How much does going from drops to tops change the drag coefficient and if you're traveling at 20 mph how much would you slow down assuming the same power output?

Technically speaking the drag coef doesn't change, just the surface area. Just estimate the increase in surface area and use the calculator I linked.

As for how much you'd slow down: never that much, because of the cube root you have to apply.

This is true, but it doesn't do anything to change the fact that for a given level of effort - which is how most people ride - the difference a large reduction in aero will make to speed is very small. Especially at 50-100W, which is a reasonable range for commuting effort (walking effort for a 70kg man is 30W.) For example, on a flat road a 60W effort will get 6.5m/s at 0.5msq area, and 1.0msq gives 5.5m/s.

And that example is using a huge difference in area: you do NOT halve area by riding in the drops, as I think someone said! The reduction is more like 25% - from 4.3 sq ft to 3.4:

https://4.bp.blogspot.com/_urSQl6wUA5...drag_chart.jpg

That's approximately like the 0.5msq rider going to 0.4, which will give him about 5% extra speed. Great in a race, but does anyone care if their 30 minute commute takes a minute and half less?

(Again, all calcs done using https://www.analyticcycling.com/ForcesSpeed_Page.html)

The percentages between the green and blue lines are wrong, though. 180W to 220W is an increase of 22.2%, not 18.2%. On the horizontal axis, 20.2MPH to 21.8MPH is an increase of 7.9%. The percentages between the blue and red lines are correct, however.

Thus, it says that to increase speed 7.9% from 20.2 to 21.8 requires 22.2% more power (speed increase to power increase ratio of 0.36), and to increase another 6.3% from 21.8 to 23.2 requires 18.2% more power (speed increase to power increase ratio of 0.35). So the message is muddled.

Perhaps it would have been better to graph speed as a function of power (flip the axes) and calculate the speed increase when power is increased by a fixed amount. Using this chart, one could say that at 180W, increasing power output by 40W gives you a 1.6MPH increase in speed. However, at 220W, increasing your power output by 40W only results in a 1.4MPH gain, which shows the diminishing returns (albeit slight).

It's the drag coefficient of a brick either way but the A (area) will change. How much depends on the person, the shape of his torso and how he's normally positioned. About 1-2 mph in practice.

Speak for yourself! On my bike this winter the difference was 4-5 mph (I did change the coefficient of drag), and I could average 20-21 on my commute including parking lots and time stopped at lights. With around only 150-160 watt effort (estimated). Not everybody commutes the same way, saving a minute or in my case 8-10 minutes is not always the ultimate consideration. Some times you want certain minimum speeds at certain places - for several reasons - and as I said the effort required varies in direct proportion to the drag coefficient and with area.

You can also fool around with this calculator: https://www.kreuzotter.de/english/espeed.htm Personally, I'm very comfortable with the physical equations and mostly use them instead.

I'm not speaking for myself; I'm giving the actually, definitely, scientifically correct answers. If they are not what you think get in experience then either

1. You are bs-ing yourself

2. Placebo effect is kicking in to make you pedal faster

I.e. you have no idea at all...

That you can speed your commute by 4mph is far from impossible; that you can do it by switching from the tops of the bars to the drops is (unless you have a VERY weirdly shaped head.) My guess is that you probably combine over-estimating the increase (eg by comparing best days with average days) with the results of pedaling harder.

The physics of air resistance, however, are the same for everyone!

This is a Great thread andI am learning so much.
I never got around to figuring out all the scientific reasons but I am starting to understand. When I commute at 15mph vs 20mph that extra 5mph felt like I was working twice as hard, sometimes even more...lol

It has been a while since I studied physics so some of the simpler explanations make since quickly, after all, I ride a bicycle everywhere so I can't be all that smart...lol

I deliberately didn't fold in the math in the original post because I have in fact worked with a wind tunnel (at Purdue) and it made me well familiar with the difficulty of generating good data that is even internally consistent, much less consistent with anyone else's. Bicycles and auto wings are popular projects for such tunnels because they are cheap enough to get, small enough to fit in the tunnel, and the students, as I was, have experience with them. Students typically get little time to use a tunnel on their one little project, and a lot of time to sit at a PC and turn their three drag curves into Javascript calculators (or I guess Python these days). These can look very convincing and give you false confidence in their correctness. I preferred a lessons-learned approach which would provide a to-do list of easy improvements with notable payoff. I supposed that people would follow up with the entire Internet's worth of data, and you all have not disappointed! But I'll stand by my original. I'll make some of the suggested edits later. I wasn't really happy with my "comfort utility cost style" writeup the first time, it needs work before it gets added to the original, and I do want to pursue the safety idea some more.

Who knew commuting around town on a bicycle was so scientific.....WOW! I feel much smarter now.... Do they make a lycra lab coats?

Grant Peterson approves of this thread.

I highly doubt if anyone would care if the commute is only 30 mins. Also, it's not so much about the speed (it's the commuting forum after all), but it's the damm persistent strong headwind that matters.

Now, no need to get upset. You're making a whole bunch of assumptions, mostly wrong.

I designed, tested and measured the performance of this with the assistance of a PHD in Physics, which was my field of study also. No bs here.

Excuse me: I wasn't making an assumption, I was being explicit about the case I was considering - that of going from the hoods to the drops! Yes, if you disguise yourself as a V2 you can do better than this....

But good luck if

1. You hit strong side winds

2. You meet a paranoid Homeland Security Agent!

That's not because the physics is variable but because wind tunnels are notoriously tricky to use!U

Understanding the maths is essential, because otherwise you can't think intelligently about the costs of reducing drag. A 25% reduction from using disk wheels (that's a guess!) sounds possibly worth $500 for a used pair... until you learn that you get less than the cube root of that as a speed increase - maybe a 6% real world speed increase.

Otoh, $80 a pair of top end commuter tyres can add 1mph, improve comfort and handling, *and* give you better wet weather cornering and braking. Everything is about benefit per cost.

Just ride.

The stuff about placebo effect, bs-ing, overestimating effects, all wrong assumptions. Limiting your exchange exclusively about riding in the drops, wrong assumption. Most people riding at constant effort, wrong assumption. Most people commuting at 80-100 watts, the continuing idea that people are only concerned about top speed regarding aerodynamics, both are dubious assumptions. Assuming that anyone you're addressing doesn't understand the cubic relationship - most of them do and you're missing their points with your assumption.

Implying that I haven't ridden it enough to know about effect of crosswinds, or that it's a problem I haven't engineered out, another assumption. I have about 2,000 miles on that model (plus 3 years worth in prior designs), with a significant amount it in thunderstorms. It's not that you're wrong about the amount of time saved vs air drag - but telling people they're not comprehending it or don't understand the math is getting old. They get it, they did before you started. And this idea that saving one minute on a commute is the beginning and end of it is a bit myopic and, while you may not realize it, probably insulting to some of the readers. You might pause and think, realizing that people do know precisely about the aerodynamic impact of various measures (and believe me, some of these guys know in far greater detail than anything written in this thread), and they don't care about the minute saved or lost on their commute, that it might be something else besides that commuting minute? Something you haven't considered or have dismissed?

People ride differently, for different reasons and with different styles. If we were all Grant Peterson types you'd probably be more correct, but that's another assumption that's unwarranted. My own counter-example was purely for illustration, and there's a whole range up to and beyond it with respect to aerodynamics. One example where the impact is significant, and even fractional gains below it can still be significant.

Mine is Nomex, for what it's worth

..Based on the belief, which I had carefully stated and you failed to correct, that you were joining in the conversation about riding on the drops vs riding on the hoods. Honestly, if you don't clarify stuff by saying, "Oh yeah, - I know you are talking about drop bar positions BUT I AM TALKING ABOUT MY HOME MADE HPV" then you only have yourself to blame when people do not realize that your are talking about your home made HPV...

I would love to see pics!

My Oma's dad (RIP) was a huge cyclist from there--I remember him riding everywhere, but I don't remember him having aerobars--but I was a little kid.

I'm talking about any aero improvements, including but not limited to riding in the drops (which was mentioned by tjspiel after I joined in) ... and I didn't even address you so I'm not sure why you're trying to arbitrate subject matter. Anyway, my points are made so I won't be pursuing this further.

Wind tunnels and graphs are nice for the mind but in real life if you commute in a city wind comes from everywhere but where you expect it to be. Sidewind between buildings, wind from cars passing, trees, signs and drop box etc... that deflect wind, vortex etc... Lots of messy turbulence everywhere. Going for an aero position with headwind is fine but what about tailwind?

The human engine is pretty limited in how much power it can deliver and is optimally designed for endurance so being able to reduce the energy required to ride a certain distance may allow one to extend their range even when top speed is not a concern.

In the real world we have wind, grades, and less than perfect roads which all serve to increase the energy requirement to ride at a given speed... the wattage calculations I have posted assume perfect riding conditions and are variable due to the differences in respective riders and their set up.

We are currently developing a design for a recumbent, electrically assisted trike which is being used because of the aerodynamic advantage it offers and lower wattage input needed to ride at given speeds. It will use a generator system to power a second reserve battery as one rides and the small amount of extra load from the dynos should not be of any concern at all.

How about regenerative braking?

We already have that with the Bionx...