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Old 11-18-2015, 02:07 AM   #1
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Default Stability in braking at high speeds

So I had the car at Thunderhill (3 mile course) last week, and with the extra power it's now bumping the limiter in 5th on the straight (6 speed and 3.636:1). GPS data showed the top speed before braking for turn 1 as being 128, which is faster than I've ever had it before.

One of the things I noticed is that while the car didn't seem unstable on the throttle, once I switched to the brakes it became a lot twitchier. I'm pretty sure it wasn't the rears locking up, because it was the front end that wanted to go places, not the back trying to come around. I've also got ABS in it, and it was not activating.

I have not noticed this instability/twitchiness/etc at lower speeds, and I'm not sure what's causing it. The two possibilities that leap to mind are alignment and aero, but I'm not sure where to go beyond that.

In front, the car has -3.5 degrees camber, 5-ish caster, and zero toe. In the rear it's got -2.75 camber and a small (1/16 inch, I think) toe in. The tires are 225 RRs on 15x9 6ULs, TSE 11.75 brakes with DTC-60 pads. There's essentially no aero at all on it, although the factory under tray is installed and I had the hard top on.

Any thoughts?

thanks,
--Ian
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Old 11-18-2015, 02:23 AM   #2
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Moar aero
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Old 11-18-2015, 03:28 AM   #3
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Probably wouldn't hurt to take some camber out of the front.

Also if your avatar is a recent picture, get a hard top and consider a front lip like garage vary.

All I have for aero is a hard top and the OEM R-package stuff that came with the car and I had no issues with standing on the brakes from almost 130MPH all the way down to around 40MPH at Autoclub Speedway.
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Old 11-18-2015, 11:10 AM   #4
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I had the hard top on.
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Old 11-18-2015, 12:40 PM   #5
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I would just assume that it's twitchy because it's a lightweight short wheelbase car braking hard at 130mph.
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Old 11-18-2015, 01:21 PM   #6
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^bingo

that and check over your shocks. you might have one blown out. if they are adjustable you could add a little compression stiffness.
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Old 11-18-2015, 01:29 PM   #7
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exact same issue here.
no aero/@250whp/SM7/hardtop/etc. First time at Mosport GP with this car magnified every weakness that I hadn't really noticed on the slower tracks. High speed braking was horrifying (128 to 90) at 10/10th's.
I chalked it up to short wheelbase/no aero, backed off a notch and carried on.
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Old 11-18-2015, 04:23 PM   #8
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The shocks aren't adjustable (FCM Bilsteins) but don't show any evidence of being blown. Yes, I have a hardtop and was using it at Thunderhill.

Aero is a possibility, but I still drive this car on the street so I really want it to be compatible with speed bumps and police officers. I'm willing to consider aero, but it would need to be something that's feasible to bolt on for a track day and then take off afterwards. The car goes to the track on a trailer, though.

The new singular wing looks interesting, but I have no idea what I'd do on the front. More research, I guess.

thanks,
--Ian
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Old 11-18-2015, 04:32 PM   #9
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Blackbird Fabworx Lexan Spoiler for NA and NB for Miata 1990-2005

this should solve all your issues. but the short wheel base is a nightmare at very high speeds. I would know, my car hits 155 at road Atlanta. going threw turn 9 at 140 is bottom clenching, and fun.
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Old 11-18-2015, 04:58 PM   #10
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I have about 2.5 camber front, 0 toe F&R, 4.5 caster, GV copy lip with the openings taped shut, hardtop and KG trunk spoiler, 550/350 springs, 11.75 V8R front / 949 sport rear rotors, used Hoosier SM7s, 6 speed, 3.63 and was reaching 150mph coming into a hard braking zone at Daytona a couple of weeks ago. The other major braking zone was 140mph. I can't recall my ride heights but I have about a quarter inch of rake front to rear, which helps. Car was very stable in both.

More camber is better for corners but it hurts the shape of the contact patch when braking. My tire wear is very even right now except for a flat spot caused by my buddy RyanG. Poor guy had a ballbuster of a weekend at the track. Again.
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Old 11-18-2015, 05:30 PM   #11
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Have you recently verified your alignment settings? I ask because I've had the experience with botching my front toe on a DIY alignment and ending up with a car that was darty under braking. I borrowed some toe plates and got it mostly sorted out in the paddock. The car improved some. I got a professional alignment right before my next event and could comfortably brake from 140.
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Old 11-18-2015, 05:33 PM   #12
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I would try just adding a lip, like the tougerun one. My ghetto garage physics airflow visualization shows me a high pressure zone under the bumper which is giving you instability when you squash down into it under braking at high speeds.

My car never hit more than 120ish at PIR but braking from 120 to 90 the car always felt very stable. Aero was tougerun lip with COT wing in the back at an extremely shallow angle.

Since you already trailer the car, I would be sorely tempted to do Crusher airdam nose + splitter in the front, Singular wing package in the back. Just get another bumper and swap the whole nose.

Edit: verifying alignment is sound advice.
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Old 11-18-2015, 09:52 PM   #13
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Quote:
Originally Posted by OGRacing View Post
Blackbird Fabworx Lexan Spoiler for NA and NB for Miata 1990-2005

this should solve all your issues. but the short wheel base is a nightmare at very high speeds. I would know, my car hits 155 at road Atlanta. going threw turn 9 at 140 is bottom clenching, and fun.
sweet jesus. video please.
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Old 11-22-2015, 02:50 PM   #14
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I don't understand why most peoples solution is aero....if it only happens under braking ???

I would have thought alignment, shocks or ball joint/rod ends being worn over aero as the casue.....?
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Old 11-22-2015, 03:09 PM   #15
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FWIW, one suggestion I got from a few other sources is that too much front bias can do this. I hadn't heard that before, I'd assumed that the only downside to too much front bias was a reduction in overall braking effectiveness. This is relevant because in addition to the TSE 11.75 Dynapro fronts with DTC-60s, I have FM's powerlite rears, using Wilwood E compound pads. I don't like the E compounds, but Hawk doesn't make DTC-60s in the 7912 pad shape, and the Es are what I had sitting around. DTC-60s have a mu of around .75, vs .55 or so for the Es.

So in addition to going through the front end to look for loose stuff/etc, I plan to get some better-matched rear pads before the next time it goes to the track.

--Ian
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Old 11-22-2015, 09:34 PM   #16
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Quote:
Originally Posted by OGRacing View Post
Blackbird Fabworx Lexan Spoiler for NA and NB for Miata 1990-2005

this should solve all your issues. but the short wheel base is a nightmare at very high speeds. I would know, my car hits 155 at road Atlanta. going threw turn 9 at 140 is bottom clenching, and fun.
Turn 9 isn't much of a turn in a 114whp. However it's pretty butt clenching when someone comes by me at 9 when they're at 140mph. I can make about 120mph by 10a braking zone, so I guess I'm closer to 100 through 9.
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Old 11-23-2015, 08:37 AM   #17
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I had a fellow carry me around there in a GT3RS that made 158 indicated before the braking zone. It is definitely a turn, lol.
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Old 11-23-2015, 11:51 AM   #18
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Quote:
Originally Posted by Ben View Post
Turn 9 isn't much of a turn in a 114whp. However it's pretty butt clenching when someone comes by me at 9 when they're at 140mph. I can make about 120mph by 10a braking zone, so I guess I'm closer to 100 through 9.
for reference stock miata is about 110mph when they hit the braking zone of 10A. nobody notices turn 9 until they turn the wheel and nothing happens. The first time that wall looks like it’s coming at you quick. lol

Quote:
Originally Posted by endura View Post
sweet jesus. video please.
How can I not share lol


and one more from Daytona.



Quote:
Originally Posted by mx5-kiwi View Post
I don't understand why most peoples solution is aero....if it only happens under braking ???

I would have thought alignment, shocks or ball joint/rod ends being worn over aero as the casue.....?
try not to think of it as "Braking at high speed" and think of it more as "weight transfer at high speed"
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Old 11-24-2015, 09:46 AM   #19
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Quote:
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for reference stock miata is about 110mph when they hit the braking zone of 10A.
I'm up about 15hp and down about 600lbs.

Thanks for sharing your videos. I'm guessing you're closer to 400hp.
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Old 11-24-2015, 05:07 PM   #20
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Quote:
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I'm up about 15hp and down about 600lbs.

Thanks for sharing your videos. I'm guessing you're closer to 400hp.
yea 500 at the crank is 425whp on lsx stuff. that and i'm having one hell of a time shifting at anything over 5500rpm. the clutch just doesn't want to disengage. you can see the loss in speed in the road Atlanta video. i just lose 10mph in the middle of the straightway. that's me fiddling with my shifter.
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