Bicycle Mechanics - High pressure slicks on aluminum MTB frame: greater frame fatigue?

I have a beast of a mountain bike (Raleigh M50) I've been taking on long smooth paved asphalt trips. The 26x1.95 knobbies originally on there were holding me back, so I decided to switch to 26x1.5 Bontrager Select Inverts, considerably smoother. Needless to say, the squishy knobbies led to a soft ride on rougher pavement around the neighborhood at 35psi. The new slicks are at 80psi and really lead to a rougher ride on rougher neighborhood pavement. I understand the high pressure slicks were never intended for rough conditions, but they do come standard on aluminum hybrids meant for around town use. Similarly, aluminum road bikes must suffer from the same forces.

I am certain my aluminum frame is under greater stress now with the higher pressure slicks, so I am wondering if anyone has noticed a trend in aluminum frame fatigue in relation to tire pressure and absorption capabilities? Has anyone noticed a higher rate of frame failure on aluminum road bikes?

I'm no expert, but I did ride a Cannondale road frame for about 12 years. I'm a big guy (6', 180-200 back then), and wrenched at two shops that sold C'dales. Never once heard or saw anything like what you're mentioning. My road frame never lost stiffness, never showed any signs of fatigue, etc.

Being that MTBs are designed with more punishment in mind, I wouldn't worry about it. And if you crack it, they'll probably give you a new one. :-)

And if you crack it, they'll probably give you a new one. :-)Don't count on it. Warranty is for defects in workmanship and materials, not fatigue!

I have a beast of a mountain bike (Raleigh M50) I've been taking on long smooth paved asphalt trips. The 26x1.95 knobbies originally on there were holding me back, so I decided to switch to 26x1.5 Bontrager Select Inverts, considerably smoother.

I am certain my aluminum frame is under greater stress now with the higher pressure slicks, so I am wondering if anyone has noticed a trend in aluminum frame fatigue in relation to tire pressure and absorption capabilities? Has anyone noticed a higher rate of frame failure on aluminum road bikes?Find something else to worry about.

If you have a frame that is good for mountain biking, you should be well below the threshhold for road riding, even if you were to run on stainless steel tires.

I put slicks on my Giant Sedona DX, and I do not worry about the vibration, since I have never broken a frame of any kind road riding, but I did break a road frame off-roading once...

Enjoy the ride.

Find something else to worry about.

:D My psychiatrist prescribed this activity as therapy.

Uh you won't see signs of fatigue in AL. It'll explode in your face.

Uh you won't see signs of fatigue in AL. It'll explode in your face.No, that's CF.Aluminum gets mushy. ;) :p

Uh you won't see signs of fatigue in AL. It'll explode in your face.

I don't know about bikes, but in airplanes there are obvious warning signs. There was a famous case a while back where an airframe cracked during flight, and they determined that for that to happen there must have been at least a foot long crack that should have been caught on inspection before the plane took off.

Obviously the geometry of a bike is different, but I bet if you looked closely enough you could catch most problems.

Uh you won't see signs of fatigue in AL. It'll explode in your face.
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Don't count on it. Warranty is for defects in workmanship and materials, not fatigue!

While the workmanship is highlighted in the warranty disclaimer, I have yet to see one that states fatigue is not included. They detail extreme uses and racing/training but not direct fatigue.

Uh you won't see signs of fatigue in AL. It'll explode in your face.

The same line I have been harping on since ever since:
THERE ARE NO SUDDEN FRAME FRACTURES IN A METAL FRAME. :o hah, I had to say it. Every crack starts small and takes time and neglect to grow. Careful attention to cleaning around welds and stress points will all but eliminate the risk if you have an ounce of observation skills.

Also, knobbies produce a charachteristic vibration on asphalt, so changing to slicks is actually smoother overall.

Also, knobbies produce a charachteristic vibration on asphalt, so changing to slicks is actually smoother overall.
While this is true I think the original poster was more concerned with the lack of cushion that a fat knobby would provide.

Right, but as far as the frame is concerned it is nicer since it is only dealing with occasional bumps(albeit sharper) rather than a resonance.

There is always so much talking about fatigue of aluminium frames, but how many people here have actually broken a frame under normal conditions caused by fatigue ?

I guess that a frame is designed to last 50.000+ km and by that time most people will have retired it anyway.

While the workmanship is highlighted in the warranty disclaimer, I have yet to see one that states fatigue is not included. They detail extreme uses and racing/training but not direct fatigue.Try reading Cdales disclaimer for one. Most also include a part about normal wear and tear not being covered. Fatigue failure is part of normal 'wear'. Some companies will replace a broken frame no questions asked. Others aren't so easy. Therefore the 'dont count in it' statement.

IM using 1.4" slicks on my homegrown. I doubt its a problem. Like another poster said, your mtb frame (and mine) are ment to go offroad over roots, bumps, rocks, etc. A little shock from a pothole or something is easing the burden on your frame

There is always so much talking about fatigue of aluminium frames, but how many people here have actually broken a frame under normal conditions caused by fatigue ?

I guess that a frame is designed to last 50.000+ km and by that time most people will have retired it anyway.

I have one that cracked and my stunt riding friend cracked one, niether were catastrophic since they were caught early. 5 yrs of XC use is the standard spec, less with stunts or freeriding. My friend had a dualie and it went 2 yrs because he was stunt riding, even though he was 110 lbs. Thats part of why I like a chromoly frame, fatigue life limits are so far longer(I still have and race a 1994 frame).
Giant's warranty, which is similar to several company's states that only competition, training for such and dirt jumping are not covered. Lifetime and no questions asked.

Giant's warranty, which is similar to several company's states that only competition, training for such and dirt jumping are not covered. Lifetime and no questions asked.'Lifetime' is current wording,but in 2003 you got 5 years with aluminum frames. AFWIW, I dont's see anywhere that it says; ' No questions asked'. . sydney calls BS on that part.

Aluminum does not have an endurance limit like steel. In other words, aluminum will eventually develop fatigue cracks after X number of stress cycles, regardless of how low the stresses are. Steel, on the other hand, behaves differently. Steel can endure virtually infinite stress cycles, as long as stresses are below its endurance limit.

But aluminum bicycle frame engineers know all this. So, they design frames to last a certain number of cycles at a certain stress. Increases in stress, which is what “tgarcia2” is worried about, obviously mean a reduction in the number of stress cycles to failure. However, there may not be an increase in stress as a result of running different tires. Moreover, the life of your frame is probably measured in millions of stress cycles, so slight changes in stress will not have a significant impact on the life of your frame.

And as our pretty aviation mechanic alluded to, frames do not fail catastrophically without warning. Chances are great that a small crack will develop long before ultimate failure. So, it’s always a good idea to inspect your bicycle frame periodically.

Jason

PS: Do you think “she” is really a “he” aviation mechanic???

Damn good reply, Jason Curtiss :beer: and thank you everyone for all your replies. You all have more than well answered my question and thanks to your reassurances, I have neither worries nor fears upon encountering rough patches of road.