* * * Aerodynamics * * *

[Anonymous]

The aerodynamics of modern race bikes, like the RS125 above, are horrible. Check out the Cd numbers on them some time.

It's the result of a combination of racing rules and market driven styling. The "dustbin" racers from around 50 years ago were more aerodynamic than modern GP bikes.

Check out some NASA publications on low drag bodies........

 

[Mathurin]

The aerodynamics of modern race bikes, like the RS125
above, are horrible. Check out the Cd numbers on them some time.[...]

Indeed it would seem:
http://www.gravitybike.com/forum/viewtopic.php?t=31



One day I found a way to get an aero advantage on my FS mountain bike
while commuting back from work. It was hot, a day where you sweat at
the least effort, and my back was getting wet. So, I stopped and carefully
attached my backpack onto my side of the handlebars, as there was
nowhere else to put it. It made 1-2 gears difference over the 24, with
400% ratio from first to last gear. So that's what, 15%? I dunno, but it
was a really pleasant surprise.



Some home-made fairings that make polycarbonate sheets +appealing:
http://www.gravitybike.com/forum/viewtopic.php?t=145


How to make wheel disks:
http://www.wisil.recumbents.com/wisil/wheeldisk/wheeldisk.htm

 

[safe]

The aerodynamics of modern race bikes, like the RS125 above, are horrible. Check out the Cd numbers on them some time. It's the result of a combination of racing rules and market driven styling. The "dustbin" racers from around 50 years ago were more aerodynamic than modern GP bikes.

In the early days people tried completely wrap around fairings, but people crashed whenever a strong wind hit them from the side. They realized that if they limited the front fairing to the center of the front wheel (vertically) and on the rear to the back of the rear wheel that you could mostly prevent this danger.

It's not a bad rule... it's accepting reality....

My opinion is that you want to get "good" aerodynamics and not seek after "perfect" aerodynamics. A little streamlining goes a very long way. The main thing is to get the "tuck" position (right there you cut your frontal area by about half) and get some smoothness in the front to keep the air out of the "pocket" that forms between your arms and chest area. That "pocket" generates a HUGE wake. It's like riding a "concave shape" (edges facing forward) into the wind. Really bad...


If we look at this chart again:

...and consider that a regular bicycle has the aerodynamics of a "cube" at about "1" if you can simply get to the level of a "half sphere" you can cut things in half. The fully streamlined shape can get as low as "0.04" which is WAAAAAAY better than the "half sphere", but it doesn't change the fact the we were starting from a "cube" shape.

Perfection is the enemy of "good enough".

 

[D-Man]

Safe, how much would you think a person would gain in mph given an upright e-bike and speed of 30 mph (no fairing) or more for each drag coefficient? Also, how much loss should there be from no load (wheel off the ground full throttle) to (load) actual mph for an aerodynamic e-bike set-up.

 

[safe]

Safe, how much would you think a person would gain in mph given an upright e-bike and speed of 30 mph (no fairing) or more for each drag coefficient? Also, how much loss should there be from no load (wheel off the ground full throttle) to (load) actual mph for an aerodynamic e-bike set-up.

Going to:

http://www.kreuzotter.de/english/espeed.htm

...and putting in 815 Watts after selecting "MTB" we get 30 mph. If you change from "MTB" to "Lowracer" you will go 44.5 mph. But those are "peak" wattage values which are NOT always the same as what will actually happen because the "peaks" don't always line up right.

The best thing about gearing is that you can make your motors peak just equal to what the aerodynamic peak can be.

A hub motor (5304 72 Volts) might "peak" in power at 40 mph, but if that same power was geared higher it "could" go 48 mph. Power drops off after the power "peak". (it's just like a car, you have to shift at some point if you want to go any faster with the same horsepower)

 

[safe]

The Formulas

"Assuming a constant drag coefficient, drag will vary as the square of velocity. Thus, the resultant power needed to overcome this drag will vary as the cube of velocity."

Drag (physics) - Wikipedia, the free encyclopedia

A Mountain Bike at 750 Watts can go 28 mph.

A Streamlined Lowracer at 750 Watts can go 68 mph.

 

[Mathurin]

Have been playing with the power calculator thing.


I could take this bike:

Add pegs on the rack or something, mtb horns in the middle of the
handlebar pointing downwards, and some kinda polycarbonate zipper style
fairing up front. Gravity bikes have knee supports, but they would
interfere with normal pedaling. Maybe something that folds up.

Then something like a MY1018Z on the BB.
Small, light, and the weight is in a good place.
http://tncscooters.com/product.php?sku=106105


The bike so equipped should allow me to use it normally with somewhat
better aero then a conventional bike. This would be pretty good for urban
use. It should eventually reach ~30km/h, add ~100w and it'll go ~35. I
could also ride it in an aero position, ridden this way at rated power it
should eventually reach ~37km/h on the flats.

Assuming rated power / 80%eff / 35km/h, that's 9wh/km.


On paper it seems like it would be a nice ride for going to work at the
winery since it would eat up most of the work en route.

 

[safe]

Foot Pegs?

Let me explain.

You really don't "need" those knee rests if you are willing to get into a "tuck" position like a jockey. All you need is to have some foot pegs that could simply be folded up when not in use.

POSITION IS EVERYTHING!

Summary:

1. Pegs too far Forward - Your legs will feel cramped and your lower back will take most of the strain.

2. Pegs too far Backward - Your arms will carry too much weight and you'll feel like you are "arching" your back up all the time. Very tiring.

3. Pegs "just right" - You are at your bodies "balance point" and so your weight is equally distirbuted in front and in back of the pegs.

BASIC "BALLPARK" GUESS

The most likely position for footpegs is about where the pedals are but straight up as far as you feel comfortable going. (there are limits to a tuck) So I would recommend something that attaches to the tube that connects the bottom bracket and the seat. It's called the "Seat Tube" I believe. Don't forget that the footpegs have to fold up and not interfere with your pedalling. A good idea is to make a clamp that fits to the seat tube and allows you to experiment with different peg heights to see what feels right. At no time would you have the pegs positioned BEHIND the leading edge of the rear wheel... that's simply too far back and you would be straining too much. (your arms would be taking all your weight, which is wrong)

WARNING

Once you take your feet off the pedals they have the option of being in a position where in a turn the inside crank could be down. You could catch your crank and crash. So "ideally" you need some sort of "lock out" that prevents the cranks from spinning. Some kind of "strap" could work. "Safety first", coming from "safe".

When you know that all you plan to do one day is run on electricity you would set the "strap" on the pedals and fold down the pegs and then you are off and riding. It would take less than 10 seconds if it was done right and you might be surprised that the position is very comfortable if you achieve good balance front to rear for the location of the pegs...

 

[Anonymous]

Rollie Free knew aerodynamics

 

[knightmb]

Rollie Free knew aerodynamics

ROFL :lol: :lol: :lol: :lol: :lol:

Now that's going fast!

 

[safe]

Trying staying in that position for an hour or two...

 

[Lowell]

Trying staying in that position for an hour or two...

Everyone has different ideas of what is practical. Heck I know guys with daily driver cars with stripped interiors, no a/c or heater etc to reduce weight. I just make more HP and go faster than them while keeping all creature comforts and 4 real seats.

 

[safe]

Well the idea I presented is VERY practical and could cut the frontal area down very significantly. It could give an extra 5-10 mph on the top end speed or longer range depending on how you look at it.

This idea is really easy to make reality.

The upper torso is a fairly aerodynamic shape to begin with. The legs are a big source of drag and it even gets worse when pedalling because it throws out these huge eddy currents. (speed calculators think it's so critical that they even include it as a parameter) So you get a double improvement of less frontal area AND fewer wild eddy currents due to the legs in motion when you tuck the legs out of the way.

 

[Anonymous]

Trying staying in that position for an hour or two...

The batteries would be dead long before then anyway.

But in the mean time, from Cool Breeze UK Ltd:

I have some pictures of the above bikes with the riders on them, but they are in a book.

 

[safe]

Imagine taking a design like that and adding a 5304 hub motor to it at 72 Volts ! That's scary though. The main thing I worry about is high speed stability, so I've been borrowing from motorcycle road racing to get the knowledge about that. These bikes that place you in weird positions might put you at real risk of losing control at speed. "Safety First".

Basically, how fast WOULD you want to go on such a thing?

My "electric road racer" handles like a road racer motorycle at speeds above 40 mph... it's so smooth and well behaved that it feels "right" to be doing that much speed. (it is very confidence inspiring at high speed, seriously, it curves through turns like butter) I purposely took the exact same geometry that the road racers use so that it would be identical in behavior and it is.

 

[D-Man]

There was a guy who put a fairing on a recumbent and his speed went from 27 to 30 mph.

 

[safe]

And with gearing it's probable that the guy could have pulled speeds into the 40 mph range for a 500 Watt motor.

 

[Geebee]

I still think a full fairing would be safer, those prone bikes are scarey, make a stuff up and the head takes the brunt of the impact.
Imagine how sore the neck would be from trying to see where you are going, and crud from the front wheel getting in your face/breathing air.
I will stick with laying on my back thanks.

 

[Anonymous]

I will stick with laying on my back thanks.

Me too. I hate to say this, but the whole RR/fetal position is obsolete. It is familiar to a lot of people - and has absolutely nothing else going for it.

After spending enough hours laid back with my butt a few inches off the ground at 40mph+ on a fast kart track (Calhan, CO), I have no desire to do the hobby horse bike thing ever again. Touching the seat down at 47 degrees lean angle - and pushing the front end was far safer on it than a close coupled wheelie/stoppie/highside prone high CG machine. Yes, it was weird at first - but anyone with a high performance mind should be able to adapt and perform. Or at least that's been my experience and observation.

Now here's a high performance bicycle:
http://www.challengebikes.com/html/index.php?taal=en&selectie=nme

 

[Mathurin]

I was seeing it like this:

Resting my weight on the saddle in aero position seems like it would be
uncomfortable, that's why I was thinking about the knee pads - also this is
how the gravity bikes I've seen were made. But I'm thinking shin pads would
be more practical then pegs + knee pads or such, since they could probably
be made comfortable for longer rides with no need for them to fold away.


Note in the pic the seat is higher then what I ride with.

 

[Lowell]

I will stick with laying on my back thanks.

Me too. I hate to say this, but the whole RR/fetal position is obsolete. It is familiar to a lot of people - and has absolutely nothing else going for it.

After spending enough hours laid back with my butt a few inches off the ground at 40mph+ on a fast kart track (Calhan, CO), I have no desire to do the hobby horse bike thing ever again. Touching the seat down at 47 degrees lean angle - and pushing the front end was far safer on it than a close coupled wheelie/stoppie/highside prone high CG machine. Yes, it was weird at first - but anyone with a high performance mind should be able to adapt and perform. Or at least that's been my experience and observation.

Now here's a high performance bicycle:
http://www.challengebikes.com/html/index.php?taal=en&selectie=nme

Had me going until this part... "only really suited for racing."

 

[Anonymous]

Had me going until this part... "only really suited for racing."

There are some very enthusiastic NME street riders - but not very many! Like +20mph electric bicycles, they aren't going to be sold as street legal. What we do with them is up to us......

 

[Anonymous]

Resting my weight on the saddle in aero position seems like it would be
uncomfortable, that's why I was thinking about the knee pads - also this is
how the gravity bikes I've seen were made. But I'm thinking shin pads would
be more practical then pegs + knee pads or such, since they could probably
be made comfortable for longer rides with no need for them to fold away.

Norton built a "Kneeler" racer - it was a great idea:

http://www.motorcycle-usa.com/Article_Page.aspx?ArticleID=3176&Page=1

 

[Lowell]

I was seeing it like this:

Resting my weight on the saddle in aero position seems like it would be
uncomfortable, that's why I was thinking about the knee pads - also this is
how the gravity bikes I've seen were made. But I'm thinking shin pads would
be more practical then pegs + knee pads or such, since they could probably
be made comfortable for longer rides with no need for them to fold away.


Note in the pic the seat is higher then what I ride with.

I almost spat my drink on my keyboard when I saw that pic. So who's going to try this out and report back with results and pictures? I wonder if the same results could be acheived with air deflectors on the down tube while simply resting your feet on the crank arms. Arms should be rested triathlon style for the best upper body aerodynamics.

On a somewhat unrelated note, has anyone ever tried measuring aero and rolling drag forces by towing a bike behind a car, with a spring scale holding the tow line? More specifically, how long would the tow line have to be so that the effects of the car (assume a low roofline sports car) would be minimal?

 

[safe]

I hate to say this, but the whole RR/fetal position is obsolete. It is familiar to a lot of people - and has absolutely nothing else going for it.

Well here's the problem with that theory.

On a "road racer" type setup you have the ability to "counter balance" the effect of a tire that breaks free from loss of traction. If you've ever done any off road riding you know how to do a "power slide". This same effect is used on the street only it's much more subtle and many novice riders have no clue about it, especially if they don't have a substantial dirt bike background.

"King" Kenny Roberts revolutionized the sport when he changed the way people ride (and build their bikes) so that the old "riding on the rail" philosophy that had been dominant for years before was replaced by the "flat track dirt racer" riding style adapted to the road. Modern road racers can put their bikes into a "powerslide" going into, through the middle and exiting the turn and it's all at the will of the rider. (excessive slide just wastes speed, so it's a delicate balance that is maintained)

If any of this is sounding like "familiar knowledge" then I'm puzzled why you would want to revert back to a riding style that offers no "corrective" behavior. The joke in the old days (pre-Kenny Roberts) was that "if you slide you crash". Kenny changed the sport and put the limit of performance waaaaay past the old "ride the rails" mentality. The lack of "counter measures" on a sit down bike is fine for slow speeds and non-racing situations... sort of like the difference between a "sports car" and the "family car". For "real" racing you need a riding style that allows for the tires "drifting" because at speed that's how tires behave. I slide all the time and it's normal to reach the limit of traction on anything when you go fast.

You have to be able to "powerslide" or you might as well ride the "minivan"... :wink:

http://www.motorcycle-usa.com/Article_Page.aspx?ArticleID=4214&Page=1

"King" Kenny, as he's been referred to since his first world championship, has an unequaled list of World Grand Prix successes as both a racer and team owner. As a racer, Roberts earned three consecutive 500cc World Championship titles, while redefining the riding style of all racers to follow. As a team owner, Roberts secured three more 500cc World Championships, along with a 250cc title with American riders, adding many more podium finishes to his successes. After taking on the unprecedented challenge of creating his own 500cc KR3 GP race bikes in 1997, followed by the 990cc MotoGP KR5, Kenny now uses Honda's new 800cc 212V motor in his KR212V MotoGP race bike, ridden by his son, and former World Champion, Kenny Roberts Jr.