[HOW-TO] Enter Gear and Drive Ratios

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[HOW-TO] Enter Gear and Drive Ratios

Post by Harry »

This how-to describes how to prepare our apps to display valid gear values from OBD input.

Vehicle data shown in our apps are either values retrieved from OBD directly, or calculated from OBD and other inputs. Samples are power developed and gear values.

The gear engaged can be derived by looking into the relation between speed and rpm:

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engine rpm / overall_ratio (gear) * WC = wheel speed

with WC being the driven wheel's circumference and overall_ratio (gear) being the ration between engine rpm and wheel rpm. The equation is valid while the clutch is not engaged.

In the vehicle's details you can enter an arbitrary number of gears plus a drive ratio. Using this, the app will calculate overall_ratio (gear) from

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overall_ratio (gear) = gear ratio * drive ratio
So what are the steps to prepare the app to calculate the gear engaged?

Step 1: get your data

Gear and drive ratios are listed in your car's manual mostly. In case they are not, try the net. In case you have problems finding your data, please post here so others can help you. Sample:

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Porsche 997/2 Carrera S come with this values:
1st gear: 3.91
2nd gear: 2.32
3rd gear: 1.61
4th gear: 1.28
5th gear: 1.08
6th gear: 0.88
Drive ratio: 3.44
Step 2: enter data into the app

Navigate to LapTimer ‣ Administration ‣ Vehicles ‣ your vehicle ‣ Power Transmission. Add gears one by one in any order. The app will order the gears from descending (small gears come with high ratios, see division above). Finally, add the drive ratio.

Step 3: enter wheel sizes and drive type

In the same section, enter your wheel / tire sizes. This will allow the app to derive WC. To select the correct wheels for calculation, specify the drive type (All-whell, rear-wheel, etc.).

Step 4: verify the Chart showing the relation between rpm and speed looks sound

The final step is to connect an OBD and drive. In case you do not have an OBD adapter available, the only benefit from the above excise is the chart. :-)

Multi drive ratios

Recently, multiple drive ratios depending on the gear have become available. We have seen this a lot in Volkswagen cars, but there are others coming in. It is not possible to enter multiple drive ratios currently, so this convenience feature is not helpful for the cars mentioned.

Instead, recall the app is actually interested in the overall_ratio (gear), not a gear or a drive ratio. So instead of specifying gear and drive ratios as described above, just multiply you gear and gear specific drive ratios and enter this values into the gear ratio fields. Finally, set the drive ratio to 1. Complicated sample:

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Some Mercedes AMG lists this specs:
1st gear: 3.86
2nd gear: 2.43
3rd gear: 2.67
4th gear: 1.05
5th gear: 0.78
6th gear: 1.05
7th gear: 0.84
Drive ratio 1: 4.13 (gears 1, 2, 4, 5 and R)
Drive ratio 2: 2.39 (gears 3, 6, 7)
So let us do the math for the overall ratio of gear 3: the gear ratio is 2.67 and drive ratio 2 = 2.39 is used:

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overall_ratio (3) = 2.67 * 2.39 = 6.38
Enter the value 6.38 as a gear's ratio and repeat for gears 1, 2, 4, 5, 6, 7. Do not forget to enter 1.0 as the drive ratio.
In case any of the values exceeds 9.99, you need to add another tweak: instead of entering 1.0 as drive ratio, enter e.g. 2.0 and divide all calculated gear ratios by 2.0 again. In our example for gear 3, this means 3.19 and common drive ratio 2.

Future support

Our apps come with integration into Edmunds to ease data entry for (US) cars. This database does not cover gear ratios, but is valuable adding approximately 75% of the other parameters our apps use. So please always start by using the lookup function in the top of the vehicle dialog. Starting with v20 our apps will collect vehicle data entered by users. We will have some kind of certification process to verify data submitted, and provide certified data as downloads afterwards. We will start to provide certified data once we have experience with data quality submitted and once we have a reasonable coverage.

- Harry
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