Hello. i just used my car pretty agressive yesterday and found out that when i pass 200km/h the car acts little of its own will. it jumps 3 lanes sideways at full speed. do you guys have any hints on a good street setup wich can be abused for 1,5 hours on the street? maybe a good tyre setup? i have 235/45/17 pirellis right now.

the second problem is that my PFC only shows 204km/h when i am in 5th gear at 6k rpm. what i have heard is that the car should do at least 280km/h stock but mine is matchported with higher boost and so on

and i forgot to say that mine is a j-spec so it also have the 180km/h gauge

 

so your car when sideways? or just drifted 3 lanes? either way dont go that speed until you get it figures out, that is totally unsafe. anyhow... try getting from and rear defuser, undertray, and bigger wheels, ilke 265+

 

yea the car jumped 2-3 lanes. well not jumped i had to let it drift 3 lanes becouse i couldent steer that much at those speeds. but shouldnt wider tires slow the car down and more sensitive for bad roads. i same with adding diffusers and such if i want to go high speeds wings are the last thing you want

 

You didn't post your alignment specs. Get the car aligned before attempting any high speed runs again.

Your aerodynamics could be pretty messed up though and causing your problem. What kind of body modifications does your car have?

 

a rear wing (spoiler) will add down force and help you get more traction, also the defuser will do just that, defuse some of the air to stabalize the car. Wider tires will give you more grip, when people say rims slow down the car it is because you add bigger rims, if you keep the same size just wider it will not slow you down that much but instead let you get more traction.

 

the body itself it completely stock but with a removed rear wing. takico adjusteble dampers. set to realy stiff front and back. wider tires should make the car more sensitive for "tracks" in the road. im thinking of getting the car alighned as soon as payday

 

the FD, w stock body parts, is rock solid at absolute top speed.

Warren Wallace is correct.

all you need to do is have your car aligned. set it to the following specs and don't let the alignment shop talk you in to anything else.

toe in:

between 1/16 and 1/8 toe in front.
no toe in rear.

camber 1.2 degrees negative front and rear.
zero rear thrust angle.

before you align the car set the tire pressure to 30 psi front and 27 rear.

and let us know what your "before" alignment was.

hc

 

He's having stability problems at high speed.

Adding downforce at the back will make it worse.

The traction afforded by wider tires will not help stability in a straight line.

To the original poster: Does your car have all of the front undertray installed?
(there are 3 pieces)

As has already been stated, check alignment, tire pressure, and make sure the body work is all installed.

 

the undertray pieces in the front are missing. i didnt even get them when i bought the car. maybe i could cut a undertray from thin alloy?

 

all i know about my current allignment is that the car eats rear tires fast. to much negative camber. what does the zero rear thrust angle mean?

 

You really need to have something there, as the front end will generate a lot of lift without them.

The main undertray is not that expensive new, about $150 IIRC.

The 2 smaller pieces near the front are probably not too expensive either.

Given the price, it's probably not worth the trouble and $$ to try an make your own.

On a stock FD the undertray is also a vital part of the cooling system.

 

Howard also is absoloutely correct.

Do not bother with spoilers, defusers etc - they dont matter a **** (to making your car stable anyway)

I regularly do 180mph+ (on gps) in my FD, and happily turn corners (slight ones of course!) at this speed.

My car is stock everything, bar amemiya hood. 255/35/18 rear, 225/40/18 front. It has the factory wing (although its proven this gives no discernable downforce)

I dont have the stock undertray either.

Everyone always comments on how stable my FD feels at high speeds - this is because they are built for it.

Sounds like your problem is a combination of bad alignment and worn bushings. Get your car to an alignment specialist, and give them the settings howard has just given, they will know what to do with them. Personally, i run 1.5 degrees camber up front as i find it helps reduce understeer with the stock sway bar. I also run half a degree toe in at the rear to help with stability out of low traction corners. My tire pressures vary from that too but for street they are fine (i have 30 front 32 rear) Have them check all the bushings while they do the alignment.
good luck!

pending your bushings come out ok, you should have a very stable car at high speeds, if not - something is broken.

 

Disagree with statement 1
agree with statement 2.

 

I can't believe that anyone would run their cars at that kind of speed without first checking that everything was 100%. Not being able to hold a straight track and having to drift over 2-3 lanes, you were just damn lucky. Follow Howards advice and get the car aligned. Make sure that you have all the stock part, that means undertray as well as wheel well liners. If you are wanting to run at those speeds I'd also look at a front lip if you don't have one already. Finally, make sure all your linkages are solid, you have fresh brake fluid, and your tires are inflated correctly.

To run a car at those speeds without checking everything out ahead of time just seems incredibly stupid to me (not to put too fine a point on it). Even at an autocross event where speeds rarely exceed 60 mph they do a tech inspection. Get your car sorted out and then test the car at gradually increasing speeds. Finally, I hope this was on a track in a controlled environment. I've never driven in Sweden but 125 mph seems rather excessive for a public roadway.

 

Well, after seeing your Nurburgring run, I am not about to disagree with you.

Please clarify though, do you not have any undertray at all in the front, or something custom?

 

my cars got a v mount but no undertray.

The argument for not removing the undertray is to keep the bottom of the car as flat as possible, flat bottom means less turbulence which = less lift.

However, the rest of the bottom of the car is nowhere near flat, so removing the undertray i doubt will cause any discernable change in lift, apart from maybe a little understeer at 100mph+ if anything (but thats better than oversteer at those speeds!)

 

The stock radiator setup with no under-tray could easily encourage a lighter front end at speeds in excess of 100 mph. Add a side-wind gust and a bad roads and you have a recipe for a disaster.

The rest of the car not being flat is not near as important as the front end. many f1 cars have a flat front section and then it goes higher towards the rear of the car to create a low pressure zone, but that is an entirely different aerodynamics conversation.

My point is the stock radiator would ramp air downward creating a high pressure are right below the steering rack, and thus right between the front wheels. This is most definitely not ideal. A V-mount would not create this effect to near the extreme.

 

As the original poster stated he has a V-mount intercooler, as do I. The stock undertray does not fit under the car with this setup. Any suggestions on how to rectify this issue?

 

ah, then i stand corrected.

 

i dont have a V-mount. i have a FMIC and no undertray pieces at all. im trying to find out a price for an alignment n my car. if that doesnt help i will cut a undertray from alloy since mazda parts arent cheap here in sweden and its long delivery

 

Theoretically correct, and sounds like it should be true; however i ran with the stock rad and no undertray for a year and noticed 0 difference between now and then.

I just don't think the aerodynamics area thrown off anywhere near enough to have the issue this guy is describing. He needs to check tyre pressures, alignment and bushings- not aero!