Bicycle Mechanics - Where does a road frame flex?

Hi all,

I was reading an article about a new Pinarello magnesium :eek: frame sporting a huge bottom backet.

"At 55mm in diameter it's much larger than the standard 40mm and according to Pinarello this makes it stiffer, and smoother rolling......."

Well, it sounds like a good discussion topic and spurs the question:

how is flex _really_ distributed between fork, stays and main triangle?

Somewhere I found a website which listed many brands and materials of frames. The guy had tested the degree of flex in non-riding conditions (on a table, not with an actual rider) in the rear triangle, bottom bracket, and fork. These are the areas where there is the most amount of flex. One interesting thing I found when looking through these data was that it was remarkably inconsistent. That is, some steel frames were more flexy than others, some Al frames were very flexy and some were very stiff, some Ti was more flexy than steel or CF and some Ti was very stiff, etc... So it seems a lot depends not only on the material but the way the tubes were constructed and how the frame was put together as a whole. It seems to me you can make a very stiff frame or a very flexy frame out of any material! The difference may in the ride quality. For example a very stiff steel frame may feel "springy", a very stiff Al frame may be described as very "harsh" by some riders, and I've heard some people describe stiff carbon frames as "dead" and "having no road feel". It's a very personal thing, apparently. Personally, sometimes I like to ride a my (heavy) stiff steel frame and sometimes I like to ride my stiff carbon frame with "no road feel"...

"At 55mm in diameter it's much larger than the standard 40mm and according to Pinarello this makes it stiffer, and smoother rolling......."

[/COLOR]'Stiffer and smoother rolling', sounds like advertizing BS.

how is flex _really_ distributed between fork, stays and main triangle?

This requires the usual answer.............................it depends.

Hi all,

I was reading an article about a new Pinarello magnesium :eek: frame sporting a huge bottom backet.

"At 55mm in diameter it's much larger than the standard 40mm and according to Pinarello this makes it stiffer, and smoother rolling......."

Well, it sounds like a good discussion topic and spurs the question:

how is flex _really_ distributed between fork, stays and main triangle?

the reason that the BB is better is becuase in an attempt to stiffen the bottom bracket spindle from bending forces, they made it oversized, 22mm instead of 17?mm. in doing that, they had to make the bearings smaller, which is not as durable. the larger BB allows them to use standard sixe bearings, and keep the large spindle, so you get someting that is smooth rolling (larger bearings) and stiffer (larger spindle)

that said, I have never experienced crank flex, and have only experienced frame flex on a very bianchi "sport touring" bike with 17" chainstays. If you want to see your frame flex, though, what you can do is clamp each brake tightly, and then stand next to the bike and put your foot on the crank. you'll see the BB/chainstay area flex. this is a rough simulation of the flex forces put on a frame when riding.

I was doing a charity ride once and saw this huge fat guy riding a nice yellow Carbon fiber Lance Armstrong Yellow Jersey Trek replica. I swear, the guy was like 300 pounds. As I came up the road from behind him, I swear I saw his rails on his saddle flex downwards.

Hi all,

I was reading an article about a new Pinarello magnesium :eek: frame sporting a huge bottom backet.

"At 55mm in diameter it's much larger than the standard 40mm and according to Pinarello this makes it stiffer, and smoother rolling......."

Well, it sounds like a good discussion topic and spurs the question:

how is flex _really_ distributed between fork, stays and main triangle?

don't forget the wheels ;)

SAB, got the link anywhere??

Thanks

Sorry, no. Try a google search. That may be where I found it in the first place.

If you want to see your frame flex, though, what you can do is clamp each brake tightly, and then stand next to the bike and put your foot on the crank. you'll see the BB/chainstay area flex. this is a rough simulation of the flex forces put on a frame when riding.


I hate that ****. Everytime I buy a new bike, someone wants to say, "hey, can I ride it?" And I always say yes, but inevitably there's some jerkwad who before or after he gets on puts his foot on the bottom bracket and pushes with all his might trying to see any flex. I hate that ****! Why don't you just jump up and down on the frame to see if it'll bend?! :mad:

that s bad form! if anyone ever put their cleated foot anyplace but the pedals on any ride o mine I'd have to give em a super chamois wedgie bomb

Sorry Rex I do that all the time. Not with all my might, just enough to see how much pressure it takes to just deflect. I place my hands on the head tube and the top of the seat tube and place my heal on the crank arm connection point. I would rather know what to expect on the outset than to drop your new bike in a turn.

Sorry Rex I do that all the time. Not with all my might, just enough to see how much pressure it takes to just deflect. I place my hands on the head tube and the top of the seat tube and place my heal on the crank arm connection point. I would rather know what to expect on the outset than to drop your new bike in a turn.


Don't do that! It's not a good idea and completely unnecessary. :mad:

I hate that ****. Everytime I buy a new bike, someone wants to say, "hey, can I ride it?" And I always say yes, but inevitably there's some jerkwad who before or after he gets on puts his foot on the bottom bracket and pushes with all his might trying to see any flex. I hate that ****! Why don't you just jump up and down on the frame to see if it'll bend?! :mad:

I actually meant to put yur foot on the pedal and then try to pedal. It still produces flex.

SAB, got the link anywhere??

I'm not SAB, but I believe he's referring to Damon Rinard's frame deflection tests...
http://www.sheldonbrown.com/rinard_frametest.html

Thx livngood

A mate tells me "don't buy an aluminium frame, it's way too harsh!". From what I read i'd draw a different conclusion...

Rinard (great read) actually tests lateral flex by fixing the bottom bracket and applying lateral weight to the head tube and to the rear dropouts. He concludes "laterally rigid is good" for handling.

He also says the diamond geometry of the frame makes it very resistant to vertical loads, implying the frame has very little vertical flex.

If the frame is close to "infinitely stiff" on vertical loads regardless of what it's made of ->
the flex felt during normal riding (that "bobbing" feel) comes from fork, wheels/tyres, seat tube and bar deflection. i.e. mostly from the FORK and TYRES

Since in theory aluminium is vertically as stiff as anything else, an aluminium frame isn't "harsh", it's actually the fork, tyres etc.

|:-/ there's something wrong with that

Any thoughts?

I've always believed that fork and wheel flexibility drown out frame flexibililty. For in the saddle riding I can't believe that most folks can create enough torque to flex a frame, unless of course your moniker is 53-11 alltheway :D Now out of the saddle jamming is something else and one can create some pretty large torques. In this case I believe the fork and wheels will dominate the flexing in most, if not all bikes. For ride comfort, it seems again fork and wheels would control the feel and not the frame.
;)

Since in theory aluminium is vertically as stiff as anything else, an aluminium frame isn't "harsh", it's actually the fork, tyres etc.

|:-/ there's something wrong with that

Any thoughts?

The only thing wrong that I can see is that these static tests are ... static! They are interesting, but only riding a bike can really tell you what it feels like. A couple of times in my life I've upgraded a frame but kept all the other components, so I can assure you that changing the frame and forks alone can certainly affect how stiff the bike feels. Different materials will also transfer shock differently, which may be why some people think aluminium feels harsh.

I understand that one of the reasons that compact frames have gained popularity in recent years is because they are stiffer. Carbon forks are commonplace on both aluminium and titanium frames now, so presumably these are no longer a factor?