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Discussion Starter · #1 ·
I saw a thread about this awhile ago, but failed to find it. I've been
curious about this myself, and because I'm an engineering student
I'd like to offer my analysis to the community.

We start with Hayes operating specifications (the only ones
I could find), which should be similar for all hydraulic brakes.
_____________________________________________________
Hayes Mountain Bike Disc Brake Operating Specs.
I. Torque Range: 0-230 ft-lbs
II. Clamping Force: 0-1200 lbs
III. Operating Temperatures:

El Camino, Mag, MX: -20ºF - 120ºF

HFX Nine & Sole: 0ºF - 120ºF
IV. Hydraulic Pressures

Mag, Nine, Sole: 0-1700 psi
_____________________________________________________

First, we must find the force on one bolt. Because
all bolts are at the same distance from the center
we can simplify things.

From figure, F=7060N

We use this load value to find the average shear stress
on one bolt with full braking torque applied.

Tau(1-bolt) = 411MPa

Of course we have six bolts normally, and the stress
is divided evenly between them all.

Tau(6-bolt) = 68.5MPa
Tau(3-bolt) = 137MPa

Next we will find the static safety factor. This
will represent the likelihood of failure under constant
deceleration with maximum braking torque applied.
We are using the maximum tensile load for a low carbon
steel bolt (Sy=400MPa), which is not very high strength.
See Figure.

FoS(1-bolt) = 0.973
Fos(3-bolt) = 2.92
Fos(6-bolt) = 5.84

So, most engineers design for a safety factor greater
than 2, so with low carbon steel bolts when we are running
3 of them we are nearing our ideal safety margin,
although this is still pretty safe for a weight weeny.

Now, we will compare with some lighter bolts. Obviously,
I am assuming that you are going to shell out for some
nice bolts. Data from Racebolts, and Matweb, although
I had to guess what Titanium they are using as they
do not specify on Racebolts.com.

__________________________________
7075-T6 (Sy=462MPa)

FoS(1-bolt) = 1.12
Fos(3-bolt) = 3.37
Fos(6-bolt) = 6.74

__________________________________
Titanium 3al-2.5v (Sy=496MPa)

FoS(1-bolt) = 1.21
Fos(3-bolt) = 3.62
Fos(6-bolt) = 7.24


Conclusions

It looks to be perfectly safe to run 3-bolts on a disc brake
rotor with all common material types. Personally, I would
recommend the titanium bolts instead of aluminum, mainly because
aluminum has no endurance limit and will not be as safe
under fatigue loading. Also, it is very easy to strip the
aluminum bolts.

Cheers!
 

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Single Speed Junkie
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3,116 Posts
Sure I am sure that 3 bolts would work fine. However if one is going to snap would you still maintain any structural integrity that the 3 bolts gave you?

With 6 if you loose one or two not a big issue, loosing 1 out of three I'm uncertain if it would work as well.
 

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Doesntplaywellwithmorons!
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Yes three works fine.... Amp discs were ALWAYS 3-bolt rotors, and Rockshox having bought the design continued that, even for DH riding (the DH size front disc rotor was about 188mm and supported by just three bolts). Historically if you look at the most successful disc brake models prior to the 6-bolt ISO mount pattern being adopted, the only choices were 3-bolts like Amp used, four or five bolts (Coda and Hope), thread-on (1970s vintage Shimano Oil-Lite hydraulic discs, and early Pro-Stop discs), or splined with a lockring (later Pro-Stops).
 

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ballbuster
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12,718 Posts
I would say...

... if they were designed to be 3 bolt, then yes, they are safe.

Other than that, I say why bother shorting 3 bolts on a six bolt ISO design? You're saving... what... 6 grams? And... you could argue that it is rotating weight, but rotating very close to the axle, so it barely qualifies as rotating weight.

I don't think the bolts are in danger of shearing off, but I think it will be harder to keep the rotor straight, and there is less safety margain if a bolt decides to back out. Also, the stress is more concentrated on the hub bolt holes.

I dunno, I wouldn't do it. The upsides aren't really significant, and the downsides can be catastrophic.
 

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FIRENZE rulez !!
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3,875 Posts
i currently use 3 bolts

i changed in february my set up from 6 to 3 for a wheel

i used and using :
SLR Mavic and now ZTR7000 wheelset ( all for Lefty fork)

Notube's 160 mm coated and now Scrub 160 mm rotors

Magura SL Red brakes

no issues , same bolts mounted 4 months ago , cheked every 20 days or more

bolts are NO Brand in gold Titanium ( grade 5 , not sure )

i had some issues on my bike , but until now no depending from bolts
 

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The calculation is kind of oversimplified. You totally disregarded the friction between the disc and the hub. There is no actual shear stress on the bolts.
How do you know the torque? Dynamic considerations (like vibrations) and thermal stresses are a very important factor.
Just my 2cents.
 

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1,135 Posts
I used three with super light steel alligator rotors and they warped a little I simply tapped them back into alignment. I think what I should have done was use 5 of them till the rotors had set up/broken in.
Anyway I went back to 5 but will switch to Ti when I have the time.
 

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TranceX Rider
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948 Posts
pimpbot said:
I dunno, I wouldn't do it. The upsides aren't really significant, and the downsides can be catastrophic.
+1. nuff said! That is if I'd be using six bolt hub wheelset. I might since I like that idea of running such uber light rotors by singltraker and crew of scrubcomponents! Well, only if they'd fabricate a CL version! :D
 

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The MTB Lab
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2,556 Posts
I use 3 steel and 3 Ti for each rotor (9.1g total). The torx heads are harder to torque down and rotors do move around a bit even with 6 bolts and I found they moved even more with a 3 bolt setup. There is a bit of slop between the bolts holes and the M5 rotor bolts that cause this issue. Now they do tend to move in the direction that they brake pad and rotors torque them towards. If you change or play with rotors much it doesn't take long before the Ti torx heads get stripped so I like to have the steel ones cause they can be torques down with a lot more force. I ran 3 bolts for about a month and just decided it wasn't worth the weight savings. But to each their own!
 

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Discussion Starter · #15 ·
karstb said:
The calculation is kind of oversimplified. You totally disregarded the friction between the disc and the hub. There is no actual shear stress on the bolts.
How do you know the torque? Dynamic considerations (like vibrations) and thermal stresses are a very important factor.
Just my 2cents.
Your statement doesn't make much sense to me. What kind of friction between the disc and the hub are you talking about? Obviously, there's no sliding friction. The only friction I can think of is due to the force of the bolts that are clamping them together, which obviously is dependent on the presence of the bolts. If the bolts shear off, then there is no friction anymore.

The torque was specified by Hayes so it made it a lot easier than estimating hand grip loads and trying to calculate the mechanical advantage provided by the hydraulics. Yes, it could be a lot more detailed but then I probably wouldn't do it for free. :)

Actually, the bolts are loaded in pure shear, I don't see how they would be loaded in any other way, especially in this application. This is because the braking torque is exactly in-line with the rotor bolt holes.

Dynamic loads are very important, but that would require a simulation done in CATIA or something like that and is really hard to quantify with a back of the envelope analysis. Technically, I suppose you can be at peak torque for a fraction of a second before the tire breaks loose, and then the torque would fall off dramatically. It is not a huge deal because I already assumed that you are braking at peak torque. In fact, you probably get a higher safety factor with a dynamic analysis because most people throttle the brake lever instead of just gripping it like an ape.

I think the thermal stresses are pretty negligible since steel doesn't conduct heat that well, but yes they do have some influence on fatigue strength, which is why I would recommend the use of titanium to aluminum.

Also, some people mentioned the possibility of one bolt shearing while the others then take the brunt of the load. This situation is highly unlikely because the load is being distributed evenly between them and unless the rotor has some sort of defect it is unlikely to put stress on one bolt more than another. Unless I saw this type of failure in the field, I would ignore the possibility.
 

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Discussion Starter · #16 ·
Mattias_Hellöre said:
So there´s roughly 3x of safety factor?

How about bolts on stems, it´s more interesting..
Yep, 3x safety factor.

I could do more bolt analysis, but they get increasingly complicated. The disc brake situation is really easy because the bolts are taking the load directly. With a stem, the bolts provide the clamping force, but the stem loads the steertube. The bolts don't actually need to carry large loads they just need to provide the necessary clamping force to keep the stem from rotating or loosening.

Something like the faceplate for a stem would be fairly easy to do for static strength, but that's not what really matters, I'm sure you'd rather know what the dynamic strength is. Obviously, in a crash or sudden deceleration (hitting a curb) is where the highest loads
would be, although I think you would bend a rim before you take a faceplate off.
 

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i can see it working, obviously dont do something stupid like line the three in a line but in a triangle form it should be ok i think.

if you lost one bolt i think you would run into HUGE problems. id try it in the stand first, then at slow speeds with weight on it, also while testing id lock the back break and slide down a hill

let us know how you go
 
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