well the actual rule states

Just curious if you just measured the OE part or measured a the actual engine mount bracket height on a stock Rx8 to see what the true loaded position is with the entire engine/trans/PPF/driveshaft/rear diff/axle drivetrain sitting on the soft OE engine mounts and soft rear diff bushings?


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I was just in the MM parts store to price out fresh arms, and saw they do have a new competition part for a S1 specific lower spindle offset bushing. Called to confirm my eyes weren't deceiving me. The S2 and NC is 30mm, and the S1 needs a 27mm. They have the parts being made, just not quite on hand (even though you can order them).

Good to know, thanks for the update.

So I’m still a bit puzzled by the previous motor mount comments and picture. It looks like there’s some kind of thick spacer being used that I’ve never seen before. This is what they look like on the website and same for every set I’ve ever seen:

https://cimg6.ibsrv.net/gimg/www.rx8...a0c803d4e.jpeg


Compared to this picture:

https://cimg5.ibsrv.net/gimg/www.rx8...7864d4ac7.jpeg

So since I was messing around with the rear suspension today, I decided to bring some closure to the rear MR debacle. I’m sure that my measurements aren’t exact, but I came up with 0.84 measuring the actual shock stroke vs. axle end centerline protruding out from the hubface slightly. Which is close enough to the 0.855 rear MR that Koni determined and has been using since late 2004/early 2005 when Clyde took the Team WTF RX8 there for the prototype shock build for me to be confident in using that value.

In addition to the shock swinging through some angle arc, it’s also located well inboard of the hubface. So the shock is technically at a shorter distance from the rotation center than the hubface, and traveling a shorter vertical distance than the hubface for a given arc angle. Was pretty sure that a rear 1.00 MR wasn’t probable and proved it well enough for myself today. Would have needed to be off around 0.35” between the two measurements to have calculated a 1.00 MR. I’m not that blind yet :hahano:

So for every 1” vertical rear wheel travel the rear shock/spring travel is 0.855”, or pretty close to that.


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I have no idea how you measured the rear MR at 0.84:1. The hub is a rigid body. How far inboard the shock is has no bearing on the MR. The video below is a good visual and you can see the wheel traces the same path as the lower shock mount, as it must... rigid body.

I measured it several months ago and got 0.98:1. If the shock were mounted to the lower control arm at the same lateral distance from the inner pivot the MR would be about 0.85. That's how I assumed everyone came up with that number.

Sorry, just rechecked enough times to have a big headache, lol; full travel and around ride height shorter travel. Not getting anywhere near 1.0, always around mid 0.8x

My rear shock has 6” total shaft travel and from full drooped to fully compressed the vertical wheel travel was just a bit more than 7”. I did that as a double-check because the amount of measurement error isn’t likely to be as magnified like it would be for a small measurement

the hub is not traveling perfectly vertical, it’s traveling in an arc while also rotating in camber change. In camber gain the axle end is rotating up gaining some travel while the shock mount end is rotating down losing some travel is part of it. It’s hard to measure by yourself. That’s why I did it over and over again, rechecking the measurements over and over again. Using a small square with level bubble to measure down from the fender lip to the level square surface at the axle.


At 0.855 you’re only talking just a bit more than an 1/8” difference between the wheel and the shock for every 1” wheel travel. I doubt you’re eyes are that sharp watching the vid. What you do see at the bottom shock mount is the angle change in the gap between the lower shock mount and the upright. So clearly the hub is angling relative to the shock just as I described above.


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Okay. But even Mazda thinks it's about 1:1.

https://lh3.googleusercontent.com/ke...b=w884-h406-no

apparently I screwed up over a dozen times measuring it every which way but loose then :dunno:

Too bad they didn’t give you an actual value rather than a marketing generality-speak.

Did you divide 6 by 7 yet? I can’t be off by a whole inch. Multiple times.

Not to nitpick, but they do say "approximately one"... And the MR is indeed closer to one than the rear suspension used in the NA/NB or FD RX-7 (internet says ~0.70). Seems like Engineering gave sales only enough information to only partially suffocate themselves, not completely hang themselves.

To be blunt, if you don't believe Team, you should do your own measurements and verify.

Marketing speak is marketing speak, but "approx. 1" doesn't sound like 0.85 to me. As for measuring...

I've measured it several times and come up with the same number. My methodology was as follows. I attached rigid welding rod to the end of the stub axle and another to the rear lower shock mount boss, both dropping down towards the floor of my garage. Throughout the range of motion of the rear suspension I measured the displacement of the ends of those rods. Relative to my static ride height I started 3" into droop and measured 2" into bump.

I wonder if the difference is that I didn't go as far into bump as he did? The rear camber gain is so extreme at the upper end of bump travel that I can see things getting really wacky there. I didn't measure up there because my setup has progressive bump rubbers limiting the rear suspension from getting that much into compression.

Again, maybe I screwed up over and over again for several hours just making sure only because some guy sitting at a computer questioned it. Not just me, the guy at Koni Motorsports too who did this for a living.

All your assumptions were accounted for. I intentionally didn’t measure from the ground because if the chassis moves on the jackstands it will throw the numbers off. I determined the actual shock measurement change directly on the shock. The first round of measurements didn’t use the last 1” shock travel. The shorter measurements were in the 11.5” -13.5” racing ride height range. The hub assy as a whole is travelling in an arc and the hub itself is rotating; any rods sticking off of those points are also arcing/rotating. Which you want the actual shock shaft distance change, not the vertical distance change. The axle change needs to be true vertical. That’s why I used a bubble level square to measure that axle distance change relative to the fender lip.

I’m done here. Use whatever MR you want.


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Well on a more positive note, further measurements showed that whacking the rear fender for 315 tire clearance is easily accomplished within the rule allowance.

At the front the rear pinch weld and front corner light look like potential issues. I don’t have any problem with cutting the fender, except that it might be easier to deal with the corner light by flaring/rolling it instead. Not sure how the fender cutting rule affects a corner light if the fender cut goes through the middle of the corner light? Are we allowed to just cut that part off too?

the rear fenders can be clearanced just with a fender roller.

The fronts, I had to do some minor fender trimming and I cut the corner light's rear mounting surface off of the fender. But I didn't modify the light. I don't see any allowance in the SP rules to modify the light.

It must be positive-reinforcement day, lol ...:rofl:

In addition to cutting out as much sheetmetal as permitted, I have an extra hood and trunk lid to strip down to bare aluminum too :)

there wasn’t anything about cutting hoods that formed part of the fender either i.e. BMW Z3, until it was clarified. What if the allowed fender cutting removed it entirely? What is the purpose of allowing fender, fender liner, etc. modification for tire clearance if 3” long section of plastic corner light is sticking out there begging to get torn off? I suppose it will need to be submitted for clarification about what the actual intent of the rule is in that regard.

Just so we’re clear, all I did is see where the vertical hub plane is at ride height to determine the permitted fender modification area, lifted the suspension until the shock was fully compressed, laid a half circle out on the body panel 1/2” larger than the tire OD relative to the fully compressed axle center, and then drew lines down to the the lower well opening to determine the permitted cutting clearance area.

That works well in the rear, the front is a bit trickier due to the additional clearance needed for the tires being steered/turned and the front hub plane being further outboard relative to the bodywork, but it’s a good starting point. Not really concerned about DOT regulations or style points ... maybe I’ll throw on some aluminum sheetmetal hoops to keep the rocks at bay.
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Probably best to submit a letter for clarification of the rule, regarding the front corner light.

What I did was cut the rear mount off of the fender for the corner light and left the unmodified corner light in place. To keep it from popping out and to hold the bumper corners to the fender, I used those ricer bumper O-ring things (no idea what they're called) with the rubber o-ring covering the outside of the corner light.

The bigger problem is the pinch seam behind the front tire. I think David Colletti found a way to align the car and limit steering so it's a non-issue but he doesn't post here often.

Yes, I believe the same Mustang limiting spacers the FD3 guys use on their steering rack also work on the RX8. Thought I had posted it before, but here it is again:

https://www.rx7club.com/suspension-w...miter-1117648/

https://lmr.com/item/LRS-3504HDW/79-...Limiter-6-Pack



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Seems like those would be prime to popping off. I made a set that were just round, not c shaped. But, different rack.

How limited is your steering vs the Tamdrew car? Hopefully a fair bit more.

I doubt they pop off the rack anywhere near as easy as either you or I do on teh intrawebz ... :hahano:


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Isn't it enough knowing that you can update to an S2 bumper cover, headlights and fenders.

Measured the same when I first got my car sans spring at ride height. Agreed, you can see the articulation of the bushing in the video.

Maybe you should clarify which “same” you agree with just to be sure, lol. I intended to measure it one more time tonight just to make sure I wasn’t going crazy, but was futzing with the brake rotor components that finally showed up.

The brake stuff is looking good overall, but still need to find someone with a big enough lathe to trim the ODs and widths down to the OE dimensions allowed by the rules. Before I started this it never occurred to me/never previously noticed that the brake rotor has a different hubcentric size than the wheel; 2.83”/71.88mm. The hubcentric for the new hats is 2.80”, so also have to get those IDs turned slightly before stuff is going to bolt on.

Yeah, that’s what I’m talkin’ about :beerchug:, too many folks screwing around with compromised street spring rates ...

So... bickering aside, who is going to Bristol?

Quote:

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Originally Posted by roflcopter (Post 4888521)

So... bickering aside, who is going to Bristol?

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some of us should consider running for Congress :rollingla

Just saw the Mineral Wells Pro is in my backyard this weekend, but need to work on the RX8 instead.

So doing some calculations based off actual weights and measurements, but approximating density and assuming I didn’t mess up the calculations a half dozen times in a row :hahano: came up with these initial estimates:

11.95” OD x 0.95” wide 2-pc front rotor - 11.6 lbs **actual

11.89” OD x 0.71” wide 2-pc rear rotor - 8.2 lbs **actual

These are full face rotors; no slots, drilling, etc. An SM build could take advantage of 11.75” front & rear “MX5” sizing to save some more, plus those are standard rotor ODs so less machining involved than OE RX8 dimension limitations of SP. For SP I have to trim the next closest sizes down both in diameter and thickness to achieve these specs.




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