From: Pat 22/12/99 13:30:32 Subject: Alumium gets saggy? post id: 19558 Does alumium get saggy and soft as it ages. Like a lot of bike's are now made of various types of alumium, but do they really "age" a lot worse than the steel frames? And if anyone out there knows aything about bicycle steel etc, i got a few questions about those too lol *8^) cheers Pat From: Mike V 22/12/99 13:44:45 Subject: re: Alumium gets saggy? post id: 19567 I would have thought the bicycle frame would loose strength due to stress through use rather than age....? From: Trevor Wilson (Avatar) 22/12/99 14:01:27 Subject: re: Alumium gets saggy? post id: 19573 Steel and Titanium are metals with fatigue limits. If they are stressed beyond a certain, they will break. Below this point, they will remainintact. Aluminium has no fatigue limit, so each and every stress, causes wear and weakening and eventual failure. For a mountain bike, I would choose a chrome-moly frame, for it's longer life span (or Titanium, if I could afford it). Composite materials are changing perceptions, almost daily, though. Not enough time has elapsed (AFAIK) to judge the longevity of such frames. It is thought that internal frictional forces, may shorten the lifespan of such frames, as well. Go with chrome-moly. Trevor From: Pat 22/12/99 14:02:03 Subject: re: Alumium gets saggy? post id: 19574 Shrugs??? i just keep hearing mention of all the pro bikes being soft and saggy.... wich would be the use i guess after training and racing for over 40 000 km a year. From: Pat 22/12/99 14:06:20 Subject: re: Alumium gets saggy? post id: 19577 but if i have a track frame built out of columbus max steel, it might wiegh in t about 3.2 kg or if i have one made out of scandium treated alumium... it will wiehg in at 1.3 1.4 kg on a track bike that is alot, as there is not a lot of weight saving to be made on components due to the lack of components lol...no brakes, gears etc. If however a good advantage of stiffness can be had through the columbus max over the steel, and it is likly to last better.... *shrugs also looking at some of the steel available from columbus, they are claiming one of them has a similar weight to alumium, and similar stiffness qualities.... but i dont understand all the stuff they say to prove this... lol *8^) cheers Pat From: Dan B. 22/12/99 15:07:38 Subject: re: Alumium gets saggy? post id: 19590 I reckon the Chome-Moly ones would be the best. But my opinion is in relation to tube frame race buggies. Aluminium is hard to work with and is relatively weak. The Steel ones are heavy, but cheap, while if you had a million dollars you'd build the whole lot from Chrome-Molybendum steel. At the moment, only the really hard worked part are Chrome-Moly, like the front and rear trailing arms, and some other suspension bits. Dan. From: huey 22/12/99 15:12:06 Subject: re: Alumium gets saggy? post id: 19591 Hi Trev W When i was still pushing spanners, i got much enjoyment out of watching apprentices trying to weld aluminium. It's a skill that took me a long time to get right and longer to teach to some. When you weld aluminium, there is a fraction of a second between too cold and tooooooo hot (the result being a big hole in your job and a ball of aluminium on the floor. Why is it that aluminium losses it consistency(?) so quickly? Huey From: Halogen Fisk 22/12/99 17:58:07 Subject: re: Alumium gets saggy? post id: 19689 Trevor said:"For a mountain bike, I would choose a chrome-moly frame, for it's longer life span (or Titanium, if I could afford it)." It really depends what you do with your bike. The weight saving of aliminium suits me, especially as I'll probably destroy the frame (already destroyed forks, stem, pedals & wheels) on the trail before any "life-span" problems. The Aluminium frame is SIGNIFICANTLY lighter than the equivelent bke in Cro-Mo, so as far as I'm concerned there's no competition. From: Alan™ 22/12/99 18:51:56 Subject: re: Alumium gets saggy? post id: 19737 The problem is not so much related to the tubes becoming sagging or soft, in fact we have exactly the opposite argument running at the moment in the sailing class with the manufacture r saying that the mast becomes stiffer with age. (there a metallurgical term called age hardening and work hardening, which are generally misunderstood and overquoted - if somebody wants a small tutorial please say so) You're unlikely (depending on the specific alloy) to have any change in strength of the actual tubes (stiffness by defination is constant). Where the frame could become "soft" is on the weld joints. Welding imposes lots of problems whether you are welding steel or aluminium, there are large brittle heat effected zones, large soft heat effected zones, porousity (holes) and inclusions (foreign particles) all associated with the weld. With use these are likely, on the microscopic level to cause weaknesses in the joints leading to the frame feeling "saggy". Aluminium because of its high thermal conductively is likely to suffer larger heat effected zones than steel and hence will probably suffer from "sagging" more. The high thermal conductively is part of the reason why it it so difficult to weld, combined with aluminium affinity for oxygen at high temperature. Steel also tends to be "over kill" in the strength department hence gas porosity and inclusions are less of a concern. Steel and Titanium are metals with fatigue limits. If they are stressed beyond a certain, they will break. Below this point, they will remainintact. Aluminium has no fatigue limit, so each and every stress, causes wear and weakening and eventual failure. Sorry, your mixing lots of different things here, where wear comes into I don't even know what you mean. Aluminium, Steel, Titanium and Composites all suffer from fatigue. Fatigue is a process where cyclic stresses well below ultimate tensile strength results in the formation and propagation of a crack. Some engineering materials can be designed not to propagate cracks and some lab prepared fatigue test sample can resist the formation of a crack (in the real world you have lots of sites to initiate cracks). Not enough time has elapsed (AFAIK) to judge the longevity of such frames. It is thought that internal frictional forces, may shorten the lifespan of such frames, as well. Composites have now been around for 30+ years, it's not so much that a problem of judging how long they will last, but more a case of when the uses of this material are going to work out how to properly use it and for the binding material (resins) will catch up with the fibres. The fibres are easily strong enough, but the resins have difficulty bonding to the fibres both statically and dynamically. Really the aircraft industry are leading almost alll other industry in this area of research. Composite layup, the order which the cloths are layed and in which directions are critical. i just keep hearing mention of all the pro bikes being soft and saggy.... wich would be the use i guess after training and racing for over 40 000 km a year. Remember most of the top guys would have several frames and they would have specific race frames and training frames. also looking at some of the steel available from columbus, they are claiming one of them has a similar weight to alumium, and similar stiffness qualities.... but i dont understand all the stuff they say to prove this... Stiffness is related to the young modulus (the constant in Hookes law) and the cross sectional shape. By using thinner wall sections and smaller diameter cross sections you can get steels and aluminium being equal in weight and stiffness. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.