Speedo Correction - SV650.org

10-05-06, 12:22 PM

I've been tryin to work out my speedo error, and I've found this rather neat way to work it out that you may all well be interested in.

Distance-time method
Find a long, straight road with light traffic. Keep the speed of the vehicle very steady at e.g. 50 mph. Start a stopwatch when you pass a milestone (distance mark, motorways are good as they tell you how long you have till the next junction) on the roadside. Stop the watch when you pass the next stone. If you can, repeat the measurements and get the average, or increase the distance to minimise the error of the measurement.

Once you've got the times use the formula below to work out the error and your real speed. Remember to repeat the test to ensure you get the correct results.

Calculated Speed

distance [mile or km] / elapsed time [secs] x 3600 = mile or km / per hour

Calculated % out

calculated speed [mile or km] / Indicated speed -1 * 100 = % out.

For example:

.5 miles / 25 secs x 3600 = 72 mph
72 mph / 80 - 1 * 100 = -10%

Which is 10% out.

So when my speedo say I'm doing 70 I'm really doing 63 and at 30 I'm doing 27.

*Please note this is just what I've worked out, I can't be held in anyway responsible for this being wrong or you working it out wrong. Then getting nicked for speeding or something else equally stupid, like riding one handed with a stop watch.

10-05-06, 02:31 PM

[pedant]

it's actually (calculated speed / indicated speed - 1 ) x 100 = % out

as per your example:

(72 / 80 - 1) x 100 = -10%

[/pedant]

Already on my way.............

10-05-06, 04:06 PM

My calculation is this:

Reading 160 km/hr = 100mph
Police clocked me at 95mph.

Ergo 5% error.

10-05-06, 04:09 PM

That is another system....

10-05-06, 04:09 PM

Go through a Gatso at an indicated 100mph, see what the NIP says you were doing and calculate the percentage error from that.

Or find a copper with a hairdryer and ask him nicely if he'll help.

10-05-06, 04:15 PM

Quote:

Originally Posted by wyrdness

Or find a copper with a hairdryer and ask him nicely if he'll help.

I can recommend the nice traffic coppers on the M1 around Northampton for that.

northwind · 10-05-06, 04:20 PM

It doesn't seem to be constant on mine. Which makes no sense, since it's an electromagnetic pickup, there's not any margin for error in the system (just in the dial) but comparing it against the Veypor (which is as accurate as a wheel-driven speedo will be, but can be thrown a little by tyre deformation) the error seems to vary between 5 and 10%.

10-05-06, 04:37 PM

Might have to check mine at low speeds just incase of problems.

Sid Squid · 11-05-06, 07:20 AM

So, let me get this right...

What you're saying is that speed is a function of time and distance...right?

Who'da guessed that one?

11-05-06, 10:07 AM

Quote:

Originally Posted by Sid Squid

So, let me get this right...

What you're saying is that speed is a function of time and distance...right?

Who'da guessed that one?

Alright sarky... some people might not know the figures in-order to get the maths right, we are not all intellectual, bike repairing, gods like yourself. Even though I bow down to your greater power I don't appreciate your tone. So nur.

10-05-06, 12:22 PM	#1
Grinch Guest Posts: n/a	Speedo Correction I've been tryin to work out my speedo error, and I've found this rather neat way to work it out that you may all well be interested in. *Distance-time method* Find a long, straight road with light traffic. Keep the speed of the vehicle very steady at e.g. 50 mph. Start a stopwatch when you pass a milestone (distance mark, motorways are good as they tell you how long you have till the next junction) on the roadside. Stop the watch when you pass the next stone. If you can, repeat the measurements and get the average, or increase the distance to minimise the error of the measurement. Once you've got the times use the formula below to work out the error and your real speed. Remember to repeat the test to ensure you get the correct results. *Calculated Speed* distance [mile or km] / elapsed time [secs] x 3600 = mile or km / per hour Calculated % out calculated speed [mile or km] / Indicated speed -1 100 = % out.* For example: .5 miles / 25 secs x 3600 = 72 mph 72 mph / 80 - 1 * 100 = -10% Which is 10% out. So when my speedo say I'm doing 70 I'm really doing 63 and at 30 I'm doing 27. *Please note this is just what I've worked out, I can't be held in anyway responsible for this being wrong or you working it out wrong. Then getting nicked for speeding or something else equally stupid, like riding one handed with a stop watch.

10-05-06, 04:20 PM	#7
northwind Moderator Mega Poster Join Date: Jan 2004 Location: In the garage where I belong Posts: 17,082	It doesn't seem to be constant on mine. Which makes no sense, since it's an electromagnetic pickup, there's not any margin for error in the system (just in the dial) but comparing it against the Veypor (which is as accurate as a wheel-driven speedo will be, but can be thrown a little by tyre deformation) the error seems to vary between 5 and 10%. __________________ "We are the angry mob, we read the papers every day We like what we like, we hate what we hate But we're oh so easily swayed"

11-05-06, 07:20 AM	#9
Sid Squid No, I don't lend tools. Mega Poster Join Date: Jul 2003 Location: Skunk Works, Nth London Posts: 8,680	So, let me get this right... What you're saying is that speed is a function of time and distance...right? Who'da guessed that one? __________________ If an SV650 has a flat tyre in the forest and no-one is there to blow it up, how long will it be 'til someone posts that the reg/rec is duff and the world will end unless a CBR unit is fitted? A little bit of knowledge = a dangerous thing. "a deathless anthem of nuclear-strength romantic angst"

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10-05-06, 02:31 PM	#2
Filipe M. Guest Posts: n/a	[pedant] it's actually (calculated speed / indicated speed - 1 ) x 100 = % out as per your example: (72 / 80 - 1) x 100 = -10% [/pedant] Already on my way.............

10-05-06, 04:06 PM	#3
lynw Guest Posts: n/a	My calculation is this: Reading 160 km/hr = 100mph Police clocked me at 95mph. Ergo 5% error.

10-05-06, 04:09 PM	#4
Grinch Guest Posts: n/a	That is another system....

10-05-06, 04:09 PM	#5
wyrdness Guest Posts: n/a	Go through a Gatso at an indicated 100mph, see what the NIP says you were doing and calculate the percentage error from that. Or find a copper with a hairdryer and ask him nicely if he'll help.

10-05-06, 04:37 PM	#8
Grinch Guest Posts: n/a	Might have to check mine at low speeds just incase of problems.