ABS and Traction control adversely affected?

[lowndex]

If one changes the tire size ratios (front to rear), the ABS and traction control will not work properly. How are folks addressing this issue when installing same size tires for front and rear?

[Old smart]

If one does something reasonable, the tires will have the same circumfrence and will produce the same rolling RPM. Thus 155/60-15 = 175/55-15; the change in sidewall ratio keeps the circumfrence the same.

[DavidV]

Originally Posted by lowndex

If one changes the tire size ratios (front to rear), the ABS and traction control will not work properly. How are folks addressing this issue when installing same size tires for front and rear?

The obvious answer would be to maintain the stock ratio when upgrading. Having said that, I was under the impression that our cars use four channel ABS in which case the ECU does not simply reference pairs of wheels front to rear.

** DavidV

[Fredvon4]

Plus sizing, done properly is supposed to result in the same or very close revs per mile as old smart illustrated above

155x60x15 stock front, diameter = 22.32" rev per mile = 931.8

175x55x15 stock rear, diameter = 22.57" rev per mile = 921.5... 1.1% error from the factory

195x45x16 = plus 1, diameter = 22.90" rev per mile = 908.2...1.45% error

215x35x17 = plus 2, diameter = 22.92" rev per mile = 907.5...1.53% error

225x35x17 = Plus 2, diameter = 23.20" rev per mile = 896.5... 2.72% error

This particular system of computer controlled traction control, Stability program, and anti lock brake system uses 4 discrete sensors on each wheel/axle/hub assy. This system really does not care about tire diameter... it is a differential comparator that knows the rpm of each wheel, vehicle speed, throttle position, Yaw sensor input and acceleration/deceleration signals...

By designing it this way, the different versions of a smart car can be supplied with different size tires and diameters and not adversely impact the safety systems

07 and 08 451 series new design smart fortwo can be configured as Pure, Passion, Pulse, Cabrio, and Brabus ...within this family are three distinctly different tire dimensions from the factory

Because the speed signal for the speedo and ECU is from a fixed gear (analog) to digital pulse generator on the transmission the tire diameters are not relevant to the vehicle speed computation.

That all said, there IS a practical error front axle to rear axle to be concerned with...

I do NOT know what the max variance can be before the comparison computer thinks there is a condition that needs ESP or ABS activity.

But I do know that we should not put large diameter tires on the back with out having the front axle diameters fairly close or weird things can happen

On the 450 series it was possible to default the ESP to OFF.

The new 451 uses different logic and the simple workaround was deliberately eliminated by MB for product liability reasons. Just a matter of time before we find a way to hack in and default OFF the ESP and then all these issues go away for those of use who want to race this car without computer intervention

Sorry but I do not need the safety gurus to chime in about how dangerous that is...

Some owners will need to do this for closed course sanctioned racing events... most of us are adult enough to know these modifications (defeating the ESP) are not for daily street use.

[xcrew229x]

Just curious if anyone has considered that the whole "drive by wire" thing is so integrated in this car that changing any one aspect of it's program will domino the whole deal. Opinion Fred?

[Fredvon4]

No it (fly by wire) actually has a series of good effects because there is such a keen safety and product liability issue...

We have all heard the horror stories of some poor sap who stuffed his car in gear and the motor magically went to wide open throttle and buildings were damaged or people killed....

If these things happed because of a electronic fault the manufacturer is liable so they built in a LOT of comparisons, tests, open loop and closed loop logic in case of failure or signal loss.

They also know the buying public and the effects of SEMA lobbying... what this means is the probability that an owner will modify some aspect of the system is very high. So they deliberately engineer for the wide range of typical modifications.


I am making some broad assumptions because I know many of the domestic, Asian and the Bosch systems...

I have no idea who provides Mercedes smart subsystems... but from reading it seems they have quite a bit of proprietary in house programming in their systems.... They do very closely follow all the CAN buss signaling I am familiar with and their OBDII conventions are not weird or shall I say non standard... they do use some different terms but the basic systems and subsystems seem to closely follow the industry conventions I am accustomed to observing

OK all of that is a lot of verbose crapolla to say that the designs are so self tested and redundant that each of the critical areas can accept a wide range of variations before defaulting to OFF...

Engine management is another animal though... some changes to certain areas can result in an undrivable car because the default limp home mode is easy to induce

Much of this is because emissions is so heavily weighted that any too lean, pinging, or over rich condition has a adverse effect on emissions and the catalyst. The LAW does require the manufacture to fix any emissions related issues well beyond the standard warranty

RE your thoughts on fly by wire ( electronic throttle) I suspect some small bit of the porpoising may be the manual matic tranny and electronic throttle timing... each time you make a foot feed change the system must do several tests to ensure the input is legitimate (especially every WOT input) these tests take several milliseconds and cause some measurable lag.

[RussBarnes]

FWIW, I measured the circumfrence of my front and rear tires and no matter what the charts say, mine are all within 1/8" of each other. Whenever the painter gets my rims done the four new tires going on are all equal and about an one inch larger in diameter than stock. So since most ABS systems look for differences in revolutions and slow the high reving one(s) I don't think my change will effect the braking one bit. And do these cars even have a traction control? If so then that would only be across the rear as they are not FWD so again a non-issue.

[limey]

Originally Posted by RussBarnes

And do these cars even have a traction control? If so then that would only be across the rear as they are not FWD so again a non-issue.

Yes they have traction control and, more to the point, stability control that is monitoring all four wheels.

[RussBarnes]

Originally Posted by limey

Yes they have traction control and, more to the point, stability control that is monitoring all four wheels.

Well then since four equal tires came off and four equal tires are going on then it's little pea brain will be happy. And when driving it sure makes me think that the name "stability" control was a oxymoron. But I know, some marketing guy meant tire slip stability, not handling stability.

[lowndex]

Having the same size tires for front and rear will adversely affect the ABS, traction control and brake assist. One must maintain the tire ratios when changing to say, 16" or bugger wheels and tires.