ex-webby

Hi Chaps

I'm typing from rally hq in cyprus so I won't be able to respond quickly all the time (don't think I'm being rude! even if i am!! )

Why RWD has more chance of allowing earlier power...

Tyres produce grip in two directions, lateral and longitudinal... they can only produce their maximum lateral force when there is no longitudinal demand on them.. and vice versa, the more longitudinal demand you place on the tyre, the less lateral force it can produce.

when entering a corner on circuit you are ideally going from MAX longitudinal force (under max braking) to max cornering force (post turn-in / brakes off - and prior to accelerating).

In FWD, you need to wait before you have got all your lateral requirements out of the front tyres as you are going to want to place longintudinal demand on them to accelerate out of the bend. In 4WD the same is true but to a slightly lesser extent, in RWD you can leave the fronts to continue generating lost of lateral force and place all the longitudinal demand on the less overstretched rears.

RT, on the other thing about steady state, I completely agree, unless you have any drive going through the tyres, and / or the diffs are putting any demand into individual tyres.

All the best

Simon

PS Adam, I think I did well to last until the third page!!

russell hayward

I was saying open front diff will give same result ( torque transfer to rear) as LSD front diff, given that you get both front wheels spinning.

BUT with the LSD front diff, it will be easier to get both front wheels spinning due to the torque transfer from the inside to the outside front wheel.

Therefore with the LSD front diff, this will happen more quickly/more likely to happen than with open front diff.

This is my understanding. I have both sorts on my two cars so am interested to know if my understanding is correct.

EvoRSX

I suppose the question is do you need both front wheels spinning for the center diff to lock up and send power rearwards or does it still lock up when a single front wheel is spinning.

If it only locks when both fronts are spinning (which is my understnading) then you won't get the same rearward torque transfer but you may still get some (I think ).

If it locks when a single wheel is spinning then you will get a faster rearward transfer of torque than with the LSD.

Andy

sempers

Going through the logic (*which may well be flawed*) in my post, I don't think you need to get both fronts spinning for the sudden rearward torque transfer to happen with an open diff. (In fact, I think it's pretty much impossible to get both fronts spinning with an open diff.)

I can't see how a scenario where 1 front wheel is spinning, and one following the roadspeed can result in anything other than the drive into the front diff speeding up - these things are geared together after all.. if the rears are still turning, speeding up the fronts will induce the centre to lock.. and so torque is shifted back..

Admittedly the spinning front will lack almost any lateral traction, but my understanding of diffs suggests that the outside is almost completely robbed of drive (in the open front diff), so should pick up some lateral tractive ability?

So, why does the LSD do the sudden tuck in more than the open one?

Surely someone can explain this? Everything I've ever *heard* of lsd's suggests that any locking action reduces the car's inclination to turn - not that I'm doubting this phenomena, just would like to _understand_!

- Mark.

EvoRSX

Sempers,
I think I'm confusing myself now, can anyone else help us out here!!!!!

Andy

EvoRSX

Sempers,
The LSD works by switching torque to the outside front which we both agree on.
In doing this, it reduces the 'spinning' of the inside front allowing it to grip the road surface again and so provides a greater level of grip to both front tyres. Therefore you get better traction at the front and the car appears to 'bite' into the corner.

Andy

sempers

...so, with an lsd, it drives normally at the point where mine would have shunted all the torque to the rear, swapped ends, and demolished a brick wall or two

I'm confusing myself here.. I give up, before everyone else is bored to death

- Mark.

DavidRB

Surely that assumes that the rears are less overstretched to begin with? If the fronts and rears are equally stressed laterally, then 4WD will allow greater acceleration than RWD (or FWD). All depends on setup and balance I guess.

Audi did rather well with 4WD in the BTCC a few years back and didn't the Skyline race in full 4WD trim on tarmac in Australia & Japan?

Adam M

dont dilute the subject,

can someone explain what a front LSD does to the handling whenm the car was understeering previously?

I can figure it out on a fwd drive car, as effectively when one wheel is spinning on an open diff no torque willbe driving the car forward, but with an lsd, there is a driving torque still.

ex-webby

David..

nearly.. but remember that when you accelerate you move load from the fronts to the rears. So the rears become less overstretched compared to the fronts.

LSD thing is a REALLY evil subject. The problem is that there a many many many different reasons for understeer. In addition it is very easy to mistake the feeling of the inside wheel spinning and the lack of command in the front end for terminal understeer (which it is not).

In my experience the front LSD provides the "feeling" of more bite under power as it will always drag the front end into the bend more until the limits of the tyres are reached. This does NOT mean ultimately more performance through the bend and does not mean it will understeer less.

Unfortunately I haven't done any specific diff testing in 4WD so I'm not qualified to talk about the transfer of torque front / back with any authority at all.. and most people know how crap my mechanical knowledge is so I can't even comment from a physics point of view. sorry.

All the best

Simon

Andy W

Guys this is another of those great threads that scoobynet sometimes produces, THIS is why I check the BBS every day not the my xxxxxx is better than your YYyyyyyy, nonsense.

Andy, feeling inadequate that my mechanical knowledge is so poor.







[Edited by Andy W - 4/18/2002 9:33:55 PM]

RT

QUOTE:

"Why RWD has more chance of allowing earlier power...

Tyres produce grip in two directions, lateral and longitudinal... they can only produce their maximum lateral force when there is no longitudinal demand on them.. and vice versa, the more longitudinal demand you place on the tyre, the less lateral force it can produce.

when entering a corner on circuit you are ideally going from MAX longitudinal force (under max braking) to max cornering force (post turn-in / brakes off - and prior to accelerating).

In FWD, you need to wait before you have got all your lateral requirements out of the front tyres as you are going to want to place longintudinal demand on them to accelerate out of the bend. In 4WD the same is true but to a slightly lesser extent, in RWD you can leave the fronts to continue generating lost of lateral force and place all the longitudinal demand on the less overstretched rears."



The Traction Circle theory sounds sound. This is how I understand it as well. But...

On the contrary, I would have thought that in a non-sliding car (let's assume), the lateral G-loading on both the front and rear axles is the same. This is why it is not sliding, right?

At the point of max lateral acceleration, all 4 tyres are at their max point on the x-axis of the traction circle. Now fast forward a infinitesimal time when you just begin to wind off the steering and accelerate.

At this point, I would have thought that all 4 tyres start coming off their max x-axis point on the traction circle and move in the accelerative (y) direction. Like I said before, this is based on the assumption of a non-sliding car (after all, we are talking about GRIP).

Therefore based on my above understanding, I would assume that you can use MORE power SOONER in a 4WD car, as you have the longitudinal (y-axis) axis on the traction circle of all 4 tyres to work with, as opposed to just 2 tyres in a FrontWD or RearWD.

All this is based on the assumption of a not-sliding car, which I know, IS a very BIG one.

RT

UGH! For the above post, I meant to say "infinitesimally small time" instead of "infinitesimal time"! Too much effort trying to spell this word....

If we fast forwarded an "infinitesimal time", I guess we won't be worrying about traction circles and grip, but about how to travel at Warp9.

RT

Double UGH!

On re-reading my post, I realised that I have forgotten to mention my assumption of weight distribution. Most cars are front heavy now, certainly the Scooby is.

I have assumed either:

1.
a 50-50 distribution for the above arguement (which obviously is out the window),

2.
or that the extra weight over the front generates more traction for the front tyres (Frictional Force = Coeff of Friction X Normal Reaction), to the point where max front and rear axle lateral grip are equal for the car's particular weight distribution in a steady state corner (not braking or accelerating).

Obviously, point 2 will start another 4 page thread on tyre performance...

ex-webby

Morning all

RT..

OK. Firstly, you are basing most of what you said (as I see it) on steady state cornering. Which as I mentioned before, unfortunately doesn't exist for more than an accidental fraction of a second in one out of a million bends, etc.

This is most evident in the statements like the non-sliding car have equal lateral load, etc. There are many different levels of non-sliding and sliding. The traction circle is a simple representation of an incredibly complex small part of a infinitely more complex subject. So complex in fact, we just don't understand it all.

Under acceleration in a bend (if the car has been set up correctly to take into account it's weight distribution and everything else - or just using the 50 / 50 weight distribution car), the rears have more grip available in both directions than the fronts.

This means that the fronts cannot produce as much lateral force as the rears car. In front wheel drive, you increase this difference further by removing more of it's lateral capabilities by placing longitudinal demand on the fronts. In RWD, you have spare grip available at the back as you can only corner as fast as the fronts will allow anyway. You can then use this spare grip for acceleration. In 4WD you are still taking lateral ability from the fronts, even though not as much as in FWD, so the rears are more likely to sit there with plenty in reserve whilst you could have been using that grip in RWD, and freeing up more grip at the front. This equals more useable grip = quicker.

Extra weight over the front providing more traction...

This is an easy confusion. Remember that more weight over the front also creates a greater overturning moment = more weight transfer across the axle = less cornering grip. More weight over the front also creates a higher lateral acceleration which puts more demand into the tyres and therefore requires more grip from the front tyres to contain.

It is important to remember that the vertical load on each tyre changes substantially depending on the stage of the corner. At no point are all 4 tyres supporting the same load which means they cannot produce the same levels of grip (unless you have a wierdo set-up just to disprove this statement). Also, it is important to start thinking way beyond the steady state examples that appear in books, etc as they are just a starting point to understand principles, when the car starts moving, things become infinitely more complex.

It is a lot easier if you are interested in this stuff already and then get involved in testing with good vehicle dynamics / chassis engineers as it all starts to make sense. But the thing that I have noticed is that the very best of these guys are the ones who have got to the stage that they know so much, they can see just how little they know!

All the best

Simon

dowser

Just to throw a bit more into the mix - there are various types of diff. The Quaife one I have at the front is mechanical (but doesn't allow a 100% transfer from 1 wheel to another) - the standard one at the rear is a clutch/viscous type affair.

To my understanding, the mechanical ones are *much* faster - I don't notice it anymore, but when first fitted my comments here were about the 'ripping itself to the inside of a corner under acceleration' and 'torque steer under hard acceleration on a bumpy road'. You really do feel it transfer torque between the wheels depending on grip levels.

Also (from another long diff related post on here a month or so back) bear in mind that it was generally agreed that a mechanical centre diff would make the car bloody horrible to drive due to the constant torque split changes front to rear (although the early Quattro's used this system).

The standard centre in the Scoob is a 12kg/cm2 viscous one which provides more resistance the bigger the difference between front and rear axle speeds.

My (flawed!) logic for fitting the Quaife was that an open front diff will allow one front wheel to spin without generating a large difference between axle speeds (one wheel isn't spinning) - the Quaife ensures that both spin, loading up the centre and sending torque rearwards. This is wrong I think - even if only one wheel is spinning there is still the difference in axle speeds so the centre still loads up.

So the Quaife didn't do exactly what I expected (make the car completely tail-happy ) - however, by having better traction at the front still provides for a much more rear biased balance on corner exits...as long as your entry speed is not fast enough to break traction, if it does the car will understeer (or 4 wheel drift in reality) out of the corner.

If I could do it again, I'd have fitted the 20kg/cm2 centre too - I suspect this would help in that breaking traction at the front would load the centre quicker, hopefully allowing the rear to break traction a bit quicker and providing a better balance out of the corner.

It's also important to note that the diffs are not the whole story - having great traction under power is good, but dangerous. Early testing in the wet saw me arrive at a corner (where I normally lift off and coast round into a reduced speed limit) 20% faster than previously achieved - this was enough of an increase in speed that when I lifted it generated a massive LO oversteer moment You quickly adapt though - after 4-6 weeks my driving style changes means I don't notice it anymore.

It also highlights limitations elsewhere - on track I'm no faster out of a corner than a friends Scoob with better suspension and aggressive geometry. And through a series of corners he's much, much faster than me. I'm soon to be fitting the same set-up as he has (KW coil-overs/ALK/-2 front/-1.5 rear) - I'm very interested to see how things compare afterwards

Note: I think the STi's have a 20kg/cm centre as standard - can anyone confirm/deny?

Richard

sempers

Not sure I fully understood webby's post, however, RT, remember that the static (mass) distribution of the car is different from the dynamic (weight) distribution. Unless the COG of the car is at ground level, accelerating will throw more *weight* onto the rear wheels and off the front wheels *without* changing the mass to 'turn' (i.e. any more side load) at either end of the car.

Extra download on a tyre will generally create more grip, however, extra mass at that end of the car will require more turning.

All things equal (over simplification) the grip of an object (tyre) on a surface is proportional to the download on it. So, under acceleration, the rears pick up grip, under decel, the fronts pick up grip - part of the lift off oversteer equation - engine braking through the rears makes this more severe in 4wd.

This is why front / 4 wheel drive is more prone to understeer on corner exit - in accelerating, you remove grip from the front wheels (weight transfer), and the traction circle *reduces in diameter*. At the same time, you ask them to provide traction.
RWD provides more grip at the rears under accel (weight transfer makes the traction circle bigger), as well as asking them to provide traction.

I guess what I'm saying is with RWD you don't have to get the fronts off max lateral loading to be able to get on the power.

Enough of a ramble about that, on to diffs...

I have a sneaking suspicion that most of the time (in a no-slip (wheelspin) scenario), appart from the feel, the diff is affecting the handling less than you might think from our discussions - I certainly don't think I get much (if any) wheelspin on my car - appart from when I'm playing on my own little 'test track' (ok, someone's very slippery carpark) I rather suspect that the suspension, suspension geometry, and weight transfers have much more effect.

The one thing that will (I'm guessing) affect things most is the pre-slip - non-limiting torque biasing characteristics of the diffs. The front in particular as you're hanging onto the wheel and will feel torque transfer to the outside as the car wanting to turn in?

Does anyone know what the non-slipping torque biasing of the centre diff is? I know I have a friend with a Celica GT-4, who reports power on oversteer - I assume the diff is splitting the torque a lot further to the rear, rather than inside front wheelspin followed by diff lock.. though nothing would surprise me

Lastly, (I've rambled enough..) have a look at this link:
http://www.autospeed.com/A_0920/page1.html
It's about scooby (centre) diffs... notably it suggests that a standard car with a 20kg/m centre will "understeer like a pig"
(Oh, and it's an aussie site...)

- Mark.

Diablo

Adam, and others, on front diffs.

I think its quite simple.

With a spinning inside wheel and minimal torque to outside, there is little torque being applied to front axle, therefore the car will not pull into the corner, although it may, ultimately, oversteer as torque is transferred to rear by centre diff.

With front lsd, inside spins, torque transferred to outside front and ultimately balanced across the front axle and thus car will pull in direction wheels are pointing - generally to the inside of the corner. Hence car pulls inward.

Up to the limit of adhesion....then it will let go with less progression.

D

russell hayward

Eureka !

Exactly what I was trying to say.

Thanks Diablo

RT

...and the plot thickens!

Quote:
"Not sure I fully understood webby's post, however, RT, remember that the static (mass) distribution of the car is different from the dynamic (weight) distribution. Unless the COG of the car is at ground level, accelerating will throw more *weight* onto the rear wheels and off the front wheels *without* changing the mass to 'turn' (i.e. any more side load) at either end of the car."


I do know that static and dynamic wt dist is different However, I did state all my assumptions explicitly in the hope of being able to understand a concept without external factors complicating it. Note that I mentioned "steady state" cornering many times, which I'll admit, hardly exists at all if you're driving right. But assuming this steady state, there's no accel/decel - therefore there will be no weight transfer.


Quote:
"Extra download on a tyre will generally create more grip, however, extra mass at that end of the car will require more turning."

Correct! Which brings me to my next explicitly stated assumption that either the mass is a perfect 50-50, or that the extra normal reaction gives extra tyre grip which in turn counters the extra mass. Again, this was an assumption made to try and remove tyre characteristics and wt dist out of the equation.



All I was trying to understand here (however ridiculous and unrealistic the assumptions may be), is that in a steady state corner - FrontWD, RearWD and 4WD will not make a difference to the ultimate lateral G / grip. I think Simon has already confirmed that.


This discussion then led to: which configuration will let us get on the power sooner. Again, I made the assumption of a non-sliding car. I still don't get why a RearWD will let you power out sooner. An infinitely small time after the apex (max steady state, non-sliding car) when you're starting to wind off steering lock, all 4 wheels will start having "extra" longitudinal traction available.

When throttle is fed-in, weight is trasfered backwards (after, not before throttle is given) and hence based on my 50-50 assumption, the rear will begin to have more grip.

In a RearWD setup, you can only use the extra longitudinal traction of the rear tyres altho your fronts are starting to ease up on the lateral loading. Therefore, you can't use as much throttle as you can in a 4WD.

I know my assumptions are great and many and probably results in a situation that does not exist in a real world. I'm however, just trying to understand the very basic and fundamental idea of traction circle - lateral and longitudinal grip - without introducing tyre traction, slip angles, wt transfer etc and how this may/may not affect
(1) ultimate conering speed without sliding
(2) application of power without sliding.

RT

Imagine this: You have a perfectly balanced (50-50 assumption) "fly by wire" magical car which does not allow you to drive outside the grip/traction circle envelope (much like a modern Airbus aircraft - what fun is that? But anyway...)


You go into a corner and turn the steering wheel, constant throttle not accel or decel. The steering wheel will only turn unit the point where the tyres start to slide (remember 50-50 assumption?), and then it won't turn anymore.

Now, you can't put down any more power as the car won't let you exceed the traction circle limits. The only way to accelerate is to wind off some steering lock. Now, the moment you do that (still not accelerating), you're removing some lateral load from the tyres.

With this lateral load removed, you can thus accelerate and use some of the tyre's (now excess) longitudinal grip for acceleration. But this magical car will not allow you to break traction! Therefore, you're only able to use the excess longitudinal grip of the rear tyres (as opposed to all the tyres in a 4WD).

This is why I'm under the impression that a 4WD allows power sooner (in this magical car at least)!

Stelios

Just to make another point regarding driveability, Its not a coincidence that STi's old and new have "shorter" steering racks for fast o/s reaction. I know from the days of old that cerain Motorsport kits threw in a quicker rack with a lsd as a kit.
Good thread this.
PS simon if you come to the Acropolis u r cordially invited by the Greek scoob fraternity for dinner....

RT

Let me just say at this point, that its threads like THIS that keep me coming back to Scoobynet.
Great read, great opinions/advise/discussions!!

sempers

Hmm. I see your point.

However, If it's steady state, it doesn't matter what wheels are driving - they aren't

Seriously, I follow your reasoning, and to a point it's right - it certainly makes sense. **But**, you can't have a steady state car accelerate..

Sidenote) I'm not at all sure that the extra download on a tire due to adding mass at an end neatly and perfecltly accomodates the extra lateral force needed. In fact, I'm sure it doesn't otherwise weight dist of cars would not be of such consequence.

Sidenote 2) You cannot have a tire that does not slip. It must (does) slip a little in order to generate lateral force. The force increases with slip angle up to some peak (on a tarmac tire as webby was alluding to) after that slip angle the grip massively decreases...

Given your model, consider the following - nothing happens instantaneously - throttle response, or weight transfer.

At the apex, your car is rolling at a steady state - the rate of turn would see you staying well inside the outside kerbing. It's a 4wd car. You pick up the throttle, for some instantaneous moment, your situation holds as described, while shocks and springs compress, then, the (dynamic) weight sits back on the rears. The traction circle for the rear becomes of larger diameter due to the download increasing. Traction and lateral force continue to be exerted.

At the front however, the traction circle diameter decreases, overall grip decreases. On top of this, traction is exerted, so less of the overall grip is available laterally. Net result, the front end washes wide.

Now we'll do the same in a rwd (with all the same proviso's as 4wd). Steady state, all fine. Power on. Weight back. Rear traction circle becomes bigger, front end traction circle becomes smaller, so we can't turn the front so hard. If we were in a steady state, we would not be using the full traction circle at the rear to keep the car turning steadily. Therefore, we can (and are - being in an accelerative state), use the remaining traction budget of the rear end to accelerate the car without it pushing wide. If we stamp too hard, there will be no lateral ability left at the rear, and we pirouette nicely!

So, during acceleration, we're using less lateral ability in the whole car - at the front, because of weight transfer, and at the rear because of the forward traction from the tire. But that's no matter as we want to release the line towards the kerb anyway.

What I'm trying to say, is that on under acceleration (corner exit), ANY car is going to pick up rear grip, and loose front grip. Therefore it will tend to understeer. Any driven front wheel will increase this tendancy. Driven rears reduce the tendancy a little.

I believe that many ?most? RWD cars will exhibit some level of power on understeer at certain rates of acceleration - where the reduction in front end grip is more than the reduction in lateral capacity of the rears due to traction. On the other hand, apply too much throttle, and the rears loose all lateral ability. Therefore a RWD car can be adjusted on the throttle, whereas a front / 4wd car needs you to come off the throttle to get it back.

Of course, lateral grip levels at each end are massively influenced by geometry - camber, castor and the like as well. A front wheel drive car balanced on throttle will compromise rear end grip - off throttle it will oversteer like nothing on earth. Ditto to a large extent, a 4wd car. RWD gives less change on/off throttle.

Ask this: why are most pure (non production derived ) racing cars RWD? Ok, rallying has different rules; Once cars are traction limited I guess the rules change, my rudimentary understanding of what's going on ends there!

Lastly, someone mentioned skyline GT-R's - bear in mind they're RWD up to the point the computer decides things are going awry / the back end is on the move, whereupon power is chucked forward to balance affairs.

I hope this makes sense - bear in mind I'm no expert, just trying to convey my understanding, such as it is..

- Mark.

Edit to say, RT - 100% agree, I like threads like this - even if someone comes along in the next post and tells me I'm talking crap - so long as I can understand, and they've got something I've missed

Also, I think you're understanding of the traction circle is spot on, however, you can't ignore weight transfer - it's fundamental. The effect of weight transfer is to make the traction circle bigger for a tire getting weight, and smaller for one loosing weight, so coming out of a corner - inside front - tiny circle, outside front - medium circle, inside rear - bit bigger than medium circle, outside rear - big phat circle!

[Edited by sempers - 4/19/2002 5:14:42 PM]

sempers

Ooops. Operator error!

[Edited by sempers - 4/19/2002 5:15:38 PM]

RT

Quote:
"Seriously, I follow your reasoning, and to a point it's right - it certainly makes sense. **But**, you can't have a steady state car accelerate.."

Yes thats true. Which is why I mentioned an infintely small time AFTER this "steady state" condition. To fulfill the conditions of "no-slip", you would have to wind off steering in order to accelerate.



Quote:
"Sidenote) I'm not at all sure that the extra download on a tire due to adding mass at an end neatly and perfecltly accomodates the extra lateral force needed. In fact, I'm sure it doesn't otherwise weight dist of cars would not be of such consequence."

You're absolutely right here. It could be this overriding factor that makes my scenario so unrealistic that it never happens in real life.



As to why traditional race cars are RearWD. I guess there're many ways to answer this. Probably rules and regulations primarily (I do believe Formula 1 rules say that the cars cannot be 4WD!). But also weight and complexity. And the fact that RearWD's are much more fun on the track than 4WD's or FrontWD's.
Hey, if not for practicality, reliability and $$$, gimme a TVR Tuscan anytime! I'll gladly swap.

roee

RT,

To my (lacking ) understanding, when you corner, the front tires use much more of their lateral grip than the rears (assuming 50/50 weight distribution). Lets assume that each tire has grip X, which you can use either for lateral grip or longitudianl grip. They come one instead of the other - i.e. the max lateral grip of a tire is total grip amount X - the longitudnal grip being used.
This is if I understand correctly.

Now, we've already said that when you corner the forces applied to the front wheels are greater than those which are applied to the rears, therefore when cornering at steady state you still have some unused grip reserves in the rear tires, that you can use for accelerating. When a rear wheel breaks traction, it increases the yaw angle (oversteer), which pushes you (theoretically) into the corner and not outside of it, like in FWD understeer. Something to consider here: when you accelerate with RWD mid-turn, you load up the rears with more longitudinal forces, but also the fronts with more lateral forces (as they are not parallel with the plane of acceleration). depending on the chassis, how much grip you still have in the fronts, and how abrupt you applied power to the rears (the more aggressively you apply power, to more likely the rears tires are to break traction getting you to oversteer) you'll get understeer or oversteer. if you press the throttle slowly, you are likely to push the front out a little bit, if you slam on it, you will probably end up spinning the rears which will not be able to handle such amount of instantaneous power..

AWD binds together all wheels, so when you accelerate you put more longitudinal grip on the fronts, not only the rears. if you are not cornering at steady state, matters are completely different - you can begin the turn with greater yaw angle (i.e. oversteering, drifting) and then apply power, using all four wheels to supply traction.

So I reckon RWD is better for steady-state, AWD is better for rallye style driving..

(I'm very likely to be wrong here somewhere, don't count on me )


[Edited by roee - 4/19/2002 5:37:35 PM]

RT

Quote:
"To my (lacking ) understanding, when you corner, the front tires use much more of their lateral grip than the rears (assuming 50/50 weight distribution)."


Fundamental question, Why?


I would have thought most cars are set up (for max cornering grip) such that the front and rear axles will have almost similar lateral grip. Isn't this why negative camber on the front axle of front-heavy cars tends to be greater?

Hmmm - just thinking aloud. If what you said is indeed true, then its very likely that a not-accelerating (steady state *again*) car can be understeering badly, instead of neutral drifting (which is what we all aim for right?)...

roee

RT,

I believe that under steady-state conditions, most cars will indeed understeer. My logic says that, because under lift-off conditions a car that oversteers at steady-state corner will just spin out. Also, I don't think manufactorers aim for neutral handling - makes car much more nervous on the limits - car will jump from understeer to oversteer and back again with every slight input.
Almost all cars (except maybe classic 911's etc.) have more weight in the front, and when you lift off you "transfer" even more weight to the already heavy front, yet only few cars will go into wild oversteer once you lift off in moderate conditions..
Makes me believe steady-state with no engine connected at all results in understeer..




[Edited by roee - 4/19/2002 6:06:14 PM]

ex-webby

hi all

sorry I haven't been able to digest any where near all of this... i've just scanned and picked up on one thing...

if you set a car up to be truly neutral, it require talent to degrees unknown to most human beings to keep them on the road. Almost completely without exception ALL road cars are set up to understeer in a theoretical steady state.

If you were truly on the limit of the front and rears at the same time ANY change in any of the controls would cause either understeer or oversteer which would immediately require instant and accurate action to control.

McLaren did some tests setting up F1 cars truly neutral and at the time none of the F1 drivers that tested it could cope with it and they all hated it, apart from senna who was blisteringly quick until the rears melted or he spun.

I have been told, although this is not official that Schuey had a similar experience to senna.

Also bare in mind that the inside tyres cannot provide as much grip as the outside tyres in a bend. So you can only use the overall grip provided by the two.

In the same way as steady state calculations cannot be applies to a real life moving car with 100% accuracy, a simple tyre slip angle and traction circle calculation cannot be used to determine an entire car's grip level unless you take all other factors into consideration (of which there are countless).

Sorry if I've missed some important stuff.

All the best

Simon