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The Official "RX8 in DSP" Thread

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The Official "RX8 in DSP" Thread

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Old 04-04-2018, 10:18 PM
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I’d be curious to see if anyone is using that number in their math... It puts the FRC of a lot of off the shelf setups closer to the 60% number, which makes sense compared to other cars.

I still want to experimentally prove it... just because. FCM says they’ve experimentally proven it to be .84, but I think they were doing an undamped bounce test and working backwards which definitely is going to introduce a lot of error.
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Old 04-05-2018, 06:28 AM
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Was there some debate that it was not 1.0?
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Old 04-05-2018, 06:40 AM
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Shaikh at FCM thinks the rear motion ratio is 0.84 based on tests.

Admittedly, his spreadsheet is adapted from his NC Miata spreadsheet and a number of things weren't changed so who knows if the notes indicating this are accidentally left over.

FCM_MSDS_RX8.xls
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Old 04-05-2018, 06:58 AM
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The NC has nearly identical front and rear suspension geometry to the RX-8.

Unless the arc traced by the rear lower shock mount pivot is appreciably shorter than the arc traced by the wheel I find it hard to believe the MR is appreciably less than 1.0. I have two cars here, so I guess I should measure them.
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Old 04-05-2018, 07:04 AM
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In this thread someone said it was 0.855 for the rear.

Using 1.0 for the rear motion ratio, my inherited STX setup (615/445 with Hotchkis bars set to medium) has a 67% front roll couple and is pretty neutral on course. This agrees with Kenneth's observations of 66% being the neutral point.

If I change the MR to 0.855 in my spreadsheet the FRC becomes 74%.
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Old 04-05-2018, 07:20 AM
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I'm in the camp that thinks it is less than 1.0. My my chemistry degree probably means I'm an awful engineer but here's my thought process.

The motion ratio describes the difference in spring movement relative to wheel movement.
A ratio of 1.0 means that there is no difference in relative motion. 1" of wheel travel = 1" of spring travel.

However, the attachment point of the lower spring/shock mount is several inches inboard from the wheel center. Even if there is camber change and the shock isn't vertical, the motion of the two points will describe circles of different radii with a common center at the pivot point.

If the radius of the circle described by the lower shock mount is smaller than the radius of the circle described by the hub then the motion of the spring must be less than the motion of wheel. Thus, we have a motion ratio that is less than 1.0.
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Old 04-05-2018, 09:44 AM
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Originally Posted by John V View Post
Was there some debate that it was not 1.0?
I've seen it quoted here, and on other websites, as anywhere between 0.83-
0.86, but had never seen it as 1.0.

Originally Posted by schickane View Post
In this thread someone said it was 0.855 for the rear.

Using 1.0 for the rear motion ratio, my inherited STX setup (615/445 with Hotchkis bars set to medium) has a 67% front roll couple and is pretty neutral on course. This agrees with Kenneth's observations of 66% being the neutral point.

If I change the MR to 0.855 in my spreadsheet the FRC becomes 74%.
What front motion ratio are you using? And I wonder what MRs Kenneth is using... I'm just hoping we can find what is 'correct' and at least all be talking the same language, would make establishing baselines and comparing setups a lot more productive.

Originally Posted by NotAPreppie View Post
I'm in the camp that thinks it is less than 1.0. My my chemistry degree probably means I'm an awful engineer but here's my thought process.

The motion ratio describes the difference in spring movement relative to wheel movement.
A ratio of 1.0 means that there is no difference in relative motion. 1" of wheel travel = 1" of spring travel.

However, the attachment point of the lower spring/shock mount is several inches inboard from the wheel center. Even if there is camber change and the shock isn't vertical, the motion of the two points will describe circles of different radii with a common center at the pivot point.

If the radius of the circle described by the lower shock mount is smaller than the radius of the circle described by the hub then the motion of the spring must be less than the motion of wheel. Thus, we have a motion ratio that is less than 1.0.
The main point I came to was that if the wheel is bolted straight to the hub, and the lower shock mount is bolted straight to the hub, then they move the same distance resulting in a 1.0 ratio. As the suspension compresses, the dynamic camber change as well as the shock angle changing would in-fact alter this slightly, but not enough to cause an measurable difference, the rear ends of these cars are quite well designed. Along with my comment on wheel offset, the wheel center doesn't play into the equation at all, the lever arm and force on the spring does not change with wheel location. It WILL change the compression/extension loads on the arms because the hub will have more rotational leverage on them, but as long as they do their job and keep it in place then the MR won't be altered.
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Old 04-05-2018, 10:00 AM
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Originally Posted by roflcopter View Post
What front motion ratio are you using? And I wonder what MRs Kenneth is using... I'm just hoping we can find what is 'correct' and at least all be talking the same language, would make establishing baselines and comparing setups a lot more productive.
Here's all of the motion ratio inputs I'm using, from post #859 in this thread:
  • Front Spring MR = 0.752
  • Rear Spring MR = 1.0
  • Front ARB MR = 0.604
  • Rear ARB MR = 0.509

For weights and track widths I'm using the FCM data:
  • Front Unsprung = 34 lbf (not incl. wheel & tire)
  • Rear Unsprung = 31 lbf (not incl. wheel & tire)
  • Front Track = 58.9 in.
  • Rear Track = 59.3 in.

I've been using 2800 lbf as the vehicle weight for my ride frequency calculations. I'll weigh my car at the NJ Pro.

Last edited by schickane; 04-05-2018 at 10:30 AM.
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Old 04-05-2018, 10:39 AM
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I can't fathom where someone came up with 0.855 MR. Flawed measurements, maybe? I had been using 1.0 in my spreadsheets.
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Old 04-05-2018, 11:44 AM
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Originally Posted by John V View Post
I can't fathom where someone came up with 0.855 MR. Flawed measurements, maybe? I had been using 1.0 in my spreadsheets.
In another thread that number is thrown out there as being the value given by Koni NA when asked. I think someone got excited with a tape measure and didn't stop to look at the big picture. I'd put money on that being the number you get if you measure from the lower arm attachment point to the shock mount and hub.
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Old 04-06-2018, 11:47 AM
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I'm actually using 0.855, though I had the same thought that it didn't make much sense that it wouldn't be somewhere around 1. I got the number from here.

Originally Posted by TeamRX8
RX-8 motion ratios:

front = 0.752

rear = 0.855


I'm sure that Mazda Motorsports can confirm the exact rates for the springs. I was simply curious to know how the information was obtained.
I hadn't put too much thought into it before, and just sorta accepted secondhand Koni info, but let's see if we can justify this:

Assuming that the distance from the contact patch to the shock mount on the upright is ~6" (just making up numbers that don't seem too ridiculous) and the rear wheel gains 2 degrees of camber in 4" of travel, that already works out to a MR of ~0.94 due to "lost" motion from camber gain. From there, it only takes a couple degrees of shock angle to get down to 0.885. This is probably worth measuring.

Anyway, changing it to 1 drops my numbers by about 2% because I run huge bars, but for me, the actual number doesn't matter as much as the trend that I use for myself as a tuning tool (obviously it's a better tuning tool if all the numbers are right). It would be nice if we were all on the same page though!

Last edited by Kennetht638; 04-06-2018 at 11:55 AM.
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Old 04-06-2018, 05:48 PM
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Seems easy enough to measure anyway; the shock shouldn't contribute much to suspension location, so it can be removed to measure the distance the shock mounting stud moves when the wheel moves x distance.
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Old 04-11-2018, 11:01 AM
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I assisted former RX8 owner Clyde with using his 2004 RX8 for Koni NA to fabricate the first set of shocks for the application. They used that information to produce the Koni S1 RX8 OTS shocks you can buy today. Those were the numbers the Koni NA tech guy (Jay, he no longer works there) gave me from his measurement sheet.

There’s no way that it’s 1.00, you can see that just by looking at the slight angle it has ...
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Old 04-11-2018, 02:44 PM
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Originally Posted by TeamRX8 View Post
I assisted former RX8 owner Clyde with using his 2004 RX8 for Koni NA to fabricate the first set of shocks for the application. They used that information to produce the Koni S1 RX8 OTS shocks you can buy today. Those were the numbers the Koni NA tech guy (Jay, he no longer works there) gave me from his measurement sheet.

There’s no way that it’s 1.00, you can see that just by looking at the slight angle it has ...
The angle of the damper doesn't necessarily matter. It only matters if it isn't equal to the camber gain at the hub (which it probably isn't).
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Old 04-15-2018, 10:54 AM
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Any suggestions on front control arm bushings? We went with Powerflex last season, and the front lower control arm front bushings are already worn out (grab the wheel and you get 1/8" of movement in and out from the bushing site). Dynamic camber! Our control arms were new at the time of install so they are still in great shape, plus we like how much freer the control arms move with the poly bushings.

Thinking of maybe trying Whiteline next, but let me know your guys' thoughts.


In other news, ran NNJR SCCA yesterday with great results. The car was great! Schickane here on the forums co-drove with me and he ended up top pax AND FTD, and I was 4th in pax and 3rd by raw. I was struggling with the course a bit yesterday and couldn't keep up with his blazing fast times (and neither could anyone else!) I'll let him share the video The event had over 110 entries, with some top notch cars and drivers all getting ready for the Pro coming up in a few weeks.
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Old 04-15-2018, 03:36 PM
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Thanks again Tamra for the co-drive. Riley is a beast!

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Old 04-16-2018, 06:03 AM
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Originally Posted by Tamra View Post
Any suggestions on front control arm bushings? We went with Powerflex last season, and the front lower control arm front bushings are already worn out (grab the wheel and you get 1/8" of movement in and out from the bushing site). Dynamic camber! Our control arms were new at the time of install so they are still in great shape, plus we like how much freer the control arms move with the poly bushings.

Thinking of maybe trying Whiteline next, but let me know your guys' thoughts.


In other news, ran NNJR SCCA yesterday with great results. The car was great! Schickane here on the forums co-drove with me and he ended up top pax AND FTD, and I was 4th in pax and 3rd by raw. I was struggling with the course a bit yesterday and couldn't keep up with his blazing fast times (and neither could anyone else!) I'll let him share the video The event had over 110 entries, with some top notch cars and drivers all getting ready for the Pro coming up in a few weeks.
For the pillowball bushing, I'd run stock. Polyurethane is a garbage material to use for bushings.

For the front bushing... delrin.

JV
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Old 04-16-2018, 07:09 AM
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Originally Posted by John V View Post
For the pillowball bushing, I'd run stock. Polyurethane is a garbage material to use for bushings.

For the front bushing... delrin.

JV
The front LCA bushing is also somewhat a compliance bushing (axis are non concentric depending on caster setting). We could try making delrin and see if it binds I guess. But tbh, I haven't had any 'feel" complaints with the poly bushings, so the only motivation to go delrin for me would be if it is more durable. In this application I'm sure it would be.
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Old 04-16-2018, 10:21 AM
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Originally Posted by Tamra View Post
The front LCA bushing is also somewhat a compliance bushing (axis are non concentric depending on caster setting). We could try making delrin and see if it binds I guess. But tbh, I haven't had any 'feel" complaints with the poly bushings, so the only motivation to go delrin for me would be if it is more durable. In this application I'm sure it would be.
I would definitely try making a delrin bushing.

With the SP rules not allowing spherical bearings I really think the pillow ball bushing should be stock rubber. I think the cons of polyurethane almost always outweigh the pros.
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Old 04-16-2018, 10:32 AM
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Originally Posted by John V View Post
I would definitely try making a delrin bushing.

With the SP rules not allowing spherical bearings I really think the pillow ball bushing should be stock rubber. I think the cons of polyurethane almost always outweigh the pros.
We left the front LCA rear pillow ball bushing OEM- completely agree there. We will probably try making delrin for the front LCA front bushing at some point, but I also ordered some Whiteline bushings in the interim (they were pretty cheap) so if we run out of time before the pro we can at least drop them in and stop our dynamic camber issue. Once we make delrin I'll report back on if it causes any binding.
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Old 04-17-2018, 08:29 AM
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I actually submitted a warranty replacement claim through Powerflex's website (they list it as a lifetime warranty and don't mention racing exclusions). They got back to me that they will send out a new set for me, no questions asked. I'm impressed.
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Old 04-18-2018, 07:27 PM
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What the easiest way to remove the diff carrier? I'm assuming it's exhaust, PPF, and prop shaft, then pivot the diff down to access the nuts on top.

Do I need to unbolt the PPF from the trans, or is there enough play to get the PPF off the diff studs?

The FSM says to drain the diff. Is this actually necessary?

Any other words of wisdom?

I ask because I don't have the luxury of leaving my car disassembled and need to get the car drivable within one weeknight garage session. Thanks!
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Old 04-18-2018, 08:22 PM
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Originally Posted by schickane View Post
What the easiest way to remove the diff carrier? I'm assuming it's exhaust, PPF, and prop shaft, then pivot the diff down to access the nuts on top.

Do I need to unbolt the PPF from the trans, or is there enough play to get the PPF off the diff studs?

The FSM says to drain the diff. Is this actually necessary?

Any other words of wisdom?

I ask because I don't have the luxury of leaving my car disassembled and need to get the car drivable within one weeknight garage session. Thanks!
Oh boy, we have experience with this one Feel free to just come over to our house one weekend if you want help or want to use the lift.

You have to remove the exhaust, the cover over the driveshaft, the driveshaft, and drop the PPF. Put a jack under the transmission to hold the tail up. Drain the diff (when you remove the axles you'll be glad you did). Remove all of the non alignment bolts in the rear suspension and free up the brakes/abs lines - you'll need to be able to yank the axles out of the diff, so you need to be able to move the hub a bit. Yank out the axles (possibly with the help of a pry bar). If yours are original this may be a pain. At this point your diff should be suspended from just the two bolts holding it in through the bushings. Put a jack under the diff, undo those two bolts, and drop it down. Install in reverse order.

It takes us about 2 hours for us to do the entire job out and back in again, but I'd budget more if you've never done it before. The main tools you'll need are a big torque wrench, preferably an impact gun to make your life better, a pry bar in case the axles are stubborn, and a variety of open ends and sockets.

Oh yeah, and then you need to get the bushings out and press in the new ones (hopefully you can hammer them in and don't need a real press, but if you need one you can also borrow ours if you come over). Check if your new bushings are made to work with the metal sleeve in place, or if you'll need to cut it out using a hacksaw.

Finally, considering you're working in a parking garage, I suggest you pick up a washing machine drain pan to use under your normal drain pan. Best thing ever for catching unexpected spills, especially when your apartment complex may not appreciate oil on the floor.
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Old 04-21-2018, 08:17 AM
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Ooh, pulling the diff... Yeah, everything Tamra just said, but according to the FSM, a tire iron works well for popping the half shafts out of the diff. We used one and didn't have any troubles.

Otherwise, follow Tamra's instructions and you'll have no problems.
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Old 04-23-2018, 09:18 AM
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Thanks for the info!

After struggling to fix a misfire (clean your ESS!) I ran my car at the NNJR TnT on Saturday and points event on Sunday. It's not outside of the ballpark, so I'm rather pleased. Stiffer springs are definitely needed. The original diff is completely shot, so I need to figure out which direction I want to go. I only want to drop the pumpkin once, so I'm going to wait on the bushings until I've got a diff installed in a spare pumpkin.

I installed poly motor mounts from rx8performance.com prior to the weekend and WOW did they transform the car! My stock mounts appear to be in good condition (no tears or leaking fluid) but I was having trouble shifting second consistently on course. With the poly mounts, shifting at high rpms while turning is just as easy as a lazy 1-2 shift pulling away from a stop. There is a minor vibration between 1-2k rpm but otherwise the NVH is near-stock. Very nice looking pieces as well with good welds. Highly recommended!

Last edited by schickane; 04-23-2018 at 05:35 PM.
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