Changing coilover spring rate - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

SilverFRS · 09-05-2015, 02:07 PM

I was curious if I was using the right calculations to determine preload and spring length calculations for using different spring rates. This particular example is using the front Flex Z and maintaining the same drop (1.2"), bump and rebound dimensions. The "B" dimension is off of the instruction manual. Preload on the 6k is set to 0 in this example just to make the math easier, in reality it is about 3/16". The stroke length shown is for the springs only from Tein website, I do not have figures for damper stroke. I would love to hear from the suspension experts out there like CSGMike, Racecomp Engineering, anybody that knows how this works. Thanks.

SilverFRS · 09-05-2015, 02:12 PM

Load was calculated as 53% of 2800lb divided by two to get single front wheel static load.

Shankenstein · 09-05-2015, 05:26 PM

It seems like you're on the right track.

You're swapping from a 150 mm, 6 kg/mm spring to a 175 mm 5 kg/mm spring.

Static ride height matches, which should mean that the damper will operate happily. I'd do a quick sanity check when everything is apart, by measuring the free and fully-compressed length of the shock... just to be sure the piston is near the middle of it's stroke at static ride height.

As a side note, it looks like your setup will bottom out slightly less often (and otherwise be more comfortable).

SilverFRS · 09-05-2015, 06:53 PM

Thanks, good info in your suspension link, I was needing that, should I revise the weight to reflect unstrung weight? I noticed that my actual stroke dimension measured less than calculated.

CSG Mike · 09-05-2015, 10:20 PM

Only problem is, the spring will fully compress too early. There are more coils in the longer spring.

burdickjp · 09-05-2015, 11:37 PM

Mass per length?
Is this a linear density problem?

Shankenstein · 09-06-2015, 10:40 AM

Mike's got a good point. Depending on the spring's design, it may fully compress or bind near the bottom.

Here's a pic of a 7" 275 lb spring from Hyperco:

If you're compressing it by ~65%, there is potential for full compression or binding.

SilverFRS · 09-06-2015, 12:57 PM

Take look at the spring travel numbers at the bottom of the page, those are straight off of the mfg website, the 6k will actually bind before the longer 5k. Mfg OK'd this setup for the front, just did not give preload and "B" dimension.

09-05-2015, 02:07 PM	#1
SilverFRS Member Join Date: Feb 2015 Drives: Silver FRS Location: Arkansas Posts: 48 Thanks: 0 Thanked 14 Times in 13 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Changing coilover spring rate I was curious if I was using the right calculations to determine preload and spring length calculations for using different spring rates. This particular example is using the front Flex Z and maintaining the same drop (1.2"), bump and rebound dimensions. The "B" dimension is off of the instruction manual. Preload on the 6k is set to 0 in this example just to make the math easier, in reality it is about 3/16". The stroke length shown is for the springs only from Tein website, I do not have figures for damper stroke. I would love to hear from the suspension experts out there like CSGMike, Racecomp Engineering, anybody that knows how this works. Thanks. Attached Images

09-05-2015, 05:26 PM	#3
Shankenstein Frosty Carrot Join Date: Jan 2013 Drives: The Atomic Carrot Location: Baltimore, MD Posts: 513 Thanks: 272 Thanked 431 Times in 199 Posts Mentioned: 19 Post(s) Tagged: 0 Thread(s)	It seems like you're on the right track. You're swapping from a 150 mm, 6 kg/mm spring to a 175 mm 5 kg/mm spring. Static ride height matches, which should mean that the damper will operate happily. I'd do a quick sanity check when everything is apart, by measuring the free and fully-compressed length of the shock... just to be sure the piston is near the middle of it's stroke at static ride height. As a side note, it looks like your setup will bottom out slightly less often (and otherwise be more comfortable). __________________ If you think you're nerd enough, join in the discussions about Suspension and Aerodynamic modelling! Wall of Fame - JDL Auto Design, Raceseng, Vishnu Tuning, Penske Shocks, Nameless, Perrin, RaceComp Engineering, Essex/AP Racing, Verus, RacerX Wall of Shame - aFe Takeda, Wilwood, FA20Club

09-06-2015, 10:40 AM	#7
Shankenstein Frosty Carrot Join Date: Jan 2013 Drives: The Atomic Carrot Location: Baltimore, MD Posts: 513 Thanks: 272 Thanked 431 Times in 199 Posts Mentioned: 19 Post(s) Tagged: 0 Thread(s)	Mike's got a good point. Depending on the spring's design, it may fully compress or bind near the bottom. Here's a pic of a 7" 275 lb spring from Hyperco: If you're compressing it by ~65%, there is potential for full compression or binding. __________________ If you think you're nerd enough, join in the discussions about Suspension and Aerodynamic modelling! Wall of Fame - JDL Auto Design, Raceseng, Vishnu Tuning, Penske Shocks, Nameless, Perrin, RaceComp Engineering, Essex/AP Racing, Verus, RacerX Wall of Shame - aFe Takeda, Wilwood, FA20Club

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09-05-2015, 02:12 PM	#2
SilverFRS Member Join Date: Feb 2015 Drives: Silver FRS Location: Arkansas Posts: 48 Thanks: 0 Thanked 14 Times in 13 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Load was calculated as 53% of 2800lb divided by two to get single front wheel static load.

09-05-2015, 06:53 PM	#4
SilverFRS Member Join Date: Feb 2015 Drives: Silver FRS Location: Arkansas Posts: 48 Thanks: 0 Thanked 14 Times in 13 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Thanks, good info in your suspension link, I was needing that, should I revise the weight to reflect unstrung weight? I noticed that my actual stroke dimension measured less than calculated.

09-05-2015, 10:20 PM	#5
CSG Mike Join Date: Jul 2012 Drives: S2000 CR Location: Orange County Posts: 14,562 Thanks: 8,942 Thanked 14,211 Times in 6,854 Posts Mentioned: 970 Post(s) Tagged: 14 Thread(s)	Only problem is, the spring will fully compress too early. There are more coils in the longer spring.

09-05-2015, 11:37 PM	#6
burdickjp Senior Member Join Date: Jan 2015 Drives: 2013 ultramarine Scion FR-S Location: Royal Oak, MI Posts: 293 Thanks: 140 Thanked 137 Times in 70 Posts Mentioned: 0 Post(s) Tagged: 0 Thread(s)	Mass per length? Is this a linear density problem?

09-06-2015, 12:57 PM	#8
SilverFRS Member Join Date: Feb 2015 Drives: Silver FRS Location: Arkansas Posts: 48 Thanks: 0 Thanked 14 Times in 13 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Take look at the spring travel numbers at the bottom of the page, those are straight off of the mfg website, the 6k will actually bind before the longer 5k. Mfg OK'd this setup for the front, just did not give preload and "B" dimension.

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