Custom Fighters - Custom Streetfighter Motorcycle Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
8 Posts
Discussion Starter · #1 ·
So I got my Thunderace fighter all setup and noticed while riding that my speedometer won't read above 30 MPH. Below 30 it does just fine. I thought the magnet was insufficient so I got a stronger rare earth magnet and same results. I have the sensor and magnet about 1-2mm from each other so distance shouldn't be an issue. Also played around adjusting 2 inches up and 2 inches down for the "rim size" calibration in the speedometer gauge...no luck.

I was thinking maybe its the style of the magnet? Maybe I should pick-up a bar magnet that will sweep across the entire sensor instead of the dot magnets, but before I go buy another magnet, Just seeing if anyone else out there ever had the same issue.

The gauge I'm using is the SP3 from DanMoto.


Sensor and magnet
 

·
Registered
Joined
·
136 Posts
That looks pretty far apart to me, my KOSO was a fooking PITA with the sensor at first.

Is the wheel balanced and true? If it isn't it could be moving out of range


If you have the sensor in your hand and strike a magnet past it quickly can you make the gauge display more than 30mph?

Does it just stop at 30mph or what?


Edit: if I were you I would consider using a KOSO style sensor bracket mounted off the bottom caliper bolt with magnets in the disc bolt heads as its nice and clean looking
 

·
The Hell You Say!!
Joined
·
2,956 Posts
I agree with shiny. I would also suggest putting it on the rotor. It looks cleaner and makes the time between rotations shorter. Is it not real jumpy with it being on the wheel? I would think the shorter its full rotation is, the more accurate and the smoother it would read.
 

·
Registered
Joined
·
8 Posts
Discussion Starter · #5 ·
As far as I can tell and feel the wheel is balanced and true. Sorry the picture might be at a funny angle but I am 100% positive its a 3 mm gap at max. I would try to put it on the disc but the gauge is calibrated to take inches in rim diameter, unless I'm stupid and I'm missing something obvious here :redface:
 

·
Registered
Joined
·
740 Posts
It doesn't matter how far from center the magnet is the magnet will pass at the same intervals which is all those sensors pick up...not how fast the magnet passes, just the time in between passes.

Try getting the sensor closer. If still nothing, I'd be suspect of the gauge. Cheapo chinese gauges can be pretty awful in terms of quality.
 

·
The Hell You Say!!
Joined
·
2,956 Posts
typically those gauges can be calibrated to rolling circumference. Find you a starting point and mark it then find you a reference point on the wheel (the valve stem works great) Roll the bike until you have completed one full rotation (i.e. start with the valve stem down and roll until valve stem is down again). Mark the end point and measure. You will probably have to convert the measurement if you are measuring in inches. Most of these read millimeters. You should be able to put that number in and voila! I know most guys will do it this way even when it is mounted on a rotor bolt.

I also believe that most of these gauges suggest mounting to a rotor bolt.
 

·
The Hell You Say!!
Joined
·
2,956 Posts
It doesn't matter how far from center the magnet is the magnet will pass at the same intervals which is all those sensors pick up...not how fast the magnet passes, just the time in between passes.

Try getting the sensor closer. If still nothing, I'd be suspect of the gauge. Cheapo chinese gauges can be pretty awful in terms of quality.
I know that they are designed to work on number of rotations. From what I understand, if you are only using one magnet, the increase is your speed reading can be jumpy. i.e. when taking off from a stop you could jump from 0mph to 20mph before it will read. Moving the magnet in closer will help correct this because the sensor is able to read the change in acceleration more accurately. You can also throw off the balance of the wheel with the magnet that far off center. I know they don't weigh much, but if your balance isn't perfect already it could be the last straw. Also the faster you go, the more force is acting on the magnet which could pull it even more out of balance.

If I'm way off, please tell me. I'm just working on my limited understanding of physics as a 2nd year mechanical engineer major.
 

·
Registered
Joined
·
4,653 Posts
I didn't read all your guys replies so it might have been said already but I had this problem with my Koso and it was not due to the magnet or how close it was it was because I switched from a 180 rear tire to a 190 tire so the gauge was reading way off due to tire size. Try recalibrating the gauge for the tire
 

·
Registered
Joined
·
136 Posts
I didn't read all your guys replies so it might have been said already but I had this problem with my Koso and it was not due to the magnet or how close it was it was because I switched from a 180 rear tire to a 190 tire so the gauge was reading way off due to tire size. Try recalibrating the gauge for the tire


Surely it'd just read wrong and not stop at 30?



Also, for increased response I have 5 magnets on my koso all on front disc bolts.
You can do this on a Chinese knock off that should have one magnet by dividing your rim size by number of disc bolts and fitting a magnet in every disc bolt
 

·
Registered
Joined
·
8 Posts
Discussion Starter · #13 ·
Thanks for the advice, Ill try getting some magnet bolts on the rotor and messing around with the sensor and gauge.
 

·
Registered
Joined
·
740 Posts
It still wouldn't matter. As a sophmore mechanical engineering student, you will probably get into this pretty soon in your dynamics class specifically rotational modeling.

Picture the path the magnet travels in when placed at the edge of the rim vs near the center on a rotor bolt. The inner circle is much smaller, with less circumference. So one rotation of the wheel has the rim magnet traveling more distance than the inner one.Lets say the outer path is 30cm and the inner is 10cm. In physics terms this is displacement or x. Velocity as I'm sure you've covered is dx/dt or displacement relative to time. Say it takes 1 second for the wheel to complete 1 rotation. Rim magnet covers say 30cm in this time and rotor magnet covers say 10cm. So the velocity is higher, but it covers a greater distance, so the angular velocity (in radians/second) of the two is identical.

As said these sensors only register that a magnet passed it and they count the time in between intervals. When you input circumference of the wheel, or displacement per rotation, it has the two parts needed to calculate velocity.

Factory speed sensors work pretty much the same way, but they read off the countershaft...which has a much smaller diameter and rotates much much faster RPM than the wheel, so the pulses occur far more frequently, so there's a much better resolution/smoothness to the data points. As mentioned adding more magnets to the wheel will increase smoothness of the display.
 

·
The Hell You Say!!
Joined
·
2,956 Posts
That makes sense. I actually will be taking dynamics either next semester or next fall so its nice to get a look at what's coming. I am just getting in to physics this semester so my understanding of it is still pretty limited.


Sent from Motorcycle.com Free App
 
1 - 15 of 15 Posts
Top