Gear inches, speed, and technique

Discussion in 'Cycling Equipment' started by PineTar, Mar 10, 2013.

  1. PineTar

    PineTar New Member

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    I'm still very new to the concept of gear inches, and I'm needing some help figuring out how this stuff works.

    I'd like to give you folks a rundown of my math and methods, and then I've got a couple of questions for the group.

    My bike has three front rings, 28, 38, and 48 tooth count.
    I also have five rear sprockets, 14, 18, 21, 24, and 28 count.
    I am using a 26 inch drive wheel. (This is an approximate; I forgot to measure the actual overall diameter).

    The formula I found is Drive wheel diameter x front count divided by rear count, rendering a little table that looks something like this;

    Gear Inches 28 38 48 14 52 70 89 18 40 54 69 21 34 47 59 24 30 41 52 28 26 35 44
    For those following at home, the numbers across the top are my front rings, the numbers down the left side are my rear gears, both represented in tooth count. When referring to a gear I'll give the front ring followed by the back ring like 48/14.

    Okie dokie. I've counted out my cadence, and a comfortable cadence for me right now is 64 roughly. I can sustain that speed without gassing myself out. Another formula I've found to calculate speed is gear inches x cadence divided by 336. This renders another tidbit; with this bike in high gear (48/14) my top speed at 64 cadence should be roughly 16.9 to 17MPH which I realize is hilariously slow for most of you. I can do short 'bursts' of cadence, but for just cruising, 64 is where I am right now.

    First off, is my math right? Have I wasted a lot of time with formulas which are faulty? I double-checked my work a couple times and got the same answers each time, but we all know of the famed JIJO phenomena.

    Second, is the gear set I have a fairly good combination front to back? It is a mountain bike, and while it will eventually be a street bike, I'd like to know if the 'jumps' are too large for somewhat efficient riding or if there are other flaws.

    Now for a technique question. I know it's never good to have the chain 'crossing' at a sharp angle, like from the largest front ring to the largest rear ring. With this in mind, if I were only to use the lowest three gears on the 28 ring, the middle three on the 38 ring, and the upper three gears on the 48 ring, that means that this 15 speed bike is functionally a 9 speed bicycle, and I'm never going to have a massive effort required to get to top speed. Am I right in my thinking here?

    Thanks for the help
     
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  2. dabac

    dabac Well-Known Member

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    I've never found the concept of gear inches particularly useful, I much prefer to run the gear calculators in cadence mode - [email protected] rpm or so.
    I can use pretty much any multi geared bike as a reference, see where I'm maxing out on that, and then estimate which drive train changes I'd might benefit from. Or what other bikes might fit me.

    And a cadence of 60 or so is really low. This isn't about style, but strick biomechanics. Knees tend not to appreciate heavy loads at big angles, which you are far more likely expose them to at low cadences.
    If you play the odds, you are likely to benefit from starting to work on that cadence now, before you get into trouble.
    One of the least discussed items in the cycling world is the recommendations to go for an average cadence in the 80-100 range.

    Triple set-ups will usually have a significant amount of overlap, so yeah, from the number of available combinations, expect to have about 2/3 as many usefully different gear ratios.

    And effort, that's a so-so word here.
    To most people, top speed = max effort, or a really wicked descent.
    I guess you're referring to your low cadence, which is limiting your speed to a fairly moderate number, considering the gearing of your bike.
    Yours is a fairly average setup, Cranks are large for a 26", but witha 14 smallest it pretty much evens out.
    Steps are a tad bigger than on a more modern bike, but not crazily so.
    With a 14/48 big, you should be able to hit 25+ MPH if you get your cadence sorted.
     
  3. PineTar

    PineTar New Member

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    Today between the raindrops I went out and logged about two hours in just low gear, no shifting, just pick the only gear on the bike that will go anywhere on the property and worked on pumping the legs like a robot. I must have done something right because I *feel* like I've been climbing a ladder a lot today. Funny how our bodies react from near sedentary status to semi-active. No pain, just some soreness not unlike that when I had after flyfishing a lot. We're due rain all day tomorrow and I don't have rain gear so I may be taking it easy.

    My main goal was to work on just cadence and making a concerted effort to keep the revs up and maintain. Before stopping I broke out the stopwatch and maintained a cadence of 71 for a minute and a half. Before my next session I intend to do some stretching beforehand as I really felt like my lower body took a third of my time just loosening up. My numbers are still pathetic, but I picked the wrong time to time myself. Earlier in the time, I believe I'd have done slightly better.

    I'm the worst at math you'll ever see, but it's intriguing to play with these numbers. I managed to work out that, on my bike in high gear, I'll need to do 226 revs to travel one mile. At a cadence of 70 it should take 3.2 minutes to do it. That's assuming that I was using perfect conditions, but it's still neat to see the numbers work. I don't know why I feel so victorious knowing I can ride a bicycle marginally faster than some people can on foot.
     
  4. dabac

    dabac Well-Known Member

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    If you're having fun, then I sure don't want to take that away from you, but I think your approach is a bit backwards.

    Cycling, particularly road cycling, is very focused on endurance. While some strength is certainly needed, it definitely comes in second to endurance.
    You're trying to go places. And generally you try to go as far and as fast as your "gas tank" will let you.
    With luck or good judgement, you'll reach your destination before you reach the bottom of your tank.
    Think of your body as a machine, an engine, where the user manual tells you to stay within the 80-100 RPM range for best balance between efficiency and endurance.
    Gear selection is secondary, gear inches doesn't matter much, number of turns is unimportant, how far you'll actually make it isn't really interesting either. The goal is to do the best you can with what you have.
    If the going gets tough, rather drop a gear than your cadence.
    So, you shoot for a cadence in the 80-100 range, you pick a gear that'll let you hit that zone, and then you just start cranking. Keep it up until you've reached your destination, run out of time, or run out of steam.

    As you get fitter, you'll be able to push a higher gear while still remaining in that rev bracket, or be able to maintain it for a longer period, or both. Either development is fine.

    You might find it helpful to pay attention to the leg travelling upwards.
    Pushing down is easy, it' uses a lot of the same muscles as we use for walking. That's our strong move.
    But every time there's one leg going down, there's another one going up, and if you're using regular platform pedals, the common way to keep your upward bound foot from coming off is to keep a bit downward pressure on it.
    So your downward-bound leg isn't only pushing the bike forward, it's also bringing that other leg back up - hence the invention of straps, SPDs etc, which allows the rider to actually lift that leg up deliberately, freeing the other leg to only provide power for the forward motion.
    The first thing you can do to improve technique is to make sure your downpressure is as low as possible, which should help your cadence.
     
  5. oldbobcat

    oldbobcat Well-Known Member

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    I enjoyed your post and I admire your attitude, PineTar. Revolutions per minute is a lot like IQ points. We could all benefit from having a few more, and with a little exercise every day, it's an attainable goal.
     
  6. alienator

    alienator Well-Known Member

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    PineTar, your math is pretty close to right. 336.14 would be better than 336, but it's likely close enough.
     
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