meangreen

Hey everyone,

I am in the process of shopping for a new bike. While I have tried to teach my self as much as I can about bikes I have had a hard time understanding these different terms and that they actually correlate to while riding.

stiffness: Aluminum bikes are said to be very stiff, but they are also criticized for being such a rough ride. What would a stiff bike actually feel like? What is good about it? What is bad?

road noise: Is this an actual noise? Or is this small vibrations that can be transmitted (or dampened) by the frame? If is the later can that make you body ache while riding? If it is just a loudness issue than is that just a personal preference?

flex: The definition of this term is quite obvious, but is it something you want? Is it something you can actually feel? Or is it something that dampens out vibrations?

There are probably even more terms that I haven't hear yet. If you know of more please give me (us) some definitions! If you want more definitions this would be a great place to ask!

Thanks!

Casey

PS I have an CAAD9 (aluminum frame with carbon fork. I have recently replaced the carbon handle bars with aluminum ones and started changing the fit around. Yesterday i tried to ride and after a short 8 miles I had to stop. I had really bad back and elbow pain. Could this be because my fit is all funky? Or could replacing the carbon handle bar allow that much more vibration mess with my body?

bradtx

Casey, I ride Cannondales, I find the ride not unlike any other racing frame I've ridden, regardless of material. Tires can make a difference, a 25C inflated to about 85% of max. I'd rather have a stiff frame that I can tame than a flexible frame. The carbon fork will also dampen some vibration.

Road noise can be a variety of things from the tires to the frame itself. I've heard carbon bikes make a hollow sound over bumps and bikes with internal brake runs make a high pitched ting. High frequency impacts like running over cobblestones can be tiring and cause numbness in the hands. Some have attributed deep V rims, with their natural stiffness fatiguing.

It sounds like the bike isn't fitted for you yet.

Brad

jischr

My 30 year old steel frame bottom bracket will 'flex' while sprinting or standing on the pedals on a hill. It moves enough to cause the chain to rub on the deraileur. It doesn't flex while seated, I can't generate the same amount of power while seated. I heard that in really bad cases it can cause the rear gears to shift automatically. Some flex is an annoying, a fair amount is truly wasted energy.

My 10-year old aluminum frame has no evidence of flexing, no noise, and it feels like it takes less energy to pedal. But that's probably due to the different wheels. But I would say that it has greater stiffness than the steel frame. The aluminum frame also rides smoother in my opinion, but that could be because I started running 10 psi less air in the aluminum's wheels lately. They both have the same Michelin tires.

My buddy's Trek Madone hums while riding. I heard others say that their carbon frames are loud compared to other frames they've had.

I doubt the change from carbon to aluminum would make that much difference in what you feel unless you had a steel fork and high pressure tires. I suspect you lowered the bars or raised the seat too much at one time and now your arms are taking more of your weight than they are use to.

DavidLee

Sheldon Brown's bike glossary, have fun.

vision646

Just FYI none of those terms are in Sheldon's glossary.

DavidLee

Just FYI, read his entire post.

JohnDThompson 

Aluminum bikes tend to be stiffer than steel bikes because larger diameter tubes are used in order to provide sufficient weld joint area to carry the loads of riding.

If you tend to mash on the pedals, or are a heavier rider, a stiff frame may be good for you. A light rider may find the ride to be too harsh.

The larger diameter tubes on modern aluminum and carbon fiber frames tend to resonate more with road vibrations and freehub ratchets. Frame stiffness determines how much road vibration is transmitted to you body, but this can be mitigated by slecting appropriate gloves, shorts, seats and other components.

Flex is the flip side to stiffness. A flexible frame will dampen road vibrations better than a stiff frame, but may feel "whippy" or unstable with a heavy rider.

vision646

That probably came out badly on my part. I was merely saying that those terms weren't there so the OP's question was still unanswered.

p.s.
I don't know the answer (thats why I clicked the link) otherwise I would have just posted it.

BlazingPedals

Oh, that's probably part of the reason, but the big reason why aluminum tubes are fatter is because of the nature of the material. Steel bikes have a very large fatigue cycle, as long as the fatigue is kept below the yield value. Or whatever they call it. OTOH, in an aluminum frame, flex is forbidden. You see, when aluminum flexes, it work-hardens. Eventually it gets to the point where it's so brittle that it cracks at the least shock. Then the frame is toast. To avoid flexing, large diameter tubes are used because the larger diameter provides triangulation and makes the entire structure stiffer.

meangreen

oh cool! Thanks guys!

Ok, I have another question. I know, I am a total noob.

So cyclocross seems pretty awesome and those bikes seem like very all around bikes. What is the difference between a cyclocross bike and a mountain bike that doesn't have shocks? I know there are differences in the geometry, but could you take cyclocross out on a pretty serious trail, or would your bike be toast?

Artkansas 

As a rider of steel-framed bikes, I was given a rude introduction to the stiffness of aluminum bikes when I rode the Capital Bikeshare bikes in Washington DC. On potholes it seemed as though the handlebars would almost get knocked out of my hands by the shock of hitting the far side of the pothole. I also noticed that my hands hurt after a while from all the shocks.

fietsbob

+1 .. Aluminum Cracks when flexed, after a number of flexing/work cycles .

So Stiff is how it is made reliable.

note: how there are no aluminum springs..

MichaelW

Stiffness and flex relate to the same feature, the resistance of the structure to bending. Bending forces are mainly the twisting caused by pedalling hard, pushing the bars in one direction and the pedals in the other. Pedals, being stuck out to the side have leverage on the frame. Loaded touring bikes have , in addition, the side-to-side sway of the rear luggage rack.
You need the bike stiff enough to resist the bending force but any extra stiffness is wasted.

In the old days when tubes fitted into std sized lugs, you made tubes stiffer by increasing the tube-wall thickness. It is much more (weight) efficient to increase the tube diameter which is now the universal solution.

Aluminium should not be bent so is used in fat tubes. Any material in fat tubing (steel, carbon, titanium) will be stiff. Generally, steel, ti and carbon are used in smaller tube diameters to increase comfort (but not always). Modern aluminium bike designs are usually much more subtle than the original fat-tube designs and are more comfortable.

MTB vs Cyclocross. CX bikes are much lighter in weight and not as strong so big stunts are generally avoided. They also have higher gears since courses avoid major climbs or riders carry the bike and run.
Competition CX bikes are totally stripped down. General purpose CX bikes usually have threaded eyelets for everyday things like rack and fenders. They are good all-rounders.

Retro Grouch 

1. For a very long time all aluminum framed bikes, like (Vitus and Alan) were made using tubes with exterior sizes that mimiced steel bikes. I suspect that part of the reason was so the clamp on components, that were common at the time, would fit. In those days everybody complained about how "noodley" aluminum frames were while marveling at how well they dampened road vibrations. Sometime in the 70's a student at MIT named Gary Klein noticed there was an optimum relationship between tubeing diameter and wall thickness. He built an aluminum bike frame that capitalized on that relationship. It had tubeing diameters that, for those days, looked huge. Since that time there have been many variations of fat tube aluminum bicycle frames. Some have been envisioned as weighing the same as steel but being stronger and stiffer, some are designed to have equal strength to a steel frame but weigh less. To assume that all aluminum frames are going to share a common ride quality strikes me as simple minded.

2. Take computer wheel magnet to a big group rides like one of the disease benefit rides. Check out the frame material of as many bikes as you can until you get bored. How many of them are aluminum? I'm betting about 50%. While it's certainly true that everything has a life cycle I'm thinking that if aluminum bike frames cracked apart as commonly as I've been led to believe, you wouldn't see so many.

009jim

The Young's modulus of steel is 200 and of aluminum is 70. So aluminum has more elasticity. But stiffness is a function of Young's modulus and the modulus of the section properties (second moment of area in the case of flexure). So if the cross section modulus of your aluminum frame is more than 3 times the modulus of your steel frame it will be stiffer. Since the second moment of area is proportional to the diameter to the 4th power, the diameter of the aluminum frame would need to be 3^(0.25) times the diameter of the steel frame, if they were the same thickness.