Monday, June 28, 2010

Coefficient of Rolling Resistance (Crr)..what is it?

Coefficient of Rolling Resistance, or Crr for short, is a dimensionless number that measures the resistance or drag that occurs when an object (such as a bike tire) rolls on a flat surface.  The higher the number the more friction or rolling resistance the tire experiences.  Most cyclists really don't pay much attention to rolling resistance when purchasing tires.  They believe if they buy the skinniest/narrowest tires and pump them up to the max that they'll go FASTER- i.e. have less rolling resistance.  That's not true at all.  In fact, wider tires actually have lower rolling resistance than narrow tires.  Really?  Yes, really!  Then why don't racers use wide tires when they race.  Good question.  It's because the wide tires are NOT aerodynamic at higher speeds.  They may be fine for slow speeds, i.e. <10 mph, but they're no good for higher speeds.  Did you know that 80% of the total drag for a cyclist riding less than 10 mph comes from rolling resistance and the remaining 20% from wind drag/resistance?  Yup.  And, these numbers reverse with speeds over 20 mph.  That is, 20% of the total cyclist drag comes from mechanical/rolling resistance and 80% from wind drag/resistance.

Here's a couple other reasons why wider tires actually have lower rolling resistance than narrower tires at slower speeds: a) the contact area is less b) less tire deflection (vertical movement) and c) active area of sidewall is shorter.  This can be illustrated below:

So which bike tires should you choose and what pressure should you pump them up to?  Well, that depends.  It depends on what you're looking for in a tire.  Do you want good wear, puncture resistance, low rolling resistance, comfort, wet weather grip, etc.?  If so, you're like me- you want it all.  But, the bad news is..when it comes to can't have it all.  You've got to choose between performance and durability/wearability/comfort.  It's no different with car tires.

For me, I like the Continental Grand Prix 4000s.  (After all, it's German! haha)  The Continental Grand Prix 4000S Road Tire provides a sticky compound, low rolling resistance, and a long tread life. Black Chili rubber and nanotechnology enhance surface contact in the corners instead of slowing you down with a super-soft compound. Continental also added a layer of Vectran Breaker fabric for puncture protection.  I train on them and I race on them.  I think they have good puncture resistance as well as low rolling resistance.  They're rated to 120 psi but I never pump them up past 110 psi.  Why?  Remember, deflection from the image above?  That's why.  You pump them up too high and you'll be doing more bouncing up/down than you will be travelling forward.  Well, not really but you know what I mean.  You can't feel these little minute vertical deflections or bounces when you ride but they're there and they add up.

Here's a great article I found on the internet that gives a better explanation for the compromises when selecting a bike tire.  I don't know who to credit since there isn't an author but it is REALLY very well written and explains things very nicely.  Read it:  You'll like it!

Power ON!  Coach Rob

Sunday, June 27, 2010

Time Trial Bike Fitting- Part 2 of 2

I've always wondered how anyone who has spent thousands of dollars on their aero position, in a low speed wind tunnel, is able to replicate that position in a race (time trial).  After all, there isn't an LED or audible warning while riding to remind you that you are, or aren't, in that stealthy position you spent so much time & money to obtain.  (BTW, I'm working on that- an indicator of sorts to make the rider aware that they are or aren't in THE position- stay tuned). 

Above left, is a photo of Jason Wood that I had taken in my studio.  Although it was dynamic (he was pedaling), it was NOT race conditions.  Above right, is a photo of Jason that I had taken in a 35k Time Trial on Rt. 29 in NJ.  (There is NO change whatsoever from Studio to Race Day position.  The only thing that Jason changed was the saddle.  You can see that Jason is more forward on his NEW saddle.  That's because his NEW saddle is softer up front and allows him to inch forward without the pain that his old saddle gave him) From the above-right position you can clearly see that Jason's race day shoulder position is more upright.  That is, his shoulder angle is more acute.  Instead of being 105 degrees, like the studio pic on the left, it is closer to 95 degrees on race day.  I like this NEW shoulder angle.  However, what I'm not quite enamored with is the NEW egg-shaped back that he created as a result of this.  Although, having said that, Lance Armstrong has an egg-shaped back in his Time Trial position.  Take a look below and see.

What I do like is the way Jason's helmet topline lies flush with his back.  You've got to believe that this makes a nice laminar flow across his helmet/back in his race-day TT position thus reducing drag. 

The experts, unlike me, can dissect/analyze these photos all they want and throw their 2 cents in on what and what should not be the "optimum" position.  However, what truly matters is the CdA- Coefficient of Drag.  (Provided of course that fit/comfort & power output are not compromised).  I'll be determining that with Jason over the next month or so with my iBike Aero/Power Tap power meter combination. 

I'm not sure how Jason finished in the 35k Time Trial but I'll bet he was top 5 out of 40 or so racers.  And, I think that is pretty damn good for a relatively new Time Trialist riding a used bike that was NOT customized to his body/size.  Jason was also NOT riding with a full disk wheel or skinsuit (which other racers had) which definitely makes ANY rider more stealthy in any length Time Trial...resulting in faster speeds and lower times.  Additionally, Jason doesn't have the thousands of dollars that other racers spent optimizing their aero position in a low speed wind tunnel over the Winter like I definitely know that other racers did.  (Who does have that kind of money to piss away?)  I'm going to try to save Jason thousands of dollars in Wind Tunnel time with my "poor mans wind tunnel"- my iBike Aero/Power Tap combination.  I can't see why I can't.  After all, I can perform testing outside in actual race day conditions on the race course itself.  You can't do that in a wind tunnel.

If I had to guess, I'd say that Jason's optimum position is somewhere smack in-between the Studio pic and Race-day pic.  That can easily be obtained by sliding the seat aft about an inch or so.  By doing that, it will flatten his back more, and put his knee joint more centered over his pedal spindle where it is biomechanically more sound/efficient where it will maximize power least I think so.  We'll see!

Oh, here's a photo of Jason's bud Tom Boonen, another big man on the bike.  Any similarities in TT position?

 Stay tuned..more to come.  Power ON!  Coach Rob 

Friday, June 18, 2010

The need for SPEED!

Wanna go fast on the bike?  Doesn't everyone?  Most cyclists, however, don't realize that 90% of the forces opposing them on a flat road is air/wind resistance.  It's those forces that kill speed. The other 10% is made up of mechanical resistance (chain, wheel bearings, etc.) and rolling resistance (tire rolling friction).  The Drag Coefficient (Cd) is a dimensionless number that is used to quantify the drag or resistance of an object (cyclist + bike) in a fluid environment (air).  Basically, it's how easy air slips around you while your cycling.  If you wear a tight fitting body suit while cycling and air is going to slip around you easily.  You wear a loose fitting shirt while cycling and it becomes harder.  The combination of Drag Coefficient (Cd) and Area (A) is CdA, where A= frontal area of rider + bike.  It is this number (CdA) that some of us spend big $$$ on reducing.  Why?  Because if we reduce this number it will require less power to go a certain speed.  And, you can reduce this number in 1 of 3 ways, either: 1) reduce the Cd of the object 2) reduce the A of the object or 3) reduce both Cd and A. 

Below is a chart of typical Cd and A values for a cyclist on their bike.  Remember, Cd is how easily air slides around you and A is your frontal area..that is, the area of your bike and body that is seeing the air/wind when you ride:

Hand position                    Cd            Frontal Area, A               Cd x A        Power req'd to ride @ 22 mph
Hands on tops                  1.15                0.55 m2                       0.632 m2                         345w
Hands on hoods               1.00                0.40 m2                       0.400 m2                         220w
Hands on drops                0.88                0.36 m2                      0.320 m2                         180w
Aerobars (TT bike)           0.80                0.30 m2                      0.240 m2                         160w

What should really catch your eye in this chart is the fact that you reduce the power requirement over half to ride at 22 mph when going from an upright sitting position with your hands on the tops of your handlebars to a lower horizontal position with your arms/hands in aerobars on a TT bike.  (Don't forget the power required to maintain a certain speed increases exponentially the faster you ride.)

(BTW, please don't email me and contest the values in this chart.  They are just estimates based on Analytical Cycling charts and references I selected over the internet.  It's based on a male, 5'10" tall, medium build, 150 lbs. wearing regular cycling shirt/shorts on a standard road bike and TT bike.  Oh, if you're curious, the Cd of sphere/ball= 0.47 and most high performance German sports cars (i.e. Mercedes Benz, BMW, Porsche, etc.) have a Cd in the 0.2 to 0.3 range.  Notice the Cd of a cyclist on a TT bike is only 0.80?  That's not too slippery when compared to a sports car now is it.)

According to the laws of Physics, CdA= 2 x Fd/ pv**2

Fd= Drag Force usually measured by a Power Meter
p= Density of Air
v**2= Velocity of the bike squared

Therefore, in order to measure your CdA on the bike, all you really need is a Power Meter and a few measuring instruments to measure velocity and air density.  However, it's not that easy to calculate unless you have a velodrome locally that is void of wind and other variables.  Or, you could always take a trip down to the A2 Low Speed Wind Tunnel in North Carolina and spend over $700/hr in a controlled environment to have them do it for you.  (BTW, that doesn't include travel expenses, lodging, meals, etc. either so you're looking at over $1000).  Unless of course, you have a Power Meter and a device such as the iBike Aero like I do.  The iBike iAero is a Power Meter designed by iBike that allows cyclists, for the first time, to see their continuous display of CdA while they're riding.  The iAero compares the Power Meter data to opposing force data and computes drag coefficients continuously, IN REAL TIME!  Is that cool or what?  At least I think so.  I call the iAero a "poor mans wind tunnel".

In the next couple of weeks I will be combining my iAero and Power Tap PM on my road bike and computing CdA values in two positions: on the hoods and in the drops.  I will post that data as soon as I achieve some statistical significance.  Unfortunately, you can't do one calibration ride record the CdA and call it a day.  It's NOT that simple.  Why?  There are just too many variables to control: weather, road conditions, road traffic, etc.  I will, however, post my results in my coaches blog.  When I'm finished with my road bike...I'm going to take Jason Wood and use him as my guinea pig to tweak his current position on his new TT bike.  We have a good baseline since Jason performed a TT the other night on Rt. 29 in NJ and averaged 27 mph for 10 miles..which aint too shabby.  I'm guessing we can dial-in Jason's NEW position and get that average speed turned up a notch to say 28 mph avg.  Who knows, maybe Santa will bring him a new Nike Skinsuit this year so he can reduce his Cd more and perhaps reach 29 mph avg. for 10 miles. 

Power ON! Coach Rob

Sunday, June 13, 2010

Time Trial Bike Fitting- part 1 of 2

Whether you're a triathlete with a "Tri-bike" or a roadie with a "Time Trial bike"- correct positioning on the bike is imperative if you want to go your fastest.  In order to go your must be comfortable, you must be positioned correctly to maximize power output and you must be aerodynamic. All three are just as you want to maximize all three without compromising each other.  Unless of course, you're able to compromise one without having an affect on overall speed. e.g. if you're racing in a short 5-mile Time may want to compromise "comfort" for power and aerodynamic optimization.

This blog is part 1 of 2 and covers correct positioning on the bike for comfort and power maximization.  Part 2 of 2, which will be covered in detail at a later time, will cover aerodynamics.

Before you begin to position your body on your bike properly, you must have the right equipment.  I hate to say this, but if you want to race in a Time a Time Trial (TT) bike.  Don't think you can slap-on a set of "clip-on" aero bars to your road bike and that's good enough.  It will help but in no way does it compare to the advantages of a bike designed for Triathlons or Time Trials.  What's the BIG difference between a road bike frame and a Time Trial frame?  The seat tube angle is steeper on a TT bike.  Why?  Because it's easier to pedal in the lower shoulder (flat back) position demanded for aerodynamic efficiency with the saddle farther forward.

The first thing you want to adjust on your TT bike is your saddle height and your saddle position (fore/aft).  There are many ways/methods to do this.  The easiest way to obtain your correct saddle height is to measure your inseam and multiply it by a particular factor/number such as 0.883.  (BTW, saddle height is measured from the center of your bottom bracket to the top of your saddle, along the seat tube)  However, there are contradicting methods on what factor/number/percentage to multiply this measurement by to obtain your correct seat height.  Another way is to measure your knee flexion with a goniometer.  A goniometer is just a fancy name for a device that measures angles.  Most bike shops use a goniometer.  Since I'm a photographer, I like to photograph the rider (on their bike) while they're pedaling and measure the knee flexion angle on the photo/picture with a protractor.  I find that a dynamic measurement is much more accurate than a static one.  (Just like a dynamic balance of your car tires is superior to a static balance.)  (BTW, you need a fast camera/lens to stop action with a flash indoors.)

Pictured below is a photo of Jason Wood.  I measuerd Jason's knee flexion angle to be 26 degrees.  Any angle less than 25 degrees of knee bend at bottom dead center, the kneecap, the fulcrum for the leg lever, loses contact with the joint, and you can NOT push as hard without the fulcrum.  The result- power loss.  So, Jason is ok with respect to knee flexion angle.  (I believe his recommended saddle height, via calculation, was 33 inches and his actual height was 33.5 inches- so close enough for now.)

Correct seat position (fore/aft) is normally derived by dropping a plumb line from the rider's forward knee when the feet are positioned at 3 and 9 o'clock.  Normally, the plumb should intersect the pedal spindle when the seat is positioned correctly.  However, for an ideal aerodynamic setup, the rider should bring the saddle forward 1-2 inches from the knee-over-pedal standard road position.  You can see in the photo below that Jason's knee is over the pedal in the standard road position.  (BTW, his crank arm length is 180mm which is longer than standard).  Therefore, he could probably move his seat forward an inch or so..or shorten his crank arm length.  Moving the saddle forward also avoids bunching up at the hips when the torso is horizontal- which is good.  Remember, when you move the seat forward you MUST raise the seat to compensate for it.  The general rule is: for every inch forward you move the seat, raise it 1/2 inch.

The next thing you want to check is your handlebar position.  Handlebar position is mostly personal preference.  Keep in mind however, that you want to ride as low and narrow as possible to achieve the best aerodynamic position.  To achieve a horizontal chest, the line from the center of rotation of your hip to the center of rotation of your shoulder should be between 6-8 degrees above horizontal.  This is also called the "torso angle".  You can see in the top photo that Jason has a 6 degree torso angle- which is good.  Jason's hip angle, measured between the lines formed by the center of the knee (when at its highest point in the pedal stroke), the center of rotation of the hip, and the center rotation of the shoulder, is also 35 degrees.  A hip angle of 40 degrees is optimal as it opens the hips more and frees the legs.

Normally, crank arm length is determined by multiplying the riders inseam by 21.6 percent.  However, for Jason who is 6'6" tall and has a 94cm inseam..that would equate to a 200mm crank arm length.  I don't even think they make 200mm crank arm perhaps 180mm crank arms is as good as it's going to get for crank arm length.

For optimal aerodynamics, you want to bring the arms/elbows close together to reduce frontal area.  Ideally, you want your elbows as narrow as your knees.  You also want your arms/elbows within your hips.  You can see from the frontal area view of Jason he has great arm/elbow/knee positioning on the bike.  Jason doesn't know this yet, but when we get on the road for our CdA test, we're going to experiment with his hand position.  I think I'm going to have him raise his arms/hands to see whether that raises or lowers his CdA.

Jason has a relatively flat back in his aero position (see middle photo).  To achieve a flatter back requires rotating the pelvis forward, moving the seat forward, increasing hamstring flexibility, and bringing the belly button closer to the top tube.  With a flat back you want your shoulder angle to be close to 90 degrees.  Jason's is 105 degrees (see top photo).  If we move Jason's seat forward a bit, which I recommend, he'll get closer to that 90 degree mark.

Jason's aero/elbow pads are postioned correctly- somewhere between the end of his elbow and the middle of his forearm.  I also measured Jason's weight distribution on the bike.

Ok, so lets summarize.  I'm recommending that Jason move his seat forward one inch and raise it 1/2 inch.  This will open up his hips a little, improve his shoulder angle and flatten his back.

STAY TUNED for Jason's on-the-road Coefficient of Drag (CdA) test results.  I will be able to measure Jason's continuous CdA with my iBike Aero and Power Tap hub.  I call it the poor man's wind tunnel.  Hey, it's faster/cheaper than driving down to the A2 Low Speed Wind Tunnel in North Carolina.  (Plus, unlike the wind tunnel..I get to experiment on the time trial course I'm racing on.)  This will give me on-the-fly continuous CdA which will allow me to optimize Jason's position even more.  Remember if you want to go the fastest you can, you have to consider COMFORT+POWER+AERODYAMICS.  I just told you how to optimize comfort and power (via biomechanical optimization) blog I'll tell you how to optimize aerodynamics.

Power ON!  Coach Rob