Drum Brake Puzzler

[clinebarger]

Thanks, did not think about ABS. I now see why GM did not go with the time tested, ubiquitous for a long time, duo servo brakes.

There were GM trucks with ABS & Duo Servo drums on the rear, Just about every GMT400 2500 & 3500 in both RWAL & 4WAL configurations depending on the year. GMT400 1500's even had a optional heavy duty 11" Duo Servo drum upgrade over the 10" Non Servo drums that were standard.

 

[George7941]

There were GM trucks with ABS & Duo Servo drums on the rear, Just about every GMT400 2500 & 3500 in both RWAL & 4WAL configurations depending on the year. GMT400 1500's even had a optional heavy duty 11" Duo Servo drum upgrade over the 10" Non Servo drums that were standard.

I am guessing but that was a while ago with more primitve ABS systems. Now ABS reaction times need to be quicker (hence the LV brake fluids) and the self-energizing effect would work against quick reaction. With duo servo both shoes have to move in unison to apply or release, hence twice the mass to move rapidly. Without duo servo each individual shoe moves independently of the other, hence half the mass. Even the mass of the self adjuster slows down reaction times with duo servo.

 

[clinebarger]

Far a ABS systems go.....The 3 channel set-up used on GMT400, 800, & 900 platforms are pretty equal in performance. The only reason GM went back to drums with the later GMT800's & GMT900 1500 pick-ups was the constant issues with the parking brake.

 

[Exhaustgases]

If you have one wheel cylinder and each shoe is loose to the backing plate, you have just what the article said. ??? Need more detailed picture. The servo action is how the rotative movement helps with the apply.

 

[George7941]

If you have one wheel cylinder and each shoe is loose to the backing plate, you have just what the article said. ??? Need more detailed picture. The servo action is how the rotative movement helps with the apply.

Just one wheel cylinder and floating shoes is not enough for duo servo, the (bottom) anchor has to be floating. It is not on my Sierra.

 

[HonorAndHonesty]

I believe that in 2005 the GMT800 trucks went to rear drums, the 99-04's all had rear disc standard. There were some issues with the rear pads wearing oddly on the early ones. I always assumed they reverted back to rear drums to cut costs or due to the fact they had issues with the rear discs, though either theory hardly makes sense.

These are awesome vehicles IMHO. They go and go.