I finally understand what a torsen diff "feels" like
09-25-2017, 08:28 PM
#1
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Join Date: Apr 2011
Location: Oregon
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I finally understand what a torsen diff "feels" like
I've been tracking various cars over many years; you see I'm an old guy. All of these had various kinds of clutch pack posi-traction/limited slip rear ends. Racing this miata has been a little quirky and sometimes difficult for me to understand; much of which is because I didn't properly understand how the Torsen acted different than a clutch-pack diff. After finally getting schooled on the "why"... (thank you all for your patience), I'm excited to say that I finally understand what these things "feel" like on track!
For me the miata had 2 handling evils that I couldn't get a handle on. 1: it would hop/chatter when accelerating hard off a tight turn. I thought it was the clutch and changed it out... no help. The rocket scientists at AR racing suggested that I stiffen up the rear spring so that I could run a lighter rear bar... I didn't ask why, but it worked like a charm (thank you). 2: The rear end would "wiggle" under hard braking/downshifting. I tried dialing in more rear toe. I checked the rear bump-steer (it was slightly out but couldn't figure out how to shim it easily). I eventually d/c 'd the ABS, installed a prop valve and dialed down the rear brakes.
Now I understand that the diff had a lot to do with all of this. #1: Apparantly, the Torsen needs both wheels on the ground to work. When I'd lift the inside rear tire, the diff would go "open" and then car would chatter as the inside bounced up/down. Soften the rear bar and no more wheel lift... no more chatter. #2: Apparantly, unllike a clutch pack diff, the Torsen doesn't work in reverse. When under threashold braking one tire would get braking and the drive wheel would get braking + downshifting, causing the tire slip and wiggle.
Again, I thank all my friends and the local vendors for being so patient with me. An epiphany... seems old dogs can learn new tricks!!
For me the miata had 2 handling evils that I couldn't get a handle on. 1: it would hop/chatter when accelerating hard off a tight turn. I thought it was the clutch and changed it out... no help. The rocket scientists at AR racing suggested that I stiffen up the rear spring so that I could run a lighter rear bar... I didn't ask why, but it worked like a charm (thank you). 2: The rear end would "wiggle" under hard braking/downshifting. I tried dialing in more rear toe. I checked the rear bump-steer (it was slightly out but couldn't figure out how to shim it easily). I eventually d/c 'd the ABS, installed a prop valve and dialed down the rear brakes.
Now I understand that the diff had a lot to do with all of this. #1: Apparantly, the Torsen needs both wheels on the ground to work. When I'd lift the inside rear tire, the diff would go "open" and then car would chatter as the inside bounced up/down. Soften the rear bar and no more wheel lift... no more chatter. #2: Apparantly, unllike a clutch pack diff, the Torsen doesn't work in reverse. When under threashold braking one tire would get braking and the drive wheel would get braking + downshifting, causing the tire slip and wiggle.
Again, I thank all my friends and the local vendors for being so patient with me. An epiphany... seems old dogs can learn new tricks!!
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09-26-2017, 05:53 PM
#2
You got #1 figured out, Although the chatter you're feeling is more a function of the tire beginning to slip rather than the diff itself chattering.
#2 not so much, it's true the torsen does act like an open diff under engine braking. One wheel won't get all the engine braking, the force is still distributed evenly just as an open diff will in acceleration. It's when you over power the tire grip that you get the unloading of torque through one tire. All this is secondary to the sway you're feeling under braking, that's simply caused by all the weight transferring to the front wheels and the fact the Miata is a very short wheel base car. You can prove the diff isn't the culprit by simply braking once with the clutch in, if you're braking hard enough you'll wag the tail just the same. Now the caveat to this is if your brake bias isn't high enough in the rear to get to threshold braking it won't do it.
I run a bias valve in the cockpit so I use the tail wag as an indicator to back off the rear brakes a bit.
#2 not so much, it's true the torsen does act like an open diff under engine braking. One wheel won't get all the engine braking, the force is still distributed evenly just as an open diff will in acceleration. It's when you over power the tire grip that you get the unloading of torque through one tire. All this is secondary to the sway you're feeling under braking, that's simply caused by all the weight transferring to the front wheels and the fact the Miata is a very short wheel base car. You can prove the diff isn't the culprit by simply braking once with the clutch in, if you're braking hard enough you'll wag the tail just the same. Now the caveat to this is if your brake bias isn't high enough in the rear to get to threshold braking it won't do it.
I run a bias valve in the cockpit so I use the tail wag as an indicator to back off the rear brakes a bit.
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09-27-2017, 04:00 AM
#3
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It's interesting - OP's #2 observation is very similar to what I noticed when going from an open to a torsen diff in my first Miata. Engine braking into a tight corner suddenly caused the inside wheel to hop. The open diff did not demonstrated that behaviour, and because the Torsen was the only thing that had changed, I put it down to that.
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09-29-2017, 08:08 PM
#8
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Location: Edmonton Ab, Canada
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One day 
Stupid Spec Miata rules, otherwise they'd be out so fast! Our local track (aka only track within 12hrs tow) rewards the clutch diff so much I'm toying with building a 1.6 SM for next season.
The hard braking+downshift+hard turn combo makes the torsen lose its luster. Oh and curbs..
The hard braking+downshift+hard turn combo makes the torsen lose its luster. Oh and curbs..
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