It is understood that the triangles would be made of the same material, differing only in the lengths of their sides. This also means the angles of the triangles would be the same.


"Why are compact frames stiffer?" - By having smaller triangles, all other variables being equal, the frame is stiffer.

"Who says stiffness is the be all and end all of bicycle design, anyway?" - Professional racers and those who aspire to be like a pro racer. People who can appreciate a bike with a quick, responsive feel. That's not the majority of bicycle riders but it is why a great deal of research time and money is spent on designing new frames and frame materials.

So the ultimate in stiffness would be to get rid of the seat tube and top tube completely, right? An infinitely small triangle should be infinitely stiff.

I could be wrong, but I think the stiffness a racer is after is out of plane bending stiffness "below the lines". One "line" being the line between the bottom of the head tube and the rear axle. The other "line" is from the head tube to the front axle. These are the the portions of the frame which would flex under load and tend to sap energy.

It is also important to have some in plane compliance, to soften the ride.

You'd also want a low hysteresis material so it will spring back after flexing. I would also bet hysteresis can be adversely affected by poor workmanship.

If the seat tube flexes a little, I'm not sure it matters in terms of energy transmission.

I don't think so. I think it is still limited by the modulus of the material (modulus of elasticity).

Let me rebutt a few of these statements, just for clarity. I believe there is a lot of incorrect concepts and incorrect statements in this thread, so I think a little clarity of thought is in order.

I don't think it is a good a-priori assumption. Large frame versus small frames, new designs versus older designs, etc. There are too many variables to just assume everyone understands this as a precondition. It is not unheard of for larger frames to be made of different tubing than a smaller frame of the same make/maker. Now, if you explicitly state this as a basis for your argument, then I have to agree.

There are many racers who intentionally introduce flex into their frames in order to improve their performance. Part of that "responsive feel" you mention is low hysteresis spring back.

I seem to recall the Paris-Roubaix (?) racers all wanted a more flexible frame because a significant portion of the race was over cobble stones and other poor road surfaces. A super-stiff frame was hard to control and beat the racer to death. Mountain bikers also introduce LOTS of compliance into their frames (front and rear suspension) for the same reason.

I think we all know we want a little compliance in between that front wheel and our hands, too. The fork stiffness, handlebar padding and gel inserts in our gloves is how we get it.

I'm not sure the OP was restricting the question to just racers, either. What about a touring bicycle?

The long tubes welded to the seattube might result in an overly stiff rear triangle. Might result in a rough ride, possibly a dead-feeling frame. It's possible that after experimentation, that tying the head tube and the rear dropouts together was the best configuration.

on a mixte, the twin longitudinal diagonals attach at the rear dropouts,where the bike has its widest structurally stable girth to triangulate the headtube/steerer from lateral & torsional deflection, making the bike safer & more precise in handling and stronger & more reliable than by simply attaching at the seat tube. the mixte design's intended user was expected to value comfort & convenience over performance & efficiency .using a semi-rigid attachment of the diagonals at the seattube or in the extreme case of the peugeot,bypassing attachment altogether, allows a greater degree of vertical compliance for absorbing road irregularities by utilising the full length of the tubes as spring members functioning under compression.

bingo!

one must always consider handling & road adhesion factors. the driveline & steering must be held locationally stable to provide precision,consistancy & efficiency but the very same tube structures must also provide suspension travel & shock absorbtion for comfort & road contact. that was my point of contention back in my first response to this thread-
"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."

I almost forgot to mention the compliance beneath the butt bones. A little compliance there is a good thing.

Are you saying the softer the cushion, the better the pushing?

Very good discussion thus far. I'll throw another question out there just for grins - if the compact frame is so much better for the stiffness required of a racing bike and given that frames resembling the compact design have existed since the late 19th century, why did the horizontal top tube design become the standard for the first 1 1/4 century of the modern bicycle? Especially since there were no oversized carbon tubes and oversized bottom brackets with outboard bearings and all that...given the limitation of their materials you'd think they would have wanted the stiffest frame design they could get.

It seems you're thinking of road bikes, right? If so, then:
I'd guess MTB trickle down effect.
Not everyone wants the stiffest frame they can get and frames should generally be stiff in some ways/areas, yet compliant in other ways/areas.

A very good discussion. Stiffness does not equate to "better" necessarily. I think we like a conventional steel frame because it is not brick stiff. The same reason that I don't personally like cannondales...they are brick stiff. But there are times that I do like a klein, especially in steep downhill corners. Wiggly top tubes are perhaps not dangerous, but they are scary.

Why the compact frame? Not because of increased stiffness. Not because of weight. Because the manufacturer can fit 95% of the world with three or four sizes, saving a boatload of capital on their production costs. A Cinelli SC or a DeRosa used to come in 1/2 cm increments...making just-in-time and inventory management almost impossible for retailers.

They are ugly, IMHO. I have four that I ride regularly, however. Once you are on 'em you don't know how ugly they are, and they ride just about like a bike.

I'm supposed to be setting up the trainers (snowed today), and I'm here drinking scotch and blathering.

Good night!

imo, compact design wasn't totally viable and largely unexplored until after "oversize" and shaped tubing became readily available. with "standard" tube diameters, lateral stability in locating the headtube/steerer was too compromised. oversized top & downtubes created a much more stable steering geometry which could be exploited without serious compromise. it's my personal belief that there's an additional factor to consider regarding longitudinal stiffness and handling/cornering qualities- ie: that any sideways frame deflection exhibited should ideally be even & progressive and occuring along the entire frame's length, from steerer/headtube to the rear dropouts or else a "hinging" behavior will occur where one or more segments of the frame deflect at a different rate or load than the rest of the structure, leading to unbalanced vehicle dynamics, loss of adhesion and unpredictable cornering behaviour.

This is getting old. Most "compact" frames come in the same sizes as non-compact. The only frames that come in XS, S, M, L, and XL are mostly low priced ones. That's the way low priced frames have always been.

1/2 cm increments are ridiculous. If you need something that precise, buy custom. you might get a 1/2cm in seat tube length that hleps, but is the top tube what you wa nt? Or the seat tube angle? It's crazy.

i have a question/request.

why is it that after 4 pages of considered responses, rabid rants, secondhand folklore & speculations that not one person, including the OP, has asked of the owners of compact bikes what they feel are the benefits (if any); their experiences & potential downsides to the design?
i have my own thoughts but given the huge scope of the topic haven't yet had a manageable framework to reply to. i'd like to hear others opinions on the merits,issues & reactions to compact/sloping bikes based on their personal observations gathered from actual usage.

k

Sorry, you lost me. Can you exapnd on that a bit?

Yeah, you REALLY don't want that. I had a Suzuki GS1100E back in the day that developed an occilation at high speed and caused me to crash in at 110-mph. It was not a pleasant experience.

RE:>>>Caterham: ..."with "standard" tube diameters, lateral stability in locating the headtube/steerer was too compromised. oversized top & downtubes created a much more stable steering geometry which could be exploited without serious compromise."<<<

i earlier pointed out one of the compact design's major effects as being to significantly stiffen the rear triangle by the shortening of the seatstays & thus creating a very rigid pyramidal structure with the seatpost as its leading "leg" and the main triangle extended forward from the seatube.
using the earlier 25mmTT/28mm DT tubing "standard" in a compact design, the main triangle would be much less resistant to sideloading & twisting than the heavy braced rear triangle,calling into play my premise that the ideal frame should exhibit a smooth, even deflection over it's entire length to minimize the "hinging effect" with detrimental effects to steering geometry,stability and cornering. modern oversized and shaped tubesets increase the TT & DT's resistance to lateral and torsional loads and will reduce the disparity of deflection between the compact's main triangle and that of the rear.

Smaller triangles.

Even Zinn, which specializes in bikes for very large cyclists (over 6'5" which can't be served by an off the shelf bike) uses compact geometry bikes.

Even Rivendell, which specializes in bikes that aren't trendy or fashionable, but appeal to the classic style uses sloping top tube geometry.

However, there are plenty of classic horizontal tube geometry bikes that are both stiffer and lighter than modern compact geometry bikes. Most modern bikes are made in Taiwan/China. The margins on bicycles are razor thin. The quality and materials aren't there compared to what they have been historically (obviously not true at the high end).

Believe it or not you can get a better bike by looking backwards rather than to the current product line.

I have a couple of 'road' bikes. You can't even begin to compare my '89 Cannondale 3.0 to my '04 Giant OCR1 frame. The Cannondale is a 63cm classic horizontal top tube bike. It came spec'd with Suntour Blaze back in the day. A horrible group. However, Cannondale didn't make different frames back then. Their top of the line road bike got the same frame as their bottom of the line road bike.

And its an epic frame. The Cannondale 3.0 was the lightest frame on the planet when it made its debut. It also set the benchmark for being the stiffest frame ever measured on the Bicycling Magazine 'tarantula' jig (back when Bicycling wasn't just a big advertisement or when editorial wasn't dominated by advertising dollars).

My '04 Giant Aluxx OCR1 frame is aluminum. It came on an Ultegra spec'd bike. You'd think the Giant, which was made fifteen years later than the Cannondale would be a 'better' frame. More optimized, stiffer, lighter, etc. Dream on. The Giant has a mass produced Taiwanese compact geometry aluminum frame and is a price point bike based on the components. The frame is nothing special. Its completely forgettable in every way. The only reason I bought the Giant is because they were marketing the XL as fitting up to being a virtual 67cm. Giant no longer makes that claim, and rightly so, the bike fits no bigger than as a virtual 63~64cm. No bigger than the Cannondale (which is 63cm c-c and 66cm c-t, and yes I need that extra 1cm I was hoping to get, more even).

Is the compact geometry Giant stiffer? Lighter? Better?

Nope.

The truth is that the Taiwanese and Chinese bikes being sold for between $1200 and $2500 today take a back seat to frames that were made fifteen years ago, and that's with compact geometry.

Now if you could find me a compact Geometry XXL Cannondale 3.0 frame, well, sign me up...but the Giant OCR1 ain't it, and the carbon bikes aren't nearly as stiff either.

So that's my rant. I've got a modern aluminum compact geometry bike, and a classic high quality American aluminum traditional horizontal geometry bike. The truth is that smaller triangles only make for a stiffer and lighter frame everything else being equal. However, it isn't. You just can't compare a Taiwanese compact geometry aluminum frame with the quality of the classic Cannondale. Its like comparing Wal-Mart Schwinn with bike store Schwinn, only dressing the Wal-Mart Schwinn up with high end components (okay, its not that bad, but its not that far removed as you might think either).

There are epically good bikes that are worth only hundreds of the thousands they used to cost that will embarrass modern production bikes (Klein, Cannondale, etc.).

thanks for the rant,sport!

ps- leonard zinn and grant p use a sloping tt for a reason -an ergonomic one-to raise the headtube,minimizing stack height using threadless steerers

I really didn't want to get into the which is better argument. I mainly was looking for a pseudo-scientifc explaination of why a bike made with a compact frame would be stiffer, not whether stiffer is better. I suppose I could have asked whether riders of compact frames have noticed the increased stiffness but there is a BIG problem with that - most compact bike are modern bikes and are likely to be stiffer because of the larger diameter tubing, oversized bottom brackets, etc and thus the effect of the compact frame design will be masked by the other factors. Somebody might say, "yeah, my compact frame is much stiffer and handles great" but is that because of the compact design or the other factors?

Support a 5ft long 2x4 with two cinder blocks placed under the ends. Stand on it mid span. Measure distance to ground. Do the same with an 8 ft long 2x4.

Assume we'll make two frames out of straight-gauge tubing one with horizontal TT and one sloping. Now consider that you'll have 4 tubes shortened by using a sloping top tube design, assuming all tubes on both frames meet in a fairly typical seatcluster.

Which one would be stiffer?

Sure, the front triangle would be moving away from the "ideal" equilateral triangle, and on smaller frame sizes, the rear triangle would be, but for larger frame sizes, the rear triangles would be moving towards that ideal.

Ok, but the 5ft 2x4 is going to fall on the ground because the cinder blocks can't move. In other words, take a bare seat tube of say 56cm and stick a seat post in it extended out about 10cm and rest the ends on the cinder blocks. Now take shorter seat tube of maybe 48cm or so and a longer seat post so that they both extend to 66cm. AND, make sure they weight the same - that is, use a lighter seat tube for the 48cm one such that the combination of the seat tube and heavier seat post weigh the same and then push on them and see what the deflection is. At any rate I think, as caterham has been pointing out, the interaction of all the various parts is probably more important than any one tube being shorter.

Short of a bicycle engineer coming along to add his 2 cents I think we've beat this topic to death.

Well, actually, all other things being equal, the sloping top tube would be less stiff than its shorter cousin (the level top tube). This is due to the extra length which, as you may know, is a major driver in stiffness.

For the sake of simplicity, let's assume we're making a bike for 200 ft. standing sprints. Rules for this hypothetical new event don't require saddle, yet require a diamond frame bike.

Now build your two hypothetical frames, do your 200 ft sprint and measure BB deflection.



oh, BTW, you can move one of the cinderblocks in the 2x4 test, just as we'll be moving the seatcluster while we build our compact frame.

This is dependent on ST angle, if I'm not mistaken. If we're making a frame with a 90 degree ST, then the sloping tube would be longer, for sure. On my Mongoose, a hypothetical horiz. TT would be approx a 1/4" longer than the sloping one it was born with.