Turbos on the McLaren MP4-12C
03-30-2011, 04:36 PM
#21
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Twice, when I referred to a limited slip design, you corrected me by telling me how an open diff works. A limited slip differential IS NOT an open differential. So what you think you know about how an open differential works does not apply. It is more than simply something to resist differential wheel speeds.
Look, this is not a simple thing to grasp. But then, the way a differential works is not simple. And they don't all work the same way.
Look this stuff up. Stop talking to people who think it's all wheel speed differences and torque never changes. Read what the companies who make these things have to say about their products. Eaton has some decent pages on their products, you might start there.
Find some pictures of the insides. Try and trace the flow of torque. It'll give you a headache, but you'll realize that what you thought you understood is far more complex.
You don't understand how these devices work. You think you do but you are wrong. If you thought about it a little more you'd figure that out.
Look, this is not a simple thing to grasp. But then, the way a differential works is not simple. And they don't all work the same way.
Look this stuff up. Stop talking to people who think it's all wheel speed differences and torque never changes. Read what the companies who make these things have to say about their products. Eaton has some decent pages on their products, you might start there.
Find some pictures of the insides. Try and trace the flow of torque. It'll give you a headache, but you'll realize that what you thought you understood is far more complex.
You don't understand how these devices work. You think you do but you are wrong. If you thought about it a little more you'd figure that out.
03-30-2011, 05:11 PM
#22
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Since you edited the other post I will reply again. Forgive me if I reuse what I wrote since I might be replying to the same material twice.
Theoretically spring clutch and torque sensing diffs (Torsen, Quafee, some clutch diffs) don't care about the relative speed of the output shafts. The spring clutch systems are in theory fixed bias (the torque required to spin the output relative to the diff housing. The Torsens and the like in theory don't care though in reality there often is some affect of speed on the bias. So you are for the most part right about those diff. However, others like the viscous diffs Mazda used in early Miatas are velocity dependent. The larger the difference between the wheels, the more bias. The is also true of the few LSD's based on hydraulic pumping action. One type uses clutch plates but the pressure on the clutch plate is generated by a hydraulic pump driven by the difference in motion of the two output shafts.
So some diffs do change their bias according to the difference in output speeds, most do not.
You are correct that the Torsen has an issue on ice. You are not correct that it's contrary to my theory. My description of what happens is based on having a bias in the diff. That is a torque that resists motion between the diff housing and the output shaft. I'm sure it's clear that with a simple spring clutch diff that bias exists. In the Torsen the torque isn't fixed. It is a function of the torque applied to the diff which is exactly what you are saying (ie we agree). Since, in the ice condition, there is no load on the diff, there is no bias torque thus 0+0 still equals zero. When we have some level of torque on the diff (say 50 ft lbs) now the ratio isn't zero and my explanation still holds.
The clutches inside the diff most certainly TRY to make the wheel and the diff housing spin at the same speed. That should be very obvious. Half the clutch plates are attached to the output shaft. The other half are attached to the diff housing. That means all the forces are between the housing and the output shaft. When both tires are on the ground and not slipping the inside shaft is spinning slower than the diff. The outside is spinning faster. That means the clutch is trying to speed up the inside tire. So the inside tire is getting torque from the diff gears AND torque from the clutch plates.
When one wheel is in the air the diff isn't locked. The Torque to the wheel on the ground is equal to the bias torque of the diff. The wheels can still spin at different speeds (believe me I know) but some torque will be delivered to the wheel on the ground.
An open diff will NOT increase torque to the inside tire. The two tires get a 50-50 split. A LSD will ensure the inside tire gets MORE than 50% of the torque until the inside tire slips. Then it will send more to the outside tire. When you have too much power or have drastically unloaded the inside wheel that's a good thing (assuming you don't spin). However, when you are at the threshold of breaking grip with the inside tire, the LSD marginally lowers your limits because it send MORE torque to the inside wheel (via the mechanism I've explained) thus the inside tire in the LSD car is getting MORE torque than the open diff case. That makes the LSD car MORE likely to have a slipping inside tire. However, since that loss of traction doesn't result in a loss of power to the outside wheel this isn't necessarily slower though it does cause understeer.
Partially true as I've explained above.
The Infinity Pro Series cars typically run ramp type clutch pack diffs. On some tracks they run so they are effectively open diffs under power and have some locking under engine braking. Basically they don't want a LSD when accelerating out of a corner because it upsets the balance of the car. They want the LSD in under braking because it keeps the car more stable.
Two points:
Again, the limited slip mechanism inside a differential *doesn't care* how fast the elements are moving in relation to each other. It's the TORQUE they are delivering that dictates the state of the system. I know it's not easy to wrap your head around, but the speed of the wheels has zero to do with how the torque is handled inside a limited slip differential.
Again, the limited slip mechanism inside a differential *doesn't care* how fast the elements are moving in relation to each other. It's the TORQUE they are delivering that dictates the state of the system. I know it's not easy to wrap your head around, but the speed of the wheels has zero to do with how the torque is handled inside a limited slip differential.
So some diffs do change their bias according to the difference in output speeds, most do not.
Consider that a Torsen differential will not be able to move the car if one wheel is on rollers. A Torsen multiplies the torque from the wheel with least traction to the other at a given ratio, such as 6:1. If one wheel can deliver zero torque (it is off the ground, for example) then the other wheel gets zero torque (0 * 6 = 0), and the vehicle goes nowhere. The suspended wheel spins like mad, but nothing else happens. This is contrary to your "LSD fights wheel speed differences" theory.
A clutch type has some preload so that doesn't happen. It will be able to deliver some minimal amount of torque to the other wheel, enough to pull away. But NOT because the clutches somehow resist the wheels rotating at different speeds. But because the side in the air is essentially locked and the side on the ground receives enough to move the vehicle. The clutches don't enter the conversation until there is a difference in traction, and that is a result of an applied TORQUE difference, not wheel speeds.
When one wheel is in the air the diff isn't locked. The Torque to the wheel on the ground is equal to the bias torque of the diff. The wheels can still spin at different speeds (believe me I know) but some torque will be delivered to the wheel on the ground.
Secondly, sure, a differential will increase torque to the wheel on the inside of the turn. However, at the point at which the inside wheel can no longer maintain traction, more torque is delivered to the outside wheel on a limited slip unit. Helping rotate the car.
This means that if you are cornering with enough power to need some sort of limited slip mechanism, there is no longer more torque being delivered to the inside wheel. The only time it is getting more torque, which can make the car want to push, is if you aren't driving the car hard enough to warrant an LSD. Or your differential is set up wrong.
The Infinity Pro Series cars typically run ramp type clutch pack diffs. On some tracks they run so they are effectively open diffs under power and have some locking under engine braking. Basically they don't want a LSD when accelerating out of a corner because it upsets the balance of the car. They want the LSD in under braking because it keeps the car more stable.
03-30-2011, 05:24 PM
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Twice, when I referred to a limited slip design, you corrected me by telling me how an open diff works. A limited slip differential IS NOT an open differential. So what you think you know about how an open differential works does not apply. It is more than simply something to resist differential wheel speeds.
Look, this is not a simple thing to grasp. But then, the way a differential works is not simple. And they don't all work the same way.
Look this stuff up. Stop talking to people who think it's all wheel speed differences and torque never changes. Read what the companies who make these things have to say about their products. Eaton has some decent pages on their products, you might start there.
Find some pictures of the insides. Try and trace the flow of torque. It'll give you a headache, but you'll realize that what you thought you understood is far more complex.
You don't understand how these devices work. You think you do but you are wrong. If you thought about it a little more you'd figure that out.
Look, this is not a simple thing to grasp. But then, the way a differential works is not simple. And they don't all work the same way.
Look this stuff up. Stop talking to people who think it's all wheel speed differences and torque never changes. Read what the companies who make these things have to say about their products. Eaton has some decent pages on their products, you might start there.
Find some pictures of the insides. Try and trace the flow of torque. It'll give you a headache, but you'll realize that what you thought you understood is far more complex.
You don't understand how these devices work. You think you do but you are wrong. If you thought about it a little more you'd figure that out.
I NEVER NEVER NEVER said torque was split evenly in a LSD. I explicitly have said otherwise. I am using the example of a traditional spider gear diff with clutches (torque sensing ramps or spring loaded, doesn't mater). Inside of that diff you have TWO sources of torque to the output. One is the spider gears. They ALWAYS split torque 50-50. The other is the clutch system. It ALWAYS applies a torque that speeds up the slower shaft and slows the faster shaft. So the NET torque applied to the output is spider gear+ the clutch torque. That means the total torque to the faster wheel is spider-clutches. The total to the slower is spider+clutches. This is why the inside tire gets MORE torque when it's not slipping. What I have said about a ramp-clutch pack diff still applies to a Torsen or helical diff but it isn't as easy to visualize. The problem is the helical gears function as both the equivalent function as the spider gears AND they act like clutches as they generate friction when pushed against their pockets.
Once the inside tire starts to slip it actually is going FASTER than the outside tire. I think that's easy to see. So lets look at the numbers again. The inside tire torque is spider-clutch torque. That makes sense since the inside wheel is spun up. The outside wheel is spiders+clutch. That also makes sense since we know the outside wheel is getting more torque.
So my explanation holds true even when the inside tire is slipping.
Also I am NOT NOT NOT saying the bias torque is related to wheel speed difference.
The torque is the clutches are between the output shaft and the diff housing. That means the torque they create is between the output shaft and diff housing. We only see that torque when the output shaft and diff housing aren't spinning at the same speed. In most cases it doesn't mater how BIG the speed difference so long as there is a difference. The exception being speed sensing diffs like a viscous diff.
Keep in mind I'm not talking about how fast the car is going. I'm saying if the diff spins at 10RPM and the wheel spins at 8 RPM then the clutches are trying to speed the wheel up with some torque (fixed value or proportional to the input torque).
I understand that what I'm saying isn't immediately obvious. It sure wasn't to me. Then again I did ace kinematics years back and I do look at similar stuff for a living. If it will help I can make some drawings. BTW, did you notice the part where I mentioned I discussed all this with a company that DOES make racing diffs. The engineer was the guy who was doing the Torsen diffs that March was using in their 1980s Indy cars.
I am right about this and I do know what I'm talking about.... this isn't just me defending GM
03-30-2011, 07:35 PM
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Until you grasp this, you'll be making mistakes about all the rest of it.
It's not an debate, it's the way it is. Your understanding of how mechanical limited slip differentials work is simply wrong.
http://torsen.com/files/Traction_Control_Article.pdf
http://www.taylor-race.com/pdf/under...ferentials.pdf
03-30-2011, 07:49 PM
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When I first wrote my last post I hadn't seen your edits. It's clear I'm not getting my point across. You are right about drawing force flow diagrams. I thought it all out a long time ago. Here are some I just put together tonight to illustrate my description:
1. A basic clutch pack diff. This diff uses the separation force of the output gears from the spider gears to load up the clutch discs. That means this example is using a torque sensing diff. Note that because the clutches are between the housing and output any time the housing is spinning faster than the output the output will feel a torque in the forward direction. The opposite is true in reverse. There are two ways torque can be applied from the housing to the output; 1. via the spider gears, 2. via the clutch plates. The total torque to the output is ALWAYS the sum of those two torques.
2. As I did before I need to start with the open diff case. Without the clutches the torque sent to each wheel is ALWAYS equal. The wheel on ice can't handle any torque so no torque gets sent to the wheel with grip. I stressed this because EVEN when we add clutches the SPIDER GEARS always send equal torque to each wheel. The clutches make up the difference. I assume you agree that with an open diff the torque to each wheel is equal at all times.
3. [edit and correction] I don't like figure 3 so it's gone.
4.Now we turn to the right (imagine we are looking at the back of the car). The right wheel slows, the left wheel speeds up. We have relative motion in the clutches. Since this is a torque sensing, not speed sensing, diff it doesn't mater if the speed difference is 1/2 RPM or 100 RPM, the torque the clutches apply to the output shaft is still just a function of input torque.
So the right wheel is spinning slower than the diff. That means the torque on the right output is forward (or trying to make the right wheel spin faster). The torque from the spider gears is also forward. So we have two forward torques added together. Call that T_s+T_cl.
The left wheel is going faster so the torque the clutch applies to the output shaft is in the reverse direction. The equation looks like this: T_s-T_cl. So in one case we have addition and in the other subtractions. It's pretty clear that the addition case (ie the inside wheel) is going to get more torque. More torque at the wheel means more force on the ground.
Since the inside wheel is pushing harder than the outside wheel the rear axle is creating an understeer moment. If the front wheels were on casters the car would try to straighten out! Of course the front wheels over power this with their own steering moment but this is why a LSD (and of course a spool which is like an LSD with REALLY REALLY high bias!) tend to make a car understeer. Its just most of the time the front wheels can compensate.
5. Of course up until the inside tire slips. This is the case where everyone says, "See the LSD sends the power to the wheel with grip!" Well yes but that's because it sends more power to the slower wheel. When the inside tire slips it speeds up. Makes sense, id doesn't have the road to slow it down. Of course all the math above holds true. The inside tire is now the fast tire. That means the torque to the inside tire is T_s-T_cl. Since the outside tire is now the slow one it's torque equation is: T_s+T_cl. So the non-slipping tire gets more torque and everyone is happy.
Yes, I really do understand this stuff even at the force flow level. Come one, I figured out how the Corvette leaf spring worked, this isn't any harder
1. A basic clutch pack diff. This diff uses the separation force of the output gears from the spider gears to load up the clutch discs. That means this example is using a torque sensing diff. Note that because the clutches are between the housing and output any time the housing is spinning faster than the output the output will feel a torque in the forward direction. The opposite is true in reverse. There are two ways torque can be applied from the housing to the output; 1. via the spider gears, 2. via the clutch plates. The total torque to the output is ALWAYS the sum of those two torques.
2. As I did before I need to start with the open diff case. Without the clutches the torque sent to each wheel is ALWAYS equal. The wheel on ice can't handle any torque so no torque gets sent to the wheel with grip. I stressed this because EVEN when we add clutches the SPIDER GEARS always send equal torque to each wheel. The clutches make up the difference. I assume you agree that with an open diff the torque to each wheel is equal at all times.
3. [edit and correction] I don't like figure 3 so it's gone.
4.Now we turn to the right (imagine we are looking at the back of the car). The right wheel slows, the left wheel speeds up. We have relative motion in the clutches. Since this is a torque sensing, not speed sensing, diff it doesn't mater if the speed difference is 1/2 RPM or 100 RPM, the torque the clutches apply to the output shaft is still just a function of input torque.
So the right wheel is spinning slower than the diff. That means the torque on the right output is forward (or trying to make the right wheel spin faster). The torque from the spider gears is also forward. So we have two forward torques added together. Call that T_s+T_cl.
The left wheel is going faster so the torque the clutch applies to the output shaft is in the reverse direction. The equation looks like this: T_s-T_cl. So in one case we have addition and in the other subtractions. It's pretty clear that the addition case (ie the inside wheel) is going to get more torque. More torque at the wheel means more force on the ground.
Since the inside wheel is pushing harder than the outside wheel the rear axle is creating an understeer moment. If the front wheels were on casters the car would try to straighten out! Of course the front wheels over power this with their own steering moment but this is why a LSD (and of course a spool which is like an LSD with REALLY REALLY high bias!) tend to make a car understeer. Its just most of the time the front wheels can compensate.
5. Of course up until the inside tire slips. This is the case where everyone says, "See the LSD sends the power to the wheel with grip!" Well yes but that's because it sends more power to the slower wheel. When the inside tire slips it speeds up. Makes sense, id doesn't have the road to slow it down. Of course all the math above holds true. The inside tire is now the fast tire. That means the torque to the inside tire is T_s-T_cl. Since the outside tire is now the slow one it's torque equation is: T_s+T_cl. So the non-slipping tire gets more torque and everyone is happy.
Yes, I really do understand this stuff even at the force flow level. Come one, I figured out how the Corvette leaf spring worked, this isn't any harder
03-30-2011, 07:54 PM
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Originally Posted by rockville' timestamp='1301534681' post='20414110
...It ALWAYS applies a torque that speeds up the slower shaft and slows the faster shaft....
Until you grasp this, you'll be making mistakes about all the rest of it.
It's not an debate, it's the way it is. Your understanding of how mechanical limited slip differentials work is simply wrong.
In other words, the spiders are applying forward torque to the output shaft. The output shaft is spinning faster than the housing. Will the clutch plates between the output shaft and housing add more or less torque to the output shaft and why.
You are correct this isn't a debate but you are wrong about my understanding of the subject.
03-31-2011, 04:20 AM
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Edit: The Taylor racing paper has a mistake that contradicts what the Torsen paper says. Taylor says the Torsen delivers a 50-50 split until a wheel slips. That isn't true. If one wheel is on gravel and the other is on asphalt, the Torsen will deliver up to it's bias ratio (say 80-20) to the two wheels with the wheel on the asphalt getting more torque even though no slipping is occurring.
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