GT posi failure
- Thread starter ARBOC72
- Start date
Derry
GT Owner
The Gt brochure says it has a 'helical limited slip differential'. All my other books are at home but I will look later.
Welcome.
Derry
Welcome.
Derry
Indy GT
Yea, I got one...too
The SAE paper(s) Dave recommends are 2004-01-1260 "The Ford GT Transaxle - Tailor Made in 2 Years". And the compilation of all SAE papers written on the FG, PT-113, "The Ford GT: New Vehicle Engineering and Technical History of the GT-40".
Unfortunately the 2004-01-1260 report is mainly written to explain the technical challenges the team faced in designing/developing and delivering the transaxle design in a very short timeframe. It has a couple CAD model pictures of the geartrain (Figure 4) and it does list one of the design specifications for the transaxle as incorporation of a "Limited slip differential". However you will not get much technical information from the report.
I am a bit confused at Silverbullitt's thoughts on why a limited slip differential may or may not be necessary for a mid-engine transaxle. I do not believe the differential location (be it mid-car located or aft-car located) bears on the significance of whether the differential should posess limited slip capabilities or not. It is a design choice. And with input power at 550hp (for us unmoded engines) that pretty much REQUIRES torque be delivered through both rear wheels.
If we always drove our cars in a straight line we could have a "locked" differential whereby both output axles always rotated at the same rpm. Early racers use to accomplish this feat (poor man's posi) by welding the differential spyder gears together disallowing any speed differential between the two rear axles. (Frank, if I have lost you, call me and I will give you a remedial course in basic mechanics.)
Problem is when we go around corners, the outer curve radius wheel needs to go faster than the inner wheel (because it travels a longer path around the turn) so there needs to be a provision to allow each axle to seek different speeds as corners are made. Thus the term "limited-slip" differential is coined describe this design aspect of the differential. The internal mechanism(s) lock the two axles to the same output rpm under heavy torque application (as in accelerations) but allow differing axle speeds for corners under normal driving conditions.
Unfortunately the 2004-01-1260 report is mainly written to explain the technical challenges the team faced in designing/developing and delivering the transaxle design in a very short timeframe. It has a couple CAD model pictures of the geartrain (Figure 4) and it does list one of the design specifications for the transaxle as incorporation of a "Limited slip differential". However you will not get much technical information from the report.
I am a bit confused at Silverbullitt's thoughts on why a limited slip differential may or may not be necessary for a mid-engine transaxle. I do not believe the differential location (be it mid-car located or aft-car located) bears on the significance of whether the differential should posess limited slip capabilities or not. It is a design choice. And with input power at 550hp (for us unmoded engines) that pretty much REQUIRES torque be delivered through both rear wheels.
If we always drove our cars in a straight line we could have a "locked" differential whereby both output axles always rotated at the same rpm. Early racers use to accomplish this feat (poor man's posi) by welding the differential spyder gears together disallowing any speed differential between the two rear axles. (Frank, if I have lost you, call me and I will give you a remedial course in basic mechanics.)
Problem is when we go around corners, the outer curve radius wheel needs to go faster than the inner wheel (because it travels a longer path around the turn) so there needs to be a provision to allow each axle to seek different speeds as corners are made. Thus the term "limited-slip" differential is coined describe this design aspect of the differential. The internal mechanism(s) lock the two axles to the same output rpm under heavy torque application (as in accelerations) but allow differing axle speeds for corners under normal driving conditions.
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It's probably three things (and I'm not trying to be a smartass here): weather, tires, and too much right foot. When forum members say be careful below 55F with the Goodyears, they ain't kiddin'. You can have some real fun real fast, or you can F-up even faster with too much loud pedal in colder weather. Tires that ordinarily hook up incur ridiculous axle hop, or completely break loose, and sometimes not predictably so.
BlackICE
GT Owner
I believe the FGT uses a gear type limited slip.
From:
http://en.wikipedia.org/wiki/Limited_slip_differential#Geared_torque-sensitive_differential
Geared torque-sensitive differential
Audi Quattro Torsen DiffGeared, torque-sensitive mechanical limited slip differentials utilize worm gears to "sense" torque on one shaft. The most famous versions are:
Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982;
Quaife differential, sold under the name Automatic Torque Biasing Differential (ATB), covered by European Patent No. 130806A2.
Eaton Corporation differential, sold under the name Eaton Detroit Truetrac.
Geared LSDs are less prone to wear than the clutch type, but both output shafts have to be loaded to keep the proper torque distribution characteristics. Once an output shaft becomes free (e.g., one driven wheel lifts off the ground; or a summer tire comes over ice while another is on dry tarmac when the car goes uphill), no torque is transmitted to the second shaft and the torque-sensitive differential behaves like an open differential.
Geared LSDs are dependent on the torque and not on the speed difference between the output shafts. Such differentials may not be adequate on extremely slippery surfaces such as ice (or thin air, when a drive wheel loses ground contact altogether).[3]
Geared LSDs may be used:
to reduce torque steer in front-wheel drive vehicles;
as a center differential in four-wheel drive (e.g., on the Audi Quattro);
in rear-wheel drive vehicles, to maximize traction and make oversteer easier to manage (as in drifting). Although, for extreme drifting, a geared LSD is less effective compared to a clutch type LSD.
From:
http://en.wikipedia.org/wiki/Limited_slip_differential#Geared_torque-sensitive_differential
Geared torque-sensitive differential
Audi Quattro Torsen DiffGeared, torque-sensitive mechanical limited slip differentials utilize worm gears to "sense" torque on one shaft. The most famous versions are:
Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982;
Quaife differential, sold under the name Automatic Torque Biasing Differential (ATB), covered by European Patent No. 130806A2.
Eaton Corporation differential, sold under the name Eaton Detroit Truetrac.
Geared LSDs are less prone to wear than the clutch type, but both output shafts have to be loaded to keep the proper torque distribution characteristics. Once an output shaft becomes free (e.g., one driven wheel lifts off the ground; or a summer tire comes over ice while another is on dry tarmac when the car goes uphill), no torque is transmitted to the second shaft and the torque-sensitive differential behaves like an open differential.
Geared LSDs are dependent on the torque and not on the speed difference between the output shafts. Such differentials may not be adequate on extremely slippery surfaces such as ice (or thin air, when a drive wheel loses ground contact altogether).[3]
Geared LSDs may be used:
to reduce torque steer in front-wheel drive vehicles;
as a center differential in four-wheel drive (e.g., on the Audi Quattro);
in rear-wheel drive vehicles, to maximize traction and make oversteer easier to manage (as in drifting). Although, for extreme drifting, a geared LSD is less effective compared to a clutch type LSD.
TifosiTed
GT Owner
MAD IN NC
Proud Owner/ BOD blah bla
ARBOC72, sounds like your Goodyears went to steel in the 40f Va weather.....
Same thing happened to me in Raleigh last week... :cheers so....
the true test is wait till the roads and tires are over ~60f, then find an open field/ parking lot and see if the 180 happens again! If not and no other occurrence then drive like me and don't hit the gas when under 55f!
As others have said, 99.9% chance that everything is probably fine. But, we're all kind of anal here and we know this would nag any of us until we could confirm correct operation.
There's a couple of relatively easy ways to set yourself at ease...
One:
Well, you can hold that one wheel real tight and have someone else goose the go pedal. If you are still alive, you have a problem. If you end up in the hospital, then you can rest easy that your posi is working....
In reality, that's a bad test. The transaxle's job is to apply equal torque to the rear wheels. Idling along in 1st gear with NO rolling resistance isn't going to be enough to give you a good test.
If you really want to check it.....
Go to a really large parking lot late in the evening or early in the morning. From first gear, light it up a little bit. Turn around and count the black stripes. If the sum is odd, you may have a problem. If the sum is even, chalk up the spin to another cold, lit pair of Goodyears.
Seriously, be careful with this little test.... I'm nearly certain the car will pass but give yourself enough error-space so if you loop it you're laughing and not cringing.
Also, at 4K miles you are likely close to needing a new set of tires. Consider replacing with the Bridgestone (ENZO OEM) tires. The cost is very similar, and everyone who has done this has a high amount of praise for the Bridgestones. You can PM me for the size/model info if needed.
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Team Jeff
GT Owner
- Sep 8, 2007
- 559
With an open differential and equal traction right to left, the result will be two black marks. It is the imbalance of traction that necessitates some form of limited slip differential, be it torsen, clutch type, or otherwise.
I believe a more accurate parking lot test would be to make an arcing turn with the intention to unload or un-weight the inside tire and roll the throttle on. If the car slides sideways things are working properly. If the inside tire goes up in smoke you have an issue with the diff. Not only would this little test be definitive, it also has the benefit of releasing feel-good endorphins in the brain of the occupants. :biggrin
fjpikul
GT Owner
Good one Kendall. More business for me.
Indy GT, I'm way ahead of you. Please read: Per Schroeder. What's the Diff? Grassroot Motorsports. 2009;26(1):105-107. Nice explanation of diffs. Dr. Frank, the scientist
Indy GT, I'm way ahead of you. Please read: Per Schroeder. What's the Diff? Grassroot Motorsports. 2009;26(1):105-107. Nice explanation of diffs. Dr. Frank, the scientist
MAD IN NC
Proud Owner/ BOD blah bla
Fast Freddy
GPS'D 225 MPH
Fubar
Totally ****** Up
I say this from experience. I had a screw is a rear tire that brought the tire pressure down to 18lbs. The tire looked fine when I inspected it at a stand still however under mild/hard acceleration the car would pull heavily to the right. It's just a thought.
ARBOC72
Bill,
glad to see you found the Forum. I just picked up my "spare" rear wheel from the Tire Shop in Elkridge, and after I put it on the car and see how it looks, I may have the shop use the design on all 4. As for this current topic, many, including myself, learned the wrong way and took the car out when it was below 40 degrees once too often. Of course, I am not an extremely experienced mid-engine car driver, but I can promise you the same has happened to some really good drivers. The information you need, peace of mind that your car is 100%, is very easy by having Rich & Denis, The GT Guys go through it. They were integral in building these cars at the factory, and are only about 10 hours north of us. As for the best local Ford dealer with quality techincians, I'd reccomend Wynne Ford, down towards Norfolk. But as I say, dropping your car off to The GT Guys ( http://www.thegtguy.com/ ) this time of year would be the best approach, in my opinion.
Pete S.
Bill,
glad to see you found the Forum. I just picked up my "spare" rear wheel from the Tire Shop in Elkridge, and after I put it on the car and see how it looks, I may have the shop use the design on all 4. As for this current topic, many, including myself, learned the wrong way and took the car out when it was below 40 degrees once too often. Of course, I am not an extremely experienced mid-engine car driver, but I can promise you the same has happened to some really good drivers. The information you need, peace of mind that your car is 100%, is very easy by having Rich & Denis, The GT Guys go through it. They were integral in building these cars at the factory, and are only about 10 hours north of us. As for the best local Ford dealer with quality techincians, I'd reccomend Wynne Ford, down towards Norfolk. But as I say, dropping your car off to The GT Guys ( http://www.thegtguy.com/ ) this time of year would be the best approach, in my opinion.
Pete S.
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DanQ
GT Owner
I'm not an expert, but a google search of "test torsen diff", returned
http://sccaforums.com/forums/thread/206394.aspx
Best suggestion was, "Put one drive wheel on pavement, and one on grass. Hit it. Black marks mean a Torsen. An open will just dig up the grass."
It mentioned that a wheels in air test was difficult with a torsen...
Hope this helps, but try at your own risk. I doubt you'd have to hit it hard enough to leave black marks to see if the grass side spins. Might be just as easy and safer to try with one side on dry pavement while the other side is on wet...
http://sccaforums.com/forums/thread/206394.aspx
Best suggestion was, "Put one drive wheel on pavement, and one on grass. Hit it. Black marks mean a Torsen. An open will just dig up the grass."
It mentioned that a wheels in air test was difficult with a torsen...
Hope this helps, but try at your own risk. I doubt you'd have to hit it hard enough to leave black marks to see if the grass side spins. Might be just as easy and safer to try with one side on dry pavement while the other side is on wet...
Neilda
GT Owner
I don't take my GT out in low temperatures.
My posi is fine, as I'm sure the OP's is.
There are too many horses and no electronic nanny for farting about in cold weather.
My posi is fine, as I'm sure the OP's is.
There are too many horses and no electronic nanny for farting about in cold weather.
griffbl
GT Owner
Was Certainly True For Me!
That 6" between the ears things is just crucial.....cost me one beautiful car! (working on replacement...will advise
)
GRIFFBL
That 6" between the ears things is just crucial.....cost me one beautiful car! (working on replacement...will advise
)GRIFFBL
californiacuda
GT Owner
- Oct 21, 2005
- 919
The test with one wheel on grass the other one pavement would work for any type of limited slip differential, not just a torsen.