Why you need a LSD if you like cornering!

[mike156]

This thread has been great and is really making me rethink some changes I had in mind for the diff.

There is lots of talk about roll stiffness from sway bars here, but wouldn't adjustable control arms to change the roll center height potentially be more effective in dealing with the F/R roll stiffness bias?

When you lower the car, the roll center likely drops considerably more up front than out back? This puts the roll angle down towards the front, increasing the relative roll stiffness at the rear. Wouldn't raising the roll center back up to level out with the change in the rear be fairly effective in restoring the roll stiffness balance? Seems like sorting out geometry first would be the better approach as it doesn't affect ride rate and simply improves the efficiency of the springs and bars?

[7thgear]

Originally Posted by mike156

This thread has been great and is really making me rethink some changes I had in mind for the diff.

There is lots of talk about roll stiffness from sway bars here, but wouldn't adjustable control arms to change the roll center height potentially be more effective in dealing with the F/R roll stiffness bias?

not even close


as control arms don't change the roll center height.. their relative pickup points do. So it would be up to the drop ball joints or custom crossmembers.


so at most is you could have a longer control arm but it's effect would be very minimal because you run into strut angle and scrub radii issues.


I did some basic math a long time ago and essentially the biggest contributor to a reduced weight transfer is CG and raw static weight.


more often than not you can actually afford to have a fucked up roll center... which is why anyone who lowers their car swears that it handles better.. because the reduced damper travel and fully compressed bumpstops effectively limit the body roll and force the tire to do all the work. This super stiff state feels "sporty" and results in increased confidence for most.




second, raising the RC towards the GC would create it's own problems and so isn't the end-all solution.




Swaybars are most effective because they are cheap and adjustable. They keep the car compliant during 2 wheel bumps and provide the necessary end results when cornering. Given the problem of WEIGHT, they are efficient...


a single swaybar to tune the car behavior to your preference is the cheapest way to go about this business.


My FRS was a little to tail happy for my tastes. A used Stranno bar for a very good price transformed the car. I am now 90% satisfied with it's performance given the fact that it is otherwise stock.






HAVING SAID THAT


you are right, the best solution would be to start from correcting geometry. But that is EXPENSIVE since the first step would be to construct custom crossmembers front and back.... and also dangerous because you could really get it wrong.

[mike156]

Ah...
There are two sides to a control arm. Yes, moving the inner pickup points will change roll center. But changing the outside pickup point at the hub carrier has the same effect.

Some front lower control arms can change the outside pivot height. That's exactly what the Robispec arms do...

Yes...front lower control arms can absolutely change roll center height.

[7thgear]

Originally Posted by mike156

Ah...
There are two sides to a control arm. Yes, moving the inner pickup points will change roll center. But changing the outside pickup point at the hub carrier has the same effect.

Some front lower control arms can change the outside pivot height. That's exactly what the Robispec arms do...

Yes...front lower control arms can absolutely change roll center height.

the change happens at the balljoint, not the control arm


if you stack the balljoint attachment bolt, you are changing the rollcenter via relative balljoint pickup


the control arm itself has nothing to do with this


it's only effect on roll center is the increase of it's length, which increases track width and in the case of a macstrut changes the KPA, but again that's something you want to be careful with

[mike156]

The ball joint is effectively part of the control arm. Other than some minor change that you can get from something like the Whiteline RCK, you really need to go to a spherical bearing setup IN THE CONTROL ARM to make this an effective adjustment.

What the hell is Ubersuber smoking thinking the tire only has to deal with 151 ft-lbs of torque though??? It's called a transmission and differential...dur...
151 ft-lbs at the engine is 1122 ft-lbs at each rear wheel in 1st gear. 677 in second, 477 in third...

That is of course stall torque and the rotating weight of everything in the drivetrain soaks up a lot of the available drive torque during first gear acceleration. But to say the tire only has to deal with 151 ft-lbs is stupid.

[7thgear]

Originally Posted by mike156

you really need to go to a spherical bearing setup IN THE CONTROL ARM to make this an effective adjustment.

actually no... you need to go to a custom upright so as to reduce overall stress in the system...


an adjustable balljoint at the control arm is just as much a hack as a modified balljoint itself

[mike156]

I disagree, adjustable spherical bearing based "balljoints" are well proven in various racing classes. Also changing arms has been PROVEN to work on this chassis...but whatever...I'm done arguing over what amounts to semantics.

Point remains, with regards to adjusting roll stiffness in an effort to reduce rear wheel lift...Adjusting roll center height to correct for a lowered ride height would likely help a considerable amount without changing spring rates or roll bars.

How that falls on your list of "cost effectiveness" I couldn't care less.

[7thgear]

Originally Posted by mike156

How that falls on your list of "cost effectiveness" I couldn't care less.

you asked if it would be more effective


if you did not mean cost effective, then your whole argument is moot, because if cost is of no issue then I'd just park the FRS and get a McLaren.


and a custom cross member being more effective than a custom control arm... because it would be stiffer and lighter than the OEM...

[mike156]

Except MANY race classes explicitly don't allow moving suspension mounting points or tubular subframes but will allow custom control arms. Not to mention a tubular front subframe would likely be 2X the cost of control arms.

In the grand scheme of things, $500 control arms are pretty cost effective though considering they allow adjustment of roll center, caster, camber, anti-dive, scrub radius, SAI, and track width while eliminating the soft rubber bushings.

[Captain Snooze]

After all the reading of this thread I convinced myself that I needed a diff (or allowed myself to be convinced). I had an OSGiken diff fitted. Here are my road impressions; track day coming up not soon enough.

The feel at the rear end has improved when turning/slipping. This is not to say the Torsen is bad (I don't know what I don't know) but the feel at the rear is much more....mmm... organic.
Analogy. I have (atm) big anti roll bars front and rear. When one side at the front hits a bump I can feel the other wheel being affected. The new diff is like uncoupling the heavy sway bar. That is, both rear wheels feel like they have been uncoupled. I do not mean that literally, it is an lsd after all, but the operation of the lsd has made the slipping inside wheel more natural?,normal? more unbiased? free to operate unsullied by the other side? Mmmm... Hard to explain.

Let me try another way.
There is a large round-a-bout near home I know quite well. When exiting there is a small bump on the inside of the curve. There used to be a weird sway oscillation which I'm guessing is the rear inside wheel tightening and loosening but now when I hit that bump the car feel more stable. That is, there isn't the same "swaying of the hips" at the rear.

(Yes, I would be a fantastic racing car driver because I can explain to the chassis engineers in minute detail exactly what is going on.
Me: The suspension seems a little off.
Chassis Guy: What's up?
Me: I don't like the way it is.
Chassis Guy: What is it you don't like?
Me: It's like ,well, you know?
Chassis Guy: Uh huh.
Me: It's like when you are craving pistachio gelato but the local gelatissimo has run out.
Chassis Guy: Oh, Ok then.
*Chassis Guy fills dampers with pistachio gelato* )

Definitely not worth it just for street use but I do like how the rear feels. (That's what I said)

I am looking forward to seeing what difference it makes on track and I will update soon.

[dradernh]

Originally Posted by Captain Snooze

I am looking forward to seeing what difference it makes on track and I will update soon.

And we will wait with bated breath.

[gramicci101]

Originally Posted by Captain Snooze

*Chassis Guy fills dampers with pistachio gelato*

I am looking forward to seeing what difference it makes on track and I will update soon.

Pistachio gelato doesn't actually handle all that well. It's kind of clunky in corners. Tiramisu gelato is smooth as silk though.

[DougW]

Originally Posted by Captain Snooze

After all the reading of this thread I convinced myself that I needed a diff (or allowed myself to be convinced). I had an OSGiken diff fitted. Here are my road impressions; track day coming up not soon enough.

Have you noticed if the stability control is triggered less having the rear wheels better coupled?
DougW

[cdrazic93]

No Doug thats just your cool suspension thats doing all the work.