macca123

I am totally mistified when you all talk about flex. Probably because I own a 22 pound aluminium beast I have never felt "flex" before. Does the frame actually bend and push up and down or does it move side-ways, I presume it is more common with the clyde's on a weak bike than in my situation but I am still very curious as to what to look for.

My vivace in my opinion feels TOTALLY solid underneath me. So this must mean my bike is stiff...how do you tell if a bike is "STIFF" or not.

Thx in advance.

Sprocket Man

Too much flex, you can hear the chain rubbing on the front derailleur.

EventServices

Stand next to the bike facing the bike. One hand on the bars; one hand on the saddle.
Put your foot on the pedal (at the bottom of the stroke).
Put sideways and downways pressure on the pedal.
Watch the bottom bracket. It'll move more on some than others.
If you do this to enough bikes, you'll see differences.

When you ride it, if it feels like the bike is absorbing your effort, that's flex.

Some bikes just know how to go forward. Every ounce of your energy is transferred into forward motion. Those are keepers.

In a nutshell, that's the best I can do.

DannoXYZ 

Yeah after a while you get a feel for how much each bike flexes. In addition to the bottom-bracket moving laterally 1-3", the wheels will flex about 1-2" laterally themselves. You can really feel all of this in sprint, especially when you lean over the bike a lot. That's why I try to keep the bike upright so that all of my leg force along with gravity is loading the pedal in the plane of rotation rather than laterally. It's kinda hard to flex a frame longitudinally since that's along the axis of the triangles.

Someone had posted a head-on shot of some guys in a sprint and you can see their wheels and frames all bent up with tubes aiming in different directions. I can't seem to find that photo anywhere...

chuck731

You don't notice the flex because the bike is moving as it is flexing, thus confusing your senses about which movement came from the bike's own movement and which from the deformation of the bike. Put your bike on a stationary trainer that locks down either front or rear whee, now pump out of saddle. You will notice quite a shocking amount of flex even on the stiffest aluminum frames.

ed073

Won't this just show you the wheels flexing when you press on the pedal 'sideways and downways"?

Wheels aren't designed to be stressed in that plane.

jbhowat

Oh man I would like to see that pic. Anyone got it?

ed073

Baden Cooke told a good story on Melb radio about a month ago about his disdain for Dura-Ace chainrings......he was flexing them so much, the chain kept skipping off, but despite numerous complaints, the team didn't do anything.

Sprinting behind the moto in training one day, chain jumped off, he hit the deck, and had to scramble off the road as a lorry locked them up bearing down on him and his mangled bike.

FDJ mechanics then fitted track rings to his bike.

macca123

I would like to see that picture. If you pushed down on the pedal that would just be the tubes of your wheel getting pushed up.

Mentor58

back many years ago, when Bicycling Magazine actually covered bicycling, not buycycling, they had a test rig called 'The Tarantula' that the various bikes they were testing would get clamped into, and the various deflections measured. As I recall, and it's been WAY too many years to swear to it, there was a correlation between ride comfort and flex, but it wasn't an absolute. ie, the 'flexist' bike wasn't the 'most comfortable'. I seem to also recall a rollup they did where they listed all the bikes, the findings from the test sessions, and compared that to their 'real world riding experence'. Like I said, that was quite a while back.

Steve W.

worker4youth

Another way to see bike flex is on the roof rack of the car at 75mph.

Gangrel

or just watch me in front of a mirror


mmmmm flexy

DannoXYZ 

You can tell it's the BB moving sideways because the axles don't move much. You can see the BB moving like 10x more. The axle's movement shows the flex in the wheels while the BB's movement relative to the axles shows frame-flex.


Well, due to the lean of the bike, some of the force is also lateral to the frame, thus flexing it and the wheels sideways. Even with the bike perfectly vertical, some of the force on the pedals will be pushing sideways due to the offset between the pedal and the centerline of the frame. This might actually be a good argument for low Q-factors.

Also if you straddle the front-wheel with your legs facing back you can flex the bars+stem quite a bit. Reach over the bars with your shoulders directly above the drops. Grab one bar-end with each hand, pull up on one and push down on the other. You'll see that the bar-ends can move over 1" in each direction for a 2" range of motion. There's a reason track guys use steel bars...

GitaneRider

No, i tryed this just before and saw no wheel flex. I only saw it flexing from the top of the fork to the drop outs

classic1

Flex?!!

Do your legs look like this?

Sean Kelly, worrying about flex on his whippy ol' Vitus, 1984.

Check out Greg LeMond getting pwn3d!!!!

DamianM

</trolling>

Phatman

I never had a problem with flex in my lemond until I rode a madone w/ 10 speed dura ace. say what you want about the madone, it is a very stiff bike. after I got on my own bike again that day, it felt like a wet noodle.

timmhaan

i can feel it, although i always thought it was my lightweight wheelset doing most of the flexing. i can't really tell how much is frame and how much are wheels.

waltergodefroot

When you do this, make sure the tire pressure is the same in all the tires on all the bikes you do this on, make sure the wheels are built with the same spoke tension and the same crossing pattern on the same rims, just to eliminate all the variables. Best to have your quads calibrated to so you know how much pressure you're putting on the crank.

And people hate on EFBE.

waltergodefroot

My favorite part of this photo was the duct/duck tape around the shoe laces.

RockyMtnMerlin

A LONG time ago I owned and road an aluminum Alan road frame. I am not a strong cyclist, but even under my poor wattage the bottom bracket would sway back and forth like a sail in a storm. Eventualy the aluminum lugs cracked and it had to be retired. I looked cool though. I bought it in Spain in 1981 when my Schwinn was damage in shippment. I wanted a wanted to buy a Vitus frame but the local pro level shop said I was not strong enough or good enough and would not sell me one . So I bought the ALAN. The owner was an ex pro and would brook no dissent. It was the only quality shop with 100 miles. The Alan had cool Zeus 2000 components. All drilled out.
P.S. a good way to see how stiff your frame is when its on a trainer. With the real axle locked in place any side to side movement in the BB area is really noticacle.

SoonerBent

I had one of those whippy Vitus bonded aluminum frames. Now, they flexed. Anything over about 30 was scary, things started getting wobbly.

SS

RockyMtnMerlin

Ha, flexy smlexy. See my post above. Trouble was they (Alan and Vitus) used regular diamater thin walled aluminum tubing.

SoonerBent

RockyMtnMerlin, I never had anything break, thank goodness. I know a lot of them did. Really comfy ride. Then I went to the other extreme and bought one of the original Klein Quantums. Stiff as stiff could be.

SS

RockyMtnMerlin

I went to a steel Basso. Road it for 8 years, then to Cannondale CAAD 6 and then to Merlin. All great bikes.