[iRider]

You can obviously run anything you like (as evident above) but the disadvantage of small rotors on big wheels is not just the obvious braking force loss - it's less heat dissipation, which means pads see higher temps, and glaze more, so you start off with less braking force but then what you have gets worse faster too (long before the pads actually wear out, particularly in the case of full-sintered pads).

Maybe I am really stupid, but I don't get the logic behind it. If the system weight (rider + bike) is the same for different wheel sizes, why do we have a reduction in brake force with bigger wheels? Wouldn't the kinetic energy that needs to be converted to heat be the same unless the inertia from the larger wheels is getting so big that it affects it? But this would also be true for a heavy and light wheel of the same wheel size, or? I get the longer lever thing of the larger diameter wheel, but this should just affect the feel of the brake, no?

 

[Flo33]

Disclaimer: lots of simplifiations ahead.

Let's assume your system weight is ~100 kg (Fgravity = 1.0 kN). Your wheel's outer diameter is 0.65 m.
Case a) is a 0.180 m rotor.
Case b) is a 0.200 m rotor.

Let's further assume we are standing on a steep slope. The system's weight (gravity actually) tries to pull you down the slope and get your wheels moving. The force pulling you downward is applied at the contact patch of your tire and wants to rotate it creating a momentum around your front axle with Fg x 0.65/2 = 0.325 kNm.


The braking force (Fb) needed to hold the whole system and avoid rolling has to be applied at the brake to create a momentum at the same point, the axle. It has to match the gravity induced ones and is:

a) 0.325 kNm = 0.180 x Fba => Fba = 1.80556 kN
b) 0.325 kNm = 0.200 x Fbb => Fbb = 1.62500 kN​

Do you see the difference now?

 

[imbecile]

So I'll jump here from the DOT vs DOT thread since my question is more frankenbrake oil oriented - has anyone replaced their Magura fluid with Shimano oil? Anyone with Formula Cura probably? Any issues? I've got a pair of Shimanos and future upgrade would be either Magura or Formula Cura, so I'll remain in the oil-based camp. I'll be buying a 1L bottle of Shimano oil and was thinking if it'd be a good long term investment if it can be used in either Magura or Formula oil brakes later on....

 

[Happymtb.fr]

Magura and shimano oils can be switched since there have been reports of people using both oils (more or less) successfully in their Shigura brakes (Shimano levers and Magura calipers)

I just switch to Bionol from Trickstuff (http://www.trickstuff.de/de/products/Bleedkit_Bremsfluessigkeit.php the translation to English is quite poor...) for my rear Shigura brake. The lever was feeling very stiff for prolonged periods. I don't know if it's caused by the exposure to freezing temp and/or air in the system? I have had moving breaking points with Shimano before but it usually went back to normal after releasing the pressure on the lever. This time the break point was way early and it felt quite stiff thereof. I cannot comment on breaking force since we have just snow and ice on the ground at the moment...
Anyhow, the Bionol solve the issue. The Royal Blood I removed had already started to turn black after 4 to 5 months and about 700km of trail riding. Something else I noticed while deaerating the Bionol was that the air bubbles rose slowly compared to other brake fluids I have used in the past.

 

[jonKranked]

thoughts on this from those who know this kind of stuff... any merit to their claims or snake oil?

https://www.bikerumor.com/2018/03/19/muc-off-bleeds-out-new-high-temp-high-performance-brake-oil/

 

[Flo33]

Like @Udi already pointed out, boiling temps is not the most critical criteria for our fields of use. Viscosity is of a higher concern. And that somewhat typical bikerumor ad... ahm, article doesn't mention any of it.

In the comments section somebody links it to Motul RBF 660, also to be found in Udi's super-sheet.

What I also found is, that some motorsport racing fluids also have a higher boiling point because of the higher temps occurring with it, but these are not suitable for "normal" application because of their also higher hygroscopic behavior. I don't know though, if this is a problem in bike application too. We have less fluid in our brakes and also less air to fluid surfaces compared to automotive.

 

[imbecile]

Probably your issue was low temperature related as air will cause exactly the opposite. Supposedly Bionol is even thinner than Shimano, so that'd explain the remedy. Still, it's twice as expensive as Shimano (per liter).

 

[Sandwich]

I can practically guarantee that's a rebadged motorsports brake fluid. There's no way that a bike company has the research capital to re-invent DOT fluid, that's just too big of an ask.

on that note, when it comes to DOT brakes, at least, I feel like a bi-annual bleed will be worth so much more than high end brake fluid only when there are problems. between seal brake down and water absorption, better to swap in a sealed bottle of valvoline (highest FLAPS available fluid) than to use the same bottle of motul blue every five years...

 

[profro]

Had another M8000 on my DJ get mushy and no amount of bleeding would correct. I dug around in the parts bin and found a Formula Oro that looked like it belonged in the trash. I had to dig around to find enough hardware to complete it, but a single bleed and a used set of pads almost sent me OTB (with just a rear brake). Sad, sad state for "modern" mtb brakes when a +10 year old brake which has been out of service for who knows how long is noticeably better.

 

[kidwoo]

I recently took the gamble of ordering some lightweight disc rotors from ebay. Turned out to be Promax parts. Didn't hear about that company/brand before. Does anybody have any knowledge about them? http://www.promaxcomponents.com/

Since no one else answered you.......

That brand has been around for years. They made some low end brake parts years ago, some cable pull and I think a hydraulic brake or two. They're fine. I've ended up with a few of those rotors over the years when I needed one quick or on a road trip. Those actually look better than the ones I've had. I had one promax rotor eons ago that was like a quarter inch thick

 

[Udi]

This is exactly why I put that information in the spreadsheet - it should make it very clear that low viscosity is the primary factor for MTB brakes, and importantly, that a lot of high-BP brake fluids (notably Motul RBF600, RBF660) are also high-viscosity! That's why I recommend if one wants Motul, that they use the Motul DOT5.1 fluid (with lower BP) instead.

If someone needs fluid for their DOT brakes in a rush without much selection, I'd advise picking any automotive DOT5.1 fluid (ideally from a reputable brand) since 5.1 is designed for small-orifice ABS systems and thus guarantees a low max-viscosity spec.

That Muc-Off product is definitely a typical scam, but no one should be surprised since their major product is watered-down pink dishwashing liquid with an exorbitant pricetag.

 

[canadmos]

That Muc-Off product is definitely a typical scam, but no one should be surprised since their major product is watered-down pink dishwashing liquid with an exorbitant pricetag.

Buttt, their chain lube smells like candy!!!! It sucks for chain lube...

 

[troy]

Since no one else answered you.......

That brand has been around for years. They made some low end brake parts years ago, some cable pull and I think a hydraulic brake or two. They're fine. I've ended up with a few of those rotors over the years when I needed one quick or on a road trip. Those actually look better than the ones I've had. I had one promax rotor eons ago that was like a quarter inch thick

Doesn't Box components belong to Promax?

 

[Udi]

@imbecile
Yes the Shimano oil will work in Shimano, Magura, and Formula's Mineral Oil brakes.
I think it's a good fluid, and it's good value in the 1L bottle as you say.

@Happymtb.fr
From the fluid thread:

@Flo33
I recently added a sheet to my brake spreadsheet (see tabs at bottom) with MTB-specific fluid recommendations.

It's logical that the Magura blood would cause slower rising of air / more difficult bleeding, as it's a higher viscosity fluid. I do not recommend this fluid. It's also worth keeping in mind that all oil-based brake fluids start having problems at sub-freezing temps, which is a practical area where glycol-based / DOT fluids are superior. Using a lower viscosity oil is probably the best defence.

Finally, the varying bite point is a "feature" of Shimano brake levers, and likely not a fault of the fluid. Resistance to movement and slower lever response can be fluid-caused.

The blackened fluid is also another one that is caused by poor quality brakes and not the fluid, the cause is the master cylinder piston scoring/wearing the cylinder walls - in Shimano the cylinder is cast and not anodized, so it's not the most durable surface. The blackening comes from aluminium oxide being exposed to oil. All you can do for this is change the fluid at greater intervals and replace the MC/lever occasionally.

 

[Flo33]

I know of a lot of other things that smell like candy.

 

[Flo33]

Since no one else answered you.......

That brand has been around for years. They made some low end brake parts years ago, some cable pull and I think a hydraulic brake or two. They're fine. I've ended up with a few of those rotors over the years when I needed one quick or on a road trip. Those actually look better than the ones I've had. I had one promax rotor eons ago that was like a quarter inch thick

Thx for the reply. At least not a completely one-hit-wonder-suicide-comp.

I'll give them a try, without the kid loaded trailer...

 

[Happymtb.fr]

It's logical that the Magura blood would cause slower rising of air / more difficult bleeding, as it's a higher viscosity fluid

Tanks Udi! I have been following this thread quite closely and I am aware that the Royal Blood is not that good. On top of the high viscosity it has a low boiling point... but I didn't know about mineral oils having problems below freezing. Good point!

But the slow rising of the air was about the Bionol. I edited my post to make things clearer. I usually pull the syringe plunger lever to create a vacuum and release air inside the break fluid when preparing the syringe before bleeding. I do it for Dot fluid and mineral oil too. It was at that moment that I found the air bubbles to rise slowly. It might also be due to the colder temperature in my workshop at around +13°C...

 

[iRider]

Disclaimer: lots of simplifiations ahead.

Let's assume your system weight is ~100 kg (Fgravity = 1.0 kN). Your wheel's outer diameter is 0.65 m.
Case a) is a 0.180 m rotor.
Case b) is a 0.200 m rotor.

Let's further assume we are standing on a steep slope. The system's weight (gravity actually) tries to pull you down the slope and get your wheels moving. The force pulling you downward is applied at the contact patch of your tire and wants to rotate it creating a momentum around your front axle with Fg x 0.65/2 = 0.325 kNm.


The braking force (Fb) needed to hold the whole system and avoid rolling has to be applied at the brake to create a momentum at the same point, the axle. It has to match the gravity induced ones and is:

a) 0.325 kNm = 0.180 x Fba => Fba = 1.80556 kN
b) 0.325 kNm = 0.200 x Fbb => Fbb = 1.62500 kN​

Do you see the difference now?

Makes sense, but would that not only change the feel of the brake. You need to replace the lever of the larger disc with more hand force.
If you consider the whole braking process, from going e.g. 20 km/h (yeah, metric!) to 0 km/h the kinetic energy that needs to be converted to heat would be the same, or? Because speed and rider weight doesn't change. The only thing you don't have with the smaller disc is the advantage of the leverage.

 

[kidwoo]

Doesn't Box components belong to Promax?

That's Toby motherfuckin Henderson bro!

They might be backing him/allowing him to run it. Don't know to be honest.

 

[troy]

That's Toby motherfuckin Henderson bro!

They might be backing him/allowing him to run it. Don't know to be honest.

https://www.cxmagazine.com/box-components-promax-mini-v-brake-linear-pull-eclipse-p-1-cyclocross

 

[kidwoo]

Oh.

Yeah.

Cyclocross Magazine. Don't know how I missed that.

 

[Jm_]

@imbecile


It's logical that the Magura blood would cause slower rising of air / more difficult bleeding, as it's a higher viscosity fluid. I do not recommend this fluid. It's also worth keeping in mind that all oil-based brake fluids start having problems at sub-freezing temps, which is a practical area where glycol-based / DOT fluids are superior. Using a lower viscosity oil is probably the best defence.

I take my shimano 9000 series down to -20F/29C with no problems. They get stiff, but they still work just fine. Having done this with an Avid on the other side of the bar at one time, they both felt about the same, supposedly it's more about the seals and not as much the fluid, although if I was going any colder I'd just go cable brakes to eliminate any possible seal issues. Doubt many others will be riding them in this cold, but most people around here now run hydros in the winter and there doesn't seem to be any big issues people are facing.

 

[Flo33]

Makes sense, but would that not only change the feel of the brake. You need to replace the lever of the larger disc with more hand force.

No. To gain the same amount of braking momentum with a larger disc you need less braking force and therefore less force at the lever. The larger disc has a larger lever to create that momentum at the wheel.

If you consider the whole braking process, from going e.g. 20 km/h (yeah, metric!) to 0 km/h the kinetic energy that needs to be converted to heat would be the same, or? Because speed and rider weight doesn't change. The only thing you don't have with the smaller disc is the advantage of the leverage.

Not exclusively. There is more area to spread/dissipate that heat, so overall less heat, like Udi pointed out as well.

simplified Math/Physics:

We are on flat grounds now. Brakes are at environmental temps.

mass m = 100 kg
speed v = 20 km/h ~5.6 m/s

W = 1/2 * m * v² = 0.5 * 100 kg * (5.6 m/s)² = 1543 Nm = 1543 J = 1.54 kJ

This work (theoretically) has to be converted into heat at the brake. The bigger the brake (=disc and pads, but mainly disc), the lower the created temps by that heat/work.

Steel has a heat capacity of c ~ 0.46 kJ/(kg*K). Let's check for the 180 and 200 rotors.

180 mm weighs 100 g
200 mm weighs 150 g

No we have to make another approximation. Heat is induced only into the friction ring of the brake and that part has to take care of the main part of the heat. But the rate of area's increase of the bigger ring is only slightly higher than the rate of the radius increase. I guess the most of the added mass goes into the carrier arms and bolt section.

friction ring mass 180 disc ~ 70 g
friction ring mass 200 disc ~ 80 g

Heat capacity is defined as c = delta Q / (m * delta T).

180: delta T => 47.8 °K temp gain
200: delta T => 42.9 °K temp gain

(In reality the heat capacity and temp gains depend heavily on the starting temps. See Debye-Model, Curie-Temp etc.)


In short, larger disc has more capacity to dissipate heat, because the friction ring's mass is higher.

 

[William42]

Here's an easier way to think about it if it still doesn't make sense.

Go flip your bike upside down, put it in a stand, whatever.

Give the wheel a big spin. Make sure its going fast.

Then, try and stop it by grabbing the spokes near the hub.

Give the wheel another big spin, and this time try and stop it by grabbing the tire. See how much easier it is to stop it?

When you bump to a 29" wheel from a 26 or 27.5, but keep the same sized discs, you're changing the point at which your brakes grab the wheel to be closer in toward the hub. To keep the same braking power, you need to go to bigger discs.

 

[Udi]

@iRider
On brief skim the explanations above are correct.
I think one question of yours that didn't get answered is regarding mass vs. leverage.

If you use a larger diameter wheel, the wheel now has increased leverage over the rotor (compared to the same size rotor on a smaller wheel). So you now need to squeeze the lever harder to stop the bike. You should be very clear on the fact that this is a clear-cut loss, since it's a very simple moment-arm calculation, basically switching to a bigger wheel (at OD) is a guaranteed loss in braking force and it is easily noticeable. Of the losses discussed here, this one is the one of greatest magnitude (by far) - so if there's one concept you grasp, this should be it.

But you ask a valid question - what if we simply increased the weight of (let's say) a 26" wheel to the weight of (let's say) a 29" wheel (with the same type of parts) without increasing its diameter at all. Well in this case, because the OD remains the same, the wheel has not gained any leverage over the brake rotor, thus our net braking force is *close* to the same.

However, the heavier wheel will have more angular momentum, so it *will* be a little harder to stop.
L = Iω
where L = angular momentum, I = moment of inertia, ω = angular velocity, and
where I, for a hoop rotating about an axis, is mr^2 (m = mass, r = radius)
So, you can see if you increase the mass (which causes I to increase, which then causes L to increase), you now have to input more hand force into the lever to counteract the increased angular momentum.

Now, where this gets interesting is, when you add the same mass, except instead of just filling your 26" wheels with sand, you actually upgrade to 29" - the resultant increase in angular momentum is not the same, but actually greater. The reason for this is that having the same mass rotating at a radius further away from the axle further increases the momentum.

If we look back at our moment of inertia formula for a hoop
I = mr^2
We can see that I increases not just as a function of r, but r^2, so it certainly has some impact.

Thus - larger wheels are harder to stop primarily because of their increased moment-arm (leverage) over the brake rotor, unless we offset it with a larger rotor. However, on top of this - they are also harder to stop because they generate more angular momentum - especially if the wheel is heavier, but also, even in a case where you built a wheel that had the same overall weight (using lighter rims / tubes / etc) as the wheel it replaced.

 

[StiHacka]

Then, try and stop it by grabbing the spokes near the hub.

Give the wheel another big spin, and this time try and stop it by grabbing the tire.

 

[toodles]

Then, try and stop it by grabbing the spokes near the hub.

Instructions unclear, lost arm in rotor.

 

[Jm_]

@iRider
On brief skim the explanations above are correct.
I think one question of yours that didn't get answered is regarding mass vs. leverage.

If you use a larger diameter wheel, the wheel now has increased leverage over the rotor (compared to the same size rotor on a smaller wheel). So you now need to squeeze the lever harder to stop the bike. You should be very clear on the fact that this is a clear-cut loss, since it's a very simple moment-arm calculation, basically switching to a bigger wheel (at OD) is a guaranteed loss in braking force and it is easily noticeable. Of the losses discussed here, this one is the one of greatest magnitude (by far) - so if there's one concept you grasp, this should be it.

But you ask a valid question - what if we simply increased the weight of (let's say) a 26" wheel to the weight of (let's say) a 29" wheel (with the same type of parts) without increasing its diameter at all. Well in this case, because the OD remains the same, the wheel has not gained any leverage over the brake rotor, thus our net braking force is *close* to the same.

However, the heavier wheel will have more angular momentum, so it *will* be a little harder to stop.
L = Iω
where L = angular momentum, I = moment of inertia, ω = angular velocity, and
where I, for a hoop rotating about an axis, is mr^2 (m = mass, r = radius)
So, you can see if you increase the mass (which causes I to increase, which then causes L to increase), you now have to input more hand force into the lever to counteract the increased angular momentum.

Now, where this gets interesting is, when you add the same mass, except instead of just filling your 26" wheels with sand, you actually upgrade to 29" - the resultant increase in angular momentum is not the same, but actually greater. The reason for this is that having the same mass rotating at a radius further away from the axle further increases the momentum.

If we look back at our moment of inertia formula for a hoop
I = mr^2
We can see that I increases not just as a function of r, but r^2, so it certainly has some impact.

Thus - larger wheels are harder to stop primarily because of their increased moment-arm (leverage) over the brake rotor, unless we offset it with a larger rotor. However, on top of this - they are also harder to stop because they generate more angular momentum - especially if the wheel is heavier, but also, even in a case where you built a wheel that had the same overall weight (using lighter rims / tubes / etc) as the wheel it replaced.

So 29ers ARE faster...

 

[Flo33]

Some stuff on disc brake rotors from the guy who runs, owns and designs all the stuff at Intend Bicycle Components and is the designer/engineer of the Diretissima brake.

https://www.intend-bc.com/tech/rotors/




Side note: Maybe @Sandwich could split this thread and transfer all the general braking stuff to the Frankenbrake thread, would be better fitting I guess.

 

[Happymtb.fr]

Some stuff on disc brake rotors from the guy who runs, owns and designs all the stuff at Intend Bicycle Components and is the designer/engineer of the Diretissima brake.
https://www.intend-bc.com/tech/rotors/

Cool info!
I just wonder if the smaller holes will not tend to glaze the pads more easily since many small holes will behave a bit like fin grit sand paper compare to the coarse grit larger holes would produce...

 

[StiHacka]

Some stuff on disc brake rotors from the guy who runs, owns and designs all the stuff at Intend Bicycle Components and is the designer/engineer of the Diretissima brake.

https://www.intend-bc.com/tech/rotors/

Speed holes! Next step: drum brakes with 50.99mm thick drums.

 

[Flo33]

Speed holes! Next step: drum brakes with 50.99mm thick drums.

I think it's actually not that stupid at all, but the price tag is steep.

 

[Flo33]

Cool info!
I just wonder if the smaller holes will not tend to glaze the pads more easily since many small holes will behave a bit like fin grit sand paper compare to the coarse grit larger holes would produce...

I was wondering the same. One has to take into account though, that Trickstuff and Mr. Intend promote the use of organic pads.

If temps don't reach glazing causing numbers at all, that may be of no concern on the other hand. I'm gonna write him an email.

 

[Udi]

Side note: Maybe @Sandwich could split this thread and transfer all the general braking stuff to the Frankenbrake thread, would be better fitting I guess.

Yeah @Sandwich maybe you could do some work around here for once!

 

[Happymtb.fr]

Let us know @Flo33
I've just seen the price 99€ a piece in 203mm fashion is not the cheapest option!

 

[StiHacka]

Yeah @Sandwich maybe you could do some work around here for once!

Don't dare him or he will CRISPR your DNA with lizard genes!

 

[SkaredShtles]

I was told that there would be NO maths.

 

[Udi]

That rotor "comparison" from Intend is BS because they're comparing to the cheapest Shimano rotor (it's evident from the weights he gives). The rotor used is one step down from ice-tech, and two steps down from their flagship freeza (sadly not available in 6-bolt). Intend's rotor is also heavy unless you buy the thinner 1.85mm version: 162g in 1.85mm and 197g in 2.25mm (no standard 2mm option). Considering a 203mm ice-tech rotor weighs 168g @ 2mm (from memory), and costs much less, how about we see a surface temp comparison with that?

I'm obviously the last guy running out to buy current Shimano brakes, but their heat management is 2nd to none - and it's mostly because they directly sandwich aluminium against steel (sandwiched between the rotor braking surfaces in ice-tech and freeza rotors, and sandwiched between the pad material and pad backing in their heatsink pads).

Remember, aluminium has much better conductive and convective heat transfer than steel, it's not just about thermal mass of the braking track, it's about dissipation.

Notice that his formulas refer to Q, but no mention of k (thermal conductivity, material specific).
Correct me if I'm wrong on anything, but I'm calling this an overpriced BS product.

 

[iRider]

@iRider
On brief skim the explanations above are correct.
I think one question of yours that didn't get answered is regarding mass vs. leverage.
....

Cheers @Flo33 and @Udi for the explanations!

 

[Udi]

The other very general point I want to make is:
Every factor should always be considered as a ratio against weight.
This is particularly important in brakes, because in many cases a rider can increase the rotor diameter for an increase in braking force as well as heat dissipation, and the weight addition is a combination of the rotor weight increase (fixed) and small adaptor/hardware weight increase (bike/setup-specific). Consider for example the rotors I run (Formula 2pc, 143g@203mm), to switch to Intend's 203x2.25 rotor would add 54g per end (108g total) or even for their 203x1.85 rotor I'd add 20g per end (40g total), for very dubious benefit in the latter case since the rotor is now thinner.

So in the only "useful upgrade" case, 108g is a HUGE jump, I know for a fact I could instead upgrade my front rotor alone to Hope 225mm (not particularly weight optimised, 229g from memory) with adaptor/hardware, and enjoy a greater net gain in overall braking performance at less cost.

The only Intend rotor that is actually weight-competitive is the thinner 1.85mm version - conveniently no test data on that one!