Differential lock!!

[pompelmo]

Why it's good to lock the differential in racing cars??
I was talking to a race driver and he told me if you have fully open (100%) diff the car will slide everywere...so it's good to lock it or little open it (25%)
SO...WHY???

[Tom]

Because if you have a LSD (limited slip diff) both wheels will always have some power delivered. If you have a normal diff and one wheel has far less traction power will only be delivered to that wheel, because its easier to turn, an LSD means you can get more power down but spinning wheels is easuier.

Also if you rip one wheel off you can still recover the car, although I don't think thats why the F1 boys do it.

An example of good use of a weilded diff here...http://www.russswift.co.uk/videos/Ramp.WMV This trick performed by one of my motoring heros, Russ Swift. http://www.russswift.co.uk/videos.html

[Venom]

It's impossible to drive F1 with 100% locked diff, how will you accelerate out of corners? It would just force the car to understeer.

By playing an F1 Challenge I learned that by unlocking diff does make the car VERY hard to control, so I keep it locked but not fully.

When f1 car is about to enter the corner, driver brakes hard while still on straight then he releases the brakes to turn in because by not braking hard (while turning) the wheels are FREE to turn differentualy. Remember, the wheels ( left and right ) dont turn in at the same speed - they turn DIFFERENTIALY. That's why drivers tap their pedals in corners.

Anyway, when the driver exits the corner he needs both wheels to trasnfer power onto the track, that's why they lock the diff. But not fully because when he accelerates out of corner wheels need to turn differentualy too.

http://auto.howstuffworks.com/differential.htm

[DaveKillens]

The whole concept of a differential is because in a turn, the inside and outside tires travlel different distances.
So let's look at the extremes at each end. If the axle was locked solid, each wheel would turn at the same speed, and would share the torque. If you went into a turn at a sedate pace, each wheel would fight each other, and there would be a lot of scrubbing. As well, the car would not want to turn, and try to keep going in a straight line. But if you do that same turn very aggressivley, you would have the same problems. But hard cornering makes for a lot of weight transfer, the outside tire would carry most of the weight, while the inside would have very little. In fact, it may even lift right off the ground. But with the differential locked, you guarantee each wheel is driven, and the torque flow is there.
Now let's go to the complete opposite, in the case of a differential like most normal, inexpensive cars. Each wheel's speed and torque is dependant on traction. In a sedate turn, there's really no problem, each wheel travels the distance it needs, independant of the other. But once you get outside of those parameters, problems can happen. In a tight turn, since the inside tire is unloaded, it can spin, or at least try to. I used to do that in an old front wheel drive econobox I had, hard corner, the inside front tire would light up, make a lot of squealing noise, and spin. Or on ice, if one wheel spins, the other just sits there, doing nothing.
So in anything but a sedate pace, there are vehicle dynamics we want to deal with. You want the car to rotate, and it's nice to have more drive (torque) on the outside wheel. And also, since so much weight is transferred to the outside, it's good to have the same amount of torque allocated to that wheel.
Ideally, you want each tire to carry it's proportionate load as dictated by the dynamics of the moment. Straight line, both equal and same speed. Hard turn, inside wheel travels less distance, and has less torque, than the outside which should carry more torque, and travel further.
But each scenario can be different, and different types of racing dictate different differentials. It would be foolish to have a rally car on the ice have the inside wheel spin uselessly. And on ovals, one neat trick is to lock the differential, and use a larger diameter tire on the outside wheel. (stagger) That car doesn't want to go straight, but it sure turns nice.

[manchild]

Speaking of blocked diffrenetial... First such thing was done 100 years ago on Renault. Sarthe circuit had only few corners so even though Renault had differential they decided not to use it (better traction and less weight).

They did keep driveshaft transmission but there was no differential in differential box so rear axle was driven by driveshaft directly. Their rivals still used chains (rear axle blocked by default with no other options).

Renault


FIAT


http://video.google.com/videoplay?docid ... 1171123312

[gcdugas]

Most race cars use a "Torsen" diff. It actually sends torque to the wheel with the most traction. Click Here

[RH1300S]

My turn to add some words.........

Some of it said already - and well enough.

The main problem with a open diff is that if a wheel loses traction (as Dave said) the power takes the route of least resistance and merrilly spins the gripless wheel and sends very little/none to the one with grip.

So, diff lock is mainly about getting the power to the road.

A limited slip diff can have power lock and coast (power off) lock (Ok, the types vary - I believe a spool diff is open on coast and goes straight to a locker on power) - but for this discussion you have power & coast lock. Let's also assume that the diff can have the power/coast lock adjusted independently (they have been doing this since the Salisbury type clutch pack/ramps diff) modern diffs can have electronic adjustment. The Torsen is a different and clever animal, but I'm not sure it's value is universal.........

Power lock - if you have too little the car will waste it's power and (probably) slide more at the rear. Too much power lock can create power-on understeer (if the rear tyres have grip the car will try and go where they are pointing!) - but you can also get snap oversteer as the power being sent through the diff can overcome the tyres. I think the amount of power side lock you want will vary with the conditions, grip available, power available and the phase of the moon.

Coast lock - when slowing down (working against the engine) the amount the wheels are tied together can affect the way the car behaves on turn-in. Probably no hard and fast rule (is there ever setting a car up?), but typically a more open setting will let the car turn easier and a tighter lock will give a "tighter" rear end. Letting a car run into a hairpin might demand a more open coast lock - but a high speed direction change (Variante Alta at Imola?) might be helped with more coast lock if the car is unstable.

The amount of locking also has an effect on tyre temperatures, so that needs watching too (a tight coast lock can get the rears hotter).

Watching Michael Schumacher in car is quite illuminating (I hope I heard this right ) - he adjusts the electronic diff and brake bias settings as he goes around the track - this will give him a different balance as he needs it.

[Saribro]

That's one damn illuminating post there, RH.

[keyser_soze]

http://img238.imageshack.us/img238/6891/ff12000fz3.jpg

This image shows the transmisson of the F1 Ferrari used in 2000.
However it is just a drawing, its quite accurate, I have also seen some photos.
Can anybody tell me why are they using the planetary gears in pairs?

[Ciro Pabón]

http://img238.imageshack.us/img238/6891/ff12000fz3.jpg

This image shows the transmisson of the F1 Ferrari used in 2000.
However it is just a drawing, its quite accurate, I have also seen some photos.
Can anybody tell me why are they using the planetary gears in pairs?

I am not sure, but I believe is to minimize planetary vibrations. You can read here, to see if you reach the same conclusion. I repeat I am guessing.

The movie posted by Manchild is excellent. Party time, ex-ce-llent! Zang!

About the explanation of RH1300, I agree is a good one. A simpler version could be:

A "classic" planetary differential delivers the same amount of torque to each wheel no matter what.

If you have one of your wheels on ice and the other one in asphalt, the one on the ice will spin as it receives the same torque as the one on asphalt. A way to "send" different torques to each wheel, when one of them spins, is using a clutch between the two planetary gears, limiting its free relative movement.

For example, you can use a liquid clutch differential, that allows planetary gears to slide slowly relative to each other, like when the car takes a curve, but resist sudden movements, like when only one wheel spins.

For pilots: I do not know why they call them "differentials" (actually, I guess it is because they allow different rotational speeds at each wheel ). However, you could call them "equalizers", because they work by equalizing the torque delivered to each tire. Your limited-slip-differential allows you to manage this "torque split".

[slick]

Having read through the posts on this thread, I get the feeling no-one really knows much about the diff in a motor vehicle. Looking up the Torsen type diff gives much better explanation of how the diff works. However a torque biasing diff like the Torsen is banned by the FIA. Well it would be wouldn't it; Before anyone complains, yes it is banned. As for the Ferrari diff, it's basically a compact open diff. The planet gears you mention are doing the job of the pinion gears in in a conventional open diff. It has nothing to do with unequispaced planets in epicyclics, and really isn't an epicyclic at all.

[Ciro Pabón]

Having read through the posts on this thread, I get the feeling no-one really knows much about the diff in a motor vehicle. Looking up the Torsen type diff gives much better explanation of how the diff works. However a torque biasing diff like the Torsen is banned by the FIA. Well it would be wouldn't it; Before anyone complains, yes it is banned. As for the Ferrari diff, it's basically a compact open diff. The planet gears you mention are doing the job of the pinion gears in in a conventional open diff. It has nothing to do with unequispaced planets in epicyclics, and really isn't an epicyclic at all.

My expertise on differentials is reduced to dismount a couple of times a Pontiac GTO's one, so do not expect brilliancy here. Please, correct anything you find that is wrong. Anyway:

I wasn't implying that the Ferrari differential has unequally spaced planets: the article I posted mentions the vibration modes for equally spaced ones and also, the analysis for paired planets. From what I understood (not much) I find that the later case have less severe vibrations. I wondered if this was the reason behind the Ferrari design.

However, you can explain a little more clearly (I did not get you) any other reason you figure out to the question posed by keyser-soze.

From what I've seen, in F1 there are no pinion gear based differentials, all I have seen are planet based. The totally-controlable-torque differentials, I believe, are banned for a good reason: apparently there is no need for higher g-forces and for cars that turn around the curve based on torque split. They want constructors to concentrate on suspension design as a result, and, besides, offer a dilemma to the aerodynamic designer, I guess. What do you think about this reason for banning them? Is it the correct one?

Another question: the Torsen differential I know has a fixed-ratio relationship between the torque delivered to each wheel. Is this still true?

If it is, there are variable-ratio differentials I find more interesting, like the ECTED, the Porsche 959's PSK (Porsche-Steuer Kupplung), the ATTESA E-TS PRO, used in the Nissan Skyline or the Volkswagen Haldex system (being considered by Audi to replace the aging Torsen-differential Quattro).

Finally the system I find more sophisticated (I'm no expert, I repeat, merely an "aficionado") is the SH system by Honda, that you can find on the Accura RL and the Legend. I find it is a beauty: it gives you front/rear split up to 30/70 and right/left rear split of 100/0 (!), all in a compact, electromagnetic clutch driven, active system that fits all of it on the rear axle housing.



This is the reason why I think F1 is being left behind by automakers: if you use this technologies, cars would do 400 km/hr on the chicanes and pilots would have brain lesions like the boxers... Just joking.