ABS Question...

[tref]

If ABS compares the wheel speed... why doesn't it come on when you go round a corner? (I assume there must be an allowable mis-match of revs... would that be made worse, or better with big wheels?) Is any comparrison made to the gearbox (speedo) sensor?

Regards,

Tref.

 

[ChasR]

I assume it probably averages out the speed from each of the 4 corners (probably averaging the speed of the LHS, also the RHS as well as F/R and then having a certain set of parameters, since the ABS works off the ABS sensor rings on the hubs/driveshafts.

I initally didn't think that my ABS worked in my S2, but it turns out that it does, just that it cuts in a little later than you expect and rather nicely too (unlike the Pug where it cut in too early and juddered in geological time LOL, very much like a 1989 XJ40 XJR that I had)

 

[sawood12]

I'm not sure it needs to refer to the actual speedo as it should be able to ascertain the speed from the wheel sensors. The speed differential of the wheels when going round a corner will never get to a point where the system thinks a wheel has locked up. I'm not even sure if the system really cares how fast you are going as it will really be comparing the relative speed of each wheel. However it must keep an eye on actual speed as it needs to disengage the system when you are braking to a complete halt.

If you were going round a very tight bend you'd be travelling so slowly that the system will know what's going on from comparing the rotational speed of the wheels. The system knows what the RPM of the wheels is from the sensors. It also knows (or assumes) the diameter of the wheels, so it will be monitoring the actual speed of the wheels. This is why in the handbook it warns about fitting wheels that are too different from stock wheel dia or the system might become confused (i think it states you need to be within a 17mm difference).

Ideally the system will kick in just before any of the wheels actually kick in (it is Anti-lock braking system afterall) rather than waiting for a wheel to actually lock up. Once a wheel locks up you have lost grip and control which defeats the object of the system which is supposed to retain control.