Rear Axle Diff Lock
Discussion
It's just for very low traction surfaces like snow. It won't make the handling more precise on track, it will just increase the level of understeer. I wouldn't bother unless you're planning on taking part in gravel stage rallies or ice-racing.
Errr just seen this. Understeer??? Clueless
Errr just seen this. Understeer??? Clueless
IMI A said:
Clueless
No, I don't think so. Locking the rear diff will promote entry-mid corner understeer, followed by snap oversteer if you overload the outside wheel traction circle with a combination of power and lateral accel. That's what it does on every rear wheel drive racing car I've ever engineered and the reasons for it are obvious if you do have a clue what's happening. I say promote, because the overall car balance may well be dominated by many other factors.uktrailmonster said:
Locking the rear diff will promote entry-mid corner understeer, followed by snap oversteer if you overload the outside wheel traction circle with a combination of power and lateral accel.
What he said...However I really don't think the OP is talking about a locked diff on a 911 - it's more likely to be an LSD which is a book in itself.....
uktrailmonster said:
IMI A said:
Clueless
No, I don't think so. Locking the rear diff will promote entry-mid corner understeer, followed by snap oversteer if you overload the outside wheel traction circle with a combination of power and lateral accel. That's what it does on every rear wheel drive racing car I've ever engineered and the reasons for it are obvious if you do have a clue what's happening. I say promote, because the overall car balance may well be dominated by many other factors.Edited by IMI A on Thursday 9th August 10:05
Edited by IMI A on Thursday 9th August 10:06
IMI A said:
Both turbos I drove with LSD and without LSD understeered equally. The car does understeer like all 4wd 911s and then break into oversteer which is quite good fun on the track if the tyres aren't your own. The LSD actually helps to control the cars wayward back end by limiting the amount of rear end slip. 911 is a road car not a race car and has its engine behind the rear axle not in front hence not comparable. Oh and its a limited slip diff not a locking diff!
This does seem to be the effect and the correct designation - At the time of my original post, I was a little bit confused as to what the option actually was (because of the label given to it by the Porsche configurator); the title of my post reflects the name of the option listed by Porsche. However, since my original post I have been looking at Rennteam and learned a fair bit more. 99.9% of US owners (who post on Rennteam) regard the lsd option as a must-have on the TT because of (a) surer handling on trail braking and bend exit (b) perceived effect on re-sale.
The experts are slightly more equivocal on the issue. Walter Rorhl apparently told Panorama magazine (the Porsche US Car Club mag) that: "He feels that the optional mechanical limited slip is not important to have on this car, "As soon as it starts to oversteer, you get power in front and the car comes back. You cannot make a mistake if you are on the throttle. The only problem, still on every car, is if you lift off in the corner."
Finally, I found that the Porsche UK site has a section on the 997 TT which is headed "limited slip differential" and states:
"The manual gearbox version of the new 911 Turbo is available with an optional mechanical limited-slip rear differential. Key benefits include greater rear-end traction when exiting hairpin bends as well as on variable-grip surfaces. It also compensates for changes in wheel loads caused by throttle modulation when cornering."
On balance, I think that I will probably add it to my spec.
I just wonder whether the lsd is audible. The diff on my M3 (as on many M3s) can make some interesting and distracting noises. Have any of you experienced rear diff noise or other untoward side-effects of the lsd?
just wonder whether the lsd is audible. The diff on my M3 (as on many M3s) can make some interesting and distracting noises. Have any of you experienced rear diff noise or other untoward side-effects of the lsd?
I have a LSD on my GT3, but I've never heard it or could confirm if it makes much difference to everyday road driving.
I do however get the impression it increases the understeer when exiting a slow tight bend with moderate speed/power, which would make sense I suppose with the inherent friction in the LSD between the rear wheels.
I haven't had the car on a track yet so can't comment on that.
I have a LSD on my GT3, but I've never heard it or could confirm if it makes much difference to everyday road driving.
I do however get the impression it increases the understeer when exiting a slow tight bend with moderate speed/power, which would make sense I suppose with the inherent friction in the LSD between the rear wheels.
I haven't had the car on a track yet so can't comment on that.
IMI A said:
I think you should actually drive turbo in both forms and then comment. Both turbos I drove with LSD and without LSD understeered equally. The car does understeer like all 4wd 911s and then break into oversteer which is quite good fun on the track if the tyres aren't your own. The LSD actually helps to control the cars wayward back end by limiting the amount of rear end slip. 911 is a road car not a race car and has its engine behind the rear axle not in front hence not comparable. Oh and its a limited slip diff not a locking diff!
From the OP's original question it sounded like the option was for the addition of a full 100% diff lock (ie. optional rear axle diff lock). But I'll take your word for it that it's a mechanical LSD we're talking about here. I can't believe the standard car has a completely open rear diff in any case. Otherwise it would just spin the inside rear wheel on power at the slightest oppportunity. So it must have some form of LSD or inside wheel braking control in the first place, regardless of being 4 wheel drive. Back to the handling effects, if it is a LSD as you say, then I agree that increased entry u/s will not be an issue. The LSD will simply reduce the level of inside rear wheel slip on power, giving improved overall traction. The downside would be potential snap o/s due to the increased combined loading on the outside rear wheel, particularly with an inexperienced driver. It doesn't matter where the engine is with regard to the effects of the diff. As I stated in my original post, the overall handling balance depends on a lot more than just the diff. We are talking here of understeer/oversteer tendencies, not absolute balance.
razor said:
99.9% of US owners (who post on Rennteam) regard the lsd option as a must-have on the TT because of (a) surer handling on trail braking and bend exit (b) perceived effect on re-sale.
and (c) because they are all extremely wealthy dentists/cosmetic surgeons/privateequityhedgefundmonkeys who have deep pockets and must have every option otherwise their manliness could be called into question and the 1st tee and later in the clubhouse.The above is clearly a (borderline offensive!) generalisation, but there is a school of thought that the only reason we have 4WD 911's is because PAG developed them to tame the car and keep their US customers happy while there were fears that powerfull RWD cars might be outlawed. (again, this may be hearsay but it makes sense to me.)
I would imagine that a mechanical LSD would be useful in the snow and might raise the point at which investing in a set of winter tyres would become a very good idea. Personally, I would always go for winter tyres first (as the EBD feels horrid when it is doing its stuff on snow/slush) but a set of spare wheels and tyres probably costs significantly more than the LSD option!
razor said:
This does seem to be the effect and the correct designation - At the time of my original post, I was a little bit confused as to what the option actually was (because of the label given to it by the Porsche configurator); the title of my post reflects the name of the option listed by Porsche.
However, since my original post I have been looking at Rennteam and learned a fair bit more. 99.9% of US owners (who post on Rennteam) regard the lsd option as a must-have on the TT because of (a) surer handling on trail braking and bend exit (b) perceived effect on re-sale.
The experts are slightly more equivocal on the issue. Walter Rorhl apparently told Panorama magazine (the Porsche US Car Club mag) that: "He feels that the optional mechanical limited slip is not important to have on this car, "As soon as it starts to oversteer, you get power in front and the car comes back. You cannot make a mistake if you are on the throttle. The only problem, still on every car, is if you lift off in the corner."
Finally, I found that the Porsche UK site has a section on the 997 TT which is headed "limited slip differential" and states:
"The manual gearbox version of the new 911 Turbo is available with an optional mechanical limited-slip rear differential. Key benefits include greater rear-end traction when exiting hairpin bends as well as on variable-grip surfaces. It also compensates for changes in wheel loads caused by throttle modulation when cornering."
On balance, I think that I will probably add it to my spec.
The above all makes sense now we've established that "rear axle diff lock" = LSD!!! Why don't they call it a limited slip diff FFS?? But you still don't need it anyway unless you're racing competitively. I wouldn't put much weighting on what US owners think either, but the trail braking on corner entry will be more stable, as it helps to prevent inside rear wheel locking. But as with the power side, if you overstep the mark and lock the outside wheel while turning in, you're going off the road! LSDs are effective in optimising the grip available, but they do make the handling limits less forgiving. But if you're tooling around at normal road speeds you won't feel any difference regardless, because you won't get anywhere near those limits.However, since my original post I have been looking at Rennteam and learned a fair bit more. 99.9% of US owners (who post on Rennteam) regard the lsd option as a must-have on the TT because of (a) surer handling on trail braking and bend exit (b) perceived effect on re-sale.
The experts are slightly more equivocal on the issue. Walter Rorhl apparently told Panorama magazine (the Porsche US Car Club mag) that: "He feels that the optional mechanical limited slip is not important to have on this car, "As soon as it starts to oversteer, you get power in front and the car comes back. You cannot make a mistake if you are on the throttle. The only problem, still on every car, is if you lift off in the corner."
Finally, I found that the Porsche UK site has a section on the 997 TT which is headed "limited slip differential" and states:
"The manual gearbox version of the new 911 Turbo is available with an optional mechanical limited-slip rear differential. Key benefits include greater rear-end traction when exiting hairpin bends as well as on variable-grip surfaces. It also compensates for changes in wheel loads caused by throttle modulation when cornering."
On balance, I think that I will probably add it to my spec.
anglebox10 said:
I have a LSD on my GT3, but I've never heard it or could confirm if it makes much difference to everyday road driving.
I do however get the impression it increases the understeer when exiting a slow tight bend with moderate speed/power, which would make sense I suppose with the inherent friction in the LSD between the rear wheels.
It wouldn't make any difference to everyday driving, only balls out driving.I do however get the impression it increases the understeer when exiting a slow tight bend with moderate speed/power, which would make sense I suppose with the inherent friction in the LSD between the rear wheels.
The understeer on a tight bend could be partly down to the LSD, but they don't have a huge effect on overall handling balance. Even a fully 100% locked diff doesn't cause as much understeer as you might imagine. The effects of partial or full diff locking are quite subtle. You can only tell easily when a diff is fully open, due to all the tyre smoke coming from the inside rear! A quick google seems to confirm that this car has electronic single wheel brake locking as standard in place of a conventional LSD. So when the inside wheel tries to light up, the brakes slow it down and torque is transferred to the outside wheel and then to the front through an electronically locking centre diff.
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