SCCA SM class rule clarification

[guibo09]

looking at my akg subframe bushings...

why not just install them upside down to lift the subframe up 5mm? small difference, but free.

[nando]

yeah, that's actually in the factory range. another half degree would let me fit the 235s a little better without having to resort to other methods. :)

[z31maniac]

From what I've "read/seen" it seems like good rear settings are 2-2.5° negative camber and 1/16" toe in.

Sound right to you?

[nando]

my front camber is 2.6/3.3 (pretty sure the RF housing is bent, oh well). -3 probably isn't too far off.

[z31maniac]

That's good to know. Since I'm not trying to build a National level car or anything, I doubt my shorter bushing idea would ever come into play.

I know right now the front of my car has -2.5° of camber, and my rear wheels look like they have noticeably more (I know that's very scientific). So I'm thinking -3° or more of camber in the back is a bad thing.

Not sure exactly how much I'm lowered compared to stock, but I'm running 215/40/17s that are slightly tucked in the back, if that gives any indication to the ride height.

[nando]

Just had my car aligned last week, and wanted to sort of confirm that raising the subframe does actually work. My car came in with -2 degrees of rear camber (stock is negative 1.8-2.8 ) and zero toe (stock is a little more but at least I'm not positive). I'm lowered about 1.5-1.75" front and rear. I may actually go the other way and lower the subframe some to increase rear camber (tire clearance) and gain some toe-in.

[guibo09]

Because of the post you erased!

haha, ya cuz i had a brain fart! that post is pretty much false.

camber gain is a function of sweep angle of the rear trailing arm.

assuming that at 0* (parallel trailing arm) the camber is 0:
- at +90* upwards (obviously impossible) the camber angle will be -15*
- at -90 of droop, (again impossible) the camber angle will be +15*

so you see, the camber travels (somewhat linearly) from +15 to -15 as the suspension compresses. so it doesnt really matter what angle you are at, as long as the suspension is compressing, you are going more negative.

nando is right about the front control arm effect. he brought up the detail of 90* between the control arm and the strut. this is the inflection point for the camber curve. this does not happen in the rear for a trailing arm.

it may not seem obvious why the two cofigs differ, but think of this major difference. a trailing arm is not like a lower front control arm because it is rigidly attached to the knuckle whereas the control arm is attached through a pivot point (ball joint). this is critical to understanding the whole thing.

[z31maniac]

why is that necessary?

Because of the post you erased!

[nando]

oh and i think i was wrong about the parellel to ground thing. it is true that the camber gain inverts at the horizontal point. ideally, the trailing arm should be parellel to the ground at ride height.

for the front or the rear? For the front it happens sometime quite after being above parallel. Basically the angle between the strut and control arm would have to be greated than 90 degrees for camber to start going positive - after parallel, the camber gain is basically flat, which isn't great, but you aren't losing camber just because the control arms aren't flat with the ground.

[guibo09]

I was under the impression you didn't want the rear TA to go past horizontal at its max compression so the mounting point would still be higher than the rear of the TA?

why is that necessary?

[z31maniac]

I was under the impression you didn't want the rear TA to go past horizontal at its max compression so the mounting point would still be higher than the rear of the TA?

[guibo09]

nm

[guibo09]

It's really the only choice unless you want to jump to XP, it seems.

well there is still a completely custom control arm with threaded adjusters that would be legal (even though it costs 10x more than ie style adjusters, which is against the spirit of the rules).

but ignoring the rules for a moment, im starting to think that eccentric bushings are a better choice over ie style adjusters for these two reasons:

1- no impact on dynamic suspension curves
2- no change in roll center
3- no bushing binding since they keep the same rotation axis

but of course, one has to accept the dependence of toe and camber as the major disadvantage. as well as somewhat limited adjustment range.

[z31maniac]

edit: all this is making me actually consider offset bushings over ie style adjusters...

It's really the only choice unless you want to jump to XP, it seems.

[guibo09]

Are you sure?

If that's true, then why wouldn't camber plates be illegal? They use the same principle. They attach to the car at the same place (from the rule), but move to change suspension geometry.

I think the spirit of the rules say you must use the original physical location on the car to attach suspension components....you can't make your own subframe for instance.

I believe that lowering / raising / offsetting suspension geometry by using offset bushings AT THE SAME ATTACHMENT POINT ON THE CAR is fine....however please correct me if I'm wrong (and point to the correct page in the rulebook).....

camber plates are accounted for by the last line in the rule:

In addition, shock absorber/
strut upper mounts are to be considered suspension
components.

suspension components are free to change, therefore by including strut mounts as a suspension component, you are free to change it. so as long as your camber plate attaches to the original strut holes, you are okay.

you are right about the spirit of the rule. in simple words, they dont want you making your own subframes, or playing with chassis attachment points to change the dynamic camber and toe curves as well as the roll centres. an example of this would be moving the front control arm front chassis mount upwards to improve camber gain.

now offset adjustable TA bushings are legal because their center of rotation is the same as the original bushings. you are therefore not messing with dynamic behaviour of the suspension.

however, IE style eccentric bolt adjusters are different because you are actually displacing the centre of rotation. this actually changes the roll center as well as the camber/toe curves, even though you probably dont want to.

it's a frustrating situation because no one uses IE style adjusters to mess with dynamic suspension curves, it's just to fix the alignment. the actual impact on dynamic curves is undesirable and is by the same token the reason why they arent legal. so double loss for us.

it's also not fair since macpherson guys could just throw in a camber bolt to accomplish the same result while being 100% legal.

edit: all this is making me actually consider offset bushings over ie style adjusters...