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  1. #1
    2oz
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    Gear Ratio for a flat TT

    Wasn't sure whether to try this here, or in the SS/Fixed section, but this one won.

    On Oct 4th I'm going to try doing the Lowes TT in charlotte with a fixed gear. I did it last time fixed with a ratio of 42x14 (wasn't planning on using that bike, but there it is). This time I want to go in with a better ratio. I've ridden it many times geared, but usually sticking with a 50x12.

    Now that I'm actually planning on doing it fixed, I'm wondering chainring to combine with the 14t. I have a few at my disposal. 50, 53, and a 55.

    I can't imagine any instance in which I'll be able to practice with the gearing on a flat course, besides just before the event. Maybe a abit around my neighborhood, which is as close to flat as it gets around me.

    So thoughts on what kind of ratio to use? I don't know what I'm doing

  2. #2
    spinlikehell mickster's Avatar
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    Really is impossible to say without lots more info - eg how long is the TT? How fit are you? What sort of time are you realistically aiming for? You need to be more specific about what you mean by wanting a 'better ratio' - what would you like to change about the 42x14 you used before (eg was it too heavy a gear or were you spinning out too early).

    FWIW the one and only time I did a TT on fixed was a 10 miler, on a conversion with regular road bars, spoked wheels, on a flat but windy out-and-back course and I used a 44x14 (ish). I did something in the high 23 minutes (hey I never said I could TT... ) and this gear was about right for me. Based on that, your 42x14 sounds closer to the mark than the 50 etc you mention...

    mickster

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    2oz
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    Quote Originally Posted by mickster
    Really is impossible to say without lots more info - eg how long is the TT? How fit are you? What sort of time are you realistically aiming for? You need to be more specific about what you mean by wanting a 'better ratio' - what would you like to change about the 42x14 you used before (eg was it too heavy a gear or were you spinning out too early).

    FWIW the one and only time I did a TT on fixed was a 10 miler, on a conversion with regular road bars, spoked wheels, on a flat but windy out-and-back course and I used a 44x14 (ish). I did something in the high 23 minutes (hey I never said I could TT... ) and this gear was about right for me. Based on that, your 42x14 sounds closer to the mark than the 50 etc you mention...

    mickster
    I knew I left out a bit of information.

    It's a 10 mile course. The best I've done on this course was earlier this year with the geared 50x12, and I ended up with a 23:17. I've had some ups and downs since then, a 23:33 at the end of August, and then when I did it at the beginning of september with 42x14, I had like a 26:30, and I was spinning out the whole way. well over 90 rpm (and sitting on the bullhorns without aerobars the whole way)

    I'm pretty fit, I'm not sure what kind of scale I could give you to gauge that on though. What I really want something below 23:00, but I don't know if that'll be possible now.

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    spinlikehell mickster's Avatar
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    Taking yr 23:17 on a 50x12 I think your cadence works out at something in the 80-85rpm range (tho my maths is worse than my TT'ing so worth checking). My suggestion would be to work on upping this slightly into the 85-90 range rather than necessarily going for a bigger gear; an increase of 5rpm should see you going sub 23:00 on the same gear. But then I would say that cos I'm a trackie - a proper road TTer may well say different.

    Re which chainring to run with the 14 rear, none of the 3 you mention are as high as the 50x12 you used previously:

    50x12=112.5 inches
    55x14=106.7 "
    53x14=102.2 "
    50x14=96.4 "

    mickster

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    2oz
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    see this is me not knowing wtf I'm doing on a track

    So I'm better off with the 55x14, rather than trying something like a 50x14 and upping the RPM further? or would it be worth it to buy a 12t cog and run 53x12, or 55x12?

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    spinlikehell mickster's Avatar
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    you need to experiment to find these answers.
    Use a gear table (eg http://www.t2r.org/train/gear_ratios.htm) to see what the different combinations will get you. I'd try and make gradual changes to the gear you've used previously - try to make changes of around 3 inches at a time rather than big jumps.

    Remember there are two variables to consider in this - gear size and cadence. You will have a sweet spot for each of these based on your physiology / fitness etc and the trick is to try and find them. Bear in mind that most of the good TTers I know ride a 10 miler at a cadence around 100-105rpm. I'd maybe work on upping yr cadence from what it is now - choosing the highest gear that will let you do this - and once that is established then you can start tweaking the gear selection.

    mickster

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    Senior Member CafeRacer's Avatar
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    I did a flat TT once on my track bike. I just had to make it look like I had a brake. Heres what I did.

    Take a look at the course speeds in the past. Using a gear chart figure out what gear you can do that speed in with a cadence you can hold no problem. Factoring in of course the "roll over" effect you get from a fix gear.

    For instance, the course I did had a winning rider doing 45kmph avg. It was also short. I picked a 90" gear done with a 50x15 for increased efficency. The only spots I was slower was in the start and the turn around.

  8. #8
    spinlikehell mickster's Avatar
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    boring isn't it? TTing, I mean

    mickster

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    Senior Member CafeRacer's Avatar
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    Quote Originally Posted by mickster
    boring isn't it? TTing, I mean

    mickster
    Better than a 120k road race thats for sure.

  10. #10
    so much for physics humble_biker's Avatar
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    If Ricky is turning a 50x12 at 80rpms and Donnie is turning a 49x12 at 85rpms and they leave the start house at a 2:00 minute difference. Which will go 10 miles the fastest and which will reach the finish line first? Ricky left first.

  11. #11
    pan y agua merlinextraligh's Avatar
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    Its a pretty simple math problem. Your goal time is a average speed of 26.1 mph. The 50x14 at 90 rpm is good for 25.8 mph, So cadence in the low 90's will get you to your goal, with the 50x14. Personally, I think the 53, and the 55 would give you too low of cadence, but you know best what your own personal most efficient cadence is.

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    TT in the streets or is in the road? 50x12 is too much gear in my oppinion, what I would do is use something like 53x15 no more than that. If u are fast enough u can do around +43km/h avr with it.

    53x15 is more rounded and easy to "move" than 50x14, it will be easyer to keep the pace with 53x15 also.

    Good luck

  13. #13
    pan y agua merlinextraligh's Avatar
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    Quote Originally Posted by ultraman6970
    53x15 is more rounded and easy to "move" than 50x14, it will be easyer to keep the pace with 53x15 also.

    Good luck
    1) the OP has the 14, doesn't indicate he has a 15.

    2) Huh? Easier to keep pace with 53x15 because its round? Last time I looked, both 14t and 15t cogs were round. I suppose there is a minute friction advantage using bigger cogs, but I highly doubt the friction loss between a 53x15 and 50x14 would even be perceptable.

  14. #14
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    A rather simple question, if you are aware of your physical conditioning and the course. Two questions and one small number will give you the answer:

    Question 1: what cadence do you expect to ride? Most people aim for 90RPM or more, so you may have to adjust if you aim higher or lower than 90RPM

    Question 2: what speed do you usually maintain at that cadence under conditions of the TT?


    Bottomline formula (without boring you with the details): Any 27" wheel will yield a speed of quite exactly 7x the gear ratio at 90 rpm in MPH

    Example: If you are riding a 48:16 = 3:0 ratio, your speed at 90rpm is quite exactly(depending upon exact wheel circumference (assumed to be 2096mm for this formula)) 7x3=21MPH

    Also the maximum speed for most seasoned riders will be around 10x gear ratio (as you are hitting close to 130rpm)

    Example as above: Your max speed on a 48:16 = 3.0 ratio would be about 30MHP (if you can be smooth at 130RPM)

    Soo - considering a 52:14=3.7 ratio, at 90 rpm you would go about 26MPH at 90 RPM with a theoretical max of about 37MPH

    Now these calculations, as riding a fied gear tt only apply to flat areas. I am a pretty seasoned rider, and I may be able to push a 3 or 3.3 ratio for a while at 90rpm, but if in doubt I think I would opt for the smaller ratio (higher rpm).

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