atomic_fungus (atomic_fungus) wrote,

#1319: Front brakes on the Jeep.

So this morning I finally got around to putting the Jeep up on jack stands and pulling the front wheels off. This marks the first time I've had the front wheels off the thing at all. And this is the sight that greeted me:

Notice the color of the wear areas of the rotors? They're freaking black. I have never seen rotors look like that in all my 23 years of driveway wrenching.

The wear areas of brake rotors are normally shiny. The entire point is friction, so anything rough on the surface of the rotor is worn down by the brake pads. Over time, the surface has concentric grooves worn into it, because the wear is not uniform (because the grain of the metal isn't uniform, for one) but it always has a shiny appearance.

This is nuts. For all of me, it looks almost like black powder coating. And when I tried to sand down to bare metal, this stuff didn't mind 80 grit at all. (The rotor on the L has a couple of visible dull patches where I tried sanding through the stuff. It dulled the surface but didn't reach bare metal.)

The rotors had apparently never been off the truck, so I had to use heat and the BFH* to persuade them off. In the process I knocked one of the wheel lugs out of the left side hub, with one hit. Dang. (Any bets on how easy it would be to knock one of those damn things out if I wanted to?)

I took the rotors to Murray's to get 'em turned. As usual, the brake lathe at Murray's was "down". (It has never been "up", as far as I know, in all the time the store has been there.) So I went down the street a bit further to Acme Electric/Lang's Auto; and the guy at the counter there told me I could have these rotors turned for $25 apiece, or I could buy new ones...for $25 apiece. Yeah, that's a no-brainer.

I got home and merrily set about reassembly; and found that the rotors did not fit: they rubbed against the dust shield. Argh etc. But I couldn't figure out why--the damn things were the exact same dimensions. On a hunch I grabbed a vernier caliper and checked the thickness of the mounting flange and--sure enough--it was 1/4" on the new ones, and 5/16" on the old ones.

A mere 1/16 inch of clearance seperates the rotor from the dust shield? That's cool but it meant I had the wrong part--so, back to Lang's; I exchanged them for the correct part. (Their system gave them two part numbers; the guy took a guess. But I'd compared the rotors, too, and they looked fine to me.) I took the vernier caliper with me and checked the flange on the exchanged part, and it was a match (just a hair shy of 5/16") so I merrily returned home and reassembled the Jeep. (Score one for having the correct tool.)

Reassembly went smoothly after that. I pressed the loose lug back in with a socket and lug nut. Sucked the old brake fluid out of the reservoir, collapsed the caliper pistons, blahdy-blahdy-blah, rotated the tires and torqued all the lug nuts to 80 ft-lbs. Filled the reservoir with fresh brake fluid. (It's not as good as a complete fluid change, but it's better than leaving all the old stuff in there.)

The brakes now work a lot better than they did, which is not surprising. I'm thinking these rotors have glazed up. Someone did something stupid with this truck at some point and overheated the brakes, which led to brake material forming a glassy, porcelain-like coating on the rotors. The stuff could have been machined off with a brake lathe but I wasn't about to try to cut through it with sandpaper; and since I have new rotors on the truck now, the old ones are at the foot of the driveway for some enterprising junk collector to recycle.

When I did the rear brakes on the Jeep earlier this year, the braking performance of the truck decreased slightly. I had chalked it up to the rear self-adjusters not doing their jobs and lived with the increased pedal effort. Now, however, having actually seen the state of the front brakes, I understand what was going on. The old rear brakes had been working harder to make up for the decreased efficiency of the fronts; they'd had a weird "channel" through the middle of the pad material which meant greater pressure on a smaller contact patch, thus making the rear brakes work better. When I switched to new pads without that "channel", the rears stopped making up for the fronts and the thing's braking performance suffered. (Not nearly enough to be dangerous; I'm not stupid. Just enough that I had to step on the brakes a little harder. Heck, I drove the thing for 9 months like this and it was perfectly fine.)

But with the new brakes up front I noticed an immediate and vast improvement in the braking performance, even before the pads had bedded in--so the brakes work the way they ought to, now, and hopefully that'll continue.

End total, about $80, and I feel like I actually accomplished something.

I took it for a shakedown drive, and noticed a shimmy over 50 MPH. Next task: get the tires balanced. *sigh*

*BFH: do I really have to explain this again? BIG F***ING HAMMER :D

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