Board logo

Ford Sierra Viscous LSD internals foto
hmh - 2/6/16 at 09:45 AM

I couldn't find any pictures of a Sierra Viscous LSD taken apart so far. Now that I started overhauling mine I though I should shoot and post them here for others who are interested. Here they go





SteveRST - 2/6/16 at 10:35 AM

Interesting, I've not seen inside one before.

What exactly do you overhaul? What wears out? Does this mean it should be less 'slippy' afterwards?

So how does it work? The thin metal plates act like a series of clutches?

A million questions

hmh - 2/6/16 at 11:02 AM

The car didn't spin well to the right although it did to the left. The plan is to refill the LSD with silicone fluid with higher viscousity than the original fluid.
In fact the original fluid looked more like dense grease or mud when we opened the case.
The principle here is like in other viscous LSDs where the metal plates don't touch each other and pass traction through the fluid.

If anyone did something like this, please share your results and may be some hints which I need to take into account before I refill and put it back into the car.

madteg - 2/6/16 at 01:19 PM

KEEP UP POSTED ON HOW YOU GET ON. THINKING OF DOING MINE AS WELL

minitici - 2/6/16 at 03:06 PM

Bara Motorsports can replace or upgrade the special silicone fluid in the Sierra LSD's viscous coupling.

snapper - 2/6/16 at 07:17 PM

There is a very specific bubble of air in the fluid, integral to how the slip works.
I have heard that some increase air pressure to change how the slip works (you can't compress a fluid so increased air pressure just provides some resistance, silicon is not affected by heat but air is, make of that what you will)
I would think the silicon fluid will make s huge difference and not easily controlled this pumping the air bubble could give you a much finer control, certainly the effect would stiffen the LSD somewhat

bi22le - 2/6/16 at 08:12 PM

Are you doing this your self withouy special tools?

So to hijack post but i am still wanting an LSD unit of and type or condition. I just want the inner LSD bit for a diff rebuild.

Any one got anything?

Rob Allison - 11/6/16 at 06:00 AM

Hope you know what the fliud is, as its very hard to find the right stuff.
And the "Air bubble" is important. When you dismantled it you need to measure the volume of fluid that comes out to put the same back in.
As for it looking like grease thats right. The closest thing thats easy to get hold of is model car lsd silicon oil/fluid.

hmh - 13/6/16 at 12:57 PM

We had an autotest event on Saturday and the overhauled viscous unit worked like a charm.
As a fluid we used Polydimethylsiloxane (https://en.wikipedia.org/wiki/Polydimethylsiloxane). As I read on the internet the usual viscousity that is used to refill different viscous units is 10.000. We used much higher viscousity 250.000:



[Edited on 13/6/16 by hmh]

Gersen - 12/7/16 at 11:52 PM

Awesome pics, and really interesting thread - keep the info coming!

PDMS is non-Newtonian fluid i.e. it's resistance to twisting applied in a diff depends on how fast you are trying to twist it - kinda handy in an LSD!
The significant bit is that it's the weight of the silane used that needs to be optimised for the locking properties.

There will be alot of people telling you "OMFG you can't understand and re-engineer the viscous pack fluid" - ignore them - you really can - as it's just engineering and not magic - though it's optimisation which is a PITA without a diff dyno. Drill a hole - flush it out - try something else - ba duh

The original stuff in the Ford viscous LSD capsule (not to be confused with the diff oil) is also a silane based grease, obviously optimised for a 1200+kg car, thus not so friendly in a 5-600kg kit car. (Noone would tell us what it was so we got lazy and put it through a mass spec)

I spoke to Bara about this a few weeks back and their approach is basically to use a dynamometer to match a silane viscosity (I didn't get a straight answer on the range of weights they offered) and given diff (they wear so they are all a little bit different) to the desired locking characteristics.

The air bubble is predominately to prevent hydraulic lock in the chamber on expansion i.e. the fluid seals could burst as it heats up if it was totally filled - the shear transfer mechanism is the fluid between the plates - so long as they are covered it shouldn't care! If there's an engineer with specific knowledge on this I'd welcome a PM

Loving this thread! And not just because I have a spare LSD that was removed as it was too locked off throttle!

Bingham - 19/7/16 at 06:17 PM

Also, as I discussed with Gersen last weekend it kills head lice and cures colic in babies.

Rob Allison - 19/7/16 at 08:43 PM

I have been looking for a uk supplier. So far I can't get it in an large quantities or the right spec

Gersen - 20/7/16 at 05:58 PM

Large quantities might be a problem

We use it in the labs for making molds for MEMS and microfluidics (generally sciencing the poo of it) so it's a lab grade reagent.

http://www.sigmaaldrich.com/catalog/search?term=PDMS&interface=All&N=0&mode=match%20partialmax&lang=en&region=GB&focus=product

Think ours comes from Sigma but might help as a starting point (see above) I'll check when I'm in the lab next

Bingham - I had another plan - we make cocktails that turn to jelly if you try to drink them too fast - market it to alcoholics and soon to be asphyxiating students!

baz-R - 27/3/21 at 03:15 PM

quote:
Originally posted by Gersen
Awesome pics, and really interesting thread - keep the info coming!

PDMS is non-Newtonian fluid i.e. it's resistance to twisting applied in a diff depends on how fast you are trying to twist it - kinda handy in an LSD!
The significant bit is that it's the weight of the silane used that needs to be optimised for the locking properties.

There will be alot of people telling you "OMFG you can't understand and re-engineer the viscous pack fluid" - ignore them - you really can - as it's just engineering and not magic - though it's optimisation which is a PITA without a diff dyno. Drill a hole - flush it out - try something else - ba duh

The original stuff in the Ford viscous LSD capsule (not to be confused with the diff oil) is also a silane based grease, obviously optimised for a 1200+kg car, thus not so friendly in a 5-600kg kit car. (Noone would tell us what it was so we got lazy and put it through a mass spec)

I spoke to Bara about this a few weeks back and their approach is basically to use a dynamometer to match a silane viscosity (I didn't get a straight answer on the range of weights they offered) and given diff (they wear so they are all a little bit different) to the desired locking characteristics.

The air bubble is predominately to prevent hydraulic lock in the chamber on expansion i.e. the fluid seals could burst as it heats up if it was totally filled - the shear transfer mechanism is the fluid between the plates - so long as they are covered it shouldn't care! If there's an engineer with specific knowledge on this I'd welcome a PM

Loving this thread! And not just because I have a spare LSD that was removed as it was too locked off throttle!


I studied viscous diffs and other various LSD types many years ago as part of my automotive engineering and electronics qualification

its an add on to a conventional gear diff and your basic description is correct. as long as the viscous unit is NOT subjected to constant slip the fluid should last a lifetime, usually the unit will fail from the fluid leaking out or contamination (oil) getting in, effectively has no wearing parts. its worth noting that normally both sides of the coupling turn at the same speed so the "slip" is zero and in average cornering the difference is only slight so its never really working and your not going to notice its there just think of the tyre wear factor if they did?

I was lead to believe the fluid or grease was a type of silicone "dilatant fluid" that would thicken or "try" to turn into a solid under stress, sheer or movement as soon as the stress stops the fluid relaxes and turns back into its original state instantly. alternate discs are coupled to each side and sit immersed in the fluid as the slip cause sheer stress on the fluid the fluid is trying to turn towards a solid and create grip between them. effectively it was marketed as it would self adjust as its only trying to resist excessive slip or speed difference across an axel and add resistance. its also worth noting that it works in equal resistance on both directions so overrun will be exactly the same as drive.

there is no dark art and as some energy can be lost by the form of heat it needs some expansion room like alot of things. I don't expect its going to be easy to get your hands on and any one who can will be trying to protect their income after all knowledge is power or money.
viscous diffs where the the OEM choice lsd around the 80-90 of choice due to is nature and cost i guess the new trend is eletronic clutch and helical biasing diffs for the everyday car

in theory there would be a way on reducing its effectiveness by switching around a few plates so all are not alternating reducing the "grip area" if the design allows it?

I have not had issues with my unit been to stiff but i usually run a track day tyre, on hill climb events attacking some very steep hairpins at Wiscombe park wheel lift is felt a bit like clutch slip it doesn't lock giving push on understear or undersreer from biting up to quick.
i managed to do a few events in heavy rain without issues to

phil clegg - 26/4/21 at 06:33 AM

Biggest problem is most are 30 years old and barely work at all. To test jack one wheel and see what force needed to turn the other. Use torque wrench on hub nut. Most are at as little as 20 llbft. This is pretty much open. Siearras were about 70llbft I think. This in a heavy car I think is a bit soft. To stop every car entering ditches backwards. After trial and error I have found 75 llbft is ideal for my 7. I use mine in autosolo events. Mine is quite heavy car nearly 700kg. For road only in a lighter car I would suggest about 60 llbft. Test on bench before refitting. Put drive flanges in. Lock one side in vice. Make a plate with bolt welded on so bench test is possible. I can WhatsApp pictures to anyone who is not sure what I mean. Or any questions give me a call. 07957617548.