99 xj brakes

I Wan't 2 change my frount hubs,found out 2 different kinds for a 99 xj.one fits full cast rotors the other is a composit rotor.


Question 1: How can I tell the difference?

Question 2: which is better?

Going to go to slotted rotors later, is there a problem getting aftermarket drilled or slotted rotors for either hub set?

I guess thats 3 questions,sorry:looney:

Cast Rotors are a single piece, machined out of a single piece of cast iron.

Composite Rotors are multiple pieces, bolted/riveted/welded together, some being cast iron, some pressed metal, some pieces are aluminum, etc.

Its very likely you have the appropriate hub for the type of rotor you have.

Slotted or Drilled Rotors rarely offer any improvement, some Porsche Racing Teams, actually take off the OEM slotted and drilled rotors and replace them with sold discs for the racing. It really depends on the application and bunch of factors, sometimes they work better, sometimes they don't. BUT, its usually extreme racing condtions they work better, NOT street conditions. Add to that, many Slotted or Drilled Rotors are really just cheap knock-offs just for looks, they usually perform worse and the simple cut or drilled hole actually weakens the rotor and the non-relieve edges actually wears down the pads faster, they crack often as well.

Real Slotted or Drilled Rotors are constructed properly, with the holes and slots cast into them and the casting has additional thickness to make up for the lose of heat sink mass and strength to re-enforce the holes and/or cuts.

A lot of the Slotted or Drilled Rotors you buy for the street are just Stock Replacement Rotors that someone cut slots or drilled holes into. Not only will these cheap knock-offs wear out your pads faster, they are probably weakened by the cutting/drilling and are likely to crack or warp as well.

why would the wheel hubs be different

Different mating surfaces and dimensions, etc. Who knows, may be the composite rotors were part of a TOWING Package, that the far more expensive big brakes required a far more expensive hub.

So why drive up the price of all vehicles for a small volume Brake Option. With the cheaper brakes you could get away with a cheaper hub, if you opted for the more expensive brakes you also had to pay for the more expensive hub that fit the brakes.

BTW, I don't know which is better, the Full Cast or the Composite Rotors. My guess would be the Composite Rotors were probably part of a heavier duty brake package, they usually are, because a Composite Rotor is far more expensive to produce, so manufacturers are far more likely to use the cheaper Full Cast Rotor unless they are forced into using the composite to meet some sort of demanding spec, like heavier duty brakes.

Rick Anderson said:

Different mating surfaces and dimensions, etc. Who knows, may be the composite rotors were part of a TOWING Package, that the far more expensive big brakes required a far more expensive hub.

So why drive up the price of all vehicles for a small volume Brake Option. With the cheaper brakes you could get away with a cheaper hub, if you opted for the more expensive brakes you also had to pay for the more expensive hub that fit the brakes.

BTW, I don't know which is better, the Full Cast or the Composite Rotors. My guess would be the Composite Rotors were probably part of a heavier duty brake package, they usually are, because a Composite Rotor is far more expensive to produce, so manufacturers are far more likely to use the cheaper Full Cast Rotor unless they are forced into using the composite to meet some sort of demanding spec, like heavier duty brakes.

Click to expand...

As far as I know, the composite rotors were simply the standard on XJ's from some time in the early 90's until sometime around 99 or 2000, when they changed back to cast, but not to the same dimensions as the earlier cast. It's not specific to towing packages. There is no real difference in the hubs except for the distance from the knuckle surface to the wheel flange.

If your XJ has the composite rotors, it's easy to tell, because the top of the "hat" has a rounded edge, whereas the cast has a sharp edge. However, many composite rotors are replaced with cast rotors dimensioned to match the composites, so if you see cast ones, you cannot be 100 percent sure what type yours was made for. For example, I just put a pair of junkyard rotors on my 95, which had composites. They're cast ones from a 98 Grand, and may well be aftermarket. They are a perfect fit.

thanks Matt thats what i need to know-everything is stock so I just have to look at the hat and order the correct hubs for the applacation. no difference in brakeing specs.

Just ran into similar problem on my 2000. Bought what I thought were 3 1/16" cast rotors online, installed them and they rubbed on the gravel shield. Ended up being cast, but made to composite specs which is 3 3/16". Went to local parts house with a tape measure and checked the ones I bought there.

The local Autozone shows the cast rotors as 11" and composites as 10". Don't know if that is true or not.

Luckily I have a buddy with a 1995 Grand Cherokee that will take my extra rotors.

Three different hub/bearing units were used over the years on XJ's. The hub/bearing units are the same part number for both sides of the vehicle. The model years for hub/bearing assemblies were:

1) 1984 through 1989 (early style rotors)

2) 1990 through mid-1999 (composite rotors)

3) late-1999 through 2001 (cast rotors).

** You need to match brake rotors to the unit bearing/hubs on your XJ. **

Lay the rotor on your bench and measure from the bench to the top of the rotor.

Composite rotors will have an overall height of 3 3/16". Composite rotors will have a thin sheet metal like appearance at the lug nut holes where the studs come through and measure about 1/8 "(.125) thick in this area. Composite rotors have a rounded off corner at the wheel mounting surface and look like two pieces joined together.

Cast rotors will have an overall height of 3 1/16". Cast rotors are thicker at the lug nut holes and measure between 1/4" (.250) to 5/16 (.320) in this area. Cast rotors have a square corner at the wheel mounting surface and look like they are one solid piece of metal.

*****Parts store replacement rotors may have a different appearance. *****

On late 1999 - 2001 unit bearing/hubs the ring around the 36MM shaft retaining nut is almost 3/4 inch tall and the hub has longer lug nut studs than older hubs.

On early 1990 - 1999 unit bearing/hubs the ring around 36MM shaft retaining nut is about 1/2 inch tall.

Two steering knuckle designs were used: 1984 to 1989 and 1990 to 2001. Left and right side knuckles have different part numbers.

Two types of brake caliper styles were used: 1984 to 1989 and 1990 to 2001. Left and right sides are different part numbers.


1999.5 to 2001 unit bearing/hub on the left, 1990 to 1999.5 unit bearing/hub on the right
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COMPOSITE ROTOR below -

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CAST ROTOR below -

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The composite rotor looks to be a single piece. I thought composite rotors, by definition, were at least 2 pieces joined together.

The pictured Composite Rotor does look like my '95 Cherokee Rotor, and I haven't removed the rotors and/or inspected them closely. Am I just missing where the pieces join together, because they sure appear to be one piece to me, in the pic and on my vehicle.

If you see them side by side with your own eyeballs you will understand.

Tim, Great job printed in my xj file exclent reply to my dumb question thanks again.

Rick Anderson said:

The composite rotor looks to be a single piece. I thought composite rotors, by definition, were at least 2 pieces joined together.

The pictured Composite Rotor does look like my '95 Cherokee Rotor, and I haven't removed the rotors and/or inspected them closely. Am I just missing where the pieces join together, because they sure appear to be one piece to me, in the pic and on my vehicle.

Click to expand...

You can't see the junction. As far as I know, it's where the hat meets the disk, but I've never detected visible welds or seams. It's a pretty neat job however they do it. In addition, some aftermarket cast rotors will have a plain edge without the lip shown above. But they're not rounded.