Math Related Motoring Question.

[elistu]

Hi all,

I hope there are some brainboxes out there who could help me with a math problem.

I have just bought a KOSO digital dash display for a project car I am working on and have so far got everything working as it should but have hit a small problem with the speedo side of things.

To fit this unit as per instructions you have to mount the magnetic sensor no more than 60mm from the center of the wheel hub, you then program in the distance you have mounted this from center (from 0 - 60mm max). Then you measure the rolling radius of the tyre and program this measurement in, the maximum tyre size I can go up to is 9999mm.

OK here is my problem that will require a much better brain than mine. My sensor can only be mounted 80mm from the hub center (60 is the max), this is the absolute closest I can get it. So what would I do on the tyre measurement to compensate for the 20mm difference of the sensor?

Is it just a case of adding 20mm to the radius of the tyre or is there a much more indepth mathamatical solution that I cant see?

I hope someone can help here.

[dave_boo]

Actually you can't just add 20 mm to the radius.

Here's why; the system takes the 60 mm and uses that as the baseline. Your value you put in for the radius of the tyre is another value that the processor will use in a ratio to compute the distance per revolution. At 60 mm from the centre of the bearing the distance traveled by the sensor is 355 mm per revolution of the wheel. So if the pickup reads 2 817 revolutions per minute the sensor translates that into 1 km/h.

Supposing you have 265/70-17 tyres (just an example I pulled out of thin air), then the circumference should be ~5612. Obviously that's much larger than the circumference transcribed by the sensor. So the processor does a quick calculation; 5 612/355=15,81. I.E. instead of displaying 1 km/h at 2 817 revolutions of the sensor it displays (sensor speed) x (15,81) =.

So to solve your problem a simple algebraic equation is in order.

(tyre circumference) / (maximum allowed distance) = (Y) / (distance installed)

or to solve for you:

Y = (distance installed--put 80 here for your case) x ((tyre circumference) / (maximum allowed distance--put 60 here for your case))

This may seem a bit backwards, but it's actually what you need to do. Input 60 for your value as distance from the centre and since you're at 80, and the radian speed is much higher, you need to fool the processor into thinking that you have 33% bigger tyres on then is actually the case.

[elistu]

Awesome,

Thanks for your help, I knew it was going to be a lot more complicated than just adding the extra 20mm to the tyre. Wish I had paid attention in math class now!!

Actually you can't just add 20 mm to the radius.

Here's why; the system takes the 60 mm and uses that as the baseline. Your value you put in for the radius of the tyre is another value that the processor will use in a ratio to compute the distance per revolution. At 60 mm from the centre of the bearing the distance traveled by the sensor is 355 mm per revolution of the wheel. So if the pickup reads 2 817 revolutions per minute the sensor translates that into 1 km/h.

Supposing you have 265/70-17 tyres (just an example I pulled out of thin air), then the circumference should be ~5612. Obviously that's much larger than the circumference transcribed by the sensor. So the processor does a quick calculation; 5 612/355=15,81. I.E. instead of displaying 1 km/h at 2 817 revolutions of the sensor it displays (sensor speed) x (15,81) =.

So to solve your problem a simple algebraic equation is in order.

(tyre circumference) / (maximum allowed distance) = (Y) / (distance installed)

or to solve for you:

Y = (distance installed--put 80 here for your case) x ((tyre circumference) / (maximum allowed distance--put 60 here for your case))

This may seem a bit backwards, but it's actually what you need to do. Input 60 for your value as distance from the centre and since you're at 80, and the radian speed is much higher, you need to fool the processor into thinking that you have 33% bigger tyres on then is actually the case.

[hansnl]

I have the idea that the maximum distance to the hub of 60 mm is just the maximum distance the sensor can pick up the turning of the wheel.

Maybe it will pick up the "signal" from the wheel from a greater distance, but I am afraid it will not work properly.

As the sensor works on magnetic waves, maybe mounting a small magnet on the wheel, near the hub, might do the trick.

[elistu]

Unfortunately I have tried that, the sensor has to be bang over the magnet or it just won't pick up a reading.

I have the idea that the maximum distance to the hub of 60 mm is just the maximum distance the sensor can pick up the turning of the wheel.

Maybe it will pick up the "signal" from the wheel from a greater distance, but I am afraid it will not work properly.

As the sensor works on magnetic waves, maybe mounting a small magnet on the wheel, near the hub, might do the trick.

[WarpSpeed]

Unfortunately I have tried that, the sensor has to be bang over the magnet or it just won't pick up a reading.

I have the idea that the maximum distance to the hub of 60 mm is just the maximum distance the sensor can pick up the turning of the wheel.

Maybe it will pick up the "signal" from the wheel from a greater distance, but I am afraid it will not work properly.

As the sensor works on magnetic waves, maybe mounting a small magnet on the wheel, near the hub, might do the trick.

I just want to say that I agree with Hansl it's probably more about range of signal pick up..

Just out of curiosity not knowing your set up or anything and looking big picture, why does it need to be on the hub? Any similar rotating piece of metal such as an axle for example should do? With a little fabrication this should be possible.. The numbers should be the same..

Actually what about mounting to a CV joint bolt, is there room for that? I'm flying blind here so be gentle..

Edited November 23, 2010 by WarpSpeed

[pattayabuggy]

I tell you what I would do, set it up roughly and follow a friend at a constant speed, calibrate till yours agrees with his.

Then do a final check at say 50, 80 and 100. Takes out all the headaches.

Edited November 23, 2010 by pattayabuggy

[dave_boo]

I have the idea that the maximum distance to the hub of 60 mm is just the maximum distance the sensor can pick up the turning of the wheel.

Maybe it will pick up the "signal" from the wheel from a greater distance, but I am afraid it will not work properly.

As the sensor works on magnetic waves, maybe mounting a small magnet on the wheel, near the hub, might do the trick.

The sensor is not mounted on the hub. Since hubs vary in diameter, the manufacturer could not use a standardised measurement for their calculations. I also highly doubt that the simple Halls Effect sensor could read at a distance of 60mm...

[elistu]

Hi,

Yes you are right, I could mount it somewhere else but the same problem applies, wherever I mount it I will be over 60mm from the centre of whatever it is and I can't use a driveshaft because it is a swing axle beetle and the driveshafts are encased and there are no CV joints.

Good thinking though.

By the way everyone, the math thing works. I did my calculations, input the figures and went for a quick blast round the block using a Honda Wave (perhaps not the most exact reading) to make sure my speedo was roughly correct and the car matched the bike KM for KM and thats good enough for me.

Thanks everyone for your input, I know where to come next time I hit a problem!!

Unfortunately I have tried that, the sensor has to be bang over the magnet or it just won't pick up a reading.

I have the idea that the maximum distance to the hub of 60 mm is just the maximum distance the sensor can pick up the turning of the wheel.

Maybe it will pick up the "signal" from the wheel from a greater distance, but I am afraid it will not work properly.

As the sensor works on magnetic waves, maybe mounting a small magnet on the wheel, near the hub, might do the trick.

I just want to say that I agree with Hansl it's probably more about range of signal pick up..

Just out of curiosity not knowing your set up or anything and looking big picture, why does it need to be on the hub? Any similar rotating piece of metal such as an axle for example should do? With a little fabrication this should be possible.. The numbers should be the same..

Actually what about mounting to a CV joint bolt, is there room for that? I'm flying blind here so be gentle..