Resources for recumbent frame design

[Dan B]

I'm thinking idly about designing a cargo bike based on a tri- or quad-wheel recumbent, and wondering if there's a book or web resource containing the hard-won experience of people who've been down this path before.

The first thing I'm curious about: upright bikes are almost always diamond-framed (hence triangulated, or nearly so), and when you see a step-through frame the down tube is usually a lot thicker. The standard way to do a bent, though, seems to be a single boom running fore/aft, which must get a lot of bending forces. Would a triangulated recumbent frame allow for thinner tubes and lighter weight? If so, what are the disadvantages of that approach that means everyone uses the boom anyway?

 

[byegad]

See the BHPC home page for details of plans to build your own recumbent.

 

[n-ick]

Try this;
http://www.atomiczombie.com/

 

[marc-triker]

if your looking to build your own then I totally agree with N-ick attomiczombie.com is a great place to start as for a small fee they give the plans. easier than designing your own and cheaper than buying something new. i've been looking at this http://www.missioncycles.co.uk/disd...IL:Category='AdultTrike'ORD:ABS:10KEY:201PAR:

 

[Auntie Helen]

I'm thinking idly about designing a cargo bike based on a tri- or quad-wheel recumbent, and wondering if there's a book or web resource containing the hard-won experience of people who've been down this path before.

The first thing I'm curious about: upright bikes are almost always diamond-framed (hence triangulated, or nearly so), and when you see a step-through frame the down tube is usually a lot thicker. The standard way to do a bent, though, seems to be a single boom running fore/aft, which must get a lot of bending forces. Would a triangulated recumbent frame allow for thinner tubes and lighter weight? If so, what are the disadvantages of that approach that means everyone uses the boom anyway?

I'm no engineer but if you had a triangular frame (with presumably one of the points of the triangle going to the boom) wouldn't it get in the way of your feet when you stop?

 

[Chonker]

For a triangular structure to really be nice and stiff (like a diamond frame bike) the triangles have to be tall. The problem is with a recumbent position you are sat in the way of where the upper members would run.

If you put the triangles underneath the seat they'd be too short to gain an advantage weight wise, or you'd raise the seating hight too greatly (imo) causing larger frontal area with increased drag and more importantly raising the centre of gravity which could cause great instability and a tendancy to roll under cornering!

The easy answer is to take the top member up over the drivers head, but this would put a tube in your line of sight. Another idea is to make the structure with three tubes, one either side of the drivers head, or coming around either side of the body, but then that's increasing the weight!

I think the large diameter tube-in-bending construction that's currently used makes alot of sense compared with these alternatives. It's worth remembering that a large diameter thin walled tube can be stronger in bending (for the weight) than a small diameter thicker walled one.

The ideal solution in my opinion is to use a composite monocoque but that's going to be tough to design.

There are some interesting designs out there, I'm trying to find a bike I remember seeing in a manufacturers prototype section which had the frame come either side of the driver and the seat hung in between, Ill post if it I can find it.

If you do end up doing a quad, I recommend a lotus 7 for inspiration

 

[Chonker]

Also, I recommend the following links from an Australian tuning magazine, the guy made four trikes from scratch honing his ideas as he went. Great engineer and well written:

http://autospeed.com/cms/A_2936/article.html
http://autospeed.com/cms/A_108591/article.html
http://autospeed.com/cms/A_111091/article.html

 

[Alf]

For a triangular structure to really be nice and stiff (like a diamond frame bike) the triangles have to be tall. The problem is with a recumbent position you are sat in the way of where the upper members would run.

If you put the triangles underneath the seat they'd be too short to gain an advantage weight wise, or you'd raise the seating hight too greatly (imo) causing larger frontal area with increased drag and more importantly raising the centre of gravity which could cause great instability and a tendancy to roll under cornering!

Agreed that a recumbent doesn't have space for a big triangle without these major compromises. But, an alternative to the single fat tube that they all seem to use these days might be lots of little triangles of narrower diameter tubing, using the same kind of principle as the Moulton. In a high racer there might be enough space for a structure along those lines. It would probably cost a lot more to design and produce though.

Alf

 

[dataretriever]

If you do end up doing a quad, I recommend a lotus 7 for inspiration

What he said!

I had a lotus 7(alike) and it is the best thing ever for grin inducing cornering even beating my Q, The Q beats it for luggage carrying though.

 

[Amanda P]

... lots of little triangles of narrower diameter tubing, using the same kind of principle as the Moulton. In a high racer there might be enough space for a structure along those lines. It would probably cost a lot more to design and produce though.

Alf

It would definitely cost a lot more. I've counted the fillet-brazed joints in my spaceframe Moulton; there are about a hundred. Each one has to be accurately cut, brazed, filed and polished. You can't weld the tube; it's too thin (and anyway it'd be really ugly on thin tubes). It takes ages (I've built racks in a matching style - 20-odd joints a time).

My 'bent has a single large-diameter main tube.

Discounting the forks, it has eight (welded) joints.

Incidentally, the price of the latest, top-of-the-range Moulton is shortly rising to about £14,000. I won't be buying one.