Chris.Palmer

ScoobySlut,

I think you owe Adam two apologies: the first for being completely wrong & the second for being so rude.

Well ?

[Edited by Chris.Palmer - 12/2/2003 12:04:31 PM]

nom

Heh.
Eddieted 3 tymes and onli won speelin meestake two goe

Ooo blimey - where's the explanation (avec spelling mistakes ) gone? This is becoming one of those confoosing threads now with mysterious disappearing sections

[Edited by nom - 12/2/2003 1:08:22 PM]

easyrider

My scoobs (MY01) 4WD system grips extremely well when going forward,its got extremely good grip which is expected.But in reverse i notice, not only with the MY01, but also with my previous MY98, that when in reverse when im going into my garage(over a raised ridge) on a wet surface the rear wheels can spin a little while before it grips again.
What im asking is, do the diffs work differently when going in reverse???

easyrider

Anyone have any ideas???

hades

Diffs shouldn't work differently - could be weight transfer? When accelerating forwards, weight transfers towards the rear wheels, giving them more grip. The front wheels have some grip as they have a heavy engine above them. When accelerating backwards, the weight transfers towards the front of the car, the back wheels have little weight on them anyway, so they can spin a lot easier.

For all the pedants out there - the weight itself doesn't actually move in the car, it's all down to direction of the net force added as vectors, but the principle is sound.

Edit to add - it's the same principle that means fwd cars tend to get better grip going up snowy hills if they are in reverse than when going forwards.

[Edited by hades - 11/30/2003 7:03:16 PM]

easyrider

Thanks Hades

scoobyslut

the standard setup is that the drive "ratio" is set at 60% to rear and 40% to fronts, so yes, you may experience slight slippage, especially on wet surfaces where traction/grip is dramatically reduced.

Adam M

sorry scoobyslut but that just isn't true.

The standard car has a 50:50 torque split when front and rear wheels are gripping, but when any axle is slipping and its wheel is spinning, no torque is being transmitted through that wheel to the ground. What you have said about torque split would be sound if you were talking about a dccd equipped car.


The diffs work exactly the same way in reverse as they do in forward gears, I would have thought that any splippage in reverse would be more a consequence of directional tires, either by trapping a small volume of standing water, or by the contact patch rubber being intended to impart a greater force when traveling in the forward direction.

scoobyslut

Adam M,
if the system ran with 50/50 split, you would never have the strength in your arms to turn the steering wheel. Get a life and see sense. You would also see the front tyres off in next to no time at all.
the only time you will EVER get 50/50 split is when you engage a "diff lock" and EQUALISE the torque outputs. try reading a proper book before you go sounding off at a tangent and not something off a weekly supplement.

Razor2001

I really dont know much but I honestly thought it was a 50 / 50 as well. I wish my JDM STI7 had more power to the rear wheels like 60 / 40. Anyway, maybe I am wrong to, highly possible

Jolly Green Monster

iTrader: (4)

Scoobyslut,

You is wrong.. and I find it amnusing that you should go off on one so extremely especially when you should take some of your own advise.

Adam is 100% correct to my knowledge and I was too going to say that the directional tyres probably don't help..

JGM

R19KET

Chris,

to be fair to Scoobyslut,

Location : Tarmac Cityville
Occupation : would be idiot specialist
Home Page : http://
Interests : water,preferably, warm
ICQ Number :
Status : Scooby Regular
Number of Posts : 719

Mark.

Adam M

tom, I started to look for that mistake only to find out the whole reply has gone!

ahhh found it!

have it cut and pasted so:

scoobyslut,

I have read books, and have had this explained to me many times.

The whole point of a diff is to allow the front and rear axles (in this case) to turn at different speeds which allows the front and rear axles to turn through different length arcs when turning a corner.

But that is speed differential, we are talking about a torque differential. The only way this can be generated is by having a mechanical advantage, in this case through leverage (via gear ratios). In the case of our cars, the only way there is a difference between the torque transmitted to the road by the front and rear axles (when they are gripping) is by having a diff which divides the torque unequally. The only Subaru diff that does this is a DCCD which is an epicyclic diff similar to an automatic diff gearbox sun/planet/ring arrangement. The tooth ratio in this unit in the impreza application provides a 2/3:1/3 split or roughly 67:33. I think this is because the sun and planet gears have equal numbers of teeth, but check with Andy forest on that.

The non dccd equipped cars have a centre viscous coupling LSD which does not provide a unequal torque distribution.

Of course this is only when all four wheels are gripping, once they are slipping I appreciate that you can distribute 100% to either wheel, until the LS (which works as a function of the speed differential on a viscous coupled diff) starts to do its job.

This is why non dccd equipped cars spin their front wheels due to the weight transfer to the back when accelerating off the line. Once this happens, the LS effect allows the rear wheels to drive to some degree until the front wheel regain traction, this will depend on the severity of the limited slip action.

On DCCD cars the rear wheels spin first due to receiving a greater torque despite the weight transfer. This can be altered by locking the diff so that (almost) no speed differential is allowed between the front and rear axles.

and by the way, I was thought I was being polite, so there is no reason to be so adversarial and rude.

what do you mean by not having the strength to turn the steering wheel?

I have a DCCD equipped car and when fully locked, I still am able to turn the steering wheel. It puts a greater pressure on the plates of the electrohydraulic clutch, and this can be felt and heard as the drivetrain clonks when attempting to turn, but it is still possible. Even if it didn't clonk, it would just force you to scrub the tyres, Understeer and over steer scrub the tyres, I have the strength to make that happen, why is this any different?

edited (phore tyms) twoo current speelin.

MorayMackenzie

Scoobyslut,

Re: "if the system ran with 50/50 split, you would never have the strength in your arms to turn the steering wheel. Get a life and see sense. You would also see the front tyres off in next to no time at all."

Okay, so how do you explain front wheel drive cars?

There was no need for the rudeness either, that simply serves to further detract from the image you project in your reply.

Moray

johnfelstead

to get back to the original question.

It is because the suspension has positive caster at the front when travelling forward to load up the tyre and generate traction, when in reverse that becomes negative caster, so the tyres dont bite in the same way. The rear suspension has zero caster, so when in reverse again the back tyres which are now trying to pull, dont generate the same mechanical grip as the fronts in the oposite direction do.

Try driving a car setup with too little caster, you will understand what an influence it has then.

And plated diffs do work diferently in reverse, you have a ramp angle for both forward and reverse loading, so the diffs can be set to work diferently under braking and accelerating. Viscous diffs and open diffs work uniformly.

Adam M

John, doesn't the plating of plated diffs only come into effect when we are talking about their limited slip effect rather than reversing into a garage?

johnfelstead

they come into effect when you get wheel speed diferences between left and right, so if you are spinning one wheel it will come into effect.

StickyMicky

PMSL @ scoobysluts expense

Adam M

John, thats what I thought, but that is after slip has occurred rather than what could be causing it to happen.

carl

No, but surely the 10% reduction gear comes into effect somewhere here? IIRC there was a decent thread on 22b.com about how this worked that we both contributed to...

johnfelstead

I have already explained why it happens Adam.

Adam M

carl,

I don't know why there is a 1.1:1 transfer case on uk cars, but I don't see how it makes a difference to the torque outputs across the diff.

The transfer case to my knowledge is only there to make sure that the front and rear axles have the same final drive ratio whilst using different crown wheels and pinions at the front and the rear.

if you box together the rear drop gear and the rear diff so that you can't see its internal workings, it present no mechanical advantage over the front axles drive train.

Again we are just talking drive speeds which must be the same front to rear on a grippy straight road, else the tyres get scrubbed or the centre diff is always having to work and hence over heats.

If there were an output speed difference then there would be a torque difference also, but since the former is not a practical solution, I don't see how it could be used to split the torque successfully.



John, on another issue, just because you have explained something, does not mean that a discussion is over.