Learn me about spindle/strut swaps?

Hello again.

I am building a first generation Mazda RX-7 GSL-SE for track days and eventual SCCA e production racing.

I have long wanted to change the front bolt pattern to 4x100 (for the purposes of using cheap readily available miata wheels) and get rid of the old style cast brake rotors with integrated wheel bearings. The rear is a live rear axle and easy to redrill or get custom axles made.

Most people would say "just stick with the 4x114.3 bolt pattern of the SE and be happy, it's relatively common vs. the 4x110 on GS/GSLs." But I am crazy.

I am hoping to get my hands on an e30 front strut/spindle assembly or two, as it looks to have similar mounting points and require the least modification. Ultimately I think I need the physical parts in hand to try it out. I am thinking I can cut the strut tube and weld it to my existing setup to get that length correct, this seems somewhat common in the drifting world.

My questions to the kind folks here are:

Can anyone name any other 4x100 cars with similar spindle design? (Front steer, similar control arm geometry)

Does the length of the steering tie rod attachment piece matter? Will my steering be twitchier with a shorter radius from tie rod to king pin?

Thank you very much for your thoughts. If I can do this, I will be sure to document it online for folks to follow.

Here are some pictures:

My shiny new roll cage:

The front suspension looks like this on these cars (not mine, someone else's photo):

The piece that goes between the outer tie rod end and the strut is properly called the steering arm. The steering arm length is extremely critical. Its length is directly related to steering effort, and bump steer. Its vertical position and its angular relation to the spindle axis is also critical regarding bump steer.

I think it would be a lot easier to re-drill the hubs to a 4x100 pattern and then find brakes that fit  (I would look at miata's), then to try and make other spindles work.

Thank you for the advice TurnerX19 - I would like to avoid messing with suspension geometry if I can help it.

Fanfoy - it looks like I will be doing redrilling after all. But unfortunately the front hubs are cast together with the rotor (they are the same piece) so I can't just put on top hats even if I found some with the correct spacing. Ultimately, that would be great, but I think I would need to have custom front hubs made to accommodate top hat style rotors.

My solution as of now is to use a custom drill guide I ordered from Rotten Leonard's Jalopy Shop (orders are now being fulfilled by Hells Gate Hot Rods: https://www.hellsgatehotrods.com/product-category/drill-guides/

I had them make me a custom drill guide to convert 4x114.3 to 4x100. I already did the rear axles, and used normal press-in studs. Because the front hubs are cast together with the rotors, and don't have a smooth back in between the original stud mounting locations, I can't cleanly use press-in studs, as they won't sit flat.

Instead, I bought the correct size 10.5mm cobalt drill bit and will tap out new M12x1.5 threads in the hub at the locations dictated by the drill guide. This way, I can use the screw-in M12x1.5 lug bolt to lug stud adapters that BMW guys use: e.g.: https://motorsporthardware.com/product/57mm-silver-stud-kit-12x1-5/

Since the drill guide is designed to take a larger drill bit, I will machine a steel round stock drill guide insert on a lathe to slip inside the larger hole to snugly index the kerf of the 10.5mm drill bit.

I'll be putting down folded emery cloth over the pilot hole before drilling to 10.5mm to prevent chattering. I will also probably take the whole rotor/hub assembly with drill guide and do the drilling on a mill to ensure the bit is orthogonal to the work.

If I obviously mess up, new front hub/rotor assemblies are cheap so I can try again.

The biggest downside, assuming things are drilled accurately, is I'll have to break out the drill guide and do more drilling every time I need new rotors.

Might be time for some KC raccware front hubs ? 

I'm sure they would be willing to re-drill the hubs to 4x100.

AFAIK there is no simple way to do 4x100 except for wheel adaptors.

There is enough room on certain 4x110 rotors to redrill for 4x100.  Not sure about GSL-SE bits.

You MIGHT be able to do some hackery with first generation MR2 struts.  Those had 4x100 bolt pattern, as opposed to the rear drive Corollas that donated the suspension design, which were 4x4.5.

Personally, I'd avoid doing anything that used GSL-SE parts.  Most GSL-SE specific parts are not available.  You can't buy rear calipers for love or money, because anybody who needs them is hoarding them.

It’s very common in the gm world of putting corvette brakes on everything to use a lathe to turn the friction surface off a rotor effectively making it a hub.  

In reply to Patrick :

RX-7 guys do/did that too.  I have seen FC 11" rotors on early 1st-gens after turning the rotors into hubs.

The problem with that, is you still have the ultra-crappy bearing setup,  The '79-83 bearings were marginal at best.  The '84-85 setup got you a larger outer bearing, but the spindle was still a flimsy POS and they are generally impossible to keep bearings in adjustment.

This was a good chunk of why I binned the whole FB front suspension and went to FC.  New problem:  4 lug FC parts are hard to find because everyone scraps them in favor of 5 lug.

If you intend to run EP with the SCCA, make sure you read the rule book on what you can do. You might want to talk to Jesse Prather @ JPM  in Topeka. He has built a very fast 1st Gen and is very knowledgeable on all things Mazda.

In reply to fanfoy :

What is this beautiful 1990s website and how have I not seen their products before? That looks incredible. Tempting and it looks trivial to machine to 4x100

Although now with what Knurled said about the spindle being insufficient, I probably shouldn't spend too much upgrading the stock assembly and should plan down the road to swap in FC stuff.

AFAIK, SCCA EP just mandates that the brake calipers and steering rack be kept the same, hopefully that's just a task of making custom brackets to put GSLSE brakes on FC spindles

The more I dig into this project the more I think I should have just bought a used spec miata. But where is the fun in that? (Answer: on the track every month, not on the forums and in the shop for hours)

Thank you everyone for your help. This is such a fantastic forum.