HI
as title, if i go at high speed my car loose stability. It means at for example at 150 Kmh (100mph) i have problem to go straight. I'm scared to go above this speed since is dangerous.
I changed All bushing with a full PowerFlex kit.

I have been to 2 mecchanich and both said that evrithing is ok and propably is due to tyre low profile size. Tyre size is 235-40-18 (ET+40) and 265-35-18 (off set +38) wheel are Enkei RP03.
I don't think is a tyre problem because is not so low and many peolple using this wheel\tyre hase no problem.
I have also modified the coilovers with a set of apexi EVX.
With car on the bridge the wheel bearing looks good since the wheel has no play.
What could be?
Onestly this sensation camed out close of using the new 18" wheel but i really don't think there is an issue with this.

Someone told me laso to check the rear drive shaft. but??? i don't know.

Please HELP!!!!!

 

Did you check/change your suspension alignment after replacing the bushings?

 

when everything is dialed in you can take your hands off the steering wheel at 165 mph. ask me how i know.

confirm tire pressure set cold at 30 psi fr and 27 psi rear.

after that if you still have the problem set your ride height at 25 inches to the top of the wheel well lip.

set camber 1.2 neg fr and rear

the key for high speed stability is to make certain the front toe in is just less than 1/8th in. rear set at zero.

if you still have a problem perhaps something is loose.

howard

 

may or may not be related

i used to run a gt-wing and the car was planted at 200km/h+
as soon as i removed my gt-wing and ran spoilerless the rear of the car would sort of "skate" side to side a little bit at 160km/h+
scary feeling, put the gtwing back on the next day.

i also run 235/40/18 in the rear, tire size is fine.

 

Thanks for reply.

habu2: Yes i check all allinement (camper, caster, Toe), but not sure of what You exactly mean about suspension allinement(if not listed above, i missed something

Howard: I check the tire pressure and setted it at 31,90 PSI (mazda stock 2.2Bar=31,90). I will try to set as You said me.

Front Caster is set to +6,00 (I guess it was degree)

I have to check for toe and camber, made just the "tyre shop"
How to translate 1\8th European metric value???
Rear 0 is easy in any value :-)
i really made an hard search but...nothing help me to translate this value. also 1.2; the 0.2 is on a 100 scale or on 0.60scale??
http://unit-converter.org/en/angle.html check this link, there too many possibility. is 1/8 circle mean 1,3° about?
Thanks
Pietro
Also 1.2 Degree You mean 1.2 on 0,60°=1 or

 

Update:
Found a table converter. It was very hard,(i also made a copy on 3d wheel brakes etc etc)
Few more simple questions :-)

1) Front Toe 1\8 is the total? of is for each wheel? Confirm is espressed in degree (angle) or Inch? its mean 0.125 inch? (value in degree is 0,15 on an hundredd scale and 0,1 on a sixty scale for each wheel)

2) Camber is 1,2 for each wheel right? (1,2 on hundreed degree or 60 degree?).

3)Hegit of the car: 25 inch is taken from the center of the wheel to the lower part of the fender? (Top of wheel well lip is this?) I should check also this since i have adjustable suspension kit.

Thanks for help, really appreciated.

 

Howard,
You have a great know how on suspension right? It could possible that some problem come from Suspension? Apexi EXV Coilovers? Should i review something?
Thanks
Pietro

 

Get an alignment done by someone that knows what they are doing.

 

Howard is very knowledgeable on many (all) things that have to do with racing.

I agree that you should take your car into a tyre shop with experience adjusting wheel alignments.

I believe what Howard meant by the ride height was that you want there to be 25 inches (63.5cm) from the ground to the highest, middle, part of your fenders right above your tyres..

 

Made a test with your tire pressure but...nothing.
My car heigt is Front 26.378 in and rear 26.772 in whic should be the stock height.
to run only 25 inches i should have the whell inside the wheel well lip since the rims diameter are 25,3 front and 25,4 rear.

I will go to check wheel alignment but please hel me with the setting

Thanks to evrebody.
Pietro

 

for purposes of finding your problem only...

just make sure your tire pressures are even side to side and 2.2 bar or 31.9 is fine. it is a little high for my tastes and should be a couple of pounds lower in the rear.

as to toe... if anything alignmentwise is causing your problem it is most probably toe.
total toe in should be just less than an 1/8th inch. i do use degrees w my laser/mirror gauge but don't have the table handy. your alignment shop should have no problem w the translation.

front camber should be 1.2 degrees negative. to be clear(er) that is one degree plus 20% of another degree.

ride height can be stock and again the primary factor is that it should be even as measured at the left and right wheel lip...

if the above doesn't help you something is probably loose.

good luck,

howard

 

Really thanks.
Really really really.
Thanks thanks thanks

 

Just fyi - you can check toe at home using a tape measure and two straight edges. Here is a description of the process:
http://www.bakerprecision.com/longacr16a.htm

In a pinch, you can use any pair of straight flat objects for rough measurements.

 

Howard, I've always been curious about this. If you get an alighnment to these specs, will it change as soon as you lower the vehicle later with coilovers?

 

Did you also replace your diff bushings? A loose diff will cause you suspension geometry problems.

 

alignment...

"will it change as soon as you lower the vehicle later with coilovers?"

the only part of the alignment that most assuredly will change is camber.

as you lower the car negative camber increases.

as you raise the car negative camber decreases.

significantly.

let's say you set the car at stock ride height and then adjust the camber to negative 1.2 degrees.

if you lower the car 2 inches (which is about the max) and then read the camber you will find it is over 3 degrees negative.

this occurs because the FD has a racecar suspension. the upper A arm traces a shorter arc than the lower arm and pulls the top of the tire in toward the center of the car.

as you round a right hand corner the car rolls to the left about 2-3 degrees. the outside tires, the left tires go into "bump".... upward into the wheel wells. since the car is rolling if there wasn't any "camber gain" the top of the tire would roll w the body and the tire would be positive cambered and wash out. because the FD pulls the outside tire in at the top on roll the tire stays where it should be in realation to the track.

that's one of the key reasons the FD is such a killer ontrack and of course it explains why when you change ride height camber moves.

toe remains unchanged.

 

Random question... If you have had your suspension tuned properly, then you raise your ride height (in order to navigate terrain where you would otherwise bottom out and mess up a bumper or diffuser) for a short time, would you need to re-tune your suspension after lowering your ride height to the original level?


I'm thinking strongly of traveling to Florida (from Northern Virginia) by way of the Autotrain, the Autotrain requires 4" of clearance for all cars transported... I'm not sure how much clearance my car has right now (since I'm deployed) but would not be against raising it up for the train ride, then lowering it again..

Thanks!

Levi

 

No.

 

uhmmmm no, i didn't. Yesterday i checked these these diff bushes, looks like the left one is loose and give play. Planned to replace this weak. Then Allignment and test again.
Thanks
Pietro