Typical tyre profile
Detail magnification
It is sometimes desirable to fit a steel tyre to a cast iron wheel. Several reasons for doing this are the repair or refurbishment of damaged or badly worn wheels, or concern (often unjustified) over possible excessive wear of cast iron wheels on steel track.
Typical tyre profile |
Detail magnification |
The usual way, and typical profile, of fitting tyres is shown in the diagram at the left. While it is possible to have a straight tyre and wheel rim, it is usual practice to turn a small lip or shoulder on the rim of the tyre (and a corresponding groove profile on the wheel). The reason for this is not, as sometimes thought, to prevent the tyre from moving on the wheel during service - there is more than enough strength in the shrink-fit to hold the tyre in position, but to provide a precise location for the tyre during fitting. The tyre is VERY hot and difficult to handle when fitting, and the tyre just sits in the correct position on the rim and stays put, and square, while the tyre cools (and shrinks into place).
How much interference fit is required for a shrink-fit tyre?
The old golden rule still applies: 1 thou (0.001") per inch plus an additional thou. [e.g. on a 6" dia wheel, the tyre would be 0.007" smaller dia]
If you are metric this means 25 micron (0.025mm) per 25 mm plus 25 micron. [i.e. on a 150 mm dia wheel, 0.200mm up to 0.225mm is about right.]
Turn the wheels and the tyres, and check, and re-check, your measurements three times. Ensure all parts are free of burrs and sharp edges etc, and are well finished. Just a normal smooth tool finish, but not polished, is best.
Heat the tyre to 280°C to 300°C [540~570°F] (full purple to dark blue colour). A normal kitchen gas stove is all that is required to heat the tyre. Just make sure the heating is even for the whole tyre.
A full steel colour temperature chart is available here.
Place or mount the wheel hub in a suitable heat resistant position with the wheel face horizontal, and drop the hot tyre on. Don't panic - it will seem as though there is too much for it contract! Just wait a bit and watch it shrink before your eyes into place!
If ever there is a need to remove the tyre, just cut carefully with a hacksaw through the tyre until almost cut completely through. Then use a cold chisel or similar in the cut to perform the final separation. The tyre will then just become loose with a 'pop' and fall off. (Note: Heating the tyre again to expand it usually doesn't work very well as the whole wheel tends to heat up and expand as well, defeating the purpose of heating the tyre.)
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