I have a set of slotted disks and was wondering if there was a certain direction to install them. I noticed that both front and back have one disk with the slot oriented in each direction.

Should the outer end of the slot be trailing or leading in its rotation? I'm guessing it should be trailing, but I'd like to hear from someone who knows for sure. Thanks.

Hi, this is always an eternal debate at the track!

My research says slots up and forward (leading) with the direction of rotation.
As in this pic:


Most hardcore racers agree, but then the rich boys with their Porsches and BMWs show that the factory put the discs in the other direction.
I say that they have such effecient brakes that the factory put the discs that way for looks even though the racing teams turn them arround when the cars are really exploited!

Regards,
Rafe

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"Turbo Slug" - '87 Fiero GT. 3800 turbo. Sticky tyres. Driven hard!
https://www.youtube.com/cha...1wZvWQlkYxTjivW_0XNg

[This message has been edited by Frenchrafe (edited 08-23-2020).]

"My research says slots up and forward (leading) with the direction of rotation."

What research have you done, and what's the reason for your conclusion, ie the orientation you recommend?

Thanks.

Ernie

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'87 Fiero 4.9 5-speed

My train of thought is that the outer end of the slot should be trailing, as pointed out on the Porsche. Seems to me that would shed debris and brake dust outward while if the leading edge was forward, it would draw it to the central part of the hub. But I'm just speculating.

Most of my driving is normal, everyday type stuff, but I do want the rotors to do their stuff when we go on our excursions into the mountain twisties. Oh, and they're cross-drilled, too but I don't think that matters as far as the rotation goes.

Rotor installation is based on the cooling fin orientation inside the rotor - not the direction of the slots. Cool air in the center, hot air pushed out the circumference of the rotor as the car drives forward.

As far as direction of the slots, there are large supporters for both orientations, which likely means it isn't terribly significant.

Given the inertial rotational forces, trailing edge high would make more sense to expel stuff like the gasses building up. Sorta similar to the way a directional tire is designed to force water to the outside as it rotates instead of drawing it into the middle. Saw a wrecked Jetta in the j-yard last week with the directional tires reversed. Wonder if that played a part in the light front end collision? But as said either way is probably workable as long as its getting scrubbed off. When we ran drum brakes on musclecars you would drill holes in the drum to let gasses out so brake fade wasn't quite as bad.

Rain tires installed in reverse is said to give much better traction on dry pavement than when installed in the correct orientation. But I can tell you that when a tire tech puts the two on the rear on backward, things can get pretty hairy on a wet road.

Sourmash, what you say about expelling gasses makes sense to me, but again, the professional race guys know a lot more about the physics of it than I. Wish I'd known about the holes in the drums. I had a 54 Oldsmobile with drum brakes. Turned 117 @12.1 and getting that big sucker stopped before the sand pit would quickly take the smile off a win. When I bought the car, it had about 6 staggered holes drilled in each drum. I thought "How stupid they were" and I took them off and threw them away. I have also seen something like a household door spring around the outside of a drum. I suppose that was to act as a heatsink. Don't know what vehicle it came from, though.

I think I'll give them a try with the outside trailing.

While we're here, what's anyone's opinion of the best roller bearings for the front? Confirmed Made In USA.

quote Originally posted by fieroguru: Rotor installation is based on the cooling fin orientation inside the rotor - not the direction of the slots. Cool air in the center, hot air pushed out the circumference of the rotor as the car drives forward. As far as direction of the slots, there are large supporters for both orientations, which likely means it isn't terribly significant.

Yes, Fieroguru is right. It's the vane orientation that dictates the direction:

"The internal vanes should lean toward the back of the car. The direction of the slots on the outside of the rotor do not dictate whether the rotor is a left or a right. For rotors with slots machined in the same direction as the internal cooling vanes, the slots should lean toward the back of the car. For rotors with slots machined in the opposite direction as the internal cooling vanes, the slots should lean toward the front of the car. Bottom line - don't use the slots to decide which side to mount the rotor. Pay attention to the internal cooling vanes."

"For slotted rotors with straight internal cooling vanes, the direction of the slots is totally up to you and what you think looks best. In this case, there is no wrong answer."

(copied from Zeckhausen Racing Brakes website)


Since my discs have straight vanes, I put my discs with the slots "leading". I also respect the disc manufacturers' mounting instructions.

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"Turbo Slug" - '87 Fiero GT. 3800 turbo. Sticky tyres. Driven hard!
https://www.youtube.com/cha...1wZvWQlkYxTjivW_0XNg

I didn't mention, this is an 87, so it doesn't have vented disks.