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Front roll center correction on a lowered FC?

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Front roll center correction on a lowered FC?

Old 06-01-08, 03:46 AM
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Front roll center correction on a lowered FC?

Does anyone make these? Last I heard of them was in this thread and jgrewe's design was as far as it got:


I'd love to make my own but know nothing about what type of material would be required or any dimensions.

I just installed new factory rubber control arm bushings in the front (old ones were SHOT) and the car feels much more responsive but the amount of body roll is ridiculous. I have 375 lb./in. front springs and 250 lb./in. rear with stock sway bars. My control arms are parallel to the ground indicating a low roll center and I believe this is the culprit. Ride height is 6.25" from the rocker panel F&R and tires are Falken 615s.
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Old 06-07-08, 12:58 AM
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Anyone?? No one has ever made one for an FC?
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Old 06-07-08, 08:44 AM
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No, sorry, don't have them as haven't felt the need. My car sits 5" off the ground at the rocker panels which you could imagine has the control arms actually angled upwards. Your springs are soft compared to my 550lb front and 400lb rear with a stock front sway bar. Have you considered different springs? Hopefully you have some good adjustable shocks as well. These two will solve your body roll issue. I am confused though on replacing with factory rubber bushings when there are many after market options which would be better for your application.
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Old 06-07-08, 01:13 PM
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I have read that the tokico illuminas can only handle 450lbs so stiffer springs aren't an option for me. They're 5 way adjustable and are set at 5F/5R. I could get thicker sway bars but I feel thats a bandaid for the real problem. The bushings I had to get from the dealer ASAP because I finally had access to a press but since I'm in tech school now I can use the press whenever I want so I will eventually replace them all with polyurethane bushings. I wonder if it would be safe to add a spacer between the factory ball joint and the steering knuckle..

Rotorsports can order the AWR roll center blocks but I don't see how they'd change the geometry at all by spacing out the ball joint from the control arm.
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Old 06-08-08, 10:07 AM
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OK since this seems like an actual intelligent thread...
1, Roll center is a lot harder to fix then increasing the size of the bars. And this is a much better place to start then geometry experimentation.
2. If you must play with RC then you should know that a spacer between the A-arm and the ball joint does nothing. What we are trying to achieve is moving the pivot point of the joint not where the joint is attached to the arm. The only way to do this is to move the inner arm pivots up or as your proposed part does move the outer point down. Both are costly and will dictate where you can race the car if that is your plan. On my old Eprod car I made a spherical bearing carrier to replace the ball joint and then made a "stub" to connect the spindle to the bearing. The stub being longer then the stock ball joint, pivot to spindle mount, achieved the goal and the car handled great. The down size was that this was pretty early in my parts design age and due to a design flaw they both snapped in half under breaking for the uphill at LimeRock and I hit the wall at about 80 mph. You can also cut the bottom of the spindle off and move it down then weld it all back to gether. I did this on another car (not the EP car) and was pleased with the results.
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Old 06-08-08, 10:19 AM
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+1 to John's suggestion on a sway bar. Unless you are very good at math or have a susension program to run to verify your changes you are basically pissing in the wind changing your geometry without knowing what the changes will do before hand.
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Old 06-08-08, 10:20 AM
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Isn't it 'unbalanced' to only worry about the front roll center?
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Old 06-08-08, 11:13 AM
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I'm surprised that you feel you are getting that much roll. I am running 400f/275r springs and don't get that much (of course my 91 is gutted and probably a little lighter than yours). I agree that the heavier front swaybar is your best bet here.

I'm not sure how the Tokiko's are valved, but you may also think about lightening up the front setting once you get the bar in.
Good luck,

-b
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Old 06-08-08, 04:14 PM
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I'm beginning to work on a way to do this that is EP legal. I need to get a clarification from national on just how "free" ball joints are. Hey John, what's your take on the ball joints?

I will also be modeling the rear to see what the roll axis will look like and see what the car likes for ride heights. A lot of my numbers will be worthless for dual purpose cars when it comes to springs and bars because I'm at 2300lbs and about 55% rear weight bias. I'll share what I think won't get people in trouble on the street or beat me at the track.

Last edited by jgrewe; 06-08-08 at 04:21 PM.
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Old 06-08-08, 06:10 PM
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Originally Posted by EProdRx7 View Post
OK since this seems like an actual intelligent thread...
1, Roll center is a lot harder to fix then increasing the size of the bars. And this is a much better place to start then geometry experimentation.
2. If you must play with RC then you should know that a spacer between the A-arm and the ball joint does nothing. What we are trying to achieve is moving the pivot point of the joint not where the joint is attached to the arm. The only way to do this is to move the inner arm pivots up or as your proposed part does move the outer point down. Both are costly and will dictate where you can race the car if that is your plan. On my old Eprod car I made a spherical bearing carrier to replace the ball joint and then made a "stub" to connect the spindle to the bearing. The stub being longer then the stock ball joint, pivot to spindle mount, achieved the goal and the car handled great. The down size was that this was pretty early in my parts design age and due to a design flaw they both snapped in half under breaking for the uphill at LimeRock and I hit the wall at about 80 mph. You can also cut the bottom of the spindle off and move it down then weld it all back to gether. I did this on another car (not the EP car) and was pleased with the results.
I understand that, I believe the suggestion he was making was to use the sway bars to decrease roll as a less costly band-aid to this problem. I was going to switch to the S4 control arms before having seen AWR's RCBs but after seeing a thread in the FS section I agree that their spacers do nothing to change roll center.

Your incident discourages me to go with your first suggestion, and the spindle being moved downwards sounds like a good idea if I can find a good enough welder.

Don49 As of now I'm certain the RC is blow the ground and I will use the factory A arm angle as reference. And there is actually a program (http://www.performancetrends.com/rc.htm) that can help me calculate the changes. I might take a picture of it now with the strut towers in plain view and try to get a rough calculation of roll center, it'll take some creativity

As for the rear being off I would rather worry about the front first as it is where I feel the car roll most. I feel the front needs as much help as it can get being that it has struts.

wrankin as of now the car has no rear camber adjuster and with -3* front and rear, shocks at 5 front and rear, 0 toe and 30psi front and 35psi rear the car still understeers and a thicker front bar would aggaravate this issue. The shock setting is a good idea, I met an FC owner at the autocross today that recommended a stiffer rear bias to help with understeer. I will try the shocks with 3F and 5R. Thicker front and rear bars (considering most keep the stock rear) will probably balance the car out nicely, its just a matter of me getting the funds. Racing with a strict college budget here, haha.

jgrewe keep us updated on your progress. The factory ball joint stud sticks out from the top of the steering knuckle a little bit, I might run a small spacer between it and the knuckle. I'll ask my undercar teacher tomorrow since hes probably seen a few broken or worn ball joints..

Last edited by KhanArtisT; 06-08-08 at 06:25 PM.
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Old 06-08-08, 07:54 PM
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You would need to raise the spindle in relation to the ball joint.

The biggest problem with just going with a bigger front bar is you have just added another 'spring' into the equation for the shocks to deal with. That can make it very difficult to get the shocks set for just controlling the coil springs. Another big thing is that the roll center moves around a lot. It actually moves sideways toward the inside of the turn(lowered strut front end) so each degree the car leans the sway bar has less effect.

The problem with a little spacer is the ball joint stud has a radius turned out of it to have the bolt act as a keeper. (see my picture you posted)
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Old 06-08-08, 10:16 PM
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Ah I see. It looks like lengthened ball joints seem to be a common modification to correct roll center in other platforms (RSX, 240, etc.). Heres a cool picture of SCC Mag.'s way of calculating roll center (AE86) :



Not sure as to how accurate it is but interesting nonetheless.
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Old 06-08-08, 10:54 PM
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Looks pretty good if you can follow the points they are drawing the lines through. That center of gravity is now where near correct height but it gives you an idea of the terms and how they relate.
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Old 06-09-08, 09:12 AM
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as of now the car has no rear camber adjuster and with -3* front and rear, shocks at 5 front and rear, 0 toe and 30psi front and 35psi rear the car still understeers
I know the old adage about "don' try and fix one end of the car by changing the other", but i think that in terms of "incremental change" one of the first things you should address is the rear camber issue. Now (someone jump in if I'm off here) we have to run a bunch of static camber in the front because of the strut design (loses camber under bump). But not so much in the rears. With that in mind, right now the next item I would be looking to purchase would be the adjustable rear subframe link so that you can remove some of that negative camber in back. Get it to around -2 to -1.5 in back and then go see what's happening with the front end at that point.

Good luck,

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Old 06-09-08, 10:20 AM
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Yeah the multilink the rear seems to be a good design as it gains a decent amount of negative camber under compression.

What I don't fully understand is this, if the rear with -3* of camber is causing understeer, this means the rear is getting MORE of a contact patch (=grip) in a turn than the front (at 3*), correct? I would think that extreme (-4 or more) camber in the rear would mean oversteer caused by the rear tires' sidewall never touching the road under cornering. So if I could dial in more in the front, this would increase overall grip?

Or do we leave camber alone after reaching large amounts like -3 or more degrees because of the resulting (poor) longitudinal traction? My car is definitely not affected by rear camber as far as longitudinal traction goes. The poor braking performance might be a good enough trade-off but the only way to find out would be to try it. This is where I really wish I had a pyrometer.

Last edited by KhanArtisT; 06-09-08 at 10:25 AM.
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Old 06-11-08, 09:16 AM
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Different tires require different amounts of negative camber for optimum performance.

You are starting to ask questions for which there is no one simple answer, and certainly nothing that be completely covered in this relatively short note. I would highly recommend that you start looking through some of better books on this subject. Personally I like "Tune To Win" by Carroll Smith (actually I like the whole series) which gives a good overview of basic suspension setup and concerns.

I don't auto-x, so I can't comment on what the "hot" setup would be. The only things I would suggest would be to get rid of some of the neg camber in back, soften up the front, and then start looking at your tire pressures and wear.

Good luck and let us know how it goes.

-b
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Old 06-11-08, 09:17 AM
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Old 06-12-08, 01:47 PM
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I have heard that somebody is building a bolt on and EP legal solution for the front roll center using the same geometry as our, yawpower, front suspension kits. I don't have any details but hopefully they will be available soon...
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Old 03-05-12, 09:56 PM
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very unlikely to ever be solved, except awr is prototyping aftermarket ball joints for s4 arms.
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