The Gasman

I saw this in person at the unveiling in Newport beach. The turbo's are tiny, but like everything mclaren does, I'm sure they are purpose driven. One cool thing people don't notice is how low the engine is mounted. I'm sure it's great for the center of gravity in an already balanced mid engine design.

mikey k

I'd agree with that McLaren - have never been great with styling
But everytime I read about this car I find out something else that makes me want one!

MDXLuvr

What did you think about the Styling Raj? Btw - I personally think they should have put in a LSD but somehow they are managing 1.7G's in the corner with Pirelli Corsa tires(according to Dealer's PDF). So I am pretty sure the car will be able to handle a corner.

AngryTurtle

Shoulda stayed NA v12

rockville

A brake steer system actually is likely to be much better than a clutch type LSD. The problem with a LSD is when it's working it is trying to make the left and right wheels spin at the same speed. Think about the clutches, they work against either each other or against the diff housing. That means the LSD is trying to slow the outside wheel and speed up the inside wheel. Yes, the wheels aren't going the same speed but because the diff is in effect trying to slow the outside wheel and speed up the inside wheel it is acting like the differential steering on a bulldozer. Basically it is causing an understeer moment about the rear axle. This is a problem with all passive LSDs and a design trade off.

With a brake based system (or active diff system) we can use other sensors in the car to determine just how fast that inside wheel SHOULD spin. We apply just enough brakes to slow it that much. Net result, rather than creating a understeer moment about the rear axle we have the rear wheels going at the exact right speeds for the job.

BTW, I think the LSD and is a very misunderstood performance part. It is very much a patch type device and has it's own set of problems as compared to alternatives.

The Gasman

The styling is bland and the interior is efficient and sparse. It's pretty tight in there too. This seriously is a track junkie car. I believe any owner that likes taking his car on the tight twisities is going to love this car. People said they felt the Aventador was tight and it is compared to the Murci because of the huge center hump, but it's cavernous compared to the mp4.

NuncoStr8

Your understanding of what a limited slip differential does, how, and when it does it is not very accurate.

Neither a clutch type or Torsen type LSD has braking forces. And neither have anything to do with wheel speed. Both act by directing torque to the wheel that is not slipping, regardless of the relative speed of that wheel. They operate on resistance to torque input, IOW the more traction a tire has, the more torque it gets when the throttle is applied. Clutches bind tighter or gears bind, but the end result is any difference in traction results in the tire with the most traction getting the most torque. The tires can turn at different rates without any issue as long as they can take the same amount of torque (going around a corner, for example). It's only when one tire loses the ability to deliver the same amount of torque that the LSD mechanism splits torque unevenly.

Again, it has nothing to do with trying to slow down the outside tire or speed up the inside tire. That simply is not how a limited slip differential operates.

A brake-based system is operating off a look-up table relative to a variety of inputs. However, it has no knowledge of which tire has traction, merely the wheel speed, throttle and steering input, and perhaps yaw sensors. And it has to wait until a wheel slips before it can react, unlike an LSD.

Further, using the brakes to slow the inside wheel mandates an active traction and stability control system to deal with the unintended ramifications of blindly applying the brake when navigating a turn. Installing a $300 LSD would eliminate the need for the system entirely.

Cars that use the brakes instead of an LSD are simply dealing with the symptom, they aren't preventing it from happening. I don't see why you wouldn't just address the issue and be done with it. There is absolutely zero advantage to dragging a brake through a turn instead of using a device that lets you put all the power to the ground.

rockville

No, I do know quite a bit about LS diffs and everything I said was correct and has been vetted by a few racing car designer/engineer I know not to mention a guy who designed racing diffs. What I said may not be intuitive but it is correct.


When I say braking forces what I mean is INSIDE of the diff the LS mechanism is trying to minimize the difference between the speeds of the two output shafts. IE it's trying to slow or brake the outside shaft WITH RESPECT TO the diff housing. It is trying to speed up the inside wheel WITH RESPECT TO the diff housing. Think about the flow of torque (forces) inside of a LSD. For instance, with a clutch plate diff you have a set of clutch plates that work against the diff housing. That means any time an output shaft is spinning faster or slower than the housing we have a torque created that tries to slow or speed that output to be the same as the housing. That force may be a fixed value for a spring loaded clutch or a variable value for a Salisbury/ramp type clutch diff. The same is true if you are dealing with a helical or Torsen diff. In neither of the previous cases is the difference in output shaft speed a factor in the speeding or retarding torque applied to the output shafts. However, in the case viscous or hydraulic LSD diffs (not common) the speed differential of the outputs does affect the torques on the output shafts.

When you say both tires get the same torque so long as the tires aren't slipping, you are wrong. That is only true of an open diff. With a passive LSD prior to loosing grip, the inside tire gets MORE torque than the outside tire. When you are cornering you want the inside wheel to spin slower than the outside wheel. With an open diff you get equal torque to both wheels. That means even when the outside wheel spins at 20 RPM faster than the inside wheel ( ie +/- 10 RPM of the housing) you get the same torque delivered to each wheel ie HALF the input torque to each wheel. With a clutch diff that's different. Now the torque to the outside wheel is half minus the torque used to overcome the resistance between the clutch housing and the output shaft. The torque to the inside wheel is the opposite. It gets the same torque from the diff gears PLUS the torque transfer from the housing via the clutch plates. IE the inside wheel is now getting MORE torque than the outside wheel. That translates into the inside wheel pushing harder on the pavement than the outside wheel. Net result the rear wheels are trying to straighten the car out, not help it turn. It a simple balance of forces. The front wheels are creating a turning moment about the rear axle. The diff is creating a turning moment in the opposite direction. On balance the rear axle loses but the car may understeer due to the LSD (up to the point the rear axle breaks free and you have over steer). This is why LSDs can make a car understeer.

What you are saying about the tire with the most traction getting the most torque is true but it looks at the problem extrinsically. It doesn't look at how those forces feed into the rest of the chassis. Consider accelerating out of a turn. The front wheels are working to yaw the chassis. They have to both provide grip to rotate the chassis and fight the understeer moment coming off the rear axle. The front axle can only do so much work. Without the LSD, assuming you didn't slip the inside tire, the front axle can do a bit more work to rotate the chassis. With the LSD some of the work done by the front axle is simply overcoming the understeer moment described above.

With a brake based system we don't have that problem. Now we do have to figure out just how hard to smoothly apply the single brake to the inside wheel. I would not claim that's easy at all. A LSD is relatively simple to tune by comparison and reasonably effective so I certainly see why it would be more common than a brake system. However, FUNDAMENTALLY, the LSD is inferior.

So I explained how the LSD results in more torque (not power, torque) going to the INSIDE wheel. With the brake system it's the other way. The open diff sends equal toque to each wheel (say 100 ft lb each). That means the outside wheel gets the full 100 ftlb. The inside wheel either gets the full 100 or something less than 100 as the brake is engaged. So now the force applied to the road is less for the inside tire. That creates a moment that rotates the car INTO the turn thus sharing some of the chassis rotation job with the front axle.

As I said, what most people know about LSD's is not correct. They can be an effective bandaid but they are just that. They aren't perfect but they are often a cost effective trade off.

NuncoStr8

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 relative speed of the wheels has zero to do with how the torque is handled inside a limited slip differential.

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.



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.

You are right only to a point. And that point is where an LSD becomes useful. And as you said, a well-setup LSD is cheap, simple, reliable, and effective.

I never said the braking-type system was ineffective, I stated a preference for a self-contained mechanical solution over an electro-mechanical one requiring a slew of sensors and a computer.

rockville

No that is not true. If you study the kinematics of an open differential you will find it, excluding friction, delivers identical torque to both wheels. This really isn't hard to show. Jack the car up and put the transmission in gear. Now the diff housing is fixed. We know that if you turn the left wheel 1 revolution the right wheel turns 1 revolution in the opposite direction. Now if we have no friction the torque deliver to the right wheel is used to spin the left wheel. Net result torque in equals torque out but in the opposite direction. That is how the diff works at the kinematic level. The average speed of the housing equals the average speed of the output shafts.

Again this is not true in the case of an open diff. The TORQUE is always equal. The POWER to each wheel isn't equal since power is Torque*Angular Velocity. Many people confuse this when talking about diffs. When neither wheel is slipping the outside wheel gets more Power because it spins faster (same Torque* Faster Speed=More Power). When the inside wheel slips and spins things reverse. Again, equal torque but now the inside tire spins faster thus gets more Power.
No, any time the diff is applying a biasing torque to the output AND the tires aren't slipping (or slipping beyond the design limits) the diff is ALWAYS providing more Torque to the inside tire for the mechanical reasons I explained in my previous post.
Certainly that could be an issue but so long as the tires aren't slipping what I said holds true. The setup of the diff can drastically affect when the inside tire starts to slip.


No everything I said is correct. I didn't state how great the affects are in real life situations. That comes down to diff and chassis setup.
And I agree here. But we do want them to be done right.

I can understand the wish for simplicity just as I prefer a manual over a semi-auto box even though the semi-auto is faster around the track).