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Old 09-11-2014, 07:19 AM   #1
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Default Power, Tire Width, and Spring Rates

I'm having difficulty choosing my suspension setup and was wondering if you could help.

Here's some info on my car:
1990 miata
Daily driver, occasional track (for fun/non-competitive)
200-250 whp goal (sr20 t25 in progress)

Current Setup:
ST coilovers (basically KW V1)
392F/336R lb/in springs
22.5mm/16.5mm sway bars (car came with these)
205 all-seasons

Power vs. Tire Width vs. Spring Rate:
I've read some (seemingly) incompatible advice on here and would like to sort them out. I've read that:
1. For 200-250 whp, 225 width tires are necessary
2. For 225 width tires, 700/450 (or higher) springs are recommended
3. For street use, ~350/250 is a good compromise

I don't know what to do. It seems, based on this, that I can't have my cake and eat it too.

Coilovers:
I chose the ST coilovers because they're cheap, comfy, built well, and were recommended to me by a friend. And their spring rates fall in between those recommended in #2 and #3. I know the F/R ratio is not ideal, but I have 450/280 springs I can swap in if I want. They're a bit longer, (6"F/6.5"R vs. 5.5"F&R), but I figured the length and rates were close enough for me to get by without issues (or a revalve).

Sway Bars:
But I'm afraid that, even with the slightly stiffer front springs, they'll be too soft for my power goals. To remedy this, I thought about getting the RB 1.125" hollow bar (.125" wall). It should fix the massive oversteer from the 16.5mm rear bar and give me back the "spring rate" I need to counteract the sticky tires.

I know this setup would be less than ideal. But like I said, I'm not looking to be competitive. All I want is to be able to track the car at 200-250 whp without bouncing on the bump stops/struggling to maintain control the whole time.

The Question:
Will this setup work okay? If so, should I get a brace/blocks for the FSB?Should I ditch the rear sway bar? Stick to 205-width tires? Or drop in 700/450 springs (without a revalve) and hope for the best? Or ditch them altogether for a bilstein/GC combo?

Thanks for all your help!

Cliffs Notes:
450/280 springs + 1.125" RB FSB + 16.5mm RSB + 225 tires + 250whp = bad idea?
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Old 09-11-2014, 12:58 PM   #2
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Will your track tires be the all seasons or r-comps? For springs, the all seasons in 205 or 225 should be off the bumpstops at 450/275 with big sways. You would get close to contact with summer tires in those sizes and be hard on the bumpstops with r-comps.

If you choose all seasons then the tires will be slick as snot after three or four 100mph-40mph braking zones in a row in a 250whp car. If 205 or 225 it won't matter with all seasons.
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Old 09-11-2014, 02:10 PM   #3
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My track tires will be summer tires (probably rs3). From what I read they should offer just the right amount of grip to keep a 200-250 whp car planted. I was originally planning to run 205s.

So "close to contact" with 225 summer tires, huh? That sounds like good news. At what ride height, though? I was thinking like 12" hub to fender. Too low?

Also, should I trim the bump stops? They're 50mm.
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Old 09-11-2014, 05:13 PM   #4
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You should probably trim them a bit.

If you are going to run relatively soft springs you will need to give yourself a decent amount of travel on the track. That will mean raising the car considerably if you are sporting flatbrim ride heights on the street. Track and street obviously require different optimum adjustments.

You will need the big sways with those springs and tires, too.

Once you are at the track put zip ties tightly around the shock rod just above the shock body. After a track session, see how high the shock body pushed the zip tie. If all of the way to the bump stop, increase your ride height (unless you hit a curb or had an "off" or some other catastrophe). This method will also help you to determine if you need stronger springs if you change tire compounds later.
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Old 09-11-2014, 07:34 PM   #5
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Thanks, sixshooter, that's exactly the kind of info I'm looking for. I'm thinking, cutting 10-15mm off the bump stops.

What I'm wondering now, though, is if I raise the car high enough, will I run out of droop travel? Is there any way to calculate/make an educated guess about this? Is there an ideal bump-to-droop travel ratio? I read an old post (by ooby_dooby, in 04) that a good ratio is 60% bump, 40% rebound. Is that a good rule of thumb?

I know all this can be determined empirically, but I don't want to set everything up the way I want (NB top hats, 450/280 springs, 225 tires) only to find the struts are too short, which would necessitate swapping for a whole new setup entirely.
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Old 09-12-2014, 07:59 PM   #6
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Bump travel will keep you from ending up kissing a concrete wall. Droop travel will give you a few tenths faster lap time. You are on street tires. Which is more pertinent to you?

Seriously, when you are tracking out of turn 12 at Road Atlanta and the car is leaned over against the bumpstops on the outside and then you hit the elevated rumble strips at the outside edge of the track as you track out at 90+mph, you need that extra bump travel before you contact the bump stops. Otherwise the only place your car can absorb the compression is in the sidewall of the tire. And that would cause a loss of pavement contact that could result in a spin. And you don't want to spin there.

That's the why.

An inside tire lifting in a hairpin is much less sensational because it has so little of the grip at that point anyway.
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Old 09-13-2014, 12:05 AM   #7
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Quote:
Originally Posted by sixshooter View Post
An inside tire lifting in a hairpin is much less sensational because it has so little of the grip at that point anyway.
+1 for sixshooter's example.
I saw Bob Bundy's inside front lifting at MRLS when I followed for half laps when he passed me. He is fast as ****, so I wouldnt worry about droop travel in your situation.
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Old 09-15-2014, 03:27 AM   #8
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Yeah, inside front matters even less than inside rear (which matters a little)

-Ryan
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