[slideshow]
We’re rolling now. We spent the last couple work days getting frame elements ready to assemble on a volunteer day Wednesday. We should have 2 carts for demos and 2 for sale this Saturday at the Marmots’ Meander 2012 event. We are now taking orders for standard and customized trailers. Contact Jim at OCBC or Bart to discuss the details and place your order. All orders will be filled on a first come first serve basis.
First test of cart on bike
This is the first test of the front fork model hitched to a bike. I put 40 lbs of weights and a bunch of frame parts in the tub and rode it around on the cobble stone road next to OCBC took it over the curbs etc. It worked real well the tongue of the trailer allowed you to turn the bike in both directions as sharply as you could safely turn if the trailer wasn’t there. It seemed to add a bit of stability to the bike. I could feel the load on the rough rode but it wasn’t jerky or bouncy or anything. We will be doing more test rides over the next few days.
This design is coming together quickly. We tested this prototype out by having someone 175 lbs or so stand on platform. Next week we will fit this prototype with a hitch and start doing on the road tests. This model will have a recommended payload capacity of 200 lbs but will probably handle more weight than that.
This model will be available within a couple weeks, and will retail for about $200.00 to start. Orders are being taken on a first come first serve basis. Contact OCBC to place order or discuss special uses and customization.
Cart is adjustable to fit different bikes, people, and loads
The Front fork rendition of the bike cart is coming together. These pics are of the first prototype ready to be bolted together. I like this design, as it is simpler to assemble, and requires less cutting. We may, in the long run, use the rear frame design for heavy duty carts, leaving the rear triangle as a whole, for added strength and structure, and make the lighter weight carts out of the forks.
Front fork model prototype ready to bolt together
Leaving the head tube on makes a nice handle. Using the down tube and seat tube from a step over (women’s) frame, brings the tongue, also the handle, around to the right place to hook up to the rear triangle of the bicycle. This centers the cart, relative to the bike. Allowing the tongue to be adjusted in angle, and height, by allowing it to be rotated, keeps the cargo area level regardless of how high from the ground the handle is positioned. This allows the cart to be easily fitted to different sized bikes, or people.
Cross brace made of bicycle frame parts
Using handle bar stems to connect the cross brace to the forks is simple and clean looking. This also allows the cart to be lengthened.
The extra length of tubing at the ends of the cross brace allows the cart to be fitted with a 26″ wheel for a cargo platform. With a 20″ wheel the overall width is 29″ allowing the cart to be wheeled through most doors. a 26″ wheel would make the cart a little less than 36″ which is a standard width for many external and commercial doors.
wheel that will be dished flt on the top
This is another way we think might work. Using the front forks instead of the rear triangles. The good thing about this is if they both work then we can get 2 trailers out of 2 bikes. This design is a little cleaner also and requires less cutting.
Rough prototype as of 1/11/2012
[slideshow]
The cross brace is made from short pieces of bike frame tube matched so the long pieces slide into sleeves, and bolted where needed, with axle bolts and nuts, from scrapped hubs. The tongue which is also part of the cross brace is made from a seat tube and down tube from a step over frame.
There is a seat post binder sleeve and bolt on one side, to allow the tongue to swivel to fit different frame sizes, and to act as a handle when wheeling the cart around on foot, that way the cart acts as a wheel barrow and the load doesn’t have to be transferred when going inside or walking around a farmers market or some other venue that would allow the use of the cart while walking. We’re not sure if the seat bolt binder set up will work yet the tongue may have a tendency to rotate under load, it may have to have a removable pin to lock it in place when it is attached to the bike.
The next step is to make a connector which swivels or rotates both horizontally and vertically, to allow the cart to turn bike and rotate relative to the bike. These can be purchased from trailer manufacturers and through bike shops, but we’re going to try making them out of scraps.
What would you do with a bike cart if you had one? If you’re reading this blog, you’ve no doubt thought about it. You’ve probably checked out what’s available at the bike shops, and box stores, or perused the internet. Maybe you’ve even tried one or two out. Ohio City Bicyle Coop has several types for rent, if you haven’t tried one and would like to.
Do you just want to go grocery shopping, or take the kids, dog, Cello, to the park? Are you an urban farmer, without a truck, who needs to get compost seed etc to the garden, and produce to the market. Do you provide a service where your stuff could be transported by bicycle, if you could just figure out a way to load it all on? There is really no limit to what you can do with human power. Even hills, weight, and distance can be dealt with by either gearing down and taking your time, or adding another person or electric assist.
So if you had the ultimate cart for you bicycle, what would it look like what could it do? How much weight do you want to carry? What kind of doors, and gates, does it need to fit through? Does it have to ride in an elevator? Are you going to need to carry it sometimes? Does it have to be weather resistant? How quickly do you need to connect it, and disconnect it, from the bicycle. Are you going to stay on paved roads or does it need to be tough enough for cobbles, dirt, or gravel?
This is a lot of questions, all of them are important, and you could answer them one or two at a time, by buying something and finding out what’s not going to work. I’m going to try to address these issues from the perspective of someone who’s been hauling stuff around on bikes for quite a while. I’ll present a lot of examples of how other people are using bike carts. I’m also going to explore what’s available off the shelf that might fit your needs. Then I’m going to ask you to tell me what you think. I’ll attach a comment area at the end for a place to open a dialogue on this subject. If you already know what you want you could just skip this read, but you might find something here you haven’t considered.
Lets start with the basics.
This drawing shows some dimensions and clearances to consider
geometry1 link to pdf of above drawing
Safety:
How much weight can you safely get moving and stop?
A bicycle powered by a human can pull a lot more weight than it should, if you haven’t improved the brakes on the bike, or put brakes on trailer. The momentum of the loaded trailer can push you right through an intersection, or in to a curb, car, etc. Bicycles are designed with brakes capable of stopping the vehicle, in a reasonable distance, only when they aren’t overloaded. Even with disk brakes your still limited as to how much momentum you can deal with, there is only so much rubber in contact with the road. The higher the payload the more momentum. This I think is why most trailers have around a 100 lb payload capacity. You can put a lot more weight than that, on even most of the really flimsy trailers. That doesn’t mean you should.
Center of gravity.
That’s the axis around which a load tends to rotate. Things that are top heavy, more weight above this pivot point then below, will tend to tip over, especially while turning or on an uneven surface. Even the best designed trailer can be loaded poorly and tip over. The majority of the weight has to sit as close to the ground as possible.
This drawing shows some considerations when loading the trailer, connecting it to the frame and alignment of wheels
The distance between the wheels of the trailer.
The greater the distance between the wheels, side to side, of a 2 or 4 wheel trailer, the more stable it will be. Being able to spread the load out over a wider area allows you to lower the center of gravity. If the load sits outside the wheel it can pull the cart over using the point where that wheel contacts the road as a pivot point.
Containment of the load. What you are hauling should not be able to contact the wheels, the road, or the tongue of the trailer. Fingers and toes, loose items, tarps, etc, should not be able to get caught in the spokes or touch any other part of the wheel. Cargo shouldn’t be able to drop under the the bottom of the trailer, and touch the road, or an obstacle in the road. The load will tend to shift around and settle on a rough surface.
Connection to the bicycle. Where, and how, you connect the trailer to the bicycle, has a lot to do with how the trailer effects the stability of the bicycle. The higher this connection point, the more force the trailer can exert, and the easier it is for the trailer to push the bicycle over when turning a corner or swerving to miss an obstacle. Mounting down near the center of the rear wheel of the bike puts it at a point below the bikes center of gravity which decreases the likely hood of the trailer taking control of the bike.
Think about what your going to be hauling and how it’s going to effect the stability of the trailer and the bicycle.
The hitch is a critical part of the trailer bike assembly. When a trailer is 2 wheeled, the hitch has to allow for turns and uneven surfaces, in order to keep all the wheels on the road. The drawing below illustrates this.
Turning geometry
Those are some of the basics having to do with the stability and safety of the cart. I’ll show some carts that are available, and how folks are using them, in future posts.
I tried everything I could think of to make the tongue with the split steel rims work. I bolted them in 4 or 5 places, put in a second connecting point to the main frame, and then took the brackets from a bolt on kick stand, and clamped it around the tongue and cross brace. It still had too much flex and when Jim grasped it at the cross brace and the end of the tongue he was able to easily bend the tongue upward.
I will try using wider, stiffer alloy rims, at Stewart’s suggestion. I like the shape, it has lots of clearance for the wheel of the bike during turns, and it would be light weight. But if its not strong enough to handle the loads, then it just won’t work.
We are also going to try something that Jim had suggested, and Kevin brought up again, which is using the down tube and seat tube from a step through frame. This will be a lot stiffer, a little heavier, but may end up answering some other design needs. The problem it may present is, will there be enough clearance for the rear wheel when turning in a tight circle? There are a lot of step through frame styles and some will no doubt work better then others.
Fortunately, there are a lot of under appreciated step through frames out there. A lot of folks would call this frame design a “girls” bike and don’t want one. The truth of the matter is a step through frame has a lot of advantages over a diamond frame, Easier to get on and off, no top tube to fall on to, and a kind of built in suspension.
The process continues, some things will work, some won’t, the trick is to have the patience to work it through and to try everything you can come up with to solve the problems as they present themselves. If it doesn’t work, don’t throw away your notes, you might come across some idea that makes it work. If you take the time to review what you’ve done in the past, that didn’t work, you won’t have to go through the process again.
Well the holidays are behind us, and we’re moving forward on the Bike Cart project.
[slideshow]
It stands to reason that the lighter the trailer is, the more cargo you could carry, if you don’t compromise durability, or weight capacity. That being said, how much difference will a couple of pounds of trailer make? There is no doubt a point of diminishing returns, where you’ve increased the likelihood of a structural failure.
A lot of people are going to fill a trailer to capacity whether it’s feathers or compost, when it starts spilling over the top that’s when it’s full. I think that has to be taken in to account when you build a trailer. Design elements such as the shape and flexibility of the supporting frame members can increase strength, a lot more than adding weight in most cases.
The first thing we are trying is leaving the wheels assembled. A spoked wheel has a lot of strength. When it’s turned horizontal instead of vertical it’s not as strong, but the design shown above was able to support my winter weight of around 180 lbs, or so, with no problem. The tongue of the trailer, the part that connects it to the bike, is made with a 27″ rim split in half and fitted together. Initially this seems to have too much flex, but we think we can eliminate that problem by bolting it together more securely, or maybe stacking the rims vertically instead of fitting them together horizontally. we’ll see how that goes.
There will be a lot more on this subject later. As I said before this process will be reported on as we proceed. Check back often for updates. Input is always welcome.