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Am considering purchasing a folding bike, but curious to know if there is any way to know (other than planned / timed test riding) if a particular model is faster (top speed, in a velodrome, with a reasonably fit rider) than the standard boris bike available in London?

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    The top speed of any bike will depend on the person riding it and the conditions, as will whether or not it's safe to ride at any particular speed. So it seems to me that you're asking the wrong question. – David Richerby Dec 14 '16 at 12:01
  • Thanks @DavidRicherby - trying to keep it in the rules (not asking for model recommendations), have updated the question to make it clearer. – Allan Bowe Dec 14 '16 at 12:07
  • @DavidRicherby I suspect this will come down to the gearing. That's more likely to set the clear-road speed limit than handling. Maybe I'll get round to running the numbers on that basis, or maybe someone else will first – Chris H Dec 14 '16 at 12:17
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    I appreciate that you're trying to make your question concrete and avoid it being a product recommendation question. But "top speed" really doesn't seem like a good criterion to be basing your decision on. – David Richerby Dec 14 '16 at 12:26
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    There are also folding bikes with bigger wheels and more conventional geometry. If you plan on taking one on public transport check first because at least GWR (main line into Paddington) has a maximum wheel size that they class as a folder. – Chris H Dec 14 '16 at 16:34
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Neither Boris bikes nor folding bikes are exactly built for speed, so we can consider mainly the gearing. For this, gear inches are a convenient measure taking into account not just the gear ratio but the wheel size.

On Brompton folders (as a well-documented example) you have a choice of gearing but the default 6-speed gives up to 100 gear inches. That's quite high for a bike with small wheels and flat bars. For short urban journeys you might not make it into top on the Brompton.

The Boris bike is a 3-speed with up to 60 gear inches. this is actually lower than the single speed Brompton. But it's not a stupidly low cruising gear. If you know the rear wheel size and tooth counts for the chainring(s) and sprocket(s) (plus any hub gears) you can use a gear inch calculator where you can also work in metric if you'd rather.

For many typical urban journeys, the time taken is determined more by the traffic than the bike. A bigger difference is trading off the unfolding/folding time against finding a Boris bike and then finding somewhere to park it, plus walking between where you park it and your real destination. You know the journeys you're likely to make better than we do, and so you're on your own for that.

Another benefit of having a folder is that it's yours. You know you've got it if there's a problem like a tube strike when the Boris bikes will all be taken. You can also customise lighting and luggage to some extent.

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    Tube strikes or other tube outages in London? That neeeeever happens. :-) – RoboKaren Dec 14 '16 at 17:29
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I have a folder, and its not fast. Riding at about my FTP for an hour returns an average speed of "high twenties km/h" where the same power does mid to high thirties km/h on a normal road bike.

My generic main-beam steel folder has 20" wheels and weighs 15 kilos which is on the high side, and its effectively too small for me. It has a 46 tooth single chainring and now has 11-34 tooth 8 speed. It start as a 14-28 six speed freewheel, which was limiting at the top and bottom of the range. Now all I need is a longer seatpost.

By comparison, I have access to a 3 speed situp bike at work, and suspect that's more like your boris bike. Riding FTP on that definitely goes faster with about 30 km/h.

  • Storage - folders take less space even when opened.
  • Weight - folder should be lighter
  • Seating Comfort - Have to try both and see which you prefer
  • Riding Comfort - Bigger boris bike will ride better and cope with road potholes better. The folder steering is much twitchier.
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    You should note the brand or general style of your bike. I have a 2 speed Brompton (S2L) and it's faster than my seven-speed Dahon simply because it's geared faster, has a better riding position (for a two handed rider, sadface), and is more solidly built. – RoboKaren Dec 15 '16 at 2:17
  • @RoboKaren I've only just associated the nick and the one-handed comments/questions recently. Are you bionic yet? – Criggie Dec 15 '16 at 8:39
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    Presumably you're spinning out on that folder. It seems like a reasonable speed. It's higher geared than a Boris bike taking into account the wheel size. – Chris H Dec 15 '16 at 10:37
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    Not bad at all. I run out of steam just below 120rpm on a downhill, which is around 60km/h on my bike (a hybrid) – Chris H Dec 15 '16 at 19:18
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    And the "same power" comparison is a good match to an arbitrarily chosen drag calculator considering that cdA for hoods (aimed for your road bike) is 55-60% of that for the tops. Strangely enough there aren't any bike per calculators for folding bikes or heavy 3-speed sit-up-and-beg shopping bikes – Chris H Dec 15 '16 at 19:34

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