It's a normal condition to lock rear drums brakes before disks in most conditions, even easier to do on ice. As was mentioned, drum brakes are self energizing. The Cobalt, like my ION have non-servo brakes on them, which actually makes them more prone to locking up then the servo set-up most pickup have. Servo means that the lead shoes transfers this self-energizing action to the rear shoe allowing to provide more & controllable braking power before it will lock. Non-servo brakes have a solid mount at the bottom of the backing so the lead shoe won't transfer this energization to the other shoe, as a result, the front shoe can wedge into the drum harder & faster making it more prone to locking up at a lower brake pressure.
Rear brakes locking first
- Thread starter NateDN10
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Are you referring to the duo-servo setup versus the non-servo leading/trailing setup? I was always under the impression that duo-servo would be more prone to lockup due to its higher self-energizing effect. Here's an excerpt from an old Haynes book I have:
'Hayes Techbook, Automotive Brake Manual,' by Bob Henderson and John H Hayes, 1994, p. 1-20:
"There's one drawback to the duo-servo design - sometimes they work too well, and can lockup the rear wheels. For the most part, duo-servo brakes are used on heavier rear-wheel drive vehicles. Many other vehicles use self-energizing brakes, but without the duo-servo action."
I also read somewhere on the web (so it must be true
) that with small front wheel drive vehicles that brake mostly with the front rotors and have a significant weight shift forward under braking that you wouldn't want to the rear brakes to self-energize too much or they will lock. That's why they use the leading/trailing setup in the rear drums in those applications (versus duo-servo).
I'm not a mechanic or mechanical engineer so if someone knows better, please correct me.
'Hayes Techbook, Automotive Brake Manual,' by Bob Henderson and John H Hayes, 1994, p. 1-20:
"There's one drawback to the duo-servo design - sometimes they work too well, and can lockup the rear wheels. For the most part, duo-servo brakes are used on heavier rear-wheel drive vehicles. Many other vehicles use self-energizing brakes, but without the duo-servo action."
I also read somewhere on the web (so it must be true
) that with small front wheel drive vehicles that brake mostly with the front rotors and have a significant weight shift forward under braking that you wouldn't want to the rear brakes to self-energize too much or they will lock. That's why they use the leading/trailing setup in the rear drums in those applications (versus duo-servo).I'm not a mechanic or mechanical engineer so if someone knows better, please correct me.
Yes, Duo-Servo brakes produces considerable more braking power than non-servo. Yes, all drum brakes will lock, but then again so will disk brakes. This why the preference has leaned towards 4-wheel disk with ABS in recent years. All that required for brakes to lock is more braking power than tire traction. So far as trucks ability to lock-up the rear brakes, their is 2 reasons behind it, 1. trucks have considerably larger brake capacity because they have a much larger load capacity and 2. their is less weight on the rear axle for the brake capacity that it their.
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