Different tires F/R messes with electronic braking?

[The Wind Breezes]

Recently I was reading a little about the ABS / DSC unit and its variable bias, which reminded me of something I've been wondering about for a while.

Let's say someone has new summer sport tires in front and old runflats in back. Will this cause any issues with the ABS that would make the car stop at less than its potential?

You would think not but I did this a couple years ago when I was into killing tires and my impression was it impairs the braking. I was driving the car with DSC OFF at a track day and had no issues other than the expected with the imbalance when cornering and accelerating (it's fun with more front grip than rear) but braking is less consistent and effective than you would expect, even after taking into account the lessened grip of the rear tires and all the adjustment and prediction the system does without being recoded, the behavior of which I was well familiar with at that point. My car had a stripped rear and stock suspension at the time, so I would imagine the weight bias was very far forward under braking anyhow, so it shouldn't have mattered. But it felt like the system was being tricked.

Any thoughts?

[The Wind Breezes]

Nobody has any thoughts on this fascinating question?

[chris_flies]

I don't think it would, other than the obvious lack of traction in the rear. ABS will just cut brakes to whichever wheel has the lowest speed (indicating it's close to lockup) and keep doing so until all the speeds are zero, indicating full stop.

[The Wind Breezes]

Originally Posted by chris_flies

I don't think it would, other than the obvious lack of traction in the rear. ABS will just cut brakes to whichever wheel has the lowest speed (indicating it's close to lockup) and keep doing so until all the speeds are zero, indicating full stop.

After re-reading this thing below I think the behavior I described may have been caused by Maximum Brake Support (HVV)

http://www.onelapx1.com/blog/how-to-...w-actually-fun

You are probably right, the rears don't have anything to do with it. I did code that out a while ago along with the rest of the stuff in that article and the car is a lot more predictable.

[chris_flies]

Originally Posted by The Wind Breezes

After re-reading this thing below I think the behavior I described may have been caused by Maximum Brake Support (HVV)

http://www.onelapx1.com/blog/how-to-...w-actually-fun

You are probably right, the rears don't have anything to do with it. I did code that out a while ago along with the rest of the stuff in that article and the car is a lot more predictable.

I did the same thing (minus certain things, since it is still a daily) and it changed the way the brakes behaved.

[iminhell1]

late to party,

the abs is pretty basic. it looks for differential wheel speed. when it sees that it tries to figure out what's going on, if there's input from a lateral g sensor, it tries to steer the car in the direction of the steering angle sensor.
in the most basic sense.


when i was on my mismatched winter setup, 215 45 front and 225 55 rear. the car never did feel right or learn to be right. there was always a feeling of constant brake intervention, until dsc was off. everything seemed to activate faster than it should have. while good for winter, proactive, it wasn't any fun.


I'd say no matter what you go with for tires, keep the rolling diameters as close as possible. I don't know the % difference where braking starts taking place but I'd guess something like a 5% grace or lower.

[ShocknAwe]

Just try to keep diameter close to stock. Perfectly fine to run a grippier tire out back for whatever the condition is.

[The Wind Breezes]

Originally Posted by iminhell1

the car never did feel right or learn to be right. there was always a feeling of constant brake intervention, until dsc was off. everything seemed to activate faster than it should have. while good for winter, proactive, it wasn't any fun.


I'd say no matter what you go with for tires, keep the rolling diameters as close as possible. I don't know the % difference where braking starts taking place but I'd guess something like a 5% grace or lower.

Yeah, I have matched tires since then. 225 square setup too. I drive DSC OFF every time I start the car since I bought it. DSC OFF does not disable all the brake nannies which are outlined in that article above, you still have to code them out manually and then the brakes work like a "normal" car.

Originally Posted by ShocknAwe

Just try to keep diameter close to stock. Perfectly fine to run a grippier tire out back for whatever the condition is.

Point taken, although I was running a grippier tire in front.

[juld0zer]

As a general rule, the DSC module is able to identify a leaking runflat tyre which is 10psi less than the other three.

By mixing runflats on one axle and conventional tyres on the other, then considering the weight shift when braking, I am not surprised that it didn't feel right. There is an inclination sensor in the DSC module. I am not able to confirm whether it plays a role in brake force distribution but the deceleration rate is closely monitored to maximise use of the rear brakes. EBD works by riding the threshould and does not require a ride height sensor to operate. It assumes vehicle load over the rear axle based on the deceleration rate of the rear wheels. If they decelerate much faster than expected, it assumes the rear is light.