Addressing the braking "surge" issue

I've read a number of threads about this behavior and I don't see any mention of the role of the transaxle’s differential. Others have correctly pointed out that regenerative braking acts only on the front wheels, but it also important to note that the regenerative braking torque is transmitted from the transaxle to the wheels via differential gears. Thus when braking traction is lost on one front wheel (due to a wet manhole cover, bump or whatever), braking torque is also lost on the other front wheel. The effect is a noticeable instant of "no braking" and takeover by the ABS. Or perhaps the driver takes over by pressing the brake pedal further and activating the hydraulic brakes (this should happen instinctively, and quickly since the driver’s foot is already on the brake pedal).

What to do?

· A software change cannot do much unless the regenerative braking function is considerably reduced, and this would be unwise for a Prius.

· Installing a limited slip differential would damp the sensation but it would still be there.

· Replacing the single MG2 motor/generator with a set of two MG2's, each geared to a separate drive axle, might make sense. This is not much different functionally since the single MG2 is already geared directly to the transaxle output shaft. A differential would still be needed to distribute the engine and MG1 torque but regenerative braking would be completely wheel independent.

· Toyota could do nothing since this does not appear to be a safety issue, but the number of braking complaints on the NHTSA website suggests that some adjustment is needed if only for consumer confidence.

Correction: Qbee42 did cite the role of the differential gearing in an earlier post; my apologies to Qbee42.

Update: Inflating tires to higher than specified pressures could exacerbate the brake surge phenomena. Higher pressures mean a smaller contact patch on a wet manhole cover and higher rebound energy when a tire hits something like a pothole (since there is less deformation of the rubber and therefore less dissipation of energy with the higher pressures). LRR also use lower hysteresis rubber compounds which might affect the rebound characteristics. If that were not enough, the S33D tires have relatively poor traction. Yokohama confirms this by emblazing this information on the sidewall. Anecdotally, I don’t recall and brake surge incidents since I’ve changed to Conti Eco Plus tires.

 

You're not being harsh. I just find this implementation of two-wheel braking extremely intriguing. I did edit my original post with a correction and some thoughts on the contribution of tires. This might be of more interest to forum members.