WTF is up with those SRAM brakes?

Karate Monkey

Well-Known Member
I agree that the differences will be minimal hence my initial statement that two piece rotors don't resist warping(even if the center is a more conductive material whatever interface is between them will likely negate any additional conductivity of the material). Even the floating ones are mostly targeting the caliper/rotor misalignment typically caused by pistons not being truly even in a caliper as it advances the pads(these are even worse for heat transfer since there is an air gap between the spider and the brake track to allow it to float) There is alot of math going on to try and determine that these different designs are better at ______(fill in the blank) but the overall improvement is going to be minimal and likely unnoticed by the rider (definitely in NJ where long downhills are kind of uncommon)

Correct. Bicycle two-piece rotors are almost entirely about weight savings. There's no heat benefit/warping protection in reality from the spiders (didn't think I was saying there was?). Just so we're clear that we're talking about the same thing:

002_2119x2500(1).jpg

Floating Rotor

P-RT-MT900_BC19Nd0175_750_750.jpeg

Two-piece

They're constructed differently (at least in theory). The brake track on the floating rotor does not--as expected--contact the spider directly. It touches the bearings, which allow it to expand--again--theoretically. The bottom brake track is fixed directly to the spider. The extended black bit hanging off the brake track is the aluminum core that is stamped into shape/painted with heat-dissipating treatment. The braking surface is literally a stainless sandwich around the aluminum core; the spider is a separate part altogether.

Warp may not be the term that would be used in the automotive world, but again, warping in the sense of a totally FUBAR'ed rotor doesn't really happen on bicycles from heat...the pads lose any ability to apply friction before that would happen (okay, maybe the Hydraulic Press channel?). Rotors definitely get wiggly [note: maybe this term is less offensive?] as they heat+cool. Fractions of a millimeter, yes, but that's really all the clearance that brake calipers have on bicycles; it makes zero difference as far as stopping/going ability, until it cools back to normal operating temps. I've seen rotors that radiated a purple HAZ ~1.5cm down the spokes (one-piece stamped rotors) that were still dead straight.

None of it matters for NJ, as you noted. You could probably stop the bike with an unusually stiff piece of cardboard.
 

shrpshtr325

Infinite Source of Sarcasm
Team MTBNJ Halter's
Correct. Bicycle two-piece rotors are almost entirely about weight savings. There's no heat benefit/warping protection in reality from the spiders (didn't think I was saying there was?). Just so we're clear that we're talking about the same thing:

002_2119x2500(1).jpg

Floating Rotor

P-RT-MT900_BC19Nd0175_750_750.jpeg

Two-piece

yes we are talking about the same thing. (someone made a reference to two piece rotors being advertised to resist warping)

They're constructed differently (at least in theory). The brake track on the floating rotor does not--as expected--contact the spider directly. It touches the bearings, which allow it to expand--again--theoretically. The bottom brake track is fixed directly to the spider. The extended black bit hanging off the brake track is the aluminum core that is stamped into shape/painted with heat-dissipating treatment. The braking surface is literally a stainless sandwich around the aluminum core; the spider is a separate part altogether.

Warp may not be the term that would be used in the automotive world, but again, warping in the sense of a totally FUBAR'ed rotor doesn't really happen on bicycles from heat...the pads lose any ability to apply friction before that would happen (okay, maybe the Hydraulic Press channel?). Rotors definitely get wiggly [note: maybe this term is less offensive?] as they heat+cool. Fractions of a millimeter, yes, but that's really all the clearance that brake calipers have on bicycles; it makes zero difference as far as stopping/going ability, until it cools back to normal operating temps. I've seen rotors that radiated a purple HAZ ~1.5cm down the spokes (one-piece stamped rotors) that were still dead straight.

each material boundary reduces the effectiveness of heat transfer, and would concentrate extra heat into the brake track. while this could work that way, in reality it is unlikely that it makes any difference. Think about it, coefficient of expansion for steel ~6.45*10^-6, lets say a tight clearance is .3mm (.11811 in), lets do some math .11811/6.45*10^-6 = 18311 def F to close up HALF of the total gap on a bicycle brake caliper . . . .( generally clearances per side are on the magnitude of .3-.8mm since a standard business card on each side can be used to make sure they are centered).


None of it matters for NJ, as you noted. You could probably stop the bike with an unusually stiff piece of cardboard.

warp is certainly used (professionally at least IME) to denote change in shape or flatness associated with internal stressed (caused by heating/cooling too much or too quickly) being out of flat from external forces is generally a bent condition.

p.s. iwould love to see someone test the cardboard theory.
 
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carvegybe

Well-Known Member
I take am taking my bike to South Africa in December and planning to ride some of the Cape Epic single track sections. There are some long descents there and I am approx. 200 lbs. Maybe here the benefit of the floating rotors makes a difference in this scenario over the quieter 2-piece rotors? Is that a good reason to keep them?
 

carvegybe

Well-Known Member
Correct. Bicycle two-piece rotors are almost entirely about weight savings. There's no heat benefit/warping protection in reality from the spiders (didn't think I was saying there was?). Just so we're clear that we're talking about the same thing:

002_2119x2500(1).jpg

Floating Rotor

P-RT-MT900_BC19Nd0175_750_750.jpeg

Two-piece

They're constructed differently (at least in theory). The brake track on the floating rotor does not--as expected--contact the spider directly. It touches the bearings, which allow it to expand--again--theoretically. The bottom brake track is fixed directly to the spider. The extended black bit hanging off the brake track is the aluminum core that is stamped into shape/painted with heat-dissipating treatment. The braking surface is literally a stainless sandwich around the aluminum core; the spider is a separate part altogether.

Warp may not be the term that would be used in the automotive world, but again, warping in the sense of a totally FUBAR'ed rotor doesn't really happen on bicycles from heat...the pads lose any ability to apply friction before that would happen (okay, maybe the Hydraulic Press channel?). Rotors definitely get wiggly [note: maybe this term is less offensive?] as they heat+cool. Fractions of a millimeter, yes, but that's really all the clearance that brake calipers have on bicycles; it makes zero difference as far as stopping/going ability, until it cools back to normal operating temps. I've seen rotors that radiated a purple HAZ ~1.5cm down the spokes (one-piece stamped rotors) that were still dead straight.

None of it matters for NJ, as you noted. You could probably stop the bike with an unusually stiff piece of cardboard.

Thanks for your contribution to this thread and for addressing some of my ignorance on the topic of rotor design. This is helping me rethink my original post about my experience with Hope rotors...
 

one piece crank

Well-Known Member
I take am taking my bike to South Africa in December and planning to ride some of the Cape Epic single track sections. There are some long descents there and I am approx. 200 lbs. Maybe here the benefit of the floating rotors makes a difference in this scenario over the quieter 2-piece rotors? Is that a good reason to keep them?
For that trip I would go tried-and-true, full-on mechanical simplicity, with the goal to enjoy riding and not fuss with fixing your bike.

Fancy is NOT better.
 

shrpshtr325

Infinite Source of Sarcasm
Team MTBNJ Halter's
Thanks for your contribution to this thread and for addressing some of my ignorance on the topic of rotor design. This is helping me rethink my original post about my experience with Hope rotors...

the floating rotors will not improve performance as noted in the discussion above, don't waste your energy worrying about swapping rotors around, find what you like and stick with it.
 

shrpshtr325

Infinite Source of Sarcasm
Team MTBNJ Halter's
yes we are talking about the same thing. (someone made a reference to two piece rotors being advertised to resist warping)


lets take a look at the math (i think only @Patrick and @jmanic will bother reading all this) behind this now that iv made some time to look up coefficients and such. as you will see a 180mm brake rotor would expand ~1mm after heating up over 320degC. (this is approx 611*f, your brakes are unlikely to ever get to this temperature since your brake fluid cannot handle this kind of temperature)

1663601155756.png

1663601171923.png


keep in mind that this math leaves out many environmental variables including (but not limited to)
1) convection (cooling of parts as air flows over them) this is actually significant!
2) heat transfer to the pads/calipers/brake fluid

1663601349744.png


when heat treating steel temperatures start at around 650*C/1200*F (double the temperature reached in this situation)
 

Patrick

Overthinking the draft from the basement already
Staff member
I
lets take a look at the math (i think only @Patrick and @jmanic will bother reading all this) behind this now that iv made some time to look up coefficients and such. as you will see a 180mm brake rotor would expand ~1mm after heating up over 320degC. (this is approx 611*f, your brakes are unlikely to ever get to this temperature since your brake fluid cannot handle this kind of temperature)

View attachment 196537
View attachment 196538

keep in mind that this math leaves out many environmental variables including (but not limited to)
1) convection (cooling of parts as air flows over them) this is actually significant!
2) heat transfer to the pads/calipers/brake fluid

View attachment 196539

when heat treating steel temperatures start at around 650*C/1200*F (double the temperature reached in this situation)

Nice. I used the online calculation and came up with heating to 300f on a 200mm rotor would expand the brake track circumference .14mm
 

shrpshtr325

Infinite Source of Sarcasm
Team MTBNJ Halter's
so 300f is about 149c plug and chug,

keep in mind i used a high estimate for the coefficient of expansion of steel (there is a range, and it could be as low as 60% of what i used depending on the specific type of steel used) specifically to magnify the differences as much as possible since we are talking about such small units here.

200*(17.3*10^-6)*149=.51554mm (diameter)

circumference = 2* pi * r = d*pi

200*pi = 628.318mm

200.52*pi = 629.952mm

difference = 1.634mm

now pick a steel with a lower coefficient say 60% or 10.38*10^-6

200*(10.38*10^-6)*149 = .309

200.3098pi = 629.289

difference = .971mm



p.s. you can thank the program for having long boring meetings today or i wouldnt be bothering with all this math
 

carvegybe

Well-Known Member
This is a recent Singletracks podcast. Fast forward to just before 48min into the podcast to hear a discussion of grippy brakes vs brakes with modulation

 

stb222

Love Drunk
Jerk Squad
This is a recent Singletracks podcast. Fast forward to just before 48min into the podcast to hear a discussion of grippy brakes vs brakes with modulation


However you missed the point, "modulation for lack of a better term", meaning less grippy doesn't mean more modulation. Validated what I have always said, modulation is a skill, not something your brake should do for you. Less grippy/grabby brakes simply means you need more force to stop the bike.
 
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carvegybe

Well-Known Member
However you missed the point, "modulation for lack of a better term", meaning less grippy doesn't mean more modulation. Validated what I have always said, modulation is a skill, not something your brake should do for you. Less grippy/grabby brakes simply means you need less force to stop the bike.
I get that. My point is that having grip is the key. Then you can train the modulation. If you don't have the grip, as was the case with my previous brakes, then any perceived benefit of being able to modulate is not really helpful.
 

w_b

Well-Known Member
yes we are talking about the same thing. (someone made a reference to two piece rotors being advertised to resist warping)



each material boundary reduces the effectiveness of heat transfer, and would concentrate extra heat into the brake track. while this could work that way, in reality it is unlikely that it makes any difference. Think about it, coefficient of expansion for steel ~6.45*10^-6, lets say a tight clearance is .3mm (.11811 in), lets do some math .11811/6.45*10^-6 = 18311 def F to close up HALF of the total gap on a bicycle brake caliper . . . .( generally clearances per side are on the magnitude of .3-.8mm since a standard business card on each side can be used to make sure they are centered).




warp is certainly used (professionally at least IME) to denote change in shape or flatness associated with internal stressed (caused by heating/cooling too much or too quickly) being out of flat from external forces is generally a bent condition.

p.s. iwould love to see someone test the cardboard theory.


0.3mm gap is ~0.012”, not 0.118” . An order of magnitude error means this lander has crashed into the Red Planet. All with no noise tho, needs atmosphere for noise.

The solution is, strap the boom box to the bike and jam The Final Countdown at full volume while pedaling the trails. Rotor noise (and friends) all gone.
 
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