With hindsight, the choice of the 16″/349mm wheel size for the original Moulton seems a little small. It was an existing size used on children’s bikes and may have been the best option available at the time, but subsequent spaceframe Moultons use 17″ or 20″ (BMX-sized, but really about 18 1/4″ with narrow road tyres) wheels for lower rolling resistance on indifferent surfaces.
The popularity of Brompton folding bikes means there is now a very good selection of road-oriented tyres in the 349 size. Schwalbe is the most prolific manufacturer and there are tyres available from about £10 to £50 depending on the weight, pressure and rolling resistance being offered.
The most important difference, however, is down to the rims. All F-frame Moultons originally came with Dunlop chromed steel rims. These rims do not have a hooked profile and are straight-sided (something currently making a comeback on road racing bikes, for low-pressure tubeless tyres). The hooked rim (or crotchet rim) was introduced to allow higher pressures, as the tyre bead locks under the hook when inflated and is far more resistant to blowing off. As a side benefit, the hook also neutralises the effect of tyre pressure on spoke tension, because the compressive action of an inflated tyre is counteracted by the outward pull on the hook.
A hookless, straight, rim cannot support high tyre pressures. The maximum pressure for a typical 1 3/8″ tyre is about 60psi (4 bar). This means that, if you are running the original steel rims, your tyre choice is far more limited. You could fit any tyre and just inflate it to 60psi, but a tyre designed for 100psi or more in a hooked rim is unlikely to perform at its best, and the sidewalls may be excessively soft. There is also a risk of blowing off, even at 60psi, if the wrong type of tyre is used. This ruins the original Moulton concept of suspension and high-pressure tyres, but Moulton had to use the technology available in the early 1960s.
The best choices for steel rims are the Raleigh Record (55psi) or the Schwalbe HS110 (60psi). Neither of these are particularly fast. If you want your F-frame to perform better, you will need to change the rims (and probably the spokes).
If you have 28 hole hubs, the older type of Brompton alloy rim is very inexpensive, at about £13 each. It doesn’t have the ideal box section but it is a reliable and fairly light rim. For 36 hole hubs on early F-frames, the only real choice in the 349 size is the Sun CR18. This is far more expensive but is a good rim and has a polished finish which doesn’t look too different to the original chromed steel.
You can use any 349 size tyres if you change to alloy rims, and run them at their maximum sidewall pressure – something that is recommended, as it reduces rolling resistance and lets the suspension do all the work, as Alex Moulton intended. Schwalbe Marathons are a popular choice. Schwalbe Kojaks are lighter but don’t roll as well as you would expect from the price and the lightweight construction. The folding bead ones are never cheap but the wired bead ones are worth buying if you see them on sale for about £15. They are reliable, if not the fastest. The Schwalbe Pro One is probably the best but, at about £50 each, you would need to be building a very special F-frame indeed. The old Schwalbe Stelvios were narrower than Kojaks and roll very well, but have been discontinued for years. if you find a NOS pair that has been well stored, it is worth buying.
There are tyres available from other makers but most are too wide for an F-frame with mudguards. Beware Chinese knock-offs – there are amberwall tyres that look almost exactly like the Schwalbe Pro One but these are actually an obscure brand using a similar logo, and are unlikely to be particularly good.
One odd issue you may encounter when fitting tyres to the original steel rims is that, because of the lack of a hook, the tyre will not centre properly. This may be because the tyre is badly made (even some Schwalbe tyres can be wobbly) but, assuming the tyre is ok, you can encourage it to sit straighter by lubricating the rim with soapy water before fitting, and by slightly over-inflating to begin with.
Moultons and other endangered species 