demonspeed

iTrader: (7)

Just as the title states, where do those dimples along the top of the rear ¼ panel come from? I've always thought it was from chassis flex, and once you threw subframe connectors on, it eliminated (as in before they've even showed up), or kept them from getting worse.

BUT, I was talking to a member on Sunday about what I had on my car. He said he doesn't run subframe connectors because it puts him in another class (autox). I said I prefered not to have a bunch of dimples in my ¼, so I put them on at like 18K miles before anything happened. He promptly said that it's not from lack of subframe connectors, but from brake hop. Then I looked at his ¼ panels and they were the most pock-marked body panels I had ever seen. He still dismissed it as brake hop.

I've had nasty axle hop, and I also had horrible brake hop when I was breaking in my new rotors and pads, but I'd really attribute the ¼ dimples to chassis flex - which is where the subframe connectors will help. Besides, when your car is bouncing around back there from nasty axle hop, I'm SURE your frame is flexing which would in turn cause the dimples - which means the ¼ wouldn't dimple in the first place if it had subframe connectors.

So assuming he had subframe connectors AND the brake hop, I would make an educated guess that he wouldn't have the ¼ panel dimples.

So, what does everyone else think?

trackbird

iTrader: (4)

I welded in a set of SLP SFC's on my car and have autocrossed on race compound tires and had plenty of brake hop. I have no dimples.....

That may not answer the question, I just threw it out as a point of reference.

I always considered chassis flex to be the source of the dimples. Now, brake hop is amazingly violent and probably flexes the chassis. So, it is possible that brake hop causes chassis flex, which causes dimples.

The chicken or the egg......

Bad Blu Formula

iTrader: (3)

I have a nice 2 inch long dimple on my driver side quarter panel which drives me crazy sometimes. I had it even before my SFCs, but I don't even have alot of power mods. Technically if its flexing, woulndn't it flex on the passenger side?

jdustu

iTrader: (2)

i'd have to agree with you.....the stiffer the chassis is, the less the body is gonna flex even when the back end is hopping.....i love sfcs by the way....

demonspeed

iTrader: (7)

I've always noticed most dimples on the driver side, but that's not saying it's uncommon for the passenger side to dimple. The passenger side seems to dimple most right past the end of the door - at least from what I've seen.

On another note, the subframes are doing more than just preventing the dimples... it helps keep the interior rattle-free.

I'm glad I'm not the only one who thought this guy didn't make any sense.

Sam Strano

iTrader: (41)

I'd be that member, and it makes perfect sense. Of course, you can choose not to beleive me when it comes to suspensions on the 4th gen, that's you choice.

SFC connectors do not come within FEET of the quarter panels, and if you've even seen a car wheelhop of brake up without of without SFC's you'll see the same action happening. The quarter panels actually flex and buckle, it's true and I know that. To compare my car having them to a car that has SFC's not having them is a poor decision. My car, or any car driven and setup like mine, goes through things that no street car, no matter how hard it's driven does. For instance I've seen brake hop violent enought to break a stock welded muffler off the intermediate pipe.

I'll reiterate to you the fact that I know of one 4th gen with over 310k miles on it, sans SFC's. Sure it has a few squeaks and rattles, but is much better than most cars with 1/3rd the miles. BTW, that car is a stiffer sprung car (1LE, and is autoxed, daily driven and drag raced almost all the time). Further, I'll add that one customer/member removed his SFC's in order to race in a lower class (ESP vs. SM) and was very suprised by the lack of difference he felt.

I know you felt the difference with SFC's. But it's a simple reason, the stock shocks transmit loads into the body that do not get there with good shocks like Koni's or Bilstein's. It's those things that make the body quiver when driving down the road. So you can put SFC's on to help, and still have lousy shocks. Or you could put good shocks on and realize you don't need the SFC's for squeaks and rattles and such since much less shock is getting to the body in the first place.

jimmyblue

iTrader: (11)

Question - looks to me like all of the performance
shocks have -more- bump damping than the stock,
which to me says the wheel-impulse would be pushing
-more- on the chassis (less suspension compliance,
more force). At least, that's how I visualize it from
20+ years since my last kinematics physics exam.

I would think a stiffer-shock combo would have a higher
(but narrower) peak force on the body. Let alone with
stiffer spring rate. What am I missing?

And is the thinking that these are "road bump dimples"
rather than "torque dimples"? I think "torque dimples"
sounds sexier.

Sam Strano

iTrader: (41)

I can't think of a good rear shock for the 4th gen that has more bump damping than the stock DeCarbon's. That's one half the issue with the DeCarbon's in the back of the car. They resist the soft springing over sharp bumps because they are rather firm in the bump at least intially. The other half of the issue is they don't control the spring when you actually get it compressed (rebound), which can also cause ride and impact issues when the spring has oscillated too far and then back into compression which causes a lack of travel for any bump you hit at the same time.

felton316

Well, I have SFC welded on, and I still have a few dimples on the quarter panel. I always assumed they were caused by the rear doors of SUV's being swung open by children when they are parked too closely. They do seem to be the right height for SUV and truck doors.

jdustu

iTrader: (2)

Sam,
I understand what you are saying about wheelhop still causing the dimples, but are you also trying to say that getting better shocks will eliminate the need for sfcs? that's what it sounds like, but i can't imagine that.......and your logic about the sfcs being "feet" away fromthe quarter panels is extremely flawed, due to the fact that they firm up the frame, which IS connected to the panels.....what you wrote would be like saying that the engine doesn't effect the rear wheels because it's 10 feet away.......i'm not disputing the idea that violent wheel hop will still distort the quarters, just wondering how sfcs could be a bad thing(other than not being allowed ina certain class)?

demonspeed

iTrader: (7)

Sam, that's EXACTLY how I took your comment on Sunday and with your reply.

The only thing I understood about you not running the SFC's is due to it knocking you into another class. That would mean your position in ranking might drop - but I'm certainly not questioning your driving ability.

That doesn't mean I can't question your theory, nor does it mean I think I know more than you. You could simply dismiss my questioning on inexperience, but you don't know me other than I prefer drag racing to autocross.

Anyway, just like I mentioned... It's a proven fact that the subframe connectors stiffen the chassis. It's also a proven fact that when the chassis flex's, that everything which is connected to the chassis can/will move.

Interior rattles caused by shocks? Haven't thought about that... but I really doubt if you've got, say 90K miles worth of rattles, that they'd disappear with a shock/strut swap.

We've got plenty of members here that can offer input into this thread. Let's here it...

trackbird

iTrader: (4)

I believe Sam's point is that the quarter panels "ring" like a bell. Even though the SFC's try to add some rigidity to the chassis (I've never watched a car brake hop, but I've driven one). So, they don't directly stiffen the rear quarters and therefore they can twist and dimple. That makes sense.

If you stiffen a section of the body, the ends can still be "free" to vibrate. I suspect that is the issue.

As for shocks, more control (notice that I didn't say "stiffer shocks") may keep the weight of the suspension from impacting the bump stops so violently (which can send loads into the chassis).

And, I don't think SFC's are a bad thing, it's just that they can't stiffen the chassis as much as we'd like to think they do since they are "flat". They still twist like the body (just it takes slightly more energy to do so). For real additional stiffness, a cage is the answer.

And, I have Koni's, that may also explain the lack of dimples (based on Sam's information....I'd not considered that).


Maybe I just need to drive my car harder......

trackbird

iTrader: (4)

Autocross classes are somewhat "funny". Adding SFC's will put you in a whole different class....for a "minimal" mod (not that I'm against SFC's, but they are flat and can flex). This would be like you adding a lid and a K&N filter to your drag car and having to move up to "Pro Mod" or "Pro Stock" to be legal (in a car that still runs 12-13's). Ok, maybe it's not that bad, but you get the idea.

I have to ask for clarification. You said "Proven fact"? That would require numbers and testing. If you've got it, share it. It is a "not so proven assumption" might be more correct. Not picking on you, but if you start calling things as facts, you need to have testing to back that up. Otherwise, (your science teacher will agree), it is a theory. There is a mass of misinformation that is accepted as the truth on the internet. SFC's can help stiffen a chassis, but they can only do so much. They may not be the miracle cure that everyone gives them credit for being. But, they have received that credit for so long, it has become a "fact".

Probably not. Once the car is "loosened up", it's probably too late. However, mine is running more spring rate than most cars out there and it quite rattle free (and more shocks than most of those cars as well).

demonspeed

iTrader: (7)

Yes, very confusing. I have my car setup more to go straight, but because of that, the mods I have put me into a more "professional" class which makes me look like a fool

Sam Strano

iTrader: (41)

The aren't a bad thing, but are done as a first thing when there are bigger fish to fry. First, the car was designed as a convertible car where many cars aren't. That is a boon to chassis stiffness. Can you make it stiffer, yes. Do you need too? Not necessarily. As felton316 pointed out he has welded SFC's and has dimples too.

I never said shocks eliminate the need for SFC's, if you are doing something that SFC's are indeed needed for. Say you have a drag car that pulls 1.4 60's, that would be damn smart to run SFC's on. But the shocks help so much that many folks find the things they think they need SFC's to cure really is helped/cured by better dampers. Classic example. The dash rattle over bumps. My car rarely does it, maybe 5% of the times it did with stock shock (all 100 miles it had stock shocks). Again, SFC's help by not letting the forces move around the uni-body so much, but to begin with most of those forces shouldn't be getting into the uni-body to start with. I'd rather fix the root cause than band-aid it. My humble opinion.

It's not flawed thinking. If you've seen a car hop, which is what mostly causes the dimples you'll see it's not the body twisting in the middle where the connectors add strength. It's the axle shaking and bouncing around that actually flexes the body, which is connected since the body is the frame. But it's not a torsional ridgity issue like you'd get upon launching on sticky tires at a drag strip. A shock tower brace is connected to the body (which again is the frame). It stiffens things up too in ways but wouldn't effect this in any way.

I realize what I'm saying goes against a lot of the things you hear online. Trouble is you hear a lot on line, and a lot of it sounds good when it's not. I've seen folks who I know to be terrible drivers make what sound like good driving arguments. I've seen folks espouse how their setup is the "Best", when they've never tried anything different. The confuse better than what they had with kick-***, can't get better. It goes on and on.

If you have SFC's or want them that's fine. They have their benefits, but a lot of them are overinflated, some are justified. I can show you car after car without them that is fast, and car after car that is in one piece after years and years of poundings.

You can take it for what it's worth coming from me. I know there will be disagreements about it. That's fine. I sell SFC's. I build cars. I race cars in an arena where chassis and handling is key. I also believe in doing what works best first. This started with a newbie (no offense, just what we call new persons) at an autox and SFC's. I said they didn't effect the dimpling, and so far we've seen at least one guy reading the thread reports dimples even with SFC'. I never said my car wasn't dimpled which is what prompted the question in the first place. I'm saying there are many ways to get the dimpling, and just having SFC's won't keep you from getting them. You can have them with SFC's. You can get with without SFC's. You can get them, and usually do from things that SFC's do not control which is the axle jumping around like a Mexican jumping bean.

demonspeed

iTrader: (7)

Makes sense... but like Sam mentioned, the forces he exhibits to the car is MUCH greater than that most everyone else exhibits, so his theory might work for him, but for the general populous, he might be painting a picture that isn't particulary true.

demonspeed

iTrader: (7)

The wonders of the interenet! It's hard for me to rewind and replay a conversation, whereas I can reread what you've typed to better understand...

Many things are getting cleared up, and I'm definitly NOT disputing I was a newbie on Sunday!

I was figuring on a 90% chance you'd find this thread and reply, but I didn't want to bring your name into it just in case you wanted to stay out of it. Nonetheless, you've defended yourself with your pertinate info.

One thing that is obcsure to me is that if the subframe connectors push you into a more "difficult" or "higher" class, then why do they seem almost like a null and void modification to you? What I am trying to say is that obviously the SCCA thinks frame modification makes the vehicle a lot stiffer and possibly more stable, in which case leades me to beleive they feel subframe connectors would give someone a competitive advantage over someone who isn't running them. Care to elaborate?

trackbird

iTrader: (4)

I've never managed to figure it out. I can't say that they suddenly would take "X" amount of time off of your run. Sam may have a better thought on it.

jdustu

iTrader: (2)

i would take my 94z28(no sfc) and jack up one corner on a jackstand...the whole car would creak and moan...not good....i do that now with my 98' and sfcs, and the whole body stays rigid.......the ideas as you guys have re-explained them are not necassarily flawed, just some of the analogies.......and i believe trackbird said the sfcs were flat, but most i've seen are tubular with boxed ends. what he said is like saying swaybars are flat and don't work...not true......

trackbird

iTrader: (4)

Actually, sway bars are flat (if the car is 4 feet tall, 3" of SFC is essentially "flat"), and they twist. That IS my point. A sway bar is designed to work like a spring, that is it's job. A subframe connector is designed to avoid it and the body twisting like a sway bar. If it works like a sway bar, it failed it's job. Your car twists those "massive" sway bars, it will twist SFC's too. I'm not saying they don't help, but they are not the miracle cure. Remove those and add a welded in 6 point cage, you'll see what chassis stiffness is about.

I think it is starting to make sense to you now.