New Tyres=Worse Driving
- Thread starter ell_p
- Start date
I have a post face-lift running the original RFTs and I would agree that the steering feels very light at 80+ MPH compared to other cars I have driven. I attribute this to the fact that most passenger cars are set up intentionaly with a fair amount of understeer. This creates a very stable on-track feel at highway speeds. While the Z4 (like all street cars) is still dialed in to understeer for safety reasons, it is far more balanced than typical sedan, which gives it it's impressive handling characteristics in the twisties. Couple this with the very light steering feel from the electronic assist (one of my biggest complaints about the Z4) and you have this perception that the car is twitchy. I do quite a bit of highway driving and have found that once you get used to it, you discover that the car is not going to simply dart off in a new direction on its own and will respond quite nicely to smooth steering input. 
Well, I have the Bridgestones, and they do well from 1 to 100. Any anxious moments are caused by me being less than used to the precision steering. Maybe the Michelins just add to the feel.
My moneys on the alignment, but it would seem a pull, or an apparent mind of its own, would be more prevalent. I am new to the Z world, so I do not speak with a great deal of experience behind me.
Good Luck, and Check Six!
Topshelf said:
Agree with everything here. Especialy the statement about speed squared. The downforce at 75 MPH will not be that dramatic, but it will go up quickly as speed increases.
BMW (like all manufactures) do extensive and expensive areodynamic testing. I doubt that you will find an aftermarket product that will dramaticly improve downforce at "normal" highway speeds.
Mine is a pre facelift with a facelift front. The problem started on RFT's and is still evident on non RFT's.
It's nothing to do with downforce and did not exist before (I've owned the car for 2 years). Something has broken, bent, out of alignment, or behaving wrongly.
It's pretty simple on mine and is either steering alignment that while within BMW tolerence is not suited for some reason, or it's spurious inputs from the EPS.
I know it's not tramlining, a normal charateristic or imagination. All I need to do is find what is wrong and causing the steering to pull rythmically from left to right, mainly on mirror smooth quality surfaces at between 70 and 85
It's nothing to do with downforce and did not exist before (I've owned the car for 2 years). Something has broken, bent, out of alignment, or behaving wrongly.
It's pretty simple on mine and is either steering alignment that while within BMW tolerence is not suited for some reason, or it's spurious inputs from the EPS.
I know it's not tramlining, a normal charateristic or imagination. All I need to do is find what is wrong and causing the steering to pull rythmically from left to right, mainly on mirror smooth quality surfaces at between 70 and 85
ay8306 said:
I know you're right about the aerodynamic testing, and I believe your right about the aftermarket products. 80 is my usual limit on my daily commute, so it's not much of an issue, but for those times where a little extra speed seems needed :evil: it is scary. Hopefully I can reinstall my Summers soon to see if that makes a difference. If it still there, I'm taking it to a specialty shop for an alignment as I think my BMW shop has screwed up two of my cars now. If that doesn't fix it, then I guess I'll be back here complaining again.
I've just been on a little hoon around today and did bumpy, smooth, slow and fast driving. I found it better than ever, super stable and solid...
One thing I will note is I was using sports mode for the first time properly and it actually made sense on quiet roads!
When the car exhibits it has anyone tried sports mode on/off? Since that alters the EPS it'd be interesting to see if any change is noted!
Also maybe worth turning DSC/DTC off.
Only time mine has felt wrong on the motorway has been in lorry tracks when it shoots about alot... not really cruised at about 90mph though as you are just asking to be Vascar'd, but have bombed down some back roads at speed and the faster you go the more stable it feels (ie ~ 90 leptons)
Have noticed the arse wiggling under hard acceleration on rougher surfaces but expect that is the heavy rear camber and lateral thrusting under bump more than anything.
Hmmm
Dave
One thing I will note is I was using sports mode for the first time properly and it actually made sense on quiet roads!
When the car exhibits it has anyone tried sports mode on/off? Since that alters the EPS it'd be interesting to see if any change is noted!
Also maybe worth turning DSC/DTC off.
Only time mine has felt wrong on the motorway has been in lorry tracks when it shoots about alot... not really cruised at about 90mph though as you are just asking to be Vascar'd, but have bombed down some back roads at speed and the faster you go the more stable it feels (ie ~ 90 leptons)
Have noticed the arse wiggling under hard acceleration on rougher surfaces but expect that is the heavy rear camber and lateral thrusting under bump more than anything.
Hmmm
Dave
Richy said:
I concur with this. Certainly when dropping the RFT's you need to lose some PSI. In the quest for a solution mine now run rear 30 Front 28
cj10jeeper, what is your car like on the A38 Sutton bypass going towards Minworth? (assuming you have driven down it) Mine wanders all over the place on that final stretch towards the Asda island, its not a great surface but didnt know whether it was down to the RFTS.
Zed_Steve said:
A lovely bit of road which I drive most weeks several times (live in Shenstone). Two islands to straight line and leave people amazed at how you exited them so quickly and vanished down the road and I've never yet seen a patrol car
Last 2 miles is a terrible rutted surface on the inside lane and tramlines really badly. Frankly I just sit in the outside lane to Minworth unless thee is anyone who need to pass to get some decent tarmac. It's way better on the Falkens than the Bridgestones, that used to just shake your teeth out.
It was an even better road before those islands were built Yes the inside lane on the last stretch is terrible, like you i stay in the outside lane when i can. It freaked me out the first time i took he motor down there as i wasnt exactly hanging around, it felt quite unstable. Glad it isnt just me then
Yes the inside lane on the last stretch is terrible, like you i stay in the outside lane when i can. It freaked me out the first time i took he motor down there as i wasnt exactly hanging around, it felt quite unstable. Glad it isnt just me then cj10jeeper said:
Wow, 28/30? I have mine set to 32/34 and thought that was pushing it as far pressures go. There's a point where too low actually causes the inside of the tire to lose all contact with the road and you're riding only on the tires shoulders. I still don't know why tire manufactures don't have a simple listing of ideal pressure per weight since 32psi on a 3000lb car is completely different as 32psi on a 4000lb car.
Here's an interesting tire pressure link showing actual wet results at different inflation levels. Link
Topshelf said:
Yes - I agree and have spent many years using different pressures to change tyre characteristics, but on large off road tyres. The zed is a new area for me in this and am under guidance from a top BMW racer, tuner, engineer and geometry specialist in dropping them like this. I plan to run for just a few hundred miles then judge the wear and stress patterns.
Very interesting CJ. You know what would be nice? You should create a thread detailing your findings on these different pressure settings and the results so we could all keep track with you and learn a little something along the way. Nothing like results from top level guys to help us out.