Advertise on Bikeforums.net



User Tag List

Results 1 to 9 of 9
  1. #1
    Junior Member
    Join Date
    Nov 2010
    Posts
    5
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Max speed vs. max gear ratio

    Hi,

    I'm getting a new bike shortly and the faster the better. I had assumed that this meant , all other things being equal, I should look for the bike with the largest gear ratio. I have more or less settled on the Giant Rapid (http://www.giant-bicycles.com/en-gb/....4/7844/45455/) but I'm a bit surprised at how the max gear ratio changes as you move up this range. The specs are:

    Rapid1:
    Cassette Shimano HG-50 12-25
    Crankset Shimano 4550 34/50

    R2:
    Cassette Shimano HG-50 12-25
    Crankset Shimano R453 30/39/50

    R3:
    Cassette SRAM PG850 12-26
    Crankset FSA Tempo 30/42/52

    R4:
    Cassette SRAM PG830 11-28
    Crankset FSA Tempo 30/42/52

    The Rapid1 is the most expensive and so I was expecting it to have highest gear ratio but the trend is the exact opposite, as you move up the range there is actually a consistent decrease in the max gear ratio. I understand that the quality of the components may be increasing as you go for the more expensive option (Rapid1) but why does the max gear ratio decrease at the same time or is this just coincidence? Or is there some reason why this would not have a significant impact on the max speed achievable all other things being equal ?

  2. #2
    dbc
    dbc is offline
    Senior Member
    Join Date
    Oct 2006
    Location
    Montreal, QC
    Posts
    136
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    You max speed will be more dependent on the engine (yourself) rather than the gears installed on the bike. You could install a 70 tooth chainring, but it won't increase your speed since the engine has a limited power output - you would only turn the cranks over more slowly.

    Watch videos of performance cyclists. They are turning the cranks over 80 rpm.

    If speed is an issue for you, look for a bike with better aerodynamics, like a drop handlebar.

  3. #3
    Banned
    Join Date
    Mar 2004
    Location
    Maryland
    My Bikes
    rockhopper, delta V, cannondale H300, Marin Mill Valley
    Posts
    5,155
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    I don't think you will be able to spin out a 50 x 12 unless you are going down a steep hill. If you can spin 120rpm, you can go 40mph with that gear. If you can't spin that fast, practice until you can. 90rpm will get you to 30mph, which is pretty fast.

    80rpm is a ponderously slow cadence.

  4. #4
    just pokin' along desertdork's Avatar
    Join Date
    Aug 2008
    Location
    the desert
    Posts
    1,071
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    It's about practical and useful gearing. With a geared bicycle, you want the gearing to be a good match with your fitness and your riding terrain/conditions.

    If you are a very strong rider with incredible power output, a very tall ("max") gear may be of some use when sprinting in a race or while descending a mountain pass. 52:11 is a huge gear, and 52:12 and 52:13 have very limited, practical use. When you factor in that the R3 and R4 only have 8 cogs (with the largest two being poor choices to use with a triple's outer ring), you might notice that you'll very seldom use that large chainring at all with an 11-28.

    If you have some riding experience, you can use an online gear calculator to see just how useful a particular drivetrain should be for you.

    The 50:12 available with the two upper models is tall enough to hit 30mph at a relatively modest cadence of 90. The 12-25 also has fairly close spacing with its nine cogs; at the higher ratios, the spacing is only one tooth, whereas the 8-sp 11-28 cassette makes you jump two teeth in the top ratios. This is more user friendly.

    Road cranks used to commonly come with 42t inner or middle rings (double or triple cranks, respectively). More recently, that has been scaled back to a 39t on most triple cranks, reducing the ratios ~8%. I get more use from a 39t ring than a 42t ring, but ymmv. And, not that it's material information to your question, you're correct about the difference in component quality. Even in its lower product series, Shimano cranks and bottom brackets offer better shifting performance and durability than FSA products.

  5. #5
    Sumerian Street Rider khutch's Avatar
    Join Date
    Mar 2010
    Location
    Suburban Chicago
    My Bikes
    Dahon Mu P8, Fuji Absolute 1.0
    Posts
    565
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    It appears to me that the lowest model is geared for beginners or casual cyclists and as you ascend the scale they are geared more and more like well conditioned road cyclists would prefer. Someone like the latter group is the most likely to buy the more expensive model and I think that explains the gearing. Within limits you can have the bike regeared as you like if you find that it is a poor fit for your "engine".

    Everyone is different and 80 rpm is not ponderously slow for many of us, in fact I find it to be about my optimum. Every person will have an rpm at which they can generate their maximum output power. It is the power you generate and nothing else that ultimately determines how fast you will go. Other factors like body position (which determines your aerodynamic drag) are critical too, of course. But ultimately if you fix all those and you cannot generate the power required you cannot go as fast as you like. The role of the transmission is to match your optimum rpm to the speed that corresponds to the maximum power you can generate (when discussing top speed anyway). Top speed on the flat may require a lower gear than the maximum gear the bike offers just as automobiles will often go faster in 4th gear than in 5th. Even though we are all different those who are elite cycling athletes tend to be similar. The fact that they all like to spin at 120 rpm does not mean that the rest of us generate our peak power at that rate. If you meant to go fast you will need to train hard and as you do that your best rpm may rise (I would not know for sure). I suspect that it will rise somewhat and so as you improve your engine enough to go really fast I think you will find that the bike is geared close enough to your needs so that it is not a limiting factor in your top speed, and if it is the gearing can be changed at a fairly low cost.

    When I bought it I feared that my Fuji ( 30/39/50 and 12-27 )was geared too high on the low end and too low on the high end. Neither has proved true in practice.

    Ken

  6. #6
    Senior Member AdelaaR's Avatar
    Join Date
    Apr 2010
    Location
    Vlaamse Ardennen, Belgium
    Posts
    3,898
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Do not only look at the highest gear ratio but keep your attention on the lowest aswell.
    We do not know your fitness level ... so it might be a good idea to ellaborate on that subject a bit

    The rapid 1 has a lowest gear of 34/25 since it has a compact crank with only two chainwheels.
    34/25 ... that is pretty high for a lowest gear depending on your riding terrain and your athletic condition.
    What you could do is to find a bike with that same gear ratio and try to use that gear to climb the talest hill in your area ... only then can you know if that gear is low enough for you.
    I have about the same lowest gear (30/23) on my racewheelset and I like it ... but for going offroad once in a while you definately want the option of lower gears.

  7. #7
    Junior Member
    Join Date
    Nov 2010
    Posts
    5
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Interesting, well I have been cycling more or less daily for over 15 years but as a method of getting from A to B rather than an activity in its own right. This is the first time I have bought an expensive bike, all my previous ones would have had a standard 5 cog casette and 3 cog crankset although I have no idea of the number of teeth. My experience though is that I would basically end up 99% of the time using only 2 gear combinations. When I have to stop at lights I would drop the crankset to the middle cog and the casette to the biggest cog. I'd leave it like this for a few seconds when pulling away from the lights and then slam it straight back to the max gear ratio as I pick up speed. I would find then that my speed would max out and I would feel no 'resistance' from the pedals any more. Now I suppose if I were to dramatically increase the RPMs of the pedals I would start to feel resistance and speed would increase further but intuitively it seems like the amount of extra effort I would have to put in would be huge to get just a small increase in speed. It seems to me (although maybe I'm wrong) that I would top out at a higher speed for the same max RPMs if I had a higher max gear ratio. I did have an odometer on one of my bikes so in absolute terms: on my commute to work the max speed I would hit would was 28mph (on a downhill portion of the journey). The smallest cog on the crankset was basically unused except on the rare occasions when I would take it up into the hills and then I could meet uphill gradients in excess of 45 degrees which were a struggle even using the lowest combination available.

  8. #8
    Senior Member AdelaaR's Avatar
    Join Date
    Apr 2010
    Location
    Vlaamse Ardennen, Belgium
    Posts
    3,898
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    If your max speed was 28mph you are going to be just fine with a 48 or 50 ... a 52 is nice though for long descents.
    My highest gear is 52/11 and I use it seldomly.
    You say you spin out ... was that on 26" wheels with typical mountainbike gears 42/11?
    You only use 2 gears mostly and you basicly only shift in front? That doesn't sound good since shifting in the back is smoother.
    Try getting a good cadence of about 80 rpm and try to find the right gear for the right situation.
    Are you serious about the "45 degrees"? That sounds insanely steep.
    If you are certain that 34/25 is low enough for you ... go with the rapid 1

  9. #9
    dbc
    dbc is offline
    Senior Member
    Join Date
    Oct 2006
    Location
    Montreal, QC
    Posts
    136
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    The way you are describing your riding appears to be a low effort type of cruising. That is, there is no sweating involved. What you will likely find out with your new bike is that you'll gravitate to a similar effort level and pedal rpm.

    My suggestion is to forget about the maximum gear when selecting your new bike. As suggested before, the minimum gear may be more interesting, but only if you are inclined to riding uphill. Try out the bikes. A nice light bike with good wheels will give you a much bigger impression of liveliness and speed than the specific gears will.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •