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Variables Affecting How Fast We Ride

Discussion on bikes, and whatever...

Variables Affecting How Fast We Ride

Postby Fourthbook » Tue Nov 01, 2011 8:38 pm

With so much time/$$$/attention given to reducing bike weight and streamlining it for aerodynamics, I’d like to better understand the relative role of the various factors that affect the work/effort necessary to propel a bike. Perhaps someone knows the equation and can provide general/typical values for the primary coefficients so we know where best to concentrate our efforts (other than on personal fitness). For example, I assume the equation must be something like the following:

Work/effort = Speed x Mass x Friction x Drag x Incline, where

S = speed the bike is moving
M = weight of the bike and rider (would the equation differentiate ‘static‘ weight and ‘rotational/dynamic' weight?)
F = friction created by contact between moving parts
D = aerodynamic resistance (drag)
I = slope of the road

Intuition/experience suggests that F is pretty inconsequential relative to the other factors given the ready availability of good bearings and high pressure tires. I is beyond our control. If there’s little difference between static and dynamic weight, then losing personal body weight is probably far more effective for all but those few athletes who have ideal body weight as spending BIG $$$ to shave off a few grams on bike components. Is reducing a pound of body fat as effective as spending $$$thousands to drop a pound off the weight of the bike?

If any of this is generally on-target, then commonsense suggests that most of us should focus on 2 variables: lose body weight and improve bike/rider aerodynamics.

Ok you engineers and physicists, what’s the math underlying the motion of a bike?
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Re: Variables Affecting How Fast We Ride

Postby Ypsylon » Tue Nov 01, 2011 9:13 pm

You can go to analytic cycling and play around with their calculators to get a pretty good feeling of what matters how much.

I don't know if I agree that bearings and rolling resistance are something that one shouldn't look at, if that's what you're suggesting.

And losing a pound of your system is losting a pound of your system on the demand side, but remember that there's a supply side as well.

Losing a pound of body fat could cost you more than you gain.

The same is true for aero. A very aero position could restrict your breathing so much that it makes you slower.

A very deep wheel could make handling so tricky that it slows you down, even though technically it's more aerodynamic.

At the end of the day, faster is faster.
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Re: Variables Affecting How Fast We Ride

Postby j0m » Tue Nov 01, 2011 11:11 pm

It was a long time ago since I left University so I won't even try to explain. As Andy said just go to http://www.analyticcycling.com. Actually I pretty much agree with all that Andy said.

While you can't neglect it the power to overcome rolling resistance is a a linear function of speed. The power to overcome wind resistance relates to the cube of the speed.

Not what you asked for but still pretty interesting I think. The body doesn't respond linearly to power produced. Blood lactate ~ power^4. So the relationship between blood lactate and speed gets pretty horrible. :D Anyhow the ^4-relationship means that there's a very fine line between being comfortable and being close to death. Which among other things explain why you have to be a good deal stronger to ride someone off your wheel.
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Re: Variables Affecting How Fast We Ride

Postby pritchet74 » Wed Nov 02, 2011 8:42 pm

All you really need to know is that aerodynamics is everything. And more power. You can always use more power. Rolling resistance is important too - so use good tires.

Oh ya, ceramic bearings are worthless.
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Re: Variables Affecting How Fast We Ride

Postby Fourthbook » Fri Nov 04, 2011 3:04 pm

Thanks all for the discussion in this thread and my wheel weight/aerodynamics thread. I explored the Analytic Cycling site as recommended, altho for my general, non-racer-related questions it's almost TMI. But having read all the pertinent threads I can find here and on other forums, here's what I've concluded as an avid, middle-aged recreational rider who has some (but not unlimited)disposable income to spend on bike toys. FWIW, John's 5 golden riding rules:

#1: The purpose of biking is personal refreshment, fitness and fun, albeit with modest challenges/goals. Keep it that way.

#2: There are no universal truths regarding THE BEST frame, wheels, components, etc. There are lots of great options in all categories, whether top-end off-the-shelf, mass produced parts, or small, boutique custom parts. Stop trying to split hairs. So, shop carefully and then buy products you believe in, that please you, and enhance your riding experience. Remember, what makes you happy and adds to your riding pleasure is what's best. And style is personal/subjective: don't look to others to tell you what color, paint/finish scheme, etc. is best... express your taste in building-out your ride. If others approve of your stylistic choices, great. If they don't, SCREW-'EM! And GET A BIKE AND SET-UP THAT FITS!

#3: Lighter weight is generally better, however don't get insane over mere grams. (I know that violates a common paranoia that I long suffered from; but I suspect many of the forum readers are like me: outsiders seeking general advice/wisdom from the experience of the 'true believers'). Don't risk safety and dependability/performance for uber-light, off-brand carbon fiber stuff. The BIG $$$, super light stuff won't make an appreciable difference in performance. For we non-elite athletes, dropping a few pounds of excess body fat and improving our cardio-vascular conditioning, especially our 'red-line' limit at our aerobic/anaerobic threshold, yields better results.

#4: Aerodynamics trump weight. And by far the biggest aerodynamic factor is the rider: use the drops and wear appropriate kit. Wheels come next. After that, the aero benefit of various components is trivial for all but the most advanced racers/TTs. And, lastly,

#5: While chosing the best quality components you can afford, be sure to use really good tires and bearings as they have a measurable affect on rolling resistence and friction. No need for ceramic bearings; high quality steel is just fine. However, if you use ceramic, get top-end ones w/the best seals and proper lubrication or don't bother.

Again, these ideas apply to we mere mortal riders... Any suggestions from you super-humans?
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