Q: You should analyze and test drag characteristics in crosswinds.
For wheel drag it is typical to state drag characteristic at 0 degrees and also at 10 or 15 degrees airflow direction.
Good idea, thanks!
Q: This could be a good container for delivery cyclists.
Yes, and there is plenty of room on the sides for branding signage.
Q: It could hold batteries. It would be good if it had an electrical socket.
With the ABS plastic construction it would be straightforward to cut whatever holes are desired to add a connector.
Q: You should integrate a taillight on the back. A LED strip light for instance.
Good idea!
Q&A
Suggestions
Construction, Capacity, Durability
Q: What is the Draftbag made of?
Functional prototypes to-date are constructed of ABS plastic.
ABS is a durable, tough, flexible plastic commonly used to mold consumer items and durable sporting goods.
It also does pretty well with thermoforming, the process of shaping flat sheets into 3 dimensional forms.
Prototypes so far are built from thermoformed ABS.
Q: What about durability? what if your bike tips over?
Bike tip-overs, no problem. It bounces off the pavement.
In one at-speed uncontrolled crash the Draftbag survived with minor scrapes. Cyclist was not so lucky. Ride safely, y'all.
Q: How much does it weigh?
Weight of the generation 5 model "size large" equipped with RackTime SnapIt adapter is 1.8 kg (4 lbs). About the same as a pair of fabric panniers, and lighter than a pair of heavy-duty dry-bag type panniers.
Q: What is the cargo capacity volume?
The initial "size large" model holds total 44 liters cargo. 33 liters in the base, and additional 11 liters stackable into the cap. That is approximately equivalent to a pair of huge, sail-in-the-wind dry-bag panniers, e.g. Ortlieb.
Q: Is it waterproof?
The body is relatively sealed, and the lid engages with an overlap. It should get through the worst rain storm with everything dry inside.
It is not protected against immersion or intended to float.
Riding Characteristics
Q: The Draftbag stands considerably higher than the seat.
I can’t throw my leg over the seat to get on my bike, can I?
Correct! The Draftbag aerodynamic advantage depends fundamentally on putting the bag in the slipstream of your upper body, not just your thighs.
It stands tall enough that swinging a straight leg over the seat is not a practical mounting strategy for most people.
You can carefully swing a leg over with your knee cocked.
Or if you are particularly limber, swing your leg forward in a jaunty over-the-handlebars move.
Q: What about having the Draftbag close to your butt? How does that feel?
You will want to mount the Draftbag so it does not crowd your butt during aggressive pedaling.
As a start, sit on the bicycle in your riding position.
Set the draft bag for 5 cm clearance aft of your butt. Adjust as necessary.
If mounted close in for best aerodynamic advantage, you may feel the Draftbag proximity at your rear.
You will quickly get used to this, both an occasional butt contact, and furthermore feeling the dead air zone which the Draftbag creates as it decreases air circulation and the generation of trailing vortices (drag).
If you ride aggressively, jump off curbs and brake hard on downhills, you probably reflexively shift your weight rearward. With the Draftbag right behind you won’t be able to slide your butt backward. Plan accordingly. Mount the Draftbag further aft or change your style.
Q: In general, what feels different about riding with a Draftbag?
At speed you should notice less wind buffeting your butt and lower back.
Because they are in a relatively calmer flow region due to the Draftbag influence.
The Draftbag and its contents are mounted higher than panniers.
In some sense similar to riding with a child in a seat on the back of the bike. Unlike the child, the Draftbag is not constantly squirming and throwing you off balance!
The higher mass location means the responsiveness of your bike may feel a little different.
And OF COURSE you feel smart carrying a great volume of luggage a lot more efficiently and faster than when using panniers.
Q: How much can the Draftbag hold?
The Draftbag is a big bucket. You can fit a case of beer, 24 longneck bottles. Yes, multiple times tested this way. We don’t have an exact volume for you, good question!
It will hold as much as a pair of typical panniers rather fully loaded.
If you filled the volume of the Draftbag with heavy stuff, you can definitely exceed the bicycle rack load rating.
And you might break your Draftbag, which would be sad.
Aerodynamics
Let’s decompose that question into two subheadings: 1. What is the aerodynamic advantage of the Draftbag in a side wind? and 2. How Does the Draftbag affect stability in a side wind?
The Draftbag should maintain its aerodynamic advantage when apparent wind occurs in the range of +/- 20 degrees with respect to direction of motion. Apparent wind is the sum of vehicle velocity and wind velocity resulting in a wind direction as measured with reference to bicycle direction of motion. Proof testing is TBD.
The Draftbag + cyclist + bicycle have larger lateral surface area, as compared to the cyclist and bicycle alone. A wind gust can push you sideways harder with the Draftbag than without the Draftbag. However, because the Draftbag center of pressure is nearly over the rear wheel, a wind gust does not caue an unnatural torque on the bicycle into or away from the wind direction.
On a bicycle we always prefer to avoid high speed, gusty lateral wind situations. On an overpass or hilltop, being passed closely by semi truck. If you encounter a wind situation that you find uncomfortable with or without the Draftbag, keep your speed down and walk your bike as necessary.
Q: How has this been tested? Have the results been independently verified?
Draftbag initial studies were done using computational fluid dynamics (CFD). A-B comparison road tests have been run using power meters, confirming the aerodynamic advantage. There are quite a few cyclists out there with power meters, and we expect to see our customers adjusting and tuning their bag positioning, and confirming decreased drag.
Q: How good is the Draftbag really? How much faster is the bike with the Draftbag than without?
The Draftbag wins hands down when compared with panniers of similar volume. You can feel it. You don’t even need a meter to determine that!
Tests show a small drag improvement on a bicycle equipped with the Draftbag versus no bag. You can ride with the Draftbag empty and expect to enjoy a small aerodynamic advantage. Or ride with the Draftbag carrying your stuff and compare its drag to your old panniers. Enjoy your ride!
Q: Do you have a wind tunnel?
Nope. It would be cool to have one, for sure. Wind tunnels are pretty expensive to operate, and rather few.
Q: Does the reduced turbulence behind your back impact aerodynamic cooling?
Probably somewhat. But not nearly as much as having a sweaty backpack stuck to your backside.
Q: Why are the cyclists naked in the CFD simulations?
We're on a budget. We never bought clothes for our CAD mannequins. How embarassing. But you don’t need to cycle naked to achieve aerodynamic advantage. Spandex clothes are aerodynamically superior to naked skin, especially if you are hairy. The Draftbag is compatible with naked riding in any case.
Mounting and Fit
Q: Can the Draftbag be mounted on a cantilever style rack mounted to the bicycle seat post?
Yes and no.
Initial Draftbag testing was done using cantilever mounting from a seat post.
This attachment scheme is simplest to retrofit to an arbitrary bike so we can use it for quick-on, quick-off A/B testing.
And by not having support struts, a cantilever style rack incurs the least parasitic aerodynamic losses.
But a cantilever rack will always be less stable than a rack supported at the rear wheel dropouts.
When heavily mass loaded, the rack can bend or break.
It can also spin on the seatpost and pivot to the side, or even rotate the whole seatpost and seat.
These are not Draftbag-specific problems; people have these problems with cantilever racks regardless of what type bag is mounted on them.
If the load in the Draftbag is limited to 5 kilos, it should be safe on a cantilever rack securely mounted to a good quality bike and seatpost, and ridden conservatively.
You should observe the rack load rating.
Q: Can the Draftbag be used with rear panniers?
It is mechanically compatible, assuming the pannier frame does not mechanically interfere with the Draftbag attachment.
Would it be a good idea? Maybe, if you need to carry even more luggage.
Would it be a successful aerodynamic strategy? Probably not. The panniers will be hanging out in the wind like they always are, not
tucked in behind your body.
Q: Can the Draftbag be used with front panniers?
Good question. We have not done any testing.
You should expect to incur the incremental drag disadvantage of front panniers, which can be substantial. They probably won't interact with the Draftbag positively or negatively.
Q: May the Draftbag be mounted to any type of bike?
It is generally adapted to standard style type bikes, for instance racing, touring, flatbar, cruiser, mountain bikes.
It is probably not suited to recumbents, which may benefit from different style airflow management.
The Draftbag mounts to a standard type accessory rack.
In the best case the bicycle should have a 4-point rack mounted to both the seat stays and rear dropouts.
Q: How high should the Draftbag be mounted?
The Draftbag lower end should interact favorably with your buttocks and upper thighs.
Your lower thighs are too far ahead of the Draftbag to successfully harvest the airflow in their wakes.
So the lower end should reach down to about 1/3 of your thigh length when your leg is extended with pedal at bottom of stroke.
This positions the Draftbag horizontal midline slightly above the bicycle seat top.
Vertical mounting adjustment is an area of current investigation and refinement.
Q: What rack does a Draftbag interface to?
The Draftbag initial models use Topeak QuickTrack mounting features.
Next generation models have leveraged a Racktime Snapit mounting system.
In general the draftbag is compatible with available rack mounting systems which make the bag-mounted component available as a separate piece.
The Draftbag needs a fairly rigid attachment since it stands up tall rather than hanging like a pannier.
Q: What about the Draftbag width with respect to the rider?
The Draftbag is sized for a typical human physique.
Men have wider shoulders and women have wider hips.
But overall the similarities are greater than the differences, and it doesn't seem productive to generate a plethora of shapes.
Different widths are possible, and will be tooled eventually. For the moment it is a "universal size".
Q: What about upright riders versus those who ride low on drop handlebars?
The Draftbag was optimized for aerodynamic performance with a rider in a middle position, with the handlebars at or slightly below seat height.
It has been tested to deliver significant aerodynamic advantage for more upright riders, and also for riders in a low pose on drop bars.
The Draftbag is a little tall for aerodynamic co-operation with a rider in an extremely flat triathlete style bike position, but these riders don't tend to carry baggage in any case.
The Draftbag fabrication and assembly process does allow some latitude for decreasing vertical height.
We are looking at providing different height options.
Q: Is the Draftbag legal for racing?
Business
Good question! So far it has been self-funded, with organic growth. We’ll see.
Q: Is this patented?
Major design elements are protected under US Patent 10,167,033.
Racing
UCI approved? Undoubtedly, no.
The UCI believes in maintaining a level playing field between cyclists by enforcing a static bicycle configuration.
They celebrate competition of human performance and generally frown on attempts to improve aerodynamics through technology.
The tri-athlete world tends to be more accepting of streamlining features applied to the bike frames themselves, as well as rider postures having improved aerodynamics, enabled by bicycle and handlebar evolution.
But that triathlete world is not likely to accept a large add-on such as the Draftbag for aerodynamic improvement.
By the way, all those streamlined tube type of features on a super-expensive tri bike make a rather minor reduction of the drag of bicycle+rider.
They sure look cool, though.
Randonneure events are fairly free-form. A Draftbag ought to be entirely legal and acceptable.
The Draftbag would be entirely legal and encouraged in any competition sponsored by IHPVA, the International Human Powered Vehicle Association.
The IHPVA is a technology-oriented group. It lives for technical advances.
Mostly you can view the Draftbag as a commuter accessory.
Ride with your luggage at the same speed or faster than you ride without it.
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