S-14 subframe into S-13 discussion...

[racepar1]

I have been planning to put an s-14 subframe into my s-13 for some time now. During this long planning process I have decided to make it an entire car project with 1" of front and rear roll center correction. This plan has brought a few questions to my attention that I just do not have the capability of answering myself though.

1. Track Width
As I understand it the s-14 has a little wider track width then the s-13. From what I can see without making a bunch of measurements the rear width seems to be in the subframe with the same length lower control arms as the s-13. In the front of the s-14 as I understand it the additional width is in the control arms. My question is would it be wise to run longer front control arms with the s-14 subframe? I would imagine that the track width could affect weight transfer some and it might be a good idea to widen the front the same amount. This is probably insignificant, but it has been on my mind and I figured it was worth discussing.

2. Roll Center Heighth
I am curious as to how the roll center heighth's compare between the s-13 and s-14 rear subframe and also between the two chasis in the front. Since I am correcting front and rear roll center heighth's in the chasis any difference may be relevant as the roll axis may be engineered slightly different between the s-13 and s-14. If there is a beneficial difference between the two chasis I would like to engineer that difference into the chasis rather then spacing the joints differently.

3. Rear uprights
The s-14's rear suspension is clearly rotated compared to the s-13's, in relation to the subframe. From what I can see the upright mounting points are rotated too. I wonder if it would be beneficial, geometry wise, to run the correct s-14 uprights or to correct the z-32 ones to match. I understand that "correcting" the z uprights is a huge task, but it's just an idea that I am tossing around.

4. Additional Rear Roll Center Correction
In the front correcting the roll center is easy. You can space the joint or modify the knuckle. Since it's a strut it doesn't really matter. With the rear multilink though there may be consequences to simply spacing the joint down. Since the joint is at an angle if you were to simply space it down you will end up with a little longer control arm to put the spindle in the same position. This could potentially screw up the camber and toe curves. I am thinking that modifying the spindles and re-locating the subframe is the better way to go. If you modify the spindles the difference in control arm length should be less at the least. Of course a longer control arm could possibly be beneficial too. I just do not have the knowledge to answer this question either.

[Def]

1. It's only 5 mm per side. Total track width change is less than 1%. not worth the effort. 5 mm spacers are cheap enough to try it out if you really are worried about it.

2. I didn't see anything obvious to change the roll center height in the rear when comparing the two subframes.

3. I'd just use a Z32 spindle and adjust the arm lengths to give you good bumpsteer and camber curves. At some point you've got to realize that the original design was probably never intended to be used in the travel range we use it in, so I wouldn't consider factory engineering that key towards a good end solution.

4. See above. I'd rather correct the rear roll center and be happy as long as everything else checked out.

4.

[racepar1]

I'd just use a Z32 spindle and adjust the arm lengths to give you good bumpsteer and camber curves. At some point you've got to realize that the original design was probably never intended to be used in the travel range we use it in, so I wouldn't consider factory engineering that key towards a good end solution.

Well my overall goal is to bring the arms closer to the original travel range that they were designed to operate in. How much roll center correction do you think is necessary overall? I am thinking in the range of 2"-3" total, but that is only a moderately educated guess.

[Def]

You mean how much to move the roll center, or space down the lower arm? I'd say the roll center probably doesn't need to move much in the rear, as some casual analysis of the design shows the roll center still seems well above ground when the front is dropping below the ground plane.

I still don't think getting things back to "bone stock" geometry isn't necessarily the best solution, especially given the trouble.

[djsilver]

Caveat; All my experience is in Autocross.

1. I wouldn't make the change for the track width. I put S14 5-lug hubs in front and also changed the LCA's so that the steering stops match up. If you go 5-lug with aftermarket S13 hubs and keep the S13 knuckle it's not necessary. The S14 knuckle/hub also uses a larger spindle and bearing which is good but the bearing is integral to the hub on the S14 so you'd have to replace the whole hub if the bearing goes out. Again, you can use aftermarket S14 5-lug hubs on an S14 knuckle and have replaceable bearings.

2. The problem with the 240sx hiking a rear wheel I think is caused by the car rolling "on it's nose" when cornering due to the differences in front/rear roll characteristics. I don't see any differences in the S13/S14 geometry that would change the roll-center. I made the change to decrease anti-squat and improve forward bite on acceleration and it's done that. I also have the rear lowered about 2" but the front is only lowered 1" and that really helped. I had originally started with an HLSD but went to a clutch style because the rear inside hiked up like my Focus in the corners. Combining the S14 sub-frame, adjusting my ride height and removing the rear sway-bar, I've really got the rear hooked up and I'm getting ready to put the HLSD back in.

3. I mounted almost the whole S14 rear; arms, uprights and all except the upper control arm because the S13 arms interfered with the coilovers. I didn't notice that the mounting lugs on the uprights were different but I never really looked. You just put something on my to-do list....,

4. It would be nice if someone made "drop spindles" or such for the rear but I've never seen any. If you wanted to get the rear lower without affecting roll-center it would probably be easier to cut the unibody and push the whole sub-frame up into the body. The rear has a lot of camber gain on compression and when I lowered mine I used camber-bolts to adjust it back out because the adjustable arms weren't legal in my class. The rules have since changed so I'll probably get some adjustable upper arms before spring. Right now I'm pretty happy with the stability of the rear setup and I "throw it around" a lot more in autocross that you would on the track.

[Def]

Good point on the roll axis inclination likely causing our cars to feel like they "fall over" on the front due to excessive roll from a large roll couple. The arm angles make it seem like the front roll center drops much faster than the rear one when lowering the car. I corrected just the front roll center and the car actually doesn't drive poorly at all the way it is.

I've thought about doing a track event with it just like it is just to get an idea of what the handling balance is like at the limit.

[djsilver]

Good point on the roll axis inclination likely causing our cars to feel like they "fall over" on the front due to excessive roll from a large roll couple. The arm angles make it seem like the front roll center drops much faster than the rear one when lowering the car. I corrected just the front roll center and the car actually doesn't drive poorly at all the way it is.

I've thought about doing a track event with it just like it is just to get an idea of what the handling balance is like at the limit.

Yes, if you can raise the front roll center, it will improve your rear traction tremendously. I can't do that in my class so I just played with ride height.

[racepar1]

You mean how much to move the roll center, or space down the lower arm? I'd say the roll center probably doesn't need to move much in the rear, as some casual analysis of the design shows the roll center still seems well above ground when the front is dropping below the ground plane.

I would like to know what you think about both measurements to tell you the truth. I am not clear on how much spacing the balljoint down actually moves the roll center. It makes sense that the front roll center would drop faster and, as expressed in other posts, that could be one of the major gremlins plagueing our suspension geometry.

I still don't think getting things back to "bone stock" geometry isn't necessarily the best solution, especially given the trouble.

Truthfully I think the stock geometry is pretty good. I mean in 100% dealer stock trim the s-13's pulled over .9G steady state on a skidpad. That's pretty darn good for a 20 year old car on stock rubber. In the end I think that getting the relationship between the front and rear roll center heighth's back to stock is the important part.

[Def]

I would like to know what you think about both measurements to tell you the truth. I am not clear on how much spacing the balljoint down actually moves the roll center. It makes sense that the front roll center would drop faster and, as expressed in other posts, that could be one of the major gremlins plagueing our suspension geometry.

I need to take more detailed measurements, but I'd say raising the rear roll center a few inches is about the most you want to be playing with it for the average car here. That might be about an inch to an inch and a half(WAG here) spacing on the outer balljoint to get around that amount of correction. Take that with a grain of salt though, going from memory here, and my initial measurements were nothing more than a quick eyeball and a little tape measure action.

Truthfully I think the stock geometry is pretty good. I mean in 100% dealer stock trim the s-13's pulled over .9G steady state on a skidpad. That's pretty darn good for a 20 year old car on stock rubber. In the end I think that getting the relationship between the front and rear roll center heighth's back to stock is the important part.

The stock geometry was a good compromise for a car that has 120 lb/in spring rates and a sky high ride height. If you're going to take the trouble of redesigning the geometry of the suspension, I think I'd consider a bit more than just making some aspects "like stock," considering you won't be able to make them all like stock.

[BeerBringer]

Driftworks makes drop-spindels.

Racepar is your car a track or a Auto-X car?

[logr]

I have been wondering about moving the rear subframe up as much as possible, making it tight against the frame. Don't know how much that would give exactly but it would be simple and inexpensive compared to all the other alternatives. Anybody know how much it can move?

Relatively soft and longer rear springs might help with traction a bit. I know one nat. 240 ran 250's in the rear. I hope to try 8in x 225's soon. I have had 450's and 300's in the rear already but the rear of my car is very light, coupe, small tank, etc.

[racepar1]

I need to take more detailed measurements, but I'd say raising the rear roll center a few inches is about the most you want to be playing with it for the average car here. That might be about an inch to an inch and a half(WAG here) spacing on the outer balljoint to get around that amount of correction. Take that with a grain of salt though, going from memory here, and my initial measurements were nothing more than a quick eyeball and a little tape measure action.

Now what about the front roll center? So you would say that spacing the balljoint down raises the roll center 2-3x as much??? I would assume that raising the subframe/FLCA mounting point would change the roll center exactly as much as you moved the mounting points, is that correct? I just wanna have a better undertanding of how and how much to adjust the roll centers.

The stock geometry was a good compromise for a car that has 120 lb/in spring rates and a sky high ride height. If you're going to take the trouble of redesigning the geometry of the suspension, I think I'd consider a bit more than just making some aspects "like stock," considering you won't be able to make them all like stock.

Yeah, that's the real problem is that everything cannot be "corrected". Well, that's why I created this thread.

Driftworks makes drop-spindels.

Racepar is your car a track or a Auto-X car?

Yeah, I just saw those the other day on zilvia. I'm going to look into those a bit more... My car is a track car. As fun as autocross is, it just doesn't quite do it for me.

I have been wondering about moving the rear subframe up as much as possible, making it tight against the frame. Don't know how much that would give exactly but it would be simple and inexpensive compared to all the other alternatives. Anybody know how much it can move?

You should be able to move it up about an inch just eyeballing and guestimating. I'm going to do the same thing with my s-14 subframe and I wanna move the FLCA and tension rod mounting points up an equal amount as well.

Relatively soft and longer rear springs might help with traction a bit. I know one nat. 240 ran 250's in the rear. I hope to try 8in x 225's soon. I have had 450's and 300's in the rear already but the rear of my car is very light, coupe, small tank, etc.

I'm down to 300 myself in the rear at the moment. I'm worried that installing the s-14 rear subframe may require re-setting up the whole car though. All this soft springs and no swaybar junk is all just band-aids for the real problem IMO.

[djsilver]

I don't know if anyone's tried it. The drifters have done a lot of crazy stuff but they like the way the stock rear binds up and oversteers ;-)
I'd say just tape a marker to a ruler and start marking around the frame while it's still mounted. I can't cut on the body in SP. I don't think you can in SM or SSM either but it may be legal in XP.

For anyone running an S13, the single biggest improvement in rear traction is going to be switching to the S14 sub-frame. Second place would probably be raising the roll-center on the front of the car.


I spent some time on www.improvedtouring.com when I started. Here's a list of the various setups I've run across in front/rear/rear bar format;
400/275/?
450/375/none
450/325/OEM
450/250/suspension techniques
600/450/?
450/300/?
450/350/OEM

Most are running either the OEM rear bar or no rear bar. I'm running Buddy Clubs with the standard 7k/5k setup on most asian import coilovers. That works out to 392/280/no bar setup for me. My front effective rate is actually a little lower because I extended the mounting ears on the coilovers about 1-1/2" so I could run front wheels with 6-1/4" backspacing! Logic would say my front rate is a little low compared to the others but I've got a little push now and don't know if it needs to go up. I am running an S14 front bar with the arms shortened about 1" and I'm thinking I might go up on spring rate and drop back down to the OEM front bar.

[logr]

DJSILVER,
I used to have my tabs extended as well but the more I thought about it the more something seemed wrong. Correct me if I am wrong but it seems to me that by extending the tabs you are reducing the camber gain which is already very low. Think of the tabs at 4 inchs long and visualize what would happen when the shock compresses. The camber gain would be a net loss unless I am mistaken. I also can't see where changing the tabs will change the spring rate as you are still pushing on the same part of the spindle. Could be wrong but that is what I surmise.

XP would be the first class that the S14 subframe swap would be legal as far as I can tell. The suspension mounting points must remain unchanged but any bushings are legal, thus moving the original subframe up a bit would be OK.

In Caroll Smith's book "Tune to Win" he says he uses the softest springs that keep the rear of the car off the ground and equal rate bars. Granted it isn't autox but it is something to keep in mind.

[racepar1]

In Caroll Smith's book "Tune to Win" he says he uses the softest springs that keep the rear of the car off the ground and equal rate bars. Granted it isn't autox but it is something to keep in mind.

I think I am going to try out my 21mm hollow JDM s-13 rear swaybar at whatever the next event that I attend is. That does give me something to think about, but I am not sure that I quite 100% agree with the statement.


So for my car I am thinking:

For now with the s-13 subframe I am going to get some solid bushings and leave the rear roll center alone. For the front I am going to look into some spindles for immediate help as they help with both roll center and steering geometry. I know a shop that makes some that I can get for $250 or so + core, but I need to talk to them directly about them and see exactly what they do and how they do it. Since the roll axis point was brought up here it is now clear to me that the front roll center is what needs immediate attention and overall concern.

Eventually with the s-14 rear subframe I will still mount it 1" higher in the car. I have decided against raising the front suspension mounting points though. Since I cannot raise the steering rack up as well that would throw the steering geometry off even worse and that cannot possibly be good. Instead I will do Def's budget lower control arms with spherical outer tie rod ends (likely SPL). I will also look into rear upright options and lower control arm options, but I don't think it will end up a priority from what I have learned here.