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1. If you believe the factory speedometer you will think you can ride on a 60 mph highway without impeding other drivers
If you cannot keep up with traffic, you are impeding drivers. If the speed limit is faster than 50MPH, this is likely the case. Doesn't matter if your speedometer is reading 63 or 66 MPH, you are still holding up traffic.

2. You cannot intelligently make engine and sprocket changes to optimize performance for your specific riding needs or wants if your measurement instruments are lying to you.
You are still only changing one variable. The speedometer may not be accurate, but it is precise. You still get a relative measurement.

3. This thread is all about "how fast have you had your Z125 going". If you answer that question by quoting your lying speedometer or tachometer, you are lying to everyone who wants to know the REAL answer to the question. If we are going to post lies, why bother to even ask the question?
This is the internet. If I was to believe everything I read, Queen Elizabeth is a cannibal, the earth is flat and I have $1B in some offshore account waiting for me to claim.
 

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As someone who makes their bread and butter doing GPS design, I would also like to see the source for this. Seems like a broad generalization.


Top speed on a Z125 is 67(ish). What does it really matter if you are riding at 35 or 32 MPH? Unless you are riding like an idiot, you aren't going to get pulled over.... so what does it matter?
I do not believe for even a second that the TRUE top speed on a Z125, on level ground, with no tailwind, is 67 mph. Your tach and speedo might tell you that, but it's not true.

On a bike that is as low powered as the Z125:

- Even a modest 3 MPH tailwind has a HUGE effect, because the difference in power required to go say 60 mph versus 57 mph is actually 15% !!

- Even when you THINK a road is "level", it almost always is NOT perfectly level, siomply because there is no economic justification for a road builder to pay the huge extra costs to make it perfectly level when a 1 or 2% grade is not a problem for most vehicles. For a Z125 though, a 1 or 2% grade IS a big deal. And, the steepest hills on an interstate highway are, by interstate building standards, a maximum of 5% except in mountain passes. Those 5% grades are enough to slow trucks and low powered cars and motorcycles down significantly. They will severely slow a Z125. And most non-intersate highways are built to much looser grade standards, with some grades in our area hitting 22%. So, the road we visually perceive as being "level" has almost certainly SOME gradient in most cases.

- Going full throttle downhill proves NOTHING about your bike's top speed. I'm sure that if you go to the right mountain pass downhill, you can indeed "hit the rev limiter", but ry that on LEVEL road. You'll be nowhere close with a stock Z125.

I was hoping to find REAL info in this thread about what a Z125 is and is not capable of doing, but have had to conclude that I'll have to do my own testing, with ACCURATE instruments, on a road or track or airport runway that I KNOW was built "level", and when there is actually zero wind. That's what you have to do when a vehicle is deliberately built with a very low powered engine.

Jim G
 

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. . .

You are still only changing one variable. The speedometer may not be accurate, but it is precise. You still get a relative measurement.

. . .
Definition of precise:

pre·cise
/prəˈsīs/Submit
adjective
marked by exactness and accuracy of expression or detail.
"precise directions"
synonyms: exact, accurate, correct, specific, detailed, explicit, unambiguous, definite
"precise measurements"
(of a person) exact, accurate, and careful about details.
"the director was precise with his camera positions"
synonyms: meticulous, careful, exact, scrupulous, punctilious, conscientious, particular, methodical, strict, rigorous
"the attention to detail is very precise"
used to emphasize that one is referring to an exact and particular thing.
"at that precise moment the car stopped"
synonyms: exact, particular, very, specific
"at that precise moment the car stopped"

Are you kidding??? Our Z125 speedometer is KNOWN to be at least 5 to 6% inaccurate, AND being digital with zero decimal places, it can only display to the nearest full integer. It is at BEST "consistent" (in its error), but certainly not precise! :)

Jim G
 

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If you line up 2 Z125s side by side, I guarantee you one is going to be faster than the other. Not all motors are built equal. These are cheap bikes and there is a lot of variation in the manufacturing process.

As far as riding on a level surface, with no tail wind, at sea level (forgot about that one), in a full tuck, then sure, you can generate ideal testing conditions... but how often do you ride on an airport runway?

But why? Straight line speed on a bike is dumb anyways (ESPECIALLY on a Z). Anyone can pin the throttle and ride the bike until redline.
 

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Definition of precise:

pre·cise
/prəˈsīs/Submit
adjective
marked by exactness and accuracy of expression or detail.
"precise directions"
synonyms: exact, accurate, correct, specific, detailed, explicit, unambiguous, definite
"precise measurements"
(of a person) exact, accurate, and careful about details.
"the director was precise with his camera positions"
synonyms: meticulous, careful, exact, scrupulous, punctilious, conscientious, particular, methodical, strict, rigorous
"the attention to detail is very precise"
used to emphasize that one is referring to an exact and particular thing.
"at that precise moment the car stopped"
synonyms: exact, particular, very, specific
"at that precise moment the car stopped"

Are you kidding??? Our Z125 speedometer is KNOWN to be at least 5 to 6% inaccurate, AND being digital with zero decimal places, it can only display to the nearest full integer. It is at BEST "consistent" (in its error), but certainly not precise! :)

Jim G

See attached.
 

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If you line up 2 Z125s side by side, I guarantee you one is going to be faster than the other. Not all motors are built equal. These are cheap bikes and there is a lot of variation in the manufacturing process.

As far as riding on a level surface, with no tail wind, at sea level (forgot about that one), in a full tuck, then sure, you can generate ideal testing conditions... but how often do you ride on an airport runway?



But why? Straight line speed on a bike is dumb anyways (ESPECIALLY on a Z). Anyone can pin the throttle and ride the bike until redline.
I agree with all or most of this. Reason for my asking about real top speed: I'd like to know what highways I can actually dare to ride on with this low powered bike. Most of the highways in our area have a posted speed limit of 80 KPH = about 50 MPH, and a maximum of 90 KPH = about 56 MPH. Only ONE highway has a higher limit (110 KPH = about 68 MPH). I think the Z125 can maintain 50 mph or close to it even on some moderate hills, using 3rd gear when necessary. But, I don't think I could safely go on a highway posted at 60 mph, and especially if there is any wind or there are any uphill gradients, unless I somehow add some power to the engine.

I have the Graves Carbon Titanium exhaust on it now, but no AFR controller, so I doubt I am getting any benefit other than the wonderful soundtrack, until I can modify the AFR to match the exhaust's airflow.

I find the "gap" between the 9800 rev limit and the actual peak power rpm (7000 to 7500?), and the gearing that Kawasaki provided, intriguing. I suspect that the Z125 engine started out as a higher powered dirt bike engine that Kawasaki dropped a milder cam into for the Z125 as their attorneys would have absolutely vetoed the idea of allowing typical young buyers to try to zip down a traffic-laden highway at 70 or 80 mph on a bike that is 5-1/2 feet long and weighs about 225 lb fulled gassed.

So, I'm thinking the easiest, mechanically safest, and cheapest way to get more performance might be to drop in a more aggressive cam with a matched ECU tune (perhaps an Aracer CPU replacement?) and actually USE more of that 9800 rev limit and that factory high rpm gearbox gearing and final drive sprocket ratios.

But, I first need to ACCURATELY know what I have NOW, and that means a more accurate testing tachometer is needed(on order now).

Jim G
 

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The internet: Because fighting with family for 4 days wasn't quite enough confrontation.
 

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How accurate is GPS for speed measurement?
As with positioning, the speed accuracy of GPS depends on many factors.

The government provides the GPS signal in space with a global average user range rate error (URRE) of ≤0.006 m/sec over any 3-second interval, with 95% probability.

This measure must be combined with other factors outside the government's control, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality, to calculate a particular receiver's speed accuracy.
https://www.gps.gov/systems/gps/performance/accuracy/

That's 6mm per second over a 3 second interval.

In the article you linked, they said they used the circumference of the wheel, I really hope they actually measured the distance that the machine moves for one rotation of the tire (better to measure 10 rotations). With rubber (or any flexible tire) those two measurements are not equal. This was never explained. Also note that this study was 13 years ago, so much has changed in the past 3 years (let alone 13) that this study would need to be done again. The math processing in those older handheld units was not great, with large rounding errors.

I have personally used the GPS in a phone to check the speedometer in a car, and at the same time driven past a road side RADAR unit that displays your speed. They were all within reasonable agreement. Also before putting my oversized snows on the current car, my dashcam GPS speed overlay agreed with the speedometer within +-1mph at 65 to 70 mph which is around a 1 percent error between speedo and GPS (and only a 1hz receiver).


It will be interesting to read what Crakerjac has to say about that article.

The 10hz lap timers claim accuracy to 1 meter and at normal speeds this is down to hundredths of a second. Accurate enough that many people no longer use the RF based lap timers at most tracks (normally an extra fee and normally prohibited during "practice" where there in no racing). If you are going to claim that much error, then you should take this company to court because you stand to win a class action lawsuit https://www.aim-sportline.com/en/out-of-production/solo/index.htm
https://www.aim-sportline.com/en/out-of-production/solo/technical-specifications.htm

And if you really want, you can go with GPS and GLONAS for more accuracy with something like this https://www.pegasusautoracing.com/2018/035.pdf
 

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I took my bike to Houston Raceway Park (1/4 mile track) And the mph they detected matched my phone gps. 68mph in the 1/4. Which also is what my Speedo showed.
 

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The internet: Because fighting with family for 4 days wasn't quite enough confrontation.
Excellent conclusion.:grin2:
I love it when the scientists disagree.
 

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I was hoping to find REAL info in this thread about what a Z125 is and is not capable of doing, but have had to conclude that I'll have to do my own testing, with ACCURATE instruments, on a road or track or airport runway that I KNOW was built "level", and when there is actually zero wind. That's what you have to do when a vehicle is deliberately built with a very low powered engine.

Jim G
Your kidding, right?

You seem absolutely infatuated about knowing the EXACT speed you are going so you be on the right road with a posted
speed that nobody actually drives?
So if in your conclusion of finding the EXACT max speed on the Z, on level ground, with no wind, then what?
When you discover it is say, 60 mph, then you only ride a 60 mph road when there is no wind and it's level?
I am finding this discussion ridiculous and selling my bike because of it.:surprise:
 

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But you have a 120/80 rear tire, correct?
The difference in tire radius between a 120/70-12 and 120/80-12 tire is about 12mm = about 1/2 inch, which is about 5.4% of the tire radius, so yeah, that would pretty much exactly "correct" the inaccurate speedometer to the correct speed.

And by the way, I'm sure that The OP knows this, but that larger tire also changed his "gearing" by the same 5.4%, making it 5.4% "taller" than stock.

Jim G
 

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I'm about 320lbs. I got my Z flying at 52mph according to the speedo. That's fast enough to get a speeding ticket on the street I was riding. Thank goodness the judge had a good sense of humor when I showed her a picture of me on the bike.
 
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