P1 has an open diff?

"YSI-what can brown do for you" (ysi-what-can-brown-do-for-you)
02/20/2014 at 13:45 • Filed to: None

Wat?

DISCUSSION (35)

Niquemarshall > YSI-what can brown do for you
02/20/2014 at 13:48

No way!

EL_ULY > YSI-what can brown do for you
02/20/2014 at 13:48

open and waiting for a good Werting

GTI MkVII > YSI-what can brown do for you
02/20/2014 at 13:49

Yep, 12C and P1 both have open diffs and use "brake-steer" instead. Rather puzzling to me as well. Can anyone shed some light on why they went that route?

GhostZ > YSI-what can brown do for you
02/20/2014 at 13:50

Doesn't the electric motor redistribute power left-right to compensate for the "lack" of a differential?

A limited slip differential matters more in wider, heavier cars than it does in narrow lightweight cars, FYI, because the different radius paths of travel are smaller, and the distribution of weight left-right during a corner is smaller as well, meaning that the inside wheel torque is less likely to take most of the power AND the outside wheel torque is less likely to cause understeer, so for narrow, lightweight cars, they offer less benefit for the same added weight and drivetrain loss.

YSI-what can brown do for you > GTI MkVII
02/20/2014 at 13:51

You would think they would just go with an LSD, it isn't like they are trying to cut down on costs.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > GTI MkVII
02/20/2014 at 13:51

Probably because the application of the brakes to control yaw motion can happen instantaneously and it much easier to build software for than a diff.

Casper > YSI-what can brown do for you
02/20/2014 at 13:52

They use frictional balancing with brakes/electric motors so they don't need an LSD system as they can be more precise without it.

IDROVEAPICKUPTRUCK > GTI MkVII
02/20/2014 at 13:53

Because the computers can adapt infinitely while a mechanical limited slip only has one setting. Essentially it gives the engineers more control over the dynamics of the car.

GTI MkVII > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
02/20/2014 at 13:53

Why not employ both an LSD and brake-steer then? Or better yet, an E-Diff a la Ferrari and even new GTI?

YSI-what can brown do for you > GhostZ
02/20/2014 at 13:53

Mclaren uses the brakes to simulate an LSD, rather than the electric engines. Or so I have read. You would think they would just go with an LSD, but I guess they probably know what the hell they are doing.

I know the Lotus Elise has an open diff, but that is much much lighter than the P1, it is also much smaller as well.

It's a "Porch-uh" > YSI-what can brown do for you
02/20/2014 at 13:54

Allows for greater control over a mechanical, or even electronically-controlled differential. Also, less weight, since you just need to add some minor electrical controls to existing braking systems.

Same reason for ditching the anti-roll bars.

Demon-Xanth knows how to operate a street. > GTI MkVII
02/20/2014 at 13:54

Just like tractors!

jariten1781 > GhostZ
02/20/2014 at 13:55

It's 76.6 in wide...I wouldn't call that a narrow car.

Arch Duke Maxyenko, Shit Talk Extraordinaire > YSI-what can brown do for you
02/20/2014 at 13:59

Because they can do it better with computers.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > GTI MkVII
02/20/2014 at 14:00

Fuck if I know.

Ask McLaren.

http://www.mclaren.com/group#/media|c…

MR2_FTW - Group J's resident Stig > YSI-what can brown do for you
02/20/2014 at 14:00

The IS-F uses a similar brake-based "LSD".

GhostZ > jariten1781
02/20/2014 at 14:02

Compared to an Agera R, La Ferrari, Pagani Zonda, etc. it's narrower by around 2-4 inches. Not really a huge margin, but I'm more trying to make a point that if you can manage weight distribution the LSD becomes less important.

GTI MkVII > ADabOfOppo; Gone Plaid (Instructables Can Be Confusable)
02/20/2014 at 14:03

Just posing the question, not necessarily aiming it at you.

ADabOfOppo; Gone Plaid (Instructables Can Be Confusable) > GTI MkVII
02/20/2014 at 14:05

I know.

I'd like to know as well, since the P1 is amazing. Someone needs to ask. Might as well be you. And if not, maybe someone at McLaren reads Oppo and will answer.

Textured Soy Protein > GTI MkVII
02/20/2014 at 14:07

Typically brake steer only works with open diffs, not LSD. With an open diff, the wheel with more traction gets less power. So brake steer works by braking the opposite wheel from the one the computer wants to get more power. LSD since it works basically opposite from an open diff will not achieve the same brake steer effect.

I think McLaren basically said they can accomplish with brake steer (which needs an open diff) results that are as good or better than LSD or e-diffs. According to McLaren.

brandondrums > GTI MkVII
02/20/2014 at 14:30

If you think about it, an LSD is basically a brake. It is absolutely no different to the car or the driver to do independent braking with the actual brakes than to have a disc in the diff assembly apply friction to reduce the speed differential between the two wheels or a helical gear system that uses the friction of worm-gears to prevent the torque from taking a backward path towards a slipping wheel.

V-diffs (virtual LSD systems that is) are superior in performance as you can very easily take infinite control over lock ratios and vector torque between the wheels rather than rely on a mechanical clutch or helical gear system that only has one or two settings.

With-a-G is back to not having anything written after his username > YSI-what can brown do for you
02/20/2014 at 14:35

Does the P1 use any regen braking to charge its electrics? If so, this makes a lot more sense. If you're using friction brakes, it's wasteful, but "whatever," they'll say, "we've got 800 more horsepower to spare."

Victorious Secret > YSI-what can brown do for you
02/20/2014 at 14:35

Porsches uses the same on the 911, brake based. The Focus ST as well.

What they COULD do with mechanical bits, they instead did with software, which is more adaptable and faster to react to changing conditions.

I'm sure they looked at the P1 and figured a e-LSD or limited slip unit was pointless if not detrimental to the cars performance. This is McLaren after all.

brandondrums > GhostZ
02/20/2014 at 14:36

Basically, when you get down to it, a LSD is basically a brake mechanism designed to slow the differential in speed between the two wheels the diff drives. A clutch-type LSD is quite literally a brake as it's a friction disk that locks up - pretty much how a brake works. A helical/torsen LSD uses the friction that worm-gears make when driven in reverse to prevent torque loss to a slipping or free-spinning wheel.

Using brakes to slow wheelspeeds means having complete control over the lock ratios and torque distribution while a conventional LSD of any type has less leverage and more moving parts to contend with to incorporate a fully variable mechanism within the housing.

Joe_Limon > brandondrums
02/20/2014 at 14:50

What surprises me is that they didn't utilize a clutch based differential. You get fewer drive train losses when you don't have to engage your brakes.

brandondrums > Joe_Limon
02/20/2014 at 14:59

Not really, a clutch type differential is basically engaging brakes within the diff itself. It slows one wheel in relationship to another...it's physically the same thing from the car's perspective. Doing it via brakes rather than internal clutch mechanism just means more control and less of a propensity to break crap. The clutch mechanism needed to lock up nearly 1000hp would be so huge that the P1 would have an enormous pumpkin housing for the read diff sticking out the back like Lambos do.

Kinja'd!!!

Joe_Limon > brandondrums
02/20/2014 at 15:06

I was under the impression that bmw's xdrive clutch based differential worked by progressively disconnecting wheels to bias torque.

TheNeonDriver - Now with More BMW! > YSI-what can brown do for you
02/20/2014 at 15:11

I think Mr. Chris "The Man we all want to be" Harris has something to say on this matter: Pistonheads: Split the diff

Biodegradable Wiring Harness > YSI-what can brown do for you
02/20/2014 at 15:38

BMW and Lexus have something like this. Of course everyone says it's inferior to a mechanical diff... until McLaren goes and does it.

YSI-what can brown do for you > Casper
02/20/2014 at 15:56

I was thinking along the same lines. Still feels kind of weird having a million dollar car with an open diff. My car comes with an LSD and it cost 24k.

thedevilinside > Victorious Secret
02/20/2014 at 16:45

I wish the Focus ST using the same idea made it drive like the P1, but while I've never driven the P1 I'm pretty sure it doesn't.

Casper > YSI-what can brown do for you
02/20/2014 at 16:53

LSD is pretty old technology, it makes sense they would move on to better tech.

puddler > YSI-what can brown do for you
02/21/2014 at 05:24

i've got a welder, i can fix that.

Weather and Darkness > With-a-G is back to not having anything written after his username
02/21/2014 at 15:42

the motor/generator is geared to the flywheel, so there's less kinetic energy fed back up stream, in the sense that most of the regen mode, as i understand it is happening with the v8 declutched from the transmission. but i could see it being a factor.

opensourceopen > YSI-what can brown do for you
04/17/2014 at 23:17

Found on http://www.mclarenlife.com/forums/mclaren…

A typical passive mechnical or active electrohyraulic cross axle differential is not actually as good as the brake steer system used by Mclaren on the P1 because it acts to equalise the speed differential across the axle, and not to control it (to a value which ensures BOTH driving tyres are maintained at their optimum longitudinal slip value).

Some people believe a conventional mech (or electro-mech) diff to be "optimum" because it allows you to do big, smokey, and easily controlled skids. Which to a large degree it does. However, whilst you are doing those aformentioned big skids, the driving tyres are effectively locked together, and the more torque you want to shuffle across to the outside (more heavily loaded) tyre, the more locked-together those tyres are (and hence the lower the speed differential across them). That means that during that time, neither tyre can be held close to it's optimum dynamic slip ratio, and as such, the perfect absolute longitudinal acceleration of the vehicle for that surface friction cannot actually be achieved.

However, by using a conventional open differential , and a high band width speed controller using the rear brakes to torque balance, the brake steer system can both maximise drive torque across the axle AND optimise the dynamic slip ratio at EACH individual rear tyre.

This is why Mclaren use the system and not a conventional locking diff. When you factor in the lack of additional mass, and the complete tune-ability of the active brake slip control, it is a no brainer to use this solution on an ultra high performance car.