Inside rear wheel unloading

[Magpie]

Before the recent session at QR I swapped the whiteline front sway bar out for the OEM to reduce its tendency to understeer. The car is now running both OEM sways, 9/6 kg spring rates.

Whilst the understeer has been reduced I experienced for the first time on the QR sprint layout the inside rear wheel lifting off the ground going through the dipper. Since I have a torsen rear diff this means that I have no drive out of the corner. At the moment this is the only layout where this has happened.

Now I could just live with this and drive to suit, that is make no changes until my lap times are consistent. Interestingly the lap times were more consistent with the current setup, but not faster than previous. Or I could make another change and test again.

My thoughts would be to either put back in the larger rear sway which will increase roll resistance and oversteer or I could increase the rear spring rates for the same effect. However the sway bar change will only effect the handling in a corner whereas the springs will effect handling in both a corner and straight line.

I'm leaning towards doing nothing and give the driver another session to get used to it and just tweak rebound/tyres. Thoughts?

[The American]

I would have thought that a stiffer rear bar would increase the tendency to lift the rear as it would further limit droop. You could try unhooking one end of the rear for part of the next session.

Mine does the same thing with the two factory bars in some corners. Giving up a little steering input usually brings back drive.

Ultimately, a clutch pack diff would solve.

[hks_kansei]

I would have thought that a stiffer rear bar would increase the tendency to lift the rear as it would further limit droop.

While the swaybar limits droop, it also limits the ability of the car to roll which means there's less need for the extra droop.


I don't know what would be the better option, but I agree that the easiest is to just pull out the endlinks and cable tie the swaybar out of the way and see if it makes the problem better or worse.

[rascal]

Before the recent session at QR I swapped the whiteline front sway bar out for the OEM to reduce its tendency to understeer. The car is now running both OEM sways, 9/6 kg spring rates.

Whilst the understeer has been reduced I experienced for the first time on the QR sprint layout the inside rear wheel lifting off the ground going through the dipper. Since I have a torsen rear diff this means that I have no drive out of the corner. At the moment this is the only layout where this has happened.

Now I could just live with this and drive to suit, that is make no changes until my lap times are consistent. Interestingly the lap times were more consistent with the current setup, but not faster than previous. Or I could make another change and test again.

My thoughts would be to either put back in the larger rear sway which will increase roll resistance and oversteer or I could increase the rear spring rates for the same effect. However the sway bar change will only effect the handling in a corner whereas the springs will effect handling in both a corner and straight line.

I'm leaning towards doing nothing and give the driver another session to get used to it and just tweak rebound/tyres. Thoughts?

Stiffening the rear (either through sways or springs) would be the opposite of what you want. The inside wheel is lifting as the car is softer at the front relative to the rear (the whole car is rolling onto the outside front and taking the inside rear with it)

To fix you would need to soften the rear to allow more droop and allow the inside rear to stay in contact with the ground, or preferably, stiffen the front to reduce the whole car rolling.

I do understand you just reversed that to resolve understeer, but with a torsen rear end that needs both wheels fully loaded to work properly you may have to sacrifice somethings to get it to work correctly. ie put the front back as it was and adjust your driving style to reduce the understeer you previously had. (You also may find that the understeer was not a function of the cars setup but rather the driving style..)

I'd also look into the aero balance of the car, as the understeer may have been caused by too much rear wing rather than spring/bar setup.
Therefore changing the springs/bar to bandaid the aero inbalance may have now introduced the wheel lift.

[Magpie]

Thanks Rascal you raise some good points for me to consider.

[rascal]

Also you could try carrying more (or some if you are not using any!) brake into the apex to keep the front tyres loaded and give you the front end grip to help it turn

[RS2000]

I found the heavier rear bar makes it worse - as per American's & rascal's post, the car rolls & the inside wheel lifts with it. I'd suggest trying even heavier springs all round, maintaining 3:2 F:R ratio with OE sway bars.

Cheers

[project.r.racing]

It is set up like a FWD car. It is not a FWD car.

I'd stiffen up the front and loosen the rear.

[Dan]

Stiffening the rear (either through sways or springs) would be the opposite of what you want. The inside wheel is lifting as the car is softer at the front relative to the rear (the whole car is rolling onto the outside front and taking the inside rear with it)

To fix you would need to soften the rear to allow more droop and allow the inside rear to stay in contact with the ground, or preferably, stiffen the front to reduce the whole car rolling.

I agree stiffening the front would help (while also increasing understeer).

Thinking aloud here but by lowering the spring rate you increase droop because the spring compresses more with the same weight (the suspension goes down more when compressed and then increases the difference between the car in it's compressed and uncompressed state) but if you soften the spring rate you also reduce roll stiffness which will counteract some of that droop.

Before reducing spring rate I'd check how much the base height adjuster has been used to lower the car as this will impact the amount of droop that the suspension allows without requiring changes to the spring rate.

[madjak]

I get a similar issue with the torsen diff. I don't think my inside wheel is actually lifting but it's unloading enough for the torsen to slip. This is occurring under throttle so it's hard to fix with different lines, braking to the apex etc because I'm typically past that point in the corner. I think the only real solution is a clutch pack diff.

I run a 22mm mazdaspeed bar on the front and a stock bar on the rear. I've had no issues at all with understeer.

[madjak]

+1 Dan... it might also be worth looking into running helper springs so that you can get a bit more shock travel. I'll report back after next weekend to see if my torsen slippage is better or worse with my new rear suspension.

[Magpie]

Thanks again to the replies. Madjack your description of the situation is exactly what I'm having. It is trying to get the inside rear wheel enough resistance to work as you accelerate out of the apex. I'll dig a video out and post it as well as do some calculations on the weight transfer.

The coilover length has been set to maximise travel, height is set by preload.

[madjak]

The problem with higher rate springs is that the weight of the car doesn't compress them enough to give you droop. Your 6kg/mm springs will only be compressing 30mm on each side which is basically the only droop you get. Unless you let the spring fall off the perch or run a helper you can't get any more.

A helper will add 40mm or so of additional droop at a much reduced spring rate but its enough to keep the tyre on the road.

[hks_kansei]

Just curious here, but how do springs affect droop?

The springs are only held onto the perches by compression, so shouldn't the maximum droop be dictated by the shock length (and piston travel)

If you have the car on stands and have say incredibly stiff springs, then remove them and have no springs on the same shock the maximum droop would be the same surely? (of course, the ease in which the car can compress the setup to it's maximum is different)


Granted i'm assuming you're measuring droop from a static point, like say the diff centreline, rather than measuring it from max compression to max extension (which would be travel rather than droop?)

[smy0003]

If the spring was stiff enough and the unsprung mass light enough, wouldn't the spring prevent the shock from reaching its maximum droop?

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