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Rigidity, Compliance, & Frame Material(13 posts)

Rigidity, Compliance, & Frame MaterialMikeC
Oct 24, 2001 1:22 PM
The following link takes you to a feature on Seven's site where they compare a couple of basic attributes as they apply to frame material.
Yeah, it's just one company's opinion, and it's obviously commercial, but it's kinda interesting, (particularly for newbies?).
Honest, I only posted once! (nm)MikeC
Oct 24, 2001 1:24 PM
Oct 24, 2001 1:57 PM
Carbon fiber (IMO) has the advantage of directional lay-up over Ti.
In my limited real world experience Ti bikes tend to be more "generally" stiff or compliant overall.
what they don't addressDog
Oct 24, 2001 1:58 PM
It makes sense that stiffness and compliance are inversely related. That's really all they are saying. They imply that they have found the sweet spot for the balance.

What they don't address is resonance or dampening, which is different than compliance. A frame might be very compliant, even noodlely, but still transmit a lot of road vibration, and vice versa. Some metals and tube shapes might provide very little dampening, allowing road vibration to be transmitted to the rider. Other frame materials, such as carbon fiber, might be very good at dampening. Thunk a frame with your finger nail hard, and listen to see if it pings or thunks. Metals tend to ping, and carbon thunks.

Carbon can be whippy or stiff, which can be designed into the frame, or can be both, depending upon the direction of movement desired or hampered. It would sort of nice to have a frame that is absolutely ridid torsionally, especially at the bottom bracket, but a little vertically compliant, and at the same time dampen vibration.

Some describe the vibration dampening as a "dead" feel. I suppose if you really want to feel every little bump in the road, cornering at the absolute limits on rough roads, that might be a characteristic "feel" that you might want. A crit rider might want lots of feel, whereas an ultramarathon rider pretty much wants none.

Keep in mind, though, that tires and points of body contact likely make much more of a difference than do frame materials or even tubing shapes. Put 25 mm tires at 100 psi on Open Pro rims on a bike, and then switch to 21mm 180 psi tubies on deep carbon rims, and it won't even feel like the same bike. Same thing with saddles, grips, even gloves and shorts.

A frame cannot compress in any measureable amount, it can only flex. For a frame to feel compliant, think what must happen. The axles must be able to travel when hitting a bump with respect to the body points of contact, that means the chain stays must flex upward, the seat stays must bend somewhere, and/or the front triangle or fork must flex. Tubes don't compress lengthwise.

A longer wheelbase helps compliance. The longer the rear triangle, the easier it is for those stays to flex. Also, basic geometry tells us that the further behind us the rear axle is, the less effect hitting a bump will have on our rear. Same thing on the front.

As we know, larger tubing, all else equal, means stiffer tubing. Cannondales are stiff not because of the aluminum they use, but because they have big tubes. Some companies are using complex shaped tubes. Ever gander at the downtube of a Bianchi EV2? It has about 3 distinct shapes and sizes, ranging from oval, to triangular, to round. Some portions of the tubing can flex more than others, supposedly where needed.

All companies are, no doubt, but especially if they make only one type of frame, going to tout some advantages of their bike. Compare Softride, Kestrel, any steel maker, Cannondale, Litespeed, and Seven. Quite a variety of claims. They each claim they are faster or better due to some attributes of their frames. I think most of the claims are exaggerated and esoteric. That doesn't mean they aren't real, just over hyped. But, they gotta do something to distinquish themselves and sell bikes, right?

Wow mr. PeabodY!MrCelloBoy
Oct 24, 2001 2:08 PM
You da man!

Seriously though, I appreciated all the great info.
Just having fun with you.
it's a theoryDog
Oct 24, 2001 2:11 PM
I may be totally full of crap. This is just an integration of a lot of reading and personal experience. :-)

it's a theorycioccman
Oct 24, 2001 2:35 PM
Good stuff. Thanks. Many of the theories and personal feelings that have gone into my decision to ride aluminum exclusively with almost no interest in titanium whatsoever and just recently acquired curiosities about carbon. Just my own personal feelings, likes and dislikes. I like the numbers the oversized aluminum frames had on the Seven comparison sheets. I want total drive train stiffness and very little vertical compliance. Stiffer the better, and my two current rigs prove it. Adding your resonance idea in, well, that would the number that I'd like to see reduced in oversized aluminum, however, to get total stiffness and great transfer of power, it doesn't bother me that I have to live with a lot of resonance. I'm not an ultramarathoner and I'm perfectly comfortable doing centuries. Not so sure if carbon will come into my life. I enjoy the thought of a C40 or Calfee or Look. We'll see.
what they don't addresstr
Oct 24, 2001 3:56 PM
bottom line, with ti,alum, and steel, to get a certain stiffness you are mostly geometry dependent. With carbon fiber, this is not the case. It all depends on how you lay the sheets. I think carbon fiber is a more elegant way to work the problem. But, this is an aerospace engineer talking. In a idealistic way, you could modify the carbon fiber angles to yield stiffness where you want it and something softer where you want it (giving the frame all the local and global characteristics that you want).In essence, you can tailor it to what you want. Each frame, no matter what it is made of, will have inherent frequencies and shapes that it will vibrate at. Whether or not the frame is excited at one of these frequency values (and/or frame shapes)depends on the forcing function (bump, etc) and the damping value of that particular shape excited (in reality, usually more than more shape is excited and the motion is a summation of them). Each pair (distinct frame shape and frequency of vibration)with have its own damping value. Like you said, some materials exhibit better damping values than others. In the past, i have seen people point to strength tests as a measure of how good a frame is. But, that is only half the story. The other half should consist of taking the test frames and subjecting them to a vibration test that would yield frequencies,shapes, and damping values. This would give us some dynamic characteristics to look at instead of just static values. Then add the wheels and see what happens. In a simple way you can look at the wheels as springs on the frame. The stiffer wheels obviously rely on the frame for damping, whereas the softer wheels will damp out some of the disturbance and feed less energy into the frame. Looking at material properties, Ti is in between alum and steel, and carbon fiber can be placed were you want it. The first ti frames used standard geometry and wall thickness, that is why they were soft during the first generation. Aluminum had similar growth pains, but they came from the other direction (too stiff).
Oct 25, 2001 2:32 AM
The real problem with carbon fiber is that there aren't enough serious geniuses in the bike industry to harness what the material can bring to the table. I talked with another aerospace junkie a month ago and he had more to say than I could keep up with. The possibilities seemed astounding; comparatively speaking much of it's application in bikes hasn't gone much farther than string and glue. Perhaps we can convince TR to consider a career change. :-)
Oct 25, 2001 5:39 AM
What you say about "bike industry has not harnessed the full capabilities of carbon fiber", i could not agree more. If they got serious, they could tune the bike quite nicely.
unless you've ridden all the best frames in the worldrlscolker
Oct 24, 2001 5:32 PM
just don't see the point in seven's custom program. it's a well done, clever marketing job made with all the myths and tales of the perfect frame. tailor made geometry is one thing that's real and measurable but chosing stiffness, copmfort on a scale of 1 to 10? well, a lot of people is happy buying sevens.. why not?
if i had the dollars i would buy a richard sachs. i "feel" it has more class. it's just a matter of taste.
unless you've ridden all the best frames in the worldrlsLeisure
Oct 25, 2001 4:36 AM
I respect your taste, colker. Everyone has their own, and it's pretty silly somtimes how much we criticize each other's. Well, having ridden several Sevens and knowing just how much of a difference the tubesets make, I really like the benefits it has to offer. It's the complete opposite of the final perfect frame for everyone; it's about you choosing exactly how your frame feels for your weight and riding style anywhere you want it to. It sure sounds like marketing hype (and yes, to some extent everything is) but then you put your leg over a few. That's when the possibilities start spooling in your head.
The manager at my LBS put together a Seven frame this season with amazingly stiff chainstays and fairly plush seatstays and a sloping top tube. The idea was to make a crit bike that didn't beat you up. Even compared to other more standard Ti setups, it feels almost softailish, while seeming to accelerate like a hushed-down jet. It felt pretty wild if you can excuse my enthusiasm, and I don't typically say that about road bikes Seven or not. Should I choose to do a Seven setup, it will likely be quite similar.
Granted, a lot of this is feel and not performance. It's the ride experience that people get enthusiastic about. I would never claim to have ridden all the best frames in the world; frankly I have a hard time thinking anyone ever truly has. I've said it elsewhere, there's just no ceiling.
The "scale of one to ten" is most certainly an issue though, and requires that you purchase from a thoroughly experienced roadie dealer that moves lots of Sevens and knows what the scales really mean. Otherwise, much of the touted benefits will likely slip through your fingers. Anyone contemplating getting a Seven should strongly bear this in mind.
At any rate, I'm interested in hearing about some of the positive aspects of any other frames you or anyone else knows of. You just brought up Richard Sachs; I'd also like to hear about Serotta. Anyone at any time can wave their hands and dismiss things as marketing hype when they don't feel like paying attention; I've heard plenty of people lay criticisms of what they don't appear to have owned or ridden, and this is hardly limited to frame discussions. People interested in future purchases, even years down the road, will benefit more from hearing some of the positives of alternative products that people have actually ridden. If I hear about a product that brings its own unique virtues to the table I'm interested in hearing about it.
Any custom builder can taylor frame stiffnessNessism
Oct 25, 2001 8:57 AM
Any deceint builder that is. They can do this by using thicker/thinner walled tubes, larger/smaller tube diamaters, and longer/shorter tube butts. No rocket science here.
Where Seven does a good job is in bringing these details down to a level that their customers can understand; simple 1-10 scale. Seven also does a good job of "including" the customer in the design of the frame which should result in a better fit.

Seven's marketing guys do a good job. The frames are not worth the money in my opinion, but good stuff none the less.