Yoshi Explains LSDs

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Yoshi Explains LSDs

Postby Yoshi » Sat Feb 12, 2011 3:04 am

***Disclaimer***
This is the property of me (Yoshi Jeffery). Please don't copy and paste this elsewhere. Quote sections are ok but please link back to this thread or e30performance.info to give me credit

To start, this is just a draft. Ask questions and I'll either answer them or research more then update this thread. I'm not 100% thrilled on the organization of this article and I'm not sure if I explained things insufficiently or not. I've read so much of this lately that my head is swimming in it. When I'm done. I will update this main post and we can all be happy together.


----Table of Contents------
Terms
General Understandings
Handling characteristics
Measuring Break away tq
Increasing lock up
Final Summary

Terms:

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clutch LSD aka Salisbury Diff - LSDs go by many different names. A clutch type LSD is simply a differential that limits slip from one side to another utilizing cllutches. Clutch types were known to be noisy but tech has come enough of a way that this is almost eliminated. I'm assuming you know enough about the description of a clutch type lsd if you are reading this.

Bevel springs/Cup spring/etc - Cup springs sit on the end of the clutch stack and act as an opposing force to keep the clutches in line. This is analgous to the pressure plate on an engine to transmission clutch system pushing the clutch back once the pedal has been lifted. Cup springs look like a big washer that has been overtightened with a bolt. It resembles a cone shape or a "cup".

Clutches - These are the items that resist movement of the axles inside of the LSD. The more friction the surface has, the more it will resist and "bite" when slipping".

Dog Ear plates - Dog ear plates are analagous to the flywheel in an engine to transmission clutch system. It is the surface the clutches act against.

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Spider Gears - This is roughtly the same system (but smaller) than an open differential. For the sake of this tutorial, I will assume you know how this works. The gears make one wheel rotate the opposite of the other in a 0 friction/force condition.

Break away torque aka preload - Break away torque is the amount of torque needed for the LSD to start slipping. This is the difference between torque applied or received of the two wheels joined by the differential. As an example, if one wheel is stationary and held still, at X force, the other wheel will begin to spin.

% Lock up - Many factories use this to describe an lsd. It is the general percentage of torque applied to the slower wheel in a starting slip condition. A more detailed explanation can be found here http://forums.bimmerforums.com/forum/ar ... 22488.html

Ramp Angles - Ramp angles are an internal cavity in the differential where the torque applied to the spider gears will separate the thrust rings. This will create a force that will eventually clamp down on the clutches and start to lock the wheels. Each diff has two sides the diff can lock. Accel and decel.

The more aggressive these angles (closer to 0 degrees), the more aggressively the diff will lock. A ramp angle of 90 degrees will cause no lock. IE 1 way differentials have a 90 degree ramp angle on the deceleration side. A 1.5 way diff will have 80 on the decel side but 60 or less on the accel side. From my reading, many road racers use 30 degrees on the decel side and 60 on the accel side.

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General understandings

The diff rotates evenly as the drive shaft spins and the car goes straight. Under light turning and straight driving, the internals of the LSD are not moving. The LSD unit as a whole is spinning however. This type of rotation will continue until the break away torque limit is reached. After this point, the two axles will begin to spin opposite of each other. EG an outside wheel spins 25mph and the inside tire spins at 23mph. The two axle halves will be spinning 2mph away from each other.

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The more force that is applied to the drive shaft or the harsher the turn, the more torque will be applied to the lsd. The more torque applied to the lsd, the closer it will become to 100% lock. This is due to the ramp angles in the differential and the torque applied to the spider gears in the center of the thrust rings. Due to this design, LSDs will spin both tires at the same instant.

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Handling Characteristics

@LSDs generally give less wheel spin on corner exit

@The more force applied to an LSD, the more the LSD will lock

@Low ramp angles (close to 90 degrees) on decel side will prevent wheel lock up under decel

@Allows for faster corner exit speeds

@When oversteer does occur, the rate at which the wheels spin will help keep even lateral acceleration.

Measuring Break away tq

To measure BAT (break away torque), lock one axle and the differential inptu shafts. Put a torque wrench on the free axle and slowly increase the torque reading from 0. When the axle begins to move, this is the break away torque of the differential.


Increasing lock up

@More clutches will increase lock up. Since both sides spin at the same time, an uneven number of clutches will not create uneven lock up.

@More aggressive ramp angles will slightly affect break away torque but will greatly affect lock up % and the rate at which the differential locks 100%

@In some cases, worn dog ear plates will rob a differential of lock up

@Thicker dog ear plates will increase preload aka break away torque.


Final Summary

In general, a loosely packed LSD will ineffectively propel the car forward. A too tightly packed differential will cause understeer and upset traction under abrupt input changes.

The graph below shows my interpretation of how an LSD locks. Until break away torque is reached, both wheels receive an even amount of torque. Past that limit, the LSD will lock minimally to the factory rated "lock up percent". Then, depending on ramp angles, the differential will progressively lock more and more until it reaches 100% lock. This should illustrate how simple changing lock up percent and how aggressive the differential reacts overall can be changed.

The graph shows a break away tq of 60 ft/lb and a lock up of 40%. This is an arbitrary example.

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Sources
lsd stack image: http://media.photobucket.com/image/lsd% ... G_1717.jpg
lsd break down image: http://www.autoatlanta.com/porsche-part ... 305-05.php

Biblio:
http://www.intothered.dk/simracing/differential.html
SAE article: Oversteer/Understeer Characteristics of a Locked Differential (942485)
Pacejka, H. B. and Bakker, E., "The Magic Formula tire Model"
http://forums.bimmerforums.com/forum/ar ... 22488.html

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Re: Yoshi Explains LSDs

Postby Yoshi » Sat Feb 12, 2011 3:33 pm

I posted this on r3v and BF.c. So far no responses. I think I confused people lol.
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Re: Yoshi Explains LSDs

Postby M20_fever » Sat Feb 12, 2011 5:51 pm

Come on now Yosh don't be giving our stickies to our competition!
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Re: Yoshi Explains LSDs

Postby Yoshi » Sat Feb 12, 2011 8:18 pm

I'll watermark the pictures so ppl point here. don't worry. Most forums hate when you cross post. So how else can I tease them to get here?
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Re: Yoshi Explains LSDs

Postby Yoshi » Sat Feb 12, 2011 9:22 pm

video added
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Re: Yoshi Explains LSDs

Postby wanganstyle » Wed May 04, 2011 7:08 pm

nice tech write up, mind if i link people to your site? I get alot of questions about LSD :)
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Re: Yoshi Explains LSDs

Postby Rick » Mon Nov 21, 2011 12:48 am

Excellent resource

After digesting this info, the main function of the lock-up feature is input torque, and the ramp angles. Put shallower ramp angles and it'll lock up tighter as a function of torque input.

I dont see the clutches really being a factor in the lock up, I mean unless they are worn, because whats really compressing them and causing them to lock is the input torque. So to add an extra clutch set doesent seem to be the main mechanisim at work behind modding them to higher lock %, right?

The advertised 40% or more lock units you see like on ebay, Im assuming the they have had the ramp angles re-worked...
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Re: Yoshi Explains LSDs

Postby Yoshi » Mon Nov 21, 2011 1:36 am

The break away torque isn't effected by the clutch material. But once it starts locking up, more clutches or different clutches will effect lock up once you reach break away torque.

if weak clutches will cause them to lock up worse, won't stronger clutches make it grab more? Thus raising lock up %.
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Re: Yoshi Explains LSDs

Postby Rick » Mon Nov 21, 2011 2:09 am

Yes, if your clutches are shit/worn then no matter the input torque, it wont be as good grab as with new friction plates.

I guess my main idea is that the ramp angle is the determining lockup, not the clutches themselves. So long as yours are in OK shape then to get more lockup, the ramp is the key.
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Re: Yoshi Explains LSDs

Postby Yoshi » Mon Nov 21, 2011 2:20 am

shimming, clutch material, AND ramp angles define lock up. ramp angles are just the most aggressive way of doing it. better clutches will grab and lock to 100% faster than stock.

What are you after? Did you read about the definitions? Lock up percentage isn't everything.
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Re: Yoshi Explains LSDs

Postby Rick » Mon Nov 21, 2011 2:38 am

Agreed.

Just grasping the tech behind it. You're the only guy who I have seen play with modding them. I never have seen clear shots and explanations before on their workings, so it just got me thinking.

For your drifting, what did you do to yours?
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Re: Yoshi Explains LSDs

Postby Yoshi » Mon Nov 21, 2011 3:08 am

Cutting ramp angles would require adding material so I just added 2 more clutches and shimmed it 0.4mm. It locks up great.
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Re: Yoshi Explains LSDs

Postby Rick » Mon Nov 21, 2011 7:35 pm

Ahh, I see. Thanks
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Re: Yoshi Explains LSDs

Postby Yoshi » Mon Nov 21, 2011 7:50 pm

I just wanted to reiterate

ramp angles are the best for modifying break away tq and rate of lock up

quality clutches kind of effect break away tq but mainly rate of lock up.

shimming effects break away tq but not really rate of lock up.

More clutches kind of effect break away tq but mainly lock up rate.
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Re: Yoshi Explains LSDs

Postby Nsquared97 » Sun Apr 22, 2012 10:10 pm

Figured I would just post this here instead of making a new thread. Anyway I am in the process of tearing apart my spare S3.73. I have the center stack out of the diff casing, but from there I can't figure out how to get it apart. Mainly, how do you get the roller bearing cages off the ends along with whatever they are attached to?

Edit: Just watched Yoshi's video again, think I figured it out. I am going to try and make a video showing the dis-assembly of the stack with everything in the proper order.
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