Quadricycle

For many years now, I've had a design in mind for a recumbent quadricycle (sometimes called a pedal-car or pedal-kart). I'm not sure what the official definition is but, basically it is a 4-wheeled bicycle where the rider sits in a seat rather than on one. In my case it looks a little bit like a go-kart with pedals and narrow wheels. My original intention was to design and build a 4-wheeled mountain bike, as I have had too many hospital visits from outings on my traditional mountain bike. It was to feature all-round, fully independent suspension and 4-wheel drive but, my first design and initials costs showed this to be unrealistic in terms of weight and money. It remains a dream in carbon fibre and titanium.

One advantage of having more than two wheels on a recumbent cycle is that you can stop at junctions and other obstacles and not have to put your feet down. This means you can leave your feet clipped into the pedals and hill starts are much simpler. The other advantage with recumbents (depending on the configuration), is that you push against the seat to extert pressure on the pedals and don't just use your body weight. This means acceleration and top speed are increased.

The perception that recumbent cycles are less safe than a normal cycle is misplaced. The very nature of the seating position means that your head is supported and relaxed in the position required to look at the road ahead, rather than the standard 'head down' cycling position. The low centre of gravity means that accidents are less likely and you are better protected in the event of a crash, through a feet forward riding position. On slippery surfaces a skid is also much more likely to be recoverable.

Why have I chosen four wheels and not three? A good question that just about everyone asks. I have no doubt that trikes are lighter, faster, have less rolling resistance and a lower centre of gravity. A trike is also naturally designed to keep all of its wheels on the ground over varied terrain. If speed is your only goal, then a recumbent bike or trike is the solution.

A quadricycle is slightly more stable and has better all round balance. It is also shorter in length and offers greater manouverability. The main reason for me though, is that it has the right handling characteristics, in that it matches my experiences of car and kart racing. It is something that you throw into corners with confidence. It has two driven rear wheels for good traction. As designed, it also has more storage space. Another factor is that it simply looks better with matching small wheels. My final reason is simply that I like engineering challenges and the elegant drive of my quadricycle avoids the huge chain lengths, chain tubes/tensioners and carriers I've seen on recumbent trikes.

My goal has always been to keep it relatively light and to build from readily available parts. This almost explains why it has taken so long to design and build my quadricycle. Even now I have had to resort to some custom made parts but, these are fairly generic in their application. The breakthrough in terms of implementing my plan was the discovery of go-kart component suppliers on-line. Go-kart parts are superbly engineered and fairly light-weight. They are also very cheap, since they are made in large numbers.

My Requirements

My requirements have changed over the last few years. Originally I was looking for a 4-wheeled alternative to my mountain bike but now I'm looking for a commuter bike that can handle the bridle paths and cycle ways, I encounter on my way to work. As a commuter quadricycle the other key requirements is that I can use it every day, regardless of weather and that I stay dry in doing so. It also has to carry a couple of bags containing computing and electronics equipment and require no change of clothing to ride it to work (a 4km ride with no hills).

Efficiency is not a critical design issue, as one of the reasons for commuting by pedal power is that it involves some exercise. Having said that the quality components and bearings used mean rolling resistance is very low. The quadricycle is as light as possible but sturdy enough to withstand some reasonable off-road use. I've compromised with the tyres which are Schwalbe Marathon tyres.

My quadricycle is approximatley 160cm long to allow me to fit in the steering, chainset, seat, rear axle and storage space. It could be shorter but this length guarantees my feet can't foul wheels, the steering, the chassis or the ground. It could be made narrower but I've constructed it to be 90cm wide for improved stability on cornering. The downside is that it may not fit between some bollards found on some cycleways. The size of the wheels affects the look but it also affects the ground clearance and the turning circle.

I'm looking to make this all-weather transport, so some kind of removeable bodywork and roof is under design.

The Design

Chassis

To reduce costs, the initial chassis is made of steel. The plan is to make an aluminium verswion when the design has gone through a few more iterations and has been throughly tested.

Wheels

The overall look of the quadricycle is heavily determined by the chosen wheel size. 14" rims were my initial choice but, I eventually decided to use 16" rims. 14" wheels are only really used on children's bikes and sourcing quality rims was pretty much impossible. Sourcing some good quality 16" rims was also hard work but, I eventually found a supplier that sold aluminium rims, in 36-spoke form to match my front and rear hubs. The rims are painted black to improve their appearance. The wheels and hubs account for a large part of the total cost.

Shimano Deore XT front disc hubs in black are used for each front wheel. The wheels are custom built to my 16" rims. As the quadricycle does not lean, high lateral forces are placed on the rim under hard cornering. For this reason 36-spoke rims and hubs are used to provide a strong and rigid wheel. These hubs come with a 10mm hollow axle which had to be replaced with a longer, solid axle. Ideally, I'd like to upgrade these later to a 12mm axle design.

A front wheel with brake disc attached.

The rear wheels use the same 36-spoke 16" rims. My original plan was to get some custom made hubs fabricated but these were going to work out as being rather expensive. Instead I've modified some existing rear kart wheels (machined down to 80mm width and purchased from the manufacturer without valve holes in the rims) and put spokes through the rims in a radial fashion. These bolt to a standard go-kart hub, on three 8mm diameter studs at 58mm PCD. The modified kart hubs are also painted black. Each complete wheel, with inner tube and tyre weighs 1225g.

A rear wheel based on a kart hub and a 36-spoke 16" rim.

Brakes

My chosen design means that in-board disc brakes or drums are the only real options. There are simply no mounting points for rim brakes with side-mounted wheels. Wear on my expensive and hard to find rims was also a consideration. I decided to use disc brakes all round but this is an expensive solution and probably over engineered.

I'm using Shimano RT-60 160mm discs. The front ones are mounted on Shimano Deore XT front disc hubs. The rear discs will be mounted in-board on custom shaft mounts. I'm going to see how I get on with front brakes only and add rear brakes if required. This will save weight and about Ł200. There wil also be a parking brake.

My quadricycle uses Shimano V-brake levers. The right one operates on the front brakes and the left one will operate the back brakes, if they are added. The brake cables run a BMX bike adaptor which converts one cable to pull on two cables. These connect to the two rear cable-driven, Shimano Deore disc calipers. It is worth noting that these brakes are designed to stop 26" wheels and I'm using them on much smaller wheels. The reduced torque on the brake disc will mean the stopping power is going to be pretty awesome and I'm going to get through front tyres very quickly.

I did consider hydraulic brake calipers but the costs are much higher. A cable calliper is Ł45 and a hydraulic calliper is nearer Ł100. The levers are also more expensive. Hydraulics have the advantage of in-built self-balancing but at the cost of twice the brake lever travel. My cable solution will require a harder pull on the lever and careful balancing by manual set up. I'm also investigating the use of a balance bar to ease adjustment of the brake bias.

Transmission

The transmission is split into three seperate sections. Firstly there is a single ring chainset. This drives a centre hub via a chain and then, two chains link the centre hub to two free-wheels on the rear half-shafts. The multiple gearing effectively makes the 16" wheels act roughly like 26" wheels and provides a range of gearing from 75.6 to 26.0 inches. There are no complicated chain paths and tensioners, the chain does not run through any piping at any point and is as short as possible. The centre hub is adjustable to tension the rear chains. The objectives are minimal weight and minimal friction. Even the derailleur is positioned right under the seat to minimise the length of cable required.

Drive is from a single front chainset with 36 teeth and a standard 170mm crank. Getting hold of a 36-tooth single chainset is impossible so I use a twin 42/36 tooth chainset. The outer ring acts as a chain guard and helps keep the chain on. The inner chain ring is interchangable to alter the gear ratios, if required. There is no front derailleur but I can manually move the chain across if I wanted to tour with a higher set of ratios (100.8 - 34.7 inches). I also use a custom fabricated 'chain-keeper' to stop the chain falling off the inner ring, something that can occur with a single chain ring driving a 9-speed cassette. The bottom bracket is mounted through part of a normal bike frame which has been cut down and mounted on plates to bolt to the chassis.

The chainset drives a 'centre hub'. This is a Shimano Deore rear wheel hub with a 9-speed cassette (11-32 teeth). The hub provides a free-wheel capability and a mounting point for two 26-tooth sprockets (with four 8mm mounting holes). The two sprockets are mounted to custom sprocket carrier disks, fabricated from 5mm aluminium plate and then cut down the middle. The two halves are then mounted around the wheel hub and then bolted to the wheels rims, using 2.5mm bolts through the spoke holes. There are also bolted to the two sprockets which are passed over the hub rims and are bolted onto the sides of the carrier. The free-wheel obviously means the quadricycle has no reverse gear and drive. A Shimano Deore 9-speed rear derailleur is used to change gear on this central cassette using a handlebar mounted Shimano Deore STI gear lever.

The centre hub.

The centre hub is mounted in a lightweight frame, which is attached to the chassis via bolts. This can be rapidly dismantle to remove the chains, if required. The centre hub drops down into a guide channel which provides fore/aft movement to tension the rear chains. For now, I've retained the quick release hub to acheive this but this will probably be replaced by a pair of nuts, which will be much lighter.

The two 26-tooth central sprockets drive two 18-tooth free-wheels mounted on the end of each rear half-shaft. The half-shafts are custom fabricated from solid 25mm aluminium rod, to provide a light-weight and short version of standard 25mm go-kart axle. They are of unequal length due to the offset centre hub and have a 6mm keyway. The free-wheels allow the shafts to both be driven but to rotate at different speeds whilst cornering. This removes the need for a differential as the outer wheel simply free-wheels to catch up with the inner wheel on cornering. The free-wheels are attached to the half-shafts via custom made mounts that are also keyed. They accept standard single free-wheels on a 1.375", 24tpi thread. These mounts are the only custom fabricated parts on the bike that I've paid someone else to make. They cost me £141 for the pair.

These are the carriers that allow the single freewheels to drive the 25mm kart axles.

These are the gold anodised kart wheel hubs. They are an amazing bit of engineering for the £14 they cost each.

Chains

The bike uses four Shimano HG53, 9-speed chains (114 links). Two are reduced down to 96 links to drive the rear axles and the other two are joined to form a single chain from the chain set to the centre hub (? links). The tension of the rear chains is adjusted by moving the centre hub fore/aft in its mounting frame. Tension in the front chain is maintained by the 9-speed derailleur on the centre hub.

Steering

The steering is a via handle bars which pivot on the chassis, just in front/under of the seat. This has the brake levers, gear lever and light switches mounted on it. An short stem mounts through the chassis rail in front of the seat. Tie rods connect this to the pivot points on the front uprights.

The front steering uprights are acheived using a Cane Creek C2 headset on each side with a custom fork column tube, to which the front axles are fixed. Nuts are welded to this tube and the axle runs through them at 23° to provide the centre point steering action. The upright also has fixing points for the disc calipers and fixing for the steering pivot. The upright is also inclined backwards by 12° to provide a self centering action.

The tie-rods are installed based on Ackermann steering geometry to ensure that the inner wheel turns more to optimise grip on cornering. Because Im envisage lots of high-speed cornering, the wheels will be slightly more parallel than pure Ackermann geometry dictates. This counteracts the bikes basic nature to continue in a forward motion due to momentum. There is no toe-in, toe-out or camber angles on my design.

There are a few web sites worth looking at about steering:

Having measured up the front wheels and steering geometry, I need a king-pin inclination of 23° to get full centre-point steering. This is quite a bit more than the recommended maximum of 15° for a recumbent trike. I'm going to go with this though because the weight distribution on my quadricycle is very different to that of a typical recumbent trike.

The reason for limiting the kingpin angle is because an angled kingpin means that the bike is actually lifted when the wheels are turned. The more it is inclined, the more it lifts the bike and the more effort is required to turn the wheels. Based on measurements from a friends recumbent trike, the front:rear weight distribution is typically around 70:30. Based on my measurements with two set of scales my quadricycle has a weight distribution of 45:55. Simple maths shows that my weight distribution and a kingpin angle will require less effort to turn the wheels, than a recumbent trike with a 15° kingpin inclination and 70:30 weight distribution.

Seating

The single biggest difference between recumbents and normal bicycles is the seat and the one thing that decides whether a recumbent works or not is the seat. You've got to get the seat right!

Recumbent bicycles offer much better rider comfort due to the fact that they distribute the riders weight and offer better back and head support. I've spent ages on the design and build of my seat, making sure that it is very comfortable, suitably robust and also light-weight. Unlike some recumbent seats I've seen, I've deliberately tried to avoid making a seat that looks like part of a hacked about rucksack.

My seat is a single piece of moulded GRP and the basic design is taken from the Tillett race seat in my Fisher Fury R1 kitcar, which was used to create a suitable mould. It's hugely supportive and very comfortable.

A recumbent seat has numerous advantages as outlined in this article.