Projekt Kastor

[Error404]

My gut feeling is around 4° total will yield a substantial improvement and not be too much of a compriomise, this is roughly what the modern front engined 4wd mahcines i.e. evo have evolved to run with

[Rusty]

Cheers John, seems that a sensible approach should see some worthwhile results here, I'm OK for excess grip but I can imagine that something like yours is starting to reach the limits of road tyres under hard acceleration and cornering. That said the machpherson design does need a little helping hand when heavily compressed, be nice to cure that understeer on yours that we experienced at Combe.

Alex, thanks for the mail, had another helping hand from KotK too, a few ideas to look at.

Hopefully 3D model of the top mount tomorrow for fun, then get on with ideas for the subframe mounts and a catch tank project.

I discounted the ball joint idea on many counts but a different ball joint design would definitely change that, it's the easiest to implement but causes wheel position headaches and I have concerns over the ball joint angle being reversed.

[Keeper of The Kitten]

Reading what Newsh said, I can certainly see the issue with possibly inducing more dive ...there is potential to counteract it with some geometry change to the wishbone hard points, notably raking the wishbone points in side view so the rear point is higher than the front. Might be something else where the subframe bushes can come into play?

[Rusty]

Thinking about this indicates to me that pure breaking transfer to the strut is a function of the sine of castor angle, thus doubling castor will double the effect at the angles were talking about (small angle theory).

Some fag packet calculations for a 1500kg car decelerating at 1g with 80% front brake effort and 4deg castor indicate an 80kg effort on the front axle, seems quite a small portion to me. Weight transfer will be the same as before. In terms of the overall front axle load under braking I would have thought this is minimal and that's for an increased level. Will have more of a think tomorrow but it strikes me as a small change rather than a big one. Worth checking out tho!

[quattro84]

I agree with Mr. Cutting, anti dive geometry would be wise for the B3/B4 front.
But be carefull, altering this means you'll end up with an non linear steering response
For Scooby drivers there are multiple upgrades on the market to reduce anti-dive/lift (lowering the 'rear' mounting point) for the OEM WRX-es to give more steering response and grip on corner exit!

[Rusty]

Can just make one bush thicker than the other, simple enough to do but it'll affect other aspects for sure

[Keeper of The Kitten]

Alternatively make a complete new subframe

[Keeper of The Kitten]

See, if the inboard points are bolted vertically and using proper spherical bearings, then it becomes possible to adjust by varying the thickness of the top hat above and below the bearing, within the limits of the clevis.

[Rusty]

I think we're getting carried away here

For the moment I'm happy enough to look at top mounts and subframe bushes. If a lower balljoint solution then the castor increase will be large enough to cause concern.

The other thing is that a custom set of lower arms would probably cost about the same as the two offset parts.

[Rusty]

I think we're getting carried away here

For the moment I'm happy enough to look at top mounts and subframe bushes. If a lower balljoint solution then the castor increase will be large enough to cause concern.

The other thing is that a custom set of lower arms would probably cost about the same as the two offset parts.

[lind4hl]

Think of that the more you move the front strut backwards (up at the topmount) the more you move the wheel backwards in the arch.

But, these small adjustments wouldnt affect much i think..

[Rusty]

The clearance on my stock car is about 45mm front and rear, that's with a 225/45R17 tyre. At a guess, I'd have thought that I could accomodate up to 25mm wheel forward, the ball joint reversing strategy would be a bit more involved, one option is to space the bumper 10mm+ (would look strange but work) and there's the possibility of the front of the arch fouling...

With the mods I've proposed the wheel will hardly move, the top mounts move teh strut top back 7.5mm and I'd hope that the subframe bushes would move the subframe forwards betwee 10 and 15mm so the wheel will creep less than 10mm forwards.

[Error404]

If your rough calcs are right Andy, 7.5mm gain at the topmount and 10mm gain at the subframe that should give 4deg castor overall (2° 15' stock + 1° subframe bushes + 0°45' offset topmounts) - ideal!

[Rusty]

That's the plan anyway...

[HPmuirt]

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