# What do you think of ano Aluminum brake rotor bolts?



## Mountain Cycle Shawn (Jan 19, 2004)

Red ones! Good idea or bad idea? Not the color of course, but the fact that they are Al as opposed to steel.


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## facelessfools (Aug 30, 2008)

plenty of people use ti ones. when i get my new wheelset ill be running blue Al and raw Ti 3 of each per hub.

i can weigh them to give you something to go by?


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## Mountain Cycle Shawn (Jan 19, 2004)

Not to worried about the weight, as I know they will be lighter. But I do like the red Al bolts. So I am doing it for weight and looks.


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## facelessfools (Aug 30, 2008)

http://torontocycles.com/

he can get you anodized ti bolts..


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## Mountain Cycle Shawn (Jan 19, 2004)

facelessfools said:


> http://torontocycles.com/
> 
> he can get you anodized ti bolts..


That's funny. I just got an 11 tooth and a 12 tooth red Al pulleys from them today. They look and work really nice. And I was just looking at their Ebay store for some other red Al parts.


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## m85476585 (Jun 7, 2007)

Bad idea. Aluminum is very soft and easy to shear, and brakes are not something you want to compromise on. At the very least you would probably need to tighten them to less torque than the spec.

It should be relatively easy to calculate how much shear force is on each rotor bolt under heavy braking. Lets assume you can stop from 22mph (10m/s) in 1 second (~15 feet stopping distance). That's 10m/s^2 of acceleration (about 1 'g'). Assuming you and the bike weigh 70kg (155 lbs), that's 700 newtons of force. Assuming that's all on the front brake. Now assume that the rotor has 6 holes drilled on a 45mm diameter circle (best I could estimate from pictures without measuring the rotor on my bike) and that the wheel is 685mm (27 inches) diameter with the tire. The force on the wheel is 700 N, which is a torque of about 240 N-m. 240 N-m applied on a 22.5mm radius lever is 10,655 N, or 1065 kg of shear force. Divided by 6 bolts, that's about 178kg of shear force on each bolt. These are M5 bolts, and I can't find much info on shear strength or really how to figure it out, so let's assume the bolts just have a 5mm diameter. I get about 1000kg shear strength each for a high grade steel bolt (tensile strength 116,000 PSI). For an aluminum bolt (37,000 PSI) I get about 300 kg shear strength (there's a factor of about .6 to covert tensile to shear strength). Actual shear strengths will be lower because the effective diameter of the bolt is not 5mm because of the threads. That's a safety factor of about 1.7 for aluminum and about 5.6 for steel. Titanium bolts have a safety factor of about 2.3. Given that every step of my calculations involved approximations or assumptions, you probably don't know the tensile strength rating of whatever bolts, there will be variations in the material, and that I didn't account for things like bolt torque pre-stressing them, *I would NOT use aluminum rotor bolts.* Is risking your life worth saving ~8 grams per set of bolts?

It is very likely that I made a mistake somewhere in the calculations, but I hope they are at least somewhat correct.

Edit: made a few minor changes.


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## Mountain Cycle Shawn (Jan 19, 2004)

m85476585 said:


> Bad idea. Aluminum is very soft and easy to shear, and brakes are not something you want to compromise on. At the very least you would probably need to tighten them to less torque than the spec.
> 
> It should be relatively easy to calculate how much shear force is on each rotor bolt under heavy braking. Lets assume you can stop from 22mph (10m/s) in 1 second (~15 feet stopping distance). That's 10m/s^2 of acceleration (about 1 'g'). Assuming you and the bike weigh 70kg (155 lbs), that's 700 newtons of force. Assuming that's all on the front brake. Now assume that the rotor has 6 holes drilled on a 45mm diameter circle (best I could estimate from pictures without measuring the rotor on my bike) and that the wheel is 685mm (27 inches) diameter with the tire. The force on the wheel is 700 N, which is a torque of about 240 N-m. 240 N-m applied on a 22.5mm radius lever is 10,655 N, or 1065 kg of shear force. Divided by 6 bolts, that's about 178kg of shear force on each bolt. These are M5 bolts, and I can't find much info on shear strength or really how to figure it out, so let's assume the bolts just have a 5mm diameter. I get about 1000kg shear strength each for a high grade steel bolt (tensile strength 116,000 PSI). For an aluminum bolt (37,000 PSI) I get about 300 kg shear strength. Actual shear strengths will be lower because the effective diameter of the bolt is not 5mm because of the threads. That's a safety factor of about 1.7 for aluminum and about 5.6 for steel. Given that every step of my calculations involved approximation, you probably don't know the tensile strength rating of whatever bolts, there will be variations in the material, and that I didn't account for things like bolt torque pre-stressing them, I would NOT use aluminum rotor bolts. In fact, anyone selling them for this purpose deserves to be sued. Is risking your life worth saving ~8 grams per set of bolts?
> 
> It is very likely that I made a mistake somewhere in the calculations, but I hope they are at least somewhat correct.


Well, I came up with about the same #'s.  And that's why I was asking, I had a feeling it might not be safe. Thanks for all that work!


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## m85476585 (Jun 7, 2007)

Mountain Cycle Shawn said:


> Well, I came up with about the same #'s.  And that's why I was asking, I had a feeling it might not be safe. Thanks for all that work!


Good to know I'm not crazy. Of course the only way to be sure would be either doing the calculations without approximation (use the shortest stopping time, weakest bolt spec, and actual shear strength) or doing testing, but even then you have to decide how much safety factor you need. With enough work you could figure out the probability of failure vs. safety factor. No matter what, aluminum will be significantly less safe than steel since it has about 1/3 of the strength, and whether or not that's safe enough is not certain. In my opinion (based on my numbers) aluminum bolts are not safe enough and I would not use them on my bike.


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## Mountain Cycle Shawn (Jan 19, 2004)

m85476585 said:


> Good to know I'm not crazy. Of course the only way to be sure would be either doing the calculations without approximation (use the shortest stopping time, weakest bolt spec, and actual shear strength) or doing testing, but even then you have to decide how much safety factor you need. With enough work you could figure out the probability of failure vs. safety factor. No matter what, aluminum will be significantly less safe than steel since it has about 1/3 of the strength, and whether or not that's safe enough is not certain. In my opinion (based on my numbers) aluminum bolts are not safe enough and I would not use them on my bike.


I just thought of something. We didn't take into account Al's poor fatique life. What could be perfectly safe today, could be a disaster a year down the road.


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## Ericmopar (Aug 23, 2003)

Not a good idea Shawn. Aluminum is not good in a shear load application. 

Eric.


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## COLINx86 (Apr 8, 2009)

Why not 3 ti bolts per rotor? just make sure to use plenty of loctite!!


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## whybotherme (Sep 12, 2008)

agreed... aluminum rotor bolts are less than ideal.

to use them properly you would want short aluminum shoulder bolts that allowed the shoulder to fit into a socket in the hub. would be a pretty sweet setup IMO. this would eliminate shear stress on the threaded portion of the bolt.


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## AlexRandall (Apr 2, 2009)

COLINx86 said:


> Why not 3 ti bolts per rotor? just make sure to use plenty of loctite!!


That would work out about the same a 6 AL bolts I'd say.


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## COLINx86 (Apr 8, 2009)

AlexRandall said:


> That would work out about the same a 6 AL bolts I'd say.


No it actually works, and people actually ride like that.









There's Steve Peats bike with 3 ti rotor bolts, I'm sure he didn't use the brakes a whole lot, but still probably used them more than _most_ people on here will. MattKock's bike also has just 3 ti rotor bolts.


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## far twiggle (Nov 29, 2006)

Got my red ti here:

http://www.imperialbikeco.com/products/Anodized-Titanium-Torx-Rotor-Bolts-.html

(Not affiliated with them). Red isn't quite as dark as Toronto's though.


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## Mountain Cycle Shawn (Jan 19, 2004)

Ericmopar said:


> Not a good idea Shawn. Aluminum is not good in a shear load application.
> 
> Eric.


Thanks eric and everyone else! I'll do Ti instead.


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## Mountain Cycle Shawn (Jan 19, 2004)

far twiggle said:


> Got my red ti here:
> 
> http://www.imperialbikeco.com/products/Anodized-Titanium-Torx-Rotor-Bolts-.html
> 
> (Not affiliated with them). Red isn't quite as dark as Toronto's though.


Thanks, I'll check it out. The Ti stuff usually isn't as dark.


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## [email protected]go-ride.com (Oct 1, 2005)

Interesting thread.

I've used 3 ti rotor bolts for 4 years with 180 rotors, i do loctite them and check them. I've seen a lot of people do it and haven't seen any failures. Maybe it's just dumb since it's such a small wieght savings. If one bolt does come loose it going to be bad! I do know that brake mfg's don't like it since it can cause the rotors to warp. I wouldn't do it on a Formula R1 rotor, since the fins aren't attached to each other by the hub.

I wonder if you could even get alu rotor bolts tight enough before they broke. The rotors seem to flaten out against the hub when tightened. Alu 8mm bolts are used on some suspension frames like Pivot.

The only alu bolt i have is for the headset preload. I use Ti everywhere else. I'm sure mounting the brake levers and shifters would be good for alu too.


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## m85476585 (Jun 7, 2007)

[email protected] said:


> Interesting thread.
> 
> I've used 3 ti rotor bolts for 4 years with 180 rotors, i do loctite them and check them. I've seen a lot of people do it and haven't seen any failures. Maybe it's just dumb since it's such a small wieght savings. If one bolt does come loose it going to be bad! I do know that brake mfg's don't like it since it can cause the rotors to warp. I wouldn't do it on a Formula R1 rotor, since the fins aren't attached to each other by the hub.
> 
> ...


It doesn't matter what size the rotors are as long as the brakes are able to generate braking force that is limited by something else (either the wheel losing traction or the back wheel lifting up) since the torque is applied at the radius of rotor bolts, which is standard.

Based on my calculations, the bolts shearing clean off is plausible, in which case it doesn't matter if you use loctite or not. Having the bolts correctly torqued _might_ provide some friction between the surface of the rotor and the hub that also takes some of the braking force, but I wouldn't count on it. Someone could make a hub with aluminum pegs that fit into three of the rotor holes and uses regular screws for the other three, but that wouldn't really save much weight and would still be compromising on strength, which is probably not something the companies making these parts want to do.

On the back brake, skimping on bolts is probably fine- if it breaks you still have the front brake and you probably won't crash, but if the front brake breaks I'd think you would be much worse off. In addition, the back brake takes much less force, since the back wheel will usually skid.


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## jadis3 (Nov 18, 2007)

I am always wondering when people those took off three small M5x10 ti bolts from a rotor still use big M6x15 steel bolts around !



COLINx86 said:


> No it actually works, and people actually ride like that.
> 
> 
> 
> ...


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## COLINx86 (Apr 8, 2009)

I'd be willing to bet those are Ti. If I remember right, Avid's Ti bolts are round on the top also. Plus the steel (avid) bolts are usually black.


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## solo-x (Feb 16, 2010)

The other thread brought this one back to the top.

We have a great bit of math covering the shear mode, but shear only factors in when the fastener isn't torqued. If shear were the mode, then we would see ovaling of the holes in our brake rotors that utilize aluminum center sections. But we don't. 

When you bolt a wheel to a car, the shear strength of the studs isn't what keeps the wheel on. It's the clamping force from the lug nuts applied to hub and wheel interface. That force multiplied by the coefficient of friction for that interface ends up being a significantly greater force than the 5 stud's shear strength.

The same mechanism is at play when you bolt your brake rotors to your hubs. The clamping force of 6 rotor bolts at 4nm. And that force is the same regardless of bolt material. Steel bolt, Ti bolt, Al bolt, all with the same major diameter, thread pitch, and torque value and the clamping force is going to be the same.


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