Differential

Hi all! I was messing about 2 nights ago in the BF1 when I decided to try and learn the Clutch Pack differential. So, at first, I put it at maximum locking power (80%) which made the car oversteer quite a bit on the throttle and then made it loose as possible (10%) which made the car understeer. Now, I understand how a differential works, more locking means that the differential will make the drive wheels spin at similair speed and vice versa.

However, something that puzzles me is that if the differential is loose, meaning that the inside wheel will spin at a higher speed than the outside wheel, why does the car understeer, I mean, isn't there less grip at the back if one wheel is losing traction and all the strain is being thrown onto one wheel? and vice versa with a locked differential.

FWD oversteer

Another thing that I tried recently was messing about with setting up a FWD car (i.e.: XF GTi). Like anyone who is trying to learn setup , I tried extremes with the suspension. From what I understood and read about suspension:

Soft front suspension: more oversteer in the phase of deceleration (i.e.: braking for a corner, lifting off the throttle)

Stiff front suspension: ^ Opposite

Soft rear suspension: more traction or understeer on the throttle or when the weight of the car is pushed towards the back.

Stiff rear suspension: ^ Opposite

I know that my setup knowledge is very basic but thats the reason why I'm messing about so that I can learn .

These rules apply fine when driving a RWD or 4WD car but whats confusing is me is that it seems that the opposite of all the above (i.e.: Stiff front suspension: oversteer) happens with FWD cars. I was told a while ago to look at in the way that since the drive wheels are on the other end when comparing it to a RWD car then the setup changes would flip around but I mean, I think I understand weight transfer so it just doesn't make sense to me.

Thanks!

...I thought that if the car tends to oversteer into corner soft front is needed, also, when exiting apex hard rear is needed to keep the front tyres on the ground as weight is transferred....opposite of what u said above

Quote from Leprekaun :

However, something that puzzles me is that if the differential is loose, meaning that the inside wheel will spin at a higher speed than the outside wheel, why does the car understeer, I mean, isn't there less grip at the back if one wheel is losing traction and all the strain is being thrown onto one wheel? and vice versa with a locked differential.

With 10% power locking, you have an almost open differential, so each driven wheel will get the same amount of torque. When you apply power and spin the inner wheel, sure it will not give as much grip, but then during cornering there is little in the way of load an it anyway. Since you cannot apply (much) torque to a spinning wheel, the outer wheel does not get much torque either, so has loads of grip. Hence no oversteer.

With an 80% locked diff, that inner wheel won't really spin much before the outer tyre does, so basically you are spinning both together. It's spinnig the outer tyre that will loose the grip at the rear, hence the oversteer. Also to consider is that, even without wheelspin, the outer tyre is getting far more torque, so the difference in forces at either side of the car creates a turning moment at the rear of the car, hence more oversteer.

Quote from Leprekaun :

FWD oversteer

Suspension needs to be broken down into the two main components; springs and dampers. If you increase both in proportion to one another (so that the damping is kept at the same percentage of critical), there will be no extra affect on transient situations. Springs (and anti-roll bars) affect the roll stiffness at all times, so the basic handling of the car during both steady state and transient cornering. Stiffer at the front increases understeer and vice versa. Damping only affects transients, and alters the balance during these transients from the static balance as created by the springs. So if you have your dampers set to give understeer during corner exit, but the springs and ARBs give lots of oversteer, you'll still get oversteer, just less so.

Of course the suspension is not the only thing that contributes to the under/oversteer balance of the car, you have to consider diffs, tyres, aero, and (when you're using them) brakes too.

Thanks for your detailed reply Bob . I'm still trying to understand it but I'm getting there .

About the differential, if you can just confirm that I understood it properly , when the diff is loose, because the inside driven wheel isn't getting much torque due to a loss of traction, the car balances the torque on both wheels? (i.e.: Inside wheel is producing 275 Nm of torque so the outside wheel has to be 275 as well?). I thought that if the diff is loose then the torque would be uneven between the 2 driven wheels, hence no real adjusting or locking is occurring.

I just thought I'd mention that during the tests with a FWD car, I only changed springs, nothing else so the setups are identical except with opposite suspension settings.

Forgive me if you've stated it in your post but I still don't get why having stiff front suspension (stiff springs and stiff dampers) on a FWD car gives oversteer and vice versa.

Quote from Leprekaun :

Thanks for your detailed reply Bob . I'm still trying to understand it but I'm getting there .

About the differential, if you can just confirm that I understood it properly , when the diff is loose, because the inside driven wheel isn't getting much torque due to a loss of traction, the car balances the torque on both wheels? (i.e.: Inside wheel is producing 275 Nm of torque so the outside wheel has to be 275 as well?). I thought that if the diff is loose then the torque would be uneven between the 2 driven wheels, hence no real adjusting or locking is occurring.

I just thought I'd mention that during the tests with a FWD car, I only changed springs, nothing else so the setups are identical except with opposite suspension settings.

Forgive me if you've stated it in your post but I still don't get why having stiff front suspension (stiff springs and stiff dampers) on a FWD car gives oversteer and vice versa.

About FWD, did you tried to soften suspension and loosen dampers just a bit? It might help you to kill the underste. On my basic FXO setup, my spring have 85.3Kn/m stiffness but the dampers are (in order) 11.7 and 9.6. I really find it easilly to put the nose of the car where you want, and it doesn't feel too much unda-unda. Try it and tell me if it works for you.

Edit: Setup attached.

Quote from Leprekaun :

About the differential, if you can just confirm that I understood it properly , when the diff is loose, because the inside driven wheel isn't getting much torque due to a loss of traction, the car balances the torque on both wheels? (i.e.: Inside wheel is producing 275 Nm of torque so the outside wheel has to be 275 as well?). I thought that if the diff is loose then the torque would be uneven between the 2 driven wheels, hence no real adjusting or locking is occurring.

With an open diff, both wheels *always* get the same torque. The catch with this is that if a wheel is in the air or only has very light ground contact, almost no torque can be applied to it, hence the other loaded wheel also only gets the same, very little torque.


Regarding the setups in general, I think it's unfortunately a lot more complicated than learning "soft springs = X, hard springs = Y". How a car behaves NEVER relies on one setting alone, it is always multiple settings influencing the output together, and more importantly the influence a setting has can completely revert when combined with another setting.

Take as example differential locking and anti-roll. For FWD cars it is often said that a locked diff will greatly hamper turn-in (which it does) and high front ARB will cause understeer (which it does). Then why do so many WR setups use exactly these settings? Because if you combine both you will get a setup that 1) lifts the inner wheel into the air when turning because of the stiff front ARB, and 2) is able to apply full power to the outer wheel due to the locked diff, which at the same time loses its understeer tendency because the inner wheel is off the ground, thus not resisting turning any more. So you combine two settings that should cause massive understeer yet result in the opposite, the infamous power-oversteer in FWD cars.

I think what helps best is if you're able to visualise in what each setting actually does mechanically, so you can play through different race situations in your mind. What happens if you drive over a bump mid corner? What happens if you turn in fast, what if you do it slow? How does the combination of two or multiple settings influence the behaviour in corner entry, -apex and -exit?

Learning that A does X, B does Y and A+B result in Z will just lead to an enormous amount of rules that in extreme situations or when combined with other settings can completely break down anyway. Learn what a setting does, what effect it has in a given situation and environment but most importantly, understand WHY it has that effect. Only then will you be able to combine different settings and correctly predict their outcome. Or you just drive lots and lots of laps changing things at random till the right behaviour comes out, but tbh I find the former method more satisfying

I do try to visualise everything mechanically but thats not the point. I want to know why stiff front springs with soft rear springs makes oversteer because as I tried to visualise mechanically, it didn't make sense to me. As the car's weight is thrown at the back, softer rear suspension should mean that the car has more traction, RWD or FWD because when you apply throttle, the weight will always go to the back. So why would softening the rear springs or stiffening the rear springs cause oversteer? the same logic also applies to the ARB (soft rear should mean more grip at the back but it increases oversteer so thats a lack of grip there). Maybe the ARB bit is a bit more complex since it involves the idea that when you're cornering, a softer rear ARB means that there will be more weight transfer that will exceed the saturation level of grip but if this was true then how come the car understeers with a stiff ARB?

Lep - read this to sort out ARB and weight transfer effects in your head. I think the discussion in that thread covered everything.

Quote from AndroidXP :

"Take as example differential locking and anti-roll. For FWD cars it is often said that a locked diff will greatly hamper turn-in (which it does) and high front ARB will cause understeer (which it does). Then why do so many WR setups use exactly these settings? Because if you combine both you will get a setup that 1) lifts the inner wheel into the air when turning because of the stiff front ARB, and 2) is able to apply full power to the outer wheel due to the locked diff, which at the same time loses its understeer tendency because the inner wheel is off the ground, thus not resisting turning any more. So you combine two settings that should cause massive understeer yet result in the opposite, the infamous power-oversteer in FWD cars.

I'm really sorry if I'm a pain... but I don't get this!

Applying power to the front wheels which are already at the maximum grip threshold would give you more understeer. Even if you have grip to spare applying power would give you a weight transfer to the rear and give you more rear grip.

I always thought (and have some Civic VT-91 track-day car tuning experience as well) the trick to make less understeer with a FWD is:

Harder rear springs and ARBs
Damping set to "keep" weight transfer to the front wheels (more front rebound and less bump, less rear rebound and more bump)
And of course tire pressure and camber/caster to match above as well as ride height (raise rear will give you less understeer).

But this "infamous power-oversteer in FWD cars" I never heard about (IRL) but I do ofcourse experience this in LFS.

Would be greatly appreciated if someone could enlighten me.

AndersL, you're just not taking everything into account. With front roll stiffness much higher than the rear, the inside front tyre will lift off the ground. With an open diff you won't be able to apply much torque to either wheel as in the inside wheel will spin, leaving just the understeer from the suspension. With a locked diff, you can apply all your engine torque to the one wheel that is on the ground, there will be no additonal understeer due to the locked diff (as the inside tyre is not on the ground to cause scrub), but there will be some oversteer generated from the turning moment caused by only applying torque to one side of the car.

On top of this, you can further counteract the understeer from the suspension by losing a little grip from the rear tyres (for example by increasing the pressure).

Thanks Bob, I think I understand the theory now with the power oversteer with locked diff.

Hmm...but I have to say that I feel that the understeer with this kind of setup (stiff front and soft rear) in corner entry (without power) is not very evident. Sometimes it even feels neutral, which I definately don't think it would.

Another thing which I believe is questionable is if a car with this kind of setup (stiff front lifting inner wheel and soft rear + locked diff) actually would be faster (IRL) than a mild LSD and opposite setup with both front wheels planted in the ground (on a racetrack)?

Thanks for the helping me understand the power oversteering logic, and even if it's not realistic it's much more fun to drive FWD with it

Quote from AndersL :

Hmm...but I have to say that I feel that the understeer with this kind of setup (stiff front and soft rear) in corner entry (without power) is not very evident. Sometimes it even feels neutral, which I definately don't think it would.

Have you tried it? Off power such a setup understeers like crazy, that's why you see people always applying a little bit of throttle to negate the understeer...

Quote from AndersL :

Another thing which I believe is questionable is if a car with this kind of setup (stiff front lifting inner wheel and soft rear + locked diff) actually would be faster (IRL) than a mild LSD and opposite setup with both front wheels planted in the ground (on a racetrack)?

Chassis flex would make such a setup even harder to make in RL, as it would twist and both front tires would still be on the ground, giving horrendous understeer, so you'd have to go even more mental than in LfS... Maybe it would be faster, but due to the excess stress such a setup would put on the cars it'll be likely to break something after a short time...

Quote from Leprekaun :

Differential


However, something that puzzles me is that if the differential is loose, meaning that the inside wheel will spin at a higher speed than the outside wheel, why does the car understeer, I mean, isn't there less grip at the back if one wheel is losing traction and all the strain is being thrown onto one wheel? and vice versa with a locked differential.

The simple reason is this. The outside rear wheel is the one with all the load. Therefore it is this wheel that is providing all your rear end grip. If the diff is open then the inside wheel is allowed to spin. This means the outside wheel will not spin so it maintains grip. This will create more understeer than the opposite situation as I will now explain.

If the diff is locked then as you apply more power the inside wheel is now unable to spin. Eventually, the power will exceed the grip of the outside rear tyre too and then this wheel will start to spin up. This will result in sudden oversteer as the wheel that was providing all the grip is now spinning and not gripping.

Hope this is clear.

Quote from bbman :

Have you tried it? Off power such a setup understeers like crazy, that's why you see people always applying a little bit of throttle to negate the understeer...

Yep, I've tried it many times since I use the inferno setups. I've tried to make a own set several times but since the normal suspension logic has not been able to apply (if you want to be fast) I have always ended up with the inferno power oversteer set-up and on the same time altering my racing technique compared to RL and non power oversteer setups.

Quote from bbman :

Chassis flex would make such a setup even harder to make in RL, as it would twist and both front tires would still be on the ground, giving horrendous understeer, so you'd have to go even more mental than in LfS... Maybe it would be faster, but due to the excess stress such a setup would put on the cars it'll be likely to break something after a short time...

I completely agree that chassi flex play an important role in the complete suspension setup, although having both front wheels in the ground would actually give you more front grip (less understeer) if you have a mild/medium LSD. I can't say how it would react with locked diff since I have only tried that on ice.