20 replies to this topic

[dw1]

I was lead to believe that for the best cornering you need an LSD (but I know very little about machanics). Would the car handle even better with an LSD or is the way it is currently set up the optimum?

[djegiant]

Car is a momentum/cornering type car! Not in lotus design brief to make them fast off the line! At speed an LSD requires a very different driving technique to a standard open diff! Can sometimes make handling worse!

[madasahatter]

Until someone with LSD experience shows up..... My understanding is that an LSD is needed for cars with enough power that they tend to spin up when cornering. A VX is capable of doing that, but moreso in slower corners, it lacks the power in standard form to spin up the rear wheels in a 3rd gear corner (unless it is slippy). The use of an LSD means that as power is applied to the diff, it tightens, locking the speed of the wheels more closely together. Should a wheel start to spin up without an LSD the diff works by tending to ship more power to the spinning wheel. An LSD resists this, since it tightens and so the other wheel still gets much of the power. Although this won't stop oversteer - far from it - it will make it easier to control once traction is being lost. I think it is heavily recommended for tuned VXT's above 250bhp or so. I am sure someone who knows far more than me, and understands far more than me will be along in a bit to put me right, or fill in the blanks. Steve

[DrJ]

I'm no mechanic but have driven Caterham Superlights with and without LSD and with LSD you get much better traction out of tight bends, especially on a track. I agree that the VX is a "momentum" car but I still think it would handle the tight stuff better with LSD IMO.

[garyk220]

I'm no mechanic but have driven Caterham Superlights with and without LSD and with LSD you get much better traction out of tight bends, especially on a track.
I agree that the VX is a "momentum" car but I still think it would handle the tight stuff better with LSD IMO.

This graph shows a speed trace from a couple of runs at Harewood hillclimb last summer. About 2/3 of the way up the hill there is an open, uphill bend with a crown in the centre of the road. Traction out of the bend and up the hill past the speed trap is critical here, especially if you run wide over the crown in the road.

The blue line about 58 secs into the run shows a spike in speed from 40mph (apex speed) to almost 70mph (ie. rev limiter in 2nd gear, with the inside rear wheel spinning like a top). The top speed at the top of the hill is about 6mph down on the red run, where I held a tight line and had no traction problems out of the bend.

On something like 22 runs, I only had this traction problem on 2 of the runs, so it is relatively easy to modulate the throttle to prevent too much wheelspin. As well, as providing more predicatable traction in this type of situation, an LSD would also allow me to use more throttle exiting the bend and increase top speed marginally, even compared with the clean runs.

I'll be fitting one when the gearbox is out to replace the clutch in the future.

PS The Sport Exige/240R or whatever Lotus are calling the new one, has the option of traction control and an LSD. So would confirm the threshold of 250ish bhp mentioned in one of the above posts.

Attached Files

•  harewood_wheel_spin.jpg   39.89KB   2 downloads

[Richy]

LSD = B road Drift god

[dw1]

What do you mean by 'momentum' car? Having been used to an lsd I love the feel of them, you can really feel them working and it almost feels like the car twists round itself giving bags of grip on the outside wheel.

[speedyK]

Apart from Range Rovers with all their trickery, I have no limited slip diff experience. My Exige (265bhp) has one (no traction control though). I'm interested to see how it feels and will report back

[DrJ]

What do you mean by 'momentum' car?

Having been used to an lsd I love the feel of them, you can really feel them working and it almost feels like the car twists round itself giving bags of grip on the outside wheel.

coming over the mid Wales hills from Hereford to Abersoch last week the car just moved from one bend to the next to the next with such fluidity that you cover distance quickly and without the need for great bursts of acceleration and braking or frantic gear changing. Thats what i mean by a momentum car.

Each to their own but that's one of the things I like about the VX.

[Ferguson]

LSD was added to the Elise mostly to quiet the bleating of the American autocrossers. It makes a big difference on a tight sprint course, but not much on a flowing track circuit. It has the potential to make the car very difficult to catch at the limit, with more pronounced snap oversteer.

[Arno]

Also an LSD can make the car much more likely to understeer under power in fast corners as both rear wheels are now locked and driven and try to 'push' the car straight across it's front tyres. So you may need to wait longer while cornering before you can get back on the throttle or the car pushes wide. On very tight corners it's not an issue, but for more 'roomy' tracks it's an effect you need to be aware of. Bye, Arno.

[sixspeed]

Not sure where "momentum car" comes into the requirements of an LSD or not.... even my MX-5 had one! And that's more of a momentum car than the VX! I do wish they had one sometimes... only because it makes doing celebratory (!) donuts easier... -andy-

[Paul.Stuhlfelder]

The ///M has a LSD. All I know is it means I can leave two great big thick lines of rubber... if the feeling should so take me.

[djegiant]

Not sure where "momentum car" comes into the requirements of an LSD or not.... even my MX-5 had one! And that's more of a momentum car than the VX!

I do wish they had one sometimes... only because it makes doing celebratory (!) donuts easier...




-andy-

Mx 5 is front mounted engine with rear wheel drive!
Makes for a different scenario as the front naturally has more grip.

With the lightweight of our front end, as arno says- LSD would make the car want to push on much more in bends, hence my mention of the different driving techniques req'd!

Dohnuts!



Paul- is the ///M powerfull enough to spin the wheels???????

[Paul.Stuhlfelder]

Paul- is the ///M powerfull enough to spin the wheels???????

321 ponies says yes

(Although foot wide wheels and 1500 odd kilos doesn't help! )

[minime]

i have both a LSD and racelogic traction control unit fitted...at the nationals i let a few peeps drive my car around brunts with just the lsd working( the new uprated tc wasnt fitted then!) no one mentioned any problems! i found that i was quicker through the twisties then ever before and feel now that with the uprated tc that i could be even quicker:)

[speedyK]

It has the potential to make the car very difficult to catch at the limit, with more pronounced snap oversteer.

Hmm... with the Yoko-shod Exige's higher limits, I'll be a little wary till I get the feel for it. Sounds like the line between "on" and "over" the limit may be very thin and very quickly crossed!

[Ferguson]

The observation about snap-oversteer is strictly heresay on my part, mind you.

Edited by Ferguson, 24 February 2006 - 12:29 PM.

[c_w]

The observation about snap-oversteer is strictly heresay on my part, mind you.

I'd say there is some truth in that since without one will just spin one wheel up but have one wheel with some lateral traction, but with an LSD when both wheels spin up, the rear end will become light and swing one way or the other.

I think the reason it was never fitted will most likely be down to cost. Apparently, Porsche won't fit an LSD to the Boxster/Cayman because apparently if they did it would be as quick as a 911 at £20k more. The Focus RS had an LSD and Ford sold these cars at a loss (probably not just because of the diff tho! but it would be significant cost in terms of production).

I don't think the LSD's main intention is for off the line traction, a VX Turbo will get low 4s easily enough without one. According to the users handbook for a BMW Z3M, it gives some information on the LSD about how drivers should be more aware of, I'd guess this could be applied to them in general;

Limited slip differential*
With unfavorable road conditions, the drive forces that should be transferred may not be accommodated by the traditional differential, and therefore one wheel may spin. This wheelspin is largely avoided by the limited slip differential cut-off value approx. 25X). Under actual driving conditions, this means better traction characteristics when starting and accelerating (under adverse road conditions) and during performance-oriented driving on winding roads. However, in the case of extremely dynamic driving, the vehicle may tend at the same time to spin around its vertical axis in the vehicle center of gravity on road surfaces with different adhesion characteristics. Controlling this rotation tendency places greater demands on the driver and therefore requires special alertness. The limited slip differential is active during actual driving without the driver taking action.

Edited by c_w, 24 February 2006 - 01:26 PM.

[woot_uk]

i recall reading one recent article where the lotus engineers said that open diff car felt better and was faster around the hethel test track. the impression was that LSD was driven by USA demand for it, even though the car designers felt it wasnt necessary. this is only for a standard powered car though.