| Author | Message | ||
     Dhutty |
Hi, I'm a big ol' lurker around here. Got a question about the XB frames. I ride bicycles a lot and have discovered that aluminum frame bikes, though light and stiff, tend to loose their rigidity over time and stress. In other words, the bottom crank will tend to feel "mushy" if you crank hard on the pedals, over time. I've read that the old Porsche 928 had similar problems (don't know if it's true) as the 928 is based mostly on aluminum - what I read was that the used market for them is abysial becuase of the aluminum in their chassis loosing rigidity. Anyhow - is this is an issue on the XB's? Just curious. David | ||
| M1combat |
WARNING - COMPLETELY UNEDUCATED OPINION FOLLOWS - I think that today a great deal of the rigidity of a chassis design comes from the shape as opposed to the material and bulk. I think that in the mid 70's when they were designing the first 928 (77?) they certainly used the most brilliant mathematical and technical minds that porsche could come up with but I doubt they used any computer programs like Pro-E to stress test the design. Anyone with more insight? | ||
| Zenfrogmaster |
928 prices are low due to enormous complexity of the beast. Servicing almost anything can be hideously expensive. Parts are expensive, and there are as many alternatives are there are for the 911 series. I don't know that they have any aluminum body or chassis parts, although the engine and some suspension parts are. I'm familiar with aluminum (and carbon fiber) bikes going soft over time - I have one of each and they both should be replaced. Now, that still doesn't answer your excellent question about XB frames - I'll leave that to an expert! | ||
| Aesquire |
It's a question of fatigue cycle #'s. Steel, as far as you're concerned, has a near infinite lifespan. Aluminum, has a certain # of bending cycles it can take, then failure. It usually goes "soft" on a bicycle before it breaks, (usually at a weld)but sometimes it just snaps while riding. ( which can result in death or serious injury....... sorry, channeling my Buell service manual ) Note that this is when a featherlight frame is stomped on by an athlete, ( or chunky guy like me ) the frame moves through a measurable arc. ( at the bottom bracket ) Even a tank like a Cannondale or Kline gets torqued hard twice per revolution of the crank. I doubt even with the massive stress from a XB12 shaking at idle, that you come anywhere close to bending the frame enough to trigger a fatigue cycle. I bet you do click off one more cycle each time you drop the front end from a big wheelie, or back from a stoppie. Bet cash it's not an issue. Buell frames are STIFF. edited by aesquire on July 20, 2004 | ||
| Blake |
Absent wear or cracking, metal (aluminum or steel or otherwise) does not lose its stiffness over time. A bicycle's crank may loosen due to bearing wear in the crank itself and also in the pedal pivots, thus making it seem like the frame is more compliant. You may also have achieved stronger more powerful pedaling thus making it seem as though the frame had become more compliant. You might want to check for very small cracks around your crank too. They can be very difficult to detect. | ||
| Djkaplan |
Plastic deflection will eventually work harden aluminum making it more brittle. Barring heat stress, aluminum does not become soft or mushy over time. | ||
| BadS1 |
A chromoly framed bike will infact give a better ride though.Bicycle frames made of aluminum have a harder ride because the medal has hardly no memory so it can sometimes crack at a weld.Where as chromoly being heavier will be much more durable.You want stiff try carbon fiber.Blake medal can lose its stiffness at stress points. | ||
| Blake |
DJ, Work hardening strengthens and embrittles, it does not affect stiffness. Losing strength does not necessarily affect stiffness. Strength and stiffness (modulus) are mostly independent. Modulus will change with temperature though as does strength. Dana, No, barring yielding or cracking (onset of structural failure) the stiffness of metal does not diminish at stress points. | ||
| BadS1 |
Without proper reinforcement yes it does.6 1/2 in aluminum Diecasting and two of it in inspection doing X-Rays for porosity and finding the weakness in Automotive parts. | ||
| Captainkirk |
Anybody ever heard of the ill-fated DeHavilland Comet.....? Seriously, while stress may eventually take it's toll on your XB frame, I doubt if your great grandchildren will really care. | ||
| Coolice |
BadS1 check your mail | ||
| Blake |
Dana, Dude, providing stress/structural analysis is how I put food on the table. Trust me, metal does not lose stiffness due to stressing. You may be confusing weakness and stress concentrations with stiffness. Stiffness and strength are two different things entirely. | ||
| Blake |
Like DJ said, and he's an engineer.  | ||
| Loki |
The comets failure was the design of the windows and the stress on the structure where said windows were located. | ||
| Curtyd |
I am no scientist or engineer, but if aluminum gets soft then what could we be doing flying across the world everyday in airliners with hundreds of thousands of miles on them and many over 20 years in service? Now the Space Shuttle, TWO catastrophic failures in 100 launches, are you sure we can't get any more politicians put on a shuttle team, you guess which ones I recommend... | ||
| Steveshakeshaft |
Is there some confusion here with regard to some Aluminium Alloys that "age"? The properties of such alloys are these days well understood and are stabilised in production of the finished article by suitable heat treatment. I really don't think your XB frame is going to suddenly turn to rubber or anything. Steve steve_s@ukbeg.com www.ukbeg.com | ||
| Bomber |
Bad -- you and Blake are talking about different things using the same terminology (which is why there are technical writers, yippeee~!) reinforcement is to spread the forces around a potential (or actual) stress riser, so that those forces do not cuase flexing/cracking, which is what causes the failure -- the material itself does not change it's properties appreciably (athough work hardening CAN take place, the material has to be worked, which intimates flexing -- no lfex, no work) thanks goodness engineers and civilians can't talk so good together! | ||
| Aesquire |
The Comet's window design was an issue because the cabin swelled & contracted with each pressurisation cycle. Fatigue cracking built up & there was a zipper effect. A big chunk of cabin would rip out, with nasty effects. ( squarish corners & a boatload of rivets ) The Comet was one of the early pressurised planes, for the british, the yanks had a few more years experience, with the Stratocruiser & B-29. The issue comes up when the part gets stressed enough to bend a bit, if it doesn't move it's not an problem. Bicycles move a lot. To make a bad example, take a 1" alum. tube, 4 feet long, hang a 5 lb. weight on one end, and clamp the other in a bench vise so it sticks out horizontally. Vibrate this mess with a novelty store sex toy, diddle happens. Use a paint shaker from the hardware store, and you can bet the tube will crack in a statisticly predictable # of cycles. ( that means one may snap in 50, another in 15,000, but the average of many tests will be known ) Curt, if I can add to the passenger list.... oh well, we'd never get the whole Senate in. The Shuttle issues are human. Bad decisions on when to launch in temps out of spec, engineering changes in insulation to meet EPA rules, then not fixing a known issue, and having cheaped out to the point where a problem is fatal since there was no way to fix anything at all, unless you could do it with a pocket Leatherman & duct tape from inside the cabin. It stems from an "rockets are artillery" (disposable) attitude at NASA, left over from the V-2 days in Germany. If the U.S. Navy was in charge of space, there would be a decent repair kit, & an attitude of "we'll get it home". | ||
| Curtyd |
The Senate?? I was leaning more to the other two primary branches of GOVT. | ||
| Bomber |
I'm thinkin DC is a target-rich environment for that kind of list making activity | ||
| Dhutty |
All very interesting. The only thing I have to add is that the "aluminum bike loosing rigidity at the bottom bracket" comes from an UBER-bike shop here in Brooklyn. This shop sells tons of aluminum and steel (and carbon and titanium) bikes. So although they may not be engineers, I do give their pratical knowledge weight. All my bicycles have been steel, so I have no first-hand experience. Did a google check on Porsche 928 and aluminum fatigue, got nothing. So maybe that was bunk. Airplanes - Aircraft are required to be inspected frequently so they may be able to prevent any problems related to aluminum fatigue before they are problem (right? maybe?) Also, I would hazzard a very un-educated guess that: a bicycle goes through more repetitive stress than an airplane. An airplane's stresses are more random (turbulence) and the stress of the actual thrust is not is more linear than the "cycling" stress of bicycle thrust (pedaling, where you are actually "pinching" the frame to force the wheel to turn.) I'd imagine on the XB frames the stress is more linear than cyclical and repetitve. I don't own a motorbike, but would'nt hesitate to buy an XB, but this aluminum issue just occured to me as I've been shopping for a new bicycle and the steel vs aluminum issue is HUGE in the bicycle world. Very interesting reading all! I'll post my politics sometime on the politics board. David | ||
| Steve_a |
A few comments on bicycle frames mushing out over time: it's not because aluminum loses its stiffness. (Note: I was a class mate of Gary Klein, and my adviser at MIT was Shawn Buckley who pioneered aluminum-framed bicycles and who also oversaw Klein's thesis on large tubes for aluminum bicycles. Klein ended up with some fundamental patents on large-tubed aluminum bicycles. I took Buckley's bicycle seminar and still have the pieces from an aluminum bike frame I built in 1977 or so.) Aluminum, unlike steel, has no fatigue limit, so it's not the best material for something that gets severely stressed at high frequency, like a valve spring for instance. But its modulus (stiffness) doesn't vary because of high-cycle loading. Some current aluminum bicycles have bonded joints, and it's possible that the bonding fails progressively over time, which certainly would reduce frame stiffness. Similarly, wear or creep allowing play of the crank bearings might be perceived as mushiness. But there is no stiffness reduction from the bicycling world that would apply to the XB frame, or any of the twin-beam aluminum frames used on many other motorcycle brands. Also, motorcycle frame design is largely stiffness limited, with unusual loading conditions (huge potholes, landing from wheelies, hitting a bump while at maximum braking, etc.) setting the stress limits. You want large sections for stiffness, but realistic casting and manufacturing may demand thicker walls on those sections than you'd really need for strength, certainly the case on the XB. After you've designed for the unusual conditions, and pumped up the wall thickness for manufacturing ease, the average stress in a frame from simply riding is going to be quite low, which will stretch the fatigue life even of aluminum so far out that it shouldn't be an issue -- unless you've got harmonics excited by engine vibration running up the cycle count excessively. edited by steve_a on July 21, 2004 | ||
| Midknyte |
"The Shuttle issues are human...engineering changes in insulation to meet EPA rules..." ••••! | ||
| Bud |
Ok .. an other question, now all the metallurgy guy’s are watching  I have don some asking around about powder coating my xb frame, I wanted it flat black, I thought powder coating is the job… Now some metal guys I spook with said that the alu from the frame could loose some strength through the baking process , they said take a flame and just heat up a plate off alu and you will notice that the strength is gone…. And it will take a special heat process to get the strength back… As I asked a lot off powder coaters… and they didn’t want to give me any warranty on the frame strength … so I wet painted it … But I really want to now, was I false informed…I know there’s some truth in the story Regards, gr, Bud | ||
| Bomber |
Bud -- could very well be some truth -- I don't know what alloy the frame is made of, nor how it might be heat treated (if at all), nor anything about the welding used, but baking it at 400 or so might very well mess up the frame -- htink about all the nwer semi-trailers with warning stickers on em -- do not weld, this thing's heat treated, boyo! | ||
| Uwgriz |
It depends on the alloy, but above 275°F is getting into an area that I would be uncomfortable with when it comes to aluminum. Powder coat baking, depending on the process, is anywhere from 300°-400° so no, you weren't misinformed. Also, the heat treating required to get the strength back would certainly destroy your powder coat. | ||
| Mikej |
So, what does this imply with lightweight alloy wheels that have been powder coated? Are they now compromised structurally? | ||
| Uwgriz |
Not neccessarily. It depends on the alloy, whether or not it was ever heat treated, and the temperatures used during the powder coat curing process. Heat treating and annealing is a science that we've come to understand very well and we can do all sorts of things. Something can be heat treated and then powder coated provided you account for it properly. I guess maybe the better way to put it is, without fully understanding what is being started with and all of the engineering that has gone into it as far as loading and cycling it is expected to withstand, I personally wouldn't do anything to an aluminum frame that will cause it to see more than 275°F because that's about the point when you start to mess with something that someone has put a lot of time and thought into making sure it's right. | ||
| Mikej |
I'm thinking specifically of the XB's lightweight front wheel for example, then the rear, then the frame. I personally don't think I'd be messing around too much with the XB front wheel. | ||
| Djkaplan |
DJ, Work hardening strengthens and embrittles, it does not affect stiffness. I never said work hardening affects stiffness, but thanks for clarifying that. | ||
| Bomber |
like griz said -- if you know what's happened and what WILL happen, you can desigin the process for it . . . . . however, I'd be a bit reticent in powercoating an XB wheel . . . . . I'm betting the black wheels are coated with the folks doing the work following very strict protocals, and the folks what made em up know what was gonna happen each step of the way now, those big ol three spokers on the tuber, go ahead on (but don't ferget to replace the bearing afterwards) | ||
| Blake |
Are the black wheels powder coated? | ||
| Bomber |
the amber one's are, I'm told, so my guess would be, yessir, they are | ||
| Ray_maines |
My 15 year old Canondale crit bike seems to be just as stiff as the day it was born, but I suppose that could also be said of the Vitus. Both are nice bikes, but what do I know? Why am I even posting this? FWIW: My next bike will be Ti. Soft is good. I don't have a clue if aluminum or any other metal degrades over time. Buy an XB, ride the •••• out of it and and sell the darn thing five years and 50,000 miles from now if you're worried about it. What? You cant even say P (urinate) iss? That sux | ||
| Anonymous |
Neat to see how many smart people are on this forum. All the posts by engineers (Blake, Steve A., etc) are right, aluminum does not ever lose its stiffness. It will start getting cracks right as it meets its fatigue life limit, but as soon as that starts to happen it is truly DONE. That incipient failure may be what the bike guys describe as mushiness, but it is not something that would slowly increase over the life of a bicycle. It would happen right before failure. This should only happen in any life of product situation with an aluminum part that is designed to flex, like a spring, or a bicycle frame. The advent of FEA and other advanced analytical tools makes it pretty easy to design this out of any part that doesn't have to flex. But, you do have to replace parts that flex (airplane wings, bicycle frames, etc.), and you should be careful what to expect for life from lightweight aluminum aftermarket parts that may not have had the appropriate analytical work done to them. |
