MBer Med Evacced- Marco Island, FL

I worry about vibration induced fatigue failures in aluminum bikes with poorly balanced HT engines
 
So do I duivendyk, but it doesn't seem to happen. If it does happen it will be after a prolonged time period because I have heard that alloy can become brittle. I still think that might be more of an urban myth and yes it becomes brittle over time but not that brittle. If any bike has been subjected to stress it would be my Merida Kalahari MTB hardtail which has been through several engines (although with HTs that could mean it's almost new!) and it's front mount meant drilling a hole through the downtube (more urban myths about the dangers of that) and there is no sign of cracking after 4 years of abuse. Maybe it will all happen after 10 or 15 years but the vibration is not as bad on the frame as riding hard over bumpy terrain. That's how I see it. The bit that you don't want to fail are the forks and they are not alloy. Frame failure you can usually walk away from but forks is where your face meets the road with terrible force.
 
It is the cumulative exposure effect,so it takes time.That's why pressurized planes for instance have a limited life expectancy,based on the # of comp/decomp cycles,Often the final fracture is a stress fracture,while the underlying cause is actually fatigue.Holes as long as there are no sharp corners are not as bad as they are cracked up to be,sharp corners where there are stress concentrations, are to be avoided.A frame+engine is a complex structure vibrationwise,it can be 'harmonically excited' by the engine vibes,which is can be bad news
 
Last edited by a moderator:
Never ever use combined brakes,ALLWAYS wear a helmet,and no bare legs either,if you're a speedfreak gloves are not a bad idea also,and make yourself as visible as possible.
 
Right thats it, I'm getting a better helmet now. Mechanical failure I'm guessing was the chain coming off and locking the bike up, that is scary. All of a sudden without warning the back wheel locking up completely. The only advice I can give if that happens is to hit the front brake and try to control the bike from skidding the back end out, pretty scary really.
 
mechanical failure = definition?


ive read aobut this happening, much more than what i would like. fractured frames....what a nasty thought!
 
I might be wrong but the most common accident caused by "mechaincal failure" would have to be engine seizure & back wheel lock-up. Another would be chain tension failure when threads strip & tensioner wheel gets pulled down letting the chain go slack enough to bunch up at the first turn inside the drive sprocket chamber which also locks the motor & wheel. Frame failure becomes a likely scenario on really cheap supermarket bikes but definitely not on mid range quality cruisers sold in bike shops. I've been expecting my alloy Merida Kalahari MTB to fracture because it has done 15000 kms and mostly with the 70cc HT motor which, you must have noticed, vibrates much more than the 48cc HT at high rpm but it just won't even crack or show any sign of failure after 4 years of use and abuse.
 
There is a vast difference in the sturdiness of frames,the mild steel ones are virtually indestructable,except in collisions,but the cheap thinwall aluminum Chinese bikes are certainly not suitable for frame mount and not to be recommended for any application except for electric propulsion IMO. I think it is a bad idea to have so much chain slack,so that there is a noticeable kink in the chain at the tensioner.The chain ought to be fitted CLOSELY so that it can't fly off the sprocket,and a 'clamshell' type hub attachment used to ACCURATELY center the rear sprocket.Then the tensioner could be dispensed with altogether.One thing less to cause trouble.
 
duivendyk said:
There is a vast difference in the sturdiness of frames,the mild steel ones are virtually indestructable,except in collisions,but the cheap thinwall aluminum Chinese bikes are certainly not suitable for frame mount and not to be recommended for any application except for electric propulsion IMO. I think it is a bad idea to have so much chain slack,so that there is a noticeable kink in the chain at the tensioner.The chain ought to be fitted CLOSELY so that it can't fly off the sprocket,and a 'clamshell' type hub attachment used to ACCURATELY center the rear sprocket.Then the tensioner could be dispensed with altogether.One thing less to cause trouble.
Click to expand...

The tensioner wheel bolts are special stepped bolts and the threads strip very easily. This causes the chain to get sucked into the chamber where it doubles up on itself at the first bolt holder inside the casing (the top left bolt).
It is not easy to do way with the tensioner because chains keep stretching although less after the initial stretch when new. On a hub geared bike the pedal chain must be really tight which means it is the one that must be pulled tight by moving the wheel backwards and it is nearly impossible to get both chains tight without using a tensioner. Also the chinese kit sprockets are not round (none of them are actually perfectly round) so your chain will vary in tightness as the sprocket rotates. A spring loaded tensioner is the answer and a rear freewheel drive chain sprocket should be a legal requirement on HTs because when the engine seizes you can't get always the clutch in quickly enough. I use a twist grip clutch because it is faster to pull the clutch in but even that didn't save me when the motor seized. I like the clamshell idea but I like a freewheel much better. I can eliminate sideways wobble on the rear sprocket by care in assembly but not the up and down bounce no matter how I redrill the centre hole to try and get a more central alignment the sprockets just aren't exactly round. I know this because I scan the sprockets and import the image into CAD and check its roundness and that is a very accurate measurement method.
Regarding tube wall thickness you can tell in the shop if it's good enough if you know about bikes. All the Schwinn cruisers are very strong - alloy or steel - but I'm not including supermarket Schwinns because they are of a lesser quality. A good giveaway of a bad frame is often the really thin rear drop outs - you just know they're not strong enough and they'll deflect and bend under load. A lot of motorcycle frames are inferior in similar ways and all sorts of nasty things can happen if they are ridden too hard on bumpy terrain. The feel of a really good frame is a lovely thing that becomes noticeable when you ride one. It is hard to put in words.
 
Last edited by a moderator:
duivendyk said:
Never ever use combined brakes,ALLWAYS wear a helmet,and no bare legs either,if you're a speedfreak gloves are not a bad idea also,and make yourself as visible as possible.
Click to expand...
Hi, Ive been away from my PC for awhile...RIDING MY MOTORBIKE!!!...yippeee...ok...I ended up drilling a hole thru the clutch lever piston and fashioned a cotter pin/washer type retainer that works great.. I am curious why the issue with a combined brake?...Did you mean a lever that combines both front and rear brakes? If the brakes are adjusted properly and safely what could go wrong? Ive never heard of cables getting tighter on their own but i guess anything can happen...The real estate on my handlebars was dangerously slim with the shifter,clutch, and front brake lever on the same side...Is there a better way to go thern combining the brakes on one lever?
 
Back
Top