Yes and yes. The thread turned back to challenging the test and its interpretations, and that is a good thing. For a long time, the tread was simply echoing themes around invalid test, real-world conditions, and slick brake track, until people got a clue, actually read the responses, and realized that they did not understand thermodynamics. The few good questions and challenges early on were quickly muffled by the ignorant. Now, the tread is back to the track of... did the system really deliver and absorb 1200W.

The thread improved because Bobby started actually engaging with the questions instead of just repeating assertions, but believe what you'd like.

And it was still never "bullying".

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The point is, ladies and gentleman, that speed, for lack of a better word, is good. Speed is right, Speed works. Speed clarifies, cuts through, and captures the essence of the evolutionary spirit.

Whatever. I am just glad that this is focusing on whether the wheels absorbed 1200W. It took about 7 pages of chaff to finally get there. Now, the next big question, after everyone is satisfied, is "so what?" I would never make a purchase decision based on this test. But, I am curious what Alto will do next with the ability to dissipate way more heat than any other wheel maker.

You know you can set the number of posts per page, right? That way, for me, it's fewer pages to get through listening to you cry foul.

I don’t think Alto is dissipating more heat than anyone else. Tom A. is a mechanical engineer. Go read his posts.

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Make Inside Out Sports your next online tri shop! http://www.insideoutsports.com/

I have been following Tom’s interactions since he focused on the question of whether the system is really getting 1200W for every wheel. Currently, they are resolving whether braking force to angular velocity is linear or exponential. I do not think they have arrived at a consensus understanding yet.

By definition, it's linear.

Power = braking torque x angular velocity, and braking torque is linearly proportional to the pad clamping force, since braking force = coefficient of friction x clamping force. Braking torque is merely the braking force x the radius from the axle to the brake track.

So, if coefficient of friction goes down, braking force drops proportionally (if normal force is constant, which it should be for the weights hung from a brake lever), and thus in order for a truly constant power to be dissipated at the brake track, then the angular velocity MUST also rise proportionally. However, none of that changes the fact that if the power dissipated at the brake track is truly constant, then the temperature should rise monotonically until it reaches a steady-state value. It shouldn't drop, especially not by over 70F.

Bobby seems to be conflating the shape of the wheel warmup curve with braking force, when the shape of that curve (assuming a constant power dissipation at the brake track) is a function of both the heat capacity (thermal mass) and heat transfer properties (conductive, convective, and radiative) of the system. An analogy in electrical terms is like charging up a capacitor (heat capacity) through a resistor (heat transfer).

Not only is it not clear that every wheel experienced 1200W (as the Mavic engineers pointed out, there should have been much more variation in wheel speeds due to the different brake tracks on the wheels), but it's also not clear that WITHIN each test the input power was constant.

Of course, since they have a torque meter in line with the system, all they need to do is publish the concurrent torque and wheel speed readings and it's a fairly simple calculation (simple multiplication, once in the correct units) to determine the point by point input power. If they didn't record the torque values during the test...well, that's a pretty big "oops".

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http://bikeblather.blogspot.com/

I guess I over-simplified and mis-typed. I meant the relationship between braking force and heat dissipation... is that linear or exponential? I know that brake force and friction force are mostly linear. If the system is normalizing to wattage (or hear), then if force to heat is linear, then angular velocity should be linear. But if Force to heat is non-linear, then angular velocity would be non-linear, right?

It's neither. The power at the brake track (heat input) is CONSTANT, since after subtracting off the tire, bearing, and aero drag losses, the ONLY place the 1200W is being dissipated is at the pad/rim interface. For the system to be in equilibrium, that's the only way that can happen.

Brake force may vary due to changes in friction coefficient, but that should be compensated for by changing the wheel speed to keep the power input constant (remember, Power = Torque x angular velocity). Braking force my vary all over the place, but the wheel velocity will change to keep the total power being dissipated at the pad/rim interface at ~1200W. Braking force drops, wheel speeds up...braking force rises, wheel slows down...but, in all cases, approximately the same power is being dissipated at the pad/rim interface.

In other words, if the power input is constant and angular velocity is allowed to vary based on the braking torque, then the power (heat, or Qdot) being dissipated at the brake track should also be constant. Granted, there will be minor variations due to differing tire, bearing, and convective losses as the wheel speed varies, but those variations should be exceedingly small as compared to the input power of 1200W, so effectively it's constant.

Does that help?

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http://bikeblather.blogspot.com/

(Just responding in general and not really to Tom A.) I think the dynamic viewpoint of this problem should be investigated as well - last I checked the rims don't just blow up everywhere all at once (ie. Failure occurs at one point). Even if the system is dissipating 1200W on average, a faster spinning wheel will dissipate the power more evenly than a slower spinning wheel.

To see this, just think about energy into the wheel over a very small constant time. Say the slower spinning wheel will make a quarter turn in this time, the motor energy will be dissipated over a quarter of the rim. A faster spinning wheel may make a 3/8 turn and so the same amount of energy will now be dissipated over this amount of the rim. So the power "density" will be higher in the slower rim, which will lead to a higher peak temperature in this rim, presumably causing it to fail sooner.

On another note, once the pads glaze over (corresponding to the second phase?) Is it possible that they abraze faster? Such that the frictional power is more dissipated into breaking chemical bonds in the pad vs. heat into the rims. Not sure the order of magnitude of power that this is but it may be a possibility to explain lower temps (other than bad motor control, shitty hubs, lubrication tricks, etc.).

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I like analyzing things - http://engineeringfitness.org

Tom: I told myself that I wouldn't continue with this discussion of energy, but I can't help myself, haha. I just really feel as though I've said everything that there is to say on this subject. You're confusing heat and power. Just because each rim must dissipate 1200W, that doesn't mean each rim is converting the same amount of power into heat. As the pads gloss over and the wheel speeds up, temperature begins to decrease and the constant 1200W motor input means that angular velocity increases. This is very standard and you'll find that it takes place in every brake track test from every brand, always.

Stumpyx13: Potential rim failure has must more to do with resin and construction properties than with the rim's speed. Take the Phase 2 Alto test, for example. The rim is running slower (19.3mph) and with more frictional force than any other rim in the test, but maintains a lower temperature and does not fail for 20 minutes. This is because the resin is able to conduct more heat away from the brake track and act efficiently as a heat sink. Regarding your assumption about energy being absorbed by the pads in order to break chemical bonds and glaze over: this is likely true, but the amount of energy going into this process is negligible and can be considered zero when compared to the other variables at work.

Hello Bobby Sweeting and All,

Please post the recorded torque values and other recorded metrics you may have for each of the tests.

That will provide some data for discussion to help put this baby to bed.

As the brake track angular velocity is a constant 20 mph plus or minus 0.5 mph (as it appears in the video) the input energy value should track the output energy value .... conservation of energy.

It appears from the video that output energy value varies over time as expressed higher temperatures followed by lowering temperatures under brake load in the Alto tests and different temperature curves for other rims.

I understand that you do not agree with some of the theories put forth here .... but leaving these questions unanswered clouds the entire experiment as more energy could have been input to different rims because of different brake pad/brake track friction while torque control held the brake track angular velocity constant.

Inquiring minds want to know .......

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Cheers, Neal

+1 mph Faster

Neal: I'm a bit confused by your comment on input and output energy, but please see my comment above regarding the energy equilibrium and its variables. A temperature bell curve would exists for all of these rims if they were to survive the initial period prior to pad glazing. They do not have different curves vs the Alto rim, they simply didn't last long enough to begin the decrease in temperature. Actually, that's not entirely true... You can see the Enve and Zipp rims did make it to this point and the temperature began to drop, but that was right when they failed. Boyd actually replicated our test but ran the wheel at lower power, and it ran for 20 minutes with the same temperature bell curve. All of the rims will follow this trajectory, it's not unique to Alto.

No, I'm not. Power is power. So, tell me, what units are used for the rate of heat energy flow (metric, of course)?

Did I mention earlier that I perform thermal analyses quite often as a part of my profession? Just checking...


Actually, it does...IF your setup is actually controlling to constant input power.


Yes, that temp drop is what your test shows...and it's a major clue that it's not operating as you've stated. That can't happen with a constant power dissipation.

You pointed to the Boyd test as "re-creating" your test and showing a similar temperature profile. But, you appear to be forgetting one important difference in that test...it's constant SPEED, not power. That means as the pads glaze and friction drops there's less braking force (and thus torque) but at the SAME angular velocity, and hence less power. LESS power is dissipated in the pad/rim interface, and so a drop in temperature after that point is expected. Fundamentally different.

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http://bikeblather.blogspot.com/

Mic drop. End of thread.

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Tom: You're claiming that every rim that is subjected to 1200W of power emits an identical amount of heat to any other rim that is subjected to the same power. This would give every rim in this test an identical result, and that is inherently false. You also ignore resin conductivity, onset temperatures, glass temperatures, friction coefficients, and construction techniques that effect heat transfer efficiency as it relates to how energy is transformed into various forms (heat, kinetic, etc).

I stated above how the variables relate to one another in order for the energy equilibrium to remain balanced as temperature decreases and motor input remains constant, so I'd rather not repeat myself here.

I honestly have enjoyed this dialogue, and I think it helps everyone to learn more about the test and the physics at play. However, it is extremely difficult to spend this amount of time on the forums during a very busy time of year for us. I don't want to leave anyone hanging, but I feel as though this subject has been discussed ad nauseam and we're continuing to go in circles. I have to focus on our retailers, customers, and ongoing R&D projects at this point. Perhaps the best thing that I can do to convince any remaining skeptics is to put my money where my mouth is. I'd like to offer a 30% discount and a 30-day money back guarantee to anyone I've interacted with here on Slowtwitch. You can literally buy the wheels, try them for a month, and return them for a full refund if they don't completely surpass your expectations. At the end of the day, the ride quality speaks for itself and hopefully a demo opportunity will help to reassure you guys that we've created something truly spectacular.

Thank you!

Nope. Never said that at all. That's a straw man argument on your part.

What I did say was that they would each be subjected to the same power input...IF your test is set up as you claim. What temperature they reach, and how quickly is a function of the thermal conductivity and heat capacity.

So...where is the wheel speed and input torque data? That could easily show whether or not the input power was truly constant.

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http://bikeblather.blogspot.com/

What's interesting to me is that if you look at the temperature graphs, and transform the Alto Phase 2 curve, you see the cooling over 100s is about 20F starting from 180F. Both the Bontrager and Zipp wheels drop significantly more than 20F in the next 100s from 180F; does that mean the other wheels (after exceeding their thermal limits) actually have better thermal conductivity?

If a motor is generating 1200W and the braking force is decreasing, then the wheel speed must dramatically go up, even under artificial load. Just imagine how much acceleration even a sustained 600W sprint would accelerate a wheel in a full rider-biker system (and that's with aerodynamic resistance). The evidence here hasn't been clearly articulated and there's a significant amount of handwaving around key issues.

Way to deviate around Toms post again.

A simple answer would be to post the data for the motor. If it’s truly 1200W then the thread may be over. Since many suspect it’s not, offering 30% off and talking about how great your wheels are is nothing more than a way to get out of being backed into a corner.

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Make Inside Out Sports your next online tri shop! http://www.insideoutsports.com/

I have a tangential question you may be very well placed to answer related to brake lever force, brake pads and braking power. Is there a problem with the use of identical brake pads in the test between texture and non-textured wheel? A bit of background:

I have played around for the past few years with brake pads with different compounds on my cross bike. What I have found is that certain hard compounds have a pronounced 'bitting point' while softer compounds are more modular. My working assumption was that the biting point is a product of pad/rim interactions causing a non-linear relationship between force at the brake and brake-rim contact area. (Toeing brake pads would be another example of a similar concept)

If this is true it would explain to me why companies producing rims with textured braking surfaces are so adamant about using specific brake pads. Basically the softness/hardness of the pad needs to be matched with the rim texture to get the best of the wheels. It would also mean the use of non-optimizes brake pads on texture rims could cause localized hot spots and uneven heat dissipation due to the rough contact between the pads and rims. Am I missing something? or at the scale of the test is it not significant?

I dont know if I'm exactly "well placed" to answer that...but, in my own experience, I've found that with textured brake tracks there's a balance required between too soft and too hard in regards to pads. Too soft, and you either chew through them quickly (or, even melt the surface!)...too hard and they don't brake as effectively.

But, this also seems to support the Mavic assertions that there should be much wider variations in wheel speeds for the different rims since there are large differences in brake track texture.

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http://bikeblather.blogspot.com/

In an attempt to not have to explain the energy transformations anymore, I've pulled the power and torque data from Matlab and input it into our excel results sheet. I couldn't do it from my phone while on the couch last night, sorry about that, haha. These metrics were not displayed in the video because they are controls, and it would have made the video even more boring than it already was! Unfortunately the file size is well beyond what this forum will allow me to attach, as it's a massive number of data points. Tom A (or anyone else), please send me an email through our website's contact page and I can send the excel spreadsheet over to you!

Regarding the question of brake pad density vs track texture, Tom is absolutely correct. We've found that softer pads conform more to a textured brake track and give improved power, while harder pads will simply have less surface contact area and therefore less power. The wheel velocities in our test and the rate of change (i.e. braking power) would be very different if Bontrager used the cork pads that they come with, if Enve used their brand pads, etc etc. That is why this test shouldn't be used to gauge real world braking power (unless you're hell bent on using a Black Prince pad), as that's not what it was designed for. We simply had to display wheel speed because it was a dependent variable, but it's not an important takeaway from this test by any means.

I've answered every question for you guys over 9 pages and even supplied control data from our test. I hope that will help to instill confidence in our brand and allow people to take more away from this particular test. And my offer on a discounted wheel set with a money back guarantee still stands!

I sent my email to you in a PM. Thanks.

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http://bikeblather.blogspot.com/

Got it! Everything should be in your inbox. I appreciate how thorough you've been, Tom, and I hope seeing the raw data will help with the transparency of the results. We probably should have just included those numbers in the video, but I think that 99% of customers would been very confused as to what they were seeing. We had to strike a balance to make sure that it would be somewhat palatable to more than a handful of engineers, haha.

Hello Bobby Sweeting and All,

Thanks .... I figure Tom will crunch the numbers and let us know what he comes up with.

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Cheers, Neal

+1 mph Faster