AeroPod, PowerPod and Newton

I understand that Aero is CDA*Wind Scaling

I have yet to find a good description of "Friction". How is this number derived? The reason why I ask is the friction number seems to have a very large dynamic range pre/post coastdowns.

Friction is the other force that slows you down when you coast on a bike. This can be friction from wheel contact with the ground and the bearings in your wheels. Friction is a constant number, no matter what speed you are going. This is how the iBike separates aero from friction during a coastdown. It looks for a constant number amongst a falling value.

When you do your out and back ride, you determine how much your stem flexes in your normal riding position (it's usually .3 to .7 of a degree of flex). This value is taken away from your total friction and the remainder is your rolling resistance (crr).

The reason why the value changes pre and post coastdown is because we gave default values for aero and friction in the unit. If you didn't do coastdowns the iBike wouldn't be accurate to you. When you do coastdowns, you are telling the unit "this is my aero and friction numbers"

coachboyd wrote:When you do your out and back ride, you determine how much your stem flexes in your normal riding position (it's usually .3 to .7 of a degree of flex). This value is taken away from your total friction and the remainder is your rolling resistance (crr).

This sounds very interesting/helpful, but I don't quite understand. Which value is subtractd from total friction? Thanks.

Here is my wild guess.

You can think of Crr as a tangent, that is, as a grade or slope. For example, a Crr of 0.0062 is equvailent to climbing a 0.62% grade with no rolling resistance.

Converting a riding tilt into a grade involves taking its tangent. For small angles in radians, the tangent of the angle equals the angle, very nearly.

Converting a grade into power at a particular speed is weight (kg) * gravity constant (9.807) * grade * speed.

Looking at the coefficient of speed, we have weight * 9.807 * grade. Total weight of me, my bike and equipment is 93kg. With a Crr of 0.0062 I get 93 * 9.807 * 0.0062 = 5.7.

My fric is 10.2 which includes a combination of Crr and riding tilt according to Coachboyd. Working backwords, (10.2 - 5.7) / (93 * 9.807) = 0.0049, which must be the tangent of my riding tilt. Converting the tangent 0.0049 back to degrees is atan(0.0049)*180/Pi = 0.28 degrees.

So here is my guess:

Fric = Weight (kg) * 9.807 * (Crr + riding tilt), where riding tilt is expressed as a tangent.

I have an iSport so I can't check these calculations to what the software displays. If anyone sees an error, please post.

You're correct that friction is like climbing a hill in the sense that a constant hill slope angle puts a constant amount of opposing force on the rider. Of course, the steeper the hill, the higher the opposing force!

Friction, too, puts a constant amount of opposing force on the cyclist, but with the crucial difference that the frictional opposing force is NOT affected by hill slope.

There are two ways the iBike reports frictional drag: in the Totals screen (FRIC) and in the iBike 3 software (Crr).

The difference between these two numbers is that the Fric drag incorporates not only the effects of tire resistance, bearings, road surfaces, etc., but ALSO accounts for the cyclist's weight and the riding tilt of the iBike. The more negative the riding tilt, the higher the fric drag is, in order to compensate for the effects of the iBike's being "pointed" down hill. Said differently: if the riding tilt is 0.0% and the frictional drag is measured as 8, then if the iBike riding tilt becomes -0.5% (due, for example, to weight shifts caused by the cyclist's position on the bike) then the fric drag will INCREASE, say to 12, in order to offset the effect of the negative riding tilt.

Once a profile is created with the coast down and calibration ride, the riding tilt is measured precisely and the riding tilt effect can be removed. Also, since the weight of the bike and rider is known, that factor can be removed from the frictional drag is well.

Once these two factors are removed from the frictional drag FRIC what remains is Crr: the coefficient of rolling resistance, the number that is commonly used in cycling publications.

mds has done a great job of intuiting the math behind all of this to discover the relationship between Fric and Crr (there might be a minus sign needed somewhere!).

Note that the more negative the riding tilt is, the higher Fric has to be in order to get the correct Crr. Conversely, if riding tilt is POSITIVE, then fric must eventually become negative in order to bring Crr back to "normal". In general the iBike doesn't like negative drag coefficients, so in this situation of excessive positive riding tilt the dreaded "Bad Coast" message will appear because the coast down has determined that the Frictional Drag is negative, a physical impossibility (or, at least, a red flag of a poorly tilt-calibrated iBike!).

John and mds,
Thanks for all the great explanations! And John, the one in other thread on wind cal was equally helpful.
Now I have one question remaining that is related with friction.

My 'normal' position 4mi rides on the bars yield less tilt than riding on the hoods. So experimentally with the gen2, I had come up with -.7 as the best all around tilt for me.

Coach Boyd worked with me and helped me get better cda and crr established after I had a rare very low wind set of CD's and 4mi ride... thanks Coach Boyd. The tilt was -.4.

I did a longer ride with the -.4 and overall avg grade was .04 with same start and stop spot. Then I 'tweeked' a copy of that profile for -.7 tilt which on another longer ride came up at .00 avg grade! Great!!!

When I 'tweeked' the profile, I changed both tilt and fric to keep the same crr and cda in both profiles.
Did I do it right?

Thanks,
Russ

Edited to remove a reported power error that did not exist on reexamination ... Russ