Secrets of the Mirage

[blacksheep-1]

Car is looking a lot more muscular..when do the IMSA style fender flares come?

Dude..this is YOU...minus the lights

[Loren]

Yeah, I wish they sold this car with two doors.

Fender flares? If I still have it after 3 years, maybe. So far, I don't really need them. Can fit 195's in the back and 205's in the front. (compare to the Festiva... couldn't fit more than 185's without flaring fenders)

[Native]

Rear wheel is only lifted about an inch.

While I was driving, not at 10/10ths...

[Loren]

Well, there are ways to consider that. People kept telling me that I wasn't even lifting a rear tire. And yet, here's clear photo evidence that you were. My guess is that while you might not have been CONSISTENTLY driving at 10/10... you may have pegged a little closer than I did to it.

Our fans will be happy to know that, yes, the Mirage could actually be driven faster than we were driving it.

[Loren]

Weekend autocross prep complete!

Had Carl flip the tires last week. Hopefully, I'll be good for the rest of the season. Or, at least a few events until I decide I want to put 205's in the front.

Today, I changed the oil. (ooooh...) Capacity is, indeed, 3 quarts. 3.2 quarts with the filter. (who ever changes oil without changing their filter, anyway?) Fun fact: It was cheaper to buy a 5-quart jug than to buy 3 individual quarts of oil.

Also, I rechecked torque on all of the steering parts, and gave the toe a slight adjustment. Ready to go be faster!

[Loren]

Oh, did I mention that the latest Bump Steer Correction mod made the car feel REALLY good at the March 11 autocross? Yep! Very pleased.

Tires seem to be holding up well, too. I'm still on the fence about whether I should increase the front spring rate before fitting wider front tires.

[Loren]

So, I've been running 6mm spacers on the front of the car for a few events now. Don't really "need" them with the 195's, but I had them, and I knew I'd need some additional offset to run 205's, so I put them on there.

I have about 7 threads of lug engagement. Which is "enough". But, it doesn't give me warm-fuzzies. I'd feel really stupid if I lost a wheel for something like that. (and I've seen it happen) So, to fit 205's, I have three options: 1) Keep the current setup with 38mm offset wheels, 6mm spacers, and minimal thread engagement. 2) Fit longer wheel studs. 3) Get proper offset wheels.

I didn't spend a lot of time looking at studs. I have in the past. To get good studs, I'd probably end up spending around $35-40 for parts, and then I'd have to disassemble the front end and have the hubs pressed out. A lot of work.

Wheels are the easy and more proper solution. Tire Rack has the same Kosei K1 that's on the car in a 15x7, 30mm offset for $125 each. So, 8mm offset in the correct direction, and as a bonus, they're a pound lighter.

Ready for the cost justification? Don't try this at home, I'm an expert!

Wheel Stud Option:
Pair of 205/50-15 RE-71's - about $285 shipped plus $55 mounting = $340
Wheel studs = $40 (assuming nothing gets broken in the process)

Total cost = about $380.

I get a new set of tires that are a half-inch wider and a pound heavier, and a little peace of mind.

Proper Wheel Offset Option:
Pair of 205/50-15 RE-71's mounted and balanced (free) from the Tire Rack = $515 shipped.

Total cost = $515.

For $135 more, I get the same tires, a pair of new wheels that are lighter (to offset the heavier tire) and proper offset. Plus, I don't have to futz around with hubs and wheel bearings, just bolt them on. Seems like a no-brainer to me. Effectively, a pair of new Kosei K1's for $68 each? They're on order. Wheels won't be available until the end of April, though... so I have to make sure my front tires last a few more events, which they should.

Oh, and if you're wondering, the rear tire fitment is way different. With the 38mm wheel and 195 tire, I'm already rubbing the fender slightly in the rear. So, I can't fit the same wheels in the rear. And if I wanted to fit 205's on the rear, I'd have to get rear wheels with an offset of something like 45. So, I'll stick with the 195's in the rear and lose the ability to rotate tires. Not a huge deal on a FWD car.

[Native]

Seems like a no-brainer to me.

This.

[Loren]

Back to the question of lowering the driver's seat...

As mentioned, the seats are really tall. But, how did they accomplish this? Can it be modified? Well... yes! The seat bottom is pretty standard. A few inches of upholstered foam on a frame bolted to a pair of adjuster rails. The adjuster rails are mounted to the floor on pedastals. Simple bolt-on 3.5" tall steel pedestals! They can easily be modified or replaced with shorter ones. Too easy. Of course, that would never work... only THREE of the corners are on bolt-on pedastals. The inside rear corner is on a welded-in pedastal. Do I want to hack up and modify that pedastal? Well, aside from "no, I don't"... my class rules don't allow me to do that kind of modification. If it's not a bolt-on part, I'm stuck with it. Grrr.

But... what if I move the rails FORWARD so that I'm not trying to move it down into the location that the non-movable pedestal is in? Move it forward about 3 inches, then I can move it down all I want! The downside: I lose 3" of rearward seat travel. Some quick measurement shows that is not a problem for me! Anybody with significantly longer legs might hate driving my car (because, you know, everybody LOVES driving my car, anyway)... but, it would work for me. I could even work it so that it's more like only 2" lost. That could totally work.

So, what I need to do is create 3 basic pedestal brackets to go between the floor and the seat rail... and one bracket that will bolt to the top of an existing pedestal go down and forward to go under the now lower seat rail. Pretty simple, really. Might not even require any welding... though, I'll probably do some triangulation and weld in some reinforcements to make sure it's plenty sturdy.

Looked at this with a bit of intent today. Found that there's another hurdle. Not an impossible one, but it adds complexity, and could make "returning to stock" more difficult. The problem is, those cute little 3" tall pedestals that support 3 corners of the seat rail are very seriously RIVETED to the seat rail rather than bolted. They are bolted to the floor. But, to remove/replace the pedestal itself with a shorter piece, I'd have to drill/grind out two massive rivets on each one, and then find a stout, but short-headed set of bolts to replace them with.

It's a bit more work than I'd expected. And I'm not feeling particularly motivated lately.

If this were a car that I didn't care about returning to stock, I'd just pull the seat out, cut 2" off of the existing pedestals, and weld a new "foot" onto them. But, doing that would make returning to stock a matter of either re-building the pedestals with some welding, or replacing the entire seat base.

I should just commit and get to cutting. I never liked the upright seating position of this car, anyway.

[Loren]

Had another great event yesterday. The car is suffering from a lack of power on courses like that, but that's a problem for another day.

I've been thinking about springs. I shared some of my thoughts here: viewtopic.php?f=3&t=3170

Got into "shopping mode" today, and what I've decided to do is go ahead and use those 450# 6" springs that I have on the shelf, and add a "tender spring" to the stack. I dug up the appropriate formulae and made up a spreadsheet to tinker with the numbers, and looked in the Eibach catalog, which has a lot of very detailed info about their springs.

The tender spring I'll be using is a 4.25" long progressive spring 175 - 650 lb/in. It fully compresses at about 850# load.

With that, I was able to extrapolate the following:

The free length of the spring stack will be 10.25"
In the static condition, spring rate will be around 215 and the spring stack height will be 7.35. (1.1" taller than current)
Rate will increase up to 265 until 1.15" of compression (about .4G in a turn) and then the tender will brick and the rate goes to 450.
Rate stays at 450, and we'll see 1.84" of compression at about .9G, and 2.12" of compression at 1.1G.

Total stack height at 1.1G is 5.23". The block height of the tender is 2.2", which means the main spring would have to be 3.03" at full compression. Calculated block height of the spring is 2.6", so we're okay there. And we'll be into the bump stop by then, anyway.

So, bottom line: I think this will work. It will provide the desired effect of "catching" the body roll that happens VERY quickly when the rear wheel lifts... and that happens well after 1" of body roll. And it will do it without completely wrecking the street ride quality. Big bumps will be a bit firm, but smaller bumps will be about the same as they are now.

Instead of total roll resistance going from ~874 lbs/in to 224 lbs/in when the rear wheel lifts, now it will go from ~1100 to 450. Still a significant change, but much better. There will be a bonus of some transitional steering response improvement, as well. It won't be "immediate", but as soon as the body starts to think about rolling, it will instead tighten up and respond!

And the added overall roll resistance should work nicely with the wider front tires that I'll have come May!

[Loren]

Ah, but I'm a cheap bastard!

I had the trick Eibach progressive tender springs in my cart at Summit Racing. But, I just couldn't do it. They're EXPENSIVE! $170 for the pair. And, being Eibach, nobody has them cheaper. They're one of those companies that is really good about price management.

I could skip the whole "progressive" notion and just get a pair of 9" 300# springs (which was where I was initially going to go) for $49 each brand new. Less if I can find them used. I just can't make myself spend yet another $70 that I don't need to!

So, off to eBay I went. And, wouldn't you know it, somebody's selling a variety of 4" long 2.5" ID springs! 300, 400, 500, 600... even 750. Where'd I put that spreadsheet?

Okay, so if I go with a straight 500# tender spring, 4" long... it will brick at 2.25" compression, pretty close to what the Eibach spring would do, it just doesn't have that progressive rate.

So, the spring stack of 6" 450# + 4" 500# will yield:

The free length of the spring stack will be 10". (plus the spring coupler that I'm going to cobble from garage parts that will add another 1/4")
In the static condition, spring rate will be 237 and the spring stack height will be 7.6" (and if that ends up being too tall, I can either raise the car a bit... or I have another pair of 450# springs that's closer to 5.5" long)
Rate will stay at 237 until right about .8G or or 2.1" compression where the tender will brick and the rate goes to 450. If I want earlier engagement, I can wrap the tender spring with heater hose and probably get it to kick in 1/4" earlier and a little more progressively.
Rate stays at 450, and we'll see 2.24" of compression at about .9G, and 2.52" of compression at 1.1G.

Total stack height at 1.1G is 5.07". The block height of the tender is 2.25", which means the main spring would have to be 2.82" at full compression. Calculated block height of the spring is 2.6", so we're okay there. And we'll be into the bump stop by then, anyway.

Aaaand... they're on eBay for $20 each. Plus shipping. And it says "make offer"... so... let's be even cheaper and see if we can get the pair for $30! If I'm lucky, I'll get this pair of springs for under $50 shipped. Much better than the $170 from Summit!

[Loren]

Awwww yeah! $48.87 shipped! I just saved myself $120! What? Oh, yeah... I spent $50.

Hey, when my wife buys stuff we don't need at the grocery store because it's 2 for 1, she's not "spending", she's "saving"!

Anyhow... shipping USPS from Illinois. Might have it by the end of the week. Next week for sure.

I'm excited. I've "planned" several of these dual-spring setups, but I don't think I ever actually followed through with one.

[Loren]

Damn, that was quick! Springs are here. Measurement and math ensues!

These springs are clearly marked 500#. Somebody actually engraved "500" on them, as if they may have been tested or something? But, if they were tested, they'd have put an actual rate on there... like 506# or 498#.

Weird, they LOOK like they are the same wire gauge as the other springs. Measurement (I do love my digital calipers... why I waited so long to invest in such a useful tool is beyond me) shows that the wire gauge is actually a bit smaller than the 450# springs... weird, you'd think that would make them a lower rate. That's where the math comes in.

Using this handy calculator: http://www.racingsuspensionproducts.com ... 20rate.htm

The 4" springs are .392" wire, 3.44" OD, and about 2 and 1/16 (2.06) active coils. (insider tip: what determines the rate of the spring is actually the wire thickness and the LENGTH of the wire... think of a spring as a torsion bar that's been coiled up for better packaging! So, knowing the OD and the number of active coils tells us the effective length of the torsion bar) I even accounted for paint thickness (.003 per layer) in my measurements. Some of the powder coat is peeling off, so I was able to measure it!

So, the calculated spring rate is actually 582#. Significantly more than the stated 500. I'm more inclined to believe what I've measured and calculated. Math doesn't lie.

Since I'm measuring, I'll go ahead and measure the shorter 5.5" springs that I want to use and get as accurate as I can.

The 5.5" springs measure 5.67" in length, .397 wire, 3.42" OD and 3.5 coils. Which puts them at 369#.

So, I'm looking at a 369# main spring and a 582# tender spring. Previous calcs were with a 450/500 combination. How does that work with the spring stack calculations?

Static rate is 226, which is almost exactly what's on the car now. That's nice.

Measured brick height of the tender is 4" less four wire thicknesses of .392. About 1.6". So, we have 2.4" of compression. Hmmm... it takes 1397 lbs to brick that spring (and take it out of the equation so that the main spring takes over). We don't get that kind of weight until 1.25G!!! That's a problem. That's why I chose a 500# spring rate instead of a 600# spring rate.

Let's see... if I wrap the coils with 1/8" wall heater hose, I can pretty much wrap all of the active coils, so 2 full coils wrapped. That adds a half inch of rubber thickness to the wire diameter, and causes us to coil bind a half inch sooner. (in reality, we'll START to coil bind a half inch sooner, and the rubber will compress to probably 1/4", giving us a progressive rate change... which is a good thing)

Doing that, our rate will start ramping up at 1106 pounds, right about .78G. Cool.

And if the rubber compresses to 1/4", that will be at 1.02G. where the spring rate becomes 369#.

So... this stack of 396/582 with the tender spring coils wrapped with heater hose will behave like a progressive spring starting at a 226# rate, and ramping up from 226 to 369 between about .8 and 1G.

The theory sounds REALLY good! No harsh transitions. Keeping the softish spring rate that I have for a comfortable street ride. Getting up close to 400# under hard cornering. I like it!

[CaptainSquirts]

Sounds cool and interested to see how it works out for you when you get it all setup. I'm sure you explained the reason(somewhere in the walls of text ) why you're going with a tender spring instead of just using 1 spring with a higher rate by itself. But is it for a comfort and performance reason so it's still streetable and has the performance of a higher spring rate? Also, since you're running an overall 10"' springs now, will it produce more compression, or less? I'm going to assume the roll center isn't changing with the springs you've added unless you changed the ride height(I think thats how it works?). So now with the new springs setup, will you have more or less body roll?

[Loren]

I'm sure you explained the reason(somewhere in the walls of text

Your lack of acknowledgement of my use of paragraphs and punctuation is hurtful.

why you're going with a tender spring instead of just using 1 spring with a higher rate by itself. But is it for a comfort and performance reason so it's still streetable and has the performance of a higher spring rate?

Yeah, it gives me a stiffer rate when pressed without sacrificing street comfort. And, since I already have the main springs, buying a pair of tender springs to go with them didn't cost me any more. (unless I bought the stupid expensive Eibach progressive tenders) If I had to buy the whole "stack", I probably wouldn't have done it.

Also, since you're running an overall 10"' springs now, will it produce more compression, or less? I'm going to assume the roll center isn't changing with the springs you've added unless you changed the ride height(I think thats how it works?). So now with the new springs setup, will you have more or less body roll?

So many questions! (I don't mind, glad somebody's paying attention)

Interesting thing about stacking springs that I actually learned the other day. Say you have one spring. It's a 100# spring, and you're loading it with 200#, so it compresses 1". If we then stack ANOTHER spring on top of it, while we DO change the overall spring rate (the resultant rate is softer than the softest of the two springs, I'll spare you the formula), that first spring still compresses the SAME amount. So, if we stacked another 100# spring on top of the first one, EACH of those springs would compress by 2" under a 200# load. Total compression of 4". And the rate (because the math is simple when both springs are the same, I know this one) of the stack is 50#. 50#... 200# load... 4". It all makes sense if you think about it.

So, if I have an 8" spring right now. It's 225#. I'm loading it with 600# (ish), which compreses is something like 2.6". Static spring height is about 5.4". Something like that.

The new spring stack will be 4" plus 5.8" plus about a 1/4" for the spring separator. Total of 10". How much will it compress? Well, under static 600# weight, the main 369# spring will compress 1.6". The 582# tender will compress a touch over 1". So, 2.6" compression. on a 10" stack. Leaves me with 7.4" static spring height. So, I'm going to need to adjust my lower spring perch way down to acheive the same ride height... because my spring stack is taller, and it's not compressing much more.

I could also look at that by looking at the overall stack spring rate. Up until one of the springs (the tender) fully compresses, that is accurate. So, the overall rate is about 225#, just like the current spring. Shoud compress about 2.6"... same same.

Spring rate doesn't change roll center as long as the ride height stays the same.

The initial spring rate will be the same until the tender spring is fully compressed (or starts to compress the rubber that I pack it with). So, for the first inch or so of body roll, it will be the same. As we start binding the tender spring, the main 369# spring starts taking over, and we'll get less roll. In reality, we probably won't get any less body roll... it will just feel more controlled as the spring rate ramps up. Or that's the theory.

[Loren]

Today's micro-project. I welded up an accelerator pedal extension. I didn't feel that I needed any more pedal height, and there was a slight lip where the top of the pedal plastic overhangs the solid side of the pedal arm. (the other side is hollow) So, I formed a piece of flat steel to match the contour of the pedal and slide just under the top of the pedal. Then I welded a vertical plate to it, and bolted the whole thing to the solid side of the pedal.

Untested yet, but seems sturdy enough. And didn't add more weight than I needed to. (yeah, I could have made it out of aluminum... but, I can't weld aluminum)

I'll put it back on the car tomorrow and see how it works.

[twistedwankel]

Geezus. Now I know why I called Stranoautoparts.com!! loren is called the professor for an obvious reason

[Loren]

The pedal extension is perfect! I can blip a heel/toe downshift even wearing my flip-flops. Not terribly useful for autocross, but I like the fact that I've added another skill that I can practice on the street occasionally so that I'm in the habit of doing it on the rare occasion that I go to a track. (or need to bang off a perfect 3-2 downshift on an autocross course)

Picked up some 1/2" heater hose today and wrapped my tender springs. I wanted to do a second wrap with 3/4" hose, but that ended up being a total fail. Too tight of a radius for 3/4" hose to deal with.

Measured space between coils as-is = about 1.75". About 1.1" of that will be eaten up by static weight, leaving about .7" of compression before coil bind. 582 x 0.7 = 407 pounds of travel before coil bind. 407# is almost 68% of 600, so I'll start binding on the hose at about .7G. If all of this math is correct, should be nearly perfect. We shall see.

[JasonS]

[Loren]

New spring stack installed. $48 for the pair of tender springs (shipped). $0 for the main springs (leftover from the Festiva, they came with the cheap coilover kit we bought just to get the adjuster barrels). $0 for the spring separators (leftover parts from the same kit). And $6 for heater hose. Total cost = $54. Not bad.

Everything went together as expected. Ride height is right about where it was, within 1/8". (yeah, I didn't measure before I started) Curiously, the street ride feels a bit softer. And Philip and I both agreed that it's impossible to push the car hard enough on the street to really gauge how it's working otherwise. The car goes where you point it, and feels "good" on the street.

So, why does it feels softer? My guess is that some small discrepancy in measurement, or some small variance in the actual hardness/stiffness of the spring steel affected my spring rate calculations. But... now that they're on the car... hey, I've now got a $7,000 spring rate tester! I had Joe weigh the car once, so I know what the weight is.

(Page 1 of thread, scroll, scroll... front weights are 573/648)

I don't know for sure which side is left on my weights, but I measured the springs on the right side, and that's the side opposite the battery and steering column... should be the lighter side? So, we'll say our weight is 573.

On the car, after driving around and getting everything fully settled, the main spring is 4.25" and the tender is 3.15".
Full length of the main was 5.8", and the tender was 4.2"

Total compression on the main is 1.55" with 573# load = 370# spring rate
(I had calculated 369, pretty damned close!)

Total compression on the tender is 1.05" with 573# load = 545# spring rate
(I had calculated 582, within 7%. Not bad for rough math.)

With this new spring rate info, that puts the static spring rate at 220# (still really close to what it was before... so my perception that it might be softer is purely placebo) Tender spring rubber will start to engage at .7G and the tender will be fully bound by .9G where the spring rate goes to 370#.

Can't wait to autocross it and see if I can feel the difference!

And just for kicks:
The front springs that I removed were clearly marked 62-200-04. That's 62mm ID, 200mm length and 4kg/mm rate.
The wire measures 0.340. We'll knock of 2 thou per side for paint and call it .396. OD measures .328
Rate calculator says 240#... it's supposed to be 226.

So, If I went from an actual 240# rate to an actual 220# rate... that's 10% softer. Now I feel better about the calibration of my butt dyno!

I love it when the math and the measurements and the objective "feel" are all in agreement!

But, the more I look at this, the more I think... I'm probably well into the bump stop before the tender spring fully compresses, anyway. Maybe I'll put a camera under there and look at it in action?