welds as this will probably end up with them breaking eventually. Try to isolate the lower stays from the welds while bending them.
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Old 03-15-13, 11:58 PM
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Originally Posted by Crankycrank View Post
Yes you can bend it 21mm but try to also bend the lower stays of the rack to be flush with the mounting eyelets. Do not bend at the welds as this will probably end up with them breaking eventually. Try to isolate the lower stays from the welds while bending them.
How do I bend a hollow aluminum tube without collapsing the tubing?

Also I've already flexed the struts out several times testing, which sounds like I may have already weakened the welds where they attach to the rack top.

This all seems extremely perilous for something, that if if breaks one day from me having weakened it, can go into the spokes and throw me in front of a truck. Everyone's saying it's "normal" to bend them. But again we hear about alloy racks breaking a lot.

It certainly seems like more of a skilled engineering task than me, a cousin of the average Idiot Consumer, should be entrusted with
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Old 03-16-13, 12:30 AM
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Its not forging .. its just bending. Forges in industrial settings slam dies together with hundreds
of tonnes of force.
How do I bend a hollow aluminum tube without collapsing the tubing?
mandrel benders have a plug filling the inside of the tube, to support the maintaining the round shape
as its pulled around the bending dies and rollers.

to bend thin metals like making brass instruments the tube is filled with pitch and sand
which melts under heat to remove.

Last edited by fietsbob; 03-16-13 at 12:43 AM.
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Old 03-16-13, 08:04 AM
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I believe that you are overthinking this. Just spring the rack legs open and bolt them to the dropouts. The non-parallelism induced by pulling them open will be undetectable over the distance the legs run along the dropouts; they are probably not extremely square as fabricated. Use flat washers against the rack legs and lockwashers under the bolt heads and torque the bolts adequately, you can add some blue Loctite to the threads if it would make you feel better. Remember this is a bike not a jet engine.

"But again we hear about alloy racks breaking a lot." Really? I haven't, more common is the bolts coming loose. Torque them right and check them regularly and you should be fine.

Last edited by dsbrantjr; 03-16-13 at 08:24 AM.
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Old 03-16-13, 08:52 AM
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Originally Posted by dsbrantjr View Post
I believe that you are overthinking this. Just spring the rack legs open and bolt them to the dropouts. The non-parallelism induced by pulling them open will be undetectable over the distance the legs run along the dropouts; they are probably not extremely square as fabricated. Use flat washers against the rack legs and lockwashers under the bolt heads and torque the bolts adequately, you can add some blue Loctite to the threads if it would make you feel better. Remember this is a bike not a jet engine.

"But again we hear about alloy racks breaking a lot." Really? I haven't, more common is the bolts coming loose. Torque them right and check them regularly and you should be fine.
This ........also, don't carry sofas on your bike.

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Old 03-17-13, 12:59 AM
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Originally Posted by 3alarmer View Post
This ........also, don't carry sofas on your bike.
Then where are my pedicab customers supposed to sit?? Think it through before you post please.
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Old 03-17-13, 01:23 AM
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Originally Posted by dsbrantjr View Post
I believe that you are overthinking this.
Oh definitely! But the problem is the thoughts make sense, notorious aluminum metal fatigue and such. Micro-fractures lurking until catastrophic sudden failure!

Originally Posted by dsbrantjr View Post
Just spring the rack legs open and bolt them to the dropouts.
OK, except it's not me above saying:
Originally Posted by Crankycrank View Post
Do not bend at the welds as this will probably end up with them breaking eventually. Try to isolate the lower stays from the welds while bending them.
And I still haven't heard how to bend hollow tubes.

Originally Posted by dsbrantjr View Post
"But again we hear about alloy racks breaking a lot." Really? I haven't
Random interwebs selection:
http://www.cycling-holidays-france.com/cheaptou.htm
"As for the rack itself avoid cheap alloy ones like the plague. They break and can't be repaired."

Originally Posted by dsbrantjr View Post
more common is the bolts coming loose.
That would be bolts coming loose... catastrophically... when the threads strip from the ridiculous twisting pressure of the rack supports crammed on at an angle.

It's really not good mechanics practice in general to be forcing something together crooked by means of tightening a bolt. That's just inherently putting strain on the bolt that shouldn't be there.


Originally Posted by dsbrantjr View Post
Remember this is a bike not a jet engine.
Meaning this case of notorious aluminum structural failure will fling me in front of a truck instead of dropping me 10,000 feet from the sky.
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Old 03-17-13, 04:24 AM
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ok.... you need to stop worrying and just do it
here's my advice from installing probably ?over 50? lost count? racks
If you think of something I didn't mention; ignore it; it's not important, just you being a hypochondriac

to bend the legs, grab one in each hand around where the tubes converge
pretend its thanksgiving dinner, make a wish and pull

for tubing of this thickness, you dont need to worry about collapsing anything

dont bend them back and fourth trying to get a perfect width; as long as its withing the length of the bolts you're using it's fine

to make the tips parralel again, use a vice; set the jaws to be slightly loose -you don't really need to clamp the tubes
dont freak out if the paint cracks on the outside of the bend; it's just the paint, you can even sand it off to verify that it's not fracturing the tubing if you're paranoid

upper support stays are generally steel, just bolt them loosely to the rack initially so you can angle them to the frame braze-ons; bend them by hand; dont worry about the tips onthesebeing perfectly flush -they will be once bolted down

use threadlocker if you're worried about losing bolts in the future

any bolts used should have a minimum 4mm allen keyway (5mm if you can); sometimes the rack comes with low profile bolts with a 3mm keyway (for aesthetics, i guess), toss them, 3mm head strips too easily
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Old 03-17-13, 08:49 AM
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I'm only jumping in to encourage you to stop over thinking this and bend the tubes. You will be surprised how resilient aluminum tubing really is once you get some time and experience behind you.

Remember, in today's world we often only get to hear and read about the relatively few catastrophic failures. Rarely do we find out about things NOT going wrong.

Also, don't be afraid of breaking something. There is much to learn from failure if you are willing to understand it. Academic knowledge will only get you so far. Get your hands dirty.
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Old 03-17-13, 11:22 AM
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Pedicab passengers on a conventional Bike Rear rack, HA !



Definitely obsessing, for no purpose, but entertainment.
thanks..

Last edited by fietsbob; 03-17-13 at 11:31 AM.
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Old 03-18-13, 10:37 AM
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I have put quite a few racks on tandems with 145mm OLD without issue. Just hold the legs about 6 inches from the bottom and pull them apart a bit. If the OLD is more than 145mm, such as for a Santana tandem, then plan on a bit more thoughful bending session as you will need to spread it more at the top and then bend it back in towards the bottom.

Generally would recommend getting a bit better rack that the one you have now. Of course the better quality of the rack, the more difficult to bend it, generally speaking. Look on Amazon and ask questions about a few that appeal to your needs at that time...
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Old 03-19-13, 06:04 AM
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Originally Posted by PJMC View Post
This all seems extremely perilous for something, that if if breaks one day from me having weakened it, can go into the spokes and throw me in front of a truck.
With thoughts like that infusing your brain as you ride, you should definitely chuck the aluminum rack and buy a Tubus titanium model.
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Old 03-19-13, 06:35 AM
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Originally Posted by PJMC View Post
How do I bend a hollow aluminum tube without collapsing the tubing?
You are way over thinking this. If you are using the rack you linked to, it doesn't use hollow aluminum tubes. It uses solid aluminum rods for the construction. The Planet Bike website states that the rack uses "6061 tubular aluminum rod". The term "tubular" is incorrectly used, however. It should say "cylindrical" rod because you can have a rod or a tube but you can have both. I've looked at these racks in the past and they are constructed of aluminum rods not tubes. You can bend it...a little...to make it fit.

These kinds of racks are very durable. I've had several of the same type in the past and have yet to break one. I used the OEM rack on my Miyata 610 for 20 years without issue and it carried a load on nearly every ride I ever did on it. On the other hand, if you are concerned about the durability of the Planet Bike rack, buy a Tubus steel rack. You won't have any issue with it at all.
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Old 03-19-13, 07:23 PM
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nope

alloy racks don't forge, they just crack, or severly weaken when bent.

shims are used to pick up slack, if too tight a fit, try another rack.
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Old 03-19-13, 07:28 PM
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21 mm is still less than 1/2" per side. I bet you can bend the legs out that much without even permanently setting the rack
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Old 03-19-13, 07:32 PM
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Originally Posted by musicgalaxyman View Post
alloy racks don't forge, they just crack, or severly weaken when bent.

shims are used to pick up slack, if too tight a fit, try another rack.
You're not helping.

(bend the rack)
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