What coils/spring rates for midsize E/UHP wheels (weekend/hobby drive)?

[Tor]

I now spent the last 14 day every evening researching which Coilovers to get. In the process I came across some info that may surprise some.

It is well know that the KW V1 is the same as the ST X, only that the KW is stainless steel. The two set use the exact same springs (4 kg/mm front, 5 kg/mm rear) as can be seen from the German TÜV documents, which list their specifications in detail. It is a German requirement, that the springs are marked. And the markings has to correspond with the documents when the car is brought in for it’s biannual “TÜV inspection”.

I am slowly getting to the point bear with me for a few more lines: Fact is that the 4/5 V1-springs are also the EXACTLY the same as those used for the european version of the KW V3.
We also have the US V3 (6 kg/mm front, 7 kg/mm rear) rates available, but it is sold as “KW Clubsport”. Over here, it’s a well know (and from KW confirmed) fact that it’s the same damper being used in both kits. They even retro-actively changed some early customers TÜV papers.

All in itself not so particular interesting…

The interesting part is about the galvanized ST branded rebound-only adjustable “XTA” - a.k.a. not-available KW V2. Interestingly enough, it uses the 6 kg/mm Clubsport/US V3 spring on the front, and the XT/V1/"Euro V3" 5 kg/mm spring at the rear.

That kit comes with front camber adjustable top mounts and for a price of 1,200$. From the respective manuals, it appears it’s the same top mounts too…

XTA manual:



Clubsport manual:




Before doing this research I was skeptical about the price, and I was under the impression it had much lower spring rates.

I imagine KW (even under their ST brand) wouldn’t release a kit where the dampers can’t support the spring rates. So I would take the relatively high rates (compared to the V3/Clubsport), as an indication that this might be a damper of good quality. Several people here are running them and all seems to be happy with them.

The 6/5 XTA spring rates doesn’t appeal particularly to me. But for 1200$ instead of 2500, there is plenty of room to spare to buy some new ST sway bars to dial out possible understeering tendencies. Btw the ST swaybars cost 300$ as opposed to the (exact same) KW swaybars that goes for 500!

Swaybars, even listed side by side on the German KW site: lol





Alternatively, since the V3/Clubsport springs are interchangeable, it could be that it is possible to use the 7 kg/mm Clubsport spring in the rear.

I think my decision has been made up... It may be a surprising decision, when I started out with a budget of 4000$. But I think I'll go for the XTA + swaybars, and spend the rest on beer and girls. And if the decision turned out to be wrong, it would be very easy to part with those coilovers used. Thanks for all the input!


Documentation for all the above claims:

ST XTA: http://docs.kwsuspension.de/ga-STXTA-58804_1094-14.pdf
ST X: http://docs.kwsuspension.de/ga-STGFw-58004_1237-12.pdf
V1: http://docs.kwsuspension.de/ga-KWGFw-58004_1237-12.pdf
Euro V3: http://docs.kwsuspension.de/ga-KWGFw-58004_1237-12.pdf
US V3 / Euro Clubsport: http://docs.kwsuspension.de/ga-KWCS-...04_1248-12.pdf

[solidONE]

^ interestingly I've recently driven 86's with Flex-Z (6k/6k) and ST (6k/5k). With the damping pressure adjusted -1 click on the Flex-Z and -1 turn on the ST from full stiff the ST is noticeably more aggressive (firmer) feeling than the higher sprung flex-Z. Both cars had wayyy more grip than they need. (255 wide 595rsr on the Flex-z equipped car, and 265 wide tires on the ST equipped car.) I would have to drive the ST equipped car some more to get a better impression, but it definitely feel like they offers more damping force over the Teins. Both cars have never seen the racetrack either.

[Tor]

With the damping pressure adjusted -1 click on the Flex-Z and -1 turn on the ST from full stiff the ST is noticeably more aggressive (firmer) feeling than the higher sprung flex-Z. Both cars had wayyy more grip than they need.

One click from full stiff is very far from the recommend default setting of 9 clicks.
I am not sure what you mean with too much grip.

If you mean difficult breaking traction rear, then that is exactly my concern with the softer springs to the rear. I don't mean drift or wheel spin, but rather to get the tail of the car to "move around the corner".
I believe I have a pretty good feel for cars, but it's new for me to put it into words and I am never sure if I use the right terminology, but I believe that is what is meant when people say "rotate".

The above would be my purpose of getting the the swaybars, so I can make the rear a little stiffer if needed.

@Racecomp Engineering

It's important to keep in mind that the stock suspensions are also very bumpstops active and very much used in cornering.

Thanks Andrew, this just really makes it even more difficult for us mortals to make any educated guess on how a coilover might perform from the stats alone.
Who knows, take e.g. the Öhlins R&T 4/3 kit. Maybe it's great, even if the numbers are not in line with the majority. As long as no one tried them? Maybe Öhlins made a golden combo and it's being dismissed just by the numbers?

This is also why I'll go with the ST XTA, it will get me started off without a big investment, and I can continue to improve upon my knowledge with practical experience. Down the line I can change, if needed and at that point I will hopefully have a better knowledge about my true needs.

[solidONE]

Originally Posted by Tor

One click from full stiff is very far from the recommend default setting of 9 clicks.
I am not sure what you mean with too much grip.

If you mean difficult breaking traction rear, then that is exactly my concern with the softer springs to the rear. I don't mean drift or wheel spin, but rather to get the tail of the car to "move around the corner".
I believe I have a pretty good feel for cars, but it's new for me to put it into words and I am never sure if I use the right terminology, but I believe that is what is meant when people say "rotate".

The above would be my purpose of getting the the swaybars, so I can make the rear a little stiffer if needed.

@Racecomp Engineering

Thanks Andrew, this just really makes it even more difficult for us mortals to make any educated guess on how a coilover might perform from the stats alone.
Who knows, take e.g. the Öhlins R&T 4/3 kit. Maybe it's great, even if the numbers are not in line with the majority. As long as no one tried them? Maybe Öhlins made a golden combo and it's being dismissed just by the numbers?

This is also why I'll go with the ST XTA, it will get me started off without a big investment, and I can continue to improve upon my knowledge with practical experience. Down the line I can change, if needed and at that point I will hopefully have a better knowledge about my true needs.

At the "default" setting for the flex z I believe it was 7-8 clicks out from fill stiff there was too much body roll and the rear end would wallow from making contact with bumpstop, even at -5clicks. It only when I jacked it up to -1 that it would be less pronounced. But the flex z weren't designed for the kind of driving I was doing with them. The ST's I've not had the chance to really toss it around some bends, but I've a feeling, even with its lower spring rate, it will not have the problem bottoming the rear under hard cornering because of the damping it provides. From initial impressions I think the ST are more performance oriented. I'm sure the T0 would be even better in this regard, though that's just my assumption since I've not tried them myself.

I'm sure there is a very good reason why the AST guys are using such low spring rates in the rear. As the saying goes, "there is more than one way to skin a cat," or something to that extent.

[ansibe]

Captain Snooze, you’re right, I should have accounted for motion ratio. This is a complex subject and I over simplified. I’m making a similar choice as TOR (at a lower price point), and I want the car to be more balanced on my tracks with OE tires. I know I’m not supposed to like the OE tires, but I’ve tried OE, all-season, max perf summer, and snows (yes, on the track) and I find the OE Michelins best for the brz.

TOR, I like your choice. You’re making a conscious decision, and I’d love to hear how it works out for you. FWIW, I’m from the bike world and I’ve never heard of a bad Ohlins set-up, or a cheap one!

I’m shooting for 25mm lower, maximum. Lower CofG resulting in less body roll is all good. Of course I’ll need stiffer springs which will compromise bump compliance and probably necessitate better dampers. That’s a double edge sword - better dampers would be nice, but they’re not free.

If I go 25mm lower at both ends, I’ll add more camber at the back than the front. That’ll make the balance worse unless I use very stiff rear springs, or sways ($$). Or I could dial out the camber with LCAs …. more $$.

There’s also a good chance Andrew from RCE has nailed my problem. The front suspension is collapsing onto the bump stops, dramatically increasing the effective spring rate and causing the front to wash out.

[strat61caster]

Originally Posted by ansibe

Captain Snooze, you’re right, I should have accounted for motion ratio. This is a complex subject and I over simplified. I’m making a similar choice as TOR (at a lower price point), and I want the car to be more balanced on my tracks with OE tires. I know I’m not supposed to like the OE tires, but I’ve tried OE, all-season, max perf summer, and snows (yes, on the track) and I find the OE Michelins best for the brz.

I tracked the OE tires 2x and autocrossed them at least 2x and I think they're seriously underrated. Yeah they're not fast or grippy but they're relatively well mannered on the road, communicate and lose traction progressively, last a really long time, and contribute to the good fuel economy.

FWIW the Primacy's could almost qualify for 'max performance summer' and are already classified as a summer tire (not all-season) you haven't really tried a tire that's objectively noticeably better as a performance tire than the Primacy yet.

And I totally agree with your assesment of understeer when lowering the car, I saw the exact same things (may have posted it earlier in this thread even...) after installing my B14's. So far camber plates have been my favorite change bar none over coilovers and tires (Dunlop ZII Star Specs in OE size) the added grip up front means you can get the car very balanced on stock suspension for autocross or track and then slide it back to reasonable levels for street driving.

[yip]

I just received my XTA's today and your posts are giving me positive reinforcement that I made the right decision based on my needs and budget!

I had ST-X's on my previous car and felt they were to soft and under damped based on my preferences. Great for going low and a stock like ride but always on the bumpstops through the fast turns.

[Tor]

Looking forward to you opinion about them.

When are you installing them?

[yip]

Originally Posted by Tor

Looking forward to you opinion about them.

When are you installing them?

This weekend!

[Tor]

Lucky you. :-) Do you have a build thread? Could you perhaps make a couple of photos before putting the wheels back on?

[Tor]

Any updates @yip?