Stiffness... what does this really mean?

[terrytnt]

Got a question for you techno SMEs, what does stiffness in a frame really mean?

Does it relate to how the frame flexes. Meaning how efficient the drive train transfers foot pedal energy to the wheel? Or does it represent the degree which the frame absorbs road conditions, i.e., stiff frames feel the road (like a BMW) but may not be seen by some as 'comfortable'?

I just build an Ottrott and when I asked the Serotta folk the stiffness I was told the frame was a 2 (1-4 scale) with 6.5 carbon main tubes (which is considered standard), however the Ottrott was designed with the addition of oversized Ti chainstays for added BB stiffness, which they said was rather atypical of a bike with 6.5 tubes.

Does anyone know of an article or information source that explains stiffness in bike frames?

thanks....

[Samster]

bike viagra atmo.

[Samster]

sorry about that last post...

i think it means the bike doesn't swing enough to shift gears on you while pumping hard to the finish (uh oh... here we go...) or uphill.

i've never had a bike that wasn't "stiff" enough. but then again, i ride 51cm frames and weigh in at 128-130 lbs.

[Big Dan]

tough guys and gals ride stiff bikes.....

:bike:

[ecl2k]

http://web.archive.org/web/19970404193744/www.bontrager.com/TheProf/1196/stiff1.htm

[swoop]

do a search on this forum and you'll find plenty o threads... me thinks.

[lemonlaug]

http://www.kirkframeworks.com/Flex.htm

[Too Tall]

Got a question for you techno SMEs, what does stiffness in a frame really mean?

Does it relate to how the frame flexes. Meaning how efficient the drive train transfers foot pedal energy to the wheel? Or does it represent the degree which the frame absorbs road conditions, i.e., stiff frames feel the road (like a BMW) but may not be seen by some as 'comfortable'?

I just build an Ottrott and when I asked the Serotta folk the stiffness I was told the frame was a 2 (1-4 scale) with 6.5 carbon main tubes (which is considered standard), however the Ottrott was designed with the addition of oversized Ti chainstays for added BB stiffness, which they said was rather atypical of a bike with 6.5 tubes.

Does anyone know of an article or information source that explains stiffness in bike frames?

thanks....

A guy like you with such an amazing depth of knowledge can not be without an opinion of what "feels" right and why right? Where are you going with this? If I was you ordering a high end custom bike I'd spend alot of time explaining what I wanted thru example eg. bikes and equipment that made a diff. in the past. Explain this to the builder in this case talk to Kelly direct...he picks up the phone :) How cool is that?

[RPS]

http://web.archive.org/web/19970404193744/www.bontrager.com/TheProf/1196/stiff1.htm
Nicely written and informative article until he blows it at the very end ATMO.

[RPS]

Does it relate to how the frame flexes. Meaning how efficient the drive train transfers foot pedal energy to the wheel? Or does it represent the degree which the frame absorbs road conditions, i.e., stiff frames feel the road (like a BMW) but may not be seen by some as 'comfortable'?
Terry, items can move in six different directions; hence they have six degrees of freedom. When you talk about stiffness, you must specify in which of these six directions you are measuring the stiffness, right?

If you don’t, it is completely meaningless IMO. To say that one bike is stiffer than another is incomplete. It may be stiffer in one direction but more flexible in another. Additionally, we need to specify which part of the bike is being evaluated – head tube, BBKT, seatstays, etc….

To assume that all six degrees of freedom are one and the same and that they affect a bicycle the same will lead to more confusion. IMHO that’s why this subject keeps coming up and why it lends itself to so much misinformation and hype.

[Ahneida Ride]

Explain this to the builder in this case talk to Kelly direct...he picks up the phone :) How cool is that?

I once caught Kelly early in the morning. As a complete newbie at the time
I asked questions like "What size handlebar do i use with my bottom bracket?"

Instead of hanging up, Kelly politely answered all my questions.

[Richard]

"...items can move in six different directions; hence they have six degrees of freedom. When you talk about stiffness, you must specify in which of these six directions..."

Could you explain six directions and how movement limited to these, please?

[J.Greene]

I have a feeling a definition of stiffness won't come out of this thread. I am in agreement with TT. The qualitative descriptions are the best we can do.

JG

[RPS]

"...items can move in six different directions; hence they have six degrees of freedom. When you talk about stiffness, you must specify in which of these six directions..."

Could you explain six directions and how movement limited to these, please?
An item in space can move in X, Y, and Z directions and rotate about each of these axis (roll, pitch and yaw), for a total of six.

Different parts of a traditional bike restrict movement in one direction more than in others. For example, seat stays and chain stays control motion of the rear wheel predominantly in different directions. Once this functional difference is noted, it's easier to understand the benefits and drawbacks of making either stiffer or more flexible.

Same for BBKT, etc.....

[Grant McLean]

One interesting element of Keith's description is the difference between "frame stiffness"
and power transfer via the pedals. Bikes are moved by the pedals.

I've always thought that too much discussion is centered around the frame
at it's mechanical effeciency to convert pedal force into forward motion,
and FAR too little information exsists on how frame stiffness relates to the
rider's ability to generate force on the pedal surface. Is a flexy frame like
running in sand? Does a torsionally stiffer frame allow the rider to generate
a higher force on the pedals?

As keith points out, off axis pedal force vectors decrease the force applied
to the pedals! Also, how does frame torsional stiffness effect the riders ability
to get maximum force to the pedal surface, like peak loads, out of the
saddle, and pulling really hard twisting the bars?

Another place to look is in the pedal circle. How close to 12 o'clock do you start
pushing? Do you push forward, down, and back? Are you pushing down at
the bottom of the stroke? That force angle generates into zero forward motion!

These forces are not that hard to measure. I think there's more to be
gained at the rider/bike interface than between the crank and rear wheel.

-g

[Richard]

OK, thanks. As I understand your description, you can locate any movement through space. For instance, if a bottom bracket swung to the side, and moved backward and down on a (massive) pedal stroke (pretty squishy bike, no?) you would locate it with xyz coordinates further refined by roll, pitch, yaw, correct?

[merckx]

I haven't read Bontrager's essays yet, but I will later today. Does he address potential tire squirm caused by a frame that is torsionally flexible? I've often wondered if the rear tire, for example, squirms a bit against the pavement when the rear triangle torsionally flexes. If this occurs, then I would suggest that tire rolling resistance would increase. Maybe this is why stiffer frames "feel" faster when laying the wood during significant out of the saddle efforts? I donno.

[RPS]

OK, thanks. As I understand your description, you can locate any movement through space. For instance, if a bottom bracket swung to the side, and moved backward and down on a (massive) pedal stroke (pretty squishy bike, no?) you would locate it with xyz coordinates further refined by roll, pitch, yaw, correct?
Richard, if we look at how a frame controls the rear wheel (using that as an example only because it is easier to describe than the BBKT), we can assign which parts at the rear of the frame do the most to limit movement (i.e. – predominantly add stiffness in that direction relative to the main triangle).

Chainstays:

Forward and backwards
Side to side
Yaw
Pitch

Seatstays:

Up and down
Roll

Please note I state predominant as there is “some” secondary effects from the other components. This information should help us visualize what happens when we make the chainstays stiffer, or when the seatstays are made less stiff. All stiffness is not the same just like all flexibility is not the same.

We should also note that stiffness in some of these directions is quite high relative to the applied loads no matter what. For example, almost any chainstay offers a lot of stiffness to control the rear wheel in the forward/backwards direction and pitch (unless you use a disc/drum/hub brake).

[cadence90]

Since we have moved on from "front-end stiffness" ("solved" by bigger head tubes) and "bb stiffness" ("solved" by bigger bb shells and bigger dts) and "power-train stiffness" ("solved" by bigger, shorter cstays) I predict the next big buzzwords will be "top tube stiffness".

The phrase has made several appearances here over the past few days already.

Pretty soon somebody is really going to come up with the perfect bike.... :rolleyes:

[dutri42590]

I have a feeling a definition of stiffness won't come out of this thread. I am in agreement with TT. The qualitative descriptions are the best we can do.

JG

this will be everyones own take on the matter! :argue:
JPR

[soulspinner]

All overrated for most of us ATMO :cool:

[shinomaster]

Stiffness? If you can't measure it it isn't real.

[Richard]

Maybe I'm asking something that can't be answered in a forum, but if you want to describe the way a bike flexes using the rear wheel as a control, wouldn' the movement be controlled first by the rear triangle as you say, but also to a large extent by the down tube connection?. Would rear wheel movement, especially in a high speed turn, transmit a twisting force to the front end or head tube through the down tube? I think that I cannot adequately ask the questions in this venue, but I am curious as to how the various tube control the multiple force inputs. I for one think that stiffness is such an imprecise term as to be meaningless because of the inability to isolate what should be stiff and what should flex to make a bike stable at speed.

[RPS]

Maybe I'm asking something that can't be answered in a forum, but if you want to describe the way a bike flexes using the rear wheel as a control, wouldn' the movement be controlled first by the rear triangle as you say, but also to a large extent by the down tube connection?. Would rear wheel movement, especially in a high speed turn, transmit a twisting force to the front end or head tube through the down tube? I think that I cannot adequately ask the questions in this venue, but I am curious as to how the various tube control the multiple force inputs. I for one think that stiffness is such an imprecise term as to be meaningless because of the inability to isolate what should be stiff and what should flex to make a bike stable at speed.I can’t answer your exact question in this forum – it’s not the right place. Plus it’s too technical (i.e. – boring for most). Maybe others will give it a shot.

If I understand the gist of your question, I’d say that the down tube in torsion is most important in controlling twisting when the rider is out of the saddle and pedaling hard – whether to climb or sprint. That’s why most all bikes have a stiffer DT than top tube or seat tube.

IMO adverse handling comes into play when either wheel is allowed to change direction without rider input (even very little changes can make a bike feel squirrelly). As an example, if flex in the fork allows the front wheel to point in a direction (even if temporarily) that the rider doesn’t input, the result will be poor steering precision.

Anyway, my view has been for some time that the only flex that is significantly advantageous is up and down – but only if the other five degrees of freedom are controlled adequately (i.e. – firmly) so as to not introduce poor handling. Stiffness in other directions adds performance without sacrificing ride quality.

Think of a motorcycle with suspension – whether a cruiser or a racing bike. Although the wheels are allowed to move up and down to improve traction and comfort it doesn’t mean that the wheels are allowed to wonder around at will. Motorcycles have more suspension travel than bicycles and are stable at much higher speed, so these two characteristics are not mutually exclusive as some imply.

[swoop]

you make the bike 'stiff' and then you gotta take a serious look at the wheels, stem, bars, fork... it really is a matter of how the forces are described along the whole thing and that magic functional zone is both simple and slippery...
which is why i think its interesting.

and its also about how you use the thing too.

[shanerpvt]

i thought it was interesting to see what Dave Kirk had to say about flex:

http://www.kirkframeworks.com/Flex.htm


cheers,
shaner

[RPS]

i thought it was interesting to see what Dave Kirk had to say about flex:

http://www.kirkframeworks.com/Flex.htm


cheers,
shaner
Dave makes excellent arguments for the benefits of vertical flex and lateral/torsional stiffness, but I’d like to note that the limitations he sites in these areas should be applied IMO mostly to conventional double-triangle frames made with straight tubes of round cross sections. In my opinion Dave’s own DKS and Terraplane are examples that these limitations don’t have to apply in the same context to alternate designs.

Where I’ll respectfully disagree with Dave is on the benefits of some BBKT flex. There is little or no evidence that the human body is efficient at converting most forms of stored energy into useful work. This subject was discussed some time ago in a different thread in more detail.

The fact that a spring returns energy during part of a cycle doesn’t mean the energy will be converted to propulsion more efficiently that if it hadn’t been stored in the first place. Flexing a bike’s BBKT side-to-side repeatedly all day in theory uses very little energy, but would make the person tired nonetheless. More importantly, this repeated action would not propel the bike forward in the least. While riding a bike, a large portion of the stored energy in every pedal stroke is much more likely to be wasted by creating pedaling inefficiency – we humans don’t recover energy very well.

A different way to visualize how our bodies can “waste” stored energy is by thinking of a person going up a flight of stairs, thereby storing a lot of potential energy by elevating his weight vertically. When the person returns to the bottom, how much of that potential energy is returned in a useful manner? Very little or none, right? If the person goes up and down repeatedly he will get out of breath because of all his exertion even though he is doing next to no net work.

The same occurs if we go up an elevator in a tall building and return down the stairs. The stored energy doesn’t recharge us – it’s simply wasted. The fact that a BBKT will spring back releasing energy doesn’t mean it will be converted to useful work. Some may, but it’s highly doubtful it will do more useful work than what was stored in the first place.

[jl123]

RPS,
I've never ridden with someone, nor heard of any racer, who
rode a flexible bike and became any more or less tired at the
end of any ride- no matter the distance; than riders of stiffer
bikes. But your theory does make sense using the variables
you present. JL

[RPS]

jl123, it’s hard to draw valid conclusions in that manner because even if they all rode identical bikes they would feel differently after a hard ride. I wish it were that easy.

I’ll state again that in my opinion based on experience with many bikes of various stiffness that a flexible bike is a very good thing; provided we are talking only about vertical flexibility which doesn’t affect power generation and power transfer. Lateral and torsional flexibility is useless in my opinion (or should be if other things are done right), which was my point. I don’t subscribe to the theory of “good” lateral flexibility that helps the rider through some form of advantageous spring action.

A major limitation discussing this subject is that most of us have been conditioned to think that stiffness and flexibility are directionless (i.e. – a flexible bike is flexible and a stiff bike is stiff). That is flat out wrong and misleading. Stiffness must be defined by direction.

As long as you limit the discussion to a simple double triangle bike with straight tubes of round cross section, then the easiest way to make a bike ride smoother is to make it less stiff. Unfortunately too many extrapolate from that limitation that it must always be so because that’s the way it has always been.

Then someone throws in alternate designs, non-round tubes, and/or anisotropic materials and suddenly it gets a little more complicated, leading some to fall back to the old premise that a plush riding bike can’t be as fast as a harsh one.

[jl123]

RPS,
I think it is that easy. And you really don't need science
to help you. Just ask someone to ride a multiple of bikes
over a period of several months on the same route. Indeed
if the rider feels and actually is, as energetic after rides,
ride after ride, bike to bike, the simple test though not perfect
is good enough. jl

[RPS]

jl123,

that’s an excellent approach when test samples are available. Once you ride a really nice bike for a few months it’s hard to go back. That’s when you can really tell the difference. Get on the old steed and it’s like how the heck did I ever ride this?

Reminds me of a problem I noticed last night. My old HDTV is on the fritz so I hauled the old Sony low-definition CRT TV out of the spare bedroom and hooked it all up. :crap: :confused: :crap: What is always that bad?

Now I’m looking forward to upgrading to a new 1080P, built-in tuners, larger screen, and flat panel LCD. I’ll have to wait a while, but I trust it will be worth it.

Yeah, there is no going back when you've experienced something really good for more than a few seconds.

[terrytnt]

Special thanks to everyone (especially the infor links)... great stuff!!!