WonderMonkey

Agreed!

Walter S

Anybody that says conserve on the uphill and put the effort into the downhill has it totally backwards.

If you're going fast (i.e. downhill) then more effort gives you only a marginal gain in speed because the effect of wind resistance above about 10 mph becomes more and more exponential. But when you're going slow (i.e. uphill) more effort gives you what's close to a proportionate increase in speed.

Downhill, double the effort and you might go 25% faster. Uphill, double the effort and you might go 90% faster. The specific gain/drag depends on the exact speeds and inclines involved. But this effect is undeniable.

These concepts relate directly to my training as an airplane pilot where we approached this a little more formally. At less than supersonic speeds, there are primarily only two components in wind drag. These are known as Parasite Drag and Form Drag (for airplanes there is also Induced Drag but that is not relevant to bicycles).

The form drag is caused by the compression of air molecules in front of a moving object. The object pushes the molecules directly in front of the object, those molecules then push on the molecules in front of them, and so on. So the object is not just pushing the air out of the way, the air in front of that air is getting pushed too, and so on to a diminishing extent as you move out in front of the object. The drag is caused by the energy required to compress the air in front of you. The faster you go, the more ALL the air in front of you is asking to be so-compressed before it gets out of your way. This gives the form drag an exponential effect, where the drag increases with the square of velocity.

Then there's parasite drag. Parasite drag is basically the friction of the air molecules as an object moves thru the air. Parasite drag is all over the object, not just in front of it. Parasite drag is proportional to the speed of the object.

So both of these types of drag are in play. But because the form drag is exponential, it becomes a stronger and stronger component of the drag as speed goes up. At low speeds, the parasite drag is much more significant. But the faster you go, the more the drag will seem to be only exponential in nature.

There's more slowing you down than just air resistance. So overall drag is not increasing as the square of speed necessarily. That's just what the form drag component of your overall drag is doing. But the faster you go, the less significant these other components become because they increase linearly instead of as the square of speed.

jrickards

Well, in that case, I'm going supersonic!!!

Thanks for the explanation!!

mgw4jc

Maybe I missed someone else suggesting it, but for real speed, you should also consider one of those four-wheeled behemoth things with an engine. An automa-something-or-other. I see them when I'm out riding. They are much faster than most bikes.

achoo

Yep.

But you're not going to go 25% faster going downhill by doubling your power because you're already getting a lot of power from converting potential energy to kinetic energy. So when you double your input from say, 100W to 200W, the total power making you go forward goes from, say, 1600W to 1700W. So you only add a few percentage points to your speed even if you hammer hard enough to blow yourself up. Because you actually went from 1500W making you go while coasting to 1800W while hammering away.

And while drag is proportional to speed squared, the power to overcome drag is proportional to speed cubed, because power is force times speed. The force is proportional to the speed squared, and then the power is proportion to the force times the speed. That's where doubling your power on the flats only increases you speed by 25%. The cube root of 2 is about 1.25.

And yeah, I'm using just form drag because at higher speeds that's by far the most dominant impediment to going faster.

Artkansas 

The two-wheeled motor contraptions are even faster.

caloso

wolfchild

If you want to ride faster then you need to increase horsepower in you engine ( IOW get fit ).. A different bike frame geometry can also help ( road vs MTB ) but not always, I've seen some very fast riders on MTB's with knobby tires.

furballi

Best and most accurate response. One red light will cost you around 30 seconds (min). That's a lot of distance to make up at 15 mph.

old's'cool

That works best if you can sprint to beat a red light. Slowing down to hit a light as it goes green saves a marginal amount time vs getting there too early and having to accelerate again; however the energy saved is non-trivial, if your goal is to conserve energy.

I should add, my comments are based on my experience with widely separated lights having no timed relationship. In a situation where lights are close together and a known timed relationship exists, then obviously you can try to adjust your speed to take best advantage of this.

gregf83 

Because very strong riders can ride quickly on a MTB you conclude that it doesn't always make one faster? I'll give you a tip - a road bike will always make you faster than a MTB with knobby tires regardless of your power level.

caloso

In my experience, if you get it just right there's no sprinting involved.

storckm

But wouldn't fighting headwinds have the same issue as downhill: exponentially increasing drag from air resistance?

jyl

I developed a rule of thumb that works in my sample of n=1. At a constant effort, cruise speed on a MTB with slicks is 2 mph faster than MTB with knobbies, and road bike is 2 mph faster than MTB w/ slicks. So 16 mph vs 18 mph vs 20 mph, if that is one's cruise speed.

old's'cool

I'm sure that's true, for your commute. As I mentioned, I was referring to my commute. If you have a method for avoiding red lights, that are separated by over 2 miles in some cases, that doesn't involve sprinting when you get close enough to actually see the light and be able to assess when it's going to change, I'd like to learn about it. This is pertaining to lights with no timed relationship. Due to the distance between lights, elapsed riding time between lights is heavily influenced by wind, at any rate.

gregf83 

Aero drag is proportional to the net wind speed squared. Without a headwind an x% increase in speed results in about 2x higher aero drag (i.e. 2% increase in speed => 4% increase in aero drag). With a headwind, an x% increase in ground speed will result in less than 2x increase in aero drag.

You can see this with an example: riding 23kph into a 20kph headwind requires about 200W. The net wind speed in this example is 43kph. If you increase your speed by 10% from 20 to 22kph the net wind speed goes up by 4.6% and the aero drag will only go up by about 9% instead of the 20% you would see without a headwind.

Downhills are different as most of the power to overcome aero drag is coming from gravity.

wphamilton

Best answer IMO. Where you're slowest has the most impact on the time - top speed almost doesn't matter comparatively.

Also, try to time things where you aren't using your brakes (wasted energy).

Leebo

The OP is pedaling a CARGO bike. Start with the bike.

achoo

Yeah, the effects of wind on speed can be a bit complex.

If it takes you 400W to go 30 mph with no wind, it's not going to take 400W to go 20 mph into a 10 mph headwind, or 10 mph into a 20 mph headwind. Both of those cases, it'd take less than 400W to maintain that 30 mph relative wind speed.

If that doesn't make sense, look at it this way: how much power do you have to generate to stand still in a 30 mph headwind? Yep - zero.

While most of the force needed to make you go fast is relative to the square of how fast you're going through the air, the power required to make you go that fast is determined by that force times how fast you're going relative to what you're pushing against - the ground.

cooker

However, if you look at absolute speed, rather than proportionate, your argument weakens. If you're going 5mph up a steep hill and double your effort (and speed), you only get an extra 5 mph. If you're going downhill at 40 mph and doubling your effort gives you a 25% increase, that's an extra 10 mph.

spare_wheel

A big ring helps:

mstraus

I go up a big hill in the morning, and down it in the evening. I find that working harder on the downhill part might shave off 10 or 20 seconds. Working harder uphill seems like it can make about a minute difference.

I have found I am more likely to pass or be passed on the climb, and typically the passer (me or the much more in shape climber) will continue to pull away. I have seen a lot less passing downhill.

One thing I notice uphill is that if you work to maintain momentum you an keep a higher speed the entire climb. Once you start slowing down on the climb its easy to keep slowing down and harder to build up speed again.

Honestly the flats an make a difference too. I have a couple long flat areas, not stops, probably making up over 1/3 of my overall commute. It can make a noticeable difference if I push myself and go a few mph faster on these sections vs ride a a more leisurely pace.

FBinNY 

True, but the amount of time is very different uphill than down, so halving the time on both (which is crazy generous on the DH) you save more total time on the climb.

idc

I agree with all the replies saying uphill or into the wind is where you make the best gains. Oh, and of course, traffic lights if you know their timing.

idc

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