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PostPosted: Mon Aug 29, 2011 6:54 pm 
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mechagouki wrote:

Treble the weight of the rotor while the bike is in motion?

If all the weight of the wheel is concentrated at its extremity - an unrealistic worst case - the maximum effective increase in the inertial mass of the wheel is equal to its non-rotational inertia, and that's only a factor when the wheel is accelerating, not when it's in unaccelerated motion. The brake disk is much smaller than the wheel, so its moment of inertia is effectively reduced by the square of the ratio of the radii. For a big rotor on a typical 26" wheel, that's an increase of about 10%.

Basic physics. :lol:


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PostPosted: Mon Aug 29, 2011 7:13 pm 
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I'm not convinced - a wheel clearly has greater inertial weight whilst decelerating (than when stationary) or you wouldn't be able to ghost-ride bikes or do the old wheel as a gyroscope trick. and surely as the rotor is bolted to the wheel it's weight would increase proportionally.
I'm no mathematician (I barely scraped a GCSE) but I thought that it was accepted knowledge that the rotational weight of a wheel was the stationary weight multiplied by Pi?


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PostPosted: Mon Aug 29, 2011 9:05 pm 
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OK... I did the maths on this ages ago on another thread, which is what I just searched for...

http://www.retrobike.co.uk/forum/viewto ... light=disc

A brief search elsewhere shows Magura Marta MT8s at just 279g per end, that's everything, hoses, levers, calipers, rotors, the lot.

XTR V's come in (using your figures) at 280g per end before you factor in the cables and the increased rim weight. I included gore cables and used Mavic 717 rims in my calculations as they're available in both disc and non-disc versions.

Using my figures in the thread referenced above, there was 20 grams in it between Magura Martas and a set of XTR V brakes. I'd say that was pretty inconsequential.

Yes, you can buy ultra light V brakes, but then you could also trick out the Maguras too. MT8s are pretty much cutting edge but discs in general are only going to get lighter.

My point I guess, is that in reality, decent modern disc brakes are really not much heavier than a good set of V's were. Its another myth that the luddites like to put about to justify not spending money on decent brakes ;)


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PostPosted: Mon Aug 29, 2011 9:23 pm 
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Russell wrote:
Info...


yeah... good job. It's fair to say disks are gonna get all the development effort, therefore gonna get lighter and lighter yet

Those KCNC brakes aren't the limit... You could maybe use carbon and a titanium skeleton and make some crazy light fabulously rigid rim brakes and levers... But where's the market? No-one's gonna invest in something with a teeny niche market

Rim brakes are now pretty much dead development-wise

I'll bet if we repeated this experiment at the mid range EG Deore LX vs SLX we'd see rim brakes come out lighter by even greater margins... For now at least

- For my money rim brakes are cool and are easy, beside which my favourite era is late '80s early '90s, not too much going on disk brake-wise back there

- I was impresssed by my bro's disk brakes, I expected them to be powerful, but they were much more progressive than I expected

- Still for me not worth it, just like the rest of the modern bike proposition, I get it, just the engineering isn't going in the direction I'd like to see. For me this is why retro is so good. You get much more for your money as new bikes are just bouncier with more gears and more complex braking, therefore wasting any weight savings on some kind of inter-manufacturer arms race of numbers that doesn't really result in better bicycles


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PostPosted: Tue Aug 30, 2011 7:09 am 
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mechagouki wrote:
I'm not convinced - a wheel clearly has greater inertial weight whilst decelerating (than when stationary) or you wouldn't be able to ghost-ride bikes or do the old wheel as a gyroscope trick. and surely as the rotor is bolted to the wheel it's weight would increase proportionally.

I think we're at risk of getting lost in bad definitions.

A moving wheel has angular momentum, and angular momentum increases with angular velocity just as linear momentum increases with linear velocity. Inertial mass is the ratio between applied force and acceleration: m=F/a

Because the wheel is rotating, you need to put more energy in to make it accelerate to a given speed that if it were simply sliding along on a frictionless surface. You can express that by making a correction to m to take account of the rotational motion.

It's tricky to write out the equations on this forum, but if you write the total kinetic energy equation for the wheel with two terms, one linear and one rotational, m becomes m(1+(r/R)²) where r is the radius of the rotor, and R is the radius of the rim. You can see that the maximum possible value of the effective inertial mass is 2m, when r=R, i.e. when the brake rotor is as big as the wheel.

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I thought that it was accepted knowledge that the rotational weight of a wheel was the stationary weight multiplied by Pi?

I've no idea where you got that from. The effective "rotating weight" (moment of inertia) of a wheel depends on how the mass is distributed around the axis of rotation, and you can calculate that using the site you just posted:

http://easycalculation.com/physics/clas ... nertia.php

It's effectively the sum of mr² for all the components of the wheel. That's why mass near the periphery is so much more important than mass near the axis. If all the mass is concentrated at the periphery of the wheel (the theoretical worst case) the maximum total effective inertial mass is 2m.


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PostPosted: Tue Aug 30, 2011 7:54 am 
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one-eyed_jim wrote:
mechagouki wrote:
I'm not convinced - a wheel clearly has greater inertial weight whilst decelerating (than when stationary) or you wouldn't be able to ghost-ride bikes or do the old wheel as a gyroscope trick. and surely as the rotor is bolted to the wheel it's weight would increase proportionally.

I think we're at risk of getting lost in bad definitions.

A moving wheel has angular momentum, and angular momentum increases with angular velocity just as linear momentum increases with linear velocity. Inertial mass is the ratio between applied force and acceleration: m=F/a

Because the wheel is rotating, you need to put more energy in to make it accelerate to a given speed that if it were simply sliding along on a frictionless surface. You can express that by making a correction to m to take account of the rotational motion.

It's tricky to write out the equations on this forum, but if you write the total kinetic energy equation for the wheel with two terms, one linear and one rotational, m becomes m(1+(r/R)²) where r is the radius of the rotor, and R is the radius of the rim. You can see that the maximum possible value of the effective inertial mass is 2m, when r=R, i.e. when the brake rotor is as big as the wheel.

Quote:
I thought that it was accepted knowledge that the rotational weight of a wheel was the stationary weight multiplied by Pi?

I've no idea where you got that from. The effective "rotating weight" (moment of inertia) of a wheel depends on how the mass is distributed around the axis of rotation, and you can calculate that using the site you just posted:

http://easycalculation.com/physics/clas ... nertia.php

It's effectively the sum of mr² for all the components of the wheel. That's why mass near the periphery is so much more important than mass near the axis. If all the mass is concentrated at the periphery of the wheel (the theoretical worst case) the maximum total effective inertial mass is 2m.


:shock:


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PostPosted: Tue Aug 30, 2011 8:30 am 
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Where does the weight of the tyre/tube come into it :?:

Doesn't seem to be included in your calculation - and surely most of the mass is peripheral?

(I haven't looked at the link - my head still hurts from struggling with the theory! - so apologies if the answer is there...)

For those struggling with the theory try the old physics experiment of filling a bucket with water, tie a rope to the handle and whirl it around in a circle faster and faster and see how much heavier it gets... :shock:

...and also how difficult it is to slow it down!

Which throws up another comparison in itself - the longer the rope the more inertial force and the harder it is to slow down; therefore a 29'er is going to need more powerful disks than a 26"...


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PostPosted: Tue Aug 30, 2011 8:40 am 
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Russell wrote:
OK... I did the maths on this ages ago on another thread, which is what I just searched for...

http://www.retrobike.co.uk/forum/viewto ... light=disc

A brief search elsewhere shows Magura Marta MT8s at just 279g per end, that's everything, hoses, levers, calipers, rotors, the lot.

XTR V's come in (using your figures) at 280g per end before you factor in the cables and the increased rim weight. I included gore cables and used Mavic 717 rims in my calculations as they're available in both disc and non-disc versions.

Using my figures in the thread referenced above, there was 20 grams in it between Magura Martas and a set of XTR V brakes. I'd say that was pretty inconsequential.

My point I guess, is that in reality, decent modern disc brakes are really not much heavier than a good set of V's were. Its another myth that the luddites like to put about to justify not spending money on decent brakes ;)


In my opinion, that just about says it it all. Anyway, you have to ask yourself, who wouldn't put up with, say, a 100 gram weight penalty if the result was a set of brakes that worked predictably and reliably under all riding conditions? But then I don't have a lot of time for all this weight weenie stuff anyway, like where somebody will post a weight for a Kona Hei Hei (or something like that) but that low weight includes a pair of skinny little slick tyres and and bars about 400mm wide. Slicks on a mountain bike, FFS....... :roll:

So yes, for a period correct 80's or 90's build then obviously you're going to fit canti or linear pull brakes, but it's just kiddology to claim that it's because they're better. If Magura or Shimano or Hope disc brakes had been around in the late 80's don't you think that mountain bike designers would have been specifying them and that riders would have been crying out for them?
And how many riders on the World XC circuit do you see using these "superior and lighter" rim brakes? Next to none, that's how many.
For DH ? None.

I rode mountain bikes with rim brakes for years (I still sometimes do) and no matter how well set up they were, no matter what the pads and rim materials were, under certain conditions (where they were constantly wet) they were crap compared to any decent disc brake. I have a bike with Magura HS-33 hydraulic rim brakes and they are as well modulated and at least as powerful and sensitive as a good disc brake.........until you're riding a series of very steep downhill switchbacks covered in long wet grass and the rims are permanently wet from that grass.
If it wasn't for the fast that I could never replicate the iconic Paul Brodie paint job on my '88 Explosif it would have had caliper mounts fitted long ago. Even I can't quite bring myself to do that though....

Anyway - these excessively light (in my opinion) brake discs like those Ashima ones. Again it's weight weenie-ism gone mad. I think maybe something like that is verging on being not fit for purpose - and that weight saving just isn't worth worrying about. It's not going to turn some old fart like me into a trail god overnight. If it could then I'd be sending a bulk order off to Ashima ASAP, 'cos I've always fancied being one of those (a trail god, that is).

This quest for lightness is an understandable thing and I'm sure that at the highest level, where the riders themselves are like human machines tuned to perfection, every few grams saved might make the difference between getting a podium place or not. It's all part of the winning edge thing - like the trails rider Yrjo Vesterinen back in the late 1970's. I remember being amazed (at the SSDT in 1980) how much lighter his Bultaco was than a standard(ish) one like mine. But I wasn't Vesterinen back then any more than I'm Julien Absalon now and 50 grams lighter or heavier (or 500 grams even) won't make a blind bit of difference to my performance (or lack of - whichever fits).

I like nice components on my bikes and I'm pretty fussy about set-up but I like stuff that works and is reliable, 'cos I may be old but I do try not to ride like an old woman. I enjoy working on bikes but I enjoy riding them more than working on them and good disc brakes are pretty maintenance free.


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PostPosted: Tue Aug 30, 2011 8:50 am 
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We_are_Stevo wrote:
Where does the weight of the tyre/tube come into it :?:

You can include it in the mass of the rim. Effectively I'm modelling the wheel as a small hoop (the brake rotor) inside a larger one (the rim/tyre/tube) to simplify the situation. It's a rough approximation, but it gets you the figure of 2m for the upper limit of the effective inertial mass.

You can treat each component of the wheel separately if you like, calculating a moment of inertia for each, but for the sake of illustration it makes more sense to simplify the rim, tyre and tube to a single hoop.

m(1+(r/R)²) above doesn't consider the mass of the wheel at all. It's just the effective inertial mass of a rotor of radius r in a wheel of radius R.


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PostPosted: Tue Aug 30, 2011 8:57 am 
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one-eyed_jim wrote:
We_are_Stevo wrote:
Where does the weight of the tyre/tube come into it :?:

You can include it in the mass of the rim. Effectively I'm modelling the wheel as a small hoop (the brake rotor) inside a larger one (the rim/tyre/tube) to simplify the situation. It's a rough approximation, but it gets you the figure of 2m for the upper limit of the effective inertial mass.

You can treat each component of the wheel separately if you like, calculating a moment of inertia for each, but for the sake of illustration it makes more sense to simplify the rim, tyre and tube to a single hoop.


That makes sense - told you my head was still hurting, but good to see a factual reply based on real world calculations instead of emotive individual preferences... :wink:


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