Bicycle Mechanics - Bending aluminum
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03-14-06, 10:41 AM
I know what the issues of aligning aluminum rear triangles are but I am not aware if certain types of aluminum tubes are more safely cold set than others. What an optomist eh? I am working on a friend's Giant OCR Touring with Avid mechanical discs. Her OLD is 135 and the dropout spacing is 130. The triangle is aligned to within 1mm and the dropouts align precisely. I tried taking out an axle spacer but of course the rotor then rubbed on the caliper. Frame material is 6061 Aluxx Extra Light. I suppose she will just have to keep spreading the frame when inserting the wheel but I wonder if this repeated stressing is not worse than a one-time "fix".
Yup. It will take awhile but eventually there will be a crack somewhere, where the bend is concentrated. One reason why airplanes fall out of the sky.
03-14-06, 11:48 AM
6061 is easy to bend even in the hardened condition 7000 series aluminium is much more critical. The repeated stresses on an aircraft are nothing like installing and removing a wheel.
03-14-06, 11:57 AM
...The repeated stresses on an aircraft are nothing like installing and removing a wheel.
Quantify. Failures modes in both instances are the same - due to fatigue. The higher forces/load/stress involved in aircraft failures only serves to lower the number of needed loading cycles before failure is reached.
03-14-06, 01:03 PM
Failures modes in both instances are the same Changing out a wheel imparts cyclic loads significantly less than flight and pressurization loads on an acft. The time before failure would be so great on the bicycle as to not be worth worrying about.
Yet another of the many reasons that aluminum should be relegated to components and beer cans.
03-15-06, 02:32 AM
Changing out a wheel imparts cyclic loads significantly less than flight and pressurization loads on an acft. The time before failure would be so great on the bicycle as to not be worth worrying about.Yeah, imagine how much force it takes to spread the dropouts apart 2.5mm each.. maybe 3-4 lbs max? Then imagine how much force is on the dropouts when you hop on the bike and ride it over some big bumps at speed... 400-500lbs!!!
03-15-06, 02:47 AM
The main problem might be the reliability of the shifting. The rear hanger will be slightly out of alignment. When you bend steel apart you usually have to follow up with an alignment of the RD hanger. May or maynot be a big deal. Why is the problem suddenly surfacing? Did you go to the new rear wheel? Try it. Don't know if I would really try to "cold set" the rear triangle. Try and find out from someone technical at Giant.
03-15-06, 04:33 AM
It's not installing the wheel that imparts the stresses that cause failure, it's the cyclic loading due to pedaling.
03-15-06, 08:35 AM
Why is the problem suddenly surfacing?
Actually I think the problem existed from the start. She complained how hard it was to insert the wheel but thought it was her ineptitude. Thanks - I will contact Giant.
Practically speaking, if removal of the rear wheel is done for the usual reasons: flats and heavy maintenance, the difference in life span is not going to be much different.
The stress from stretching the rear triangle is relatively small in the scheme of things
but is something Al doesn't like, compared with steel and ti. Al frames are made so that they are very rigid: large diameter tubing, gussets, heavy members for that reason, to avoid metal stressing flex. Flexy areas are replaced by pivots, carbon or steel. I had two frames I switched to 9spd from 5spd, the Ti was easy to spread, the steel required one foot and one hand to do, Al is stiffer than that by design.
03-17-06, 08:00 PM
I have been unable to find an email for Giant. Lots of information but it seems no one wants another email. Anyway, today a LBS told me that when Giant decided to make a touring bike, the only hub available at that time that was disc compatible was one designed for a mountain bike (135 mm) He figures they used this hub for building up a 700C wheel and stuck it in a touring frame with 130 dropouts. Would they do that? Hmmmmm! At any rate I have decided to NOT cold-set. My friend will just have to do a bit of stretching! She may contact the dealer who sold it to her.
03-18-06, 05:00 PM
Well, let me end the thread I started by announcing that I FINALLY figured it out! Seems the LBS, in their attempt to fit a set of fenders and a rear rack around the disc brakes, just muscled the rack (a high quality one with very stiff aluminum tubing) into position on the outside of the stays without bending or modifying it. In a fit of "I can't work around this thing", I removed the entire rack. SPRONG - suddenly the spacing returned to 135. Live and learn eh?
03-19-06, 07:06 AM
Yeah that clears it up. I cannot ever imagine a factory selling a 130 Al frame with a 135 wheel as the stock setup.
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