Playing with S14 subframe

[Epstein]

So after seeing djsilver's post (and thread on FA) I decided to track down the necessary parts for an S14 subframe swap into my S13. But while its out, lets play with it first. This is complete subframe with stock arms from a stock car with stock alignment settings.

Propped it up loosened all the bolts and started sweeping it through different heights with a jack. The one thing I've heard is that "S14's have crazy camber curves". I know that this is not the most accurate measurement technique, but I strapped my ipod touch to the spindle with some bubble level software and took some readings. The overall numbers might be off, but the trend lines are most likely accurate.

At stock ride height, the camber (on both sides) measured -1.0* which is what a stock setup should measure. Excellent. Then I swept it and took readings every 1/2". Then I put on an adjustable traction rod and shortened it 10mm, which is what the internets says I should do. Repeated the measurement process.

Took my data, plugged everything into Excel and did a 2nd order trend to smooth the data. Y-axis is height and X-axis is camber. Think of it as looking at the right rear wheel from behind. Heights have been rescaled to reflect my ride height, which would put this setup roughly 1.75" lower than stock in the rear.

Blue line is stock arms. Red line is with traction rod at -10mm.

[Epstein]

I used an increasingly ghetto method of measuring bumpsteer. Centered at the lowered ride height, I got a curve that toes out by 5mm if you go 1.5" higher or lower than the stationary position. I need to build a better jig or buy a real bumpsteer gauge. Measurements taken assuming a 25" tire.

[Def]

So I'm assuming positive height numbers on the Y-axis reflect the suspension compressing and gaining camber?

Otherwise, it's an interesting comparison. Any chance you could do a little playing with the S13 rear subframe? What are you doing for the subframe bushings?

[Epstein]

Yeah, positive height is the suspension compressing. I appologize to people who have seen real plots like this. I took a guess on how it should look. You can see that the shortened traction rod setup gains 3/8* more camber than a stock-type setup at 1.5" of compression.

I should also add that shortening the traction rod made it bind pretty hard. With the bolts loose and stock length, I was sweeping 5+" without needing to hold the subframe down. With the short rods, I needed about 50 lbs on one corner to hold it in place.

I'll have my S13 subframe out soon, and I have the lengths of the stock arms written down somewhere. I'll pull numbers on that later on.

As for subframe bushings on the conversion, I'm going to drill out most of the stock rubber so that it deflects laterally easier, then fill it up solid with Windo Weld. Way back, I filled up some stock subframe bushings with Windo Weld and it worked really well..... about as solid as my Nismo subframe bushings but not as stiff as Peak Perf urethane locking spacers.

[Epstein]

Finally got the S13 subframe setup to take some readings. I only did it with the stock arms to get an idea of how close the S13 and S14 stock systems are.

Black line is stock S13
Blue line is stock S14
Red line is S14 with 10mm shorter traction rod
all lines offset so that they cross at zero on the Y-Axis (vertical wheel displacement)



So it looks like things are largely the same between the 2 subframes as far as camber curve. I should also mention that I found that both sets of arms (toe, ruca, traction) are all the same length between the S13 and S14.

[Def]

Hmmmm.. I've been debating going with an S14 subframe in the many months ahead that my car will likely be down for work/mods.

Anybody know how much anti-squat the S13 subframe gives a typical lowered car? I think it's expressed in percentages.

[Epstein]

OK. Built a double dial bump steer gauge from a bunch of scrap 1/4" plexi and a few sticks of angle aluminum to brace it. It's beautiful! I like that since it's clear, you can see what's going on behind it. Plate has +/-3" range, like you'd ever have that much travel. Plate is 24" wide, for those converting to toe angle at home.

Data says that the S14 subframe has a WAY flatter toe curve than the S13. One caveat to this data is that I've extended the Traction Rod about 6mm to restore the bushing alignment from dialing out camber at the RUCA. This should serve to flatten the toe curve as the angle on this arm would build slower. This is already the shortest arm by 4". The S13 toe curve is more of a broad arch.

Heights (Ht) data is listed in inches as you would look at the tire. That is, inches of compression at the top, inches of droop at the bottom. Toe is measured in inches with positive numbers shown as toe-in. All data zeroed at the ride height. I'll make pretty plots of this later.

S14 subframe, Traction Rod +6mm, rest is stock. Zeroed to 1/2" fender gap ride height.
Ht : Toe-in
2.0 .086"
1.5 .053"
1.0 .028"
0.5 .012" compression
0.0 .000" ----ride height
0.5 -.007" droop
1.0 -.007"
1.5 -.003"
2.0 -.020"

S13 subframe, All arms stock. Zeroed to 1/2" fender gap ride height.
Ht : Toe-in
2.0 .192"
1.5 .105"
1.0 .062"
0.5 .026" compression
0.0 .000" ---- ride height
0.5 -.051" droop
1.0 -.096"
1.5 -.101"
2.0 -.105"

[Epstein]

Here ya go. If someone can tell me what a standard plot looks like (x-y axes, magnitude) I'd be glad to reformat it.

Positive toe numbers refer to toe-in. Positive height numbers refer to compression.

[racepar1]

My question is this; what is the difference between the two subframes that makes the toe curve "better" on the s-14 suspension? Is it because the RLCA has been re-positioned? Has the pick-up point for the toe arm on the subframe been altered? Has the pick-up point for the trac arm on the subframe been altered? Have the pick-up points on the spindles been altered? I ask because I have a set of j-30 spindles right in front of me and the pick-up point for the toe arm on those uprights appear to be slightly different then the pick-up points on the s-13 spindles, which are also right in front of me. As far as I can tell the j-30 spindles are the same as the s-14 spindles. If it would make a difference I could just swap the j-30 spindles onto my s-13 rear subframe and get a better toe curve. Unfortuanetly I am not skilled/experienced enough to measure everything out and really figure out what actually makes it "better".

The one thing I noticed about those curves that could make the s-13 toe curve theoretically better is that it is more consistent. It gains more toe, but it seems to gain the same amount of toe throughout the entire range of suspension movement. The s-14 toe curve begins to gain drastically more toe as the suspension compresses more, which would be a problem if your car is pretty low.

[Epstein]

I'm thinking that the RLCA probably puts the lower pivot in a different place. I can't really see anything majorly different between the 2 at just a glance. I'll check it out in the next few days. I'm pretty sure that the uprights are the same as far as locations. For the record, I'm using a Z32 upright on the S14 subframe, and an S14 upright on the S13 subframe.

I'd take the S14 toe myself. While the S13 curve looks smoother, you've got to see that in the last 1/2" of compression you're adding twice as much toe as you would with the S14. I'm getting a little tired of taking everything apart, but I might do some more curves with the traction arm at different lengths.

I should also add that the caster gain (I guess you'd call it) on both setups is substantial. A point 15-18" forward of the hub center probably wouldn't move at all through the measured 4" of suspension height change.

[Epstein]

Did a couple more plots on the S14 subframe yesterday. I wanted to see what the effect was of changing the traction arm length. The stock length is 8.25". I also tried 8.50" and 8.00".



You can clearly see that with the stock length (8.25"), toe is very flat across the -2" to 0" section. This would be the first 2" of compression on a stock height vehicle. With the arm lengthened to 8.50" toe is constantly changing with height. Looks like I was going the wrong way with my adjustments (i guess that's why we measure things!). What I'll call the Ziptied theory of shortening your traction arms looks to be the best bet. At a length of 8.00" the toe curve is nice and flat through the first 1.5" of compression. The Ziptied theory actually specs a 7.85" (10mm shorter) length arm, which would probably flatten the curve higher up. This correlates to the fact that they run their cars a lot lower than mine.

I'd like to point out the magnitude of these plots as well. None of these curves ever go near 1/8" of toe in, and the toe out that we're seeing is miniscule. I would say that this subframe has pretty good toe geometry all around.

[Longfellow]

Im due for an alignment soon, so this is good news. Do you think I have enough adjustment in the stock traction arm to get it to 8 inches? Eventually I'll get the SPL unit but I have other things to do before that.

[Def]

Stock traction rod is not adjustable. Only camber/toe arms have eccentric bolts, and those don't offer much adjustment really.

[Longfellow]

Hm, I never really looked at the traction arms. I thought all the rear upper arms had camber ecc. bolts. I dont even have the camber bolts in my RUCA any more, I had to cut both off when I put on my new arms and I've yet to replace the standard bolts that are currently in there.

[Def]

Maybe I'm on crack, but I can't seem to recall the traction rods having eccentric bolts, and I was just in there about a week ago. I have too much rattling around in my head though, so entirely possible I am on crack...