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  1. #1
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    Replacing Shimano HG-70 13t cog with 11t cog. Do cassette subtypes matter?

    I currently have a 7 speed Shimano HG-70 cassette which, based on the following chart, I believe is an HG-70-7-J. http://www.m-gineering.nl/cassette.pdf

    I'd like to replace my smallest 13t cog with an 11t cog. I've found two possibilities so far.

    An 11t cog for an HG-70-AC
    http://www.niagaracycle.com/product_...ucts_id=403720

    An 11t cog for an HG-70-I-AN
    http://www.niagaracycle.com/product_...ucts_id=403757

    According to google, the HG-70-AC is a 7 speed cassette while the HG-70-I-AN is 8 speed. My questions are:

    1) Have you done this type of replacement before?
    2) Do you know if the replacement cogs above are suitable, or do I need to contact Shimano?
    3) Even if the cogs above will fit on my cassette, are there other issues I should be aware of?

    Thanks in advance.

  2. #2
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    The HG-70-AC (11-28T) should work, so long as your derailleur cage is long enough. I'm guessing you might have short cage rear derailleur since the cassette you're looking to change out is a very narrow range HG-70-7J (13-21T). Pretty much any 7 speed Shimano cassette is interchangeable, it's just the teeth range and derailleur wrap you need you to worry about, which also takes into account range of teeth up front on your crank. What type of bike and what derailleurs/shifters/crank do you have now? The HG-70-AC (11-28T) was a very popular MTB cassette around '94-'95, just before the switch over to 8 speed. The HG-70-I-AN was the 8 speed equivalent and started up around '96. The Shimano LX derailleur from these years was a very popular (and durable!) spec for these cassettes.

    <EDIT> after checking your other links I see you just want to trade out a single cog on your existing cassette, to up your high gear? I'm not 100% sure of the compatibility of just changing out the small cog. If it does work, I don't think you'll like that big jump shift from 11 to 14 teeth.

    According to Sheldon :

    "There is no problem mixing 8-speed sprockets into 7- speed cassette."
    Last edited by RaleighComp; 10-28-10 at 01:30 PM. Reason: add lines

  3. #3
    Constant tinkerer FastJake's Avatar
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    RaleighComp, he's not replacing the whole cassette. He just wants to replace the smallest cog.

    Quote Originally Posted by daveizdum View Post
    1) Have you done this type of replacement before?
    2) Do you know if the replacement cogs above are suitable, or do I need to contact Shimano?
    3) Even if the cogs above will fit on my cassette, are there other issues I should be aware of?
    1. Yes
    2. I've only used Shimano cogs, but that's only because I just take apart spare cassettes to use them for parts. I can't say either way whether a non-Shimano cog will be satisfactory, but I would assume you'll be fine.
    3. The only thing you have to watch out for is the lockring. Some lockrings are not small enough to accommodate an 11T and when you assemble everything you'll find the chain jumps all over the lockring. So, you might need an 11T specific lockring depending on what you have now. Otherwise I haven't had any other issues.

    7-speed and 8-speed spacings are virtually the same. You shouldn't have any problem using a 7-speed or 8-speed cog with your current cassette.

  4. #4
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    There is one other potential problem beside the lockring. The cassette body must be a "compact" type, that is the splines must not go quite to the end of the body. 11T cogs do not seat all the way on the splines and the splines must be short enough to allow the cog to seat fully. If the body has full length splines as used with 12 and 13T small cog, the 11T won't fit. This can be fixed with a Dremel if needed.

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    BTW, is the purpose of this upgrade just to have a bigger top gear where you won't spin out down a big descent? On 3 different road bikes I have top gears of:

    50/11 700x23c tires, 175mm cranks - overall impression: *plenty* tall enough
    52/13 or 14 (different wheelsets) 700x23c tires, 170mm cranks - overall impression: It is nice having the 52/13 over the 52/14 for descents
    42/11 26"x1.5" tires, 175mm cranks - overall impression: eh, so what if I spin out every now and then on my rain/winter bike?

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    Quote Originally Posted by HillRider View Post
    The cassette body must be a "compact" type, that is the splines must not go quite to the end of the body. 11T cogs do not seat all the way on the splines and the splines must be short enough to allow the cog to seat fully. If the body has full length splines as used with 12 and 13T small cog, the 11T won't fit.
    Here are two photos I've found on the web of my hub model (Shimano FH-6401). If I'm understanding you, these photos show that an 11t cog will not work without dremeling.

    http://fotos.mtb-news.de/p/419766


  7. #7
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    Quote Originally Posted by RaleighComp View Post
    I'm not 100% sure of the compatibility of just changing out the small cog. If it does work, I don't think you'll like that big jump shift from 11 to 14 teeth.
    +1
    I would hate a ratio jump like that.

    All of my cassettes start with a 13.

  8. #8
    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by daveizdum View Post
    Here are two photos I've found on the web of my hub model (Shimano FH-6401). If I'm understanding you, these photos show that an 11t cog will not work without dremeling.
    Yes, you will need to grind off about 3mm (about 2-threads) from the end of the splines to allow a smooth and round outer-surface for the 11t cog to fit onto and over the end. Here's a picture of a Hyperglide-C body versus regular Hyperglide:

    http://www.sheldonbrown.com/images/2k7-bodies.gif


    Also you will find that being able to feel resistance on your pedals on a downhill will really not result in higher downhill speeds. The goal is to generate more power, which is RPMxPedalForce. If you are pushing as hard as you can on the pedals at 80rpms, you will only generate 67% as much power as spinning it at 120rpms. The most powerful and fastest sprinters spin at 130-150rpms (and practice 200rpms+).

    And the aerodynamics plays a huge role at 15mph+ and even more so at 30mph+. Often times, I find tucking in deeply and low behind the bars and pulling in my elbows and knees will give me another 5-7mph on top of 40mph. If I tried to pedal harder or faster at 40mph, I will gain only 3-4mph (doesn't matter if I use 53x11t or 48x12t), but tucking gives me even higher speeds because the gains from pedaling isn't as large as the aerodynamic savings.

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