SurlyLaika

when talking about the quality of a frame, what does flexy mean? Might as well talk about the "vertically compliant yet laterally stiff" for the unintiated, as well. Can anyone else think of any other terms that are thrown around but maybe not fully understood?

pdlamb

Flexy: "I want a new bike"

Vertically compliant yet laterally stiff: "I want this one"

Both are marketing lies.

SurlyLaika

That was my inclination but I hear them so often that at least some consumers most associate some quality with the terms. It can't be completely vaporous, right?

bradtx

Albert, Flexy is a pretty generic term that could mean a rider can torque a BB shell enough to cause the chainring to rub the FD, to a tourer it could mean that the camping gear and stuff on the rear rack makes the bike wiggle side to side or to someone who likes to rail turns and the bike feels like it's hinged in the middle. Other examples?

Brad

Robert Foster

Flex can happen in several ways. Stock wheels often are a good example. But what people are trying to describe is a side to side movement that happens is response to torque applied while pedaling or weight shifting when loaded.

Any movement from side to side is a loss of power going into forward movement. How important that is depends on how you ride and what you want in your bike. In the most basic terms at least in road bike terms flex if side to side movement. Damping is up and down movement. Vertical and lateral are all terms related to this most basic concept.

cyccommute 

Here's a nice primer on frame materials.

Usually, a 'flexy' bike is one that does to much of what bradtx said. All bikes flex at the bottom bracket. It's the nature of the beast. Some flex way more than others. If the bike flexes too much, most riders don't like it. The amount of 'flex' depend on the material of the frame, the diameter of the tubes, lengths of tubes, etc. All materials have what is called a modulus which is a measure of their flexibility. The link I provided shows what the modulus for common bike metals is. Basically, steel is the stiffest, titanium is about half as stiff as steel and aluminum a third as stiff as steel.

For example, think of a steel 'tin' can. It just doesn't bend all that easily. Aluminum is a very flexible material...think beer can. However, most aluminum bikes are described as 'stiff' and most steel bikes are described as compliant (flexible). This has to do with the diameter of the tubes more than the materials. Aluminum uses a tube with a huge cross section to stiffen up a flexible material (larger tubes are harder to bend). Steel uses a small cross section tube to cut down on the amount of material needed to build the frame and the frame is a bit more flexible because of the smaller tube. You could make a steel bike that is super stiff if you made it of the same diameter tubes as aluminum. It would have effectively no flex in the frame at all. It would also be heavy.

Vertical compliance is more myth than reality. Sure a long wheel base touring bike flexes more in a downward direction than a short wheel base race bike. But tires influence the vertical compliance more than the bike's material of construction, given comparable bikes.

fishymamba

That's the excuse I have to get new wheels

SurlyLaika

okay, so my Surly CC is sort of flexy because when I stop at a red light on a higher gear and I stand on the pedals to get started, it starts swaying side to side after maybe three cycles? BTW, that's good enough to give me a kick start, then I can start pedaling at a fair cadence.

JonathanGennick 

Flex can also happen in the wheels and the fork, especially in the interface between the two. That's why you see larger, 32mm stanchions on quality forks as opposed to the 28mm or less that you see on cheapie ones. It's why some hubs have gone to larger diameter QR interfaces than the traditional locknut presents. It's why thru-axles are becoming the norm in mountain-biking.

If you lived near me, I'd haul out two mountain-bikes, one having a quick-release fork and the other a thru-axle fork. Then I'd point you at a rooty section of trail, or at a rock garden. You would probably notice then, the difference in deflection as your front tire hit the roots or rocks at speed.

cyccommute 

No, not if by swaying, you mean the whole bike moves out of a vertical line. That's normal for a bicycle balanced on two wheels and has a lot to do with trying to establish that balance from a stop. Moving the bike from side to side while accelerating is even desirable since it allows you to push down on the pedals harder and return your rear foot to the top of the crank faster.

You can see how flexy your frame is (kinda) by standing next to it or over it and pushing down on the pedal while watching the bottom bracket. The bottom bracket will deflect away from the foot that you are pushing down with. Most frames will only deflect a tiny amount. 1/2" to 3/4" would be a lot and would probably indicate a very flexy bike. Old Alan aluminum bikes made of 1" diameter aluminum tubing were probably that flexy. Most bikes are better engineered than that or they go the way of the Alan...extinct.

bradtx

Is the nutted axle on a disk braked bike? Suspension fork flex can also be caused by fork tube diameter, slider overlap and worn bushings. Any fork flex can be compounded negatively if the frame is also flexy.

Brad

SurlyLaika

Oh okay, cool. I love how there are no dumb questions on BF...usually. I heard somebody say that the CC or the BB on the CC, actually is flexy...and I thought that's what I felt when I could keep balance standing on the pedals for 3 seconds, about three pedal rotations. Is that something I can improve with practice. I see some riders, especially fixie riders, standing in their pedals for a lot longer than I can.

rnorris

In a discussion about flex in a given bike frame, it's also useful to consider the strength and weight of the rider. A frame that's too stiff for a reasonably fit rider at 150 lbs. might feel like a wet noodle to a 220 lb. strong pedal masher.

fenny

I didn't think there was much to it, until I went from an '86 Trek 1000 to a Pinarello FP-2.

Now I can't believe how much the '86 squirms when I crank up a hill, and I keep checking the frame looking for some crack I must be missing...

EDIT: You will probably not feel the difference between [your bike] and [the bike you want], the gap between my two bikes is extreme.

cyccommute 

You can stand and pedal for as long as you can keep it up. It takes a bit more energy than sitting but you get great acceleration out of it. I use it for those times when I'm climbing a short hill or sprinting to get up to speed after a stop or to catch someone in front of me.

Watch the way those guys are riding and try to copy it. That's the best way to learn anything on a bicycle. Follow someone else and copy what they are doing. Find some videos of people climbing out of the saddle and watch the rhythm that they use to climb with. Also just go with what feels natural.

clydeosaur

Fenny, I echo your statement. I have a Jamis Satellite (steel frame/carbon fork). Seems more relaxed/touring geometry than anything. When I mash, the bottom jumps around quite a bit (not to mention the fork if I'm out of the seat). My C'dale six seems very stiff in comparison, and more responsive as a result. I don't think the flex is a bad thing, depending on what you likes are in terms of comfort. I'm sure the amount of stiffness or lack of is inherently put there for a reason, comfort and life of the product.

SlimRider

To me, it means that she just took a shower and she's wearing that cherry perfume that I warned her about once before. She knows what it does to me! I can see her curves through that black silk negligee that she wears only on just these special occasions. She smiles with invitation as she moves closer and closer. I hold her thin waist as she runs her insole up my calf past my inner thighs. She looks at me with those big brown eyes of yearning. I kiss her high cheek bones and turn yellow to red. I run my hand through her straight black hair and... Oh! Wait a minute..

You said "F L E X Y"!

Ok! I'm sorry! I thought you said something else!

Cyclaholic

Dear penthouse...

SurlyLaika

I tried it again today. I can do it for longer than I thought especially on flatter roads and I don't care if I sway a little on a quiet residential street. It's just the hills and busier streets that make it tricky. I'll practice doing it more, though.

rogerstg

FWIW, from my experience, low wheel tension often mimics frame flex, or makes it feel much worse. Once, I accidentally "fixed" a flexy frame by tightening the spokes. It was a happy revelation. I've since done the same to a number of other bikes.

pdlamb

You could have done the same by buying a bike with new wheels!

fenny

Good thinking, these are 25-year-old wheels that haven't seen a lot of love lately. I'll give it a try.

rogerstg

The revelation came when I fixed a new bike with new wheels.

FWIW it seems that most mid and low range LBS bikes (~$1600 and less) come with wheels with low spoke tension. I suspect that it's the nature of machine built wheels.

pdlamb

Too true. BUT! those machine built wheels are a great source of cheap parts. If you've got a good LBS with a good mechanic, they can tension the wheel for you. Then you'll have a good wheel at a good price.

rogerstg

Better yet, I do it myself. It's really very easy.