peejay

Technically, the FC strut is not inline with the ball joint. The FC ball joint is a tiny bit rearward of that centerline. I know this because I had to grind the top of my ball joint down so I could bolt the struts in, and they sit forward of the ball joint stud.

It's like maybe 1/4-1/2" and this translates to a minimal change in caster.

If you use the existing FB forward subframe mounting studs, however, the wheels will not be in the same spot. They will be an inch more forward in the chassis. Thus why the fender will need the forward corner cut back and the inner fender massaged. Doing it this way gives more caster (good in our case) and shifting the frotn axle forward will move the weight distribution back slightly. It also requires less fabrication on the chassis side, but you will now need to modify/make new motor mounts since the motor mounts will no longer line up.

Ey8s

Peejay and Stevan, thanks for responding with input. The one variable I couldn't account for was a potential difference in ride height since both vehicles I'm working with were acquired without drivetrains. I contemplated loading weight into the engine bay but took it for granted it was a moot point since there was really no mention of ride height differences in anything I read in regards to this swap. Now I'm a bit perplexed, I'll have to carefully consider this situation. Ultimately I don't mind cutting/welding if I have to. I've already cut up two vehicles and have to make this work. Here's a pic of where the strut tower caps are in relation to the hood line - that's pretty much maxed out. edit - if the one guy raised it correctly - what did he do about strut tower height? I would think you would need to cut holes in the hood and have them almost sticking out to be "correct" IDK

peejay

The strut tower height doesn't affect suspension geometry (practically speaking), it only affects where the top and bottom of suspension travel are.

With the Escort strut tops and stock towers, this is where FC struts top out (tires are 195/60-15)







And this is the angularity of the control arm/tie-rod at that point:



Roll center is somewhere around China at this point

Stevan

iTrader: (17)

Peejay, thanks for straightening me out about the ball joint offset. I have the parts and remembered seeing after I read your post. I went and dug out the FC strut assy and it looks like about 1/2" offset. A quick ghetto measurement with a protractor and a string yields "about" 2º.
So that may account for Ey8s's measurements indicating the need to move the towers rearward. I'm going to use coilover springs and a trimmed strut top/pillowball so there should be enough room to slide the strut top rearward camber plate style if necessary.
As far as the height of the strut towers I believe we were both thinking about suspension travel and ride height, not geometry/handling. I'm going to cut the subframe to raise it 1-3/4" higher in the chassis so this will shorten the distance to the strut top. Just want to make sure it has full travel. I guess mounting the subframe and installing the strut w/o the spring will tell what needs to be done.

peejay

I don't see how altering the subframe will change anything related to the strut top.

Ey8s

I also noticed the balljoint offset early on when I was experimenting with different FB/FC mix/match assemblies.

OK - Here's a before (initial mockup) and after (subframe setup + FC strut tower install) pic:



This setup (the after pic) uses ALL stock FC suspension and steering parts. It also duplicates the stock FC geometry - which I was as careful as I could be to duplicate. You can see that the wheel is centered in the wheel opening and that it's not hanging as low.
1) I think the ride height should be just right when it's on the ground with the engine/trans/accessories in it.
2) The roll center should be FC-like since the relationship of parts is the same.

I wanted a very stock-like ride height and appearance from the beginning. I don't see how this won't work. I can see where the roll center is wacky in peejay's pic but the tire is stuffed to the max with that setup. Input appreciated guys.

Eric

peejay

And that is not necessarily a good thing, because the FC's rear suspension also had a really low roll center so everything was balanced. The 1st-gen has a rather high roll center in the back so the front also had to be higher to balance things out.

A car is a whole thing and can't be considered in sections without also considering how it affects the whole.

Ey8s

I'll have to consider that when I setup the 8.8 in the back. Good perspective to have going in, thanks Peejay.
I went out and measured the hub center to fender lip distance and it was 17.25". If I deduct the 12" radius of the wheel/tire that leaves 5.25" of tire to fender distance unloaded. I read somewhere that the factory front spring rate is 130#/inch. If my FB ends up weighing 2500# with a 50/50 wd that would put 1250# on the front suspension static. 1250 / 2 /130#/inch = 4.8" of compression (totally theoretical based on linear compression) which would net me at 1/2" of tire to fender lip clearance. I'm sure there will be a bit more clearance than this because the spring rate is probably more progressive but if I end up with 1 1/2"ish clearance I'll be happy with that. If it ends up being a little more front heavy it'll help with stance more than hurt. I can certainly see if you want to stuff the tires into the wheel wells more you would want to raise the subframe to raise the roll center. For the rear I will be making my own torque arm setup and I'm thinking panhard vs. watts for simplicity at this point. I can play with the roll center when I get into it, even make it adjustable.

Ey8s

I did some measuring. The radius from the subframe pivot to the hub dust cap is @17" and from the pivot to the ball joint is @12". 4.8" of compression at the hub would equal @3.375" at the ball joint. So theoretically if I realize this amount of compression at static ride height the lower a arm should be more or less parallel with the subframe which would net a much higher roll center than the FB in Peejay's pic.

Aaron Cake

iTrader: (1)

Whoa...Escort strut tops? After reading that I did some searching and found that you guys are referring to ZX2/Escort strut tops.

Does anyone have a comparison picture between those and FC strut tops? I did some Google image searching but didn't come up with any thing definitive but it appears that they locate the strut/spring much higher than FC strut tops, which move it down about 2" (compared to SA/FB mounts tops).

Because if the Escort top moves the strut/spring assembly up, then it may save me the trouble of raising the top of the shock towers on my Cosmo.

Ey8s

The FB in Peejay's pic is at full stuff so the ride height should obviously raise the roll center. I would think that planned ride height has the biggest effect on subframe orientation. The next consideration would be strut/coil travel which would dictate strut tower cap location depending on the strut/spring package to be used. Packaging is obviously limited to the hood line unless you don't mind running hood blisters there. I originally considered going with the FB/FC hybrid setup but figured it would be easier in the long run to keep everything FC as well as having more off-the-shelf options in the future as well. I will be fitting the FC tilt column in next before I can start playing with engine/trans placement. The rear will be after that so I can properly phase the pinion angle. I may have to borrow some plate weights from work and put 650# (engine weight, misc and 1/2 the trans weight) on the subframe and see how she sits up front.

peejay

The Escort tops are dimensionally identical to SA/FB but they are laid out like FC struts, holding the strut shaft stationary in a D-hole and putting the spring perch on its own bearing. The D-hole is smaller but that is easily fixed.

The design of the FC strut tops looks like it was an attempt by Mazda to move the spring higher up on the strut for tire clearance, without altering actual strut length. So the spring seat is actually higher than the top of the strut shaft.

To repeat: Using Escort mounts makes the suspension bottom and top out about an inch-40mm higher than using the FC mount, with no other changes. I have no data on the spring perches because I fabricated my own.

Stevan

iTrader: (17)

Meant to type strut tower top.
The closer the subframe/control arm assy is to the strut tower top = less travel.

j9fd3s

iTrader: (3)

but the higher the inner pivot the better the roll center is going to be.

the Rx8 training manual of all things has a good explanation of roll centers. Mazda draws a line between the front and the rear roll centers and the movement of that line in a corner is the important bit. so we want the front and rear roll centers to be around the same place and not move anywhere drastic. the rear in the FB is just the pivot of the watts link, so it is fixed.

as PJ's picture shows there is only so much room to lower the car before the tire hits the frame, so we'd guesstimate the suspension travel we need, (we can compute it too, there are some suspension spreadsheets out there that take lateral G, and will give you roll angle, and roll in inches, you have to add for bumps), and come up with a ride height.

depending on where that ends up, you might raise the shock towers to accommodate stock length shocks, if the rules allow, most classes won't. there is also lots of precedent for bending the upper rails/shock towers for more camber, and the miata people bend them up, so the car is lower.

Stevan

iTrader: (17)

[QUOTE=j9fd3s;but the higher the inner pivot the better the roll center is going to be.[QUOTE]

I know this, but not much more . I was talking about raising the strut towers to accommodate stock FC struts with the subframe higher up in the chasiss.

[QUOTE=j9fd3s; as PJ's picture shows there is only so much room to lower the car before the tire hits the frame, so we'd guesstimate the suspension travel we need, (we can compute it too, there are some suspension spreadsheets out there that take lateral G, and will give you roll angle, and roll in inches, you have to add for bumps), and come up with a ride height [QUOTE]

Yeah when I saw that pic I thought there's no sense raising the towers if the tires going to hit the frame. But if Peejay's car had the subframe raised 1.75" then the strut would bottom out before the tire got that close. Like I said earlier, it will be easier (for me anyway) to put the subframe and strut assy w/o springs in and see if/how much the towers need to be raised.

peejay

The subframe position has nothing to do with the strut position.

I bolted the subframe to the body as per normal practice, then ended up raising the *ride height* an inch or so in the front to get the handling back to where it felt good. I did this at first by modifying FC strut mounts to bolt into the FB towers, now I have the longer struts that do the same thing while also giving me more suspension travel. I discovered that the FC strut mounts like to separate.

The strut towers determine where body height is at a given suspension travel, the subframe height determines where the suspension geometry is at a given body height. I assume most people don't want to be able to just walk right under their car and start working without needing a floor jack

The person who modified the body to put the subframe higher in the chasiss is Urgent! How to control a Mazda3 (focus?) Electric PS pump? :: motorgeek.com
Sadly, it appears that the images have been taken down. There's a link to a Facebook page, but I don't want to infect my computer with whatever Facebook does if you click on one of their links.

edit - forgot about "private browsing"

Aaron Cake

iTrader: (1)

That sounds promising then. As the SA/FB (and Cosmo) mounts place the strut shaft nut above the top of the shock tower (more or less). While the FC mount moves it down about 1.5 inches. That's reducing the available strut travel once the car is as low as I'd want it. So if I can gain the same change by swapping the strut tops vs. having to cut the top of the strut tower off and move it up, I'm all over that. Looks like I'll need to hit the parts store and take some measurements.

I saw that when I was measuring. Having never taken a good look at the spring seat before I hadn't noticed how it's pressed up. Not really important for me as I'll be hacking them up (or potentially Escort spring seats) for air bags.

Excellent.

twinkletoes

iTrader: (17)

Welcome to the forum! Excellent info in here! I followed 82transams thread on the install of my FC subframe swap with a few adjustments done differently. My setup was mounted below the FB frame uncut. It works good, but not great like peejay stated. I had a set of custom BC coilovers made for my setup too. My coilovers have FC bottom mounts and FB strut top mounts. I love my swap and the advantages I got from it, but there's a couple things that bug me.

1. I have a bouncy ride only up front on certain highways/roads that aren't completely flat. Bouncy as if someone cut their springs. My spring rates are 6k or around 350lb up front and 150 in rear.

2. My control arms are in the slanted "up" position when the car is just under its own weight. Your pic in post #34 is how it should be. My control arms are just slightly above the center line just under the cars own weight. I know this is a issue.

I'm not sure if moving the strut top an inch up will fix my problem, but i'd like to think it will. Keep up the great work!

Stevan

iTrader: (17)

[QUOTE=peejay;11847309]The subframe position has nothing to do with the strut position.

The strut towers determine where body height is at a given suspension travel[QUOTE]

The strut tower height also determines suspension travel at a given subframe and body height.
Please correct the following methodology if I'm mistaken;

Determine ride height/body height desired, (I want the car at a certain height).
Place the subframe in the chassis at a height (within chassis/engine limits) to achieve optimal suspension geometry at the given ride height.
Adjust/modify/change strut and spring combination and if necessary strut tower height to achieve optimal suspension travel.

I don't know if raising to strut towers will actually be necessary.
I do not want to have to change the ride height (much) to get travel.

Stevan

iTrader: (17)

1. Sounds like your shocks are not strong enough to control the spring rate.
2.Raising the tower will make it worse. Three ways "I've read of" to correct this;
Raise your ride height(I know with your coilover adjuster, a longer strut/spring combo.
Raise the subframe in the chassis.
Space the balljoint/control arm away from the spindle. I don't think there is a part available for this but the FC racers talk about it and it has been done from what I've read.

Ey8s

OK, lots of good input Peejay.
So here's my summary of what I get from this:

1) If you want to maintain a decent roll center when doing a FC subframe swap you want to raise it as much as possible within SA/FB frame rails to move the lower control arm pivot points up.

2) If you want to use FC strut/spring assemblies while doing this you need to move your strut tower tops up (and back at least 1/2" IMO) or swap in FC strut towers (like I did - which is definitely a bit more involved). IN ADDITION to this you would want to use some kind of spring hat/pillow block arrangement, like the escort setup as described by Peejay, to gain the strut stroke lost (if using FC struts/springs) by raising the subrame into the frame rails. Otherwise you'll have to fabricate a hybrid strut/spring setup to work with the available space.

If I was pursuing a track and/or rally car (nice vids BTW Peejay!) this would certainly be the route I'd go - especially with rotary power.
In my case I'm installing an LQ4/LS and it's a tight fit to keep everything under the stock hood as it is - with the FC steering rack location in the subframe this is a no-go for me since this will be mostly a street driven vehicle. For a V8 powered competition FB or FC I would probably move the engine back as much as possible - at least 4" and redo the firewall/trans tunnel. I've already looked at this and it looks like 2" is possible without even moving the throttle pedal - the hiccup is the wiper motor which looks to only allow an inch without interference or some other setup.

Thanks for the continued input - I enjoy learning!

Eric

peejay

Oh yeah, if you are doing an engine conversion at the same time, hood space constraints will have to take precedence over theoretical gains.

Like the Mopar guys who sometimes have to shift their K-frames down an inch for hood clearance with radical intake manifolds if they don't want to cut a hole in the hood. This drops the engine an inch... also puts the oil pan into the steering linkage, so that has to be modified as well... And the headers might foul, and this and that and the other... Good times And that is before you get into the theoretical no-no of having the torsion bars working off-center to how they are mounted!

Maztang5.0

iTrader: (1)

I'm so confused on all of this. I want to do an fc swap so bad but this has me second guessing everything. I'm looking for a better suspension setup that I can control better to put into my drift fb. We need more steering angle and a quicker steering ratio. So I thought fc with power steering would be just the ticket. Is there any way to get this swap right?

82transam

iTrader: (18)

Go the route that I and twinkletoes (and others) went and just attach the FC subframe to the bottom of the FB rails. Then use some short stroke coil overs (assume you'll have it low since it's a drift car) with stiff springs and you're good to go. It's not ideal as they've pointed out above, but for a drift car it should be fine.

This thread has some excellent discussion in it, but honestly it's making my head spin too! LOL

One thing I've been thinking about lately - to correct the control arm angling up, why don't we just bolt the ball joint on the top of the control arm rather than the bottom?? That would make the arm move down by that 1/2" or so. You could then use one of the many bump steer kits available for the FC to correct the tie rod angle... Just a thought...

j9fd3s

iTrader: (3)

the thing we care about is actually pivot point to pivot point, so its the center of the ball joint to the center of the pivot axis of the inner control arm (center of the front bushing is close enough, imo), so the arm itself is irrelevant.

with the FB we can just buy these https://technotoytuning.com/mazda/fb...-first-gen-rx7

with the FC you're stuck making a ball joint extension, or going to a rod end, both are more complex, expensive, and less reliable.