Suspension Set up 2/29/16

Yes, that’s the correct date – 2/29/16.  It’s leap year.   🙂

Proper suspension set up is critical.  Doesn’t matter if it’s for the street or the track, either.   Getting ride height, wheel alignment, and corner weights set properly will make the car so much more enjoyable to drive that it’s worth the time, effort, and expense.

If you’re going to be racing, I think you should invest in the tools and time to do it yourself.  Different tires, different tracks will require different set ups.   At high Plains Raceway, I used an asymmetrical set up.   More camber on the left than on the right.   At first you’ll need to be doing some experimentation to see what works best.   If you have to take the car to alignment shop, you’ll waste a lot of money and a lot of time.

To start with, do a Google search on “wheel alignment”.   You’ll need to understand terms like camber, caster, toe in, corner weight, etc.   I won’t bore you here with all that basic stuff here.

You’ll need some specialized tools for this job.  Nothing really fancy, but not cheap Harbor Freight stuff, either.   You’ll be measuring stuff to the accuracy of 0.1*, so don’t skimp here.   You can cut some corners here and there and save some big bux and still accomplish your goals.

First, you need a flat surface.  This is critical.   If the car is not level, all of your measurements will be wrong, and the car will handle like a parade float.   Most garage floors are not flat.  If yours is not too out of whack you can place plywood or metal spacers under the corners to set the tires on.  Use a laser level from Home Depot to get all the corners level.

Start by setting your ride height at all 4 corners.   Hard to define a specific ride height, though.   I gotten a couple of different answers.   I settled on 5.0″ in the front, and 5.5″ in the rear.   FM recommends a little higher, and Exomotive recommends a little lower.   At this ride height, there is good clearance for the oil pan for those oh-so-special off track farming excursions.

I’m using Afco shocks, which seem to be longer than stock or most aftermarket shocks available.   Afco’s do not need shock spacers.   In fact, I’m considering raising the rear shock mounting point for more range.   We’ll see how it goes on the track    I could just cut the bump stop in half if I need to.

Here’s what I suggest.   Set the adjusting collars of  shocks to be exactly the same height left to right.   Measure ride height at all 4 corners.  Adjust the left and right springs by exactly the same amount.   When you get close, adjust front and rear on the left side the same amount.   Your ride height will not be exactly the same left and right,  but it will be very close, and your corner weights will be very close as well.

Fastrax bubble tool, with toe bars
Fastrax bubble tool, with toe bars

There are a number of alignment tools available from places like Longacre and Fastrax.  For the street, a bubble level will work just fine.  For greater accuracy, use a digital gauge.  Accurate camber is the most important part of an alignment, IMO.

You need to set toe.  Make sure you have the ability to do that.  Again, not overly complicated.  But needs to be accurate and repeatable.  I use the toe bars attached to the Fastrax tool.

Hub Stands from FM
Hub Stands from FM

The hub stands from Flyin’ Miata would probably be the best way to go.  BUT… you need them on all 4 wheels.  They are much too tall to be used on one end only.  I found that out the hard way.  At $800, that’s a bit pricey.   When they’re in stock, I plan to buy a set.  When you’re racing, you do a lot of wheel alignments.  So (IMO) spending the bux when can for the good tools makes your life a lot easier.

 

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I made some hub stands for my Cobra out of some heavy duty aluminum.  They work really well, and make the job so much easier on that car.  Making them for a Miata shouldn’t be that hard.

 

You cannot just change the caster, camber, or toe while the car is sitting on solid ground.  The tires stick, and will disrupt your readings.  For ride height and corner weights, you can roll the car back and forth to settle the suspension.  But that’s not really practical with alignment tools.   You can buy plates from Longacre, but they are really expensive!   I bought some 1/8″ thick aluminum plates from  Speedy Metals  and made a grease sandwich.   Not as trick, but just as accurate, and <$100 for all 4 wheels.

OK, now we get to the hardest part.  You need to have something to measure the wheels against for toe.  Caster and camber are pretty much independant, so that’s easy with a little bit of practice.  But the wheels need to be straight and square to the chassis, and then straight and square to each other.   On a stock chassis car like the Miata, using a string set up via FM hub stands is the easy and high dollar method.  But I don’t have that, and can’t get them right now.  So I went for the slightly more difficult but cheaper method.

The Exocet is not symmetrical,  and is not straight.  The sides are curved, and floor pans are not the same on each side.   So now you get to use some of that high school math you stored away so long ago.  Fortunately, there’s no algebra or trig involved.  Yeah!  Algebra gives me a headache.

Measure the front and rear of the car at the corners of the frame.   Measure it a couple of times to make sure your being accurate.   Be sure to measure the frame, and NOT the little jacking strips.  I had to move my front measuring point back a bit because the T56 trans and the exhaust pipes hang below the frame and make accurate measurements impossible.

Divide those numbers in half, and you now have the exact center of the frame.  Unfortunately, there’s no easy way hang something from this point.   So you make a mark on the frame 8-10″ in from the center at both ends of the frame.

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See where that mark is on the rear?  Turned out to be a bad spot.  Later on I welded a bracket there.   I’ll have to measure again, and make the mark 2-3″ outboard.

If you did this right, these two points will form a line that is exactly parallel with the centerline of your car.  I measured and marked both sides so I could hang the string from either side.   It ended up being easier to hang the string on the left.

Now, get a couple of pieces of nylon fishing line, and attach some weights to the end.   Use some new line that’s not bent out of shape by age.  And use some real thin light weight line, too.   Big fat string adds more inaccuracy to your measurements.

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Hang the lines down from those marked points.   Then set up your laser level at the front or rear of your car (doesn’t matter which end).  It does not have to be perfectly level.  But raise it up so that it’s about the same height as your toe bars – where your tape measure will be.   Cast the laser beam so that it intersect with both pieces of hanging nylon line.  It takes a little patience, so take your time.   Hard to see in this picture, but this is what you’re after.

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The far left arrow points to the laser at the front of the car.   The middle arrow points to the front string.  and the arrow on the right points to the rear string.   Now you have a laser line that is exactly parallel with the center line of the car, and at the right height for measuring anything.

 

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Now you can start measuring toe.   Make sure your steering rack is on dead center.  You want the steering wheel to be straight..  Attach your tape measures to your toe bars.  Measure front and rear toe bars against the laser line.  The difference between the two measurements is your toe.   If the front is greater than the rear, you have toe out.

Do all 4 wheels the same way, measuring off the same laser line.   Be sure and check your laser line often.   It’s easy to bump the laser and knock it off line.  Easy to put back, but if made adjustments on that off kilter line…….   Well, you just get to do it again.   And we all know how much fun it is to do this multiple times because you screwed it up.

Setting caster and camber is pretty straightforward.  Nothing magical or difficult about it.  Read those other sites, and look at the instructions that came with your alignment tools.

Corner Weights

Corner weighting is tedious and time consuming.  But well worth the effort.  Again, start with a flat floor, or shim up to flat.   I originally started with Ruggles Scales.  Low buck, but very accurate if done correctly.

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Originally designed for small aircraft, they work quite well.   Notice that the drivers seat is filled with stuff.   You want to do all your adjustments with expected weight in the seat.   I just piled in everything I could find until it matched my weight.

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This time around, I bought some Longacre digital wired scales.   Expensive, but much easier to use.  And I can use them on the lift, where the Ruggles scales don’t fit.

Again, do a Google search on corner weights.   There are some excellent articles available.   And the Longacre scales come with instructions, too.

Specs

So, where did we end up?  Alignment settings were tough to come by.  Not many people racing a V8 Exocet yet.   I went to the  949 Racing  web site, and started with their recommended race set up.   After some track time, we’ll use tire temps to make appropriate changes.

Front camber: -3.5° (or as close as you can get to it)
Caster: >+4.5° or max available once you reach camber targets
Front total toe: 0

Rear camber : -3.0°
Rear total toe in: +1/16~1/8″

Corner weights took a while to do. The car is definitely heavier on the left than the right.   The only way to change that is to move stuff over.    In a stripped down car, that’s not easy.   You could re-engineer the frame and chassis, and move the engine over a couple of inches.    But here’s what I cam up with:

RF:   605                       LF:  552

RR:    483                      LR:  427

Left side = 52.6%                 Rear Weight:  44.0%

Cross Weight:  50.07%       Total weight:  2067  (heavier than I thought it would be)

Weight was done with 1/2 tank of fuel, and my race weight in the driver’s seat.

Getting the cross weight to 50% is the most important thing.  You cannot change left side or rear weight percentages by adjusting the springs.   The only way to do that is move stuff around.

Keep a close watch on your ride height.   You can make enough adjustments to get your cross weights looking good, but your ride height to be way out of whack.   Then the car will exhibit some really weird handling.

Take your time doing all this.   It’s careful, exacting, and tedious work.  But well worth it.

3/26/16

I’v been to the track a couple of times.   Nothing like track time to get real world data.

Camber

There are reasons for adding camber to the front.  Ideally, you would want the tires to be straight up and down, or 0* camber.   But in a corner, the the car leans over, that chassis tilts,  and  the outside suspension is compressed while the inside is extended.   Lots of things happening at the same time.   A heavier car with a higher roll center and longer roll couple will roll over more, which requires more negative camber.

But, the Exocet is exactly the opposite.   Very light weight, low CG, low roll center, and short roll couple.  That’s all adds up to very little roll over in a corner; and you need a lot less camber.

In the rear I started out with -3*.  That was too much.  I changed it to -2*.  and that was still too much camber!  I’m surprised.   But -2* seems to be the limit with stock parts.  Fortunately, I had a set of FM’s fabricated rear arms in my basement.   I wanted to experiment with the stock parts first, then change over.

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Now, there is a big advantage to these arms.   Using stock parts, if you want to change camber, you have to turn the concentrics on the lower arms.   That’s fine, but it also changes the toe.   So you have to be able to measure and adjust both of those;  not really practical at the track.

With these arms, changing the rear camber is a simple matter of rotating the outer bracket.  Each 1/2 turn = 0.375* of camber change.  No need to actually measure camber at the track, just change by 1/2 turns and see what works.   That won’t change toe enough to matter.

You could also move the lower arms out as far as possible on each side, and set your toe in.  Then use the upper arm to set camber.   That will give you a slightly wider rear track, by about an inch or so.   Makes room for wider tires, and eliminates the rubbing issue common to the Exocet.  And a wider track will corner better.

Here’s a little trick.   You might notice that all of the rear suspension bolts are inserted from the front, with the nut towards the back of the car.   I did that for a reason.  I have heard that it’s common for racing Miata’s to bend some of these bolts, and they can be a bear to get out.   I can use a piece of round steel and drive them out from the back like a drift pin, with plenty of room to swing a BFH.

Alignment Specs

Initial alignment specs were for a racing Miata.  Good place to start.  But certainly not where this car needs to be.   Based on tire temps, I’m going to try these specs the next time I go to Pueblo Motorsports Park:

Front:  -1* camber, 5.0* caster, 1/64″ toe out.  (Toe out improves Ackerman)

Rear: -1.25* camber, 1/64″ toe in.