Help me understand Lotus' LSD choices - LotusTalk - The Lotus Cars Community
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post #1 of 11 (permalink) Old 11-16-2017, 08:06 AM Thread Starter
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Help me understand Lotus' LSD choices

OK, so I know how an LSD works, but what I don't understand is some of the choices that Lotus made with regards to fitting either no LSD, or a particular type.

For example, a standard Elise does not come with an LSD, and I've read several times that the car turns better because of it. This has always been a mystery to me.

Secondly, Lotus chose a torque-sensing LSD for their road cars, but a different plate-type for their Cup race cars. Any idea why this is?

Anyone with good knowledge on this subject, please chime in.

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post #2 of 11 (permalink) Old 11-16-2017, 11:40 AM
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My 2011 Elise SC came with a limited slip differential because - towards the end of their final federalized production run - Lotus ran out of transmissions without the LSD.

A Torsen LSD locks up when a wheel slips. For example, in a very tight turn where the inside wheel slips due to weight transfer, the diff locks and both tires are forced to turn at the same rate. You’ll be grinding rubber off at least one of the tires and the locked diff will try to straighten out your path. If you pick up an inside tire all the way off the ground (like in autocross), the LSD will probably be quicker around the course. When the car rotates drastically, having one or more rear tires in contact with the pavement still driven by virtue of a LSD will reduce slip angles and perhaps make it easier to recover control (this has been my experience in my Porsches comparing LSD to no LSD).

So an open diff could be more nimble with less power as long as you can still get enough power to the ground through the slipping tire.

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post #3 of 11 (permalink) Old 11-16-2017, 12:05 PM Thread Starter
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My 2011 Elise SC came with a limited slip differential because - towards the end of their final federalized production run - Lotus ran out of transmissions without the LSD.

A Torsen LSD locks up when a wheel slips. For example, in a very tight turn where the inside wheel slips due to weight transfer, the diff locks and both tires are forced to turn at the same rate. Youíll be grinding rubber off at least one of the tires and the locked diff will try to straighten out your path. If you pick up an inside tire all the way off the ground (like in autocross), the LSD will probably be quicker around the course. When the car rotates drastically, having one or more rear tires in contact with the pavement still driven by virtue of a LSD will reduce slip angles and perhaps make it easier to recover control (this has been my experience in my Porsches comparing LSD to no LSD).

So an open diff could be more nimble with less power as long as you can still get enough power to the ground through the slipping tire.

Glen
Got it. But why change the type of LSD from the road car to the race car. I've heard that some of the race car diff's were adjustable, which is a bit lost on me, bu could that be the very reason?

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post #4 of 11 (permalink) Old 11-16-2017, 12:08 PM
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Got it. But why change the type of LSD from the road car to the race car. I've heard that some of the race car diff's were adjustable, which is a bit lost on me, bu could that be the very reason?
Cynical speculation: they got the roadcar versions really cheap and figured most people weren't going to push the cars enough to notice the difference.
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post #5 of 11 (permalink) Old 11-16-2017, 12:22 PM
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There is a difference in how a torque sensing LSD and a clutch type LSD work. I'm not an expert on these but Fred @ Blackwatch could most likely answer your questions. He sells both types and has a clutch type that he has configured.

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Eldon
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post #6 of 11 (permalink) Old 11-16-2017, 01:06 PM
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Cynical speculation: they got the roadcar versions really cheap and figured most people weren't going to push the cars enough to notice the difference.
That is overly cynical IMO. Do any roadcars these days, lets say under $150k, come with a clutch pack diff? Just about every car I can think of comes with a torsen style. They don't wear or require maintenance and you get most of the performance benefit vs clutch type.

https://grassrootsmotorsports.com/ar...hats-the-diff/
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post #7 of 11 (permalink) Old 11-16-2017, 04:10 PM
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@VisualEchos - See this post for a quote from Nick Adams, project manager for the Elise. The whole thread is interesting with a fair amount of conflicting information, so you’ll have to think through each author’s arguments and viewpoints to develop your own opinion.

And another thread ...

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post #8 of 11 (permalink) Old 11-16-2017, 04:40 PM
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Got it. But why change the type of LSD from the road car to the race car. I've heard that some of the race car diff's were adjustable, which is a bit lost on me, bu could that be the very reason?
A locking differential helps with stability and putting down large amounts of power. A clutch plate type LSD can be set up to

- lock upon acceleration (1 way)
- lock upon acceleration and partially lock upon deceleration (1.5 way)
- lock on acceleration and fully lock on deceleration (2 way)

The first type is usually whatís used on the street, the second for track and the last (2 way) for track or drifting.

Maybe someone who has experience driving one of these types of LSD can talk more about how they affect handling.

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post #9 of 11 (permalink) Old 11-16-2017, 07:32 PM
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Glen is pretty well spot on IMO. I have three different types of LSD in 3 different cars, and two of them initially had open diffs. The helical/torsen style will lock as long as both wheels have some resistance, and they last nearly forever. I suspect that reliability and "good manners" is why they are put in the road cars. The clutch types do best at locking, react the quickest, but the clutches wear. I suspect this is why they are chosen for race applications--best performance, and rebuilding the diff is just one of the many additional high maintenance items that people put up with because racecar. The viscous doesn't lock so well, seems to hold up better than a lot of what I've read on the intarwebs, but since Lotus doesn't use these I'll just say it feels weird sometimes at the limit and now I will shut up about that.

A 1.5 or 2 way would seem to have a bit more of that bind mid turn that Glen is referencing, but I honestly don't think there is much restriction in the turn in. My BMW has a 1.5 way clutch style LSD. It can be noisy at low speeds--not ridiculous loud, just kind of disconcerting if you don't know what it is. I never hear it when flogging the car because the motor/exhaust is louder, but for instance in a parking lot you can hear the diff when turning tightly. I've got a ton of miles on this car and I'm sure it's not doing its job as well as it once was. It's one of those things that gradually fades so you don't really notice it "going bad" but if I were to rebuild/replace it I bet I would see a noticeable improvement.

My Lotus came with no LSD and I added the TRD unit later, so I can give a direct comparison there. On the street, there is no discernable difference as I'm not going to push the car to the limits. In autocross, I don't feel a difference regarding turn in, but it's obviously different for putting the power down. Since there are lots of "digging out of corners" in autocross, there are plenty of opportunities to spin the inside wheel for just a bit, and in a sport where we are worried about .1 of a second, it really can make a difference to be able to put the power down just a few feet sooner. The same is true when pulling out of a corner on a road course, but I think the percentage of relevance to overall lap/run time is highest in autocrossing.

Personally I prefer the helical as I feel it provides both performance and low maintenance reliability.
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Last edited by Parko; 11-16-2017 at 07:41 PM.
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post #10 of 11 (permalink) Old 11-17-2017, 06:33 AM
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...The helical/torsen style will lock as long as both wheels have some resistance, and they last nearly forever. I suspect that reliability and "good manners" is why they are put in the road cars....

The clutch types do best at locking, react the quickest, but the clutches wear. I suspect this is why they are chosen for race applications--best performance, and rebuilding the diff is just one of the many additional high maintenance items that people put up with because racecar. ...
Correct. OEM have to provide warranties. Torsen LSD are more reliable.

I'm a big fan of the OS Giken product after installing it in our test mule (original ArtCar). We cleaned up with that car in Lotus Cup racing many years ago so it proved itself while helping us set track records. It delivers performance and I have yet to hear of any reliability issues. The price is steep but it is worth the investment...
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post #11 of 11 (permalink) Old 11-21-2017, 06:47 PM
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That is overly cynical IMO. Do any roadcars these days, lets say under $150k, come with a clutch pack diff? Just about every car I can think of comes with a torsen style. They don't wear or require maintenance and you get most of the performance benefit vs clutch type.

https://grassrootsmotorsports.com/ar...hats-the-diff/
That's why I put "cynical speculation" in front of it - it's very cynical and complete and total speculation. It's a bit of playing the devil's advocate.

That said, I think the center diffs in the WRX STi and Lancer Evolution were clutch packs (some evos had viscous center diffs). I don't know about the rears.

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