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?

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?

Raising the subframe will add grip, but less camber will lessen grip...probably a wash.

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.

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:
シャーシパーツ Solid Bushings - Parts Shop MAX

They also make spherical bearings for the rear trailing arms now which is cool.

Oh cool! Thanks for the link!

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.

Also, I'd definitely fix the fronts roll center before I bothered with the rear...

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.

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:
https://lh4.googleusercontent.com/-w...53077973_o.jpg

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.

+1, its the imaginary line between the two pivot points.

Cool, thanks for the explanation! I'll do a little more reading.

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

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

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.

^^^^ 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

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.

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.

thanks for the info guys! any goods front roll center options?

No :)

I'm having trouble visualizing what you're trying to say... care to elaborate? (or draw something? :) )

and it leaves room for a low flat trunk floor. i know we don't care much, but a good design will satisfy several different criteria



on the front of the arm there is room to do this a little, in the rear though, it would be kind of tricky, just because they mounted it the way they did.

by a little, is like 1/2"

the line we want is pivot point to pivot point, so putting the ball joint on the other side of the arm does nothing

Does not change the roll center, but does add camber on compression which is normally foreign to a Macstrut car.

I was thinking of making lower ball joint brackets using a 1/2" spherical bearing...that way you could move the spindle relative to the ball joint vertically.

if you're just flipping the ball joint to the top of the control arm... it won't. at least nothing significant.

an easy way to add more camber gain is to increase the SAI (ie, camber plates)


also a 1/2" spherical bearing seems awfully small for a ball joint..

Eage8...looking at 1/2" spherical bearings...radial load in excess of 25000 lb. Could go up to the 5/8" for 44600 lb rating. I would think that would be enough for my 2700# race car! By using the spherical, you can add shims between the LCA and spindle to change RC.

The bearing itself isn't what I would be worried about, it's the 1/2" bolt you're putting through that bearing.

and how do you plan on connecting that bolt to the spindle?

Tow ways...drill the spindle for a square hole, or use a tapered bushing. By drilling I think you would want to use the 5/8" bolt.

the stock balljoint connects to the spindle with an 18mm pinch stud... there is no taper.

Haven't had it apart so I did not know that. In that case, I would machine an 18mm stud on top to accept the 5/8" bearing on the bottom, with a lock nut.

+1 i just drew a bolt up in solidworks. its 18mm, like stock, and as you would expect, the longer the bolt the weaker it gets, so spherical bearing + bolt solution has a limit to how long you can make it before the wheel falls off in a turn :)

material is super important.

TBD on using a larger bearing, and bigger bolt

my new and improved (hopefully) idea is to use the AWR ball-joint/bearing retainer and weld/drill out the knuckle to 19mm / 3/4" and use a 3/4" grade 8 bolt with a nut holding everything together. (the AWR ball joint uses a 3/4" bearing)

even though the pinch bolt area is 18mm. the recess where the pinch bolt goes through goes down to 15mm which is where mine broke.

with the strength testing i'm not 100% sure i'm using the right numbers, if you have a 2800# car with 50/50 weight distribution, 1600# but add 1g, and i'm just testing #2800lbs.

i'm not sure if that's too much, or not enough...

and of course there is the oddity that if i have the computer add a stress diagonally, vs straight it makes a difference, how that is possible on something round, i do not understand

Some guys over at nissanroadracing did something similar with the same size bolt:
HOW TO: Ultimate budget front LCAs - Nissan Road Racing Forums

The only problems they've had with the setup is the bolt loosening up... so I just have to make sure it stays tight.

oh you know i'm not testing for shear, i'm bending it, that must be the difference. it does look like you could hang a couple S13's from one bolt, and we just played with a 350Z today, and i bet the body would fold/spindle/mutilate before the bolt bent

http://nucor-fastener.com/Files/PDFs...r_Strength.pdf

Besides camber, would there be any advantage to making it so the control arm was longer? I'm not talking about camber

It changes SAI which helps all kinds of things... (roll center and camber gain) But it also increases track (which is good or bad depending on what you're doing)

I have no idea what it does to bump steer.

I just need to see how my wheel fitment is with 17x9 +22 on the shine fenders.

I'm thinking why not drill out 18mm to 19mm or 3/4" and using a huge bolt.

I did not measure the stock thickness of the metal on the ball joint, but I was thinking 3/8" would be ok? I guess the hardest part would be machinging the bore for the bearing.

Look at your other thread :)

https://www.rx7club.com/race-car-tec...cones-1058123/

the stock ball joint is aluminum, so the thickness could be smaller if you make a steel ball joint.

I don't know how thick my AWR ball joint is, but I'd probably say somewhere around 1/4" steel

I also wanted to raise the rear for more anti-squat. Ultimately, 1/2" just isn't enough to justify the effort. The frame needs to move at least 2" up to help with the lowered stance and give the increased anti-squat I'm looking for. I've thought about cutting it all apart and just welding brackets to the frame / roll cage to locate the outer control arm mounts and keeping a cut down center section of the subframe to support the camber links, and diff front.

I've also looked at the front roll issue and tried getting the setup from AWR. I'll have to give him a call as e-mail doesn't appear to get his attention (everyone has their best way to contact them).

-Trent

Yeah, I only raised my subframe .25" and everything just barely fits, you would need some major fabrication to do more than that. maybe 3/8" tops.

I have the idea of making my own front roll center, but I am too lazy to buy the bearing to figure the size I would need to get machined so I could fit a snap ring, then source a bolt and spacers. I really should right now since I am waiting on my fenders.