Speedos

[spankydean]

Icfbgwdg

[Mr Pants!]

Not too sure how the speedo works on a bike, can not the tyre pressures being low cause an inaccuracy as the wheel diameter will be less?

I think it is a good question, also, they are always inaccurate by over reading the speed. Which is good as we would be getting done for speeding everywhere otherwise.
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Mr Pants! (Alex or Al)
Its better to burn out, than to fade away....

[MarJay]

By law speedometers need to be accurate to within 5 miles an hour, but they are not allowed to underread, IE you aren't allowed for it to say you are doing 40 when in fact you are doing 45.

However... Over 70mph there isn't any point in them being accurate because you aren't supposed to go faster than that!

Generally speaking Japanese bikes' speedos begin to overread significantly over 70mph. This is often worse on a lot of italian bikes too (RS125s Mitos etc etc). This could be a sort of added safeguard to stop loads of "Mad bikers" from being caught speeding. I reckon though that its just that the manufacturers don't have any need to make the speedometers accurate above 70. Also, they only reallyt have to obey the law when new because an MOT test doesn't test the accuracy of the speedo, so some might possibly deteriorate.

Basically, don't get stressed when people say that bikes speedo's are innacurate because they generally are only over 70mph!

HTH
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[MarJay]

Oh and tyre pressures won't make a difference as they don't make a significant change to the diameter of the wheel.

Also a lot of modern speedos are run from the front sprocket anyway. These speedos become inaccurate when the bike is downgeared.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[1cyl]

I've wondered about this for a long while, there must be an accurate way of measuring speed on your bike, my speedo is mechanical, running off the front wheel. this is supposed to be the most in accurate but simple design, i think some of the bikes with digital speedo's are meant to give much more accurate readings all the way through the range.
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Chase my Witch up, Catch my Snitch up.
The SMC | The Pit | Stable

[Mr Pants!]

I have a Road Angel in my car which tells you your speed via GPS, I believe that is accurate if you continue at a constant speed over a given time.
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Mr Pants! (Alex or Al)
Its better to burn out, than to fade away....

[MarJay]

GPS's are only accurate if you're not going up or down hill, because they measure speed in two dimensions, and don't take into account for vertical movement!
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Stew]

[jimbo]

I read up on this a while ago. The EU directive covering speedometer accuracy is 75/443/EEC.
Here it is:

Section 4.3 Accuracy

"the following relationship between the speed indicated on
the dial of the speedometer (V1) and the true speed (V2):

0 ≤ V1−V2 ≤ (V2/10) + 4km/h

This means the speedo reading shall never read less than the true speed of the vehicle.
The indicated speed can be a maximum of 10% + 4km/h (2.5mph) above the true speed. The design engineer would probably aim for an error of about +5%, purely for simplicity in defining manufacturing tolerances.
Tests are made at 40, 80 and 120km/h. there is no stated requirement for testing above 120 (75mph), so accuracy is not guaranteed above these speeds.

Mechanical speedos usually use a magnetic drag-cup system, which will certainly be non-linear and accuracy would decrease at higher speeds.
A digital speedo should be linear across its entire range, so the error would be constant at any speed.

[Kickstart]

Hi

Not sure if the European regulations have superceeded the UK rules, but the UK limit was they were allowed to over read by 10%, but not under read (slightly stricter than the EU rule that Jimbo quotes).

Speedos are inaccurate for numerous reasons. Tyre wear (on a bike tyre that is probably 1~2% between new and old)), tyre make (maybe another 3% from one make to another), tyre inflation (could be loads), speed (tyres increase in size with speed) and even the age of the speedo with mechanical ones (they rely on magnets which are likely to loose some of their strength over the years).

My 1200 Bandit (mechanical speedo driven from the front wheel) was indicating around 155mph when it was actually doing 136.5mph.

It is difficult to accuratly measure speed. GPS devices measure the speed on the ground, so take no account of hills. Speedos drive from the wheels / sprockets / gearbox are prone to all the problems listed above. Laser / radar only measures closing speed.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Dr Nick]

[Kickstart]

[Frost]

The only truly acurate way to measure speed is through a device similar to an optical mouse, which when pointed at the ground measures changes in the surface below to calculate your exact speed.
Due to cost and practicality such things could only be used to calibrate a speedo.
Calibration does improve speedo acuracy, but speedos are normally only calibrated to be acurate up to 70mph, with the possible exception of police bikes.
So far as i know they only bike you can buy with a truly acurate speedo is the RSV mille which is acurate to within 1mph all the way up to 160mph

[jimbo]

[Mr C]

indeed they do

all the way to 198mph
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187.3bhp = 201mph

[MarJay]