Vented Rotors, I have a question Options

[John]

Do vented rotors offer more warp resistance over solid rotors?

In my mind they would due to the additional support structure internal to the rotor (i.e. the vents).

Specifically, I am looking at 914-6 rear rotors (solid) vs 911SC rear rotors (vented).




Also, has anyone recently considered using the wider 911SC rear wheel bearings (5mm wider)? I have been thinking of a thick plate (3/8") with the retainer bolt pattern and a 5mm deep recess to accept the 5mm wider rear wheel bearing. This would allow use of 911SC rear hubs without spacers.

Here is a sketch:

[Brian Mifsud]

Do vented rotors offer more warp resistance over solid rotors?

In my mind they would due to the additional support structure internal to the rotor (i.e. the vents).

Specifically, I am looking at 914-6 rear rotors (solid) vs 911SC rear rotors (vented).

Looking at rotor design is not enough when considering propensity to warp. The manufacturing PROCESS is equally important. The casting process, especially how it is cooled, how quickly, and if any outside stresses are imposed on the rotor all factor in. After machining, adequate heat-treat/stress relieving is quite important. If there are assymetrical stresses "built" into the rotor, careful "breaking in" can/will still release these stresses, and result in warpage.

That being said, whatever you end up buying, get a good brand (like Brembo). They know what they are doing, don't cut corners, and the cost/rotor is actually pretty decent. Check out TireRack and Tires.com for their pricing.

Avoid the "Slotted and Drilled" style of aftermarket rotors. Every sharp corner machined creates an apportunity for stress cracks to propogate. The stress induced can be wholly from the heat dumped in by hard braking, and/or, any inadequate stress/relieving done while manufactured.

I've gone through enough rotors autocrossing in a (heavy) Mustang GT to have some personal experience with this (IMG:style_emoticons/default/biggrin.gif)

[davep]

Brian is correct. The vented rotors should warp less partly due to more thermal mass and better cooling. There was very few 911's that got the solid rear rotors. That is why Eric, and I, developed vented rotor kits for the 914.

[John]

I prefer non drilled, non slotted rotors.

Are you saying that Brembo Rotors are higher quality than Zimmerman rotors?

[John]

Brian is correct. The vented rotors should warp less partly due to more thermal mass and better cooling. There was very few 911's that got the solid rear rotors. That is why Eric, and I, developed vented rotor kits for the 914.

That is what I was asking and thinking. In general, I believe the vented rotors should tend to warp less by design.

These rotors would have no forced air cooling (like the fronts).

I believe that I am having issues warping the rear rotors. The front 930 rotors don't seem to exhibit the warping issues. Unfortunately the 930 rotors only come in the drilled variety.

I am planning on swapping out the rear 914/6 rotors (with 914/4 front calipers) to 911SC rear rotors and calipers and wanted to know before I do this if I should have a reduction in rotor warping issues (I believe this will reduce the tendency for the rear rotors to warp).

[Brian Mifsud]

I prefer non drilled, non slotted rotors.

Are you saying that Brembo Rotors are higher quality than Zimmerman rotors?

Hi John,

I haven't compared Brembo vs Zimmerman. Just Brembo vs. Canadian-Made Rotors as sourced at places like:

Brake R Us

If Zimmerman is Porsche OEM, I'd go with those also. You just want to avoid the sources who cut corners on quality. Unfortunately, there is no way of knowing "quality" just by looking at them.

Tire Rack has Brembos and ATE's for $60-63 apiece. I can't attest to "ATE's" quality not having used them, but of course, they were OEM for the Calipers on most Porsches from the 60's/70's.

Tire-Rack

[Brian Mifsud]

Unfortunately the 930 rotors only come in the drilled variety.

"Drilled" rotors can be found in two forms. One (the best) is where the holes are not drilled, but actually cast in place. These have good fillets to reduce stress crack propogation sites. (Brembo did these as the OEM for Porsche and others).

Vented Rotors which are CNC drilled (or slotted) are the ones with the problems. You CAN still machine a rotor, carefully fillet each hole (or slot), then stress relieve it, but after all the hours you put into it, it is hard to make any money, so few if anybody does it.

[Dave_Darling]

The "bracing" of the rotor doesn't really have any effect on "warping". Most "warped rotors" actually have uneven amounts of brake pad material deposited on them, leading to that nasty pulsating feel in the pedal and a shaking car.

Keeping the rotor cool will help with that uneven deposition, and that's where vented brakes come in. They have more mass to absorb heat before they get to "too hot", and they have more surface area to dissipate heat into. Not to mention that there tends to be some air flow through the vents regardless of ducting, which also helps carry heat away.

Heat is the major thing about brakes. Vented rotors help quite a bit when dealing with heat.

--DD

[John]

No comments on the bearings or retainers or hubs????

Hmm.

I stumbled onto this page while doing research:

Warped Rotors and other Myths.......

It would seem that the "warp" or thickness variation experienced in the rotors in my case stems from some or all of the following:

1. Improper Bedding in of the pads (gradual increasing pressure stops and then cool to ambient)
2. Improper new rotor break-in (preferably with used pads of the same compound?)
3. Overheating of the pad/disc
4. Inclusions in the rotors creating cementite under extreme temperatures
5. Uneven material transfer from pads to the rotors (due to the above conditions)



Very interesting.... (I've been living in myth land for a long time)

[Eric_Shea]

No comments on the bearings or retainers or hubs????

Could be a way to use a standard 911 rear rotor without the spacer under it or without spacers under the caliper ears (which can be tricky with 914 or 914-6 calipers... asl Felix).

Cost to machine Bearing cap vs. the cost to machine rotor spacers vs. the cost to machine hub spacers vs. some shims under the caliper ears... (IMG:style_emoticons/default/confused24.gif)

For "me" that would be what it boils down to. I don't see any drastic improvement in the later bearing that would really "matter" for 99.9% of the applications.

Neat idea...