SCCA SM class rule clarification

[z31maniac]

I think you have it right, I was thinking its not a suspension component so it is free to be changed. But since it doesn't specifically say you can, that means you can't.

So really the only way to get the camber adjustment is to use the eccentric TA bushings. Those are legal because its a method in the service manual to align the car.

I need to get under my car and see what level my TA's are sitting at, from what I understand, you basically don't want them going past parallel to the ground during compression. But my car will have to be raised up somewhat when the bigger tires go back on either way.

Although I think it would be a tortured interpretation of the rules:

H. Subframe bushings may be replaced with bushings of any material
as long as they fit the original location. Offset bushings may
not be used.

Does not say they have to be the same "height" only that they have to fit the original location, any material and not offset. I don't think shortened = offset.

[guibo09]

I think you have it right, I was thinking its not a suspension component so it is free to be changed. But since it doesn't specifically say you can, that means you can't.

So really the only way to get the camber adjustment is to use the eccentric TA bushings. Those are legal because its a method in the service manual to align the car.

I need to get under my car and see what level my TA's are sitting at, from what I understand, you basically don't want them going past parallel to the ground during compression. But my car will have to be raised up somewhat when the bigger tires go back on either way.

Although I think it would be a tortured interpretation of the rules:

H. Subframe bushings may be replaced with bushings of any material
as long as they fit the original location. Offset bushings may
not be used.

Does not say they have to be the same "height" only that they have to fit the original location, any material and not offset. I don't think shortened = offset.

i dont think that logic of the control arm being past parellel applies. that logic is more for the front control arms on macphersons to some extent, although not completely right either.

a semi trailing arm gains its camber from the trailing angle and not from the sweep angle. and is actually fairly linear over most of the possible travel.

as for your interpretation of offset, i think you're really pushing your luck. i can easily say that a shorter bushing offsets the subframe vertically.

anyway, in the spirit of the rules, it's pretty clear that any relocation of the subframe is illegal.

[IsItElectric?]

i think the SM rules do not allow you to move the subframe at all. infact, even the IE style camber/toe adjusters are illegal. which is very annoying, because in the spirit of the rule, they should be legal since they arent designed to provide geometry adjustment, only alignment.

edit: and infact, a completely tubular/adjustable trailing arm is legal even though it is countless times better than the illegal adjusters.

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).....

[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...

[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]

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.

[guibo09]

nm

[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]

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?

[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.

[z31maniac]

why is that necessary?

Because of the post you erased!

[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.

[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.

[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]

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