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Disc Rotors wrong way round??


Mr Zee
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This has been puzzling me for a while, as I have owned several motorcycles and the arms of the disc rotors on them sweep backwards, rather than forwards, as indicated clearly on all bicycle disc rotors with angled or swept arms (see the two images attached).

A mechanically minded motorcycle friend explained that most metal is stronger under extension, rather than compression.

The motorcycle disc rotor arms in the photo are thus under extension (caliper grabs rotor and it pulls on hub via rotor arms, which are trailing, thus being stretched- good and strong...) In the case of the bicycle, the rotor arms are under compression (caliper grabs rotor and it slows hub with leading arms which push back against hub- not as strong...) This may be purely academic, as there certainly haven't been any instances of disc failure that I have heard reported, but it interests me that two industries using discs are so adamant about the direction in which the rotor arms HAVE to point and, yet they have them opposite way around. Is there something about bicycle disc technology that is different? Can anyone shed some light on this for me?? :unsure:

post-14438-0-27566400-1313317840.jpg

post-14438-0-02193900-1313317858.jpg

Edited by Mr Zee
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Funny but i was thinking exactly the same thing the other day when i had to put my rear disc back on.

 

Then again look at the Hope discs which does not work on either principle.

Edited by specializedfan
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Funny but i was thinking exactly the same thing the other day when i had to put my rear disc back on.

 

Then again look at the Hope discs which does not work on either principle.

 

Could it be a conspiracy?

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The braking forces present on a mortorbike is way less than on a MTB!!

100km/h vs 40-50km/hr

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This has been puzzling me for a while, as I have owned several motorcycles and the arms of the disc rotors on them sweep backwards, rather than forwards, as indicated clearly on all bicycle disc rotors with angled or swept arms (see the two images attached).

A mechanically minded motorcycle friend explained that most metal is stronger under extension, rather than compression.

The motorcycle disc rotor arms in the photo are thus under extension (caliper grabs rotor and it pulls on hub via rotor arms, which are trailing, thus being stretched- good and strong...) In the case of the bicycle, the rotor arms are under compression (caliper grabs rotor and it slows hub with leading arms which push back against hub- not as strong...) This may be purely academic, as there certainly haven't been any instances of disc failure that I have heard reported, but it interests me that two industries using discs are so adamant about the direction in which the rotor arms HAVE to point and, yet they have them opposite way around. Is there something about bicycle disc technology that is different? Can anyone shed some light on this for me?? :unsure:

As a rule of thumb, you can assure that the tensile and compressive strengths of steel are the same. However, in your example of disc shape, I'd say shear strength will be the limiting factor provided you can keep the disc laterally stable.

 

And the shear force will be about the same in both shapes so it doesn't matter which way the disc faces. But just to me sure, I'd like someone to perform a high-speed test and report back. I may have to modify my stance on all this.

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as JB correctly pointed out, the young's modulus for steel, and most other engineering materials, is essentially the same in both tension and compression. The benefit of the tensile properties becomes more apparent when heat is involved along long slender metal structures. Stretching over buckling any day.

wrt shearing stresses, if the area is the same in either direction, which of course it is, then the shear stresses will be the same as well.

 

Maybe it's a fashion statement... thinking of mud clearance, increased windage for cooling... *shrug* dunno.

Edited by Capricorn
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The braking forces present on a mortorbike is way less than on a MTB!!

100km/h vs 40-50km/hr

I appreciate that, but the principle remains the same- why have the two industries opted for different configurations?

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as JB correctly pointed out, the young's modulus for steel, and most other engineering materials, is essentially the same in both tension and compression. The benefit of the tensile properties becomes more apparent when heat is involved along long slender metal structures. Stretching over buckling any day.

wrt shearing stresses, if the area is the same in either direction, which of course it is, then the shear stresses will be the same as well.

 

Maybe it's a fashion statement... thinking of mud clearance, increased windage for cooling... *shrug* dunno.

I presume the sort of heat you refer to is way beyond that which is generated by MTB discs? Perhaps with motorcycles the sort of forces and temperatures involved make this tensile 'advantage' more relevant? This still implies that the motorcycle engineers have stipulated their disc orientation through some extensive r&d and it surprises me that the MTB industry have ignored this- I suppose at the speeds and forces involved with MTBs, it does not really matter and it is thus some sort of fashion statement...

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as JB correctly pointed out, the young's modulus for steel, and most other engineering materials, is essentially the same in both tension and compression. The benefit of the tensile properties becomes more apparent when heat is involved along long slender metal structures. Stretching over buckling any day.

wrt shearing stresses, if the area is the same in either direction, which of course it is, then the shear stresses will be the same as well.

 

Maybe it's a fashion statement... thinking of mud clearance, increased windage for cooling... *shrug* dunno.

 

Trust the janitor to come up with the goods! :P

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I presume the sort of heat you refer to is way beyond that which is generated by MTB discs? Perhaps with motorcycles the sort of forces and temperatures involved make this tensile 'advantage' more relevant? This still implies that the motorcycle engineers have stipulated their disc orientation through some extensive r&d and it surprises me that the MTB industry have ignored this- I suppose at the speeds and forces involved with MTBs, it does not really matter and it is thus some sort of fashion statement...

 

Yes, beyond that on the average MTB, but not beyond that on the average tandem. Tandems in the mountains generate enormous heat and if there is a disc on the back and it is being abused (dragged constantly to scrub off spead rather than used quick and decisively), it easily glows red-hot and buckles.

 

The buckling doesn't happen in the disc's spider as you would expect, but in the brake track.

 

The safest brake as a drag brake on a tandem is a drum brake and the best, Araya, are made by a Japanese company. They screw onto the left side of tandem hubs. Shimano tandem hubs have an Araya screw as standard.

 

Of course most tandems never see the real mountains and the problem is largely academic and moot.

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I appreciate that, but the principle remains the same- why have the two industries opted for different configurations?

I think Cap was being kind to the industry by calling it fashion. I think it is all BS. The same disc will work perfectly well either way round. Tyres too, with the exception of noise generated by the likes of Crossmarks turned the

wrong" way around.

 

Marketeers love throwing some bogus engineering mystique into their literature.

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Yes, beyond that on the average MTB, but not beyond that on the average tandem. Tandems in the mountains generate enormous heat and if there is a disc on the back and it is being abused (dragged constantly to scrub off spead rather than used quick and decisively), it easily glows red-hot and buckles.

 

The buckling doesn't happen in the disc's spider as you would expect, but in the brake track.

 

The safest brake as a drag brake on a tandem is a drum brake and the best, Araya, are made by a Japanese company. They screw onto the left side of tandem hubs. Shimano tandem hubs have an Araya screw as standard.

 

Of course most tandems never see the real mountains and the problem is largely academic and moot.

 

That is not true, we are outstanding climbers and use our Arai drum brake with a drogue chute to slow us down on tortueous descents. The Tandem normally comes to a complete standstill when the ropes of the drogue chute tangles with the rear wheel. It stops faster than the Nisshin Maru with props fouled by the crew of the Gemini of Whale Wars fame.

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  • 3 weeks later...

bit late but, you are going over the handlebars/locking the wheel/into the tree before you get anywhere near the extension/compression limits of disk brakes, and if the arms do brake, the chances are its not your fault for putting them on the wrong way round, its the guy who designed it's fault, cause he is bad at his job.

 

think about it, if you do xc the disks are not as beefy, but they don't have to stop as much weight from such high speeds(in most cases).

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bit late but, you are going over the handlebars/locking the wheel/into the tree before you get anywhere near the extension/compression limits of disk brakes, and if the arms do brake, the chances are its not your fault for putting them on the wrong way round, its the guy who designed it's fault, cause he is bad at his job.

 

think about it, if you do xc the disks are not as beefy, but they don't have to stop as much weight from such high speeds(in most cases).

No.

 

If you lock the brakes, you go over the handlebars. If you drag the brakes over distance and time, you heat them up. You can make brakes glow red-hot without doing any sudden stopping.

 

It is a real-world scenario.

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thats correct, heating the disk will make it glow, but they are still going to glow if they are the other way round. what I'm trying to say is, you are going to run out of ball pressure before the arms brake, you or something else on the bike will before they do. so have them the way the looks coolest to you.

 

(I hope I'm not offending anyone with my opinion here)

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