Projekt Kastor

[Error404]

We're after more steering weight and feel primarily Si, straight line stability and dynamic camber gain are welcome additions

[quattro84]

Rusty: (sorry, I edited this by mistake meant to press reply with quote )

Discussion points were bump and roll steer and Ackermann effect with respect to geometry changes.

No prob, will rewrite it

Perhaps is it possible to swap left & right guide joint to increase caster and still put the wheel in center of the wheel arch...



But, this will have an effect on the bump/roll steer and Ackermann geometry. Could be positive!
Next issues will be hitting the ARB and possibly the lenght compensation of the driveshaft.

So, who is willing to measure all the suspension parts and capable of putting it in a FEA simulation such was Adams?

[Rusty]

What are you actually trying to achieve by changing the castor Orificer Rusty?

If it is just improved cornering through the effect that positive castor has on negative camber, then would adding a little more negative camber to start with mean that less castor is required? I know there is much less scope to do this on 4wd cars before you loose traction, but surely someone has looked at this?

We all know that road cars are designed to ultimately understeer as it is considered safer for the average driver. They do this by adding positive camber as the suspension deflects and thus reducing the front end grip.

It therefore seems to me that the ultimate goal should be to redesign the front geometry to reduce this increase in positive camber (I know I Know it is a black art, but we have enough designers on the forum). I had always assumed that this could be done by changing the ratio between the length of the upper and lower wishbones but I expect the experts to now jump in and tell me it is impossible on our cars.

If it was possible, it would then mean that only smaller castor and camber changes would be required to allow us to tune the handling to suit our own driving styles.

Cheers n

Si

Hi Si

Upper and lower wishbones, not on a MacPherson strut sir, you're thinking B5 (B4 rear), I have experienced that the B5 is a lot more stable car with this suspension setup and indeed rides smoother too with good cornering characteristics. That's not to say that it's as much fun as the old coupe that rarely does the same thing twice

The aim is that more castor will add steering weight during cornering and also improve front end grip with the effect of camber that you describe, both are areas where I have identified a requirement for improvement over my 6 years with my S2.

I already have a rather large rear ARB that has improved cornering by improving front end grip and adjustability at all stages of the corner. Other areas to look at would be a proper coilover setup with spring rates chosen correctly and corner weighted, I'm not convinced with the HnRs, I think they're more style than substance but I've yet to test them on a track for well known reasons...

More camber etc. could be dialed in on the base setting and the toe settings altered also but all of these require a lot of indepth testing to get right, perhaps something for the future and the cooperation of an experienced company or individual who can correctly advise on where best to make changes. The problem here is that getting your local alignment place to play about is rather unlikely and costly.

With this little project the aim is to identify how to add castor to the S2 front axle, to quantify the changes that are possible without major modification and also to test whether additional castor helps overcome some of the less desireable characteristics of the Audi chassis setup that we have all inherited. Ultimately, my goal is to increase my enjoyment of my car and understand what changes have been made and why they work, this is the area I enjoy the most, the learning both through academic and empirical means

[Rusty]

If you would swap the wishbones left for right and vise versa it would give some change is caster, and put the wheel back in the center of the arch...
But will the driveshafts be able to compensate the change in lenght.. will it hit the ARB.. and how is the steering arm position changed (think of: Ackermann and roll/bump steer).

Under bump you get more toe in, thus stabilising the car, this is due to the positioning of the steering arm on the rear of the strut and the rack above it. Interestingly the rack on the BMW (due to the recessed engine) is in front of the wheels and below! This has the same effect in bump.

In roll the outer wheel will turn more, the inner wheel less, I'm not sure what effect this has.

The Ackermann effect will be affected but I would doubt that the changes discussed here are too far beyond the basic manufacturing tolerance of a car, we're talking in the region of 10mm for the samller changes. Accepted that the 50mm change would make rather a large difference to many other areas and this would need either testing or calculation to understand the exact changes made.

As mentioned above, swapping the ball joints isn't something that appeals due to various designed in significant angles that would be reversed and could cause interference.

[Keeper of The Kitten]

As Andy says, the Macpherson strut doesn't have the capacity for camber recovery like a double wishbone, but you could reduce the lost camber with very long wishbones ...unfortunately that also requires a lot of fabrication and complexity, just like playing with the inboard points for anti dive!

I think ackermann starts to play less of a role when the tyres are already slipping, indeed some racecars have reverse ackermann to make use of reduced steering input simply because the inside wheel is doing nothing. However these cars are heavy and the speeds are lower so the inside wheel surely has an input to make. Perhaps increased ackermann is therefore a good thing, but you can get some turn-in by adding static toe-out (if you don't mind a bit of tyre wear).

Andy, are you seeking to improve performance on road tyres or on slicks? I am wary of the differences between getting heat into a slick and getting the best out of a hard compound road tyre. Notably in colder conditions, adding camber to heat a small area of a slick up to working temperature is preferable to using the whole tyre, but that will never really apply to a road tyre.

[Rusty]

A suitable setup for sports road tyres and track day tyres. I'll ignore the winter tyres

[quattro84]

Killing bump/roll steer is hard, i agree, look at this guy: http://www.motorgeek.com/phpBB2/view...=asc&start=125
Typ85 though, hacked the firewall so replacing the rack wasn't a big problem. He is using different rods and spacers and is trying to weigh out the bump steer and eliminating it with spacers.




I'd personally would try to kill it as well.. under hard cornering and perhaps a bit of acceleration as well it could kick you out of the Kamm's circle ending up beside the tarmac
Ackermann issues will not be a great influence i agree

Keeping the wishbones on the same side and simply swapping the joint should be an option, i am going to try it in some time.


Hmm... how to overcome the understeer on these heavy front end cars...

[Rusty]

That's quite Heath Robinson for sure! Is the steering arm to strut joint floating? Not sure I'd like that idea.

The ball joint on the pressed wishbone is a lot more offset and is angled with the suspension strut, at a guess 10-15 degrees. The offset is about 25mm so reversing it would give more offset than the 30mm ish you've shown. Looks like forged 'bones are different in a few respects there.

Nice cad model, where did you get that to then

[quattro84]

Nice cad model, where did you get that to then

Just some DIY on a free afternoon @ work

[Rusty]

Ah I see. I miss my 3D CAD at work, 2D and free programs make life pretty hard work nowadays. Shame proper CAD is a small fortune

[HPmuirt]

Hi Si

Upper and lower wishbones, not on a MacPherson strut sir, you're thinking B5 (B4 rear)

DOH! I'll get my coat

As I was typing my original post, I was actually thinking of my Triumphs as they have double wishbones and I have played with them more recently than the S2 front suspension (or anything else S2 related for that matter )

The other thing I was going to ask regarding castor was the effect on torque-steer. Am I correct in thinking that torque-steer increases as positive castor increases on FWD cars and therefore I assume on 4WD cars? If so, wouldn't adding as much positive castor as you probably need to give the desired turn-in, simply dial in torque-steer, or this that only on cars with transverse engines and un-equal length drive shafts (you can see I am very rusty [no pun intended ] on all this)

It sounds like we need a combination of camber plates (as discussed above) a small amount of positive castor and as much anti-roll stiffness as possible, such as: -

1. Has anyone seem welded their subframes and reported better stiffness?
2. Is the S2 front strut brace strong enough? It looks pretty flimsy to me.
3. Anyone tried a rear strut brace and reported an effect?

I will now be returning you to the real experts and normal service will resume......

Cheers n

Si

[Rusty]

Camber plates? Think I've missed that one?

There are quite a few areas on the chassis where additional bracing looks like it could be a good idea to stiffen up the car. The horseshoe shaped subframes are one area, rear struts another. All little projects to have a look at in the future

No idea on torque steer but thinking about why torque steer occurs I don't see how a uniform change left and right would make much of a difference, assuming a small change. A for the more major ideas, I'm sure that it'll cause other headaches that need to be solved...

[Rusty]

As promised (more to myself than anyone else ) here are some pics of my top mount design (with its pink ring ), it's hardly revolutionary but does use a nice large bearing and also allows for attaching a strut brace at the rear (work in progress ). Offset is 7.5mm in whichever direction you chose.

[quattro84]

Looks like a beefy setup!

[Rusty]

yup, no messing about here