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Brake disc questions

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Old 03-13-18, 07:37 PM
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rollagain
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Brake disc questions

(1) Whoever assembled my new bike put the front brake disc on backwards. My question is: What difference does it make?

(2) I'm going to be removing said disc and reinstalling it in its correct orientation (I have the instructions). Question: they don't mention it, but should I loc-tite the disc mounting bolts?
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Old 03-13-18, 09:43 PM
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1. I doubt it makes a huge difference. It might make a difference, however, so I'm wondering how you know that it is the front disc that is reversed and not the rear?

2. You should not loc-tite the disc mounting bolts because they get incredibly hot in use. Ordinary loc-tite is not designed to survive those temperatures. That's why there are six bolts and they have a very fine threading and they are rather long as these things go. Loose disc mounting bolts are not likely to be a serious problem for you. Check on their tightness every now and then and call it good.
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Old 03-13-18, 11:13 PM
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The disks are usually labeled and those 6 bolts often have blue loc-tite on them already. I don't bother to put loc-tite on them myself BUT I've also had the bolts on the rear disk fall out on my rigid mtn bike a ruin my ride and disk so I probably should.
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Old 03-14-18, 12:32 AM
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Originally Posted by Leisesturm View Post
1. I doubt it makes a huge difference. It might make a difference, however, so I'm wondering how you know that it is the front disc that is reversed and not the rear?
I compared it to the drawings in the installation instructions. Also, I was able to find the directional arrow stamped on the rear disc, but can't see the one on the front.

As well, I was in the LBS and talking to one of the people working there about it. He didn't know the reason for the directional preference, but I saw that EVERY bike with discs had them on opposite to the way my front disc is mounted. Big store, too, with a LOT of bikes.
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Old 03-14-18, 01:06 AM
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If the cut out spokes in the rotor are slanted, they should be oriented with the outer end pointing forward. Otherwise its POSSIBLE - but not common - for the rotor to collapse.

To the extent that I’ve thought about it, I’ve assumed it’s about steel in these dimensions being better at resisting tension than compression.

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Old 03-14-18, 07:03 AM
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I put loctite on the bolts.

Shimano rotor bolts come with thread locker from the factory.


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Old 03-14-18, 08:05 AM
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Originally Posted by rollagain View Post
(2) I'm going to be removing said disc and reinstalling it in its correct orientation (I have the instructions). Question: they don't mention it, but should I loc-tite the disc mounting bolts?
I have never added loc-tite to rotor bolts (beyond what they come with), and I have re-used them too many times to count over the past 17 years.

I have never once had a rotor bolt come loose.
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Old 03-14-18, 08:30 AM
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Loctite isn't going to hurt anything and adds only a minute or two to the job.
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Old 03-14-18, 09:31 AM
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Originally Posted by Kapusta View Post
I have never added loc-tite to rotor bolts (beyond what they come with), and I have re-used them too many times to count over the past 17 years.

I have never once had a rotor bolt come loose.
The caliper bolts on my discs came with pre-applied grey loctite (heat resistant) but the rotor bolts do not have any thread locking compound at all. Coincidence? The blue loctite that is the kind you will find if you don't go out of your way to source the heat resistant kind will be a waste of time and money. At best it will not do any harm... or good. FWIW. But you know that. My comments are for the other (approximately) 734,000,000 people that might read this over the next 5 years.
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Old 03-14-18, 09:48 AM
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typically there is a dried spot of some kind of thread compound on the bolts originally..

In the Shop we use a pre set torque wrench.. observe the bolt tightening sequence, in the manual..

[ those who work on auto engines know there is a much greater emphasis on the bolt tightening sequence on cylinder head installations]
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Old 03-14-18, 11:02 AM
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Originally Posted by Kapusta View Post
I have never added loc-tite to rotor bolts (beyond what they come with), and I have re-used them too many times to count over the past 17 years.

I have never once had a rotor bolt come loose.
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Old 03-14-18, 12:15 PM
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Originally Posted by Leisesturm View Post
1. I doubt it makes a huge difference. It might make a difference....
It certainly can. Iíve seen a couple of rotors with folds and creases in the brake track.
But between the spread in rotor design, rider weight and riding style, disaster is not guaranteed.
Originally Posted by Leisesturm View Post

2. You should not loc-tite the disc mounting bolts because they get incredibly hot in use.
No.
The brake track can get seriously hot. The caliper can soak up some decent heat too. But big amounts of heat making it down the thin cut out spokes and heat up the bolts to any important degree, that doesnít happen.
Too little material to carry the heat, too long path for it to be conducted, not enough continuous heat production at the brake track.
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