A typical pickup truck will have the standard 4 wheels. A modified version that is designed to carry heavy loads will have 6 wheels, 2 in the front and 4 in the back, 2 on the left, paired together, and 2 on the right, paired together. Pairs of wheels are also common on other trucks, like the typical "18-wheeler".
A bicycle is in some ways the most efficient form of transportation, because while it is lightweight, it can carry many times its own weight. So, power used to move the bicycle and the rider mostly goes toward moving the rider. Compare that to a big truck, which tends to weigh about the same as the maximum load it can carry; half of that vehicle's power goes to moving the vehicle, not the load.
Nevertheless, bicycles do have weight limits; a bike that carries too much weight will often experience broken spokes, most often in the rear wheels. That's because of the way the typical bicycle is designed, more of the rider's wieght is carried by the rear wheel than by the front wheel.
So, this Idea is simple; replace the single rear wheel with a pair of wheels. Both wheels would be on the same axle and inside the frame of the bicycle; this is not a "trike" design; at first glance this bike will appear to be quite ordinary. But this bike can carry a quite heavy rider, with twice as many spokes for support. And anyone who thinks riding a trike is "sissy" will not object to riding this bike!
Naturally, the rear part of the bike frame will need to be a bit wider, to accommodate two wheels. And the brake system might need modification as well, although many new bikes these days have a kind of disc brake (could easily put the disc on one wheel while the gear changing mechanism is attached to the other wheel).-- Vernon, Apr 24 2009 And the advantage of this over a single wheel with a stronger rim and spokes (but using the same frame and breaks) is.....-- MaxwellBuchanan, Apr 24 2009 [21 Quest], a guy weighing 150kilograms will need the bike to be able to carry the load. An existing bike probably can, for a modest time, until the spokes start breaking. In this case the distribution of weight is nearly normal; the trike design is not needed if the rider has decent balance and bike-riding practice.
[maxwellBuchanan], the advantage is that the heavy-duty bike wheels are not easy to find in the average bicycle shop. But all shops have plenty of ordinary wheels available.-- Vernon, Apr 24 2009 [21 Quest], it might be a kind of chicen-and-egg issue, because if the big guy is always breaking the bike he tries to ride, because it can't carry the load long enough, then of course you won't see many big guys on bicycles. (There are plenty of heavy-set motorcyclists, though.)-- Vernon, Apr 24 2009 Um, 320 lbs here when I started riding, it's dropped a bit since then. However, a deep v wheel, with the correct spokes and keeping them correctly tensioned will stand up to an awful lot. I routinely drop 30-40 lbs on my rear rack without any wheel breakage problems.-- MechE, Apr 24 2009 four words, Ho Chi Minh Trail. A properly tuned and loaded bicycle can transport 350lbs, plus rider, over cross country terrain for 150 miles without failure.-- WcW, Apr 24 2009 // But all shops have plenty of ordinary wheels available.// Yes, but they sell very few bikes capable of taking two rear wheels.
If you've got to either make and distribute modified bike frames which will only be useable with a doubled-up standard wheel, or a modified bike wheel which can be fitted to a standard frame, it's a nob rainer.-- MaxwellBuchanan, Apr 24 2009 Don't forget that except at very slow speeds, you steer by leaning. Two back wheels would make it hard to steer the normal way, but not really stable enough to steer like a trike, unless you had them wide enough, in which case it's just a trike.-- mitxela, Apr 24 2009 Also, due to uneven ground and leaning, only one of the rear wheels would be supporting the weight much of the time, which may defeat the purpose to some extent.-- spidermother, Apr 25 2009 Ah, which leads to where I was going. With some smart suspension, this could be a racing enhancement.-- normzone, Apr 25 2009 This design specifically says both wheels have the same axle. Though this can be modified to accommodate a slight unevenness of the ground, it would be inferior when compared to independently suspended rear wheels with their own axles to handle extreme unevenness, especially often encountered when leaning upon cornering on rough roads. The only way to accommodate unevenness is to have a horizontal support beam structure to which the axle can pivot, but within limits such that the downward force should only lie somewhere between the two ground contact areas of the two wheels. Well, this is much simpler than independently suspended rear wheels, but when only one of the rear wheels would be supporting the weight at a certain instance, as pointed by [spidermother], the purpose could be defeated.-- rotary, Apr 26 2009 Optimally, you would want a tilting pair rear wheel setup. I can picture one of these which would be a drop-in replacement for a single rear wheel.
Passing through the original rear fork fitting is a strong axle with one standard rear-wheel multi-cog hub and gearing inside the fork, and two single cogs outside the fork on either side.
Two beams extend backwards a short distance outside the bike, each beam being attached to and allowed to pivot around a small strong horizontal beam. The point of pivot is co-axial with the aforementioned axle. A chain from each external cog drives a single-cog rear wheel on each side. The pivoting arms allow you to lean into curves while still keeping both wheels on the ground and spreading the load between them.-- BunsenHoneydew, Oct 26 2010 A far better solution (as [Max-B] suggests) is to just make the back wheel a bit stronger. Strong wheels are very easy to come by - you just have to ask for a wheel that's been built for a touring tandem. A touring tandem is significantly heavier than a single-person road bike, and will carry two riders plus front and rear panniers, most of this weight being taken by the rear wheel. Wheels made for tandems are noticably stronger than normal wheels.-- hippo, Oct 26 2010 Most wheels are made by machine, because inserting and screwing down 36 wire spokes is the kind of thing a machine does a lot cheaper than a person. Unfortunately, adjusting all the spokes to be at optimal tension so that the wheel is tight and balanced and even (known as ‘trueing’ it) is done a lot better by a person than by a machine. So any machine-made wheel can be made a lot stronger by taking it to a wheel-builder and asking them to true it for you. Or by learning to do it yourself (it's fun and easy!)
As [hippo] says, touring equipment is much better and more useful for the majority of the world’s cyclists than mountain or road racing equipment, which is why most cycle shops don’t bother carrying touring cycles and equipment, and concentrate instead on dedicated mountain and racing bikes.
And this idea is silly, since as soon as you lean the bike will a)become unstable, and 2) rest all its weight on one of the pair of wheels, utterly negating any supposed benefit.-- pocmloc, Oct 26 2010 random, halfbakery