50x11 at 100RPM = what speed?

R8JimBob88
R8JimBob88 Posts: 285
edited March 2011 in Road beginners
Evening people of BikeRadar.

Just wondering how to work out what speed you would be traveiling at 100RPM on a 50 ring at the front and an 11 tooth cog at the back?

What formula do you use to calculate it?

Reason being is im currently in the process of building a 10 speed compact bike. I live close to the peaks and often find myself averaging over 30 mph for a fair few consecutive miles on some long decents and want to know if I should go for a 11 or 12 tooth small cog?
If you do what you have always done, you will get what you always got....

Comments

  • Zachariah
    Zachariah Posts: 782
    This is a very good applet for figuring things like that out.

    For a 700x23 wheel, you'd be doing 35.5mph.

    For the 12-tooth, 32.6mph.
  • R8JimBob88
    R8JimBob88 Posts: 285
    Zachariah wrote:
    This is a very good applet for figuring things like that out.

    For a 700x23 wheel, you'd be doing 35.5mph.

    For the 12-tooth, 32.6mph.

    Very helpful! Thanks.

    Still not sure though :lol: I think i'll go for the 11 tooth
    If you do what you have always done, you will get what you always got....
  • MikeWW
    MikeWW Posts: 723
    I run 50/12. That gives me a top speed about 38mph pedaling wise

    Seems to be enough
    www.lichfieldccc.org.uk
  • Bar Shaker
    Bar Shaker Posts: 2,313
    I changed from a 12 to 11 and it has given me a few extra mph down the hills. I was hitting 120+rpm down some hills on the 12 tooth.
    Boardman Elite SLR 9.2S
    Boardman FS Pro
  • R8JimBob88
    R8JimBob88 Posts: 285
    11 tooth it is then!
    If you do what you have always done, you will get what you always got....
  • Zachariah
    Zachariah Posts: 782
    Make sure your lowest gear (biggest rear cog) is still the same. The minimal extra speed downhill won't make up for the minutes lost struggling up the hill in the first place!
  • R8JimBob88
    R8JimBob88 Posts: 285
    It was either a 12-27 cassette or 11-28. I'll go for the latter
    If you do what you have always done, you will get what you always got....
  • paul64
    paul64 Posts: 278
    That's potentially quite a lot of gap for the chainline if I understand correctly i.e. 34/50 would ask a chain to cope with bigger gap between the chainrings than, say, a 39/53 or 42/52
  • R8JimBob88
    R8JimBob88 Posts: 285
    paul64 wrote:
    That's potentially quite a lot of gap for the chainline if I understand correctly i.e. 34/50 would ask a chain to cope with bigger gap between the chainrings than, say, a 39/53 or 42/52

    I dont want to sound stupid but, you lost me with that one... :?
    If you do what you have always done, you will get what you always got....
  • rake
    rake Posts: 3,204
    i riiden 52/42 in the past. absolutely no good for steep hills although there was nice cross over ratios.
  • MikeWW
    MikeWW Posts: 723
    R8JimBob88 wrote:
    paul64 wrote:
    That's potentially quite a lot of gap for the chainline if I understand correctly i.e. 34/50 would ask a chain to cope with bigger gap between the chainrings than, say, a 39/53 or 42/52

    I dont want to sound stupid but, you lost me with that one... :?

    If you run 50/34 11/28 then that is 33 teeth. Thats more than a normal short mech can cope with (28) So you will need to go for a longer mech or say change your front inner to 39
    Standard cassette with 50/34 is 12/25
    www.lichfieldccc.org.uk
  • R8JimBob88
    R8JimBob88 Posts: 285
    Ah I wasnt aware of that! So whats the biggest granny gear I can have with a Shimano 105 derailleir and a 34/50 chainring?
    If you do what you have always done, you will get what you always got....
  • jejv
    jejv Posts: 566
    R8JimBob88 wrote:
    paul64 wrote:
    That's potentially quite a lot of gap for the chainline if I understand correctly i.e. 34/50 would ask a chain to cope with bigger gap between the chainrings than, say, a 39/53 or 42/52
    I dont want to sound stupid but, you lost me with that one... :?
    What I thought he meant was that on some compact doubles, the range of gears on the small ring is limited by the chain rubbing against the big ring. Which can happen, but I have no idea how common that is. Probably not very, since 34/50 is pretty common.
  • flasher
    flasher Posts: 1,734
    Depends which 105 RD you have.
  • R8JimBob88
    R8JimBob88 Posts: 285
    I I need a longer cage then?

    Something like - http://www.wiggle.co.uk/shimano-105-570 ... r-gs/#more

    Would I be ok with a 50/34 ring and 11-28 cog on that? I'm lucky enought to have a range of 25% gradient hills nearby so the easier the better :D
    If you do what you have always done, you will get what you always got....
  • flasher
    flasher Posts: 1,734
    No, the latest 105 short cage can take 11-28.
  • R8JimBob88
    R8JimBob88 Posts: 285
    This one?

    http://www.wiggle.co.uk/shimano-105-570 ... illeur-ss/
    If you do what you have always done, you will get what you always got....
  • johnmiosh
    johnmiosh Posts: 211
    The short cage can take a 33 tooth difference. 50-34 is 16, 11-28 is 17, 16 +17 = 33. Right on the limit.
  • MikeWW
    MikeWW Posts: 723
    Yes sorry
    For some reason I had 28 in my head-my mistake :oops:
    www.lichfieldccc.org.uk
  • flasher
    flasher Posts: 1,734
    R8JimBob88 wrote:
    This one?

    http://www.wiggle.co.uk/shimano-105-570 ... illeur-ss/

    Yes
  • JAGGY
    JAGGY Posts: 167
    I run Ultegra 34/50 with 11-28 and a short cage mech. No bother whatsoever.
  • paul64
    paul64 Posts: 278
    Mike got there quicker than me and explained it better anyway. I recall some advice in the distant past that a combination with a lot of teeth and requiring a long chain can then be too slack at the other end when less teeth are used. From subsequent replies though it sounds like the mech can cope so that's good news.

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