I recently bought a mountain bike after a decade of riding hybrids. I knew hybrids were faster and I assumed it was because they had larger wheels: basically you turn the crank, that turns the back wheel and since the wheel is larger on a hybrid you get a bit of extra mileage, unless you’re going uphill in which case there is no advantage because you’re in a lower gear anyway.
26 inches (the wheel size on mountain bikes) is 660 millimetres while hybrids have 700 millimetre wheels, which is about 7% more ground to cover. In addition the hybrids have narrower wheels so they offer less rolling resistance.
That was enough for me until I bought this mountain bike. It was frighteningly slow and I noticed that the crank was a lot smaller then my city bike. So I investigated and discovered that the number of teeth was written on the front crank. The mountain bike had 42 teeth and the hybrid had 53. Actually what I have isn’t a hybrid, its a high performance city bike, it isn’t designed to double as a mountain bike, if anything it doubles as a racing bike, its half a city bike and half a racer and it has an awfully large front crank. 53 front teeth is very rare on bicycles and even racing bikes usually have 50. That means that when you turn the pedals the back wheel covers a lot of ground (if you have the strength to move it of course). Compared to a mountain bike with 42 teeth, that’s an extra 25%. Add that to the wheel size and you’re talking a third more speed being delivered to the back wheel.
Hybrids and some mountain bikes have 48 teeth which is OK but quite a few bicycles have 42 which is OK if you want to ride in sand dunes but won’t get you very far in the urban sphere unless you only ride uphill in which case it might be quite useful.
As the wheel size issue becomes more widely known, people have started riding 29 inch mountain bikes which solve the wheel size problem by having larger wheels. 29 inches is 737 millimeters so in theory the mountain bikes gain the advantage.
Armed with this knowledge I have discovered that people are paying vast sums of money for bicycles with really fancy hydraulic brakes and 29 inch wheels but which have a 42 teeth crank which completely defeats the purpose of the larger wheels (and the brakes for that matter). Incidentally some of the bicycles in Tel Aviv with hydraulic brakes cost more then motorbikes which not only have hydraulic brakes: They have engines too. So somebody is over-charging somewhere.
I periodically race fixed gear riders on my way to work. We randomly meet up and start racing. Fixed gears are very cool, problem is that to enable them to go uphill they need 42 teeth front cranks… So that’s advantage me. I usually win.