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Old 07-09-2018, 09:12 AM   #1
n0thing
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stiff spring + front sway = understeer?

i changed from kwv3 (4k/5k), eibach front sway on soft, -3.2f/-2.5r, 245/40r17 re71r to tein monosport (8k/9k), -3f/-3r, 255/40r17 toyo ra1, with the same front sway. although i was able to achieve the similiar time at a local track, i was experiencing a huge amount of entry understeering. i played around with tire psi and shock setting, but that didn't change much.

the only thing i can think of is due to the spring rate change? not that 8k/9k is super stiff, but it is double of what i had before. i'm thinking of swapping the front sway back to stock. Or should i change the rear to a thicker aftermarket one, so then front and rear sways would be adjustable?

Tldr: does increasing the spring rate magnifies the effect of sway bar?

Last edited by n0thing; 07-09-2018 at 07:08 PM.
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Old 07-09-2018, 09:35 AM   #2
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The F/R spring rate ratio is different, so your fronts are supporting a higher percentage of the weight vs the rear (under lateral load). Im no suspension expert, but that alone would lead me to expect a bit more understeer. Reducing the front anti-sway seems like a logical first step.
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Old 07-09-2018, 10:14 AM   #3
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Your alignment changed too. Add more front camber unless your tire wear indicates otherwise.
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Old 07-09-2018, 08:09 PM   #4
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Your alignment changed too. Add more front camber unless your tire wear indicates otherwise.
i see your point, but i cant imagine 0.2 front camber makes that much difference, heck if i put it on the rack now the number will probably be all off. the LF tire took the most beating (CW track) from the understeering.


LF tire



RF tire
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Old 07-09-2018, 08:38 PM   #5
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i see your point, but i cant imagine 0.2 front camber makes that much difference
Probably not, but you did increase the rear 0.5 degrees, that's something you can feel combined with the other changes. Camber was the first thing I went to, because the suspension geometry is drastically different front to rear it's odd to have the same camber F/R, strut front+multi-link rear cars seem to like having ~1 degree more camber up front than rear, as a starting point.

I read it this way: you added grip to the rear with the camber change along with a new coilover setup, now go add grip to the front. Softening that front spring/sway may be one way to do it (as would stiffening the rear), but alignment is a lot cheaper to adjust than buying parts (since your front sway doesn't go any softer). @Spuds is right though, accounting for motion ratios you stiffened the front a lot more than you stiffened the rear. Since you're achieving similar times look to add grip to the front to go faster, reducing grip in the rear (by stiffening the rear sway) may get a more balanced car and net faster lap times, but you'd go even faster if you added grip instead of taking it away. Or maybe the rear just lacks stiffness, none of us know for sure and stiffening that rear sway could be the perfect antidote to your problems. Hell maybe stiffening the front helps, street cars can do weird unintuitive things when pushed to the limit.

Can't really tell from the pics of the tire but it does look like outer edge wear to me, if I had to make a change to the car and then drive it based solely on what you've posted, I'd put it at -4F/-3R or -3.5F/-2.5R first.

But that's just some internet nobody spouting bullshit. Good luck.
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Old 07-09-2018, 09:04 PM   #6
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+1 on it looking like the outside blocks took a beating there. I've heard stock geometry and spring rates generally lose camber on the outside front under load (yay struts) so possibly a bit more camber would help. Less anti-sway up front might actually make that uneven wear worse, even though the balance might be better.
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Old 07-09-2018, 09:05 PM   #7
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Besides the camber change and tire compound change, consider the spring change. Spring rate change in itself is not linear. You have to consider motion ratio of your suspension components.

From just looking at the spring rate change, your front wheel rate has increased by 100%, but your rear wheel rate has only increased by about 80%:

Wheel rate of 4/5 kg/mm (F/R): 3.5/2.9 kg/mm
Wheel rate of 8/9 kg/mm (F/R): 7.0/5.2 kg/mm

Your ride frequencies have also changed to where the front now has a higher rate than the rear than before.

4/5 (F/R): 1.8Hz / 1.7Hz (approximate)
8/9 (F/R): 2.5Hz / 2.3Hz (approximate)

To put it simply, you should have gone with 8/10 kg/mm springs if the intention was to keep the difference in wheel rate between front and rear the same as 4/5 kg/mm.

I'll try to answer your last question to the best of my understanding. Anti-roll bar rates increase with torsion, and they may have less torsion if you have stiffer springs that reduce roll. Your bar can be less effective when used with stiffer springs, and depending on the bar, it may increase in rate more quickly than springs do for a given amount of compression/torsion. It's possible to have the same total roll resistance on two cars with each using different spring and bar combinations.

TL;DR: Soften the front bar. More front camber, depending on tire temps. (Without further discussing in detail how the car is understeering)
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Old 07-10-2018, 07:51 AM   #8
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Doubling the spring rates resulted in similar lap times? Maybe the new spring rates are stiffer than optimal. I'd try swapping the 8kg/mm front springs for 6kg/mm.
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Old 07-10-2018, 08:32 AM   #9
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put a stock bar back on the front, you were compensating for a lack of spring rate with the bar and soft springs. now you have way more spring and the same amount of compensation. your effective front rate is probably WAY too high for the tire you're on. if you were on a michelin competition slick it would probably work but that tire isnt enough for that much rate.
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Old 07-10-2018, 01:03 PM   #10
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thanks for everyone's input

@Roadcone that's what I was thinking too, but @AFRichZC6 brought up an interesting point that the swaybar may be less effective with the stiffer spring.

i might just try to disconnect the front sway (just one nut), essentially softening it, to get a feel for it, before i change the bar or alignment.
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Old 07-10-2018, 03:21 PM   #11
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tein monosports arent crazy stiff or anything. my roomate has them on his BRZ with stock bars and it has a ton of roll still on AD08R. Here's his car at Road Atlanta.

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Old 07-11-2018, 12:38 AM   #12
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Springs and bars are the only things that contribute to roll resistance. The dampers only control the speed of how weight transfers or how quickly the car rolls.

@n0thing. Feel free to send me a PM. I'm interested in discussing your vehicle dynamics in more detail.
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