Setting your tire circumference for cycle computer. Compression measurement?

[MuniAddict]

Originally Posted by Dane M

I don't expect it to be perfect, but within range of reasonable error.

Just use the formula in post #6. It will work just fine, but you could also do the same measurement a few times, just to confirm the results, and rule out any possible mistakes. It's also better to have a second person helping, to mark the spot when/where your stem completes one full revolution, with you ON your uni, of course.

[skilewis74]

+1

Originally Posted by tholub

Now you're getting into the vagaries of measurement...

Also mile markers on the road are often estimates. For motorists between any two markers if the distance is off a few % between markers (like 1.05 or 0.95 mi) it doesn't matter as long as the errors average out over a few markers.

[Dane M]

I'll just take my tape measure to LAX and measure out a mile on the runway.

[scott ttocs]

Or just use "2800 mm", which will be accurate within a few percent. For a new tire with high pressure and a light load it will be a little bit high. For an old tire under lower pressure with a heavy rider it will be a bit low. From my own measurements I would guess an uncertainty of less than ± 2% for most riders under most conditions.

Scott

[johnfoss]

MuniAddict's step-by-step in post #6 is the easiest way to do it and be accurate, though you don't get a "real-world" wobble that way. I suppose if you want to factor in your wobble, do something like 100 revolutions (in a straight line) at your comfortable cruising speed. Start from a mark on the ground, line up your valve there and walk back a few revs to get started. Then cruise as straight as you can for 95 revs, jumping off as you hit 100 and marking the valve spot again. Then have fun measuring the rollout. After that, simply move the decimal over two places for a single revolution (use the metric system, eh?).

I have a 30m measuring tape, but that's not nearly enough for 100 revs. Maybe do a shorter rollout and figure out the math from there...

[magnustudios]

What about using google maps and picking two landmarks on a straight road? I guess Google maps directions will only give you two or three significant figures, so maybe you'd have to dig into the api to get better resolution.

My favourite idea is the one tholub posted. I vote for water: make a wet line in the road so you can ride over it at a typical speed and form, and then measure out as many marks as you can distinguish. I'm going to try that next time.

[Klaas Bil]

Originally Posted by MuniAddict

Ride the bike tour of a 26.2 mile marathon. The event organizers are pretty good about getting the distance right. The last one I rode in came out pretty much on the button with my computer. None of this is an exact science anyway, so I wouldn't stress too much about it.

The vagaries of measurement, as Tom so aptly calls them, are not that easily resolved. Marathons have prescribed margins of error and in no case they may be too low. Therefore they are always a little bit longer than the nominal distance. In addition, the official marathon distance is 42195 meters, which is approximately 26.218758 miles. This is often colloquially rounded to 26.2 miles (just like in metric it is sometimes rounded to 42 km), but if you are after a perfect mile you should not use rounded numbers. Lastly, that Terry's cycle computer "pretty much" confirmed this, gets us into a circular argument. Oh, and doing the same measurement several times doesn't rule out "any possible mistakes". It may seem more accurate (and indeed 'noise' will be decreased that way), but most systematical errors are just repeated. (Most, not even all... your tape ruler is most likely temperature dependent, but temperature is probably going to vary if you do several measurements.)

Are athletic tracks in the US a quarter mile? In Europe, they are nominally 400 m, which is about 2.34 m less than a quarter mile. Again, no big difference, but not exact.

I'll stop here Like others said, if you get it right within a few percent, that's usually good enough for our purposes - and achievable if you measure carefully. But precise measurement... man, it's a minefield.

[jogi]

Originally Posted by MuniAddict

Ride the bike tour of a 26.2 mile marathon. The event organizers are pretty good about getting the distance right. The last one I rode in came out pretty much on the button with my computer. None of this is an exact science anyway, so I wouldn't stress too much about it.

i think thats because they do exact what you do!
they set up their computer and ride adistance on 26.2 miles and mark this on the bottom

[Ereksonj]

The best computer I've ever seen is a garmin gps watch. It measures distance, average speed, top speed, current speed, date, time, and it keeps track of your course. When you are done, it wirelessly puts all of the data on your computer through a thumbdrive reciever. You can actually see your course on google earth or maps, and it will tell you how fast you were going at any point and time during a race. Forget wheel circumference. Just use this one wristwatch for all of your unis, running, and biking needs. You'll have to google it. I have no link.

[MuniAddict]

Originally Posted by Ereksonj

The best computer I've ever seen is a garmin gps watch. ...

You beat me to it! I've had my Garmin 205 GPS watch for over a year, and it is very accurate, with the "sirf" chip technology, which provides much faster satellite acquisition times. The downside is, the battery only lasts a few hours, which is why I bought the cateye strada, which has turned out to be very reliable, and the battery lasts for well over a year!

So as a comparative test of distance traveled, you could simply zero out both the gps and cycle comp, and ride one, two three miles, or whatever distance you want, and see if they agree. I've done just that, and they were so close, that it was, again, negligible. Now go out an RIDE!

[bouin-bouin]

I'm using a GPS watch and no concern, I can switch from my 24" to my 29" or my 36" without any concern and very easy to read the information while riding

[MuniAddict]

Originally Posted by bouin-bouin

I'm using a GPS watch and no concern, I can switch from my 24" to my 29" or my 36" without any concern and very easy to read the information while riding

I like my cateye wireless, which logs total mileage (ODO), and won't run out of power during long rides, such as centuries, like the garmin would. But the gps is great for shorter rides, and I prefer it for more extreme riding, where a uni-attached cycle computer could get damaged or easily knocked out of adjustment. Plus, unless you use a handle on your MUni, which I don't, there's not many places to mount it. I use my cycle comp. Primarily for 36er road riding.

[Klaas Bil]

I've used a Garmin Vista (now the HCx model) since 2004 for uni rides. Like the GPS watch it's literally a no-brainer to switch from uni to uni. It runs 16+ hours on one pair of rechargeable AA batteries. And it's more versatile than a wrist watch, let alone a cycle computer. On the down side: it's heavy (170 grams with batteries), and not so easy to read while riding. Related to unicycling I keep it in my Camelbak or waist bag, and use it mainly for post-ride analysis. With the new chipset it's quite accurate but still can give erroneous readings sometimes. They're easy to filter out in Excel, but a wired cycle computer is probably more reliable (in the few things it can do).

[johnfoss]

Originally Posted by Klaas Bil

...the official marathon distance is 42195 meters, which is approximately 26.218758 miles.

Previously 26 miles, 385 yards. It got changed from 26 miles when a marathon course was modified in one of the early "modern" Olympics to go past a royal viewing area (London?). Silly stuff.

Originally Posted by Klaas Bil

Are athletic tracks in the US a quarter mile? In Europe, they are nominally 400 m, which is about 2.34 m less than a quarter mile.

That's a good question. I'm pretty sure when I started using tracks to practice for unicycle racing (1980), they were 440 yard tracks. Now Track is universally done metric (I think), but I don't remember seeing any tracks being redone in the process...

Originally Posted by Ereksonj

The best computer I've ever seen is a garmin gps watch.

I have a Garmin 305. As a guy who rides multiple unicycles it's much preferrable to keeping track of multiple cycle computers, wires, batteries, etc. Highly recommended, and it eliminates the need for measuring wheel size.

Downsides:
- You have to remember to start it and stop it. This may be easy for some of you, but I'm notorious for forgetting one or the other.
- Mine doesn't play well with Mac. It's the main reason I have to run Windows on my computer at home.