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Heavy duty brakes, bullshit?

12K views 25 replies 8 participants last post by  AAIIIC 
#1 ·
Can anybody explain what does make sense with enlarging REAR rotors and use the same pad size as for standard? Yes, for 13.3" rear rotors the same pads are provided as for 12.4" by vendors like AC Delco and glorified EBC or many others. Why not to use whole available rotor surface?

I found only two different part numbers for OE. Does any difference in size of these pads?
 
#3 ·
The larger rotor helps dissipate heat, and (I assume) your heavy duty calliper clamps harder, so it is capable of generating more heat. The assumption is if you buy heavy duty brakes then you do heavy duty braking (either racing or you live in an extremely hilly area), so you have more heat to dissipate.

The only thing a larger pad would do is wear a little bit slower. Unless your rear pads are wearing out before your fronts, the pads are correctly sized. More even wear on the rotor won't gain you anything. If you want more even rotor wear, you can buy smaller rotors.

Under normal braking, the only thing that effects braking performance is pad material (and rotor material, if you use switch to carbon ceramic rotors).
 
#4 ·
It sounds reasonable, but:
- larger pad surface = lower temperature - so why not to make pads which will use whole available rotor surface?
- front's have different pad size for HD.
- antirust coating doesn't cover unused rotor surface, I expect to see rust ring there soon...
It still remains just stupid for me.
 
#5 · (Edited)
Can anybody explain what does make sense with enlarging REAR rotors and use the same pad size as for standard?
Because it's more economical for GM to use the same caliper, thus the same pads. The performance benefit comes from the larger rotor, so no, the HD rear brakes are not "bullshit".

Are you using OEM rotors?

Why not to use whole available rotor surface?
What do you think that would gain you? Using the whole rotor surface would actually slightly reduce the effective rotor diameter, which would thus reduce the torque of the rear brakes. It would be a small change, but it might be enough to change the front/rear bias in a way that GM/Cadillac didn't desire. That, combined with the economic incentive to stick with the same caliper, would be more than enough reason not to change the rear calipers & pads for the HD setup.

- larger pad surface = lower temperature - so why not to make pads which will use whole available rotor surface?
Are your rear brake pads exceeding their recommended temperature range? If not, then who cares?

- front's have different pad size for HD.
The fronts do the majority of the braking work, so it makes sense to maximize the effectiveness of the HD upgrade on the front brakes.

- antirust coating doesn't cover unused rotor surface, I expect to see rust ring there soon...
Ahhhh, so this has nothing to do with performance, it's just a concern about looks. In that case, see your thread about rusty rotors for suggestions about how to avoid rust on that portion of the rotor.
 
#6 ·
Thanks for answers, but I am still not convinced. They could just leave JE5 rear brakes without changing anything and It would be also okay. I can't believe someone calculated they need bigger rotor but can use same pads, calipers, whatever.
You missed one part. There are 2 different part numbers for OE pads (one for non HD and second for HD). Are there any difference between them?

My configuration at rear now is AC delco durastop rotor + ebc redstuff pads.

Regarding rust - the last thing I want to do when I buy coated rotors is painting them. But probably I will have to do it anyway. Or to replace ebc pads with OE if they are better matched, are they?
Maybe redstuff pads should be used only with ebc rotors... I don't know, this topic begins me sweat.
 
#11 ·
Thanks for answers, but I am still not convinced. They could just leave JE5 rear brakes without changing anything and It would be also okay.
Brake torque is proportional to the effective rotor diameter x the piston area of the caliper. If they changed the front rotors and calipers on the HD setup but left the rears the same, the brake bias would be unacceptable. This isn't a complicated concept.

I can't believe someone calculated they need bigger rotor but can use same pads, calipers, whatever.
Sounds like you need to educate yourself on how different factors affect brake torque and how front/rear brake bias changes affect the braking effectiveness and feel.

You missed one part. There are 2 different part numbers for OE pads (one for non HD and second for HD). Are there any difference between them?
Yes, there's a difference between them, otherwise they wouldn't be the different part numbers. Since they're the same pad shape, the obvious answer is that they are different compounds.
 
#10 ·
Larger rotor, smaller rotor, you will still have the same amount of energy expended to stop your car. A larger rotor will absorb more heat as it is a bigger heat sink. Once it is saturated it will take long to cool down.
 
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#15 ·
Handling, different process, probably shipping to and from. Most folks don't care, just give them OEM at the lowest price. I'm like you, I'd rather pay $10 more and have the entire rotor coated. Now when I put in new rotors I usually paint the hats.
 
#19 ·
In this case I don't think it will make much of a difference. I would use spray on my calipers, for instance, but on the hat area no one will really see it. Go with the cheaper price, or whatever may make you feel comfortable. Just be sure to mask off areas. If you get overspray onto the rotor, don't worry. It will get worn off quickly.
 
#20 ·
This is obviously a far more complex calculation than it might appear at first glance, but it's a simple matter of physics that a pad that covers the entire disc would create a greater amount of friction (assuming the same pressure is applied as with smaller pads) and thus provide greater stopping power. Of course, that ignores the problem of heat dissipation and balancing the rear and front brakes, two things most of us probably wouldn't have thought of unless you guys pointed them out.
 
#21 · (Edited)
It is simple physics, and you are wrong. Friction is not related to surface area. The equation is μxF.

Just about the only thing surface area helps is heat dissipation, which really just protects the brake fluid. Larger rotors or pads only help when the pad or rotor is pushed to its functional limit, which shouldn't be encountered in normal use. Race car rotors glow red and still function better than almost any production brake set-up.

Larger surface area negatively impacts braking as it makes it more difficult for gases and debris to escape from between the friction surfaces (ie it has further to travel and less force pushing it out).

Usually brake bias is adjusted by changing the output from the master cylinder. However, you can change the balance by having different pad materials on the front and back, among other physical changes to the brake system.

More physical pad will last longer (either thicker or wider). It doesn't really make any sense to buy a wide rear pad that significantly outlasts the front pads, which do the most braking.

Edit: Sorry, just saw the other posts. Unless something else is going on I can't figure out, brake torque won't be affected by the size of the rotor if the caliper mounting position is the same. It just makes extra rotor that isn't used (which that observation is what i think started this madness). A larger pad will marginally increase torque applied to the wheel, but nothing significant. The larger the rotor the more angular momentum it will have as well making it harder to accelerate and stop (again, insignificantly). The different pad materials is what makes the difference.
 
#22 ·
That unused couple of square inches of swept rotor area make very little or no difference in "stopping power" unless the area would significantly - significantly - increase overall pad friction. Located at the inner edge of the rotor machined area (what if that area was NOT machined smooth ????) that small swept area is of little consequence.

Red hot rotors - common on any road course track - and maybe achieved in stock car models during HD autocross work. One reason why rotors are machined from cast iron - no grain orientation so they don't warp during temperature extremes.

https://www.google.com/search?q=red...SHu8PLAhWHHx4KHc3aBQ4QsAQIHA&biw=1366&bih=618
 
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