Maths for gear position voltage to road speed calculations

[Chris Wilson]

I have set up a voltage based gear position input on my M800. I have 2 questions, in Datalogging : Other Sensors : Gear Position Voltage what rate of logging do I need to enable for reasonable road speed accuracy in I2 Standard?

Being totally new to datalogging the maths needed to go from RPM, and gear position, to a road speed readout is a bit daunting. Can someone give me a hand please? I *THINK* the info below is enough?

1st gear ratio : 3.00
2nd gear ratio : 2.14
3rd gear ratio : 1.65
4th gear ratio : 1.32
5th gear ratio 1.10

I have drop gears in the gearbox (Tractive sequential ) of 22 driving, 23 driven giving an overall reduction. I calculate this at 1.045 to 1

My diff ratio is 3.545 to 1

I *THINK* i ma right multiplying the two together to get an overall ratio of drop gears and diff ratio of 3.70 to 1??

That's what I have used for now, anyway.

My rear tyres revs per mile figure is 829 RPM. The car is RWD.

What do I put in as a maths function to get I2 Standard to read road speed in MPH from the above data, and is logging engine RPM and gear position voltage enough to achieve this?

Thanks. I think once I see the procedure I'll be fine to input other values if I change ratios etcetera, it's just I would like to see this demonstrated by someone that knows what they are doing first please

[PaulGM]

Mr. Wilson,

There are a couple of caveats to get out of the way in the first place- this cannot be done in i2 Standard, because you cannot create 2D tables (which you need to hold gear ratios). The channel must be created in i2 Pro, so you must have either Pro Logging enabled in your M800 or have the "Open i2 Standard Files" license for i2 Pro. You must also keep in mind that if you leave the car in gear and rev the engine, your speed channel will show you as moving when you are not. Traditionally, Wheel Speed or GPS are much more accurate and less error-prone means of finding ground speed.

That being said, this is possible to do in i2 Pro. First, you should create a 2D table which contains your overall ratios for each gear (that way, you don't have to write the constants for your drop gear and differential into your equation), which will look something like "1-11.114, 2-7.928, 3-6.112," etc. You will need to select your "Gear" channel as your X Axis, so that the output of your table is the gear ratio.

Next, you must actually create your Ground Speed channel, which uses an equation like this->
GRND SPD = (Miles/Axle Rev) * (Axle Rev/Engine Rev) * (Engine Rev/Min) * (60 Min/Hr), resultant units = MPH

The "Miles/Axle Rev" value is the circumference of your tire, which I calculated to be 0.00121 miles. The "Axle Rev/Engine Rev" value is the inverse of your overall ratio (as gear ratios are driven/driver), or 1/OAR. The "Engine Rev/Min" value is simply Engine RPM. And finally, "60 Min/Hr" is a fairly obvious conversion value.

The 2D table, when setup properly, is typed into an i2 Maths equation like a constant, so your i2 equation should look something like this-
0.00121 * (1 / 'Gear Ratio Table') * 'RPM' [rpm] * 60

As for logging rate, 10 Hz is what I'm told is standard for logging gear changes (unless you are doing Gear Change Ignition Cut, etc.), just be aware that in certain spots your speed may be calculated using the wrong ratio for 1/10th of a second.

[Chris Wilson]

Wow, what a great and comprehensive reply, thanks so much for taking the time to type that little lot, it's much appreciated. I have an immediate snag in that I neither have Pro Logging enabled in my M800, nor do I have i2 pro. On a different forum someone suggested that it may be possible to create a table in the M800 itself to output a suitable data stream for i2 Standard to read the gears from, and thus input the data logging software with parameters ready tabled? I think I understood him correctly in this regard.... It's becoming obvious though that the simplest way for me to log speed without using a lap side beacon (which is disallowed on most UK track days) is to use GPS.

If I abandon the above (and sincere apologies if you feel you have wasted your brain power and typing time , is it straightforward to get i2 Standard to juts show what gear I am in?

This subject is even more complex that I had expected, and anything like this gives me the willies

[PaulGM]

Mr. WIlson,

No worries, I am happy to help no matter what you decide.

As for the M800 sending out the ratio, this is difficult and limited. You can make a custom auxiliary table which sends out values, but you lose precision here (i.e. your ratios would be something like 3.0, 2.3, 2.1) and you wouldn't actually have enough precision to send the first overall ratio. Our dash could do this conversion and calculation for an output, but I am assuming you are using only the ECU. For all intents and purposes, your best bet is to have the ratio table in i2.

As for showing your gear in i2 Standard, if you have logged it, you can see it. You simply display your gear channel on any worksheet you wish. If you are unfamiliar with i2's operation, there is a demo video you can see under "Help->View Training Video."