sandy_pangle

(TRUE STORY) Can anyone answer this question:

On a normal lunch ride, I am capable of riding a 10 mile out and back route at an 18 mph average........
The route goes predominately uphill for 5 miles (3% grade).......
The return is of course 5 miles downhill on the same road.......
I average the climbing portion of the route at 13 mph.......
I average the descending portion at 23 mph.......

Here's the question for discussion.......

Throw in a 10 mph headwind on the climbing portion like I had today and the average up the hill is 8 mph or 5 slower than the average.....
The decsending portion with the 10 mph tailwind only increases the speed to 21 mph or 3 faster than the average.....

Why??? Shouldn't it affect the speed the same amount both ways??? I really wasn't that tired on a 10 mile lunch route..... Maybe some of you physics experts here can answer this one!!!!

acape

I think the reason is that less time is spent on the descent/tailwind portion of the ride, when the wind and downhill are helping you. On your ride back, it's shorter because it's downhill and downwind, and therefore there isn't enough time for your average to be increased by the same amount that it's decreased on the uphill and upwind portion.

Sort of like if you are riding a flat out and back route, you will always be slower if there is wind because you spend less time with the wind at your back than you do with the wind in your face.

jschen

I think it's because the numbers aren't correct. If you climb at 13 MPH and descend at 23 MPH, you won't be averaging 18 MPH. Also note that your descent speed today dropped to 21 MPH from the normal 23 MPH.

That said, a wind's effect on speed is different uphill versus downhill. Consider the following extreme. On a 10% grade, a 5 MPH headwind won't slow my sustainable speed down significantly since aerodynamics at my pathetic speed is inconsequential and virtually all the power is going into fighting gravity. (So the aerodynamics is as if I'm going through still air at 10 MPH. Big deal.) A 5 MPH tailwind could conceivably increase my descent speed by about 5 MPH since I'm coasting downhill and limited primarily by aerodynamics.

sandy_pangle

I felt it was because I had a net wind force of +23 mph going up and a net wind force of +13 mph going down when you factor in the bike speed versus wind speed. All things being equal (no wind) I pretty much average a split speed difference, so I don't think it's a time thing.

ovoleg

Ahh a hypothetical debate!!

jschen

If that's the case, you're just making up numbers. If you go out averaging 13 MPH and return averaging 23 MPH, you do not average 18 MPH round trip.

ovoleg

Your analysis of a 10 mile trip is very interesting...


Do you even have a cycling computer or are these numbers again hypothetical?

msheron

It's all about ball bearings! The world has gone to ball bearings! And of course the fitzer valve!

acape

Indeed, I spoke too soon and didn't read carefully enough. I think the numbers have to be off as well.

ovoleg

He's making up numbers...Thats the interesting part!

DrPete 

Ceramic bearings would increase the average speed to 26mph.

But what time did Train B leave New York, and how fast was it traveling?

iab

Don't forget the Quaker State motor oil. Oh yea, and some gauze pads.

sandy_pangle

Mission Accomplished!!!!

I just thought since we're all couped up at work....we should start a lively discussion on a totally meaningless subject! I'm just glad I didn't get hit!!!

2Tired2Shift

Even if his numbers are off his hunch is correct - we'll never get back from the physical system all the kinetic energy we put in. In one word: Thermodynamics.

Flak

On the way down you're already going faster than the wind, so it doesnt push you.

DrPete 

Just goes to show that we should all ride frictionless bikes in a vacuum. Simplifies the calculations.

Psimet2001 

.....*sigh*...I already posted this once today, but here it goes...

"Ah, come on guys... It's so simple. Maybe you need a refresher course.
Hey! It's all ball bearings nowadays...

Now, you prepare that Fetzer valve with some, uh, 3-in-1 oil and some gauze pads,
and I'm gonna need about 10 quarts of antifreeze... preferably Prestone...

No... No, make that Quaker State... And wash those windows. They got filth and muck on them. "


As for speed...remember that speed and effort are non-linear...regardless of whether or not the numbers are made up.

Psimet2001 

Hey! My whole college career was based on the world being frictionless.....well until we started covering all of the friction and flow calcualtions....damn that Mach dude....

DrPete 

The frictionless world is where my knowledge of physics remains.

Snicklefritz

the 3 laws of thermodynamics:
(1) You can't win
(2) You can't break even
and
(3) You can't get out of the game

Psimet2001 

ARrrrrrrrrrrrrrgggggggh! You beat me to it! Easiest laws to remember...PITA to apply though.



Diesel was a wheel sucker.

Cromulent

We're all perfect spheres in a vacuum.

Okay... mostly perfect.

Cromulent

But you must play the game.

my58vw

Oh no physics debate

You average speed is based on distance vs time... if you are going 5 miles at 13 MPH = 23 minutes, 5 miles at 13 MPH = minutes --- total 10 miles at 36 minutes = 16.7 MPH avg - not 18 MPH avg.

Now speaking of head and tailwinds... the force is always there, regardless if you are climbing or not... you can not "outrun" the wind! When climbing it is true that most of the force is gravity, but their will be a small force of air friction pushing back, and it will make you slower (maybe .5 MPH, or less. but the math is very complicated). On the way down the tail wind also increases your speed (very much like someone is pushing your back), once again compared to the force of pushing through the air on the way down (wind resistance) it will make you a bit faster... maybe .3 MPH...

Now in this scinerio you avg speed will be a bit lower - why? - it takes longer to climb and shorter to go down, but because climbing takes significantly longer, your total ride time will be longer. If the wind speed was the other way, you might be a bit faster - having someone push you up the hill...

This is why averages mean very little - instantanious speed and average per pertion of a ride is more important!

chrisvu05

NOOOOOO!!! I can't escape...damn thermo!!!