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LSD question
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03-09-2007, 02:24 AM
Post: #11
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LSD question
Yeah, NoHachi basicly oulined my quiet irritation on the matter. My experience is mainly [or should i say 'solely'] race sim based. I have an idea of how and why each of them works, but i'd love to see it worded properly so i can find answers to some of the questions i'm looking for. One of them [NoHachi already mentioned] is why on Earth the 2-way diff considered the best for racing applications when it appears everywhere as hard to use and understeer inducing in engine braking procedures. It is true that most sports mainly use straight line braking [or as NoHachi said it running schoolbook lines, if i got it properly], but there are always tracks in which you cannot draw a straight line for losing speed - then the 1.5/1-way should shine, but they still go at it with 2-way and make their best times. Something just doesn't add up. If anyone was to find a proper explanation, please post.
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03-09-2007, 02:49 AM
Post: #12
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LSD question
Some answers - possibly.
NickBW Wrote:Clues: Perhaps the 2nd sentence should say more forgiving balance under ENGINE braking. In a FWD application most of the traction and car control is in the front wheels, with a 2 way application you will have issues with car control in a FWD vehicle. There are a few good articles that I've come across describing the reasons why, I just don't remember where I got them from. domi Wrote:Yeah, NoHachi basicly oulined my quiet irritation on the matter. My experience is mainly [or should i say 'solely'] race sim based. I have an idea of how and why each of them works, but i'd love to see it worded properly so i can find answers to some of the questions i'm looking for. One of them [NoHachi already mentioned] is why on Earth the 2-way diff considered the best for racing applications when it appears everywhere as hard to use and understeer inducing in engine braking procedures. It is true that most sports mainly use straight line braking [or as NoHachi said it running schoolbook lines, if i got it properly], but there are always tracks in which you cannot draw a straight line for losing speed - then the 1.5/1-way should shine, but they still go at it with 2-way and make their best times. Something just doesn't add up. If anyone was to find a proper explanation, please post. Here's my observations on it. 1-way engaged only on acceleration, unlocks on decel - could lead to potential spins due to throttle changes mid-corner. But still better overall traction than open. (good for drag racers - most common for OEM equipment) 1.5-way engaged on accel, partially engaged on decel. More predictability than a 1-way during cornering, but less harsh with drivetrain shock due to throttle control irregularities. (jerking / not smooth inputs). Better suited to learning drivers or cars with harsh reactions & rear-end snap/sudden oversteer due to throttle input changes mid-corner. (Best suited for cars with twitchy rear ends or FWD vehicles due to the nature of a FWD vehicle.) 2-way fully engaged under accel & decel, but still allows a LIMITED amount of slip allowing for the best traction (or when drifting, control of lack of traction) to an experienced driver who knows, understands and is able to use the additional traction and smooth enough with his control of the car to push it to the limit. (not recommended for inexperienced drivers on slick surfaces, or FWD vehicles) EDIT: I think I've posted this before, for some reason I have a large feeling of deja vu... But it probably wasn't here (maybe c4ag?) Also, in response to: Quote:is why on Earth the 2-way diff considered the best for racing applications when it appears everywhere as hard to use and understeer inducing in engine braking procedures. A 2-way is NOT understeer inducing off throttle. A 2 way is: Understeer inducing on accel (on-throttle) Oversteer inducing on decel (off-throttle) Andrew |
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03-09-2007, 03:40 AM
Post: #13
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LSD question
Thanks for the feedback, but my question still persists:
Going on the idea that the 2-way diff has similar coast [engine breaking] and power [engine acceleration] torque deliveries on both wheels, then it'd be just as under/oversteer inducing in deceleration as it would be in acceleration. Understandably you can control whether one will exhibit itself in any way with suspension adjustments, but to be fair let's examine this as having identical suspensions on two identical RWD cars, one has a 1.5way, one a 2way. My way of thought is such, and please correct me where i am wrong, since it's obvious i am and therein lies my confusion: Let's say both cars approach a 90° left turn in the same manner and coast in halfway through and at the apex throttle their way out. Car #1 with its 1.5way has sufficient control while braking and when applying torque it shows understeer due to both rear wheels going at about the same speed. Car #2 with the 2-way diff understeers on the way in due to rear wheels having about the same speed for a brief moment has neutral steer when applying the throttle and then continues having the same amount of understeer when torque is applied due to the same reason. What am i misunderstanding? Don't get me wrong, i can see how both rear wheels having near 1:1 speed ratio can cause oversteer when weight is taken off the rear end and then cause oversteer when weight is reapplied to it with the throttle being open, but it still doesn't add up, not to mention neither under- nor oversteer is considered good on the racetrack. Tirewear aside everything that causes fatigue to the driver's concentration by requiring more effort is a no-no. The way i've understood it is that neutral steering is best, then the racing cars manage to have neutral steering with their 2way diffs and that's what gets me confused: what could possibly be more neutral than open diff? Ok, it is a pain to have the inside rear tire spin on the exit of a corner - go for 1way or viscous link for all i care, but why 2way? I hope you can follow my line of thought and sort this out, cause i'd seriously love to know what is wrong with it... Cheers and best regards. |
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03-09-2007, 03:37 PM
Post: #14
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LSD question
Assassin, I'd love to read those articles if you find them eventually. Regarding some of the other remarks, I figured out how to get my head around these things, but a few drinks yesterday are stil making my head hurt.
One thing I can say immediately: if you are braking in a straight line, and the rear wheels are locked, any resistance to a change in the rear wheel speed differential (aka diff-action) will result in understeer. The rear of the car will just plow straight on and resist rotation. Also your examples of how diffs work are wrong..its the stuff you see a lot on the internet but its just not how the little buggers operate. Read the stuff in the links I posted earlier, they explain things a lot better then I can with an hangover. @Domi: first: Good sims like GTR or GPL are GREAT places for getting a feel for this stuff. I know several racing drivers that claim most of their setup knowledge was gained by countless hours of tinkering in GPL. My take on the 1.5/2-way diffs: They lock the wheels together untill they are overpowered. While they lock (during braking and no- or little steering) they stabilise the rear of the car preventing the car from making sidesteps on small steering imperfections. This allows you to setup the car comfortably towards the turn-in. Once you turn in, the torque diffential across the rear wheels breaks the clutches loose and the rear starts to turn (and also, you usually reduce braking effort towards turn in, so the diff is engaged less severly). So diffs would really benefit a clean driving style. Dunno...difficult stuff it be... A wheel to steer the front of the car A pedal to steer the rear |
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03-09-2007, 06:49 PM
Post: #15
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LSD question
NoHachi Wrote:One thing I can say immediately: if you are braking in a straight line, and the rear wheels are locked, any resistance to a change in the rear wheel speed differential (aka diff-action) will result in understeer. The rear of the car will just plow straight on and resist rotation. If I'm wrong I'm wrong, but I don't think I totally am. Perhaps it's just based on my experience driving a cusco 2-way which has ramp angles which allow the rear to rotate out when partially off throttle. And I'm used to driving it which allows me to make the car rotate into corners and follow the lines I want. I just didn't want to get into ramp angles and all that fun stuff. My 'observations' would be on what I would call 'properly' setup diffs. You're thinking of it like a 1.5/2-way setup very harsh and almost acting like a welded diff/spool. Which it can be, but most of the time it isn't because the manufacturer of these knows not to have too much angle which causes the understeering type of LSD setup your talking about. 1-way, 1.5way and 2-way aren't quite true of the amount of lock they are providing. They are just giving you a description of the basic operation of the LSD you are being provided. The information I provided above is what I think of as 'good enough' for the average not so mechanically inclined driver looking to purchase an already setup LSD. NOT the type of info someone who is mechanically inclined and wanting to do a complete racecar setup where they control all the variables. I agree an almost fully locked lsd/welded diff would cause under steer on decel until you make it to the point too much weight has shifted to the front or you let off the throttle too fast 'shocking' the drivetrain and causing the tires to slip or just plain overpowering. Found some bookmarked info that I saved on another forum since you guys like this stuff. FWD/AWD related stuff - not LSD related FWD: http://whiteline.com.au/docs/articles/AS...1_0202.pdf AWD: http://whiteline.com.au/docs/articles/AS...1_0202.pdf A BUNCH of good reading. Quote:Whiteline has a ton of past technical/how-to and why it works articles here (some vehicle specific, some not): http://whiteline.com.au/dir.asp Andrew |
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03-13-2007, 10:04 PM
Post: #16
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LSD question
NoHachi Wrote:My take on the 1.5/2-way diffs: They lock the wheels together untill they are overpowered. While they lock (during braking and no- or little steering) they stabilise the rear of the car preventing the car from making sidesteps on small steering imperfections. This allows you to setup the car comfortably towards the turn-in. Once you turn in, the torque diffential across the rear wheels breaks the clutches loose and the rear starts to turn (and also, you usually reduce braking effort towards turn in, so the diff is engaged less severly). So diffs would really benefit a clean driving style. Dunno...difficult stuff it be... Actually - most lsd's dont do a whole lot unless they are loaded (have torque/power applied) - or are preloaded by springs in the LSD itself like the cusco-RS. So unless you have a preloaded LSD, if your coasting or braking into a corner without a lot of engine braking your LSD isn't doing much at all. Yes the clutches alone will have enough grip so when you grab the tires by hand you can spin them both together, but it's not enough grip to lead to the understeering type problem your talking about. Not without a lot more force applied to the clutches. Andrew |
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