I bought 2 different computers, one for my bike and one for my daughters. Mine reads about 2 MPH faster than my daughters (not sure which is correct, but I suspect my daughters is closer). I have a recumbent with a 20" front street tire, and hers is a Mountain bike with a 20" front knobby. Both computers were set to the default (in the manual, yes I read it ) for a 20" tire. So why the 2 Mph differance? Is there a way to get them closer? The instructions in both (different brands) have something about setting it to the MM of your tire rather than using the defaults, would that help, and the largest thing I have with MM on it is a 12 inch rule, so can I you measure in Inches and convert with enough accuracy?
ToPeak Panoram and Sigma BC600 are the computers if that matters.
Thanks

A difference of 2 MPH sounds like a fault on one of the computers.

There will be a difference between two 26 inch tyres, because the external tyre diameter will vary a bit, but 2MPH is too big a difference for that to be the case.

Richard

p.s. I wouldn't advise measuring in inches then converting to get a millimetre figure because of a loss of accuracy, but there's 25.4mm to the inch.

Put a chalk mark on the floor and the tire. Line them up and make one complete revolution of the tire and make another mark on the floor. Measure that distance and see if it varies from the default, If it does, recalibrate the computer.

Doing a manual rollout calibration is the only way you'll really know the computers are set up properly. There are too many variables that can change the actual circumference of a wheel -- brand and tread of the tire, width of the rim, weight of the rider and the bike, inflation pressure -- for the default numbers to be anything but an approximation.

When you do the rollout, be sure to do it with the tires properly inflated and with the rider's weight fully on the bike. Do it at least three times and average the results.

Finally, keep in mind that different computers use different sampling rates; they don't update the "real-time" speed readout every revolution. So there could always be moment-to-moment differences between two different makes/models. A computer like the Panoram, which devotes more "cpu time" to displaying a bunch of data all at once, might conceivably reclaim that processing power by doing its averaging less frequently. (This is just speculation.)

IMO, the best way to gauge a cyclocomp's accuracy is by riding some measured miles. If the odometer function is accurate to within a few percent, the rest will take care of itself.

RichC

Fofa, Rich is right. Think about it. If you take two mountain bikes with 26" wheels and put a 1" or 1.5" slick on one and a 2.4" downhill tyre on the other. You are obviously going to get wildly differing circumferences.

Another way to measure is to unroll a roll of wallpaper on the floor, blank side up. Draw a narrow line across it at one end with kids poster paint. Now get on the bike and ride along the wallpaper. Your tyre will pick up paint and draw a second line about seven feet from the original line. Measure the distance between the two lines. Do it with both bikes and I'll bet there's a difference of a few inches between the two .

Or if your local area has either a mobile radar speed trailer or radar speed signs, I just ride past them (usually when the traffic isn't there) and take note of the speed it says, and check the reading of the computer. Then correct if necessary.

OK, the manual in one says to measure the MM of the tire height, and multiply by 3.14 (inital setting was something like 996 or something similar). Not being a math wiz, is this the circumference of the tire in MM? The other computer was roughly within 20 of the other computer for the setting.
Thanks :confused:

3.14 looks like Pi from my limited maths knowledge, 3.14752...?

I can't remember the formula for finding the circumference of a circle, I think it's Pi x radius squared? Or is that the area of a circle?

Sorry

Richard

Yes that would be the circumference; however, 3.14 is a rounded number. Plus you don't have the weight on the tire. When you put weight on it, the tire would flatten a little and the circumference would change. The rollout method with weight on the tire would be more precise. If you are concerned about 2MPH, you may ultimately be concerned about the .5-1MPH difference.

3.14159265358979323846 is a little less rounded

Pi x Diameter is Circumference (what you need)

Pi x Radius Squared is Area (not a lot of use )

Richard

Thanks Rich, I was just about to point that out, too. Pi x R squared is area, Pi x D is circumference.

We should have let Fofa try the R squared method, the answer that would have given on the bike computer would be out by a factor of 19 or 20 ie when travelling at, say, 24mph, the computer would have read 456-480 mph .

Astra,

Yeah but think how fast he'd be going

Marty

It's not Pi x D, it's 2 x Pi x r.

Cheers...Gary

P.S. Don't - I know.

Believe me, the rollout method is the only accurate one, because it's *real*. The number you want to enter into the computer is the actual distance the tire actually covers when being actually ridden over one actual revolution.

There's only one way to ascertain that number. Measure it. No formula will be as accurate.

RichC

Gmason, two R IS D !

Oops, sorry m8, didn't notice the , .

The most accurate way will be to measure it with the rider on and th tires inflated. 25.4 is by definition accurate so use whatever ruler/tape you have. If you can then find a measured mile or radar to check it would be interisting to know how that worked. Most calculators I've seen have Pi to a silly number of places so just use one if you decide to calculate the diameter as well as measure it.
HTH
Joe

Well I decided to use the most scientific method I could think of. Then reality set in when I priced the laser range finders and we just lined the valve stems up until they looked like they are at the very bottom, drew a mark on the driveway, rider rolls forward until valve stem looks like it is down again, make another mark. 3 1/4 inches differance in the 20 inch tires (give or take a 1/4 inch). I guess that is close enough. Thanks all for your help in this.