Kiwi HPV - Events

The Human Powered Challenge 2006

Designing a Vehicle

Home Rules Design Construction Testing

What? Me Design a Vehicle?

Yes! It's nothing like as hard as you might think. All sorts of people design and build their own HPVs.

It can be very addictive, so be warned!

Here is a set of ideas you need to consider when designing your vehicle.

If you take some care with the design, you will make the building stage easier and quicker. You won't have to spend so much time fixing mistakes.

Design doesn't stop when building starts. Construction doesn't have to wait until designing is finished. Testing can start while design and construction are still going on. You might have to build something to try out a idea. If the idea works, you use it. If not, you try something else.

Choices	Design is about choosing from a range of possibilities. Good designs often look very simple, but this simplicity is usually the result of a lot of thinking about all the things that could go into the design, and then only choosing what is absolutely required. A well-built simple design will often perform better than a poorly-built fancy design. Do some looking around at what other people have done and well it worked. Sketch some ideas on some paper and talk about them with your team mates.
	Some choices for the vehicle design are: How many wheels? How many riders? Faired or unfaired? Big wheels, small wheels? Rider position? Hands or Feet? Tyres and suspension? What Materials? These choices are detailed below:

How many wheels?

The Rules of the Challenge allow the vehicle to have any number of wheels, but practically, the choice is either two wheels or three.

	Two wheels	Three wheels
Balance	more difficult with a fairing	no problems, even with a fairing
Size	narrow, so less air resistance	wider cross-section, so more air resistance
Strength	no special requirements	wheels must be stronger than normal because of sideways forces
Steering	straightforward	more complicated
Weight	straightforward frame	more framework, so usually heavier

How many riders?

The Rules of the Challenge allow any number of riders but practically, the choice is either a single rider or a pair of riders.

	Single rider	Dual rider/tandem
Power-train (pedals, chains, etc)	straightforward	more complicated
Weight	shorter and lighter	longer and heavier
Power		twice the power of a single, but less than twice the drag for a good design

Faired or Unfaired?

A fairing is a streamlining shell that attaches to the frame, to reduce the air resistance of the vehicle. The Rules of the Challenge allow fairings to be used.

A fairing is not an all or nothing choice. Partial fairings are possible, and can provide useful performance improvements. A fairing can also be added to an existing bare frame at a later date. See the Building page for more about fairing construction.

	Good	Bad	Comment
Weight		X	a fairing adds some weight, but usually less than 20% of total frame + rider weight. This means that acceleration is reduced
Access		X	a fairing makes it harder for riders to get into or out of the vehicle. A partial fairing can be designed to allow access
Drag	X		a well designed fairing will reduce drag and allow higher cruising speeds, which is significant in headwinds and in the endurance event

Big wheels or small wheels?

	Big wheels	Small wheels
Rolling	roll easier over rough surfaces	more affected by potholes and bumps
Gearing	easier to get higher speeds	easier for low gears for acceleration
Strength	weaker for sideways loads (very important for trikes and quads)	stronger for sideways loads
Size	taller vehicle has more air resistance	lower vehicle has less air resistance

Rider position

	Upright	Laid back (recumbent)	Prone (head first)
Power	familiar position from "normal" bikes	requires some training to develop different muscles	requires training
Drag	higher, so more air resistance	lower, so less air resistance	less air resistance
Comfort	narrow seat, not very comfortable	good back support, more comfortable for endurance event	?
Adjustment	easier (?) to adjust vehicle to different riders	possibly more complicated adjustments required for different riders	possibly more complicated adjustments required for different riders

Hands or Feet

The vehicle will be powered by the effort of the riders, the "motor". A rider can provide power with arms or legs or both. Most people are much stronger in their legs than their arms, so pedals will be the best way of linking the power to the wheels.

Hand cranks could be used as well as pedals to get some more power from the motor, but this adds weight and complication to the vehicle. Unless the riders have very strong arms (most of us don't!), the extra complication is not worthwhile.

Tyres and suspension

High pressure tyres without much tread roll more easily on a firm surface than soft tyres with knobbly treads. However, high pressure tyres mean that any roughness and bumps on the road surface are passed to the frame of the vehicle. This makes for an unpleasant ride, especially during the endurance event, and can also lead to frame damage from the repeated vibration.

This can be dealt with in several ways:

full suspension - add suspension and shock absorbers to the wheels. This make the frame more complicated than an unsuspended frame and adds some weight. However, since the Challenge is being run on sealed roads, the suspension does not need to have lots of travel like a mountain bike; 20-30mm of travel will be plenty.
partial suspension - add padding to the rider's seat to absorb the road vibration. Make the frame strong enough to withstand the vibration, or leave enough flex in the frame to have enough "bounce" to soften the vibration. This is less complicated than full suspension.

Frame Materials

Some materials are easier to work with than others. See the Construction page for more discussion about this.