Can someone help explain Limited slip and/or Positrac?
03-13-2005, 05:04 PM
#1
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Can someone help explain Limited slip and/or Positrac?
I really would like to know the difference between Limited slip vs. Positrac. I have been told that they are the same basically except for the fact that Positrac was a name. I also have been told they are completely different in terms of function and performance. (ie. if you jack up the rear one wheel will spin opposite of the other with posi or won't spin at all with LSD...I still don't understand this if true!)
I just want to know the benefits of Limited slip and/or Positrac.
What causes a funny car to shift its rear all over the track and lay down tracks when accelerating ?
I know there are some real drag specialists in here so please don't be shy.
My limited slip is coming at the end of March...
I just want to know the benefits of Limited slip and/or Positrac.
What causes a funny car to shift its rear all over the track and lay down tracks when accelerating ?
I know there are some real drag specialists in here so please don't be shy.
My limited slip is coming at the end of March...
03-13-2005, 05:10 PM
#2
2004 TL w/ 6MT & Navi
http://auto.howstuffworks.com/differential.htm
And, to answer your question, Positrac is a brand name.
And, to answer your question, Positrac is a brand name.
Thanks...
03-13-2005, 08:30 PM
#5
Registered Member
A limited slip differential and a Posi-traction rear end are one and the same. Posi-traction was a term trademarked by Chevrolet in the 50's as a marketing tool for their LSD. Other companies have done the same thing.
If you raise the rear end of a RWD car without LSD and turn one wheel, the other will turn in the opposite direction. With LSD, they'll both turn in the same direction. In fact, you can actually turn the rear wheels with the transmission in gear if you have LSD.
Funny cars produce virtually the same amount of horsepower and torque as to Top Fuel cars.. roughly 7000 horsepower and more than 5000 lb/ft of torque. What you are describing in your question is wheel spin due to all of this power and torque that the driver is trying to regulate in order to gain/maintain traction for the quickest run. Basically, these cars launch and run the quarter under full throttle and the driver uses the clutch pack to regulate traction. Too much clutch and you go up in smoke. Too little and you lose the race.
Incidently, try this with your friends who know little or nothing about drag racing. Ask them to place a bet on this. Which would go off the end of an aircraft carrier first? An F14 Tomcat (remember they go to afterburners at launch and use a catapalt), or a car?
Of course the answer is the car, but they'll argue with you. It takes an F14 about 2.5 seconds to reach rotate speed (around 140 knots or about 175 MPH) and launch from the carrier deck. The car (Top Fueler) goes 0-100 MPH in 0.78 seconds!. Yes, that's just over 3/4's of a second to hit 100 MPH from a standing start. 200 MPH comes up in just under 2.5 seconds. Ain't no carrier-launched aircraft that quick.
If you raise the rear end of a RWD car without LSD and turn one wheel, the other will turn in the opposite direction. With LSD, they'll both turn in the same direction. In fact, you can actually turn the rear wheels with the transmission in gear if you have LSD.
Funny cars produce virtually the same amount of horsepower and torque as to Top Fuel cars.. roughly 7000 horsepower and more than 5000 lb/ft of torque. What you are describing in your question is wheel spin due to all of this power and torque that the driver is trying to regulate in order to gain/maintain traction for the quickest run. Basically, these cars launch and run the quarter under full throttle and the driver uses the clutch pack to regulate traction. Too much clutch and you go up in smoke. Too little and you lose the race.
Incidently, try this with your friends who know little or nothing about drag racing. Ask them to place a bet on this. Which would go off the end of an aircraft carrier first? An F14 Tomcat (remember they go to afterburners at launch and use a catapalt), or a car?
Of course the answer is the car, but they'll argue with you. It takes an F14 about 2.5 seconds to reach rotate speed (around 140 knots or about 175 MPH) and launch from the carrier deck. The car (Top Fueler) goes 0-100 MPH in 0.78 seconds!. Yes, that's just over 3/4's of a second to hit 100 MPH from a standing start. 200 MPH comes up in just under 2.5 seconds. Ain't no carrier-launched aircraft that quick.
03-13-2005, 08:40 PM
#6
2004 TL w/ 6MT & Navi
Originally Posted by SouthernBoy
Incidently, try this with your friends who know little or nothing about drag racing. Ask them to place a bet on this. Which would go off the end of an aircraft carrier first? An F14 Tomcat (remember they go to afterburners at launch and use a catapalt), or a car?
Of course the answer is the car, but they'll argue with you. It takes an F14 about 2.5 seconds to reach rotate speed (around 140 knots or about 175 MPH) and launch from the carrier deck. The car (Top Fueler) goes 0-100 MPH in 0.78 seconds!. Yes, that's just over 3/4's of a second to hit 100 MPH from a standing start. 200 MPH comes up in just under 2.5 seconds. Ain't no carrier-launched aircraft that quick.
Of course the answer is the car, but they'll argue with you. It takes an F14 about 2.5 seconds to reach rotate speed (around 140 knots or about 175 MPH) and launch from the carrier deck. The car (Top Fueler) goes 0-100 MPH in 0.78 seconds!. Yes, that's just over 3/4's of a second to hit 100 MPH from a standing start. 200 MPH comes up in just under 2.5 seconds. Ain't no carrier-launched aircraft that quick.
03-13-2005, 10:05 PM
#7
WDP Director of R & D
Originally Posted by SouthernBoy
If you raise the rear end of a RWD car without LSD and turn one wheel, the other will turn in the opposite direction. With LSD, they'll both turn in the same direction. In fact, you can actually turn the rear wheels with the transmission in gear if you have LSD..
With the car out of gear, both wheels could then be spun and both would rotate in the same direction as if the axle was "solid" between the tires.
I did not research the links above but basically an LSD has a small "clutch pack" within the diff that "limits" torque split/differences between the axles while a non LSD / normal diff has only the spider gears allowing the wheel with the least amount of traction to spin
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03-14-2005, 07:14 PM
#8
Registered Member
To KJSmitty;
No, I really did mean that if you leave the car in gear and the emergency brake off, you could actually turn the rear wheels (albeit slowly).. and yes, they will both turn in the same direction.
At least I did this with my 1966 SS396 Chevelle. Perhaps there was enough wear in the clutch packs that this was able to be done. But I did do it. Freaked me out when it happened, though.
No, I really did mean that if you leave the car in gear and the emergency brake off, you could actually turn the rear wheels (albeit slowly).. and yes, they will both turn in the same direction.
At least I did this with my 1966 SS396 Chevelle. Perhaps there was enough wear in the clutch packs that this was able to be done. But I did do it. Freaked me out when it happened, though.
03-14-2005, 07:17 PM
#9
Registered Member
To jmn4667;
You wrote, "Well, the catapult is using steam. Duh!" What's your point?
Yes, I know this.. to the tune of over 1,000,000 pounds of thrust. What I was trying to convey is the idea that a car can accelerate faster than a jet fighter launched from a carrier. You have to admit that it is pretty incredible that these babies (top fuelers) can go from 0-100 MPH in less than 8/10ths of a second.
You wrote, "Well, the catapult is using steam. Duh!" What's your point?
Yes, I know this.. to the tune of over 1,000,000 pounds of thrust. What I was trying to convey is the idea that a car can accelerate faster than a jet fighter launched from a carrier. You have to admit that it is pretty incredible that these babies (top fuelers) can go from 0-100 MPH in less than 8/10ths of a second.
03-14-2005, 07:51 PM
#10
2004 TL w/ 6MT & Navi
Originally Posted by SouthernBoy
To jmn4667;
You wrote, "Well, the catapult is using steam. Duh!" What's your point?
Yes, I know this.. to the tune of over 1,000,000 pounds of thrust. What I was trying to convey is the idea that a car can accelerate faster than a jet fighter launched from a carrier. You have to admit that it is pretty incredible that these babies (top fuelers) can go from 0-100 MPH in less than 8/10ths of a second.
You wrote, "Well, the catapult is using steam. Duh!" What's your point?
Yes, I know this.. to the tune of over 1,000,000 pounds of thrust. What I was trying to convey is the idea that a car can accelerate faster than a jet fighter launched from a carrier. You have to admit that it is pretty incredible that these babies (top fuelers) can go from 0-100 MPH in less than 8/10ths of a second.
03-14-2005, 09:10 PM
#11
WDP Director of R & D
Originally Posted by SouthernBoy
To KJSmitty;
No, I really did mean that if you leave the car in gear and the emergency brake off, you could actually turn the rear wheels (albeit slowly).. and yes, they will both turn in the same direction.
At least I did this with my 1966 SS396 Chevelle. Perhaps there was enough wear in the clutch packs that this was able to be done. But I did do it. Freaked me out when it happened, though.
No, I really did mean that if you leave the car in gear and the emergency brake off, you could actually turn the rear wheels (albeit slowly).. and yes, they will both turn in the same direction.
At least I did this with my 1966 SS396 Chevelle. Perhaps there was enough wear in the clutch packs that this was able to be done. But I did do it. Freaked me out when it happened, though.
--Now, if you have LSD, the car is in gear and you're able to spin a rear tire by hand,,,, your LSD clutches are bad and the other tire could "only" be turning in the opposite direction. ( or your ring and pinion are stripped
)66 SS Chevelle - great year/car!
03-14-2005, 09:32 PM
#12
Registered Member
Yeah, like I said.. I freaked me out, too. But it really did happen the way I described.
Chevy made top-notch LSD's but I believe the nod goes to my '88 Mustang LX 302CID. For calls theirs, Traction-Loc and in my Mustang, it was superb. Never once did I get clutch pack chatter, but then again, I wasn't racing the Mustang like I was the Chevelle which will tend to heat the packs up a might.
Chevy made top-notch LSD's but I believe the nod goes to my '88 Mustang LX 302CID. For calls theirs, Traction-Loc and in my Mustang, it was superb. Never once did I get clutch pack chatter, but then again, I wasn't racing the Mustang like I was the Chevelle which will tend to heat the packs up a might.
03-15-2005, 03:35 PM
#14
AZ Community Team
A colleague sent this to me years ago.
Here's something that will blow your mind. Drag racing is definitely different!!!
++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++
> * One Top Fuel dragster's 500-inch Hemi makes more horsepower than the
> combined first 8 rows at the Daytona 500
> * Under full throttle, a dragster engine consumes 1 1/2 gallons of nitro per
> second, the same rate of fuel consumption as a fully loaded 747 but with 4
> times the energy volume.
> * A stock Hemi will not produce enough power to drive the dragster's
> supercharger.
> * Even with nearly 3000 CFM of air being rammed in by the supercharger on
> overdrive, the fuel mixture is compressed into nearly-solid form before
> ignition. Cylinders run on the verge of hydraulic lock.
> * Dual magnetos apply 44 amps to each spark plug. This is the output of an
> arc welder in each cylinder.
> * At stoichiometric (exact) 1.7:1 air/fuel mixture (for nitro), the flame
> front of nitromethane measures 7050 degrees F.
> * Nitromethane burns yellow. The spectacular white flame seen above the
> stacks at night is raw burning hydrogen, dissociated from atmospheric water
> vapor by the searing exhaust gases.
> * Spark plug electrodes are totally consumed during a pass. After 1/2 way,
> the engine is dieseling from compression - plus the glow of exhaust valves
> at 1400 degrees F. The engine can only be shut down by cutting the fuel
> flow.
> * If spark momentarily fails early in the run, unburned nitro builds up in
> those cylinders and then explodes with a force that can blow cylinder heads
> off the block in pieces or split the block in half.
> * Dragsters twist the crank (torsionally) so far (20 degrees in the big end
> of the track) that sometimes cam lobes are ground offset from front to rear
> to re-phase the valve timing somewhere closer to synchronization with the
> pistons.
> * To exceed 300 mph in 4.5 seconds dragsters must accelerate at an average
> of over 4G's. But in reaching 200 mph well before 1/2 track, launch
> acceleration is closer to 8G's.
> * If all the equipment is paid off, the crew worked for free, and for once
> NOTHING BLOWS UP, each run costs $1000.00 per second.
> * Dragsters reach over 300 miles per hour before you have read this
> sentence.
> * Top Fuel Engines turn ONLY 540 revolutions from light to light!
> * The redline is actually quite high at 9500rpm
> * The current Top Fuel dragster elapsed time record is 4.477 seconds for the
> quarter mile.
>
> Putting all of this in perspective:
> You are driving an average Lingenfelter powered "twin-turbo" Corvette. Over
> a mile up the road, a Top Fuel dragster is staged and ready to launch down a
> quarter mile strip as you pass. You have the advantage of a flying start,
> but you still run the 'Vette hard up through the gears and blast across the
> starting line and past the dragster at an honest 200 mph. At this moment,
> the dragster launches and starts after you. You keep your foot hard down,
> but you hear an incredibly brutal whine that sears your eardrums and within
> seconds the dragster catches and passes you. He beats you to the finish
> line, a quarter mile away from where you passed him.
> That, gentlemen, is acceleration.
> Think about it, from a standing start, this phenomenal machine has spotted
> you 200mph and not only caught, but nearly blasted you off the road when he
> passed you within a mere 1320 feet.
>
Here's something that will blow your mind. Drag racing is definitely different!!!
++++++++++++++++++++++++++++++++++++++++++++++++++ +++++++++++
> * One Top Fuel dragster's 500-inch Hemi makes more horsepower than the
> combined first 8 rows at the Daytona 500
> * Under full throttle, a dragster engine consumes 1 1/2 gallons of nitro per
> second, the same rate of fuel consumption as a fully loaded 747 but with 4
> times the energy volume.
> * A stock Hemi will not produce enough power to drive the dragster's
> supercharger.
> * Even with nearly 3000 CFM of air being rammed in by the supercharger on
> overdrive, the fuel mixture is compressed into nearly-solid form before
> ignition. Cylinders run on the verge of hydraulic lock.
> * Dual magnetos apply 44 amps to each spark plug. This is the output of an
> arc welder in each cylinder.
> * At stoichiometric (exact) 1.7:1 air/fuel mixture (for nitro), the flame
> front of nitromethane measures 7050 degrees F.
> * Nitromethane burns yellow. The spectacular white flame seen above the
> stacks at night is raw burning hydrogen, dissociated from atmospheric water
> vapor by the searing exhaust gases.
> * Spark plug electrodes are totally consumed during a pass. After 1/2 way,
> the engine is dieseling from compression - plus the glow of exhaust valves
> at 1400 degrees F. The engine can only be shut down by cutting the fuel
> flow.
> * If spark momentarily fails early in the run, unburned nitro builds up in
> those cylinders and then explodes with a force that can blow cylinder heads
> off the block in pieces or split the block in half.
> * Dragsters twist the crank (torsionally) so far (20 degrees in the big end
> of the track) that sometimes cam lobes are ground offset from front to rear
> to re-phase the valve timing somewhere closer to synchronization with the
> pistons.
> * To exceed 300 mph in 4.5 seconds dragsters must accelerate at an average
> of over 4G's. But in reaching 200 mph well before 1/2 track, launch
> acceleration is closer to 8G's.
> * If all the equipment is paid off, the crew worked for free, and for once
> NOTHING BLOWS UP, each run costs $1000.00 per second.
> * Dragsters reach over 300 miles per hour before you have read this
> sentence.
> * Top Fuel Engines turn ONLY 540 revolutions from light to light!
> * The redline is actually quite high at 9500rpm
> * The current Top Fuel dragster elapsed time record is 4.477 seconds for the
> quarter mile.
>
> Putting all of this in perspective:
> You are driving an average Lingenfelter powered "twin-turbo" Corvette. Over
> a mile up the road, a Top Fuel dragster is staged and ready to launch down a
> quarter mile strip as you pass. You have the advantage of a flying start,
> but you still run the 'Vette hard up through the gears and blast across the
> starting line and past the dragster at an honest 200 mph. At this moment,
> the dragster launches and starts after you. You keep your foot hard down,
> but you hear an incredibly brutal whine that sears your eardrums and within
> seconds the dragster catches and passes you. He beats you to the finish
> line, a quarter mile away from where you passed him.
> That, gentlemen, is acceleration.
> Think about it, from a standing start, this phenomenal machine has spotted
> you 200mph and not only caught, but nearly blasted you off the road when he
> passed you within a mere 1320 feet.
>
03-15-2005, 03:37 PM
#15
AZ Community Team
In the near future it will be electric linear inductive motors, some former engineering colleagues are working on the system.
Originally Posted by jmn4667
Well, the catapult is using steam. Duh!
03-15-2005, 09:34 PM
#16
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Originally Posted by Legend2TL
Here's something that will blow your mind. Drag racing is definitely different!!!
>
>
03-15-2005, 11:23 PM
#17
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Originally Posted by Legend2TL
> * To exceed 300 mph in 4.5 seconds dragsters must accelerate at an average
> of over 4G's. But in reaching 200 mph well before 1/2 track, launch
> acceleration is closer to 8G's.
>
>
> of over 4G's. But in reaching 200 mph well before 1/2 track, launch
> acceleration is closer to 8G's.
>
>
Either way they sure can move...I just want to get in a car that can do the 1/4 in under 10 in my lifetime!
03-16-2005, 08:00 AM
#18
AZ Community Team
It's possible, it is done with downforce from aerodynamic aids (wings) and very high static and dynamic coefficient of friction of dragsters tires.
Originally Posted by dAveSPEC
Forgive me for I am not a physicist, but I thought the maximum theoretical G-force in straight line acceleration was 1G and the only way to go over that, albeit slightly, was the fact that the rubber combined with the DHT? compound actually grabs the pavement causing increased friction as opposed to just rolling over it. I know higher lateral g forces are possible when cornering at speed.
Either way they sure can move...I just want to get in a car that can do the 1/4 in under 10 in my lifetime!
Either way they sure can move...I just want to get in a car that can do the 1/4 in under 10 in my lifetime!
03-16-2005, 04:37 PM
#19
WDP Director of R & D
Originally Posted by dAveSPEC
Forgive me for I am not a physicist, but I thought the maximum theoretical G-force in straight line acceleration was 1G and the only way to go over that, albeit slightly, was the fact that the rubber combined with the DHT? compound actually grabs the pavement causing increased friction as opposed to just rolling over it. I know higher lateral g forces are possible when cornering at speed.
Either way they sure can move...I just want to get in a car that can do the 1/4 in under 10 in my lifetime!
Either way they sure can move...I just want to get in a car that can do the 1/4 in under 10 in my lifetime!
Your at 1G sitting still...... Anything less than 1G and something would be amiss with gravity
So yes, braking 1G is quite easy in a straight line, even the TL can do it
03-16-2005, 07:01 PM
#20
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Originally Posted by KJSmitty
Your at 1G sitting still...... Anything less than 1G and something would be amiss with gravity
So yes, braking 1G is quite easy in a straight line, even the TL can do it
So yes, braking 1G is quite easy in a straight line, even the TL can do it
That makes perfect sense, I guess the skidpad G-Force threw me off.
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