Speedometer Error
05-22-2008, 07:56 PM
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Speedometer Error
Ok, everyone knows that motorcycle speedometers read high. This is no secret and several magazines have published articles about it. What I've been doing up to now is assume that my speedo is 5mph high at normal road speeds. If I want to go 60mph (because the speed limit is 55mph!) then I stop when the speedo is at 65mph. Hopefully that makes sense to everyone...
Lately it's occurred to me that this is dumb, my speedo should be reading an accurate speed. Therefore I've been considering getting a Speedohealer. After checking out their site I noticed that you need to dial in a correction factor. That got me thinking (which of course is where it all goes downhill). What is the actual correction I would need? How far off is the stock speedometer system? Knowing all of the ratios involved and the tire circumference you should be able to easily determine the actual road speed at a given engine rpm. That means you need to make two assumptions 1) the tach is correct and 2) the tire does not 'grow' at normal road speeds. I think both are close enough for what I'm after.
What I did was to pick an rpm, in this case 4000rpm. I then started with the indicated speed in 6th gear - 75mph. I'm running a nearly new Dunlop Qualifier right now, which is handy because Dunlop publishes the dimensional data on their site. They state the tire diameter to be 24.95in. Running the math I found the actual road speed to be 71.65mph which is 3.35mph slower than indiated. This is pretty close to my estimate of 5mph. I thought "Sweet! A Speedohealer would tweak that right in, no problem." More thinking insued...
I then captured the indicated speed in the other gears. While this isn't much of an issue because I almost always cruise in top gear I thought it would be interesting to know. To my surpise here is what I found:
Indicated Road Speed =
1st= 25mph
2nd= 39mph
3rd= 49mph
4th= 59mph
5th= 65mph
6th= 75mph
Calculated Actual Road Speed (error) =
1st= 25.20mph (-0.81%)
2nd= 38.01mph (2.61%)
3rd= 48.22mph (1.61%)
4th= 57.08mph (3.36%)
5th= 63.79mph (1.90%)
6th= 71.65mph (4.68%)
Notice the variation in error! I was really surprised to see that the lower gears are not that far off. I really can't explain this because the speed sensor is on the countershaft and should send a signal to the speedo that is directly porportional to the shaft speed. Some people (mag articles again) have suggested some voodoo is built into the speedometers from the factory. I've always poo-poo'd that as paranoia but this experiment might support their theories.
Here is what I'm considering, and this might sound crazy to some. I'm thinking about running a smaller rear sprocket. Right now I'm at the stock 41T out back, if I went to a 39T the speed in 6th would only be off 0.32mph, damn close. Obviously, the other gears would be off by as much as 3-5% but as mentioned I don't spend much time (or rack up many miles) on those gears so I'm not too concerned about them. This is part of something else I've wanted to do with the gearing (think of the newer bikes with their tall first gears) so maybe this is just the excuse I need.
I'd love to hear your comments about this as, frankly, there are parts that seem pretty weird to me. Anyone else investigate this with their bikes? By the way, I've got an Excel spreadsheet with all of the calculations in it, if anyone wants a copy let me know and I'll be happy to e-mail it over to you.
Lately it's occurred to me that this is dumb, my speedo should be reading an accurate speed. Therefore I've been considering getting a Speedohealer. After checking out their site I noticed that you need to dial in a correction factor. That got me thinking (which of course is where it all goes downhill). What is the actual correction I would need? How far off is the stock speedometer system? Knowing all of the ratios involved and the tire circumference you should be able to easily determine the actual road speed at a given engine rpm. That means you need to make two assumptions 1) the tach is correct and 2) the tire does not 'grow' at normal road speeds. I think both are close enough for what I'm after.
What I did was to pick an rpm, in this case 4000rpm. I then started with the indicated speed in 6th gear - 75mph. I'm running a nearly new Dunlop Qualifier right now, which is handy because Dunlop publishes the dimensional data on their site. They state the tire diameter to be 24.95in. Running the math I found the actual road speed to be 71.65mph which is 3.35mph slower than indiated. This is pretty close to my estimate of 5mph. I thought "Sweet! A Speedohealer would tweak that right in, no problem." More thinking insued...
I then captured the indicated speed in the other gears. While this isn't much of an issue because I almost always cruise in top gear I thought it would be interesting to know. To my surpise here is what I found:
Indicated Road Speed =
1st= 25mph
2nd= 39mph
3rd= 49mph
4th= 59mph
5th= 65mph
6th= 75mph
Calculated Actual Road Speed (error) =
1st= 25.20mph (-0.81%)
2nd= 38.01mph (2.61%)
3rd= 48.22mph (1.61%)
4th= 57.08mph (3.36%)
5th= 63.79mph (1.90%)
6th= 71.65mph (4.68%)
Notice the variation in error! I was really surprised to see that the lower gears are not that far off. I really can't explain this because the speed sensor is on the countershaft and should send a signal to the speedo that is directly porportional to the shaft speed. Some people (mag articles again) have suggested some voodoo is built into the speedometers from the factory. I've always poo-poo'd that as paranoia but this experiment might support their theories.
Here is what I'm considering, and this might sound crazy to some. I'm thinking about running a smaller rear sprocket. Right now I'm at the stock 41T out back, if I went to a 39T the speed in 6th would only be off 0.32mph, damn close. Obviously, the other gears would be off by as much as 3-5% but as mentioned I don't spend much time (or rack up many miles) on those gears so I'm not too concerned about them. This is part of something else I've wanted to do with the gearing (think of the newer bikes with their tall first gears) so maybe this is just the excuse I need.
I'd love to hear your comments about this as, frankly, there are parts that seem pretty weird to me. Anyone else investigate this with their bikes? By the way, I've got an Excel spreadsheet with all of the calculations in it, if anyone wants a copy let me know and I'll be happy to e-mail it over to you.
Last edited by JamieDaugherty; 06-29-2009 at 04:41 AM.
05-22-2008, 08:07 PM
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i bought a healer a little while ago and put it on. I just measured the time it took me to travel a measure 3 mile interval holding steady at 70mph and repeated 3 times and averaged it and used that to calculate the % off.
What you are saying is interesting though. I'm not sure I quite understand, its late and my head hurts. But I know that if I'm going an indicated 70mph and shift gears to shiift rpm while matching revs, my speedo doesn't change - but maybe I should look a little more carefully next time.
bill
What you are saying is interesting though. I'm not sure I quite understand, its late and my head hurts. But I know that if I'm going an indicated 70mph and shift gears to shiift rpm while matching revs, my speedo doesn't change - but maybe I should look a little more carefully next time.
bill
05-22-2008, 08:18 PM
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My Hawk had 15/44 gearing when I bought it. Speedo showed I was running 80 in a 65 and I was getting passed by soccer moms in their minivans. I picked up a speedohealer just because I didn't want to be calculating in my head how fast I was really going. Used my GPS that I take mountain biking. It gives me instantaneous MPH and I set the speedohealer using it. My Hawk is pretty much dead on now. Highly recommended.
Thinking about swapping the front back to 16. Any thoughts on the 16/44 combo?
Thinking about swapping the front back to 16. Any thoughts on the 16/44 combo?
05-22-2008, 10:33 PM
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Along with my 16/44 combo kit, I also installed a SpeedoHealer in order to keep the speedo/odo the same as before. All I did was use the online calculator from the SpeedoHealer website, input the gearing changes, and got the directions that way. I recently checked for error against my gps navigator... at an indicated 70mph, actual speed is 67-68... 40 is dead on... when the clock shows 20 , gps says 22-23! Seems to me that you can only have a correct speedometer at one certain speed. I ran the same tests on my 2005 Crown Vic Interceptor... it is dead on at 70, and becomes optimistic as the speed decreases down to indicated 20, actual 17. Mind you, the speedometer has the words "Certified Calibration" printed right on the face... LOL.
As far as the 16/44 goes, that combo requires an extra pair of chain links, and will put the rear axle further back, thus reducing the wheelie-prone-ness? of the bike, as well as slowing down your turn-in when cornering. I have been thinking about swapping out the rear sprocket with a 43t, cutting out 2 links, and seeing how much difference it will make. The quicker cornering ability seems tempting, and the possibility of 2nd gear power wheelies is also enticing.
As far as the 16/44 goes, that combo requires an extra pair of chain links, and will put the rear axle further back, thus reducing the wheelie-prone-ness? of the bike, as well as slowing down your turn-in when cornering. I have been thinking about swapping out the rear sprocket with a 43t, cutting out 2 links, and seeing how much difference it will make. The quicker cornering ability seems tempting, and the possibility of 2nd gear power wheelies is also enticing.
05-23-2008, 01:34 AM
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Steener, If you swap to a 16/44 you are likely going to be let down by the reduced acceleration, it is very noticeable- I did this swap in the other direction just to be able to shift gears around town & have a little more instantaneous punch. The bike revs a little higher on the highway but not enough to be bothersome. Most sportbikes are needlessly high geared for noise regs.
05-23-2008, 03:42 AM
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What you are saying is interesting though. I'm not sure I quite understand, its late and my head hurts. But I know that if I'm going an indicated 70mph and shift gears to shiift rpm while matching revs, my speedo doesn't change - but maybe I should look a little more carefully next time.
bill
bill
I recently checked for error against my gps navigator... at an indicated 70mph, actual speed is 67-68... 40 is dead on... when the clock shows 20 , gps says 22-23! Seems to me that you can only have a correct speedometer at one certain speed. I ran the same tests on my 2005 Crown Vic Interceptor... it is dead on at 70, and becomes optimistic as the speed decreases down to indicated 20, actual 17. Mind you, the speedometer has the words "Certified Calibration" printed right on the face... LOL.
Last edited by JamieDaugherty; 05-23-2008 at 03:48 AM.
05-23-2008, 10:28 AM
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Man, this is really turning into a bit of an obsession/ white whale with you huh? You can always mount a bicycle computer.The important thing is that you NEVER let ths die & KEEP writing more about it, (just kidding-good focus)
05-25-2008, 07:01 PM
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Update - it looks like there is more to the story than I thought. I captured the indicated speeds over a four day period and during that time the 6th@4000rpm was always a perfect 75mph, right on the line. On Friday I checked again and it was clearly reading about 1mph slower (74mph). Friday morning was warmer than we've had recently and a lot more humid. I'd say it's pretty clear that the stock speedometer is a crude device to say the least.
Now where's that 39 tooth sprocket....
05-27-2008, 04:01 PM
#11
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You're correct, I did miss it. Sorry. That was a lot of reading though, dammit! 
I think that the growth would be linear, much as a stress/strain curve would look like for...well, most any material in the elastic range.
I think that the growth would be linear, much as a stress/strain curve would look like for...well, most any material in the elastic range.
07-15-2010, 02:59 PM
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Sorry to kick a long dead horse but I got a speedo healer and decided to read up on what has been said about it here. I hope since 2008 you have all realized that the original calculations here (under ideal conditions aka no tire slip or growth) should yeild the same number at any speed or rpm since it's a percent. observed/theoretical. the miles per hour you speedo is incorrect will increase with speed but you percent off will remain the same. (except for tire slip, expansion, over compensation......)
07-15-2010, 03:21 PM
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I think my original conclusion is still valid. Sure, you would think that it would be the same percentage.... problem is it's not. The funny part is that the odometer is dead on. Basically the speedometer itself is inaccurate and should be used as "for reference only".
07-15-2010, 03:54 PM
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After I changed sprockets I did the lazy man's speedo healer. I looked at my speedo when I passed one of those stationary radar installations common on Ca highways. Speedo said 75, radar said 60 so that was all the math I needed. I'm sure it varies by more or less than 15 mph depending on what gear your in but so far speedo -15 has kept me ticket free. Also found that for every mile marker my ODO reads .9. So every mile I lose one tenth. That math is simple too, just subtract 10 miles from every hundred, or one mile from every ten.
07-15-2010, 04:25 PM
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No, the speedohealer and speed sensor are dead on... It's a linear signal... A pulsed signal from the speed sensor, and the Speedo Healer just changes that with a factor of x%... So up to the point where you feed that signal into the dash it's dead on... The conversion process and calculation in there then is as accurate as the guy writing the software made it...
Last edited by Tweety; 07-15-2010 at 05:19 PM.
07-15-2010, 04:36 PM
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I think my original conclusion is still valid. Sure, you would think that it would be the same percentage.... problem is it's not. The funny part is that the odometer is dead on. Basically the speedometer itself is inaccurate and should be used as "for reference only".
07-15-2010, 04:42 PM
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No, the speedohealer and speed sensor are dead on... It's a PWM signal... A pulsed signal from the speed sensor, and the Speedo Healer just changes that with a factor of x%... So up to the point where you feed that signal into the dash it's dead on... The conversion process and calculation in there then is as accurate as the guy writing the software made it...
07-15-2010, 04:44 PM
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Ok now I'm as curious as Jamie, I might try and do some tests this weekend to figure this out for sure. I figure I'll measure the observed over actual (gps) three times at every ten mph from 40-80 (or something like that) in 4th 5th and 6th. If there is really an inaccuracy in the coefficient they are using for the gears then I should get the same %error at every speed in a given gear but different error for every gear. Before I waste time doing this does this all seem logical to everyone?
Last edited by spladle160; 07-15-2010 at 04:45 PM. Reason: sp
07-15-2010, 05:02 PM
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The problem is that the dial is pretty inaccurate in it self, and us reading a dial scale is pretty inaccurate, I'm pretty sure you cant reliably tell the difference between 60 & 61 mph... Ad that so a software "error"... And you get a guessing game...
Ok now I'm as curious as Jamie, I might try and do some tests this weekend to figure this out for sure. I figure I'll measure the observed over actual (gps) three times at every ten mph from 40-80 (or something like that) in 4th 5th and 6th. If there is really an inaccuracy in the coefficient they are using for the gears then I should get the same %error at every speed in a given gear but different error for every gear. Before I waste time doing this does this all seem logical to everyone?
It's a single linear sensor all the way from a standstill to 6'th gear rpm limiter... The problem is in the software, see above... The software calculates the speed based on a linear signal... I don't know how much of this anyone of you know... Let me know, and I'll try and explain if neccesary...
BTW if you do that experiment with a GPS, take the latency of the GPS into account... No commercial GPS will give you an accurate speed reading unless it's been traveling at an absolute constant velocity for several seconds... So unless you let the GPS settle before taking a reading, it's not anymore reliable than the stock speedo...
The most accurate reading you will get on your speed is actually mathematics... Disregarding tire slip & growth, calculating the speed based on RPM is more accurate than most GPS's...
Last edited by Tweety; 07-15-2010 at 05:18 PM.
07-15-2010, 05:17 PM
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Bleh... For the sake of all of us being on the same page... A very crude explanation...
The sensor sends a signal that is either 0V or 5V... nothing else... it shifts once per revolution of the driveshaft... This means as soon as the bike is rolling the signal is "slowly" shifting 0-5-0-5-0... The speedo measures the lenght of the 5V part vs the 0V part and that's the rpm of the driveshaft ie basicly a tacho for the driveshaft... Then the software calculates the speed based on the final gearing, tire/rim size and an algorithm for RPM vs speed... And then that is represented on a dial, ie another algorithm for the scale (or a combined one most likely)... And it's also probably taking into consideration slip & growth... And in most cases on the conservative side in the lower region where you are likely to get a ticket if it's wrong, and definetly optimistic at the higher end...
The meaning of this is that the algorithm in the Excel spreadsheet that's been discussed before is probably a good bit more accurate than the speedo, since that's limited to a very basic microprocessor for it's calculations...
The sensor sends a signal that is either 0V or 5V... nothing else... it shifts once per revolution of the driveshaft... This means as soon as the bike is rolling the signal is "slowly" shifting 0-5-0-5-0... The speedo measures the lenght of the 5V part vs the 0V part and that's the rpm of the driveshaft ie basicly a tacho for the driveshaft... Then the software calculates the speed based on the final gearing, tire/rim size and an algorithm for RPM vs speed... And then that is represented on a dial, ie another algorithm for the scale (or a combined one most likely)... And it's also probably taking into consideration slip & growth... And in most cases on the conservative side in the lower region where you are likely to get a ticket if it's wrong, and definetly optimistic at the higher end...
The meaning of this is that the algorithm in the Excel spreadsheet that's been discussed before is probably a good bit more accurate than the speedo, since that's limited to a very basic microprocessor for it's calculations...
07-15-2010, 05:52 PM
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Wait so your telling me the speed calculation the speedo uses uses it's own last answer as part of it's next equation? (I was wondering how they did it without a gear indicator) So now a new question. If your speedometer is not putting out a linear response to begin with how could a simple coefficient do anything but move where it is that your speedo is actually correct. And I actually had guessed that the speedo worked off a digital on off signal like a counter on a stepper motor (I think) I just assumed it had to have some way of taking transmission gearing into account instead of using it's own results as a variable. I know your right about the GPS, my method with it is to go out and ride and not get to close to the target speed until I get to my flat level open straight then hold it as dead steady on the target being sure not to go over for as much of the half mile or so as I can, then upon stopping I check the recorded top speed. I figure it's more accurate than my eyes reading that dial at the least.
07-15-2010, 07:12 PM
#23
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Oh, I agree totally that the problem lies in the speedometer itself - not the speed sensor.
Now here's another wrench in the works: since I first posted this I've also measured the odometer (actually the trip odo) to see how it compares. It's off by only about 0.1 in 20 miles. That's pretty darn close! Considering the variation in tires, etc. I'd say the speed sensor and other electronics have no problem at all with determining what's going on.
Because of that, a speedohealer could correct your speedo but mess up your odo. If I ever changed to something other than stock gearing I'd probably go the speedohealer route but focus on the odo and not the speedo.
Now here's another wrench in the works: since I first posted this I've also measured the odometer (actually the trip odo) to see how it compares. It's off by only about 0.1 in 20 miles. That's pretty darn close! Considering the variation in tires, etc. I'd say the speed sensor and other electronics have no problem at all with determining what's going on.
Because of that, a speedohealer could correct your speedo but mess up your odo. If I ever changed to something other than stock gearing I'd probably go the speedohealer route but focus on the odo and not the speedo.
07-15-2010, 11:47 PM
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Wait so your telling me the speed calculation the speedo uses uses it's own last answer as part of it's next equation? (I was wondering how they did it without a gear indicator) So now a new question. If your speedometer is not putting out a linear response to begin with how could a simple coefficient do anything but move where it is that your speedo is actually correct. And I actually had guessed that the speedo worked off a digital on off signal like a counter on a stepper motor (I think) I just assumed it had to have some way of taking transmission gearing into account instead of using it's own results as a variable. I know your right about the GPS, my method with it is to go out and ride and not get to close to the target speed until I get to my flat level open straight then hold it as dead steady on the target being sure not to go over for as much of the half mile or so as I can, then upon stopping I check the recorded top speed. I figure it's more accurate than my eyes reading that dial at the least.
No the speedo doesn't use the last answear as input for the next reading... The signal it interprets is completely linear, but that represents the RPM of the driveshaft, not actual speed... Then a fixed algorithm is used to take into account final drive, tire & rim size, slip & growth and convert it to a movement on the dial... And in a computer that algorithm can be as accurate as your readings, in the speedo which is using a more limited processor it's fairly crude maths... Because of the many things that change over the range from 0-150 that algorithm will never be fully accurate all the way...
Then add in the inconsistancy of trying to accurately read a dial type speedo... I have checked the stock VTR gauge against the CBR 1000RR gauge, and the CBR gauge is much more linear, much better accuracy...
Partly because it's a digital number, what I read is accurate, not "guessing" on a dial... Partly I'm assuming because of better electronics with better maths...
BTW the Speedo Healer does exactly what it's supposed to, it changes the base signal by the chosen coefficient, to account for a gearing change or tire change...
07-15-2010, 11:54 PM
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Yeah I would think that is right, but Jamie's recorded figures don't bear it out.
Calculated Actual Road Speed (error) =
1st= 25.20mph (-0.81%)
2nd= 38.01mph (2.61%)
3rd= 48.22mph (1.61%)
4th= 57.08mph (3.36%)
5th= 63.79mph (1.90%)
6th= 71.65mph (4.68%
Pick any zero point you wish from zero to infinity, the error percentage doesn't consistently rise from that point, it's erratic.
I'm assuming Jamie measured speed with GPS, not sure about rpm.
Could mean slight error in the GPS or some slight variation in the assumed 4000 rpm constant. Or something else.
Calculated Actual Road Speed (error) =
1st= 25.20mph (-0.81%)
2nd= 38.01mph (2.61%)
3rd= 48.22mph (1.61%)
4th= 57.08mph (3.36%)
5th= 63.79mph (1.90%)
6th= 71.65mph (4.68%
Pick any zero point you wish from zero to infinity, the error percentage doesn't consistently rise from that point, it's erratic.
I'm assuming Jamie measured speed with GPS, not sure about rpm.
Could mean slight error in the GPS or some slight variation in the assumed 4000 rpm constant. Or something else.
07-17-2010, 08:12 AM
#27
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No GPS, just calculated versus actal reading on the speedo. I wish that I had a GPS because it would be a nice reference as well.
This whole topic is just something I found pretty interesting and I thought it would make some decent discussion. Considering how often you use the speedometer (every few momens, really) I'd say it's pretty important to understand how it works..... or better yet how well it works.
This whole topic is just something I found pretty interesting and I thought it would make some decent discussion. Considering how often you use the speedometer (every few momens, really) I'd say it's pretty important to understand how it works..... or better yet how well it works.
07-17-2010, 08:36 AM
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So if we have no gear sensor and "No the speedo doesn't use the last answear as input for the next reading" then how does the bike know if I'm going 40mph at 4krpm or 60mph at 4krpm. I wasn't suggesting the last known speed was the only input into the equation for determining current speed but one that goes into the calculation (along with tire slip, growth...). If I'm being ignorant again could someone please explain how the speedo accounts for the gearbox.
07-17-2010, 09:22 AM
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I've always wondered if there is some kind of electrical voltage fluctuation. I know that my bike is fairly stable (because of the modified ground wiring from the charging system) but it will still vary quite a bit. Each recording was made a the same engine speed (4000rpm) but it's possible that for some reason the voltage was different each time. Since my assumption is that the speedometer is a cheap device, it would stand to reason that it doesn't compensate very well to changes in chassis voltage. Not sure, just another theory.
07-17-2010, 09:32 AM
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So if we have no gear sensor and "No the speedo doesn't use the last answear as input for the next reading" then how does the bike know if I'm going 40mph at 4krpm or 60mph at 4krpm. I wasn't suggesting the last known speed was the only input into the equation for determining current speed but one that goes into the calculation (along with tire slip, growth...). If I'm being ignorant again could someone please explain how the speedo accounts for the gearbox.
This means that the only variables left are slip & growth, the final gearing (ie sprockets and chain) and the size of the rear tire... That plus the conversion to the dial scale... But that's the mathematics that screw it all up...
