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Driveshaft Rpm as VSS

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Good Day

I have a Link g4 + on my AWD Sti. Looking for a non driven speed signal to get my launch control set up as best as possible. If I use a hall sensor with trigger wheel to detect driveshaft rpm and in turn use this as a signal to the ECU, how can I use the signal to establish vehicle speed?

I plan on fitting trigger wheel on DS where it mates to rear diff. how many teeth should be on trigger wheel?

Any info/advice will be appreciated.

On an AWD vehicle you don't have a non-driven wheel (when everything is working correctly). Have you considered towing a small trailer and measuring it's wheelspeed?

Yeah, agree with David here, no such thing as "non driven wheel speed" on a AWD car. A driveshaft sensor will not give you anything different than the stock VSS gearbox sensor already does. You can still assign both driven and non-driven wheel speed sources to the same sensor in the Link software if thats what you are trying to achieve.

Thanks David and Adam, Im really looking to improve my launch control strategy the same as you would measuring the difference in slip of a driven versus non driven as you wud in a rwd or fwd. The difficulty being that non driven cant be measured in an awd. Perhaps a small jig with a small wheel and trigger could be setup behind the car to measure road speed. Any other options that could possibly work?

While I didn't do launch control, I have used a very expensive Inertial GPS speed measurement unit (Race Technologies Speed Box), which uses an accelerometer to integrate a speed value, then uses high-frequency GPS data to correct the drift errors you get with that. It provides a speed pulse measurement just like a wheelspeed sensor. We were doing traction control on a 4-wheel drive streamliner at Bonneville.

I have also used GPS for speed input with truck pulling setups, but conventional GPS only provides data good data after you're going about 2-3 MPH. That might work for your launch control.

There are some interesting ground speed sensors used in agriculture that might prove useful. If I remember, prices start about the same as a set of tires, though. Here is an example:

http://www.dickey-john.com/product/radar-ii/

http://www.dickey-john.com/product/radar-iii/

https://ph.parker.com/us/en/tgss-true-ground-speed-sensors