Why soften front/stiffen rear to fix understeer?

  • Thread starter dwillms
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dwillms

11
Hello,

This question comes from reading the Tuning Guide, and the problems I have been having fixing the understeer on my Subaru WRX STi.

I have not been able to understand the example listed in the guide, so I am hoping somebody can give me a different explanation that I might be able to understand.

Most of us are already familiar with the terms understeer and oversteer, but let's start thinking about them in a different way now. With an understanding of how tires produce tractive force and why, let's define understeer as when we are putting too much download over the front tires and oversteer as too much load over the rear tires.

Now, let's pretend we stiffen the front springs. What does this do to our car when we take that turn?

/*700lbs*----800lbs]
<-front-------rear--+
\-100lbs-----400lbs]

The stiffer front springs have caused MORE weight to move to the outside front tire. Suddendly, we have a problem: the front tires can only handle about 600 lbs of download before they start to slip. 700 lbs. is far too much. RESULT: our race car understeers.
Click to expand...

First of all, wouldn't stiffening the front suspension improve the weight transfer, and in this example, woulnd't that cause LESS weight to be transferred to the outside tire, and not more?

I understand that the more weight put on a tire the better it will handle, but only up to a certain point. In this example, the RF tire has too much download, causing it to slip and lose traction (understeer). So, my thinking goes, if you already have too much weight on RF, if you soften front/stiffen rear, you would be increasing the weight transfer to that tire. It seems to me the whole point for stiffer springs is to decrease weight transfer, and improve handling, so that is why this seems to go against all logic for me.

If anybody has a good example/diagram to explain why this is, I would love to take a look at it. The simpler the better, because I think the more physics terms and numbers are thrown around, the more confused I will get!

Thanks for your time,
-Dustin
 
ok, its like this. say, you have 6KG/mm spring in the front, and a 10KG/mm in the rear. the higher the spring rate(stiffer) the more weight that end will support. so the front end wont have as much weight on it as the rear, thus improving grip in the front.
 
Understeer means: less grip in the front than in the back. Which is a problem for nearly all FF cars.

The solution in GT3 (and perhaps also in GT4) is : soft front spring settings (8 to 12) and very hard rear settings (17 to 20). This gives less grip to the back, which gives the car a more neutral behaviour. For bound and rebound, apply the same: 5 boud, 6 rebound in the front and 9 or 10 bound and 10 rebound in the back gives very often a good result.

I don't know whether this works in real live, but I bought the French review "AutoHebdo" today, in which there was a test of a Citroën C2 JWRC. The journalist said that the back settings are very stiff.

Other tip to avoid understeer on FF and 4WD: adjust your gearbox such that you dont come out of corners whiith too much torque. For instance, on Midfield I take setup which permits to take the big corner with low revs in 4th instead of whith high revs in third.
 
Thanks Suzuki for the explanation, that makes a little more sense. But I still stick by what I am saying, I just can't understand why softening springs would decrease weight transfer instead of increase.

Instead of thinking about lateral weight transfer, maybe think about simply stopping a car, and front-to-rear weight transfer. If I am putting too much weight over my front tires, and I would soften up the front suspension in relation to the rear, would this again not add MORE weight over the front, and not less? Where stiffening the front would help weight transfer, and result in less weight over the rear.

Again, not meant to be an argument, just trying to explain why I think the way I do, and trying to figure out where I'm going wrong.

-Dustin
 
[edit]never mind im confused[/edit]

so someone help me out here...
say we have a car with 50/50 weight distribution
same spring rates front and rear

say during breaking it becomes 30/70

so for every centimeter the front springs compress, the rear has to decompress. correct?

now say the front is stiffer and the rear is the same as before. that means that there will be less body roll front to back right?

but if there is more weight in front becuase the front springs are stiffer, that means that theres less weight in the rear to decompress the rear springs...which means that the rear will lift up higher than before?

sorry.. i am trying to rationalize this physically interms of the rotational torque about the car's center of gravity.
 
Yes, it is confusing, but don't get caught up with the front/rear thing. It's lateral transfer that's important.

Each tire can take 600kg (in their example) download before the contact patch distorts and it begins to lose grippiness. Before stiffening the front springs, both front tires shared the burden in a turn of moving the car in line with the wheels. After stiffening, more weight is transfered to the outside faster.

No big deal until you reach the limit. In their example, there's 800kg up front:

400 & 400 (straight line)
500 & 300 (light cornering -- no problem)
600 & 200 (med. cornering -- outside tire at limit)
700 & 100 (hard cornering -- outside tire distorts beyond limit)

Again, stiffening the front springs only means this happens faster. Now the outside tire is tasked with supporting more front-end weight right off the bat:

400 & 400 (straight line)
550 & 250 (light cornering -- no problem)
700 & 100 (med. cornering -- outside tire already beyond)

Only after the front end breaks away do the rear tires (and rear weight) come into play. If the rears are still within their g-force limits (download being one factor)*, they will continue to counter the lateral forces as they have been, while the fronts won't: understeer.

*It must be said that a tire may be outside it's download limits, but not skid if the lateral forces aren't yet great enough. This example does not address to amount of lateral forces at play, only that they are there.

Another source of confusion is our terminology. We might say that a tire that has lost its grip is in a skid, but here they just mean that it has lost its grippiness (a lowering of its coefficient of friction).

Skippable:

Tightening up the torsion bar will help offset the effect of stiff springs to an extent, by moving the burden inwards or "sharing the load". However, both wheels will now act more as one, which can be a problem over, say, rumble strips; the vibration on the inside tire is transfered to the outside, causing a loss of grip on the outside tire (bouncing even though it is on smooth pavement). There are probably other effects, too, but someone smarter will have to address these.
 
Midget_I
After stiffening, more weight is transfered to the outside faster.
Click to expand...

ahh that makes alot of sense...
so with stiffer springs, there is more weight transfer per length of compression... so its faster and thats the key.
 
Ok, that makes a lot more sense. That a harder suspension will not transfer any more or less weight, just transfer it quicker (compared to softer suspension). I guess that would make sense explaining why a soft-suspension car feels "floaty", because it takes a long time to transfer weight.

Why does common sense dictate then that people need to buy stiffer springs to "improve" the handling of their car? I know in most cases, these sport springs or coilovers will also come with a lower ride height and center of gravity, but is there any truth to this generalization? Yes, you will decrease body roll with a harder suspension, but all else being equal (ride height, tires), will you generally also improve your handling capabilities?

-Dustin
 
dwillms
Ok, that makes a lot more sense. That a harder suspension will not transfer any more or less weight, just transfer it quicker (compared to softer suspension). I guess that would make sense explaining why a soft-suspension car feels "floaty", because it takes a long time to transfer weight.

Why does common sense dictate then that people need to buy stiffer springs to "improve" the handling of their car? I know in most cases, these sport springs or coilovers will also come with a lower ride height and center of gravity, but is there any truth to this generalization? Yes, you will decrease body roll with a harder suspension, but all else being equal (ride height, tires), will you generally also improve your handling capabilities?

-Dustin
Click to expand...

Basically, that's it. You get to lower your center of gravity without the suspension bottoming out.

I don't know about your "all things being equal". If it were possible to lower your ride without stiffening (in a computer model maybe) I'd be interested to know.

The fact that sway bars exist to partially reverse this transfer of weight is the key I suppose.
 
well i guess with softer springs, the weight of the car reacts more slowly... maybe you will start sliding before you can transfer enough weight to make the tires turn eficiently...

generally i guess stiffer springs will improve handling by improving the response [time] of your car.
 
Good point, Jaded. Having weight shifting around like crates on a truck would lead to unwelcomed surprises, esp. in chicanes and such. Your wheels'd be going one way, while the rest of the car was going another.
 
So, to reiterate my previous post, am I correct in understanding that a harder suspension will not transfer any more or less weight, just transfer it quicker (compared to softer suspension)?

I think you misread my question a little Midget. I understand that in order to lower ride height, a stiffer spring will be needed to offset the loss in suspension travel. My question was though, disregarding the ride height reduction commonly found in purchasing stiffer "sport" springs, would they result in any other benefits when compared to a softer spring, other than a reduction in body roll?

But Jaded answered that for me I believe by pointing out the same thing I did. That stiffer springs result in better response times by transferring weight quicker, so as to decrease the "floaty" feel and slow weight transfer times in softer springs. Which, as you stated, would be a total mess in chicanes and quick, consecutive corners.

Thanks again for the help,
-Dustin
 
I'm no authority. I really don't know what further benefits are gained besides those stated.

I was just responding because the same thing had occurred to me. To wit, it seemed counterintuitive that tightening the back end makes a car loose. And with all the talk of front to back weight distribution, it appeared no one was really getting to the heart of your question.

It's really about side-to-side distribution, independent of front-to-back. There's really no need to address the other axle unless its counterpart has breeched its lateral g-force limit, in whatever fashion. (You already know this...I'm mostly talking to myself.)
 
Just to add a little more confusion to the pot, stiffening the springs or softening them does affect the amount of weight transfer rather than the speed of transfer; it is mainly adjustments to the Dampers that alter the rate of transfer.

Stiffening the springs at a given end of the car also works to reduce grip at that end and reduces body roll (thus improving handling response).
 
That's an interesting question Mr. X.

The general rule of thumb is that you run the suspension as softly sprung as you can without bottoming out and adjust the handling with the Dampers, Stabalisers, Ride Height, Camber etc.

However, it is also a rule of thumb to never soften the suspension just to get more grip, you can utilise weight transfer (lateral or pitch) to get that and WT is controlled by the Dampers with some assistance from the Stabalisers (for lateral) or Ride Height (for pitch).

I'm afraid I'm not an expert at tuning FF cars (see my sig for a clue why :D) so I hesitate to give detailed settings advice on them. Essentially, an FF is always going to understeer no matter what you do unless you are prepared to make major sacrifices in other performance areas.

An easy fix is to simply fit stickier rubber to the end you want to grip but that is a procedure fraught with pitfalls with the accelerated tyre wear modelled into the game. Another easy fix is to manipulate the tyre pressures ... oh wait, we can't do that in the game :(. Also, getting the camber spot on will help ... but, again, the game doesn't make that easy to do.

P.S. Why on earth did I get the impression this thread was about an FF car :embarrassed:. Still, the comments are still equally valid for a 4WD so it wasn't a total waste of time :slinks away:
 
Well what i was asking was more about FR or MR cars that lose front grip easily. Such as my Buick GNX.
 
Ah ... that's more my sort of territory, or at least the drive-train is anyhow. I'm afraid that until next week I won't be able to do any practical spannering as we poor PAL'ers don't get GT4 until the 9th March :(.

When you talk about losing front end grip, do you mean going into or coming out of corners?
 
Good discussion gentlemen. But there's a couple of things that is worth keeping in mind.

Why does making one end of the car stiffer promote loss of traction in that end? The answer sounds counter-intuitive, but every reliable source of information I have read unequivocably says this is so. A stiffer spring WILL cause more weight to press down upon it than a softer spring at the other end of the car.

Try to picture this: you have some ballons inflated to identical sizes. Two are filled with water (our soft springs) and one is filled with sand (our hard spring).

Let's start with our water filled ballons. Say you put them on flat ground about a foot apart. Now, take a peice of plywood and lay it on top of the two ballons (the car). Assuming they are identical and the plywood is equally massive all the way around, our ballons will deform by the same amount. 50-50 weight distribution, right? Now take your hand and press down in the center of the wood. They will continue to distrubute whatever load you put on them in equal amounts. Push down with 10 lbs of force, each ballon will resist with 5 lbs. each.

Now replace one of the soft water filled ballons with our hard sand filled ballon. What happens? The sand ballon will deform much less, causing your wood to slope slightly. The harder spring will bear more download, because it yields less than the softer one. If you could slip a minuature scale under each ballon, you will see one appears to weigh slightly more than the other.

Here's another, somewhat more tongue-in-cheek illustration. Imagine you are sitting on the shoulders of two friends (just bear with me.. there is a point to this). One guy works out 5 days a week, is on the crewe team, is 6'2" and 240lbs. The other guy eats Cheetos, drinks Jolt, plays Counter-Strike in the basement all day and is 5'11" and 120 lbs. Now ask yourself: who is going to end up supporting more of your weight as you sit on them??

Okay, but what's the point of this, you ask..

Here's the counter-intuitive part. Even though the stiffer end of the car can generate more grip than the softer end, because there is greater weight on the stiffer end, it will reach it's limit of adhesion BEFORE the softer end.

Remember that a tire will grip in proportion to how hard you press on it. After a certain point, it gives up and begins to slip. The end with more weight on it will slip first (provided all tires are identical). If this is the front end, you have understeer. If this is the back end, you have oversteer. It's that simple.

This is to the best of my understanding, but I freely admit I'm no suspension expert with a degree in engineering... I've just read a bit on the subject. I hope that helps but if someone has a better, more accurate explanation of this, I'm all ears.

The other thing we should keep in mind is that in racing, you very rarely have equal weight transfer from side to side. In other words, you are almost never just turning. You are usually turning and braking or turning and accelerating OR lifting and getting ready to accelerate.

And in racing, the goal is to transition from braking to throttle as soon as, as oftern as you can. For this reason, you should always consider fore-aft weight transfer in addition to laternal weight transfer when you tune.


M
 
I think that a higher spring rate in the rear PREVENTS an much weight being shifted to the rear under acceleration .
 
Illuminating parable M-Spec 👍.

That is probably the shortest epistle on the subject I've read and gets the point across very effectively.

Like you, over the past few years, I've learned all my suspension theory from books rather than on the race track but, as we seem to reach the same conclusions, that gives a measure of veracity to the research I hope :crosses fingers that we haven't just read the same books: :lol:.

For everyone who wishes to broaded and deepen their understanding of suspension tuning, I would highly recommend visiting this is site:

http://www.smithees-racetech.com.au/

They have distilled most of the knowledge it took me years to glean into a few pages on their website. There's plenty of deeper background that you can research but this is splendidly accessible stuff (they leave out all the maths and equations that scare most people off from learning more).
 
sukerkin
Illuminating parable M-Spec 👍.

That is probably the shortest epistle on the subject I've read and gets the point across very effectively.

Like you, over the past few years, I've learned all my suspension theory from books rather than on the race track but, as we seem to reach the same conclusions, that gives a measure of veracity to the research I hope :crosses fingers that we haven't just read the same books: :lol:.
Click to expand...

Thanks :)

Actually, I did learn a little from practical application... but BMW did most of the work for me. All I did was play with the adjustable stuff (dampers & swaybars).

My old SCCA ESP M3.... long gone.

ElsaM3SCCA5.jpg


They have distilled most of the knowledge it took me years to glean into a few pages on their website. There's plenty of deeper background that you can research but this is splendidly accessible stuff (they leave out all the maths and equations that scare most people off from learning more).
Click to expand...

Sadly, as interested as I am in this sort of thing, math has never been my strong suit. First time I saw Reckman's Physics of Racing Series I got MEGO within 2 pages... :indiff:


M
 
:envy:

Do my eyes deceive me or am I seeing a pronounced forwards rake on that there Bimmer with just a hint of opposite lock (DR1FTER :P) :D?
 
sukerkin
:envy:

Do my eyes deceive me or am I seeing a pronounced forwards rake on that there Bimmer with just a hint of opposite lock (DR1FTER :P) :D?
Click to expand...

:lol:

Actually, the rake is from me on the brakes.. I was probably turning my head there so I can see the apex I just blew by. :indiff: That car required a lot of trailbraking because the tire set up I used caused MASSIVE understeer (no much choice because of sizing options) .... Drifting would have been less embarassing... :dopey:


M
 
You need weight transfer to generate grip, it's a simple fact. Too much weight transfer will overcome your mechanical grip eventually but making the car too stiff will not allow you to generate enough grip. Yes softer springs = more grip, stiffer springs = more response, but I tend to tune on the side of softer springs and adjust response with my damper settings.

So if you want to induce oversteer or understeer, you can either go too soft or too stiff at the end you want to lose grip first. Too soft will have a lower limit for mechanical grip. Too stiff won't allow weight transfer. Each spring rate has it's own range of grip for example. But it depends on how much weight is on it. Spring rate x will have a higher range of grip than spring rate y but only when both springs are carrying the same load. Throw an engine over one end of the car and it changes. Throw that car around the corner and it changes even more.

Damper settings determine how fast either axle responds to weight transfer. Weight transferring to the rear too soon (ie. too early in the corner)? Increase front rebound and increase rear bound settings.

Also remember ride height makes a huge difference in weight transfer. Lower ride height means less weight transfer so you can run with less spring rate, less camber, less roll bar. Opposite applies obviously with a taller, higher cg vehicle.

Unfortunately, the game doesnt allow you to corner balance the car or even balance the car. Why bother giving you weight and ballast setting if you can't see how much weight it adds over each axle?
 
///M-Spec
Now replace one of the soft water filled ballons with our hard sand filled ballon. What happens? The sand ballon will deform much less, causing your wood to slope slightly. The harder spring will bear more download, because it yields less than the softer one. If you could slip a minuature scale under each ballon, you will see one appears to weigh slightly more than the other.
Click to expand...

I believe what you've described here is what racing technicians actually do at oval tracks.
 

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