Cast vs composite, can someone please reassure me? I'm losing it

JoshNZ · Sep 27, 2019

Hey guys,

Failed my regular safety check due to front ball joints being lose. It was expected, so going to replace them and my wheel bearings and U joints all in one nice front end maintenance mission.

Front Diff is a HP30 our of an XJ of unknown year.

I’ve read the forums and concluded* that the hubs are the older style with the shorter hub ring measuring about 12mm or 1/2 inch. ( I know, I know didn’t have my ruler handy but I can confirm it’s not 3/4” or about 20mm)

Cool. Pre 99.5 and the composite type right?

If these are composite them what do cast rotors look like??

I stopped halfway through typing this and gave them another good wire brush. Starting to see what looks like a machines surface between the holes? This must be the composited material? Those two rings also suggest that.

Of course both overall height and rotor to wheel mounting distance fall just in between the measurements I find online when converted to mm. (Yes, I need to find a metal ruler or verniers)

And why this has got me in a panic? I need to order all the parts from the states, so can’t afford to get it wrong. And I only have 4 weeks (from yesterday) to sort it all. So hoping to get parts on their way ASAP!

PleasE someone reassure me, this is definitely the composite style hub and rotor? Ordering parts tonight!

And just for my learning, any pics of cast vs composite in a new and used state? All the google pics tend to go either way

Thanks as always

JoshNZ · Sep 27, 2019

To be honest, spelling it out on this post made it all make more sense.

I’d still live to see pics of cast and composite rotors, to definitively see the difference.

mrblaine · Sep 27, 2019

JoshNZ said:
To be honest, spelling it out on this post made it all make more sense.

I’d still live to see pics of cast and composite rotors, to definitively see the difference.

This is a cast rotor-

This is a composite rotor-

You can NOT measure the overall height and derive any meaningful measurement to pick a new rotor. The reason for that is the cast rotor has a hat that is about .230ish thick. The composite rotor has a hat that is about .100ish thick. If you measure the overall height of the composite rotor, it will be roughly what the overall height of the shorter later full cast rotor is due to the thinner hat.

The only correct way is to measure from the inside of the hat to get the actual depth that matches the unitbearing.

A composite rotor is made by deep drawing a steel rotor hat like a kitchen pan is made, putting it in a mold and pouring the cast rotor around the flange. A full cast rotor is machined from a one piece casting.

Rollin20z · Sep 27, 2019

mrblaine said:
This is a cast rotor-
View attachment 117484

This is a composite rotor-
View attachment 117485

You can NOT measure the overall height and derive any meaningful measurement to pick a new rotor. The reason for that is the cast rotor has a hat that is about .230ish thick. The composite rotor has a hat that is about .100ish thick. If you measure the overall height of the composite rotor, it will be roughly what the overall height of the shorter later full cast rotor is due to the thinner hat.

The only correct way is to measure from the inside of the hat to get the actual depth that matches the unitbearing.

A composite rotor is made by deep drawing a steel rotor hat like a kitchen pan is made, putting it in a mold and pouring the cast rotor around the flange. A full cast rotor is machined from a one piece casting.

View attachment 117486 View attachment 117484 View attachment 117485 View attachment 117486

Not trying to correct you MrB but is it just me or do both of those rotor pics you have showing measures look exactly the same and show same 2 3/4"(81mm) backspace? Pics showing fronts(hats)? composite versus cast are different bit I think you just inserted that other pic both times.

mrblaine · Sep 27, 2019

Rollin20z said:
Not trying to correct you MrB but is it just me or do both of those rotor pics you have showing measures look exactly the same and show same 2 3/4"(81mm) backspace? Pics showing fronts(hats)? composite versus cast are different bit I think you just inserted that other pic both times.

The post put up the same pictures twice. I fixed it. The measurement pic is to show HOW to do it correctly. I only inserted one of each pic.

Rollin20z · Sep 27, 2019

mrblaine said:
The post put up the same pictures twice. I fixed it. The measurement pic is to show HOW to do it correctly. I only inserted one of each pic.

That's what I figured that why I stated not correcting you. I've don't the same thing trying to see those small pics to insert in page sometimes look the same anymore. Young eyes are a wonderful thing you never pay attention to till yours start getting old and look back on it.

I just thought you were posting pics of each different rotor with measurement showing which was which.

GASnBRASS · Sep 27, 2019

So the million dollar question is......overall which rotor is better? Better at heat dissipation, resisting warpage, wear longevity, resurfacing, etc?

JoshNZ · Sep 27, 2019

R

mrblaine said:
This is a cast rotor-
View attachment 117484

This is a composite rotor-
View attachment 117485

You can NOT measure the overall height and derive any meaningful measurement to pick a new rotor. The reason for that is the cast rotor has a hat that is about .230ish thick. The composite rotor has a hat that is about .100ish thick. If you measure the overall height of the composite rotor, it will be roughly what the overall height of the shorter later full cast rotor is due to the thinner hat.

The only correct way is to measure from the inside of the hat to get the actual depth that matches the unitbearing.

A composite rotor is made by deep drawing a steel rotor hat like a kitchen pan is made, putting it in a mold and pouring the cast rotor around the flange. A full cast rotor is machined from a one piece casting.

View attachment 117486

Thanks Blaine,

Good to see the two side by side from someone who knows.
To me the cast rotor looks more of a “composite” than the composite, that’s what was doing my head in.

Your measurement makes a lot of sense too. To no surprise, a lot of the other internet info gives overall heights but as you say, they’re quite similar for such a weathered part.

mrblaine · Sep 27, 2019

JoshNZ said:
R

Thanks Blaine,

Good to see the two side by side from someone who knows.
To me the cast rotor looks more of a “composite” than the composite, that’s what was doing my head in.

Your measurement makes a lot of sense too. To no surprise, a lot of the other internet info gives overall heights but as you say, they’re quite similar for such a weathered part.

Composite has a very thin steel rotor hat and in this case, means made from more than one piece. The hat is everything that isn't the rotor. The outer edge is rounded like a kitchen pot due to the fact that it is a deep drawn part done in a press.

This is a steel rotor hat

This is a rotor-

Instead of bolting them together, the steel hat is put into the mold and the iron is cast around the flange to attach them together.

It has nothing to do with being weathered. The post 99 unitbearing has the flange moved closer to the knuckle by the difference in thickness between the steel rotor hat and the cast rotor hat to keep the track width the same when they moved from the composite to full cast rotors. If you look at the registration shoulder on the early rotor, it is about .100 or so and the full cast rotor barely registers on it to center the rotor.

JoshNZ · Sep 28, 2019

Cheers @mrblaine, your explanation makes a lot of sense. I would’ve assumed the composite part referred to the hat and rotor as you’ve described, but I was too caught up on what the rotor hat looked like compared to Google...

Either way, the hubs I’m replacing match the new ones I’ve got on the way, so all should be good!

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Cast vs composite, can someone please reassure me? I'm losing it

JoshNZ

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mrblaine

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Rollin20z

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mrblaine

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