I had once heard that ABS systems are precalibrated for a particular wheel/tyre size, and that if, say, you upgraded to larger rims/tyres that the ABS would not be as effective.

My Mustang has no ABS -- which, quite frankly, I prefer -- but my Dad's 3.2 TL DOES. For a while he was running slightly lower profile tyres and it definitely seemed to me that his ABS and ESPECIALLY his traction control was *NOT* as effective.

So my question is this: if the wheel/tyres of an EVO (or any other car, for that matter) with ABS are changed from stock- is it true that this will reduce the efficiency of the ABS and/or traction control systems?

My intuitive belief would be that this is surely *NOT* the case, because all the sensors, I thought, are constantly receiving data in real time, and hecne this makes no sense at all.

Thanks,

--Marc

 

Anybody?

 

You're correct. ABS systems are calibrated to a set of software rules and any change creating a large deviation from those parameters will affect the ABS system.

 

My only concern was about upgrading the brake system (calipers, etc.). I had been considering the Endless 6-piston front and 4-piston rear, but because of the ABS calibrations (esp. because of the EBD), I was fearful the brakes would be extremely grabby and unbalanced. I know ECU flashes are abundant, but the ABS settings would require a real pro to reset. I ended up sticking with upgraded pads, rotors and lines. I don't think larger wheels will affect anything as long as the overall diameter (wheel + tire sidewall) stays the same (there is a LONG thread about wheel sizes elsewhere).

Chris

 

ABS are calibrated for a certain coefficient of friction with stock tires. With stickier tires or less than it would be optimal to change to programmed CofF to the new value.

If your overall wheel/tire(tIre is how you spell it in the US) remains the same then no changes will probably need to be done. If it is different then changes will be needed.

 

I'm not sure I believe that. If ABS were calibrated for a particular co-efficient of friction then it would function quite differently on dry pavement vs wet vs ice. I know that ABS has sensors that can detect wheelspin and perhaps there's a certain threshhold that's based on the designed rolling circumference of the wheel+tire. But whoever heard of having to recalibrate ABS when you change the tires? That seems unrealistic.

 

I'm still looking into it. I'm *guessing* that manufacturers put in an underestimate for the CofF for safety reasons. But it could be optimized. But you're thinking just like I am that differet CofF's occur under different conditions, pavement, concrete, ice, rain..etc. So I'm not sure.
Anyway, here's a nice article I stumbled on while looking for wheel size differences with ABS.
http://academic.csuohio.edu/aerl/papers/cca2000.pdf

 

Page 4 of the article I posted. I think this gets more into what I was talking about.

"To make the estimation less sensitive to noisy peaks, the deceleration rate limit is defined as the slope between the peak and the start point of ABS (point 1), rather than the slope between two adjacent peaks. To accommodate potential changes in road surface condition and the overall nonlinearly curve of vehicle deceleration, the rate limit R is set to be increasing at a fixed rated, BUT NEVER EXCEEDS THE MAXIMAL DECELERATION."

I believe this is telling us that there is a limit in deceleration programmed into the ABS ECU that *could* be surpassed with superior tires. BUT it is typically programmed with a lesser decel value because typically consumers are not running heated, R-compound tires on a dry road course. If a greater R value was used on lesser tires, braking distance could be increased when ABS is applied using different wheel velocity changes.

Read over the article and let me know what you think.

Here's another article found for me by 'Q45Tech' a master Infiniti tech @ www.nissaninfiniticlub.net
http://www.edccorp.com/pdfs/2002-01-0559.zip
Download and save file.

Reading into this and you'll find how different rolling radius will affect ABS. It will immediately impact it by changing calculated slip. Ref: Eq.1. Wheel spin velocity would change and rolling radius would stay the same. If a LARGER radius is used ABS would active prematurely and increase your stopping distance considerably. Mostly because pressure will not be applied at the peak friction coefficient.
Just the opposite as in your Dad's case with the Acura, when a smaller radius is used the acceleration to the threshold maximum slip occurs MUCH faster than the ECU expects. This will give you more wheel lock. It is possible that the ABS will actually cause a skid of it's own because of this situation. Potentially catastrophic if the radius change is severe enough.