Q Where can I get solid inner tubes? I once saw one made from a solid compound that fitted inside your existing tyre. It required no pumping and never went flat. - JT, Donegal
A You could try www.greentyre.co.uk or www.hancox.co.uk , which both supply solid tyres - not inner tubes - made from microcellular polyurethane. Bear in mind that the rolling resistance is going to be higher than that of a fully inflated conventional tyre. And people who have tried tyres of this type say they are hard to fit. The use of solid tyres and variants has virtually died out as puncture-resistance in conventional tyres has improved hugely over the past decades. Tyres with a strip of Kevlar to prevent punctures are now common. And Schwalbe's Marathon Plus has a thick, squishy layer of material under the tread that makes it as near impregnable as you could reasonably hope for.
Q I have a 21-gear bike - three chainrings at the front, seven sprockets at the back. There is some duplication. Where the effective gearing is the same, is there an advantage in using larger chainwheel with larger sprocket as opposed to smaller chainwheel with smaller sprocket? - DL, Nayland, Suffolk
A A chain wrapping round larger diameter sprockets is a tiny bit more efficient, but much more important is to avoid having the chain wrenched out of line between front and back - for example by running round the smallest (inner) chainwheel to the smallest (outer) rear sprocket. That does cost you effort - and wears out the chain.
Send your bike clinic questions to ingear@sunday-times.co.uk