Tailhappy Miata

At autocross Saturday, the course had 3 or 4 fast segments, separated by some variation of 120°-180° sweeper.

It was fun, but it really highlighted the Manic Miata's one weak point:  it's very tailhappy under trail braking.  I spun on 3 of my 6 runs, in 3 different turns.  See the hotlink thread.

Other than that, it is remarkably balanced.  But that characteristic is keeping me conservative in driving it on course, and hence, hurting the times I think the car itself is capable of achieving.

It's a little under-tired:  225/45/15s on 15x9 wheels, but going any wider would cause rubbing (it already rubs the insides of the wheels at full lock) unless I break out the sawzall.  It also has no aero at all other than stock Miata R front and rear lips.  It has a Torsen.

It's got Racing Beat swaybars front and rear, Koni Yellows, and Ground Control coilovers.  I don't remember the spring rates off the top of my head, but they are stiffer than stock, but not full-zoot track/autocross stiff.  The Konis are currently set 1-2 clicks off full stiff in the front, I believe, and maybe 1 click off full stiff in back.

In steady-state cornering, corner exit, and slaloms, it really is very nicely balanced.  So... what should I do to tame the trail braking oversteer?

1. Soften rear swaybar?
2. Stiffen front swaybar?
3. Stiffen front shocks?
4. Soften rear shocks?
5. Add a big rear spoiler?
6. Learn to drive better?
7. Something else?

Thanks!

Since you can't increase diff locking on decel, I'd start by reducing the rear rebound adjustment, and depending on the outcome of that, consider softening the rear swaybar.

In reply to red_stapler :

Thanks.  Koni Yellows are single-adjustable, so I can soften them, but only in both directions, not just rebound.

Put on less aggressive rear brake pads.  Our rx7 does that when we try to get too aggressive with pad selection.

What about brake pads? Could you switch to a less aggressive compound in the rear? 

Duke said:

In reply to red_stapler :

Thanks.  Koni Yellows are single-adjustable, so I can soften them, but only in both directions, not just rebound.

IIRC only rebound is adjustable. Bump is fixed. 

My E street setup: brakes are hawk HPS rear, parts store pads in front. Hard S springs, single adjustable Konis (which only adjust for rebound). Front racing beat hollow bar, stock rear hard S bar. Front bar is full hard, the rear isn't adjustable. The front shocks are 1/2 turn from full hard, rears are 1/2 turn from full soft. Camber is maxed out all the way around.

It's very easy to spin under braking. I've got to be coming off the brakes at turn in. Last fall I had an alignment done at a miata race shop, and they were able to get the rear negative camber to 3.2. It's still very touchy under braking, but the rear end doesn't snap spin like it used to. Now , if I finesse it, I can lightly trail brake and use the loose rear end to begin to rotate the car. I still have to coordinate the brake release with turn in.

My suggestion is to start with rear shocks 1/2 turn from full soft. Try a run or two, and if it doesn't make it worse, then disconnect the rear bar. Adding more aggressive pads in the rear helps with the NA and NB tendency to lock up the fronts, but can make this problem worse.

Trail braking in the rain. Wheels don't look like I'm turning much, but you can see the right rear unloading and the car taking a set. Brake lights are on. This is prior to the added rear camber.

get us those spring rates. 

the setup I had on my STS NA was 550F 375R with the adjustable racing beat front sway on the middle setting and no rear bar. That said, it was open diff.  that was rear biased enough to be tail happy in higher speed stuff, but to a somewhat desirable point, an LSD would likely make it worse. IIRC 550 was about that max you could reliably damp with off the shelf Koni Singles, you needed to get a revalving to go higher to the desired 700-800 lb range (front)  I think people  were running 400-450 rear with that (like I said, my 375 was on the tail happy end of the spectrum, but it met my preferences)

This helped me a lot on my setup to see where I was at regarding F/R spring rates https://www.fatcatmotorsports.com/FRC_TUTORIAL/FCM_MSDS_TUTORIAL.htm

I'd get what you are running and plug it through there.

CAinCA said:

Duke said:

In reply to red_stapler :

Thanks.  Koni Yellows are single-adjustable, so I can soften them, but only in both directions, not just rebound.

IIRC only rebound is adjustable. Bump is fixed. 

Capische.  Thanks.

Brake pads and bias will only help if the car is tailhappy because the brakes are locking up or getting close to it, which I don't think is what's happening here - it sounds more like a suspension problem, which is best addressed with a suspension solution. So +1 for softening the rear shocks, from there you should look into your alignment - could the rear be getting toed-out under droop? And then look into changing rear springs and sway bars.

I think of trail braking as a way to help a car that doesn't want to rotate to rotate and not a primary means of slowing the car down. If you're happy with the steady state behavior (and non-trail braking transient behavior) I'd just stop trail braking. It sounds like your car is rotating well on turn in on its own. You could even use some maintenance throttle on turn in if it's needed. I think tighteing the car up here is going to make it slower through other sections where you're saying you like the balance currently. Loose is fast. 

How's the steady state cornering balance? I'd make sure that was where you want it. I'm thinking brake bias might be the answer here, but I'd hate to compromise straight line braking to make it work. It could be a driving technique, trail braking is purposefully trying to get the car to rotate and that's not usually a weak spot in Miatas.

Has the car been corner weighted? If not and you always spin when trail braking in the same direction, if could be a corner weight problem. It's quite possible to have a car with a level stance and totally jacked up corner weights.

Spring rates are probably 375/250, that's the standard spring pack for Miata Konis from GC. 

Keith Tanner said:

How's the steady state cornering balance?

The steady-state cornering feel pretty good and balanced.

Has the car been corner weighted? If not and you always spin when trail braking in the same direction, if could be a corner weight problem. It's quite possible to have a car with a level stance and totally jacked up corner weights.

Spring rates are probably 375/250, that's the standard spring pack for Miata Konis from GC. 

I haven't done it with really accurate scales, but our DMV inspection lanes have combination brake dyno / scale pads for each wheel.  Thanks to the cool guys in the inspection lane at Wilmington DMV, I learned that the Manic Miata weighs 2525 pounds with a full tank of gas and no driver. Corner weights as follows:

LF: 657 lbs    RF: 633 lbs
LR: 615 lbs    RR: 620 lbs

These are fairly approximate given that it was from the scale / brake dyno set in the floor of the inspection lane.  But that seems like pretty decent distribution.  It's equally twitchy in both directions.

I will have to dive into my paperwork and verify, but I that's about what ballpark the spring rates are in.

Thanks, everybody!

Knowing your alignment numbers and ride height would help, but if your rear swaybar is adjustable, I would start there:  soften it and see what happens.

Given that your steady state balance is good, I'd stay away from adjusting sway bars, spring rates and alignment. Look at things that are transitional: shock settings, brake bias, driver behavior. You can also get some transitional behavior from bumpstop engagement but I wouldn't expect that from trail braking as much as you're unloading the outside rear compared to steady state.

Cornerweight numbers are decent - 49.4% cross weight. You're probably within the margin of error of those DMV scales. Spring rates are printed on the Eibach springs.

For me I get a good tuning of turn in/trail brake by playing with rear rebound*. Since you want less rotation I'd soften rear rebound a couple of clicks at a time until it is not rotating enough, then go back a click or two.  

*This is my experience on track and not auto-x with AFCO double adjustables.

Keith Tanner said:

Given that your steady state balance is good, I'd stay away from adjusting sway bars, spring rates and alignment. Look at things that are transitional: shock settings, brake bias, driver behavior. You can also get some transitional behavior from bumpstop engagement but I wouldn't expect that from trail braking as much as you're unloading the outside rear compared to steady state.

Cornerweight numbers are decent - 49.4% cross weight. You're probably within the margin of error of those DMV scales. Spring rates are printed on the Eibach springs.

I agree with all of this.  I'll add that reaching droop limit can cause weird behavior when trail braking.

As it's a transient handling issue I too would start with the shocks.

I'm a strong advocate of trial braking; if you can only master one thing master this. 

With that said you may need to come off the brakes 2-3 feet sooner (even after adjusting the shocks). In my Datsun you trail the brakes depp into the corner whereas in my Showroom Stock C Miata you only needed to trail the brakes into the first third of a corner.

Is it snap oversteer?  More front bar, more spring or less rear bar?

APEowner said:

Keith Tanner said:

Given that your steady state balance is good, I'd stay away from adjusting sway bars, spring rates and alignment. Look at things that are transitional: shock settings, brake bias, driver behavior. You can also get some transitional behavior from bumpstop engagement but I wouldn't expect that from trail braking as much as you're unloading the outside rear compared to steady state.

Cornerweight numbers are decent - 49.4% cross weight. You're probably within the margin of error of those DMV scales. Spring rates are printed on the Eibach springs.

I agree with all of this.  I'll add that reaching droop limit can cause weird behavior when trail braking.

Good point. It's unlikely we're seeing a droop limit on the shocks but the springs on those GCs will come fully unloaded fairly early, which means effectively the end of droop travel. A set of longer rear springs would be an interesting experiment if they can be packaged with those perches.

I removed the rear sway bar entirely from my car, and when my car is really loose in the rear under braking, I start cranking up the front shocks. Even without the rear sway bar, you can still finesse the back end around, it's not a terminally-understeering dump truck. I'm not any sort of suspension tuning wizard or top-notch driver, but I still manage FTD at local autocrosses with that setup.

Rodan said:

Knowing your alignment numbers and ride height would help, but if your rear swaybar is adjustable, I would start there:  soften it and see what happens.

I'll dig up the alignment specs and pinch weld heights as soon as I am in the same zip code as the car.

Justjim75 said:

Is it snap oversteer?  More front bar, more spring or less rear bar?

I wouldn't say "snap" oversteer as much as "easy to provoke" oversteer.  Admittedly the big lurid spin I posted was an off-camber part of the track.

Just for stance reference here's a shot of me not spinning:

Here's the alignment specs. This was set by maxing out front camber with stock components, then setting rear to slightly more.

Sorry, garage is a little tight and I couldn't get pinch weld heights.  Here are wheel arches, at the centerline of hub.

Front:

Rear:

I don't see anything in those alignment specs that would make the car tail happy and the stance looks reasonable as well.

It's hard to get too much detail from a static picture but your "not spinning" pick looks like you've got remarkably little steering input compared to the amount of lateral load on the car which makes me think it might not be as neutral as you think it is.  If you've got front bar adjustment I'd try stiffening it up.  It's a free and easy change to make.  Actually, if you've got adjustment on both ends I'd try stiffening both ends.  The adjustments aren't necessarily equal on both ends but if they are then the overall ballance will stay the same while decreasing the body roll to keep the suspension in it's happy range alignment wise and it might keep the inside rear tire from unloading on corner entry.

In reply to APEowner :

Cool, thanks for the advice.