- EBD only reduces rear brake line pressure and leaves front pressure unmodified (up to the point where ABS kicks in of course)
o.k. maybe it's just me being picky, but if you want to make it a "general statement", I would say EBD only adjusts rear brake pressure......, is more accurate.
Now, this may very well apply only/mainly in the context of racing, where you don't brake with a constant deceleration value into a corner, but nonetheless, I feel it would be a more correct.
If you "read" the brake force distribution diagram backwards, that would be the case when coming from maximum braking force and easing off the brakes, you will see, that you would actually
need to increase rear brake pressure to stay close to the ideal distribution.
Ignore the lower graphic, you will see that the "ideal" brake force distribution parable will change with load.
Any form of "fixed" distribution, which would mean a straight line, will intersect the parable only at one given point. This given point would correspondent with a certain deceleration value (G).
Only for this one value, would the brake force distribution be "ideal", meaning both front and rear would lock up at the same time (assuming no ABS).
Until you reach this point, you will lock up one axle before the other (usually the front), after this point the other axle would lock up first.
This should be seen in the context of overall grip (road condition). As you will see, when you look at the second diagram.
Let's assume we estimate overall grip with 0.85 (longitudinal braking force/vertical force) and optimize a fixed distribution so that the both axle would lock up at the same time, when deceleration is 0.85G.
This means both axle use all the available grip and nothing is left "on the table" (max. braking efficiency).
Now, if it rains and the overall grip reduces, so that we can only get a max. deceleration of 0.5G, we would lock the front wheels first (no ABS) which means under this conditions we underutilized the rear axles possible contribution to overall braking.
On the other hand, if we encounter a higher then anticipated grip condition, where we could brake harder, decelerate with more then 0.85G, the rear axle would lock up first (because, due to the higher deceleration, more weight/load will get transfered to the front tyres). This can cause stability issues, if we have a side force (cornering, banked road, side wind etc.) at the same time.
Now, with ABS, in theory, we could say, that we would not need to worry about this, because the ABS will just take care, of the axle which is going to lock up.
That's true to an extent, and would be the rear if we opt for equal brake moment distribution front and rear (as a side note, brake moment is not only dependent on brake pressure, brake disc/drum and tyre diameter/radius have a effect as well).
But for most people it would be very "unpleasant" and "confusing" if they sense ABS activity under "normal" braking. This would lead to the impression, that they have reached the limit of grip, and that more braking effort is not possible. This in turn would lead to greatly increased stopping distances, as most people, would not "dare" to brake harder.
To avoid this, brake moment (pressure) distribution is used, to take into account the effects of load/weight transfer. But with any fixed distribution, you will get only one intersection with the ideal parable.
If you use a proportioning valve, you can get two (bi linear) intersections/or near intersections and in general minimize the area between the two curves. With a two set point proportioning valve, you would
get three intersections (tri linear) and can minimize the area even more.
If you can track the parable perfectly, you have optimized your overall braking efficiency for all deceleration values, and both axle would (without ABS) lock up at the same time, now ABS intervention would start at both axles, and really signal that the overall limit of available grip is reached.
If the brake force distribution is not "ideal" the axle which has the larger brake force proportion will trigger the ABS action, but that still leaves the other axle underutilized, reducing overall braking efficiency.
The advice given to "brake as hard as you can" and let the ABS sort out the rest, is good, but some (most?) people will not follow it, and stop increasing or even reducing pedal pressure when they sense ABS intervention. While a "normal" ABS can prevent the lock up of the over utilized axle, it can do nothing to use the other, underutilized axle more.
Therefore, as long as people don't brake harder when ABS kicks in, this axle will not contribute it's full potential and overall braking performance/efficiency will be less, then it could be.
EBD tries to account for this, by getting both axle "to the limit together", ABS then controls the overall deceleration value, in accordance to the available grip, but both axles will contribute there max potential to the overall braking effort.
This diagram shows the optimum brake force distribution for a given vehicle configuration.
The thin black diagonal lines, represent the longitudinal acceleration/deceleration values and are labeled at the intersection with the ideal parable (0.1-0.8 G)
The thin dotted lines which meet at the intersection of the diagonal lines and the parable are the "grip" values for the individual axles.
The red/black line (ß) would represent a fixed brake force distribution front/rear. (this schematic is from a hybrid, therefore it shows the combined effect of generator and brakes, but you can ignore this for now)
As you can see, the shown distribution intersects the ideal parable at 0.5G - only at this one value, would the front and rear axle reach the limit of grip together.
At all value below <0.5G, the red line crosses the "grip lines" coming from the front axle first.
In practice this means that for example at 0.3G deceleration the front axle would lock up in the moment the red line crosses the 0.3 Mü (grip) line coming from the front axle. The overall deceleration at front lockup would be ~ 0.28G (the front wheels would lock up or ABS would intervene at the front axle only).
If the driver keeps increasing brake pressure, the overall deceleration would continue on the dotted line, until when intersecting the parable and also the "grip line" coming from the rear, the rear would lock up too the overall deceleration peaks at 0.3G which would be the maximum at this grip level.
If the driver would not increase the brake pressure (until rear wheel lockup) just keeping at the lockup threshold of the front axle only (or let ABS do that for him) the total deceleration he can achieve is ~0.27-0.28G, which is less then the overall grip would permit in this situation.
As further away, the brake force distribution line is from the ideal parable at any one point, as larger this difference (utilized deceleration vs. possible deceleration) will become.
This is where EBD can help to increase the overall braking efficiency and augment ABS.
Edited by TC3000, 18 March 2013 - 03:37.