The Mustang Forum for Track & Racing Enthusiasts

Taking your Mustang to an open track/HPDE event for the first time? Do you race competitively? This forum is for you!

  • Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!

True rear coilover - daily ride and handling comparison

72
48
So apologies for a long post but maybe others will find this helpful.

I was always interested in the daily ride and handling effect of a true rear coilover suspension on an s197 sra versus the oem style setup. But it's hard to find much in the way of true apples to apples comparisons. Before and after reports are usually in the context of wholesale changes to dampers, spring rates, wheels, etc.

Note I'm only focusing on rear spring location and its effect on daily driving ride and handling. Car is a daily driver '11 GT that sees 5-10 track events a year. But I'm only talking about the street side of things here.

So I swapped out rear FRPP P springs (~200 lbs, progressive?) with Bilstein monotube sport shocks and the stock 24 mm bar, and installed lightly used Cortex Koni street coilovers with 200 lb linear rear springs and Koni 30 rebound adjustable monotube dampers (4 settings). I took rear bar off completely. I set the Konis to 50 percent damping. (Settings are 0 (softest), 20 percent, 50, and 100.) I have an adjustable PHB and stock LCA and UCA. Ride height is similar between old P springs and new coilovers. (Cortex is very slightly lower.)

So similar spring rates, similar quality monotube dampers (Bilsteins reputationally maybe a little better?), move the springs outboard, and get rid of the rear bar. One variable is that I don't know how the damping rate of these Koni 30s set at 50 percent compares to my old Bilstein sports.

Result is holy cow, the rear axle is way, way better controlled on rough roads in daily driving. My old setup was a lot better than stock but I still was getting extra up and down movements on a lot of bumps, particularly in corners with lots of broken or uneven pavement coming at the wheel fast. All of which created an annoying skittishness in any corners that weren't nice and smooth. The new setup just eats this up by comparison. It is a nice, smooth, taut but not harsh ride, with a noticeable single up and down movement for each bump. The rear tracks way, way better and doesn't step out nearly so much. In fact it doesn't really step out at all as far as I have see thus far, in really quick but sane public road speeds. The improvement is way more than I expected.

Compared to the improvement from stock springs and dampers to the P spring/Bilsteins, this feels like about the same level of improvement again. The rear end is noticeably more stable, more comfortable, and faster around rough pavement corners.

I'm no engineer but I suspect most of the improvement is due to moving the spring outboard, with the corresponding decrease in unsprung weight. It just feels like a lot less weight flopping around back there, and I think if you do the math it kind of is.

I respect that "ride and handling" is subjective stuff but this is not confirmation bias or a placebo. The improvement is significant and real.

I hope this might be helpful to anyone considering a similar swap.

By the way I have to give a shoutout to PLexo50 on here, great seller and thank you!
 
112
73
Exp. Type
HPDE
Exp. Level
Under 3 Years
25 min. to 1½ hrs. from Sonoma (ugh... traffic!)
When I installed CorteX coil-overs on my 2014 GT I noticed a huge improvement too, but I like your comparison better because I made a lot more simultaneous changes. One thing I'd like to point out in regards to "apples to apples' comparison, I'm not sure that you can directly equate the spring rates of the coil-overs to those of the stock location. Filip at Cortex told me that because of the more outboard location of the coil-over the effective spring rate will be different
 
72
48
When I installed CorteX coil-overs on my 2014 GT I noticed a huge improvement too, but I like your comparison better because I made a lot more simultaneous changes. One thing I'd like to point out in regards to "apples to apples' comparison, I'm not sure that you can directly equate the spring rates of the coil-overs to those of the stock location. Filip at Cortex told me that because of the more outboard location of the coil-over the effective spring rate will be different

I agree, and I might not have been clear. What I meant is the "spring rate" in isolation (say off the car), is approximately the same (although the P spring is progressive while the cortex is linear). Often you see comparisons where the individual spring rates are very different, which makes it hard to compare the effects caused by the different spring mounting point.
 

Grant 302

basic and well known psychic
It’s great to have new opinions and a comparison that is closer to apples n apples even if not ‘exact’.

It’s a closer change than I did since I went from P-springs, Boss dampers and the 24mm GT rear bar to 550/350 springs, CorteX/JRi DA, and the 18mm rear bar.

Your comments remind me a lot of the things I remember from the upgrade about 5 years ago. Even before I got the damper adjustments done.

It’s my belief that the advantage of the ‘true’ coilover is that it can be ‘matched’ more precisely to the spring. First for the linear rates. And second that mounted coaxially, there’s no need to adjust for different motion ratios.
 
I think a lot of the skittishness as you say in the rear come from the large rear sway bar. When I removed mine and used a 18mm I noticed an improvement in ride quality and was blown away at how early I could get on the throttle. It was very nervous before on corner exit and bumps. I think if you placed the sway bar back on you would find that some of the characteristics from before come back. It would be a good test, if you get a chance let us know.
 
I have had quite a few shock/spring/sway combinations on my car over the last 4 years as I have a habit of continuously modifying and experimenting. I started tracking with the stock struts/shocks and P springs with the stock swaybars. Then I switched to an adjustable Strano 32mm front bar, single adj Cortex coilovers with 450/250 springs. I still found the rear of the car too sensitive and the front not responsive enough. Then I started going down on the rear bar as well as increasing negative camber and that's when I realized.......the rear bar has a huge impact on not only the front end but also throttle confidence and lap times. I've recently gone to slightly stiffer 500/300 springs and a full JRi DA set-up with watts link, torque arm and 18mm rear bar or no rear sway. I've done some reading and adjusted the rear shocks for the right rebound and compression and now my car finally has the perfect feel. Good front end response, the ability to take curbs smoothly, great traction out of corners and balanced lateral grip. IMHO running the smallest rear bar or none at all is the key. I know some like a super stiff set up but I found a soft, compliant rear with the right rebound/bump is what made the biggest difference in my lap times. Having confidence that the rear will stick on exit and during high speed corners is key. The watts and TA just added to that feel but it's the rear bar and the shocks/springs that matter.
 

Norm Peterson

Corner Barstool Sitter
835
618
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
When I installed CorteX coil-overs on my 2014 GT I noticed a huge improvement too, but I like your comparison better because I made a lot more simultaneous changes. One thing I'd like to point out in regards to "apples to apples' comparison, I'm not sure that you can directly equate the spring rates of the coil-overs to those of the stock location. Filip at Cortex told me that because of the more outboard location of the coil-over the effective spring rate will be different
In two-wheel bump mode, the effective spring rate - maybe think wheel rate here - stays the same. It's in roll and one-wheel bump that the wheel rate would change. Moving springs to a more outboard location without changing their stiffness increases the rate in roll, compensating somewhat for a lighter (or eliminated) rear sta-bar without affecting pitch and flat ride considerations.

There is a separate caveat involving inside clearance for wide wheels and tires. This may not be an issue with 285 or narrower tires on 10" or narrower wheels, but I think likely with 11" wide wheels and any size tire you'd even consider putting on that much wheel.


Norm
 
72
48
I'll try putting the sway bar back on at some point. I know it's discouraged with this setup but it would be an interesting experiment.

@Norm, the rear coilover is inverted so the spring is down the by the hub and the damper/shock is up by the wheel and tire. I think big wheels and tires fit pretty much the same as oem.

Rather than think in terms of effective spring rate, I found it helpful to think of effective unsprung weight. Whatever rate of spring you use, the effective unsprung weight of the wheel, tire, brakes, etc. that it has to control is significantly increased the further inboard you locate the spring. If a wheel and tire weighs 50 lbs, when you hit a bump it appears as a lot more than 50 lbs at the oem spring perch which is way further inboard.
 

Grant 302

basic and well known psychic
There is a separate caveat involving inside clearance for wide wheels and tires. This may not be an issue with 285 or narrower tires on 10" or narrower wheels, but I think likely with 11" wide wheels and any size tire you'd even consider putting on that much wheel.

Lots of room with the CorteX rear setup and Apex 18 x 11. At least with the JRi tubes.

I’d think specific measurements and offsets would need to be considered for say 12” wide, IMO.
 

Norm Peterson

Corner Barstool Sitter
835
618
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
@Norm, the rear coilover is inverted so the spring is down the by the hub and the damper/shock is up by the wheel and tire. I think big wheels and tires fit pretty much the same as oem.
Makes sense. I wasn't thinking in terms of 'inverted'.


Thinking a little further, when the springs are moved outboard, one wheel bumps on one side of the car don't cause as much variation in wheel load (think mechanical grip) on the other side. As far as the axle is concerned, springs are supports that it would like to pivot about except that springs obviously aren't fully rigid like we tend to think of supports as being. But even so, putting these weenie-pivots closer to the loaded ends reduces the loads seen at the far ends of the levers (the opposite-side tires in this situation).


Norm
 
72
48
Updating this thread:

As others reported above the rear bar seems to make a big difference.

For winter I swapped back to oem stock suspension (2011 gt), no rear bar, bilstein shocks in rear. Results were kind of surprising. I expected lots of skittishness but it's actually quite good. It definitely is very soft, but the back end doesn't really step out very much around bumpy corners and to me it feels better than the P springs with a bar.

In terms of rear end stability on broken pavement, from best to worst, my subjective impression is:

Cortex coilover, 400/200, no rear bar, koni set 50 percent
Stock springs, 123/156, no rear bar, bilsteins
P springs, ~230/200, 24 mm oem rear bar, bilsteins
stock springs, 24 mm oem bar, stock shocks

I freely admit I'm out of my depth here but from messing around with these various setups it seems to me that increased rear end stability comes with making sure you have plenty of shock damping relative to the effective rear wheel rate - whether you get to that ratio by reducing the effective wheel rate (softer springs, smaller ARB, outboard spring location), or by increasing the shock damping (koni adjustable > bilstein > oem dampers), or both.

Also just confirming, I had a chance to do a last track day at Palmer with the Cortex and 305/30/19 square setup and no rubbing in the rear, 50 mm offset LMR 19x11 wheels.

Winter project is to address the rest of the suspension pieces to line up and play nice with the coilovers.

The winter setup is Softy McSoft, 4x4 Caddy Stang. I kind of love it!
 
I freely admit I'm out of my depth here but from messing around with these various setups it seems to me that increased rear end stability comes with making sure you have plenty of shock damping relative to the effective rear wheel rate - whether you get to that ratio by reducing the effective wheel rate (softer springs, smaller ARB, outboard spring location), or by increasing the shock damping (koni adjustable > bilstein > oem dampers), or both.
From your spring rates, I gather you have a non-Brembo car? Funnily enough, the rear Bilsteins are nearly identical in damping to the OE Brembo/TP dampers - actually have a bit less compression than stock - if the dyno charts floating around are to be believed. Yet almost everyone who gets them talks about how much better then rear end feels after.

It's in the front where the Bilsteins (and Konis) are dramatically different than stock, with almost 2X as much compression damping, and about 30% more rebound (again, relative to a Brembo damper, they're the only OE ones I've seen a dyno plot of). I've always thought the Bilsteins way to much for stock springs based on that (and the fact that most guys are running them with srpings in the 200-300 lb/in range). It doesn't sound like you felt the care was super over-damped with OE springs and Bilsteins though.
 
Nice post...do you have the charts to show?
Not my data, but they've all been posted publicly before so sure.. Bilsteins were dynod by Stuart Maxcy IIRC, Konis & OE dampers by a member of another forum. Compression is on the top in all plots, rebound on the bottom. The Koni/OE plots show a full hysteresis loop, unfortunately the Bilstein plots are just the avg, but beggars can't really be choosers and I don't have a set of Bils to get dynoed:

Fronts:
Bilstein-front-s197-scan.jpg

OE Brembo damper Front Dyno Range.jpg

Rears:
Bilstein-rear-s197-scan.jpg

OE Brembo damper Rear Dyno Range.jpg

It’s been so long since I had the Brembo package dampers on, but it was my opinion that they were significantly stiffer than base cars.
The base dampers are linear and considerably softer from what I've read. Someone dynoed PP vs. base S550 dampers back in 2015-ish or so (BMR maybe?), but I've never seen the same for the S197.
 
Last edited:

ChrisM

Mostly harmless.
844
871
Exp. Type
HPDE
Exp. Level
3-5 Years
Japan
Probably a stupid question, aside from what's already been described here, but if all else were equal, would the way the coilovers are attached to the vehicle have any significant impact with how the rear end handles/feels versus the OEM-style setup? The Cortex coilovers are mounted using heim joints at both ends, as opposed to OEM-style shock mounting using bushings that seem likely to be subject to binding as they move through their range of motion.

When I switched from stock to Koni yellows with H&R springs, the ride on the street was noticeably harsher (bouncier), I'm sure due to a number of reasons explained throughout the forum. Switching from that to coilovers was a Godsend for street comfort, in addition to a much more predictable rear end. Made me wish I had just skipped the intermediate stage but at least I know for comparison now.
 
72
48
From your spring rates, I gather you have a non-Brembo car? Funnily enough, the rear Bilsteins are nearly identical in damping to the OE Brembo/TP dampers - actually have a bit less compression than stock - if the dyno charts floating around are to be believed. Yet almost everyone who gets them talks about how much better then rear end feels after.

It's in the front where the Bilsteins (and Konis) are dramatically different than stock, with almost 2X as much compression damping, and about 30% more rebound (again, relative to a Brembo damper, they're the only OE ones I've seen a dyno plot of). I've always thought the Bilsteins way to much for stock springs based on that (and the fact that most guys are running them with srpings in the 200-300 lb/in range). It doesn't sound like you felt the care was super over-damped with OE springs and Bilsteins though.
@Senna, yes I have a non-Brembo 2011 GT. I originally went from stock springs and dampers to P springs and Bilsteins and the improvement was noticeable in the rear. I don't have any numbers to back it up but it definitely felt significantly better damped compared to stock.

This past fall when I put the stock suspension back in for winter, I used the Bilsteins in the rear with stock springs and no bar. But the front is original dampers and springs (I didn't bother taking apart the original strut and spring). So I've never combined Bilstein front struts with the original front springs and have no experience with that combo.

I started this thread thinking about rear spring location, but posts such as docs302 above, combined with my own experience, have caused me to think good rear axle control is likely as much or more about having plenty of damping for a given wheel rate. But I'm no engineer, just my impressions!

Hope this helps.
 
72
48
Probably a stupid question, aside from what's already been described here, but if all else were equal, would the way the coilovers are attached to the vehicle have any significant impact with how the rear end handles/feels versus the OEM-style setup? The Cortex coilovers are mounted using heim joints at both ends, as opposed to OEM-style shock mounting using bushings that seem likely to be subject to binding as they move through their range of motion.

When I switched from stock to Koni yellows with H&R springs, the ride on the street was noticeably harsher (bouncier), I'm sure due to a number of reasons explained throughout the forum. Switching from that to coilovers was a Godsend for street comfort, in addition to a much more predictable rear end. Made me wish I had just skipped the intermediate stage but at least I know for comparison now.
Chris, do you happen to know the spring rate on the H&R you were using? And which rear bar? Thanks.
 
3,232
3,000
Well there is definitely a measurable difference in where the springs are located on the axle, because it's basically just a big lever. I used to have access to some of that info, but not anymore. In any case, this appears to be another example of the 18mm or no bar recommendation that works well.
 

ChrisM

Mostly harmless.
844
871
Exp. Type
HPDE
Exp. Level
3-5 Years
Japan
Chris, do you happen to know the spring rate on the H&R you were using? And which rear bar? Thanks.

They were the H&R Race springs. I don't have the official numbers, but a quick Google search turns up 325F/285R if that sounds about right. Stock rear sway bar for an 07 GT, which is still on the car. Going to try without it when I get back and see what the difference is.
 

TMO Supporting Vendors

Latest posts

Buy TMO Apparel

Buy TMO Apparel
Top