String-spoked wooden wheels
by Hans-Jorg Hinz
A new technique of wooden wheel construction using string of a suitable fibre for the spokes yields a product with a number of desirable properties. Technologically somewhere between the "cartwright" and the "bicycle" design, It is technically superior to the wooden spoke wheel in all but two aspects - narrow track and wooden friction bearings - where it has only slight advantages.
The basic design of the wheel and some of the steps in its construction are shown in figures 1 - 4, followed by technical specifications for the most extensively tested version. These data are provided in the Table.
Construction of the wheel
For production, apart from standard carpenter's tools like saw, file (plane), and brace with two rather big bits, 20 - 30 mm and 50 mm in diameter, only two simple jigs are used: a mitre-box for cutting the rim segments and a support for assembling the wheel (see Fig. 2). Hub and bearings can be produced faster and more accurately on a simple lathe, if available.
After arranging the rim segments on the support so that they overlap, the hub with the two oil-soaked bearings is placed on the centre pipe. String loops are then pulled through the rim-holes and around the bearings, where they are tightly knotted (see Fig. 3, support not shown). The number of loops (strands) per spoke depends on the actual strength of the string used and the desired load-bearing capacity of the wheel; as a rule of thumb, the payload of the wheel equals the breakload of the single spoke.
After having inserted all 24 spokes they are gradually twisted to approximately half their break load, careful lay controlling and squareness of the hub. After cutting off the protruding edges of the rim, filing (or planing) it into circular shape, nailing the rubber tread, and fixing the twisting sticks with a locking string, the wheel (see Fig. 4) is finished by oiling all the spokes-an essential treatment.
Tests and results
While payload and breakloads given in the Table have been tested both statically and dynamically, further controlled tests on durability near the breakload still have to be carried out.
Table: Specifications for the "average" wheel tested
Some field tests have been carried out in the Usambara mountains (Northern Tanzania) within the TIRDEP-SECAP project, a GTZ rural development project, where the wheel design was part of a single traction oxcart to be used on the mountain footpaths (see "gate" 4/85).
All other tests were workshop tests conducted by the author, simulating shocks and loads under controlled conditions. Most tests were performed on wheels 55 - 60 cm in diameter, 15 - 25 cm hub length and spokes with a break load of 120 - 480 kg.
The results may be summarized as follows:
• The design allows easy and fast production using standard tools and local materials, requiring neither special skills nor extreme accuracy for the parts. Calculations for overall dimensions and cross-sections to meet actual size and load requirements can be done by simple rules of thumb.
• With a service life of over two years and low maintenance the wheel offers fast and foolproof replacement of all wearing parts: rubber tread, rim segments, spokes, and bearings.
• The wheel's efficiency ratio (payload: deadweight) of over 70:1 at a shock coefficient of 7:1 (break load: payload) is considerably higher than that of cartwright wheels (10 - 30:1 at comparable shock coefficients, but at lower lateral loads) and even higher than that of standard steel rims with pneumatic tyres (efficiency ratio 30 - 40:1). Reliability of the string-spoked wheel is increased by the fact that a spoke rupture will soften but will not break the wheel, even if the critical load persists.
• Smooth running is one consequence of the spokes' elasticity, the order is a quite substantial angle of inclination when subjected to lateral forces: 6° at the lateral breakload, three times more than in a comparable wire spoke wheel.
• Utilization of this kind of wheel is mainly restricted by the rather narrow tread resulting in high ground pressure and high rolling resistance on soft ground. Another restriction lies in the use of wooden friction bearings, which allow speeds below 30 km/h only. String-spoked wheels may thus be used with all kinds of hand-, animal-, bicycle- and slow motor-drawn carts and trailors, including wheelchairs, wheelbarrows and farm implements.
A wider tread (see Fig. 5a) will increase the wheel's versatility, but at the expense of deadweight and production time. Shaping the rim to fit any standard pneumatic tyre will yield a proper substitute for the wire-spoke, steel-rim wheel (see Fig. 5b).
Other variants of the basic design pertain to spinning-, wind water- and gear-wheels, all sharing the advantages of simple strong, and lightweight construction.
Similarly, rope-braced lattices for windmill towers, roof trusses, house frames, carts, or furniture may offer a design alternative in cases where scarcity of materials or weight I limits exclude traditional or standard solutions. A small research project on rope-braced wooden lattices is planned by Prof. Dr. A. W. Date, CETARA, 11 Bombay, in cooperation with the author.
A paper with design details, static evaluations and some test results is available from the author.
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