front and back ratios question/brain fart

[andres14oj]

brain fart question time: why must front and back final rations match in a system with a variable torque split transfercase? lets say we're using a torque split controller and we have a constant f/r torque split. i understand the importance in a conventional transfercase with a fixed split, but other than having a bad torquesplit gauge reading, and maybe not reaching a full 50/50, how would this affect the awd system? i can see a problem using a lower ratio on the front during 50/50 making the front wheels turn faster, but how does having a higher front and lower rear affects it? 

flame suit on.... 

[Duncan]

either way, ATTESSA will throw a fault if the front and rears are continually turning at different rates

[GTSBoy]

Different front-rear speeds is one of the significant inputs for the ATESSA CU to decide what it's going to do about that torque split. Remember.....it was designed in the 80s.

[joshuaho96]

2 hours ago, andres14oj said:

brain fart question time: why must front and back final rations match in a system with a variable torque split transfercase? lets say we're using a torque split controller and we have a constant f/r torque split. i understand the importance in a conventional transfercase with a fixed split, but other than having a bad torquesplit gauge reading, and maybe not reaching a full 50/50, how would this affect the awd system? i can see a problem using a lower ratio on the front during 50/50 making the front wheels turn faster, but how does having a higher front and lower rear affects it? 

flame suit on.... 

That's just the assumption the system makes in the controller programming (any front-rear speed differential is slip, not just different ratios) and the transfer case is not designed for a ton of slip for long periods despite being a wet multi-plate clutch. Modern cars absolutely do have different front-rear final drive ratios, Acura even did a variable rear final drive ratio with some versions of their SH-AWD system. Ferrari also did a different front final drive ratio on one of their AWD systems so it only works in certain gears before disengaging entirely. It's also not like Nissan didn't know this was possible back when they were designing ATTESA ETS for the R32 Skyline, the Porsche 959 rather than using a different front final drive ratio ran slightly larger front tires to reduce transfer case slip at the steady state torque split. Nissan could've done the same trick but their design goal was a true RWD feeling normally with a bit of AWD when needed, not true AWD. You can kind of see their logic in this video:

 

[andres14oj]

Gotcha, i was forgetting these are inputs for the attessa ecu. totally makes sense