RotaryZZ

I just got back from a three days driving event at the Watkins Glen (WS) with the PCA folks. It was great fun and I've learn a lot more about the 8 on the track.

Overall the car is pretty impressive on the track and I've got some good compliments from folks in the same run group as I was. However, the biggest problem I've encountered was in hard braking!
For those who have been to WS before, there is a back straight at the the top of the hill just before the bus stop area. And I was traveling at almost 120 mph approaching that bus stop. When I hard squeezed on the brake preparing to enter the bus stop, the back end of the 8 would wriggle A LOT. The back end was sliding left and right and simply won't stay planted!! It was so bad that by the time I was ready to make the turn in, the car was away from the left edge of the track by at least half a car width! Bear in mind that I've been holding on to the steering dead straight and not jerking or yanking it in any way whatsoever.

For comparison, I've been to WS before with my last car, the E36 325is. I've approached that same bus stop area with similar speed if not much slower than in the 8. And I squeezed the brake real hard, probably in between the 100' and 200' marker. The bimmer would just stop without any drama. All four corners would simply dig in and stayed planted and the car would plowed to a slow enough speed for me to make the turn into the bus stop, which is exactly what I would expect the car will behave anyway. And btw, I did not activate the ABS systems in either situations at all!

Now with that uneasy feeling with hard braking on the 8, I've to apply the brake earlier in order to avoid any drama. I've tried adjusting the tire pressure but that didn't seemed to alleviate the problem at all. Someone did mentioned that it may have something to do with the tire itself. I have no clue about this but I've used Dunlop SP8000 on the bimmer before and I would think they're comparable to the RE040 on the 8.

So does anyone have any idea of what's going on or have similar experience?? I've been thinking could this be due to the EBD system that's acting up! Anyway I've absolute no clue at this point and I would like to hear some of your opinions or experience.

TIA,
Pete

Ned M

iTrader: (2)

Back end going squirrelly under braking: my first thought is a 'lot' of weight transfer to the front end in those particular braking conditions and that corner approach. Your E36 may have been sprung quite differently from the RX-8 and not as prone to transfer as much weight to the front end as you braked for the corner.

I have some entertaining video of my brother's car getting hugely out of shape approaching the bus stop at Watkins Glen, but his car was a 3400 lb. fwd'er with a lot of front-end weight bias already.

GeorgeH

iTrader: (2)

Alignment? My '8 was more stable under hard braking when I had a bit of toe-in front and rear. I got a little greedy to get better turn-in and noticed less stability on the freeway as a result.

And, technically speaking, weight transfer doesn't have anything to do with spring rates, but I think Ned was referring to nose dive, which is absolutely related to spring rates.

davefzr

Yeah.. same here.. My car was very stable braking from 110 down to 35. Though after the track event my cars brakes felt like crap but that slowly goes away.

Modena4RE

Ive had this sorta thing happen in the 8 once.

I was driving home and there is a really nice right hand turn that has nothing around and i was going about 80 before braking. I hit the brakes hard before the corner and the backend got real light and tried to step out as i approached the corner. Gave me a bit of a shake, but i let off the brakes right when i began to turn and the backend settledown and hung on to the road like a champ.

I think it is simply a weight transfer issue. The 8 has quite a bit of nose dive under hard braking and it loosens up the rear. I think a stiffer suspension and rear sway bar would eliminate it. The 8 still has some awesome brakes though.

Speed Racer

Pete,

I had my 8 out at Watkins Glen last summer with NASA Racing and noticed the same tendency when I was late braking going into the bus stop (i.e. 130 MPH on back straight and standing on the pedal around 250 ft). The stock springs are just too soft for this and the nose pitches forward which causes the backend to get light and squirly. This also created some problems with brake fade.

Conversely the car felt great if I started braking earlier around 400 ft and concentrated on a smooth weight transfer. With that technique the car felt very stable and it sets you up for a nice four wheel drift through the bus stop. On my best laps I found myself carrying another 10-15 MPH out of the bus stop and as they say "smooth in, fast out".

When I hit the Glen in September I hope to get the car dialed in a little better with coil-overs, hi-temp brake fluid, and race pads. :D

Ned M

iTrader: (2)

Yes, thank you for expressing that better.

GeorgeH

iTrader: (2)

I wonder if there is a toe change when the rear suspension compresses/extends? This would explain why some people have stability problems and some don't. Also would explain why braking at 400' doesn't cause the problem, but braking at 250' does. Make sure you have enough toe-in, or don't ask for full braking force (hence reducing dive) and you are OK?

mysql101

My 8 usually brakes well, but once I was driving in the rain and braked hard, the car started to wiggle back and forth. It was disturbing to say the least... While it was raining, the car was planted on the ground, the wiggling was very unlikely caused by the wet conditions.

Hymee

iTrader: (1)

Nice thread guys.

On the track, mine brakes nicely in a straight line. In the multimedia section I posted some videos where you see I get a bit crossed up once or twice. In these situations, I was braking into a corner. That was OK - it was the downshift that unsettled the car, until I smoothed that out some.

Cheers,
Hymee.

RandyP

I was at Buttonwillow Sunday before last. This was the first time I've had the 8 on a track, so it was a day of increasing confidence and demands. By the afternoon, when we had the whole track available, it was about 100 degrees, so I generally was going for smooth lines.

There was a 210-degree curve a bit after the start/finish line, followed by a fairly decent straight, then a tight chicane over a hill. I got the urge to go for 90 on the straight, which set me up a bit late for the curve and rather hot. I was on the brakes smoothly but very firm, the ABS pushing back for a little while (DSC/TCS was off). I remember thinking how well-behaved the car was as I pulled it into the curve, still braking, then smiling as I found the late apex set me up really well for the next curve.

You were at 120, I was at 90. I'm assuming we had the same nose dive, since I felt the ABS pushing back. Could speed have anything to do with the difference?

Another possibility would be track temperature. The air in the central valley is very clear, air temps were 100+, track temp about 180. Maybe the RE040 likes hot temperatures?

I'm going to Thunderhill this weekend, I'll let you know if I see anything there.

EDIT: When I posted this I saw my avatar. That photo was taken just after this happened. And it reminded me of why I was going for 90 - that WRX in the background... I wanted it to stay in the background.

RandyP

The same thing happened with me. I looked at the photos and found the WRX was in view for two successive laps:



Chatting between track times, the driver said he expected to eat me up with his 300 HP, but was surprised how well I went through the curves. You can see the difference in distance after 3 miles of chasing me. My RX-8 is pure stock... still trying to pick what to buy and how to pay for it.

RotaryZZ

Thanks guys for all your input. I think the 8 did quite a bit more nose dive than I would expect under the hard braking situation. And it really lightened the rear end more than my old bimmer which may utimately causing the wriggling of the back end.

As some of you have suggested, I may want to align the suspension so it will have a slight toe in front and back and see how it'll improve the situation. Btw, how much of a toe-in would you recommend without causing any stability problem on the freeway? And any knowledgable Mazda shop in the tri-state area that you can recommend me to go to for the service??

Next thing that I may try is to put in a somewhat stiffer suspension. The chassis itself is really good, its stiff and felt like a vault and it warrants a stiffer suspension to complement the body, especially if you intend to use it on the track ;-)

p.s. I've used the Mazdaspeed brake pads on the track and I've to say that it really performs well. I've experienced no brake fade whatsoever for three full days. I would have more confidence to go in deeper in the braking zone if not for the wriggling of the back end.

PUR NRG

For what it's worth murix reports no brake fade even with stock pads at Thunderhill.
________
Judaism Forum

GeorgeH

iTrader: (2)

As for alignment specs, search this forum; somewhere somebody has posted a scan of the page from the factory service manual with the factory reccomended specs. These may or may not yeild the desired results, however.

Quite often, you have to venture out of the factory specs to get what you want. So far, most of the "non-stock" alignment reccomendations on this forum have been for autocross, which probably won't suit you on the track. I reccomend taking a print out of the page with you when you get an alignment, and then see where your car is at right now. Basically, if I am correct, you want more toe-in at the rear than you have currently. This will increase stability, not decrease it, BTW. As for how much, I probably wouldn't go for more than 1/8" per side. If you already have that much, I wouldn't go further, as you could get into wear issues. Since, to my knowledge, you are forging new ground here you have to ask yourself how much you are willing to experiment. Note that toe-in will slow the turn-in of the car, so it's a balancing act.

Good luck!

Ophitoxaemia

there are many fine brake pads available these days. you will not generally find a stock pad that works well at the track not because of quality, the rx8's seem excellent, but because all brake pads have a heat range they work in. modern pads have a wider range of temperatures, but there isnt a pad that i know of that has good friction from 0 deg F to over 1000deg F.

many people after experiencing brake fade go out and buy bigger brakes, when really all that was happening was they operating temp moved outside the range of their low-telp street pads. brake ducts are very effective in reducing brake fade.

about the fishtailing- ive nailed the brakes from 120 down to 30 and it stopped straight as an arrow. but it is very important that "nailing" doesnt mean slamming- slamming the pedal dips the nose the raises the tail, which reduces traction at the rear. that tiny moment as you depress the pedal needs to be smooth to let the entire car settle, then the rx8 brakes are the most amazing i have ever felt in a car, this includes my race cars. also, if the car is even slightly turning as you start braking, you can start a slide.

autocross braking is a different animal- you can be really rough with the car in comparison to the big track.

that said, im ditching the progressive rate springs- i dont like how they feel, it is hard to control the drift bouncing between the rates, and they are so soft they let the car get on the bump stops too easily.

james

RotaryZZ

>.....but it is very important that "nailing" doesnt mean slamming-........
I absolute agree. You're supposed to squeeze the brake real hard, not slamming it. That's track braking 101.

I've been thinking the surface of the track may have something to do with the wriggling of the back end as well. The track overall is pretty smooth, but on closer inspection, the braking area isn't as smooth as one may expect. It maybe due to the hard braking from all kinds of cars over the year!

durrada

Ok, bear in mind that these comments are from a motorcycle racer and not a car racer However, weight transfer issues and chassis setup can be somewhat similar - minus, of course things like toe in/out.

When I raced, a wriggly back end was a great thing and a heck of a way to set the bike up for a corner. I've experimented with the feeling in the 8 and have been able to accomplish a very similar effect. It's one of the things that I truly love about the car. I am a very late and very deep braker. I try to be on the brakes all the way to the apex. Once again, I point out the motorcycle experience . Now, I have no lap times to compare a higher corner speed approach to my approach, but nonetheless, I love how it feels and haven't any problems as of yet. I trail brake the hell out of the car. Basically, I get on the brakes as quickly and smoothly as possible, as the rear steps out the direction I want it to, I modulate that step out with the brake (ever so lightly). Once I have approached the need to turn in (which I already am very slightly trying to keep the rear where I want it) I begin to get off the brakes as I turn (when this is done the rear end catches and I am now pointed more into the turn - ie - square it off a bit), much in the same way as you would modulate the clutch and gas. Once I hit the apex I'm off the brakes and on the gas.

Again, all this is from motorcycles translated into cars. And it is great fun I guess the one thing that I learned most from bike racing in these situations was about front spring rates. The weight transfer was a huge issue on bikes (basically the rear brake is damn near worthless on a sport bike and under heavy braking). We were always able to modulate that weight transfer with a stiffer spring up front and stop the bike from bottoming out. On the 8 I have noticed that I can get it to bottom the front all the way out if I'm not a little careful and that will definitely cause you to get wriggly rather than a nice slide.

BUt, hey, that's just me and this may not translate well at all. Fast times and true racing are very different. In a close race, if you stay on the "fast" line you are going to get passed. Block passes are the best

Cheers all and hopefully I haven't muddied the waters to much,

David

GeorgeH

iTrader: (2)

Yes, Road&Track mentioned that the RX-8 late-trail brakes on the track very nicely.

VividRacing.com

I remember having the same problem the first time out before the suspension was done. I agree that is was a wieght transfer issue. After the JIC coilovers were installed the problem went away. Also, the StopTech kit up front helps along with the Motul RBF600 fluid. I miss the track. We don't race here in th summer months as it gets way too hot. Most of out track days are in the late fall, winter, and spring months.

On another note, I am very pleased to see so many acctually taking the car out to the track for some tire eating fun. It makes me smile that there are at least a few of us owners who aren't afraid to let it all hang out even with a car payment looming overhead. Rock out guys!

One last side note: Check out the Agency Power steel braided brake lines and clutch line. They will help out with some of the brake fade and clutch issues some of us have had on the track.

Hymee

iTrader: (1)

I couldn't agree more. These cars are gr8 on the track just as they are! Not many cars can go straight onto the track without changing anything, especially paying attention to the brakes.

Cheers,
Hymee.

GeorgeH

iTrader: (2)

Sorry if I sound like an engineering geek, but changing to stiffer springs doesn’t actually reduce load transfer, right? Load transfer is dictated by the CG height, wheelbase, and rate of deceleration. Installing stiffer springs does reduce the amount of dive, but it doesn’t change how light the rear end becomes under braking.

Why do I bring this up? Simply because all evidence is pointing to this being a suspension geometry issue – it looks like the rear suspension has a tendency to toe-out under extension. If true, there are at least three ways to fix this:

- Increase spring rate to reduce dive.
- Reduce the rate of deceleration (i.e., brake sooner with less pedal force) to reduce dive (this can also increase fade, so be careful).
- Increase static toe-in.

Arguably, one could tweak something in the rear suspension, like bushing stiffness, to reduce this toe change, but that’s a bigger adventure than most of us are likely to undertake. Those kinds of solutions tend to arise later in a car’s life.

George

Ophitoxaemia

has anyone measured the rear bump steer? when i swap my springs maybe i will take a few measurements. i would be surprised if a modern sports car suspension had appreciable toe-out in the rear under extension!

james

VividRacing.com

Acctually you've said it right in your own post. "Increase spring rate to reduce dive" this is typically what you get when you replace the springs, a higher spring rate. Sorry man not going after you or anything just helping clarifiy. The othe tweak do and will help also. But springs do help, give them a try.

GeorgeH

iTrader: (2)

No, I'm not disputing that springs help - we have testimony that reducing dive makes a difference - either by increasing spring rate or by reducing braking forces. This is why I come back to geometry being the underlying issue.

As to James' point, yes, at some levels it does seem funny that the toe would be changing with compression/extension. On the other hand, it may have been intentional. On the Miata the designers used different durometers in the rear bushes to get certain behavior under load. Perhaps on the RX-8 they wanted to get a bit of toe-in on the outside wheel when cornering - this would result in toe-out when the rear unloads. If it's there, it's probably not much, and I suspect getting the car to factory specs may be all that's needed. But these things do happen.

I have a friend with alignment gear, perhaps I can convice him to rig up my '8 so we can load the rear end up and watch what happens to toe & camber.

George