When do front discs need to be machined?

It's less rocket science and more "are the surfaces grungy, or are they not flat? has the brake pedal been pulsing?"

The repair manual for an old Ford I had took pains to discourage the lathe if there was only the amount of surface glazing or light rust you could take off easily with sandpaper by hand on a flat block.

If the pedal has been pulsing, it might be warped, or you can check for that with a dial indicator.

If you end up thinking of using a lathe, a micrometer check is worthwhile to find out if the rotor has enough metal left to be worth machining.

If you just want everything shiny and new, you can just put new ones on, and be all set until it rains.

 

I wouldn't do anything, unless:

1. the rust is starting to extend into the pad contact zone. It's normal for there to be rust at the outer and inner edges, beyond the contact area.

2. Rotor thickness or runout varies beyond spec (see attachment).

For #1, as long as the thickness and runout are ok, I think you can just go over with an abrasive disk on a drill. There are pads made expressly for this IIRC, and they're kinda like those green bristly plastic scouring pads? 3M makes a system, for one. Probably best to take the rotor off to do this. And you want to keep moving around, keep it uniform.

FWIW, I've never seen the need to "dress" a rotor, on the maybe 1/2 dozen cars we've owned over the years.

 

Run your fingernail across the disc from in to out, if your fingernail hangs, the disc needs to be turned or replaced.

Also, you can mount a dial gauge so that the tip touches the disc and rotate it to see the amount of runout or warppage.

 

In my not so humble opinion, most modern vehicles do NOT have rotors that are thick enough to allow for safe and productive "truning".
If they have anything wrong with them, they need to be replaced.

 

If that's a trend, Prius rotors seem to have bucked it, at least from Gen 2 to Gen 3. When I wrote this post a couple weeks ago, it was about Gen 2, where the rotors are born 22 mm across and they go to pasture at 20 mm.

A Gen 3 front rotor is born with 25 mm thickness and goes to pasture at 22 mm, so it begins life with 3 mm of expendable meat to the previous 2 mm ... a 50% increase.

If you make a 0.15 mm cut on each side, that's 0.3 mm off the rotor in total. If you do two such passes, 0.6 mm off.

A machine shop might decline to turn it if there's less than about 0.8 mm remaining, so at least it could have a couple machining passes and still some margin remaining for wear. Given a Gen 3 front rotor is born with 3 mm to go, machining can be practical as long as it is not too worn or too grooved by the time the job is considered.

 

Nowadays new discs are so affordable that having old discs machined hardly yield any savings. There is of course the eco thing but i am not sure how big the difference because metals are so well recycled. One interesting aspect is the lower weight of the old machined disc. Less mass to spin, better MPG

 

with so much energy returned by the regenerative forces, it's inexplicable how someone could mess up a pair of discs. Many a Ppads will go for over 100k miles & still have life on the original pads. How does anyone get down to the rivets ... hardcore breaking?

 

Depending on the environment and usage, rotors can get badly rusted, long before you would need them because of pad wear.

 

yes indeed. I was told at 140K that it’s time replace discs soon. Not sure if discs are original. Car is almost exactly 11years old and been driven in wet and salty roads quite a bit.

one of my others is a Volvo SUV and was just told thar rear need brakejob. But those discs were replaced just 35K (2.5years) ago! I’ve been told that extensive use of adaptive cruise make them wear out quickly. I say poor design

 

I say that story is pure BS.
If you insist on using adaptive in bumper to bumper slow traffic maybe.

Automatically believing everything that a service shop tells you WILL make your wallet empty out quicker though.

 

Yeah, take some of that money (~$50), and cut through the BS: invest in a dial indicator (with magnetic base) and 0-25 mm micrometer, check against the Repair Manual specs (see post #3f)

 

Due to how the Prius uses regenerative braking, I don't see this as a necessity under normal driving conditions. However, if your pins get frozen or you do drive in salt road areas, this may need to be done.

 

The surface of your rotors are important for disc thickness variation. You really only need to resurface them if you feel brake shudder (a vibration or pulsing when braking). Some vehicles are more sensitive to disc thickness variation (DTV) than others so there is no uniform agreed value. The only other thing you need to check is simply thickness. If too much is worn away you need new rotors (min value is often on the rotor if not check a repair manual or ask here). If not and you are not getting shudder you can just change the pads. If shudder develops then you can think about resurfacing them or replacing them. Resurfacing and replacing are recommended as preventative measures for the most part, but if you don't have shudder you don't really have to do it.

The other issue with rotors is runout (essentially warping). Most of the time warping is not a big deal in its own but can lead to DTV over time. What happens is the high spots of the warping hits the pads in off brake situations (highway driving). That wears down spots in the rotors leading to DTV and shudder. Once again if you don't have shudder you probably don't have warped rotors. Just make sure you use a star pattern when tightening and torquing the lugnuts.

 

what’s DTV?

 

He explained that earlier in his post !!

 

Found it. ​

 

Turning them will probably get them hot, assuming the level of care and experience of most brake shops.

Blanchard grinding with flood coolant would be a much better idea.

Personally, I haven't had the need in the last 40+ years of doing driveway brakes.

 

It was not service shop telling me this.. They just inspected and delivered the message. They don’t speculate why wearing parts wear.

But anyway, there is obvious difference between the latest one with adaptive cruise and previous that had normal cruise. Same brand, same platform and rear brakes simply wear much faster with adaptive. Others have noticed the same and some even quit using it in certain driving conditions. Be it reason for brake wear or not i use it all the time. I think one of the best features in modern cars

 

New discs are so cheap that i rather buy new ones in good time and enjoy good brakes. Here discs rust..a lot