Doughnuts !

The_Badger · 1 October 2019 16:33

Ok I’m not technically gfited as you guys so these might sound like dumb questions please be patient with me - ok here goes

can I do doughnuts in my mk3 2.00 sport ?

What is a limited Slip Diff ???

thanks

anon35904086 · 1 October 2019 17:03

Lsd will help avoid tripping into a hedge…

Drumtochty · 1 October 2019 17:06

The limited slip diff adds air to the doughnut mix and gives them a light and fluffy texture.

Countryboy · 1 October 2019 19:04

My answer is a question. Why on earth would you want to? The only people I can remember who do doughnuts are rally drivers in their team car and yobbos in stolen cars.

richardn · 1 October 2019 19:10

and members at the National Rally on the Autotest course

Countryboy · 1 October 2019 19:14

Re limited slip differential, just type those words into a Google search and lots of info (with pictures) will come your way.

The_Badger · 1 October 2019 20:31

???

Thanks for the info !!

richardn · 1 October 2019 23:05

So the answer is Yes it will as the Sport has a LSD

ast · 2 October 2019 01:37

No it doesn’t…

Vaughan · 2 October 2019 07:02

Yes you can, turn off DSC(or whatever its called) and enjoy!

Paul_G · 2 October 2019 08:04

AutoTests

Targa’s

Stage Rallies

AutoSolo’s

Halfway to the Pub, and, Whoops I’ve left my wallet at home.

Halfway to the Pub, and, Whoops I’ve remembered my my wallet. (delete as applicable)

There’s no jam in this one !!! I’m taking it back.

Countryboy · 2 October 2019 08:50

New tyres anyone?

Chris_Phillips · 2 October 2019 09:15

The only doughnuts I’m interested in are covered with cinnamon !

Scottishfiver · 2 October 2019 10:17

The common Jockhanistani reaction.

Scottishfiver · 2 October 2019 10:18

Fair one.

The Cake with the Less Fattening Centre?

The_Badger · 2 October 2019 22:12

Thanks everyone I’ll give it a go when sainsburys is shut

Paul_G · 2 October 2019 22:22

Mmmmmmm, Sainsburys doughnuts … but you need them to be open.

ast · 3 October 2019 01:31

When referencing limited slip differentials and MX5s… Note, these aren’t really the same as LSDs used in motorsports.

When it comes to NA and NB, the factory fitted 4 different LSDs (3 for the NA, 2 for the NB).

1989-93 1.6s in Japan and US were optionally fitted with a viscous LSD. Lance Schall provided a description of this:

From 1990 to 1993, the Mazda Miata LSD was a viscous unit. This replaces the clutch pack friction material and preload springs with a bizarre fluid between closely spaced rotating plates. One series of plates is connected to each axle shaft. Speed difference between the axles causes the plates to rotate at a rate relative to each other. This shear rate (and shear rate alone) of a thin film of silicone enhanced fluid increases the fluid’s viscosity. Picture a beer mug sliding across the bar on a thin film of water. The increased shear rate (speed of the mug) increases the viscosity of the fluid, creating more friction. This allows the fluid to transmit torque (makes the beer mug harder to push). The beer mug and water really do not behave this way. Neither does gear oil. That is why a special silicone based oil is permanently sealed into the factory LSD unit. Regular gear oil lubricates the ring and pinion gears.

This is a speed sensitive (not torque) system. The difference in axle speed controls the fluid viscosity and degree of TBR. The friction available to the viscous clutch is limited by the operating characteristic of the fluid. At some point, the shear rate heats the fluid too high and it begins to breakdown. The fluid and size and number of the clutch plates limits the torque carrying capacity of the viscous unit.

It is excellent for helping a driver start moving from an ice covered road at a stop sign. This unit does not transmit enough torque to satisfy race drivers nor does it stand up to racing loads reliably. This comment is primarily directed at autocrossers (the poor bastards). The tight cornering in low gears is particularly hard on viscous LSD and is another reason the autocrossers ought to find something more interesting to do.

The test for a viscous LSD says a rear wheel must turn no faster than 3.75 rpm with 11 foot pounds of torque on an axle (page M-26 Shop Manual) with both wheels free to rotate and the driveshaft constrained. Although the shop manual does not give any more data points, the resistance to turn should increase with the speed of rotation…

…Viscous couplings are generally not well understood and the Miata unit is no exception. First, it is thought to wear out. I think this concept is very overstated here. While there is no clutch plate contact, the usual notion is that the heat cooks the silicone oil and destroys it. This may be true and if so would be evident in demanding competitive applications. However, I suspect the opposite is true. The Miata VLSD doesn’t get hot enough during an autocross to work effectively. Which brings us to the second point: the lock-up is lame anyway. Well, not if it’s hot enough.

Historically, the technical papers that indicate the beginnings of understanding how VLSD work date to the late '80’s and early '90’s, precisely when Mazda came up with the VLSD in the Miata. And they were proud of it, sending their service technicians to a special course on the VLSD. I remember one dealer mechanic confiding in me, “They were so excited about the VLSD, I thought it would be on every Mazda car…”

Now, the alternating plates in the VLSD are toothed wheels with a couple dozen radial tabs. The reason for this is to eliminate warpage of the plates with temperature. Turns out there was some serendipity to this design. It has long been known that VLSD transmit more torque than estimated by shear forces alone, sometimes 10 or 20 times more torque! But sometimes NO more!(?) Engineers had been muddling along with empirical test results until about the '90’s when several scholarly attempts were made to quantify and predict performance.

It turns out that the plate tabs twist and shift axially giving rise to all sorts of interesting phenomenon with fluid phase changes, bulk pressure, dissolved air, Coulomb friction, blah blah blah (see SAE 2004-01-0867 - Mohan). The short story is the VLSD differential enters an operating phase Mohan calls “Self-induced Torque Amplification” (STA). This is also described by engineers working with Nissan, Takemura and Niikura, although they call it “hump” (SAE 900558).

In any case, the VLSD has 3 phases. First is initial viscous dissipation where the oil heats and viscosity drops. The VLSD transmits less torque. This is the operation you can measure with a torque wrench as outlined in the Shop Manual and gives rise to the idea that the VLSD is lame. As the fluid heats, the transmitted torque levels and at last in the final phase, as the fluid temperature exceeds 200 C, the torque graph shoots up, with a very steep slope, to transmit 300-400 ft-lbs or more. The ability of the VLSD to transmit this torque will continue as long as the high shear rates are present. To achieve this temperature, the VLSD may have to be subjected to high shear rates for several minutes. Clearly, this is not accomplished at an autocross or even, perhaps, on the track. So, the poor reputation of the VLSD for typical Miata performance applications is supported by the scientific explanation. However, this continuous shearing may be experienced in off-road center differentials and, indeed, Mohan worked for New Venture Gear and viscous differential are still found on 4x4 vehicles here and there.

So, the Miata VLSD is probably not weak or broken, just poorly matched to its application. If it DOES break, it can be overheated oil, or broken tabs on the ‘cluch’ plates, or I suppose the fluid could leak out. In any case, the unit is not serviceable and one suspected of being defective is discarded.

I believe the VLSD may have been an expensive option in the UK, but if so, the numbers of cars would be puny. All of these diffs used a 4.300 final gearing.

From 1993, on the 1.8 in Japan and the US, the Torsen differential was made available, first in Torsen T1 guise, then from late 1995, Torsen T2. T1s were 4.100 and on the NA, T2s were 4.300.

Again, quoting Lance:

There are two types of Torsen (TORque SENsing) differentials. Both are used on the Miata. The torque bias ratio in either case is about 2.5 to 1. This means that the differential is capable of supplying 2.5 times more torque to one wheel than the other. It is important to realize three things: First, the Torsen divides torque without regard to speed difference between the axles. Secondly, it does have a limit of 2.5. If one wheel is in the air or on ice, 2.5 times zero equals zero. The wheel on dry ground gets no torque and you are stranded just like an open differential. However, zero is never exactly zero and the Torsen is very much less likely to leave you stuck than an open differential. But still, if you try to support a torque bias greater then 2.5, the torque applied to the outside wheel will drop. And lastly, the Torsen does have friction clutches with the axial movement of the worm gears (under drive and coast loads) providing the necessary actuating force.

The 1994 and 1995 Miata have a Type I Torsen, which is the traditional worm gear unit. This device can be manufactured with a torque bias between about 2.5 and 5.0. The unit in the Miata is designed to about 2.5 or a little more. I do not have the exact number as it is actually verified through testing on the completed assembly. The 1996 and 1997 Torsen is a Type II. This unit has a different design with the worm gears being replaced by three sets of drive gears that are mounted with their axes parallel to the ring gear axis. It is simpler and less expensive than the Type I to manufacture. The Type II torque bias ratio (TBR)can be manipulated the same way, by changing the amount of helical twist in the drive gears although they are not as worm-like as the Type I. This design can be varied from a 1.0 torque bias (like an open diff) to about 2.5. Again, Miata is using approximately 2.5 or a little less.

For racing, the highest permissible torque bias ratio is usually desirable. Since the Type I and Type II are configured for similar torque bias ratios, it doesn’t matter which one you use. The Type II is a fundamentally more robust design, but I have not heard of either unit breaking from track use. They seem to last forever. Off-road is another matter. The Torsen does not take kindly to rotational impact loads.

With the car on a lift, it has been suggested that the relatively puny force of you rotating one wheel, will not over come the internal friction of the Torsen. Both wheels rotate together. However, with the driveshaft constrained, the opposite wheel should rotate the other way just like an open differential once you exceed the TBR. Perhaps a bored member reader would like to experiment with some ropes wrapped around the wheels and those unused workout weights…

More techie stuff:

[quote] Open differentials have a torque bias ratio of 1 to 1. That is, they will not distribute more torque to one wheel than the other. It is common to confuse wheel speed with torque. Do not fall into this trap. When one wheel is on ice, it may be rotating very fast indeed, but it does not take a significant amount of torque to turn the wheel on ice. The opposite wheel, on dry land, does not turn because it also has negligible torque (and in this case no speed either).

Another way of looking at the same idea is that the open differential can deliver no more than 50 percent of the available torque to any wheel. When both wheels are on a hard surface and the car is pointed straight, this is not a problem. 50 + 50 = 100. As you exit a corner and the lightly loaded inside wheel begins to spin, the torque to the outside wheel drops also. If one wheel is performing at 25 percent, the other is at 25 percent. The rest of the available torque goes to wheel spin or the driver is forced to lift throttle to avoid overreving/spin out. It is the loss of tractive effort from the outside wheel that racers want to eliminate. The spinning of the inside wheel does not matter much by itself. That wheel is lightly loaded anyway and can not contribute much to propelling the car out of the corner.

For cars with a solid rear axle, the driveshaft torque also attempts to rotate the axle about the driveshaft axis, lightening the load on the right rear wheel. It is as if the car were turning right even though it is going straight. This is a source of annoyance to drag racers who still use one piece rear axles.

Performance numbers for open differentials: 1 to 1 torque bias ratio. Since the second ‘1’ in ‘1 to 1’ is implied, sometimes you just see the first number 1 or 1.00 by itself. Sometimes the 50 percent figure is presented (see above) as the maximum amount of driveshaft torque (disregarding the rear end ratio) that can be delivered to any one wheel. 1, 1.00, 50 percent or 0.50 can all mean an open differential.

To the driver, the open differential is the safest, most forgiving system. Floor it coming out of a corner and you get less acceleration than you wanted and the outside wheel is still in rolling contact with the road, holding the rear end of the car in line.

With the car on a lift with the driveshaft constrained (in gear), turning one rear wheel will cause the other wheel to turn the other way. You should be able to turn it rapidly with relative ease. With the driveshaft free to rotate (gearbox in neutral), the internal friction of the open differential is probably not high enough to turn the driveshaft anyway and the result is the same.

Included here only for comparative purposes, is the all out racing setup; the spool rear end. The left and right tires are locked together completely, they rotate together. A spool rear end can deliver 100 percent of the available torque to one wheel (The engineer should note the maximum design torque load for a given half shaft has just been doubled!). The torque bias ratio is infinite (100 / zero). Performance number is expressed as 1, 1.00, or 100 percent.

A spool equipped car will break both rear wheels loose at the same time, exhibiting power-on oversteer (tail out power slide) exiting corners. Spool cars obtain some semblance of balance with ridiculously stiff front roll bars. This makes sure that as the rear end breaks loose, the weight transfer in the front is 100 percent to the outside wheel, so the front of the car slides away at the same rate.

You would not have a spool on your street driven Miata. If a spool is installed and the car is on a lift, the wheels will not turn unless the driveshaft is free to rotate. The wheels always turn in the same direction and speed......

.....Since a spool will send 100 percent of the torque to one wheel (but has an infinite torque bias), the clutch style limited slip people needed a figure to describe how close their clutch pack approaches a spool. Take a torque bias of 5 to 1 for example: 5/(5+1) = 83 percent. An open differential at torque bias of 1 to 1: 1/(1+1) = 50 percent. Or a spool: infinity/(infinity + 1) = 100 percent. So the “stiffness” of clutch pack limited slip can be expressed as a percentage between 50 and 100.

To further complicate matters, the clutch pack limited slip (Kaaz) does not really have a specific torque bias. The clutch pack will support a given torque difference, behaving like a spool, until its threshold is reached. Then it slips, reducing the effective TBR, heats, and wears. To the driver, the car tends to push toward the outside of a turn with application of power. The left and right wheels are coupled together, it wants to go straight. You compensate by increasing the slip angle on the front wheels. When you lift, the front wheels take over, and the car pulls to the inside of the turn. As you approach the cornering limit, the push turns into power-on oversteer. All of this just to get out of the corner faster!

And the clutch pack wears out too. An additive is in the differential oil to allow the clutch pack function in an oil soaked world. By the way, friction modifier will also slightly increase the TBR of the Torsen.

With the car on a lift and driveshaft constrained, a wheel will show a considerable and consistent resistance to rotate (you are slipping the differential clutch). The other wheel turns in the opposite direction. [/quote]

For the Torsen, both tyres need to be in contact with the road for it to work. Many years ago, I was caught out one chilly March. Leaving a village and accelerating from 30mph, my nearside rear caught some residual ice on a manhole cover, with the result that the car neatly span into the grass verge on the opposite side, shattering my fancy GHO 1001 copy nose (being kevlar/carbon fibre probably saved the car, with only the wings slightly bent)

The late NBs and subsequent NCs and NDs have the fuji diff. Its "torque sensing" but a completely different design from the Torsen (for the 2004 Mazdaspeed Turbo, Mazda ditched the Fuji diff, and used an Hitachi-built Torsen). The Fuji diff is the one that spits bits of metal from the drain plug. Not sure what the TBR is on the NB-application, but in the ND, I hear it is a rather weedy 2.0.

Bettabuilda · 3 October 2019 07:16

A TBR of 2.0? That’s shocking!

By the way, what’s a TBR and how does the 2.0 come in to it?

Barrie