Traction Control / Vehicle Stability VSC kicking in when not necessary?
 

Hey guys,

So, first I got the car and threw new wheels/tyres on the day I got the car.

They were slightly different rolling diameters, which I expected would possibly throw out the traction control/VSC.. 215/40/18 and 225/40/18.. Anyway, straight after fitting them I noticed the trac light flashed a little prematurely, but on mountain runs it only flicked on once, even after some seriously hard driving.

Today I decided to put some softer compound tyres on, but same front to rear - 235/40/18 both front and back:

http://www.streetfx.com.au/galleries...res/tyres4.jpg

http://www.streetfx.com.au/galleries...res/tyres1.jpg

So anyway, I take it out for a drive, and of course these tyres are VERY sticky in comparison (200 treadwear as opposed to the previous rubber of 400 treadwear - ie TWICE as soft).

As always, brand new tyres are kinda slippery for the first few blasts, as they usually come a little greasy.

I turned traction control off and they stuck brilliantly. No worries.

So tonight I take it up thru the mountains, and give it some stick.. Since they are unfamiliar tyres, I leave stability control on (same as the previous times), the VSC is kicking in CONSTANTLY, under braking, under cornering, I can feel the pedal pulsing frequently, even on corners than there is NO WAY the car is going to get loose or sideways..


My thoughts are this:

1) Does the car 'learn' the diameter of your wheels, and when you change tyres will it take a while to know that you have changed them? (My BMW M5 does this, but you have to press a button to do it).

2) I haven't had a full wheel alignment (just a partial one) since the coilovers have gone in, so is it possible the wheel alignment is throwing the VSC out? However that said, why would it not have been as affected when I had the previous 215/225 setup?


I am kind of confused, since these sticky tyres should leave next to no reason for the trac/vsc light to flash up. Some of the corners were no way about to make the car step sideways. It was frustrating trying to power out of some of the corners when the VSC kept kicking in.

I tried it with VSC sport mode, and the same thing was happening, so it was the VSC not the Trac Control that was causing it.

It basically felt like ABS'ing on individual wheels etc..

I actually expected it when it had the different size rolling diameter tyres, but not the same all round..

Any thoughts?

Any help appreciated :)

Mark

I was actually just thinking about this today. It all depends on how the computer decides you are "losing traction" I guess. If it calculates it based on the change in acceleration (yes change in acceleration, not change in speed), then having different size tires would definitely cause some problems with the algorithm. Does anyone have any insight into how cars with TC/VSC typically figure out when you're losing traction? I'm very curious about it.

I would assume this is purely based on front/rear/left/right rotational speed differences, as well as g forces and so forth.

I would see no reason why 235 stickier tyres all round, would not give the same feedback to the traction/vsc computers as 215 stock tyres ?

That does make sense, and also explains how forward traction control would work too. All it has to do is sense that the rear tires are moving faster than the front tires. Well if that's the case, this tire issue is all the more perplexing...

That is why I am confused.. It is behaving basically how I expected it to with the mismatched front to rear tyres.

Theories:

1. Get an alignment then retry. Maybe small differences in the alignment are causing rotational differences through turns.

2. If you're in a turn, and your tires are a different size than the stock ones, will it cause the rotational differences between front and rear to be different than they would be with the stock size tires? (does my question make sense?)

I either read in the manual or in an interview with one of the FRS-BRZ engineers that changing the tires basically invalidates the entire VSC system. The VSC system is mated to the stock tires. If you change tires you might as well just turn the system off and learn the limits of your tires manually.

Turn off the traction control, problem solved

Stock tires on the FR-S are 215/45/17. By changing out to 235/40/18, you've increased the diameter of your tires by more than 3%. Most places I've seen agree you don't want to change your rolling diameter by more than 3%, as that can flip out the traction control and stability control systems.

235/35/18 and 245/35/18 would be better choices. Both are within 0.6% of stock diameter.

I just dont see how a system can tell it has different tyre diameters, unless it knows what the max g forces it's factory tyres can offer, on a typical road surface, but this would not work unless it also knew the coefficient of friction on every surface, wet or dry etc..


I am unsure how advanced the VSC is in the 86, but the computations should purely be based on what is inputted - speed, direction, rotation of each tyre, plus the yaw/gforce sensors (if they are used in this car).

The fact I have increased my rolling diameter by 3%, all round, should not affect the VSC unless I am missing something? :S

I will get a proper wheel alignment done asap.. Hopefully that can help me get to the bottom of this.

I will also try see if there is a way of resetting the factory ECU in case it has 'learnt' the tyre sizes maybe?

Surely I am not the only person who has gone larger/stickier tyres, has anyone else had similar experiences?

Neutral - I think I understand where you are going with your 2nd point.. maybe since the car knows how big it is, it knows how many turns each wheel should do, to go around a certain angle, as such could get annoyed if the rolling diameters are different.. You would think though they would calculate it based on a ratio, that way alterations like this wouldn't matter.

If someone has a more indepth insight into the workings of traction control, then I am all ears :)

Thanks again for all feedback thus far :)

Off topic but what are the specs on those wheels?

18x9.5 +38 all round.

So even changing front and rear to be exactly the same rolling diameter, can upset the VSC system? If that's the case, I guess it's a lot more complicated than I gave it credit for. :S

Half the reason I changed to these tyres was to get the same rolling diameter front to rear. I didnt expect it to actually upset it even further :(

It's possible. With a 3.12% difference, the VSC will be expecting to see 845 wheel rotations per mile, but only be seeing 819. That happens even if all four tires are matched to the same incorrect size.

Whether that difference is enough to trigger the VSC system, I doubt anyone here would know, but 2% is generally considered the threshold on many systems.

The easiest way to check would be to throw your stock wheels/tires back on and drive on the same roads to see how your VSC responds.