Why are compact frames stiffer?
 

Can someone explain to me why comapct frames are supposed to be stiffer? I'm not asking why manufacturers use compact frames today (current style, mountain bikes, less sizes to stock) and I understand the advantage of more standover clearance without a giraffe neck stem and I am not saying compact is better or worse than traditional . But why would a compact frame be stiffer than a traditional frame? That's one of the biggest advantages claimed about compact frames. Supposedly Abraham Lincoln was once asked how long a man's legs should be and he replied, "Long enough to reach from his body to the ground". It's pretty similar with bicycles: the seat tube/seat post/saddle has to be long enough to reach from your buttocks to where your feet touch the pedals right? So if you take a compact frame in isolation, I suppose it is stiffer but if you include the longer seat post, why would it be any stiffer than the traditional frame? We had sloping top tubes back in the day too - we called them "girls' bikes" but we always said that one of the main disadvantages of that frame design was that it flexed too much. So what gives? Can anyone enlighten me? Why would a compact frame plus seat post be stiffer?

most modern bikes are made with oval aluminum tubes,


aluminum isnt as flexy as steel is, if your aluminum tubes flex, you're in some deep trouble.
plus the oval shape helps stop flexing (which could destroy your bike)

where as steel flexes quite easily and can endure the stresses of bicycle riding. so the tubes are circular, which allows it to flex more than oval aluminum tubes.


nothing will ever match the ride of a steel bike.

I love steel bikes because they flex.

smaller triangles are stronger than larger triangles

That might be true but you could build an oval tube aluminum (or carbon fiber) frame with a horizontal top tube too so it doesn't really matter. That has nothing to do with the compact frame.

Yes, true - but longer seats posts are more flexible than short ones. Anyway you slice it, you've got to reach from the pedals to your backside so if you slope the top tube down for a smaller triangle then you have to use a long, flexible seat post. So how is that stiffer or stronger?

Where did you get that information from?

Shorter tubes are always stiffer than longer tubes of the same type. Take a foot long pice of PVC pipe and a three foot long piece of the same diameter and thickness of PVC pipe, I gurantee you will be able to bend the longer one more than you can bend the shorter one, 'Tis the same for steel, or carnon fiber, or aluminum, or titanium, or unobtainium, or any other tube. Thus the shorter seat tube of a compact frame is stiffer than the longer seat tube of a regular frame - if they are made of the same tubing. That makes for a stiffer frame.

Throw in the longer seat post, and that may or may not end up with an overall bike that is stiffer than a regular frame with a regular-length seat post. But you asked about frames, not bikes. I'd guess - and it is only a guess - that the compact frame with a long seat post is still stiffer because seat posts are significantly thicker than are frame tubes. It also would not feel as good as a properly sized regular frame made of good steel, at least to my backside and legs.

As for why what used to be called "girls' frames" (as opposed to mixtes) were less stiff is because the top tube and the bottom tube met so close together on the seat tube, thus making for less natural leverage and bracing to resist up-and-down frame flex. Compact frames have signficantly more spread between where the top and bottom tubes meet the seat tube.

Flex in a bicycle frame occurs mainly on the down tube and chainstays in a side to side motion at the bottom bracket. Because the seat tube is more verticle there is less flex in that side to side direction but the flex that is there will be less if you change the geometry to make the seat tube shorter. You will have less flex in the tubes that have the most flex to begin with, and not necessarily more flex in a longer seat post. As for the girls bike thing, they are much more flexible because the old style bike frames were not a triangle but a parallelogram with a curve. Just my observations.

I don't care how much my seatpost flexes as long as it don't break.

All other things equal, a smaller frame is going to offer more stiffness in all areas except the saddle.

The harder you pedal, the less weight there is on the seat, seems like to me.

there's more than just the main triangle that gets tighter & more rigid-the seatstays become shorter making the rear triangle come closer to that of an equilateral construction in both the vertical and the lateral.
if you study a pic of my *semi-compact* steel cinelli, you'll be struck that the rear triangle comes close to forming a classic pyramid structure.

http://i154.photobucket.com/albums/s...lli066-1-1.jpg

the toptube length becomes shorter as well,thus more rigidly locating the steerer/headtube so that there's less torsional & lateral deflection when force is applied to the bars or from steering inputs & cornering deflections.

in spite of the *potential* for some loss in rigidity from the saddle to seatcluster due to the need for a longer seatmast, the vertical structure from seatcluster to bottom bkt and the longitudinal structure from the headtube to rear axle dropouts & from seatcluster to headtube is dramatically more rigid & better braced in nearly every deflection mode, enuf to easily offset your long, flexible seatmast concerns and which in addition, can be partially or completely addressed by reinforcing/stiffening of the seatpost itself.

to me, the real question is whether or not all the added stiffness in a compact design is necessarily consistantly beneficial and without trade-offs . in my experiences, the thorough, well-considered, goal-oriented implementation of any given design philosophy is the real key to any superior and satisfying result.

How many aluminum springs have you seen? How many steel ones? It's rather obvious.

At one time Cannondale said that there was no strength or weight benefit to compact frames, and that their popularity was simply a matter of fashion.

Yep, shorter tubes would be stiffer.

It probably has something to do with the fact that Torque=Force x Distance from pivot. So when you're pedaling, you're applying some Force at some angle away from your downtube and chainstays. The resultant torque causes the flex. With shorter tubes, you have a shorter distance from the pivot, and thus a smaller amount of torque is applied to the tubes.

A longer seat post would probably contribute to some flex, but when you're pedaling hard, a lot of your mass is offset by the opposing force from the pedals. So your effective weight on the bike is lower anyway.

I'm just speculating here-- correct if I'm wrong.

The seatpost is way thinker, and much less flexible than frame tubing. Same thing with a stem. Couple this with the extra strength of smaller triangles as others have mentioned, and all else being equal - you have a stiffer frame.

+1

Notice how mtb. bikes have smaller triangles and bmx bikes have much smaller triangles.

And today's oversized carbon seatposts are incredibly stiff as are the frames. Oversized bottom brackets, larger headtubes and customized shaping of each "tube" in the frame makes a modern carbon fiber frame stiffer than a traditional steel frame.

http://www.engineersedge.com/manufac..._materials.htm


aluminum doesn't bend well, when it does bend, it can break, suddenly and catastrophically. It's a step above carbon fiber on the list of things that scare the s#!% out of me.

whew... now i'm worried. should i switch over to steel or wooden rims?

meh, my response about the failure is more related to the material in general, not for bikes specifically. but it does still stand that an aluminum frame that suffers a hit that's bad enough to bend it, is going to break. it's inevitable.

The top tube does not count much for stiffness so if it's longer, it's no big deal. You'll need to use a longer seatpost but isn't most of the force generated at the bb area and that's theing thing needs to be beefed up? I think most of the forces occur from head tube, downtube, bb, chain stays. Not much with the top tube or seat stays.

+1
And rectangles, and trapezoids, parallelograms, etc.
He said it short and sweet.

That's not for strength - that's to reduce the likelihood of your boys interacting with the top tubes! An equilateral triangle is the strongest geometry. The more you flatten it the weaker it gets. The triangles are NOT smaller - well it depends on how you define "smaller", but the length is the same to fit a person. A small triagle would be 3" x 3" x 3" and yes very strong, but kinda hard to ride. Compacts are just flatter. And flatter is not stronger. If it were, you'd dispense with the seat tube and top tube altogether. It'd be infinitely flat and therefore infinitely strong. That's not how it works.

Good point. Seatposts are many, many times thicker than frame tubes, and so there is not an equal give and take between the flex of a large triangle traditional frame and the flex of a compact frame.

Now, stem flex might be a different matter given the torquing nature of the forces on the stem.

jim

And it is because this seat post is many times thicker that Cannondale said that a compact frame will not be lighter than a conventional frame. The very long, very thick seat post is also very heavy. And, as GV27 said, "the equilateral triangle is stronger" than a non-equilateral triangle, so a compact frame cannot be stronger than a conventional frame.

uhhhhhhh...

I should have been more specific - I'm quite sure if you put a bare compact frame in a test jig of some sort it will be stiffer than a traditional frame for all the reasons mentioned. Obviously shorter tubes are stiffer than longer tubes. But since people don't ride bare frames, I assumed you'd need to include all the other parts that make up a bicycle :)

Let me be even a little more specific - I realize campact frames don't exist in a vacuum so as compact frame design was becoming popular other changes like shaped tubing, oversized tubing, new frame materials, oversized bottom brackets, and even things like outboard bottom bracket bearings also came along and all of these things improve stiffness. They are not, however, in any way tied to compact frames. Traditional horizontal top tube frames can be and have been built with all of those other features.

Who says a compact frame is stronger? Tensile strength and tensile stiffness are two different things. The shorter the tube, with a given diameter and wall thickness, the stiffer. Round tubes are stronger than oval tubes in all directions. An oval tube is stronger in 2 directions but weaker in the other 2 directions with a given circumferance and wall thickness. Steel tubes of a given strength or stiffness have a thinner wall. Aluminum will bend, depending on the alloy, but it will not bend as far or as many times before failing.

Compact frames are lighter and stiffer because there is less mass and the tubes are shorter. Aluminum frames tend to be stiffer because larger diameter and thicker walled tubing is used (in most cases). Remember the early Vitus frames..... flexy and made for a nice ride in my opinion. They had used small dia tubes.

Seat post has no bearing on frame stiffness. It is an external part and since most of them are alloy, they had better not bend much or the rider will find himself quite uncomfortable.

You also need to consider the manufacturing process. Forging vrs. casting. Forgings will bend more without failing than will a casting. Castings are strong and stiff but brittle and will fail abrubptly but not as strong as a forging. A forging is stronger and will flex more before failing.

Any given tube is much stronger in tensile than in compression. Round tubing is stronger in torsion than either oval or square.

This is kind of what I'm thinking...

Could it all be marketing hype? Could the modern compact frame be stiffer than the traditinal steel frame because it uses huge oversize, shaped tubes, an oversize bottom bracket, etc and not because of the compact design itself?

And please, let me restate, I'm NOT trying to argue one is better than the other. It would not matter to me anyway. I ride a traditional frame for essentially the same reason women wear high heel shoes - it looks better (IMO anyway, horizontal top tube and women in high heels :)) and not because I think it is better, whether it is or not.

Simple: because the Marketing Department says they are.

But a longer unbraced tube (i.e., the longer seatpost required by a compact frame) is more flexible than a shorter tube.

presuming other variables are the same (tubing material, thickness and diameter, etc), this is true...

Basically shorter tubes are stiffer... (as dit mentioned)

How is this not for lower standover height on uneven terrain?