Disc brakes and rotational mass
 

With the trend going to disc brakes, what happened to the concern of rotational mass? The disc will certainly add weight to the wheels, albeit in the center, where it will be least noticeable, but there none the less.
Am I missing something here or are the people hopping on the trend wagon? I suppose you could make an argument that the "overall" weight of the bike might (?, is the jury in on that debate?) be less, but I am specifically talking about the rotational mass of the wheels....

Thoughts please, as I am considering a Giant Defy Advanced Pro2, which comes with disc brakes standard.

Anyone have any thoughts about the Giant?

As its close to the axis Whats the Problem? other than over-thinking a bike purchase writing online .. :innocent:

Giant is A Huge company they got that way making bikes under other Customer Importer brands first ..

They still do , just added their own dealer network and distribution warehouses around the Various Countries.


https://www.giant-bicycles.com/en-us...2/22174/83952/

not my style of bike , , but do as You Wish.

That's kinds what I was thinking, so close to the center... so maybe it doesn't matter? I do not notice it on my mountain bike, but on those, disc brakes are mandatory with the crap we ride through. Mud, water, etc would surely weaken braking power.

Rotating mass will still affect braking, wheel directional changes and what not. Still sitting on the fence on this one since I'm not sure but the question still is unanswered in my mind. I can't help but think the cons outweigh the pros...

I'm about as far from being a fan of disc on road bikes, but purely on a basis of rotational weight, they may turn out to have an advantage.

Yes the disc adds weight, but as pointed out it's near the center. OTOH they allow the use of CF rims without brake tracks. That shaves a decent amount of weight where it matters most, so on balance a CF wheel with disc may have lower total inertia than one made for rim brakes.

But that may be their only advantage, along with better rain performance. The flip side is that they introduce new issues such as increased stress on fork blades, wheel mounting issues, and higher cost to own and operate (at equal quality levels).

It's long been my opinion that the push for disc on road is driven, not by technical superiority, nor consumer demand, but by an industry that, like a shark needs to keep moving to surviving. Makers need to sell ever more expensive bicycles, and convince those who already own bicycles to upgrade to the latest and greatest. In this way the bicycle has become like the computer and cell phone industries in trying to force a planned obsolescence on their customers.

This isn't to say that there's no place for disc on road or "pavement" bicycles, just that we should be honest and stop claiming that rim brakes which have served so well for a century have suddenly become barely adequate relic of the dark ages of bicycle tech.

Rims designed for disc brakes can be lightened since they dont have to have a brake surface, and they can be made more aero. Therefor there will be less rotational mass. The disc near the hub has little rotational mass.

OTOH I am all for disc brakes. The major point bikes with disc brakes have is the fact they dont destroy rims. Rims can be very expensive, and rebuilding a new wheel costs money too.

With manufacturers putting the disk on the "wrong" side of the fork, I wonder if riders who grind off lawyer lips for quicker wheel changes, realize they are needed with disc brakes to stop the wheel from ejecting during hard braking.

One reason why thru-axles are becoming more popular.

The rim wear argument is totally spurious, and IMO is the Restless Leg Syndrome of the bike world.

Brake rim wear was never an issue for the vast majority of road bike riders. Very few (outside of the Pacific Northwest) ever wear rims out, either because they don't ride enough, or because the wheels were destroyed by other means before the brakes wore them enough to worry. Note: the story is different for mtb, but we're only talking about road here.

Typical brake wear life of road rims exceeds 25,000 miles, and many here on BF report wheel life that exceeds 50,000 miles. By comparison, disc owners can expect to go through a number of rotors before then, so the total cost of ownership, even factoring rim wear is still higher for discs.

Again, I don't say there's no place for disc brakes, because they do have a place. There are road riders for which wet performance is a major consideration, and those same people will be more concerned about rim wear. Disc brakes also make tricycle design simpler, so will always be popular on 3 wheeled recumbents.

But other than those and possibly other niches, it's all one big snow job, and one more example of a "solution" to a problem that didn't exist beforehand.

BTW- With 10s of thousands of wet miles under my belt I've yet to wear out a rim. That's including my road rims with wall thickness of only 1mm when new. I got close on one of my commuter's wheels after 25,000 miles or so, but a nice lady in an SUV saved me form "disaster" by killing off both wheels first.

Those are some good points.

I guess, for me, I think the ability to use lighter weight rims can offset some of the rotor weight. I still don't think hydraulic disc brakes are going to be of any benefit to more than maybe 5% of the cyclist out there.

I am concerned about something else, when using the brake, you can argue that is when there is a lot of force on the front wheel but that "pinch"point at the brakes might help by offering a temporary support point during unusual applied forces. Time for some research...

Any comments about the Giant Defy Advanced Pro2? Other recommendations for an A rider riding 30-60 miles at a time? I want speed, lightweight, some comfort (but will trade comfort for more speed), Ultegra 11 speed, strong wheels and carbon.

Disc brakes require a stronger fork, because torque during braking is applied to the end of the fork instead of at the crown. Research away!

Sort of what I was saying. With a disc, the forces at at the axle, not high on the fork, hence mechanical advantage working against you.

Yes and to add insult to the injury, all that braking stress is applied to only one blade.

So, after they beef up the forks to accommodate the brake, the "hardened" front end will cause the bike to ride harshly. No worries, they're ready to sell you a suspension fork, and tell you how it solves all sorts of problems that you never knew you had.


........"there was an old lady who swallowed a fly........"

Commuting daily in the Northwest, and having recently converted a bike to disc brakes I can say without hesitation disc brakes are superior in every way, with absolutely no disadvantages. Any additional weight, or rotational mass are undetectable. I had to get a new fork designed for a disc brake to do the conversion, and since it was an upgrade from the OE, ride quality was improved, and weight reduced.

Before
http://bikeforums.net/attachment.php...hmentid=513511

After
http://bikeforums.net/attachment.php...hmentid=513513

Here in the PNW where I commute, in the 8 wet months I would go through a set of brake pads in a month, and rims in a year. The disc pads last 3X longer, and cost less. A disc is cheaper than a rim, and doesn't require a wheel rebuild. For me discs are significantly cheaper, significantly less maintenance, and work better than rim brakes.

They're also much cleaner, which is important for domestic harmony since I bring my bike in the house. ;)


BTW, my 1935 Raleigh Sports Model X has drum brakes which were an OE option for most of their bikes in that era, alternatives to rim brakes have been around a long time.

No rim brakes here.
http://bikeforums.net/attachment.php...hmentid=513538

I mentioned that the PNW was an exception twice, once in this post and once in the prior post. Should I have said it once more?

But just as riders in the PNW shouldn't have to live with bikes not suited to their needs, riders elsewhere shouldn't feel compelled to buy stuff they don't need for their conditions.

After decades of riding with rim brakes I made the switch over to discs about three months ago and I have to say there is zero chance I'll ever go back to a rim brake. It's not that disc brakes stop better - it's that they stop a LOT better. Way better. And that's in dry conditions - throw some rain into the mix and they're light years apart.

As for weight? Yeah, it's a bit heavier. So my bike weighs 18.5 lbs instead of 18.0 lbs. No biggie.

Rotational weight? Whatever, I'm not a physicist.

Asthetics-wise, I've come to really like the way they look - sexy, even. In fact, bikes without discs are starting to look wrong to me.

5-6 of the guys in my cycling club are running discs now and they've all been saying the same things as me: 1) works better, 2) looks good, 3) better feel, 4) wouldn't go back

Yes, I agree, perhaps I expressed myself poorly. :p
My point is cost really isn't any greater, just different, and that there's other considerations in their favor.

I have 3 bikes with rim brakes on steel rims, 1 bike with rim brakes on alloy rims, 2 bikes with drum brakes, 1 bike with roller brakes, and 1 bike with disc brakes.

All meet my braking needs, discs just do it better.


Edit, I guess I'm not the only one who thinks it's odd to get emotional about someones brake preference. Thanks to the management for cleaning it up.

Simmer down please, guys. Put some brakes on the personal attacks.

Braking is just negative acceleration, and all things being equal discs are better at negative acceleration, so they are technically faster. :thumb:

Yes, so close to the center that it doesn't matter.

FWIW: The technical quantitative measure of what's colloquially known as rotational mass is moment of inertia. It's straightforward to calculate for simple shapes and fairly easily measured with a torsional pendulum.

If interested, Google "moment of inertia" and "torsional pendulum" for further info.

Look at all the mfg that are introducing their road bikes with discs. People against disc, are the type that was against click shift and clipless pedals back in the 80s. I have always wondered why so many cyclist are against new technology.

That guy was obviously drunk last night or something. Definitely over the top with the emotion and then some of the statements came out of nowhere. He probably understood very little English and misread/mistook some of our conversations here and then went ballistic.

Thanks moderators for removing that junk.

FYI, I did not start this thread to argue if disc brakes are superior or not, just inquiring about the additional rotational mass and asking about the Giant Defy bike.

I mean, who wouldn't want a shiny new bike with all the latest and greatest components on it, but that doesn't necessarily mean it is a better bike, or that to get one thing, you have to give up another. In this case, more weight on the wheel in exchange for greater stopping power. How many really need more stopping power and is the weight noticeable.... I think we have answered those questions.

Any input on the Giant Defy Advanced Pro2?