Gimme a Brake - Part II

by Jim Lunson

I wrote last month about the various options for brake pads available for the MG.  The other face of the braking system is the rotor or drum that the brakes push against to make the car stop. Here again, there are several options to consider, at least for the MGB which has disc brakes in the front. Other MGs have the older drum and shoe brake system (unless they have been upgraded), and here there is little choice of drum material beyond the cast iron.

Last month I described how the stopping power of the various brake materials depends on the bite of the material. In actuality, this bite by the brake pad is against the drum or rotor surface and the wear is not totally on the pad, but also bites into and eventually grinds down the metal thickness of the rotor or drum. So even with super pad materials, there is going to be wearing on the rotors or drums as well and that means they will have to be replaced also. They are all steel or iron and last much longer than the pads, but not forever. As they slowly wear down, they reach the minimum thickness they need to withstand the heat generated during the stopping. Then they start to warp. Then when you hit the brakes, you feel a pulsing sensation in the petal. That is the sign the rotors or drums have warped and are not longer perfectly true. It is time to replace the rotors or drums as well as the pads or shoes that work against them. In order to keep weight down, especially in the unsprung area of the wheel system, manufacturers keep the thickness of the drums and rotors pretty close the minimum they need to stay true. There is not much thickness to play with

Drums and rotor wear get another boost when brake shops change the pads or shoes and grind down the rotors or drums to provide a new clean smooth surface for the new pads to work against. This step is necessary if the wear has reached the steel backing of the shoes or pads and has scored into the drum or disc. I have a running debate with the guy who does my service as to the pros and cons of resurfacing these brake rotors and drums when there has been no scoring. He claims that the wear from the pads and shoes makes the metal surface uneven, even if you are careful and replace the pads long before they wear down to where metal scratches against metal. By resurfacing the face, it assures that the new pads or shoes will wear evenly and allow them get their maximum life. I argue that the rotors and drums are heavy and therefore made to nearly the absolute minimum thickness required to work properly. Shaving metal off them to get a new smooth surface only shortens their lifespan that much more. To my thinking, having to replace the pads and shoes a little more often is a lot easier and cheaper than speeding up the wear on the rotors and drums too. There is no right answer.

There are not a lot of options for replacing the drums on the brake systems. They are made of heavy cast iron to withstand the pressure of shoes forced outward against them and the heat generated by the braking process. As for rotors on the front of MGBs, there are several
options available:

Standard steel - this is the normal rotor found on the car when it left the factory and provides a suitable replacement. They run about $30 a pair. If you can find them, some manufacturers offer a higher grade steel rotor that lasts longer, but costs about twice as much. Still this is not a great expense and will provide a much longer lifespan than the factory original. Look for brand name rotors.

Slotted steel - this is an upgrade that is similar to the standard rotor with the exception that it is scored with 5-6 slots radiating out from the center to the edge. The slot is there to help shed water in wet conditions and to dissipate heat buildup by providing ventilation. These run about $100 a pair. They provide much better stopping ability, especially when wet and are not a bad upgrade. Heat dissipation is minimally improved.

Slotted and cross-drilled - this is the ultimate brake rotor. It has the same slots to improve the performance as noted above, but also have about 40 holes drilled in them. These holes really reduce the heat buildup as they allow for a great deal of air circulation through to the pads. They run about $200 a pair. They are usually made of a much higher quality of steel also.

I feel either the upgraded standard rotors or the slotted steel ones are the way to go. Either will give good service, and with a more limited usage than a daily driver car, should last as long as needed for the MG. The slotted ones do offer better performance in wet conditions, but I try not to drive my MG in wet conditions anyway, so this is not a big factor for me. Consider your personal driving conditions. I have heard questions about the cross-drilled rotors. Yes, they run cooler in theory because of the air holes, but due to these holes, they have a reduced surface area for the pads to grab against, requiring more pressure to get the same braking effect. And with less surface area, they tend to wear much faster than standard rotors, even in spite of the higher grade steel used. I have no experience with these, only the comments heard by various owners.

Another option available is the conversion of front drum brakes to disc/rotor systems. Disc brake systems came into widespread use during the 1970s. They provide a more sure braking system as the calipers allow the pads to pinch together against the rotors rather than expanding outward against drums. There is less distortion, better gripping, and the pad replacement effort is much easier than with drum type systems. The calipers provide a better system of movement providing a better seal of the brake fluid and less potential for leakage. It is a better system all around. It is one of the amazing positives MG did when they introduced the MGB in 1962 was making the switch to front disc brakes. They were ahead of their time on this item. So conversion of the front brakes for MGAs and T series are something to consider if braking power is critical in your mind and originality is secondary. Kits are available to do this change without a great deal of modifications to the cars. Again, this conversion is really dictated by your own personal driving habits.

I dont have space to go into the possible conversion of the rear braking systems to disc type function. I will leave it to say that kits are available and provide some braking advantage, although with about 80-90% of the braking effort done by the front brakes; I question the need for this change. And these conversions are not cheap, as it requires a considerable modification to the axles, and gets further complicated by the emergency brake system which uses a cable in lieu of the fluid system to engage the brakes. Happy stopping your MG.