Kolia

iTrader: (2)

+1

aki

hehe I guess it'd be obvious that I wouldn't have problems otherwise. yes I've mixed tires - OEM fronts with Avon 555 AS rears. I looked up specs and the Avons seem slightly taller for 245/45-18's compared to the RE040s based on revolutions/mile. OEM sizes for both ends.

The left front wheel chatter only happens on right turns, making it the outside front wheel. I've managed to induce it as low as 30 mph. This was on a tight exit ramp - third chatter in the same exit, a right, left, right ramp - it chattered twice on entry to the first right, was fine in the left, then chattered in the second right. Normally it's at higher speeds (i.e entering the ramp). When the chatter starts the car severely understeers - basically goes straight. It stops as soon as I straighten the wheel a bit. I end up going quite wide when the chatter thing happens.

Initially I thought I had a brake issue - worn inner pad or sensor malfunction on the left front. but I can brake hard in a straight line with no problems - ABS kicks in if I really stomp on it. The problem is related to turning right.

As I mentioned before I have no occasion to brake as hard while veering left so have not been able to replicate the problem on the right front wheel.

I've done this with VDC both on and off (but sensor has not been disabled).

I think my next step will be buying matching front tires (all season so I can drive when it's dry and cold), then a different set of wheels/tires altogether.

any ideas would be appreciated.
thx
aki

Kolia

iTrader: (2)

You certainly have uneven grip front and rear.

ABS and VDC will not be able to adjust to that kind of situation.

It's still odd it would only do it on righthanders.

enZineer

You mentioned that you replaced the rears with the stock size but you did say they were Avon (?), which doesn't match the brand on the front.

The VDC issues you seem to have are probably related to the differing cornering forces front/rear than what the controller is expecting, which is due to the mismatched tires.

As I mentioned in a previous post, this is a very dangerous condition on any car, especially one that will be driven near physical limits.

Also, with regards to your ratio calculation: This calculation is fine, however the tire has another measurable, which is the dynamic rolling radius. The dynamic rolling radius is the radius of the tire under a load and given velocity. This number is different from what you can calculate knowing the sidewall height and wheel radius.

There are xx teeth on the rear tone wheels, from which a pulse train is generated by the wheel speed sensors. The system is calibrated to calculate a velocity from this pulse train, but it is based off of the dynamic rolling radius of the stock tire. Since you've changed the rear but left the front the same as what the system was tuned against, the controller is likely reading a different velocity at the rear wheels, which may also cause ABS (longer stopping distance) and TCS (sensitivity/insensitivity) issues.

enZineer

That is, the outside front wheel.

THX723

iTrader: (3)

Yup. Sorry ... I meant to say 'outside' wheel as per the earliest converations towards the very top of the thread.

aki

enZineer - when I searched the forums for posts on the brake issue, I wasn't thinking VDC. so didn't see that previous post. I think the tire mismatch explains it. I'll go get the matching fronts.

I am aware of the rolling diameter issues but didn't roll out the tires in a measured way. Went by averaging rolling diameter and tire height. In fact, when I calculated rev/mile using the manufacturer's loaded tire radius, it was off from their own revs/mile spec by quite a bit - 40 revs/mile for the rears. It's easy to calculate things while at my desk - it's another thing to go out to the parking lot and drive my car back and forth on a line

I believe that the tire size in the rear is within spec for ABS - hence straightline braking works fine. I think the difference was 778 rev/mile AVON versus 783 rev/mile for RE040s. I believe my front tires are worn enough so that it takes about 10 extra revs/mile, not a lot, and it compounds the faster rolling fronts for a total of 15 rev/mile difference front+back. Any difference in traction will be compensated for by the ABS as long as I'm going in a straight line. However the lateral traction difference will cause a different reaction to hard cornering with VDC, yaw, expected slip angle, etc.

anyway, this was an illuminating discussion. Now I need to figure out what I want to do with new tires on new wheels - was looking to reduce understeer but not sure what sizes would be best - was thinking a slightly undersized but wider 275/35 and 295/35 18's for front/rear on new/wider wheels. wondering if reducing slip angle will simply kick in the VDC.

kolia - just to clarify, it only happens on right handers because I don't have any hard left turns in my reverse commute (the hard rights are all exit ramps). I only push when I have a clear road and relatively clear real estate around me. I'm actually a sort of slow driver most of the time. If I autocrossed or tracked my car I would be able to try those lefties.

thx for all your help ppl. I'll have to update this when I get my new tires and get enough nerve to go fast into a turn again.

aki

DavidM

Thanks guys for all you help, and in particluar Arizzee for sending me a section from the service manual which had the wireing diagrams for VDC and yaw-sensor. From there I could figure out what the wireing harness for the yaw-sensor looks like, and also what each wire does. So now I have a switch in the car that toggles the yaw-sensort on/off. Well ... it can toggle it off, but not back on without restarting the car.

Anyway, it took about 1/2 an hour to isntall the switch and wire it up once I had all the information. All that we did was wire the switch into the +12V wire (which is wire #6) so that the switch cuts the power to the yaw-sensor when switched on. All the lights light up as they should (ie. 'VDC off' and 'slip' light), and also ABS works as it should. Oh, we needed to wire the switch in the 'ground' as well ... that is wire #3 for ayone interested ... we did not even need to unplug the harness as we cut straight into the wires.

So thanks again to everyone who participated in this thead, and I'll really get to see how much of an effect this switch will have next time I'm on the track or skid-pan (ie. in 3 weeks).

ps. The new switch is mounted in the center-console right next to my seat-warmer switches ... there was already a space for one more switch there.

Mr_Q

iTrader: (1)

How is pulling thus fuse differenct from just pushing the VDC off button? Especially if it stills keeps the ABS online?

enZineer

Pushing the off button is preferred, but many people have noticed that VDC is still active albeit with higher thresholds. Traction is disabled, at least in my 2003 Touring, so you can do all the tire-smoking launches you want.

Pulling the fuse is the worst way to disable the system, because as you mentioned it also disables ABS and EBD.

03Z33

iTrader: (49)

Just like enzineer said

Pulling the fuse will disable:

ABS
VDC
TCS
EBS

Disconnecting the Bosch Yaw sensor will completely disable:

VDC
TCS
EBD

Pushing the VDC button will only partially disable

VDC
TCS (low speed)

GY-Z

nice threads, i was experincing the EBD problem John had on the trac too..
its kinda annoy.

so Disconnecting the Bosch Yaw sensor will completely disable VDC TSC and EBD while keep the ABS?

enZineer

Guys,

EBD is Electronic Brake Distribution

It is likely not a problem on the track, nor is it disabled by disconnecting the sensor. EBD is simply an electronic version of the prop valve; it prevents the rear axle from locking. Unlike ABS, usually EBD can still function with all sensors down including a wheel speed sensor. It is the final system to go down.

Please look through earlier posts in this thread for definitions of all systems.

Kolia

iTrader: (2)

I'm really skeptical of our analysis of the electronics behaviour here.

On what basis do we "All know the VDC is not completely turned OFF" ?

I can think of many other explanations for understeer while trail braking. Lifting the rear inner wheel and triggering the ABS is one of them...

And it seems everybody is forgetting about the ABLS (wich in my opinion is the sole system responsible for exessive rear brake wear). That system is always ON and it will change the behaviour of the car in mid-corner by changing left and rear torque distribution of the drive axle.

GY-Z

hm..ok..now i'm confused about which eletric system is the cause.
modern cars is way to confusing for me.
though i dont think its abs since i can feel the brake force distribution changing while i trail brake.any other ideas?

enZineer

I agree. In my 2003 Touring, I have not noticed interventions after I disabled the system via switch. I've had the car quite loose autocrossng and did not notice interventions during under or oversteer w/ the system disabled via switch. It may be that I didn't push the car hard enough.

One other occassion (playing around again) I completely rotated the car on purpose with the system disabled. This was at a relatively low speed.

They may gradually reduce the activation thresholds depending on speed (maybe I wasn't fast enough), or they may automatically re-enable the system (even if turned off) when the driver brakes w/ a certain pressure. Mercedes-Benz does this.

Other people definitely indicate that the system is still active after disabling via switch. Many other systems have increased VDC thresholds and disabled TCS when the switch is pressed; the latest Chrysler vehicles (Magnum, 300C, etc.) as an example.

DavidM

On what basis do we "All know the VDC is not completely turned OFF"

I experience it every time I go to the track or on a skid pan ... that is how I knew about it. The situation where it would interfere for me (and that is the only one I'm aware of), is when I have the car poited at the apex with the back sliding, and I touch the brakes (that brakes is what activates it). What it does is straighten the car (ie. stop it from rotating) and points ther nose away from the apex. The reason why I'd touch the brakes is to rotate the car around the apex even more ... not to straighten it up and point it away.

On the streets I can't get these dormant features to activate so I've never felt it interefere anywhere else but on the track and skid-pan.

ps. If you watch the 'Best Motoring International' that introduces the 350Z, then you'll see a segment where they discover this 'dormant' stability-feature.

Kolia

iTrader: (2)

I've experienced the VDC's action on a track (Forgot to run it off ).

It actually allows for pretty fast progress. When it does come ON, there is no mistaking it. Appart from the sound of the ABS, a significant amount of torque is transmitted through the steering wheel.

I got none of that with the VDC switch OFF. Even in power oversteer situations. Applying opposite lock on the wheel didn't even make the brakes twitch.

The car does tend to settle the rear when the ABLS comes ON tough. Simply because the load changes and thrust is recentered closer to the car's CG.

I'm curious GY-Z, how do you feel the EBD working ? What are you "listening" to ? I haven't felt it myself.

Kolia

iTrader: (2)

Thanks, I'll try that at MidOhio Monday and see if I can reproduce the behaviour.

GY-Z

there's some kind of clunking sound and u can feel the braking force transfer from rear to front during trail braking. this makes you feels like you brake hard while connering(i.e. braking hard while turning), there's only certain conner when i trail brake does this, but i have no idea whys that.