Converting to disc brakes: has anyone used the stock 11" E-body rotors?

[HPP]

When I read the long argument among Ehrenberg and others on, Moparts I think, I don't remember him conceding anything; his main issue was increased bump steer, but he eventually had enough of the argument and said he was done. Has he said anything new on the topic since then?

I was one the parties involved in that thread along with Steve Dulcich, Andy Finkbiner and Bill Reilley. Over the course of the thread, all of Eburg's issues were addressed and the only remaining item left was bump steer. The tall spindles created .064 of extra bump at 3 inches of compression and .027 of extra bump at 3" of dive. Bill Reilly documented this and published it on his big block dart site. This was about a 20 page thread that ran its course in 2001 or 2002, IIRC. There have been subsequent threads on it where he has said there may not be the issues he originally though, but he wasn't going to change his article, and none of the ensuing conversations ever reached the height of conversation that original thread did.

About a 7% increase (6.8%), though I'm not sure there's a 1:1 relationship between diameter and braking torque, but regardless of that, the pads would have to also increase in size in order to increase braking torque, as braking torque is determined by the size of the pads-to-rotor contact area, clamping force and all else being equal.

Now we're splitting hairs. I calculated 10.87 and ended up with 7.5, which I rounded to 8. Certainly 11.98 vs 11.75 is 6.8. Since that may be closer to whats really out there, we'll call it 6.8. IMO, the size of the pads as a function of torque is not necessarily a factor here despite being the same size because they are .75 further from the center. That does produce an increase in leverage with the same pad coverage. Many 13 and 14 inch brake rotors don't have significantly bigger pad sizes either, but the increased leverage is present in greater value. This is an important factor, IMO, and is why wheel limited racing classes have figured out how to stuff a 12.19 rotor inside a 15" wheel. Pad size and clamping force via piston area are additional factors, but getting into all the calcs for those could take upa book.

Absolutely true (though your math is a bit off; an 11" diameter circle has ~95 square inches of surface area, while an 11.75" circle has 108.4 square inches of surface area; a 14.1% increase in surface area), and as far as I can tell, this is the only actual advantage of the 11.75" conversion over the 11" conversion (with regard to the Disc-O-Tech conversion only; the 11.75" rotors have more potential braking torque, but the calipers and pads aren't taking advantage of the rotors' increased surface area). On the other hand, the 11" rotors have the advantage of being lighter (I assume), which would actually translate to effectively having slightly more braking power (emphasis on "slightly"), all else being equal, until heat saturation-induced brake fade enters the equation.

Again, I didn't get as specific in the math. Thank you for the clarification.

Surface area is actually a big factor in applications that have repeated, heavy braking requirements, like a short track oval. In these situations where wheels may limit the diameter, racers will go to a thicker rotor to provide even more heat sink capability, despite the penalty in weight. The flip side of this where a shorter duration and less heavily used braking application can get away with less weight, then by all means reduce the unsprung mass by as much as possible. This is why an oval track brake set up that is 1.25x12.19 looks massive compared to a Super class drag race set up that may be .75x10. So in cases such as this, the compromise becomes what is the lightest effective size for the intended vehicle in the localized traffic. For a great number of guys, 11" disc on an occasionally driven toy car will be perfectly fine. If you change the application, then you can do the analysis to see if a change will benefit you any.

[MaximRecoil]

IMO, the size of the pads as a function of torque is not necessarily a factor here despite being the same size because they are .75 further from the center. That does produce an increase in leverage with the same pad coverage. Many 13 and 14 inch brake rotors don't have significantly bigger pad sizes either, but the increased leverage is present in greater value. This is an important factor, IMO, and is why wheel limited racing classes have figured out how to stuff a 12.19 rotor inside a 15" wheel. Pad size and clamping force via piston area are additional factors, but getting into all the calcs for those could take upa book.

For whatever reason, the leverage issue never even occurred to me, but you're absolutely right. It is the same reason that you can apply more torque with a fat-handled screwdriver than with a thin-handled screwdriver. I don't know the math to figure out how much additional braking torque you would get from an extra ¾" distance from the center of the disc, but if anyone does it would be interesting to know.

As for cooling, the type of wheels you are running can make quite a difference too. This is another case where I don't know the math, but I suspect that an 11" rotor with turbine-style aluminum wheels (such as American Racing Vectors) would cool better than 11.75" rotors with stock stamped steel wheels. The aluminum wheels have a few advantages here over stamped steel wheels, i.e., aluminum is a far better thermal conductor than steel (it is commonly used for heat sinks and cookware for that reason), plus the "fins" increase surface area (another feature common to heat sinks), and the open spaces between the fins allow for more airflow. If you wanted to really take advantage of the heat sink function of aluminum wheels, you could polish the surface of the face of the hub part of the rotor to a mirror finish; do the same to the back of the aluminum wheel which contacts it, and spread a layer of thermal grease on them both. A setup like that would be very difficult to saturate with heat, because of the wheels functioning as properly mounted massive heat sinks, and the sort of fan action of turbine-type wheels.