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fc rear roll center

Old 02-19-14, 09:28 AM
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fc rear roll center

So I was just thinking about roll center the other day and I was thinking about raising my rear subframe and diff closer to the chasis by turning down my subframe and diff bushings so there is less clearance between the subframe and chasis.

I took a peek underneath my car and it seems like I won't have any clearance issues if I did it. I would have to look at it again, but I would say I could raise the subframe 1/4-1/2"

Anyone tried this yet?

I was thinking about other factors too, and I would also be changing the camber slightly more positive, which I would not mind. I'm holding off on individual camber adjusters because I am avoiding spherical bearings because of my budget at the moment. Only the rear center link would be needed.

Also, I was looking at the driveshaft angle, and it looks like driveshaft angle would be inproved also.


What do you guys think?
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Old 02-19-14, 01:05 PM
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This would probably make the car push/understeer more. If this is what you want, then go for it.
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Old 02-19-14, 02:24 PM
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Originally Posted by Travis R View Post
This would probably make the car push/understeer more. If this is what you want, then go for it.
I'm still a little new to suspension geometry, do you mind an explanation?
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Old 02-19-14, 02:45 PM
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Raising the subframe will add grip, but less camber will lessen grip...probably a wash.
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Old 02-20-14, 12:00 AM
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Well, I'm not worried about the camber as much since it is adjustable. I was just thinking since I will be lowering my vehicle and most people are getting camber adjustment to decrease the amount, there is a possibility that I might get lucky and decrease the camber to the desired number. If not, I can always add more

I was talking to my buddy about it and he told me how they already sell bushings for 240sx's with different sized washers that can be placed between the bushing and subframe, or between the subframe and bolt.
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Old 02-20-14, 10:14 AM
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you might have an issue with clearance for the fuel and brake lines.

MAX makes some bushings that raise the sub-frame (they also make ones for the diff too). But they're also a little wierd because they are tiltable... I'm not sure I trust them:
‚ƒƒ‚ƒ‘ƒƒ„ Solid Bushings - Parts Shop MAX

They also make spherical bearings for the rear trailing arms now which is cool.
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Old 02-20-14, 10:18 AM
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Oh cool! Thanks for the link!
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Old 02-20-14, 10:31 AM
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Also, because our trailing arms are so much longer than the links of S-chassis I don't think the roll center changes nearly as much when you lower the car... and likewise raising the subframe won't have as large of an affect. but I could be wrong.

I haven't really looked into it much because changing the location of a subframe isn't legal in my class.
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Old 02-20-14, 10:36 AM
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Also, I'd definitely fix the fronts roll center before I bothered with the rear...
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Old 02-20-14, 11:56 AM
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raising the subframe seems like a good idea to me, its should give you less camber at a given ride height. might not be a big difference, but when you stack a couple of small differences....

if you use an adjustable camber link, the subframe needs to tilt, so the tilting solid bushings are needed.
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Old 02-20-14, 12:00 PM
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Originally Posted by j9fd3s View Post
raising the subframe seems like a good idea to me, its should give you less camber at a given ride height. might not be a big difference, but when you stack a couple of small differences....

if you use an adjustable camber link, the subframe needs to tilt, so the tilting solid bushings are needed.
I only wish someone else bought those subframe bushings and reviewed it. I'm curious on how much movement there is while driving. maybe loosen subframe bills before center camber adjustment then tighten again. I almost jumped the gun and bought it.
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Old 02-20-14, 12:10 PM
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Originally Posted by GrossPolluter View Post
I only wish someone else bought those subframe bushings and reviewed it. I'm curious on how much movement there is while driving. maybe loosen subframe bills before center camber adjustment then tighten again. I almost jumped the gun and bought it.
https://www.rx7club.com/2nd-generati...table-1014464/
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Old 02-21-14, 01:13 AM
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After reading it, I think I am going to just shorten my delrin ones. I did see a few other things I would like to buy form their website.

Now I'm back to looking for good individual camber adjusters. recommendations?

What about front roll center? I'm going to do a search , but I was thinking about making a spacer for my s4 control arms for front roll center
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Old 02-21-14, 09:07 AM
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Originally Posted by GrossPolluter View Post
After reading it, I think I am going to just shorten my delrin ones. I did see a few other things I would like to buy form their website.

Now I'm back to looking for good individual camber adjusters. recommendations?

What about front roll center? I'm going to do a search , but I was thinking about making a spacer for my s4 control arms for front roll center
A spacer between the ball joint and control arm doesn't do anything. the arc the ball joint takes is still the same. the spacer needs to be between the ball joint and the spindle.... not easy on FCs.

I'm working on something. I'll post it in my build thread when I get started.

I used to run a set that AWR makes, it didn't end well:
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Old 02-21-14, 10:21 AM
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Originally Posted by eage8 View Post
A spacer between the ball joint and control arm doesn't do anything. the arc the ball joint takes is still the same. the spacer needs to be between the ball joint and the spindle.... not easy on FCs.

I'm working on something. I'll post it in my build thread when I get started.

I used to run a set that AWR makes, it didn't end well:
Please keep us updated!

I'm newbish to the suspension game. I always thought the spacing was anything to make the control arm angle better. Wouldn't all the angles be the same if you raised the control arm- subframe, spacer between the ball joint- spindle, and a spacer on the control arm-ball joint the same?
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Old 02-21-14, 10:27 AM
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Originally Posted by GrossPolluter View Post
Please keep us updated!

I'm newbish to the suspension game. I always thought the spacing was anything to make the control arm angle better. Wouldn't all the angles be the same if you raised the control arm- subframe, spacer between the ball joint- spindle, and a spacer on the control arm-ball joint the same?
you have to change the angle of the virtual control arm, which is a line between the 2 pivot points, the bushings of the control arm and the ball joint pivot. spacing down the control arm with a spacer between it and the ball joint won't change that.
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Old 02-21-14, 10:56 AM
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Originally Posted by eage8 View Post
you have to change the angle of the virtual control arm, which is a line between the 2 pivot points, the bushings of the control arm and the ball joint pivot. spacing down the control arm with a spacer between it and the ball joint won't change that.
+1, its the imaginary line between the two pivot points.
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Old 02-21-14, 11:17 AM
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Cool, thanks for the explanation! I'll do a little more reading.
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Old 02-23-14, 08:01 PM
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Suspension conversations can be very tricky since there are a lot of variables and people may be coming from different perspectives depending on what they are doing with there car (drag/rally/autox/show/etc). So I'll try to keep things generalized.
The roll axis (line connecting the front roll center with the rear roll center) in nearly all cars is pointed down at the front. This makes the car understeer. When you lower a car like the FC that has struts up front, the roll center tends to go even lower making the problem worse. So you can see that raising the rear roll center is kind of doing the same thing. But like others have said it will probably be pretty minor, and the camber change might balance it out.
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Old 02-25-14, 12:25 AM
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Originally Posted by Travis R View Post
Suspension conversations can be very tricky since there are a lot of variables and people may be coming from different perspectives depending on what they are doing with there car (drag/rally/autox/show/etc). So I'll try to keep things generalized.
The roll axis (line connecting the front roll center with the rear roll center) in nearly all cars is pointed down at the front. This makes the car understeer. When you lower a car like the FC that has struts up front, the roll center tends to go even lower making the problem worse. So you can see that raising the rear roll center is kind of doing the same thing. But like others have said it will probably be pretty minor, and the camber change might balance it out.
Hmmm, I'm not as worried about the camber, even though I haven't found a camber adjuster that I want to purchase

I'm seeing these diagrams on the internet for roll center, but I haven't seen one for our style rear trailing arms
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Old 02-25-14, 10:24 AM
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Travis, I've been thinking a lot especially since I'm trying to learn more.

So if I correct my front roll center, my roll axis will not be as effected as much? I am thinking like some people mentioned that the rear has long arms so the rear roll center number would not be drastically changed. Front roll center can be influenced more is what I am thinking. Then not significantly changing the roll axis.

Or should I keep my rear roll center the same, and help roll axis by correcting front roll center.

I was always under the impression that the rear suspension had issues because of the excessive camber and toe as the suspension moves, and when we lower our cars, it made it even worse. Anyone ever measure numbers? Please correct me if I am wrong

Oh, I found those diagrams of rear trailing arm roll center
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Old 02-25-14, 10:31 AM
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Originally Posted by GrossPolluter View Post

I was always under the impression that the rear suspension had issues because of the excessive camber and toe as the suspension moves, and when we lower our cars, it made it even worse. Anyone ever measure numbers? Please correct me if I am wrong

Oh, I found those diagrams of rear trailing arm roll center
The rear suspension (at least my suspension) had damn near 0 bump steer when I measured it on my car. Our suspension is so much different than an BMW E30s trailing arm setup to correct it's bump steer issue.
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Old 02-25-14, 11:10 AM
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^^^^ yes compared to the BMW, Mazda added the extra links, so it is a semi trailing arm, but the camber and toe curves are flatter than BMW's.

that being said the big problem when you lower the rear of the FC seems to be more negative camber than we want. fixing this, without the diff in the floor, is the hard part
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Old 02-25-14, 11:12 AM
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The FC rear suspension is actually kinda genius. You can change the roll center independently from the camber curve. Raising/lowering the inboard pickup of the lateral control arm (on the sub frame) raises/lowers the roll center. And the roll center moves very little as the suspension goes through it's travel (that's a good thing). Changing the length of the camber link (the short little arm that connects the trailing arm to the subframe) changes the camber curve.
The front is not so clever... The roll center in the front is basically tied to the angle of the lower control arm. Once the arm goes past parallel (point down towards the centerline of the car) your roll center drops quickly. It's like this on heavily lowered cars.
The lower your front roll center is, the more load transfers to the outside tire in a turn. That means more understeer.
So you can see that you can make massive changes to the front roll center, and hardly any changes to the rear roll center. Unfortunately the "easy"changes to the front are all bad, going the wrong way from what you want for a good handling car. The "difficult" changes to the front, involve trying to RAISE the front roll center. One of them is lowering the outboard balljoint. And eage8 posted a picture of how that worked out. You could also try raising the inboard mounting points of the front lower control arm. But the frame/chassis is in the way.
...
That was a lot of words, and I'm not even sure I answered your question.
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Old 02-25-14, 01:54 PM
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Travis, I think you mean if you lower the front roll center you will increase steering.

Fixing the height of the lower ball joint is actually easy, but it does require fabrication. I was thinking of removing the stock holder and switching it to the top of the control arm thus changing the angle of the control arm to result a better camber curve in compression.
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