I've just received a new set of Zimmerman front rotors for my '85 and they have slots. The PET calls out two different numbers for L & R and the boxes are marked that way, but I understood the difference was in slot direction. Turns out these rotors are symmetric, so if I mount them with the slots striking the outer edge of the pads first on the left side, they strike the inner edge on the right.

I've gotten conflicting reports on this. Brembo says the slots must always strike the outer edge of the pads first, Centric has an article that claims you can't tell the directionality of the rotor by looking at the slot, instead you need to look at the vanes.

Anyone out there feel like advancing a deciding vote?

 

Look at the vanes first. Their orientation is much more important than the slots. The vanes should have outside end trailing inside end, if that makes sense. So with the rotor spinning in the direction of the car going forward, air is drawn in the middle and spit out the outer ends. IMHO, keeping the rotor cool with proper airflow is the priority when choosing which side the rotor goes on.

The slots are there (supposedly) to allow gasses generated during hard braking to vent rather than build a boundary layer that might reduce the pad contact with the rotor. They are not directionally sensitive for that purpose.

 

Orient them so the slots/vanes scoop up the air in the direction of rotation.

 

Thats the wrong way around. It should be flinging the air out from the center, not scooping. The vanes work as an air pump, creating a low pressure in the center of the rotor which sucks cooling air in from the ducts.

If you stick a pencil into the vanes on the top of the rotor, the pencil should point towards the rear of the car.

 

 

take a peek at where the cooling air enters the center of the hub,
air in the center and out at the edges

 

My mistake.

 

Thanks everyone, that confirms what I'd hoped. To summarize:

- The vanes are most important and should be oriented to spin air out at the edge, not pick it up. This directs heat away from the center of the rotor.

- It really doesn't matter how the slots are oriented.

 

Say what? My verbal and/or visualization skills are lacking and i have no idea what the inner and outer edges of the pads might be.

Now that's clear. And matches RKD's graphic.

Thanks.

 

Thanks Bob. I consider this the definitive reply

 

Pictures are always better, I didn't take one.

By "inner" I meant the smaller part of the circle made by the rotor, the part of the pad that's closest to the hub. "Outer" refers to the part of the rotor with maximum circumference, the "outside" diameter of the rotor. I actually stole those terms from an ad for Brembro slotted rotors that describes how to orient them during installation. Thankfully, we have Dr. Bob, who appears to know what he's talking about I'll try harder next time...

 

Mine too. That was exactly what I thought at first; it was designed to scoop air into the rotor rather than throw it out. Until Bob (and Stan) explained it it never occurred to me that I was thinking backwards. It's funny when that happens, a sort of "well DUH!" moment

It makes sense you'd design it to move heat away from the center of the rotor and, most importantly, the wheel bearings. Sometimes I'm still amazed by the simplicity and elegance I find in the design of these systems.

 

I sometimes have a hard time visualizing things without seeing them in person.

 

And of course, a picture is worth...

 

On a street driven car you will not notice a difference if the rotors are on backwads. It's good to install them tbe proper direction but it's not that big of a deal if the car isn't tracked.