joejack951

I am curious about that, too. I would think that any LBS that can fix a frame stiffness issue is one that you'd want to keep using.

StanSeven

A video of someone doing sounds good; especially when they lose their balance and fall over

rpenmanparker 

So when you see the BB move in response to your force, how much of that is compression of the tires? What BS!

BoSoxYacht

You realize this "flex test" is a bunch of crap, right? It's nearly as useless as testing your wheels stiffness by pulling the rim toward the frame with your hand.

3alarmer 

...well, IME the ugly sister is more likely to put out.

PepeM

How does one do a wheel 'spin up' test?

BoSoxYacht

IME, they all put out.
YMMV

McBTC

In my experience, given the same material and geo, the bigger frames are more flexible (compliant) so maybe that's just what I became accustomed to and came to appreciate, beginning with steel frame bikes, at a time that probably was way before engineers began to weigh in and make more of a science out of it, which probably started with the use of aluminum and bloomed with the use of CF.

That said, I've never had a road bike that didn't kill my arse and that's always going to be an issue for me no matter how many saddles I try (what I'm using now seems to be okay although it also seems to be harder than a rock). Here's an interesting perspective from Felt's '14 catalog as it seems to use terms like flexibility and compliance in different ways:

3alarmer 

.
...if you are genuinely suffering on a bicycle cycle from what you think is "stiffness", and the accompanying transmission of road shock, far and away your best bet is to calculate the largest tyres you can fit to your bicycle given the limitations of clearance, and to use them. I still have one of the olde Cannondale touring bikes, that I ride around from time to time. The only way I can do so is by running very large tyres.......I'd run even bigger ones if the frame stay clearance in the back would permit it.

But really, any comment you see from me with regard to lateral stiffness and vertical compliance comes straight from the fact that every bicycle review in the last 20 years has noted the "remarkable lateral stiffness and vertical compliance" of the product. It's like the words "vote for me" in a political speech. You expect to hear them, but they still make you smile.

hooCycles

The stiffness of a frame is a very elusive number. First, you have to decide to what kind of load you are going to apply i.e. torsion, axial load, bending moment, combination, etc.. Then you have to decide which direction this load is being applied. Then you have to decide how you are measuring the stiffness i.e. where is the node of interest whose displacement is important to you. Only then can you say how stiff a frame is. For example, I could say that my frame has stiffness xx.xx force/length when measuring deflection at the junction of the top tube and seatpost when applying a point force on the top of the saddle oriented down the seatpost. Of course, this number would mean nothing because this is a terrible model of how a bike is ridden. The act of riding a bike will always involve a complex loading scenario acting on fairly complex geometry in fairly complex directions. The bike manufacturers likely use FEA solvers to quantify some benchmarks.

fietsbob

Put Your foot against the center of the crank and PUSH, Bike Sideways.

Left side is fine ..

rpenmanparker 

And the tires don't absorb most of the force and get compressed to the side?

OldsCOOL

The problem of the chainline of the smallring being too close to grinding on the chainstay. After they shimmed the BB it was moved outboard so I could mash with no grind.

rpenmanparker 

That's what I thought you meant. The flex is still there, just not the fouling that it was causing.

OldsCOOL

Right. And as you asked/stated, the tire and wheel has enough lateral flex to render that test as frame flex invalid (in my opinion). I winced when that kid did that to my bike.

pdedes

doesn't bicycling magazine have a "tarantula frame tester"?

wphamilton

That's interesting and I've kind of thought that my Nashbar frame is pretty stiff, but how do you know that it's the frame that's flexing and not the wheels? And tires?

Carbonfiberboy 

It's actually pretty obvious. It's easy to see the down tube flex over the wheel upper edge, a little less obvious in the rear triangle, but still . . . flex a few bikes and you'll see. Of course there's some mushiness from the tires, so don't pay too much attention to the fact that the bike moves away from you. Just watch things change in relation to each other.

BoSoxYacht

I haven't seen them use that contraption in at least a decade. They probably finally came to the conclusion that their findings were irrelevant.

McBTC

Interestingly enough, as I understand it, 'stiffness' has to do with how far something can be flexed before it breaks or is permanently deformed because it is no longer able to return to it's original state. So, when you look at it in that way, flexing the frame as the kid did wouldn't cause any permanent deformation because in the range we're talking about, the stiffness of the bike's frame (overall at least and not necessarily at the joints) is more like a compliant rubber band than a stiff and brittle toothpick.

wphamilton

I dunno, I stomped on a couple of my bikes like that a while back, and there was so much from the wheels and tires that I couldn't tell much from the frame. I figured I really needed to clamp the frame at the dropouts, but at that point I wasn't curious enough to mess with it further.

Campag4life

Sorry your thinking is wrong. Stiffness is unrelated to how far aka displacement something can be flexed before it breaks.

Like all stiffness threads, the nay sayers always come out and say a flexible frame is as fast as on that is laterally stiff which is nonsense and the reason every single bike company tries to make their race bikes as laterally stiff as possible. There is no exception...thousands of engineers that make up the bike industry are in uniform agreement. All anybody has to do is climb a hill on a SL2 Roubaix and then a SL4 Roubaix which climbs like a Tarmac...night and day...same 2D geometry.

People tend to think of a frame as one that stores energy consistently. Powering a bicycle at 20 mph is the integral of hundreds of small accelerations making up the pedal stroke comprised of power zone and dead zone. Anybody who has ever ridden an uber stiff race bike knows they out accelerate a whippy bike.

McBTC

The engineers may wish to weigh-in on the relatedness or lack thereof but it seems to me -- other than the fact that something that is stiff or rigid is less flexible or pliable and vice-versa -- when it comes to the stiffness of a manufactured object, we must also look to the geometry involved. For example, when used as a support beam a wooden 4x2 may be a stiffer than a 2x4.

rpenmanparker 

Wait, I thought multiplication of integers was commutative. No?

McBTC

...sure, piece of cake but, not with pie!