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I never liked those screws anyway!
June 14, 2012 10:56 AM   Subscribe

I torqued off the heads off of the screws that fix one of my car's rotors to the hub assembly, and I had to reassemble it and leave for work before I could extract the broken remainders and find replacements. Now that I'm thinking about it, it seems like they only serve to hold the rotor in place when the wheel isn't there. Do I need to replace them?
posted by pullayup to Home & Garden (25 answers total)
 
No, they don't do anything useful. I used to leave those damn things out because they were so annoying. Double check your wheel torque after 50 miles and ignore it, is what I'd do.
posted by Brockles at 11:01 AM on June 14, 2012


Voice of reason: what's the cost to you to have the car repaired the way the manufacturer intended? Yes, the likelihood is that the screws may do nothing that compromises the safety of your vehicle, but they also may be important. The bits may come off at speed. Who knows! I think if you're going to do repair on your car's brakes, you might as well do it right. Any money you're saving doing it yourself could be lost if you end up tearing up your rotors or something because the bits and pieces came out.
posted by Geckwoistmeinauto at 11:19 AM on June 14, 2012


Removing the bits is easy, I can do it myself monday-ish. And trust me, they're going nowhere before then. But I would prefer not to replace them, as taking them back out is always the most time-consuming part of replacing a rotor, since they're not proper bolts and and require an impact screwdriver (which often destroys them) to remove.
posted by pullayup at 11:23 AM on June 14, 2012


Yes, the likelihood is that the screws may do nothing that compromises the safety of your vehicle, but they also may be important.

I take it from that you don't know which piece the OP is referring to? It is impossible for them to come off without the wheel falling off. They retain the brake disc to the upright only during the period that the wheel is removed from the hub - when the wheel is on the disc is clamped to the upright and the loading of the wheel nuts/studs is far in excess of anything that small screw is capable of.

They are purely on there to aid assembly (primarily at the factory during automated fitting). They are a non-essential part.
posted by Brockles at 11:27 AM on June 14, 2012 [2 favorites]


This could be a pain when changing a flat tire on the side of the road. Will your rotor fall off without the wheel attached?
posted by twblalock at 11:53 AM on June 14, 2012 [1 favorite]


twblalock, the rotor will not fall off when the wheel is removed because it's still clamped in the caliper.
posted by Daddy-O at 12:25 PM on June 14, 2012


My suspicion is that they serve to reduce vibration and prevent wobble. I'd wager it'll make no difference over a few months but might make a difference over a few years.
posted by phearlez at 1:52 PM on June 14, 2012


My suspicion is that they serve to reduce vibration and prevent wobble.

That is incorrect - when a tire is mounted, the rotor is clamped tightly between the rim and the hub. The screws do nothing to alter the way that this works.
posted by davey_darling at 2:20 PM on June 14, 2012


On my last car the rotor could move when clamped in the caliper but not attached by the screw. It could move enough that it made it difficult to line up the holes for the wheel bolts.

If the OP's rotor doesn't move in that situation, there is no reason to use the screw.
posted by twblalock at 2:24 PM on June 14, 2012


The only cars I've seen this on have been Acuras (it may be a Honda thing). Other cars I've done rotors on, (Subarus, Fords) have not had screws holding the rotor to the hub. But, in all cases the rotors have ultimately been secured the same way, clamped between the wheel and the hub by the lug nuts.
If the screws are still serviceable after I take them out, I may put them back in with a liberal amount of anti-seize. Or, if they were a bugger to get out I'll just leave them out and be happy the job is done.
posted by highway40 at 5:17 PM on June 14, 2012


I also leave them off. They corrode, are hard to remove and I think they only serve to hold the rotor in place during assembly at the factory. I have never had a problem and I have done more than one brake job in my life.
posted by bartonlong at 9:25 PM on June 14, 2012


In the unlikely event your lug nuts get loose-- I've seen wheels wobbling around really badly on other people's cars about five times over the years, I guessed from loose or missing lug nuts, most likely because of failure to tighten or tighten enough, and it amazed me how little noise they made or seemed to affect the ride, the couple of times I saw this while on my bicycle-- and I think in that circumstance those screws could keep the wobbling wheel from causing the rotor to slam into the calipers laterally and damage them or itself, and possibly improve braking performance on that wheel, but only briefly until the screws failed.
posted by jamjam at 10:21 PM on June 14, 2012


Hard to comment without seeing pictures and knowing the details but...

What size and how long are the bolts? What head is on them? What's the specified torques for them and the wheel nuts/bolts?

Bolting something up is not a simple mechanism; bolts can take huge loads and, if not used appropriately, can very quickly fatigue and fail.

I can imagine a couple of scenarios where the rotor bolts are required;
The rotor bolts would ensure that any axial loads generated by the rotor are isolated from the wheel wheel bolts/studs.
Bolting the rotor down separately could ensure that any compression from the wheel studs/nuts occurs in the wheel, rather than the wheel and rotor. (Not entirely correct, but the principle's there).

In this case I'd say that if you dont know what they do, dont mess with them. Although you'd probably be fine; hubs and wheels are overengineered and, after a few years any precision in the mechanism will be lost due to corrosion and general wear and tear, I think that having more rather than less holding your brakes and wheels on is a good thing...

Dont know how things are in the US, but if there were an accident and it was discovered that you were missing fairly major bits from the hub, the insurance and liability could be a real problem.
posted by BadMiker at 4:06 AM on June 15, 2012


I'm pretty sure those screws are there to push the rotor off the hub if it's stuck. You'd actually tighten them, separating the two parts.

I've done a lot of brake jobs. Sometimes you see the screws, sometimes not. Your lug bolts and caliper hold the disk in place until you mount the rim back on.
posted by PSB at 5:04 AM on June 15, 2012


I'm pretty sure those screws are there to push the rotor off the hub if it's stuck.

I've never, in my entire life, seen jacking screws on brake discs. Not once. The threads are in the hub, not the disc.

The rotor bolts would ensure that any axial loads generated by the rotor are isolated from the wheel wheel bolts/studs...

I'm not sure why several people in this thread are commenting if they don't know what the bolts in question are or what they do. It's not an answer if people are 'theorising' based on what they assume the bolt could be or do, that's called 'guessing'.

They are a single countersunk bolt per disc on the same PCD as the wheel studs, with a cross head drive, of either M8 or 5/16 diameter at most, often smaller. There is only one per disc and they are a fraction of a size of the wheel bolts so the idea that they can somehow carry or separate anything like the loading of 4 much larger studs/bolts is ludicrous. The clamping force from such a bolt - especially with a low torque head design) is miniscule by comparison. This clamping load applies directly down the load path of the screw in question and enormously overpowers anything it does while the wheel is on.

Bolting the rotor down separately could ensure that any compression from the wheel studs/nuts occurs in the wheel, rather than the wheel and rotor. (Not entirely correct, but the principle's there).

The principle is not possible - The clamping load applies directly down the load path of the screw in question and enormously overpowers anything it does while the wheel is on. In addition, you can't limit clamping load to within one component in a chain. Something ... something... opposite reactions equalling forces? Pressure equalising? That sort of thing.

I think in that circumstance those screws could keep the wobbling wheel from causing the rotor to slam into the calipers laterally and damage them or itself

Not a chance. It is a single small bolt that doesn't haven anything like the load capacity required to stabilise anything occurring from a wheel coming loose. Especially as it is one off set bolt. Additionally, there would be nothing causing the rotor to move from side to side in that example - the wheel can produce no real 'pull' on the rotor, only a push onto the hub so the wobble would mostly end with the wheel itself and would damage the rotor or calliper when it finally comes off by the rotor landing on it or something.
posted by Brockles at 5:35 AM on June 15, 2012


I've seen jacking threads on rotors Brockles and it's Newton's third law: every force has an equal and opposite reaction. My 'principle' was completely wrong but, depending on the size of the bolts, they could serve to isolate the wheel bolts from brake loads.

"I'm not sure why several people in this thread are commenting if they don't know what the bolts in question are or what they do. It's not an answer if people are 'theorising' based on what they assume the bolt could be or do, that's called 'guessing'. "

OP has not confirmed what bolts he's talking about here. You're just theorising too, friend.
posted by BadMiker at 6:27 AM on June 15, 2012


it's Newton's third law: every force has an equal and opposite reaction.

I know. I was being a touch flippant.

depending on the size of the bolts, they could serve to isolate the wheel bolts from brake loads.

It is not a system that is used on any track or road vehicle I have seen. The only time I have seen any such device it is a plain wheel drive peg (no threads but a plain shank style) that is bolted to the hub and just pokes into the wheel. It makes no sense to try and separate those loads with more threads when there are perfectly good ones already holding the wheel on, especially as the loads are taken up by the clamp force against the hub and the chamfer on the wheel nut rather than the threads themselves.

OP has not confirmed what bolts he's talking about here.

He said the bolt that needs an impact driver to get off to remove the rotor. Brakes are not varied in their mounting style - the only things that go through rotors on the hub face are the wheel studs/nuts or the countersunk retaining screw. That is why I am not guessing or I wouldn't have made my first answer so decisive being as the brakes are such a critical system.
posted by Brockles at 6:52 AM on June 15, 2012


depending on the size of the bolts, they could serve to isolate the wheel bolts from brake loads.

I forgot to add this bit - it is impossible to isolate the wheel studs from the braking loads unless the part doing so is connected to the wheel and the disc/hub. The brake loading is reacted by the wheel (through the tyre) so any bolt through the disc to the hub isn't isolating anything as the path must go from the disc, through the hub to the wheel.
posted by Brockles at 7:29 AM on June 15, 2012


They are a single countersunk bolt per disc on the same PCD as the wheel studs, with a cross head drive, of either M8 or 5/16 diameter at most, often smaller. There is only one per disc and they are a fraction of a size of the wheel bolts

I looked over a variety of rotors using Google images before I made my comment last night.

I found a range of numbers of screw holes in the rotors I saw: some with none, such as the first and third Audi rotors on this page, which also shows Audi rotors with two and three screw holes, and I also saw some with one, such as another Audi and a Mercedes I'm not linking.

I'm pretty sure those screws are there to push the rotor off the hub if it's stuck.

I've never, in my entire life, seen jacking screws on brake discs. Not once. The threads are in the hub, not the disc.


Of particular interest however, are the last Audi rotors on the linked page, which has two countersunk screw holes, and two other small holes of like diameter which look to me as if they are threaded, but I haven't searched to confirm that.

But in any case, rotors with screw holes and two extra threaded holes seem fairly common, and the purpose of those threaded holes is, as PSB indicates, to jack the rotor off the hub:
AutomotiveHelper.com Discussions and Help > Toyota > Camry/Camry Hybrid/Vista > Disc brake rotor removal

rum runner
04-01-2005, 08:13 AM
1996 Camry 2.2 L
I thought I would replace my front brake pads last weekend. I went with Toyota parts ($40 front brake pads). The discs are not heavily grooved but need turning.

I get to a point where the rotors should come off.....but rust seems to have them nice and snug and won't let go. Before I "heat and beat" I wanted input from anyone who has done brake work on a Camry. Are there rotor pullers available? I noticed the rotor has two threaded holes 180 degrees apart on the hub that could only be for rotor removal.

Any help is appreciated.

Brian R.
04-01-2005, 09:54 AM
Insert a couple of bolts into the holes, snug them down, and then gradually tighten them evenly. The bolts will push the rotor off.

rum runner
04-01-2005, 02:23 PM
wow, that was simple.............ould have never thought of that. It looked like there was another object threaded down the hole. That must be part of the hub? Thanks for the tip.

While I got you on the line, got any idea what size bolt it is? I have to drive the car to the store for the part, so it would be a guessing game.

Brian R.
04-01-2005, 08:59 PM
Call a Toyota parts department and ask them. Sorry, I have no idea. ...

rum runner
04-02-2005, 09:56 AM
Bolt size

Hex head bolt 8mm-1.25 x 30mm long or longer
And it would be a strange design decision, I think, to put screws in the countersunk holes which could not be used in the threaded holes to push the rotor off the hub.
posted by jamjam at 7:52 PM on June 15, 2012


Yeah, I made myself look kind of stupid there by getting completely lost halfway through the post by getting distracted by the non-answers.

I meant to say "I've never seen jacking screws on the brake discs that stay on there". As in the screws don't stay in the disc so they won't be there when you take the wheel off so those can't be the screws we are talking about or they'd not need removing to take the disc off. I can totally, with marvellous hindsight, see what a dumb comment that is otherwise. Arses. My bad.
posted by Brockles at 8:34 PM on June 15, 2012


And it would be a strange design decision, I think, to put screws in the countersunk holes which could not be used in the threaded holes to push the rotor off the hub.

Having said all that above, I'd be very surprised if the disc retaining bolts were long enough. The threaded section of the bolt is probably only about as thick as the disc bolt face itself (maybe 10mm, 3/8"? 1/2" at most?). Also, they're cross head csk bolts so you can't get too much torque on them to jack with.
posted by Brockles at 8:37 PM on June 15, 2012


I think you're right.

The only retaining bolts I could find pictures of were significantly shorter than half an inch.
posted by jamjam at 9:13 PM on June 15, 2012


On cars like audis, BMW, and volkswagen (and probably more) that use wheel bolts, not studs to fasten the wheel on some type of retaining screw makes a lot more sense and is probably useful as lining up the wheel and the rotor is a total pain in the ass to start the bolt.

As to using the screw to help remove a rusted on disc has anyone not had a hammer work? that really is a case where a bigger hammer is the correct answer. Those discs are heavy cast iron and really tough.
posted by bartonlong at 9:32 PM on June 15, 2012


Without seeing a pic, most likely it is a retaining screw.... but Brockles, I do a lot of brakes and I see jack screws (or threaded holes for jackscrews) all the time. I can't believe you've never seen them. And, you're kind of a dick.
posted by PSB at 8:10 AM on June 18, 2012


I can't believe you've never seen them.

Did you read my clarification? I posted a badly worded comment and reworded to show that I actually meant that I've not seen any that remain in the disc (ie would already be there when you take the wheel off to remove the disc). You see the holes, but you don't see the screws themselves.

you're kind of a dick.

Possibly. Not sure that is relevant, though.
posted by Brockles at 8:29 AM on June 18, 2012


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