It's time to talk about roll/bumpsteer on the SVX!
First, if you haven't been following, here's the background on the modifications I made to the SVX in order to run some big StopTech brakes:
http://www.subaru-svx.net/forum/showthread.php?t=53570
The short version: I adapted some STi front knuckles to replace the SVX's knuckles. This involved fabricating a ball-joint adapter (both cars are coilover McStrut with an LCA and swaybar). The result is that the knuckle rides about half an inch closer to the centerline of the car than before, and I think because of it, I've got a rollsteer issue.
The car drives fine enough on the street, but at the track, it could use some help. Here's my summary of the handling at the track from the other thread:
Quote:
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Originally Posted by sperry
I'm not super happy with the car on track. First, with those wheel spacers pushing the front wheels outwards and the super soft stock suspension, I'm actually rubbing the shoulder of the tire on the rubber fender liner under hard turning. It's actually minor enough that I can probably just pull off the liner strip... but if I'm going to do that, I'll probably also put a really minor roll on the front fenders as well just to be sure.
Second, the car pushes like a plow... I mean sure, it's not any worse than the car was before the new front knuckles/brakes, but I was really hoping the additional camber up front was going to help a little... but with a 70/30 weight bias, I don't think the SVX is ever going to be super fun on track... it's probably always going to be a bit gruesome in the slow corners.
Third, and this is the most disconcerting, I've got a weird shimmy in the steering on corner exit. I don't know how to explain it better than to say that if feels like a speed-wobble on a motorcycle. Basically, as you feed in power exiting the corner, if you hit even a minor bump, the steering starts to shudder back and forth. Just backing out of the throttle a tiny bit stops it, but leaving your foot on the floor allows the shudder to worsen. My best guess is that the car has a bumpsteer issue that coupled with my 231,000 mile old struts can put a wobble in the steering. Hit a bump with the car's weight shifted off the wheel, and the bumpsteer pushes the steering one way, then the strut starts to oscillate on the one corner and as the wheel travels up and down it pushes the steering back and forth. And I think the -2deg of camber probably allows the shudder to self perpetuate, as the wheel turns back and forth just a little bit, the camber keeps the strut from ever damping the vibration out. The only thing that stops it is lifting a little and shifting some weight back on the strut.
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Now, I'm not super educated on the details of bump/rollsteer. I understand what causes it, but I don't know enough to figure out how to fix it. I'm not even sure I
can fix it, simply due to the limitations of the space and parts I have under the car.
I did a little drawing in Visio just to see if I can understand what moving the knuckle towards the centerline of the car might do rollsteer-wise. If the lower balljoint moves inwards, then the dynamic camber change of the wheel should be more dramatic. Because the length of the steering rod has only been shortened enough to give me zero toe at static right height, I'm guessing that there will be excessive toe-in on the outside tire when cornering.
Here's my guess at the geometry:
The blue diagram is the stock SVX geometry, the black is the new STi geometry. The lower arc in each diagram is the LCA range of motion (from static height, pushing up under compression), the upper arc in each diagram is the steering rod range of motor through the same compression.
Assuming the blue diagram represents "proper" rollsteer, i.e. it's just right to compensate for the change in camber, at a delta of 3.6 deg, you can see that a shorter LCA and more static camber results in a greater change of rollsteer angle under compression, 4.5 deg.
Now, the diagrams above have no real basis in reality, as they're not from any measurements off the car... they're just me attempting to see what would happen to the rollsteer under the conditions I'm guessing are occurring on the SVX. How far off base am I? Is this totally incorrect or am I on the right track? I think that without accounting for the strut angle itself, I'm probably missing a big part of how this works.
But if this is the cause of my issue, what can I do to address it? Narrower steering rack? Push the knuckle back outwards and come up with some means of extending the upper strut mount (which is the reason the knuckle is closer to the centerline to begin with)? Neither of those options are very likely... but I can probably raise or lower the ball joint adapter to change the angle between the LCA and the steering arm... but I don't know how that would help/hurt the geometry.
People smarter than me... throw me a bone!