Tuesday, December 04, 2012

Brief rant, brief recipe

Ok, ok, you'll get a recipe in a minute. First, a brief rant:

This is a common misunderstanding: a frame with a "stiff" (meaning, doesn't move side to side much when you're sprinting up a hill) bottom bracket is faster/more efficient/better for racing.

This is a bunch of crap, as anyone who has taken high school physics can tell you. 
-To make your bb move side to side, there has to be a force applied that makes it move that way.
-You generate this force when you pedal, because you're not a robot and you can't generate all your power in exactly the direction it needs to go to move the pedals. If you could somehow generate no side to side forces while pedaling, you wouldn't see the BB move side to side no matter how flimsy your frame was.
-If you have a very stiff frame, the bb will not deflect to the side as much, but that does not mean that any extra energy goes into propelling the bike forward. The force still goes into flexing the frame/bb sideways - it's just that since the frame is stiffer, it resists that force better. The end result in terms of watts to the ground is the same regardless of BB flex. 

Put another way: you cannot make your bike go by pushing sideways on the bottom bracket - it won't work if the BB hardly moves, and it won't work if the BB moves a lot.

That's not to say that super flexy is a good thing, but if you are buying a stiff bike because you think it's somehow going to make you faster/more efficient, you are throwing your money away. 

Now! Recipe:

Waltworks Peanut Satay

Put all of these in a pot:
-1 can coconut milk (NOT light!)
-1/4c red or green Mae Ploy curry paste (cut back on this for the milquetoast relatives from Iowa)
-1/2c peanut butter (sugary skippy type)
-1/4c sugar (you can get away with none if you want - taste test first and see if you want to add any)
-1/2 tsp salt
-2 tbsp white vinegar

Bring it all to a boil, simmer for a minute or two, and you've got some kickass peanut sauce for dipper or pouring directly into your gullet.


John said...

Even if your legs are theoretically pushing perfect circles with only downward vertical forces applied to your pedals there is still a lever arm that is roughly equal to the distance from each pedal to the lateral center of your frame. This creates a moment in the lateral plane that will cause deflection in a bike frame(i.e. you don't need to push sideways to create lateral frame flex). W=f*d , A force does work when it results in movement. P=W/t , Power is equal to the change in work divided by the change in time. Unnecessary frame movement=unnecessary use of power. Love ya Walt, but I disagree. You agree that super flexy is not a good thing, what is your reason? Is there a benefit to lateral movement of a bike frame?

Walt said...

Again, vectors. If you've got a lever doing the work to push sideways, that makes no difference. You are pushing sideways, and having the frame resist that force does nothing to propel you forward.

So you are sort of half right - because your pedal is off to the side, you're generating some lateral force. But that still doesn't mean that a stiffer frame can somehow take that lateral force and turn it into forward movement. What you see at the BB is the result of you pushing - the energy is gone either way.

mike said...

Stiff feels good! Ha!

Super excited to make some Peanut sauce! Thanks

Feldy said...

I suspect you're both partially wrong and partially right. (Not Mike, that is, I'm sure the peanut sauce is wonderful) My guess is that most people have a tendency to pedal sideways a bit. Having a rigid frame helps keep you pedaling in the right plane. Human bodies are complicated and really hard to model.

To Walt's point, you can be exerting yourself and doing no work (the stated physics definition of work) -- see isometric exercises. But I think John has a point, too, as I attempted to explain in the previous paragraph.

js said...

thanks for the recipe professa' now just recommend me some books!

maybe the chicken will soothe my aching joints. just too much stiffness. ;)

Lance said...

Stiffer not better--she said, never.

Anonymous said...

I disagree too, I'd brush up on your physics. Downward force on the pedal is going into twisting the frame instead of driving the sprocket.

BigHank53 said...

My goodness, how quickly people have forgotten the Alan and Vitus frames that many a race were won upon...and those early titanium frames where a strong rider could get both chainstays to rub on the tire.

There's a force orthagonal to the drivetrain generated by pedal forces. It's inescapable. The force is there no matter if the frame is made of out paper-mache or reinforced concrete. And having the frame bend or not bend doesn't make any difference at all: there's no way that force can be recovered and put to use moving you forwards, because it's acting at right angles to the drivetrain.

Back when I sold bikes, it was easy to get someone onto a Cannondale or a Klein: send 'em out into the parking lot and ask 'em to notice how the bike just squirted forwards when they mashed the pedals. The bikes sure feel different. But the stiffer frame isn't really faster, and it's usually a damn sight less comfortable, too.

Now, you can get too noodly, and things like ghost shifting and the chain falling off start to happen. But if the power losses were as high as people like to think they are, a flexy steel frame would be hot to the touch at the top of a big climb. They're not, so therefore they're not absorbing power.

DanH said...

I think there are scientific justifications for both perspectives. I think we're somewhat close to the situation of a forced harmonic oscillator, where the forcing frequency (pedaling) is much lower than the natural (resonant) frequency of the frame (ie if you flex the frame it will vibrate at a much much higher frequency than the 2-3 Hz at which you can pedal in a sprint). Here's what happens in that circumstance: after a somewhat chaotic looking start to the sprint (frame moving out of sync with rider's pedaling), the frame settles down to oscillate at the forcing frequency with stable amplitude. The amplitude of the oscillation is lower for a stiffer frame, and the energy stored in the oscillations is then lower also (proportional to force * displacement). But the power being absorbed is zero in both cases because there is no damping in the system. The questions then become, how damped are the actual systems, and how significant is the energy that became stored in the frame oscillations rather than in forward movement? My sense is that with a highly elastic frame material like steel or titanium, the damping is very low and can be ignored (so frame stiffness doesn't matter once you are up to speed and the oscillations become steady). But the less-stiff frame will have an increased duration and amplitude of the initial out-of-sync response to the start of a new effort (while the oscillation energy is being stored)... I'm guessing this is what leads to a stiffer frame "feeling" faster/better when you first launch your sprint.