Sponsored

Thoughts on replacing Torsen for a clutch type LSD 15 GT PP

mustang1

Well-Known Member
Joined
Feb 12, 2016
Threads
6
Messages
1,494
Reaction score
270
Location
Dallas, TX
Vehicle(s)
2016 Mustang GT
seems this topic has come up before:

http://forum.miata.net/vb/archive/index.php/t-405258.html

"Had the factory torsen in my FMII turbo Miata. With snow tires, and the right type of snow, the car was capable of shooting 10' tall rooster tails of snow off both wheels. I never had ANY issue getting going in snowy or icy conditions
Sponsored

 

moffetts

Well-Known Member
Joined
Nov 30, 2015
Threads
14
Messages
978
Reaction score
325
Location
San Mateo CA
Vehicle(s)
Black Whippled 2015 GT PP
If the Torsen were influential in all of the Mustang spinouts, it would be a bunch of track packs and PP cars going up over the curb. It's not. The problem is that people don't know how to drive, or pick the right car for the environment. I have never driven my car in the snow, but a 400+ HP RWD daily driver in the snow belt seems like a poor choice to me.
 

BmacIL

Enginerd
Joined
Sep 21, 2014
Threads
69
Messages
15,010
Reaction score
8,920
Location
Naperville, IL
Vehicle(s)
2015 Guard GT Base, M/T
Vehicle Showcase
1
If the Torsen were influential in all of the Mustang spinouts, it would be a bunch of track packs and PP cars going up over the curb. It's not. The problem is that people don't know how to drive, or pick the right car for the environment. I have never driven my car in the snow, but a 400+ HP RWD daily driver in the snow belt seems like a poor choice to me.
It's completely driveable in the snow with snow tires, better than most FWD with all seasons. It's about knowing how to drive, that's it.
 

Sponsored

Brazenbuck

Well-Known Member
Joined
Dec 24, 2014
Threads
4
Messages
187
Reaction score
40
Location
United States
Vehicle(s)
Mustang
If the Torsen were influential in all of the Mustang spinouts, it would be a bunch of track packs and PP cars going up over the curb. It's not. The problem is that people don't know how to drive, or pick the right car for the environment. I have never driven my car in the snow, but a 400+ HP RWD daily driver in the snow belt seems like a poor choice to me.
Having 400 HP in the snow isn't that bad, common sense takes care of everything. Trying to use all 400 of your HP in the snow can be fatal.
 

EricSMG

Well-Known Member
Joined
Aug 22, 2016
Threads
3
Messages
361
Reaction score
106
Location
San Diego
Vehicle(s)
2017 GT w/PP, 2004 BMW M3 Coupe
I agree with the OP in that the Torsen, by its very design, isn't well suited to very low single wheel traction situations. This is my first Torsen diff and it's nowhere near as goodd as any of my previous LSDs in these situations.

Where the Torsen shines is at speed in dry conditions - here it's fantastic and may offer an advantage over a clutch type diff but that's only a loose assumption since again, clutch types work well here, too.

I would say that the Torsen works best in a somewhat narrow set of conditions while a clutch type has much broader capabilities and so from this, it's my opinion thus far that the clutch type is the better street car diff - it always works the same way in all condions, and this predictability is key.

However - the IRS in these cars exhibits very poor wheel control and so this is very likely highlighting the Torsen's natural weakness. Were the Torsen in a better IRS I'd bet it'd work vastly better and thus my opinion might be different. In other words, the Torsen requires constant force at the drive wheels to work properly and the IRS cannot provide this in stock form and so not a good pairing. I'm hopeful that LCA bearings, diff bushings and better tires will wake the Torsen up but I fear that it will always be a poor choice in extremely low traction situations.
 

EXP Jawa

Well-Known Member
Joined
Jun 9, 2014
Threads
3
Messages
1,011
Reaction score
205
Location
Rochester, NY
Website
www.torsen.com
First Name
Rick
Vehicle(s)
1999 Cobra Convertible, Electric Green
At the end of the day, both the base model Traction Lock and the Torsen differential essentially operate by biasing torque - they try to generate internal friction to support a low traction tire to allow torque to go (be biased to) the high traction tire. They just don't do it in exactly the same manner.

In general terms, the 2 functional differences are 1) the Traction Lock's friction level is constant, where the Torsen builds friction progressively with input torque (this where the improved track behavior comes from). In this specific case, the total amount of friction - and wheelspin resistance - that the Torsen offers is markedly greater than the Traction Lock. The later has a torque bias ratio of about 1.8:1, whereas the Torsen is about 2.8:1. This refers to the amount of torque the high traction tire can receive in proportion to what the low traction tire can handle.

Someone posted earlier a hypothetical case of 50 lb-ft and 3:1 ratio. Well, with 50lb-ft on the low traction side, the Traction Lock would send about 90lb-ft to the high traction tire, whereas the Torsen would send 140. These numbers are relatively small compared to how much an axle could potentially see, but in slippery conditions could mean the difference between moving and stuck.

And, 2) the Traction Lock has a preload spring - which is what loads the clutches (and leads to its constant friction), which gives it a small ability to transfer torque in one-wheel unload conditions BUT has to be overcome to allow the car to turn/maneuver when both wheels are on low traction surfaces. This makes the car harder to steer on an icy parking lot, for example. Which is the lesser of two evils, in daily practical use? That depends who you ask and if they're really considering what happens more often.

To say that one has a clear advantage over the other in winter is folly, both have pros and cons. I will offer a meaningless anecdote - I drove an older 5.0L Explorer with front and rear Torsen diffs for 125K miles through all kinds of winter situations that you'd never put a Mustang in - or that Explorer had a right to be in - and it was absolutely unflappable. I'd personally take a helical-gear diff over a clutch-type any day if given the choice. I may be, however, "biased"... :D

If an S550 has a tendency to lift one tire while making a diagonal uphill transition, I can see why that would be annoying. This type of product does favor having more suspension compliance so the tires maintain contact with the ground. Also, a car that's lowered or has increased roll stiffness would work against that. I would think TCS would catch that or you could carry a little more momentum to pop past that. But, as Torsen's sales engineer Paolo says "it doesn't walk on water". However, the overall behavior is quite sound and its winter performance is on the whole quite good and quite predictable. If some folks don't care for it, well, to each is own. Can't please everyone, I guess. In fact, the only person they'd had to please was the Ford vehicle dynamics engineer that was signing off on the project, who probably assumed it would see dry weather use only... :shrug:
 
Last edited:

EricSMG

Well-Known Member
Joined
Aug 22, 2016
Threads
3
Messages
361
Reaction score
106
Location
San Diego
Vehicle(s)
2017 GT w/PP, 2004 BMW M3 Coupe
If an S550 has a tendency to lift one tire while making a diagonal uphill transition, I can see why that would be annoying. This type of product does favor having more suspension compliance so the tires maintain contact with the ground. Also, a car that's lowered or has increased roll stiffness would work against that. I would think TCS would catch that or you could carry a little more momentum to pop past that. But, as Torsen's sales engineer Paolo says "it doesn't walk on water". However, the overall behavior is quite sound and its winter performance is on the whole quite good and quite predictable. If some folks don't care for it, well, to each is own. Can't please everyone, I guess. In fact, the only person they'd had to please was the Ford vehicle dynamics engineer that was signing off on the project, who probably assumed it would see dry weather use only... :shrug:
It's not lifting the inside tire.... but it (IRS) does unload it (the tire) via wheel hop in slow, sharp turns. The LCA bushing is so soft that it compresses during acceleration and then quickly releases, overcoming the tire's traction. This results in a near zero load at the inside drive tire and the result a momentarily ineffective Torsen. The cycle then repeats for a second or two (mild wheel hop) and little torque is applied to the outside wheel.

This is precisely the limitation of the Torsen. If this weren't he case, Wavetrac would be out of business since they only exist to solve this very problem - the zero or near zero load situation.

http://www.wavetrac.net/technical.htm

This is not to say that the Torsen can't be effective. It just can't be, by design, in very low single wheel traction events. Minimize these events and you've a better Torsen. Which means you're exactly right - the engineers designed a 'focused' model in the PP which by definition means "not well rounded".
 

Sponsored

Rebellion

Well-Known Member
Joined
Aug 30, 2016
Threads
14
Messages
1,052
Reaction score
273
Location
Houston
Vehicle(s)
2016 Competition Orange GT
It's not lifting the inside tire.... but it (IRS) does unload it (the tire) via wheel hop in slow, sharp turns. The LCA bushing is so soft that it compresses during acceleration and then quickly releases, overcoming the tire's traction. This results in a near zero load at the inside drive tire and the result a momentarily ineffective Torsen. The cycle then repeats for a second or two (mild wheel hop) and little torque is applied to the outside wheel.

This is precisely the limitation of the Torsen. If this weren't he case, Wavetrac would be out of business since they only exist to solve this very problem - the zero or near zero load situation.

http://www.wavetrac.net/technical.htm

This is not to say that the Torsen can't be effective. It just can't be, by design, in very low single wheel traction events. Minimize these events and you've a better Torsen. Which means you're exactly right - the engineers designed a 'focused' model in the PP which by definition means "not well rounded".
Focused on what exactly?
 

EXP Jawa

Well-Known Member
Joined
Jun 9, 2014
Threads
3
Messages
1,011
Reaction score
205
Location
Rochester, NY
Website
www.torsen.com
First Name
Rick
Vehicle(s)
1999 Cobra Convertible, Electric Green
Well, to be fair, the expressed purpose of a "Performance Pack" model is to emphasize track performance. By definition, that makes it a focused package for the task at hand. But, with that said, some of this is on Ford; they specified the performance criteria they wanted the Torsen to meet, they also were responsible for the chassis controls and suspension integration. That also includes sorting the wheelhop issue (and its cascade effects).

Part of where I think Ford might've fallen short was in scope of imagination - they envisioned a low-volume package that would be purchased by a handful of people autocrossing and doing track days, etc. Not something that would be adopted by the general consumer. In retrospect, that seems kind of silly, because most people buying the performance version of a sport coupe are probably going to opt for the most performance that is practical. The take rate of that option seems to have been a bit higher than was anticipated. That may be why we've seen indication of a PP2 option on the '18MY. It might be to push the overall performance level up, while also making more of the performance content more readily accessible.

In any case, it does remain true that the limitation of a typical helical-gear differential is the one-wheel unloaded situation, but in my 20-year experience using these types of products, I can't say its been a significant concern. I've not come across the wheelhop-induced unloading as described, so I wonder how relevant it is to the typical buyer. Obviously, it must happen if some folks are reporting it, but on the whole how widespread is it? There are certainly ways around the issue, but all have their tradeoffs. The Wavetrac design seems novel, I'd like to get my hands on one at some point to see how it performs...
 
 




Top