Gearbox Ratios

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Joined: Thu May 19, 2016 2:17 pm

Gearbox Ratios

Post by maxgas » Fri Dec 06, 2019 2:57 pm

Hi all have only just started with the NVH, I am planning to do some classroom work on my old P38 Range Rover (good subject). I have a full breakdown of the gear ratios (attached PDF) my question which would be the correct one's to use with the NVH software?.
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Steve Smith
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Re: Gearbox Ratios

Post by Steve Smith » Fri Dec 13, 2019 2:52 pm

Hello and thank you for the post, sorry for the late reply.

Thank you also for the data sheet as this helps to provide you with a number of options for transmission ratio entry

The minimum data you could enter is the differential ratio and tyre size assuming we have road speed from the OBD connector.

If we have road speed via OBD we can calculate the speed/frequency of the road wheels.

If we also have the differential ratio then we can calculate the speed/frequency of the propshaft

For example:
Tyre Frequency (T1) at 10 Hz x by your differential ratio of 3.54:1 = 35.4 Hz (P1)

Your data entry would look like this:
IMAGE 1.png
Image 1
If you now add transmission ratios, the NVH software can automatically display and announce your gear position as you drive through each gear range (See image 2) For this feature to function you must obtain Road Speed via OBD
IMAGE 2.png
Image 2
Your data entry fields would look like this with Transfer High:
IMAGE 3.png
Image 3
And this for Transfer Low:
IMAGE 4.png
Image 4
Whilst the Transfer Ratio entry field is available, it has no direct effect on the calculation of propshaft frequency or automatic gear display (image 2 above) when Road Speed is acquired via OBD

If you wish for increased accuracy for the display of the selected gear position (incorporating the Transfer Ratio) you could choose the “Calculate Road Speed” option, enter the Overall Gear Ratio (Gear Ratio x Selected Transfer Ratio) and select each Gear position manually within the software.
This was the approach with HGVs and Agricultural vehicles until the J1939 protocol was included within our NVH software

To select “Calculate Road Speed” see below:
IMAGE 5.png
Image 5
With “Calculate Road Speed” selected your data entry fields would look like this:
IMAGE 6.png
Image 6
Above we have taken each ratio and include the Transfer High ratio to obtain the overall ratio (without the differential)

For Example:
1st Gear Ratio = 2.480:1
Transfer High Ratio = 1.216:1
2.480 x 1.216 = 3.015:1
3.015:1 is our entry for 1st gear (including the transfer ratio)
When using this technique, we enter 1.00 as the Transfer Ratio entry

For Transfer Low the data entry would look like this:
IMAGE 7.png
Image 7
During your road test you will now be required to select the gear position in which you are driving within the software (See below)
IMAGE 8.png
Image 8

The image below describes how component frequencies (Propshaft and Wheel/Tyre) are now calculated using the “Calculate Road Speed” method
IMAGE 9.png
Image 9
Using the Road Speed via OBD method (as the alternative)
Wheel/Tyre Frequency (T1) 6.25 Hz x Differential ratio 4:1 = 25 Hz Propshaft Frequency (P1)
Propshaft 25 Hz x 3rd Gear ratio 2:1 = 50 Hz Engine Frequency (E1)

I hope this helps and please feedback for any clarification on the above,

Take care…….Steve

Posts: 5
Joined: Thu May 19, 2016 2:17 pm

Re: Gearbox Ratios

Post by maxgas » Wed Dec 18, 2019 9:14 am

Hi Steve Wow that's what you call a comprehensive explanation, I will sit down and look through it and then try it out on the RR. I wasn't sure where it gave the listing of overall ratios, how that would come into NVH. It would be handy if this info was available from more manufacturers maybe we will need to start our own database.

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