Strength: Are smaller frames less likely to fail?

[RK1963]

Here's a question for all of the framebuilders / knowledgeable individuals out there?

If everything else (rider size, riding style, ect) remains equal, would a 52cm frame be less likely to fail than a 56cm frame? The frame in question is made of super vitus (steel).

With thanks,

rk

[Falanx]

It's not only the comparitive frame sizes against a fixed set of data (rider diemnsions), but also those fixed data that control the result. You'd have to specify both.

[RK1963]

It's not only the comparitive frame sizes against a fixed set of data (rider diemnsions), but also those fixed data that control the result. You'd have to specify both.
ceteris paribus

[Peterpan1]

Normally if the frame size goes down, the tubes stay the same. About the only thing that might affect the strength is there is less of each butted tube in, say, the main triangle, that is full thickness which does in effect scale down the tube thickness. But in general the smaller frame should be stronger since the shorter the tubes the stiffer and stronger they are in proportion to the length change. However, if you run that idea in the opposite direction, you may well still be using the same tubes for the larger frames, with much larger riders. so at some point the smaller frames are actually overbuilt assuming the frame is used for the same service.

[RK1963]

Normally if the frame size goes down, the tubes stay the same. About the only thing that might affect the strength is there is less of each butted tube in, say, the main triangle, that is full thickness which does in effect scale down the tube thickness. But in general the smaller frame should be stronger since the shorter the tubes the stiffer and stronger they are in proportion to the length change. However, if you run that idea in the opposite direction, you may well still be using the same tubes for the larger frames, with much larger riders. so at some point the smaller frames are actually overbuilt assuming the frame is used for the same service.

So a small frame may be overbuilt for a small rider and a larger rider could get away with (and may benefit from the lightness/stiffness) a smaller frame?

I am 6 feet tall and found that a 12cm stem and a longer than average seat post have given me a comfortable fit on a 52cm frame---I was worried that my size would be a problem. Also, I ask this as, its hard to get beyond the formulas that tie riders to frames based on their seat tubes.

If I use these old formulas, I should be on a 56cm frame. Is 4 cm a big deal? Thanks for your help. Additional comments would be welcomed, as you sound quite informed on this subject.

rk

[Falanx]

ceteris paribus

The problem with all other things being equal is that they generally aren't :) .


I will elaborate.

For example, two men at six feet do not exert the same loading on a frame if they have different torso and limb lengths. In the same way, one man who fits a two different frames because his fit falls somewhere between them does not stress them in the same way. If, for example, you are closer to the small size because of standover measurement, then the seatstays will spend most of their life in tension. If you ride that smaller size but are closer to the bigger, then they will take more compression as your centre of mass will be continuously sliding back over them. Their mode of failure, and lifetime changes. It's not so easy to say yay or nay assuming you fix all the service dimensions.

Second: A bad weld is just as likely in a short frame as a long one.

But: I've never broken a small frame. There's a lesson in there, somewhere.

[RK1963]

The problem with all other things being equal is that they generally aren't :) .


I will elaborate.

For example, two men at six feet do not exert the same loading on a frame if they have different torso and limb lengths. In the same way, one man who fits a two different frames because his fit falls somewhere between them does not stress them in the same way. If, for example, you are closer to the small size because of standover measurement, then the seatstays will spend most of their life in tension. If you ride that smaller size but are closer to the bigger, then they will take more compression as your centre of mass will be continuously sliding back over them. Their mode of failure, and lifetime changes. It's not so easy to say yay or nay assuming you fix all the service dimensions.

Second: A bad weld is just as likely in a short frame as a long one.

But: I've never broken a small frame. There's a lesson in there, somewhere.

Thanks for your input. Following-up, have you ever broken a large frame? If so, what precipitated it?

rk

[Falanx]

Corrosion and fatigue. Snapped a chainstay-dropout weld.

[Peterpan1]

"So a small frame may be overbuilt for a small rider and a larger rider could get away with (and may benefit from the lightness/stiffness) a smaller frame?"

What's the average size frame? 54? I don't know. Anything above that should start to be a stretch, and anythign smaller is getting heavy and stiff. Obviously the specifics mater more than the theory, but something like that should be in play.

"I am 6 feet tall and found that a 12cm stem and a longer than average seat post have given me a comfortable fit on a 52cm frame---I was worried that my size would be a problem. Also, I ask this as, its hard to get beyond the formulas that tie riders to frames based on their seat tubes."

Your size won't be a problem structurally, it just sounds like the wrong frame if we are talking a road frame. Unless it's the real height of a dropped tube frame. Like if the TT length is also 52, and not like 56 then it's small for you, if you have normal proportions. If you are really different in some dimension then it might explain it.

What kind of frame are we talking about, Road? I'm 1" taller and ride a 58. When I was a kid the frames were sold over here in inches, and the spread was like 21, 23, 25. 2 inches is 5 cm, and your on the worng frame, but it took 5 cm to get you there. i think 52 sounds way too small just off the cuff.

"If I use these old formulas, I should be on a 56cm frame. Is 4 cm a big deal? Thanks for your help. Additional comments would be welcomed, as you sound quite informed on this subject."

As above. One of the fit programs gives numbers for French Fit. I use those for standard height TTs where the ST and TT are basically the same length. Standard road stuff.

If you fritz around with stuff like stems, and I would say 120 is long on a 52, then you might get a good position, but you loose out on handling assuming the design is correct for your intended use.

Good luck

[RK1963]

"So a small frame may be overbuilt for a small rider and a larger rider could get away with (and may benefit from the lightness/stiffness) a smaller frame?"

What's the average size frame? 54? I don't know. Anything above that should start to be a stretch, and anythign smaller is getting heavy and stiff. Obviously the specifics mater more than the theory, but something like that should be in play.

"I am 6 feet tall and found that a 12cm stem and a longer than average seat post have given me a comfortable fit on a 52cm frame---I was worried that my size would be a problem. Also, I ask this as, its hard to get beyond the formulas that tie riders to frames based on their seat tubes."

Your size won't be a problem structurally, it just sounds like the wrong frame if we are talking a road frame. Unless it's the real height of a dropped tube frame. Like if the TT length is also 52, and not like 56 then it's small for you, if you have normal proportions. If you are really different in some dimension then it might explain it.

What kind of frame are we talking about, Road? I'm 1" taller and ride a 58. When I was a kid the frames were sold over here in inches, and the spread was like 21, 23, 25. 2 inches is 5 cm, and your on the worng frame, but it took 5 cm to get you there. i think 52 sounds way too small just off the cuff.

"If I use these old formulas, I should be on a 56cm frame. Is 4 cm a big deal? Thanks for your help. Additional comments would be welcomed, as you sound quite informed on this subject."

As above. One of the fit programs gives numbers for French Fit. I use those for standard height TTs where the ST and TT are basically the same length. Standard road stuff.

If you fritz around with stuff like stems, and I would say 120 is long on a 52, then you might get a good position, but you loose out on handling assuming the design is correct for your intended use.

Good luck

I just worked out another formula stating that I should be on a 55cm C-T. Thanks for your input.

rk

[Momentum]

Many framebuiders will use tubes specific to the frame they are building. For example, I have a 64 cm Bob Jackson that's built with oversized steel tubes to keep it stiff and strong at such a latge size.

[Peterpan1]

Good point. This particular thread states "everything else being equal" other than the size change. And frames using the same tubing for all their sizes, even fairly expensive frames are not unusual. Certainly with a custom frame one likes to think one is getting something specific to tailor the tubes.

[powers2b]

Don't confuse Strength with Rigidity.
A small frame will be more Rigid (all else equal) and therefore will flex less under the same loads.
Flexing leads to fatigue failure.

Frame builders try to strike a balance between rigidity (size, geometry), and strength (material, thickness).

[mattonabike]

i don't have anything to contribute on this subject. still, i'm interested in framebuilding and must say that this thread has been very explanatory and helpful. it's been very positive and informative. thanks for the read.

[Peterpan1]

"Don't confuse Strength with Rigidity."

Both are improved by shorter runs of tubing, which is what you get with a smaller frame, all else being equal.

[Cactus]

The answer to the question is No.

What is the cause of the question? There aren't too many Super Vitus examples sitting around. It doesn't seem likely that you have the choice of a 52 or 56, but perhaps?

Shouldn't fit be your first concern? Most steel frames, on the road, should be strong enough for just about anyone.

[Cactus]

Many thoughts in the prior threads. One that warrants some discussion:
"Don't confuse Strength with Rigidity.
A small frame will be more Rigid (all else equal) and therefore will flex less under the same loads.
Flexing leads to fatigue failure."

Firstly, steel frames take a permanent bend before the suffer from fatigue failure. It's one of their advantages over aluminum or carbon. So, don't worry about flexing leading to fatigue failure on your Super Vitus.

Let me offer a new axiom. Don't confuse movement with rigidity. Fix one end of a seat stay in a vice. Set up a guage for where the cantilevered end sits above the floor. Next measure the stay height midway between the vice and the cantilevered end. Now, connect a large weight to the cantilevered end of the stay, and measure the new height off the floor (at both the end and the midpoint). Clearly the end of the stay will move closer to the floor.

Now, put the weight on the mid-point of the stay and repeat the measures. Now, both points will be higher from the floor than they were when the weight was at the end of the stay.

The rigidity of the stay has not changed at all through the experiment. But the motion of the stay varies along its length, and the motion varies based on where the weight (force) is applied to the stay.

So, next comes the question of how does this play out in the real world? If the same force can bend the stay a lot or a little, does it matter which it does? There are two issues: Is energy lost? Is stability/handling compromised? Studies seem to indicate that energy isn't lost (despite everything the magazines say). So, then is handling compromised? And the answer is: It depends.

[rmfnla]

The problem with all other things being equal is that they generally aren't :) .



But: I've never broken a small frame. There's a lesson in there, somewhere.


I, too, have never broken a small frame, but at 6' 2" I've never ridden one, either.

Apparently, your translation/rebuttal holds true.

[Cactus]

RK1963,

So you asked a question about fit that hasn't really been answered. Nor can it be based on the info. But some input could help you answer it for yourself.

Generally, a smaller frame has a shorter top tube. This is a major driver for small frame designs to use different head and seat tube angles from larger frames. The angle changes can cause handling to be different across the range of sizes in a given bike model. Actually, its more than "can", the word I should use is "does." But, these changes are subtle and can't be readily perceived by most riders. Note that once performance orientation and ability (in other words racers) isn't a good predictor of ones ability to recognize these handling differences. This is the reason that modern carbon forks, with their limited range of rakes, can be used effectively in expensive high-end frames. Depending on their philosophy, a good builder may or may not adjust fork rake to frame size/angles.

A 12CM stem isn't all that long for an average 6 footer. My best estimate is that A typical 58CM frame has top tube ranging from 57.2 to 58.5CM. A typical 52 CM frame would have a top tube ranging from 51to 53CM. I'd usually put someone 6 foot tall (normally proportioned) on a 58CM frame w/ a 10-12CM stem. So, your combined TT&Stem is anywhere from 2.2 to 7.5CM short of the norm. That's quite a gap.

Another difference between the frame sizes is the height of the top of the top tube. The smaller frame will have a materially shorter head-tube. Let's say a 58CM frame has a 16.5CM headtube (this is based on many assumptions regarding a various frame design parameters). Let's also say that a 52CM frame has a 10.5CM head tube. Without any compensations, your bars will be located 6CM (about 2.5 inches) lower on the smaller frame. Given that most modern frames don't really compensate for the fact that threadless headsets and stems result in lower handlebars than traditional threaded headsets and quill stems, this could be quite an impact.

Finally, the smaller frame will generally have a steeper seat tube to help shorten the top tube. This typically places the rider farther forward relative to the crank. I, personally, like this position for a number of reasons, but many fitters don't.

So... If you're especially flexible, or have a very tall stem, the shorter head tube might be right for you. But if you run into problems with pains in your arms/hands/wrists, or excessive muscle pain after a ride in your lower back, you may have the handlebars down too low. Your body is your best guide on this.

If you're comfy with the positioning of your seat relative to the cranks, either because forward works for you, or because you have a seat post with a lot of set back, and you seat is slid way back in the post, then the smaller frame may be right for you. However, if you seat (and therefore your weight) is slid way back, and your weight is very high for the size frame, you might encounter problems climbing where hard pedalling causes the front end of the bike to rise. When this happens, your front tire loses traction with every pedal stroke causing 2 problems: a) working harder to steer and using energy that would be better used to pedal; b) losing directional control and possibly crashing. Again, you can determine what's right for you. If you like the fit, but want to avoid a light front wheel - a good choice would be to go with longer chainstays.

If you have a short torso relative to your legs, then a short top tube/stem combo makes sense. I use one myself having a 58CM seat tube with a 56.5CM top tube and 10CM stem. Again, you can measure youreself and figure this out.

A 52CM frame is an unusual way to fit someoen 6 feet tall. It's not necesarily wrong, however, depending on how your 6 feet are built. There isn't a magic formula that works all the time for fitting. The formulas make good guidelines and can be pretty darn accurate for people with average proportions. But, when one gets outside of average proportions, more judgement is necessary. Good luck.

[Falanx]

I'm 6'6". Presently I'm riding a 14".

[Cactus]

Falanx,

You told me it was 14CM. What gives?

[Falanx]

I'm pretty sure I said 14 inch?

Anyway. Long-assed seatpost. Comfy positioning.

[rmfnla]

I'm pretty sure I said 14 inch?

Anyway. Long-assed seatpost. Comfy positioning.

Is that the frame or the wheels?

[Thylacine]

Nobody should buy a frame because it's 'less likely to fail' than an the alternative. In fact, I don't know of any commerically available frame that is designed specifically to outlast the competition, so that makes the choice even easier.

If you're looking for an heirloom or collectable, get a hand made Reynolds butted 6-4 Ti frame or a Richard Sachs or something along those lines. If you're looking to get one specifically to ride the bejesus out of, get one that fits.

[Falanx]

Is that the frame or the wheels?


Frame.

Hell. yeah.





And you know what - it really doesn't look all that odd using a dirt jumping frame as a XC mount for a giant....