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Some call it speedometer or odometer but I mean the thing to measure velocity, trip distance and such things. When you come up with a usage-case, please, point out the function which you use. The only thing where I may need a speedometer is to know when to lubricate e.g. after every 500km and to know the speed not to break speed limits. So very simple cheap one should be enough.

Not sure about terminology, whether speedometer, odometer or cyclometer. The first ones just return the cheapest units: ~2.75USD (ebay) and 5.04USD (dx), perhaps Chinese are abusing the language or different English.

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The term I usually see is "cyclometer". –  freiheit Feb 7 '11 at 16:18
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Or bike computer - unless you are into racing you might as well just get the cheapest one. In my experience they typically either get broken because you turn the bike upside down and crush them, they fall off, or they corrode. I've never had one wear out! –  mgb Feb 7 '11 at 17:19
    
It's kind of fun to have one. I miss the old cable driven ones though. Batteries don't last long on the new electronic ones, and I hate replacing those expensive batteries all the time. –  Brian Knoblauch Feb 7 '11 at 18:38
    
@mgb Perhaps you're not doing it right. I've never crushed one, and never had one fall off. They stay nice and tight right where I put them and work forever... –  Brian Knoblauch Feb 9 '11 at 13:54

4 Answers 4

up vote 5 down vote accepted

Speedometer = device which measures speed.

Odometer = device which measures distance traveled.

Cycle computers can normally do both (plus many other features you may or may not need).

Are they absolutely necessary for cycling? No, but they can be quite helpful in many cases. When riding long distances (e.g. touring, randonneuring), it's often important to know how far you've traveled, so an odometer comes in handy. You may be trying to determine where you are on a map, how far until the next turn, how far to the next town, etc.

If you're training, being able to track your performance over time can also be handy.

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darkcanuck: you hit me, even forgot that I made the driving mistake today in estimating distance. I knew today that it took 2.2 km to the nearest (never-visited) bike-shop in a direction. I missed it and I drove much over 2 km. If I had known that now 2.2km, watch out and ask someone, it would have saved a lot of time. What is the thing called to measure trip distance DST? Excellent usage case by example! +1 –  user652 Feb 7 '11 at 16:42
    
@HH01: A simple trip odometer would do that. Most cycle computers have a long-term odometer (only resets when you reset the device) and one or more shorter trip odometers that you can start/stop individually. –  darkcanuck Feb 7 '11 at 16:46
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@darkcanuck: A non-obvious benefit is that they all have a clock. So when I'm going somewhere I know whether I'm on time or not without having to fish out my phone then pick it up off the road when i drop it. I find the service interval measure quite useful, and when to rotate chains which can greatly increase the distance you get out of expensive consumables. I also track average speed on a daily basis as one guide to my health/sleep levels (and it tells me when to pump up the tyres if I forget to check pressure). –  Мסž Feb 7 '11 at 23:11
    
moz: what do you mean by "when to rotate chains which can greatly increase the distance you get out of expensive consumables."? Very good point about pressure, again a hit, drove with underpressured tires for a while and it does not do any good to the sides of the tires. –  user652 Feb 8 '11 at 3:20
    
@hhh: rotating chains - having more than one chain for the bike, and swapping them periodically. Generally the chain wears out faster than the cassette, but a worn chain will grind the casette down to match it. So if you have (say) two chains and swap them every megametre or so you can almost double the life of the cassette. No real point for a $20 7 speed cassette, but for a $200 10 speed one that's quite a saving. –  Мסž Feb 8 '11 at 23:02

I am partly generalizing current replies but added some new ones. I am not sure yet which unit correspond to my needs, perhaps just trying the cheapest one like this (2.75USD, shipped). I can imagine all of my usage cases below would be satisfied with an odometer.

A. Business

I bought my bike as second-hand and it made a good impression when the seller gave all stats about the bike usage like 10k driven, it is 2 years old but been in heavy use. I calculated the price with 25% demortization rate p.a. (pessimistily to 3 years) and added a risk factor that depended on the driven distance. So it was easy for me to pay a fair price. Please, do not interpret the last thing wrong. I bought the bike from auction-site and since I knew the lawful price for it, I made considerably higher bid than the next highest bid (not wanting to waste time) so I think a good example where quantifying bike usage is good for both parties. And am I happy? Sure I am, superb bike and I do like fixing some trivial bugs in it like fenders and pedals.

B. Maintenance

Seen suggestions such as lubricate every 300miles (about every 500km).

C. Orienting

You don't always know places, exactly, just something like 4.2 km along that street and then left. If you know exactly your driven distance and don't want to look at the map all the time, I think odometer can make the journey much more pleasant, killing unnecessary zig-zagging.

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I use my speedo/odo for training purposes.

  • It has a cadence sensor, so I can keep my cadence at 90rpm.
  • It has a lap timer, so I can track my pace on the 5k loop I ride on.
  • It accumulates mileage, so I have a sense of my weekly distance.

The fancy ones have wireless sensors so you don't need to route wires, built in GPS, a heart monitor, a computer interface for uploading your workouts, etc.

Mine cost me $20 USD, took about 15 minutes to install, and does everything I need. I know where I'm going, and I know where on the loop my heart's about to explode.

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I can think of two possible reasons why you might want to use an odometer/speedometer. (Note: sometimes it is also called a cyclometer.)

A. Tracking fitness/training goals (especially if you are into racing).

  • If you want to compete in an endurance race, you will want to prepare for it by conditioning yourself to cycle consistently at a sustained speed for long periods of time.

  • Or, if you are just looking to improve your fitness, a heart rate monitor in combination with a speedometer is very useful for finding the heart rate or pace at which you can get the best cardiovascular exercise.

B. If you don't care much about fitness or racing, a speedometer and/or odometer can be nice if you are just curious about how far you can cycle or how fast you can go.

  • Most speedometers will show the maximum speed of your trip, so after a descent/sprint you can check how fast you were going.

  • And of course, an odometer can help you figure out how far you have cycled in a given time period (daily, weekly, monthly, etc.). From that, you can find out how much money you have saved on bus fare/gasoline, etc. by cycling instead.

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