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Frame flex? Can you tell??(20 posts)

Frame flex? Can you tell??VictorChan
Nov 2, 2001 10:20 AM
Some interesting topic. How do you tell when a frame is flexing? I heard such a term throwing around here and there. From what I understand thur if your chain is rubbing your FD during high torque or gear shifting automatically or you are feeling the bike is loosing control at high speed (this can be caused by the fork and headset installations too), your frame flexes.
Yes, yes...Kristin
Nov 2, 2001 10:27 AM
This was one subject to which I was most doubtful! I thouhgt, "This is metal tubing. I can't possibly bend metal. Of course my frame doens't flex."

Flex test:
(I only know about steel--don't blame me if this doesn't work for Al frames)
While riding, unclip on one side of the bike (whichever won't make you fall over) and carefully rest that foot on the chainstay. Be careful to not jam your heal into the spokes (ouch). Keep pedaling with the other leg. You'll feel the flex!
Yes, yes...VictorChan
Nov 2, 2001 10:30 AM
Will give that a try. :-)
Yes, yes...badabill
Nov 2, 2001 10:37 AM
Much of the perceived "flex" felt by most people is in the wheels,fork, and bars. A frame does flex but what many people think is frame flex has more to do with other components IMHO..
Nov 2, 2001 10:49 AM
You're saying that the flex I feel (perceive) from the chainstay is not "frame" flex, but something else? I tend to ride with my foot resting on the stay when my ITBS flairs up. I was suprised at how much movement I felt.
Nov 2, 2001 11:05 AM
A frame does flex, and the chainstay movement you feel is Flex between the CS and BB, But the "flexy" feel many people atribute to frame flex is the overall ride quality of a bike. Wheels, fork, bars, and even seatpost all add to this. Case in point, a friend who rides a litespeed tuscany was allways complaining of frame flex. His bike was outfitted with mavic heliums and prima 199 bars. He changed to ksyriums and easton carbon bars and frame flex is gone. Ride quality is effected by the whole package IMHO
Humour me..Jofa
Nov 2, 2001 12:23 PM
I'm afraid I can't quite visualise this scenario. You are pedalling with your right foot, and your left foot is on your c/stay. Through you left foot you are reporting flex: relative to what? Is your leg bending detectably? Also- I am sorry- I may have missed some points: what is ITBS?

Nov 2, 2001 1:17 PM
* Yes, pedalling w/Right foot
* Yes, Left foot is resting on Right c/stay (kidding) Left c/stay
* And Yes, the c/stay moves around. My frame is Columbus Steel. I'm not sure what grade that is.
* No, I'm not flexing my left knee while its on the c/stay. This is actually the bike.

(BTW--I'm no tiny, petite little thing, so I can flex a frame--I'll never tell my true weight though :-) Try my little test sometime and see for yourself!

ITBS = Illio-Tibial Band Syndrome. It feels like a stabbing pain at the bottom outside of the knee. (Or like someone wrapped a rubberband tightly around the muscle.) Not very pleasant, and its difficult to heal. Avoid this injury.
Nov 2, 2001 2:13 PM
Whatever your petiteness, slight or not, -though I'll freely believe the former- I don't think you bend the bike as much as you think. I don't know what you mean by 'moves around', but I'v never seen a bike frame bend much in its rear triangle under the pedalling forces generated by big bluff guys: though the BB does move relative to the front end under the most svelte pedallers like myself. The left crank hangs pretty stationary under this condition however.

The medical condition you describe rings a bell, both in recent posts to this forum and my own experience: I've had similar symptoms but so far not this one. Currently I'm entertaining a visitor to my left Achilles tendon. He is intermittent and will hopefully get bored. In any case I hope your experience has resolved itself.

off the bike flex testTig
Nov 2, 2001 11:17 AM
Here's an old test that is usually seen in a bike shop showroom. Stand facing the side of the bike, holding the bar and saddle. Place a foot on the pedal which is at 6 O'clock/bottom. Step down on the pedal hard enough to see the flex. You can feel and actually see the flex. Big 200+ pound guys need to do this gently! Look closer or watch while someone else is doing it and you will see that most of the flex is in the components. Noodle bikes will flex in the frame as well as the wheels and other components. Super stiff frames will be easier to see the component flex.

My old Tommasini had much more flex that the old Cannondale crit frame. Changing to stiffer wheels made only a little difference. Oh, but the ride was magic!
good test for BB noise also (NM)badabill
Nov 2, 2001 11:50 AM
Yes, yes...VictorChan
Nov 2, 2001 11:13 AM
It does move. I put my left foot on the chainstay, all the way to the rear dropout and the quick release. On the down stroke, my left foot felt that the chainstay was going away (down) from me. When I stopped pedaling, it flexed back to the original position.. No chain rub here in the FD though.
oops, may notVictorChan
Nov 2, 2001 3:10 PM
Well, the method is not really accurate. The "flex" I felt could be caused by the movement of my pedaling and how my weight being shifted from the saddle to the leg.
re: Frame flex? Can you tell??morey
Nov 2, 2001 10:30 AM
Gear changes automatic = Frame flex
re: Frame flex? Can you tell??PbOkole
Nov 2, 2001 10:54 AM
It has been my experience that auto-shifting happened a lot more often before SIS/STI. When friction shifting was de rigeur, frame flex would eventually pull the cable enough to move the shift lever to the point where it would shift. Today, you would have to have a seriously flexing frame to overcome the indexing systems in use. For this reason, frame flex is possibly less consequential than it was in the friction shifting days.
re: Frame flex? Can you tell??morey
Nov 2, 2001 10:56 AM
This is quite true, it happened primarily in a friction shifter bike!
chain suck, toomr_spin
Nov 2, 2001 1:30 PM
but chain suck can also have other causes.
also...heavy weight
Nov 2, 2001 11:08 AM
I had a Ti mtb that would get serious chain rub when I would pedal hard. I thought the Ti was too flexy to me until I realized that it may be the bottom bracket. I switched to a BB with a shorter spindle and the chain rub was gone.
Little Secretgrzy
Nov 2, 2001 4:31 PM
All frames flex to some degree and this is the critical point. Can you tell the difference between two bikes and how does one make an objective measurement outside of the lab and given that components are different? Most believe they can. Another consideration is given that everything flexes somewhat, and that nothing is infinitely stiff, what is the response like. Does the frame come back quickly or slowly? A bike with zero flex, though not really possible, would be an uncomfortable and harsh ride (think about why tires have air in them). Some of the things like FD rub is often caused by the crank/BB/CR movement. I did have an old Motobecane that I could make shift by really jumping on it. I often get a little reminder about flex and compliance when I lock out the front and rear shocks on my MTB - even the tire pressure makes a difference. It's a matter of finding a ride that feels right to you - that may be "stiff" or it may be "flexy".
Tarantulanee Spoke Wrench
Nov 3, 2001 5:37 AM
Way back in the olden days, Bicycling Magazine actually had content. One of the things they did was to have Gary Klein build a fixture for them that used dial indicators to accurately measure frame flex under a controlled load. It was called the Tarantula.

Unfortunately, this kind of data while extreamly interesting to a few, is just a number unless you can somehow relate it to riding experience. What they found was that subjective perceptions of frame flex by riders were more closely related to front end geometry than to bottom bracket stability.

This is the second time this week that I have referred to an article from the Bicycling Magazine of the mid 70's. I must be turning into a geezer because I wish it was still like that. The latest issue has a feature article about disc brakes that is the equivlant of steel is too heavy, aluminum is too stiff and titanium is too flexy. We deserve better.