Why are speedos inaccurate?

[tokarev]

I was just wondering.

How is it possible for NASA to land men on the moon in the SIXTIES (1969 to be exact), but it is so hard for large manufacturers such as Honda, Yamaha etc... in this day and age to create a mechanism to read the speed accurately? I hear lot's about a 10% or maybe even more inaccuracy in the speedos of recently made bikes.

I'm not talking about GPS sytems, just the normal speedos that come with the bikes.

[Rookie]

It only happens to that extent with 50ccs - bikes which aren't aupposed to go over 32mph anyway, and they only get inaccurate above a certain speed.

Yamaha and Honda don't have the technology that NASA do? And rockets don't require speedos?

[freestyler_onli]

It's funny you should say this:

Im on my gixxer earlier and im only doing 32, but it seems more like 55/60

[Paracaine]

Fuck knows, let's slap them.
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Bike: Suzuki Bandit 600, 2002, Blue
Type: Unfaired
Extras: Datatool Alarm & Immobiliser, Airblade Tinted Fly Screen, Motrax Crash Protectors

[McGee]

I find like -1 on the back and +2 on the front mess things up with speed summit bad. Going 70 but it reads 100.
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◙◙► K6 GSXR 600 ◄◙◙◙◙► K5 GSXR 1000 ◄◙◙◙◙► K5 GSXR 600 ◄◙◙◙◙► 96 RF600r ◄◙◙
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Its pronounced Jixxer!

[Rookie]

What do you mean McGee?

[tokarev]

[Paracaine]

I think he means he has changed the sprockets.
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Bike: Suzuki Bandit 600, 2002, Blue
Type: Unfaired
Extras: Datatool Alarm & Immobiliser, Airblade Tinted Fly Screen, Motrax Crash Protectors

[Bendy]

So people can brag more.

[Rookie]

And swell the already oversized heads of ped bois!

'My ped hit 79mph last night!'

edit: Raises a good point actually, if people think they are going faster on a bike than they really are, the bike gets more praise and the manufacturer makes more sales.

[McGee]

Yeah the sprokets also makes the mileage climb and climb. And i done it for about a week for more ummph from pull off didnt make a big difference that was on my first 600, my cbr.

*Let me correct my self in no way does it increase bhp it makes acceleration quicker.*
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◙◙► K6 GSXR 600 ◄◙◙◙◙► K5 GSXR 1000 ◄◙◙◙◙► K5 GSXR 600 ◄◙◙◙◙► 96 RF600r ◄◙◙
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Its pronounced Jixxer!

[Paracaine]

Yeah, and "I got 120 out of my RS25 on the flat!!"
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Bike: Suzuki Bandit 600, 2002, Blue
Type: Unfaired
Extras: Datatool Alarm & Immobiliser, Airblade Tinted Fly Screen, Motrax Crash Protectors

[muddycoffee]

I think that Bike Speedos alway read over because it's not a very accurate method. The old fashioned mechanical speedo with a square rod connecting a flywheel inside the speedo dial to a gear in the wheel, rely on a spinning flywheel, and it tends to overspin by a few percent at 70mph with the momentum, but they are usually farely accurate at 30mph.

Newer bikes tend to have an electonic pickup and cable instead of a mechanical method. I would expect these to be more accurate, but it seems they also read over a tiny bit too..
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Suzuki SV650 naked Blue curvy with Givi Screen and G force can.

[mattfuzzy]

first off, they never landed on the moon, that was just a stunt to try and make the americans win the space race between the russians and americans. (not that you need speedos on the moon )

second, speedos over read if say you got a bike thats not capable of doing 120 but modified it to do 150 without upgrading the speedo, trying to make the bike go over the speed it wasent designed for will make it over read and inacurate, atleast thats wat i was told.

[Kickstart]

Hi

Speedos are allowed to read up to 10% high but are not allowed to read low.

Most work based on how quickly the wheel is turning. Different makes of tyres, differently worn tyres, different loads, tyre deformation with speed, etc, all affect how far the wheel travels in one rotation so they are never going to be that accurate. And there is not much need for them to be that accurate.

Speedos are all over the place. I know that the Bandit at an accurate 166.2 was indicating around where 180mph would have been on the speedo. Silvers GSXR750 at the same event was (from memory) pegged at 186mph for quite a while but still accelerating and eventually getting to a real 171.x .

All the best

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

[Bendy]

The sprocket thing is a lot more simple than you lot are making out.

A lot of bikes have a speedo that reads from the front sprocket. The speedo is programmed to think the sprockets on the bike are the size they were when it left the factory. If you change these sizes, it throws your speedo out of whack, over-reading if you downgeared it, under-reading if you upgeared it.

So what you do is fit a gadget like a speedohealer or a yellow box, which lets you program in what gearing ratio you are using and should bring everything back into line. Equally, you can use one of these devices even if you haven't changed the gearing, just to tweak the accuracy of your speedo and eliminate the over-reading.

Of course, if your speedo reads from the front wheel, you can mess with the sprockets all you like, it'll have no effect.

[gavin]

.......for the same reason that rev counters over read and manufacturers BHP figures are always very optimistic.

all by the same percentage, i might add. its marketing hype.

i recently saw a standard 8,000 mile yamaha rd350ypvs on a dyno thats not known for being stingy. everyone "knows" they make 55-60bhp and do about 120mph.

well this one, ( and four others we tried on the same day! ) made 35rwhp. just for a laugh we wound it up in top on the dyno, it went off past the redline and was showing about 135mph on the dyno's speedo. in the real world that would probably be around about 120 on the clock on the road, which is probably about 108mph in reality.

same as 190 odd on your R1's speedo is more like 170........

[Mr noOdles]

[Rookie]

Oh God, we're on to moon landing conspiracy theories?!

I don't care whether they did or not, it's all in the past.

[tokarev]

I see, so there are many variables affecting the the wheel travel and hence the speedometer is also affected.

OT;

[Mr noOdles]

well i think that speedo arent accurate because every living soul on this planet will break the speed limit.

i think the manufacturer play a little mind game by making speedo read higher,

e.g you are doing 50mph (speedo reading) where infact you r doing 48mph

so who no, that 2mph might save some one live

well that what i think anyway

[Mr C]

I've said this before mine under reads

it only goes up to 190mph and I've had tickets for loads more than that
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187.3bhp = 201mph

[kawashima]

My ex bike was made in thailand(LS125) and had a terribly happy meter. I received a response from LS owner in thailand in this thread who had believed that LS speedo is accurate speed.
Then I felt Honda does this for giving dream to young people in thailand.

[Dotzler]

I would say that the 10% innacuracy in the speedo is down to one thing only! Cost. There is no need for a manufacturer to make a speedo that is accurate to within +/- 0.1Mph as this would make it far too expensive to manufacture, test, calibrate regularly.

The bike companies probably buy the speedos from a sub contractor and put very aggresive cost targets on the parts, but they will also probably specify the accuracy of the part too.

Make the part as cheap as possible, with a tolerable accuracy, and that will be enough. Over engineering adds cost.

[sickpup]