LionZoo

Yes I know. In the original thread, I started the query about installation ratios. It's just always good to confirm.

LionZoo

For the latest iteration, I adjusted the spring rate to the aforementioned 7/5 kg/mm F/R spread and took down the rear damping by about 20% across the board. The rear springs are the same as before, but the front springs are now 7kg Tein 70-90mm tapered springs coupled with a set of tenders that are the same spec as those on the rear. Like I suspected, these springs work with the stock front top hat, but the rubber boot over the damper was trimmed since the bottom of the boot had some interference issues with the spring. This isn't a big deal, but in the future I might look into some alternative method of boot covering the front dampers to seal out some dust. It requires a little bit of force to seat the front spring onto the perch as the rubber boot on the bottom is quite thick and takes up an extra few millimeters, but once it's seated the spring is flat on the top perch. An extra rear race was added to the tender spring to main spring adapter on the rear and the bottom spring perch race was changed from a 70mm ID to a 65mm ID part. I managed to get it on the car and test it a bit over it's intended environment before packing up and leaving for Boston.

So onward to the impressions. I must say that the latest changes have really transformed the car. The springrate change was huge for the balance of the car. No longer does the car seem to want to oversteer at steady state; it's now much more neutral. The rear is still more active under power than stock, but that's probably due to the stiffer springs eliminating a lot of the squat. Nonetheless, steady state balance feels great and when you do get it to slide it's much easier to gather it back up than stock. Brake dive is much less than stock and the responses in general are faster, just like before. The thing that is so amazing is that it feels so intuitive. The stock RX-8 is a great handling machine, but now the car is just quicker responding and more telepathic than before. In stock trim, the car can be a bit edgy at the limit since the soft spring rates meant it had a slight hesitation when it came to responding to control inputs at or a little over the limit of grip. That hesitation is now gone. Also, the damping control seems much improved in a handling sense. Previously I can get the car out of sorts and feel a bit less in control when really being on it through transitions. Now, the first impression is that the drama is gone. I will confirm these impressions and find out more the next time I get to drive the car. Sadly that might not be for a month or more.

As expected, the ride frequency is probably changed. I didn't get numbers, but using the old 6/5 spread the car seemed to hit bumps and come down flat at around 55 mph. Now that behavior seems to happen at around 80 mph, suggesting that the front frequency is now closer to, but still lower than, the rear's. Turning down the rear damping has really helped the ride quality.

At the moment, I'm quite satisfied with the setup and won't be touching it for awhile as I'm just going to observe it and try to get some testing mileage on it, as much as I could. If I were to change it, I think the first thing I'd do is look into possibly switching the bumpstops. I'm not getting bad behaviors from the stops at the moment, but the HD comes with the same length bumpstops front and rear, but the front damper compresses less per inch of wheel travel than the rears. This has me a bit worried about the front stops engaging before the rears, but then I think that's a very minor issue. Next time I run the car hard, I'm going to use the ziptie trick to see how much travel I'm actually using up. Next time I drive the car, I should also have my next wheel and tire package up and running. I'm switching out the 18x8 +42 TC005s with dead 225/45 Sport Z1s for 18x9 +50 SSR Type Fs with 245/40 tires. Not sure which tire yet, but contenders are currently the AD08, NT05, or Z1 Star Spec.

Astral

Thanks for great info! Z1 Star Specs are excellent tires and they come highly recommended from many autocrossing and tracking locally here.

LionZoo

I forgot to mention that in terms of rideheight, my current setup won't be very popular with the looks crowd. If you want the springs to stay seated under full droop, the most the rears lower is about 1 inch, which is far from what is needed to get that slammed look that some people seem to be crazy about, but perfectly fine for my application. The fronts can lower a lot more than the rears. Also, the addition of the extra rear races has eliminated the noise I was previously reporting.

LCheung

Have you thought of running the tender springs in the back as well? Run them solid at ride height and you will be able to keep the springs from coming out of contact at full droop, but still be able to run your high rate springs and have that linear rate at ride height. It doesn't sound like you care for that slammed look, but if you did, you could technically have it. I don't know how close you are running to the bump stop though. You could also sacrifice some of the droop travel by shortening the case on the damper. If you're running a stiffer suspension, the likeliness that you'll need to retain the stock droop is unlikely.

I'm curious what your thoughts were on these issues, the approach you've taken with your suspension is really interesting. No one ever really bothers to properly engineer their set ups. Especially all the damn aftermarket tuners (including all the reputable ones) that seem to be hell bent on creating understeer, crappy ride, and supplying people with spring rates that create natural frequencies for open wheel race cars with aero.

LionZoo

I already have tenders on the rears. The rears obviously needed tenders, the front tenders were a design change addition in the second go around. There's a lot less preload on the rear springs than the fronts stock, so as you increase the springrate, you end up needing the rear tenders before needing front tenders. Now what I can do is go to a lower rate tender; my current tenders are 80 lb/in springs and many people offer wimpy tenders in the 15 lb/in range. That will get the rideheight down a bit, but since the rear tenders will come into play more than the fronts, I wanted some rate back there just to better control the wheel in those few instances that it does, rather than have the damping completely overwhelm the spring and have the suspension droop become as slow as molasses.

Shortening the damper body is something that is definitely on the cards; though beyond my resources at the moment. As it is though, buying some NC (third generation) Miata dampers will be a more cost effective solution than shortening RX-8 dampers. The NC dampers have the same mounting points so they should bolt right up. However, they are a bit shorter which would work in this case as the springrate has gone up so the sacrifice in droop travel would be acceptable. Right now though, I have more droop travel than I know what to do with. That's somewhat of a good thing, but a shorter body and hence lighter damper weight would be nice. What I don't want is something like the Tein Flexes; so little droop travel that you routinely pick up a wheel when going into a driveway, but at the moment I don't feel using a NC damper will lead to this situation.

The bumpstops are a minor issue, but they are something to consider should I do another adjustment of the setup. I feel as it is right now, this is pretty well sorted and I'm going to use this configuration for the time being. I can say that I've learned quite a lot from making this set of dampers and this final setup probably costs something in the region of $1300-1400. Yes it's not damping adjustment, but for a full custom setup made to your specifications, it's a pretty good deal. Of course I also spent another $320 on a set of Swift springs that aren't being used...

LCheung

Are your 80 lb/in tenders solid at ride height? I really dislike progressive spring rates, but that's just because of the feel they produce in transient if they go solid under cornering.

Have your measured your bump and droop travel? What is it at right now? Where I went to school we made road going monocoque cars (however, I ended up working on the Formula SAE car instead), the yard stick we used were the numbers they used on some old Lotus cars. 3 inches bump travel and 4 inches droop. I'm curious if your ratio is somewhere around there and if not it's probably pretty safe to head for a ratio of 3:4 bump to droop. But it was always noted that those numbers aren't set in stone, it was just a good place to start.

LionZoo

Tenders are solid at normal rideheight and the rears stay solid until about 1.5-2 inches into their droop travel. The fronts are solid for all but a tiny part of the full travel and should never unload in any normal circumstance.

I didn't measure the travel, but all I can say is that it's the same as stock since the HD dampers have stock damper. However, there is about 0.8 inches less compression and 0.8 inches more droop.

Juice

iTrader: (1)

Do you happen to have any pictures of how the rear springs sit on the rear top mounts?

Are they flush with the flat part of the mount like the stock springs are?

I have a similar setup on my NC with the Tein springs but am a little concerned with how the rear springs are sitting on the top mounts. The springs sit about halfway up the tapered part of the mount. They aren't moving or making any noise but it doesn't seem ideal and I would rather have them sit all the way up like stock.

If the 8's mounts allow it to sit flush then I might try switching to them as they seem like they have the same mounting points as the NC. Might need to trim off the mounting points on the side of the mount that attach to that bracket in the trunk of the 8 since the NC doesn't have those but that is fine.

LionZoo

Both my front and rear springs sit flush on the mounts just like how they are stock. I'm currently using Tein taper springs, not the standard springs, so the top part where it sits on the mount is 90mm ID instead of 65 or 70. The stock springs have an ID of around 91-92mm at the top. What is your spring dimensions?

Are you using adapters for the top mount of the springs? If you follow this thread, you'll know that at the beginning I was going to use adapters to use standard ID springs, but I found out that the Tein tapered springs are a great solution that would eliminate some extra parts.

Though the NC and RX-8 upper mounts have similar mounting points, the NC rear upper mount is significantly shorter than the RX-8 mount. If you were to still use NC dampers in the rear, you would run out of shock body length. You might be able to get away with using RX-8 dampers instead, but I think those might still be too long for the NC, even with RX-8 top hats. Also, the NC top hats are cast aluminum, while the RX-8 ones are welded steel. The RX-8 hats are significantly heavier.

My recommendation is to measure the ID of the NC top hat and see if you can possibly use the Tein tapered spring. I say so because having seen the NC top hat, I don't think you are using the Tein tapered springs if you're reporting interference on the ID of the spring. Confirming the ID will allow you to figure out whether you can use the Tein tapered springs and avoid interference.

Juice

iTrader: (1)

Yes, I am using the tapered springs, 70-90mm. Rear rate is 4kg, 175mm long. part number ST040-0175. With or without the rear rubber boot the springs dont sit flush. I will have to measure the stock springs tonight when I get off of work to actually see what the ID at the top is.

My first guess is that the problem is that the mounts are cast like you said. They are quite a bit thicker at the ring around the bottom of the mounts than I remember the 8 being. It is about and 1/8 inch thick all around from what I can tell.

The way I see it, my options are:

1. Try a 70-100mm spring. They only come in 10 inch lengths in the rates I want though so that isn't really going to work.

2. Try the 8's top mount. I know that the springs will fit on them now thanks to your reply. The weight doesn't concern me too much since I have the PRHT and its never going to be as light as the soft top. I know the length will be a concern but how big is the question. Do you know how much longer the 8's mount is?

3. Try and take off some material from the stock NC mount so the spring sits flush. I don't know if this is a good idea though and it might ultimately make it much weaker and I might not be able to take enough material off to still sit the spring flush.

4. Make some sort of an adapter that will sit between the spring and top mount so that it is all flat against one another. Seems like this could be the easiest way if the 8's mounts won't work.

5. Live with it. It isn't causing any problems for the cars suspension from what I can tell so I could just leave it be and see what happens if anything.

Anyways, thanks for the information.

LionZoo

1. This would be my first option. Starting from 6 kg/mm onward, the 70-100 tapered springs come in lengths shorter than 10". While the rate is a bit high for a rear application, it might also be the easiest solution.

2. The RX-8's top mount is significantly longer. I'd say it's around 2 inches, but I didn't take measurement. They were very noticeable though.

3. I'd say go ahead. Being cast aluminum, I'm guessing the reason the collar is so thick is more for manufacturability than anything else. If you have the aluminum thickness there be too thin, you'll run into big difficulties casting it as the molten aluminum in the mold will want to freeze before it reaches the outer edge of the part. A thicker part means the material stays hotter longer and so will flow to the other ends. From a structural point of view, I don't think the collar adds much. It's more there so the springs have a reference to line up to.

4. I've tried the adapter solution for the RX-8 and while it might work, it's a bit of a pain in the *** to figure out. It seems Goodwin's NC rear adapters rub. You'll probably need to go with a 70mm spring regardless since the damper position changes as it goes through it's travel and getting it machined and fitted will take some effort. If you were going to book machine shop might, it'll probably be better to use your resource to simply turn down the inner collar.

5. I'd get it fixed, but I'm a bit of a perfectionist.

Good luck and keep us updated!

LCheung

Upon reading the first post, my first inclination was also towards modifying the stock NC top mount.

From what I understand the RX-8 top mount might not even fit in the Miata. I could be completely wrong, but the length difference I saw I thought was probably due completely to a packaging issue than anything else.

Juice

iTrader: (1)

Thanks. I will certainly keep you updated.

I measured the ID of the stock rear spring and it was about a 1/16th off of 3.75 inches, so about 94-95mm. I am pretty close with the 90mm so I think just grinding a little bit off of the mount would be the best bet. It is no doubt pretty thick, at least 1/8th inch at the bottom, so it might even be thicker as it gets near the top where the spring should seat.

I really want this fixed as it is just nagging in my mind every day.

I thought about this as well, but when disassembling the rear suspension I did notice a bit of room near the top. Of course I didn't measure, but there was enough room that it put the thought of the 8 mounts in the back of my mind. If the length is as different as LionZoo says though, then it probably wouldn't work unless I modified the 8's mounts to be shorter. If I ended up having to do that then I might as well just modify the NC's mounts to fit the springs better.

Well, thanks for the opinions and suggestions guys, I will let you know how I make out. If it is nice this weekend I might do it. Of course I am just as inclined to sit by the pool and drink a few beers if it is nice out.

Blackroadster

i can confirm that 70-100mm taper spring work with NC rear top hat with dust boot installed as well

I'm currently running
8" 500 lb/in 2.5" ID front spring with custom top adapter
Tein taperd spring 70-100mm 6kg, in the rear with oem dust boot

If you want 4kg, why not buy the tapered springs from Ohlins owners? I'm sure there are people who has Ohlin coilover that upgrades their rear spring rate.

Juice

iTrader: (1)

I hadn't even thought of the Ohlins springs. Any idea how long they are?

Right now though I am planning on just working with what I have and not spending any more money. I should be able to make it work, I mean we are only talking fractions of an inch for clearance, it shouldn't be too hard.

If I can't get it to work then I will go with most likely go with the Ohlins 4k springs.