mtnbk3000

Hi does anybody know what the % increase of power when you increase from 160mm to 203mm rotor

Hanzo

17.381%

Dannihilator

What? That just completely went over my head.

Jeremy D. Swain

He's saying how much extra braking force do you gain, by going from a 160mm rotor to a 203mm rotor. Also, he wants that gain in % form.

Dannihilator

Being that I suck at math and don't remember much physics. I'm going to estimate a 10-20% increase.

However I would not put an 8" rotor on a fork that doesn't run a 20mm thru axle.

Flak

hahahah good one

Nickds7

I would have to argue with you and say 19.7396%

Gravity Worx

There is definately a difference you can feel.

That difference is primarily in the form of having much less brake fade under heavy braking conditions and a little more actual stopping power too.
Now if you go to a good stainless wavy rotor, it will stay even cooler than the standard slotted rotors and work a little better yet.

Putting that into a % form though?
No, too many variables.

The differences will vary depending on various things ranging from your weight, bike weight, tire/wheel weight (rotating mass), how hard you are braking at that particular point, how much you are using the brakes and getting them hot on that section of trail, weather will have an effect on the temperature.

If you are trying to make the most powerful brakes possible, go to a good stainless braded line that will not expand when you squeeze the lever and cause that mushy feel, run good compound brake pads, and also the wavy rotors in the largest size that your frame or fork will allow.

203's are fine with QR as well. 20mm is NOT needed for 203mm rotors.

Maelstrom 

I actually remember a study done back in the day by some brake company. I think it measured at 33%.

Obviously outside a controlled study there are a lot of variables however.

And no I can't find the study, actually better yet, I don't have the inclination to look for it.

mcoine

There aren't a lot of variables. The only variable that was changed was the radius of the rotor.

Torque = force x radius

The force is the force applied to the rotor by the caliper and remains constant because we haven't changed the brake system. So the % change in torque is the same as the percent change in radius..

(101.5mm - 80mm)/80mm x 100% = 26.9%

Of course, as we've discussed in another thread, none of this means anything in terms of decreased stopping distance.

Stacey

Thread Closed

probable556

https://bikeforums.net/showthread.php?t=303777

If you read this thread, the answer will be obvious.

Gravity Worx

So brake temp causing brake fade is not a variable according to you?
You might want to re-think that one.
All of the things I listed off will make a difference in braking temps as well as the rotor size it's self.
These are some of the differences from your formula to actual real world applications.

Also, brake pad compound makes a difference as well.
A difference is a variable.

born2bahick

One finger!

BugsInMyTeeth

Chicks dig 203s.

mcoine

Sure, the slight increase in surface area will cause a slight increase in heat dissipation, causing a slight decrease in fade. But read the OP's question, he's specifically asking about the increase in "power":

I took this to mean, what is the increase in torque applied to the wheel by the brake.


Yes, brake pad compound makes a difference, but not in this situation, because the op is not changing that variable. The only variable that was changed was rotor size. If you want to make up your own hypothetical brake experiment, go ahead, but don't add variables to this person's question.

Gravity Worx

I didn't make anything hypothetical.

You gave this formula:
(101.5mm - 80mm)/80mm x 100% = 26.9%
The formula is fine for a single use application on paper and I think IS accurate for that, but that's all it's good for.
It simply can'y hold true in real world conditions.

The heat and all make a big difference in braking power and in power fade due to exess heat from braking hard and/or from lack of dissipitation.
Rotor size makes a big difference in factoring these variables into an actual percentage of braking power.

The reason that the rotor size makes such a big difference is because the larger rotors will cool at a faster rate than the smaller ones and your formula does not bring in how heat induced brake fade effects braking power at any point on the trail.

Simply put:
Smaller rotors will get hot and fade faster and then take longer to come back.
Now back to my first answer:
There is definately a difference that you can feel. But to put a definate % on it? NO, it has too many variables.

That's my take.
I'm out.

cryptid01

Combine mcoine's number with the knowledge that larger rotors are less likely to heat soak, and we can infer that the larger rotor should provide at least 27% more power. His answer has utility.

Gravity Worx's response has none.

BugsInMyTeeth

I did mention that chicks dig 203s, right?



Nuff said.

mtnbk3000

in that case, i guess i will get two

Dannihilator

Unless you are running a full on DH bike, Dual 203mm rotors on a hardtail is overkill.

willtsmith_nwi

Nope, this one is easy. Most brakes work by converting kinetic energy to heat. At a fixed pressure, the amount of heat a caliper produces is dependent on the amount of surface that passes under it. For a given rotation of the wheel (which is strictly dependent on your linear speed) the amount of surface that passes under the rotor is 2πr. Here r would be the center of the pads from the hub to the edge of the rotor.

Expressed as a percentage, this would be the ratio of the two linear distances of material passing under the pads, r1, r2. The we have 2πr1/2πr2 = r1/r2.

A 160mm is 80mm radius but I'll give 10 mm for the pad centers so we'll estimate an effective radius of 70mm. Likewise for a 185mm rotor we'll estimate a 83mm effective radius. For a 203mm rotor, 91mm.

From 160mm to 185mm we get 83/70 = 1.185, which is 119% of the original or a 19% increase.

From 160mm to 203mm we get 91/70 = 1.3, which is 130% of the original or a 30% increase.

From 185mm to 203mm we get 91/83 = 1.096, which is 110% of the original or a 10% increase.

Note that the big bump is from the 160mm to the 185mm rotor.

Gravity Worx

I came back to this thread to see what you all came up with and I see some mistakes on here.
Gastro is correct in that the heat it's self should expand that % a little, but that's about as far as I agree with his input on this one as the other variables have a larger effect in the other direction than just rotor heat.

I guess I should have gone much further in detail in illustrating the differences between the 2, rather than just showing the heat differences.

I see 2 mathmatical responses on here and I do see relevance in both as far as actual braking possible at the rotor it's self.
26.9% and 30%.
3.1% difference between the 2 will be with in allowable variences due to pad compound, rotor thickness as it pertains to heat dissipation etc.

I say that there is a much lower difference in usable braking power between the 2 sizes, but that there is a difference that you can feel.
I'll give this answer from experience in riding on both sizes of rotors on the same bike, annd also from the info that I've gotton from Galfer brake as I do have a nice relationship with them.

So here is the expanded answer with utility:
There is not 26.9% to 30% improvement in actual braking power because of the other variables involved with the bike in motion.
The weight of the bike, the weight of the rider, the weight of the tires/wheels (rotating mass carries a 7:1 ratio to weight effect), traction, how flat or steep of drop are you trying to brake on, road/trail surface, and a few others.

Think about it, going flat or uphill on pavement with reletively cool brakes, you get killer traction and see the largest difference between the 2 sizes. Here you MIGHT even get that 26.9% to 30% difference, but no where else.
The opposite end of the scale will be a nasty steep section at the end of about 5 miles of DH course on loose silty terrain where you will probably not get any difference in usable braking power because you are at this point most likely going beyond the traction limitations and have a perverbial ton of heat build up in the rotors.
This illustrates the difference between the actual force possible at the rotor as opposed to actual usable braking power.
The real world application under actual riding conditions carries much utility.

The actual difference in usable braking power when going from 160mm to 203mm rotors is a sliding scale depending on all of these variable combined that will range from 0% difference to approximately 15% difference in about 80% of normal riding conditions.
Again, the biggest gain will be in the reduced brake fade in the larger rotors.

Try this.
Run 160mm rotors on your bike and then swap out to 203's and see the difference for your self.
Ride it with both set ups in as large a variety of conditions possible and you will then get it.

willtsmith_nwi

True enough. But realize that applying brakes constantly for 5 miles may lead to hand cramping. The larger rotor allows the same braking force with less pressure applied to the hand levers. Hence less cramping.

myidolis

I can't see how knowing a theoretical percent would come in handy in the real world...i mean, a rough estimate would be good enough to know if it's worth upgrading, but in reality, there are indeed way TOO many variables to consider. Humidity, atmospheric pressure, speed of travel, smoothness of the terrain, tread of the tires,(to name a FEW) for example would all come into play here...assuming you're looking to gain an actual accurate percent, you would need all these conditions (and several others) to be identical, which we all know isn't going to happen in the real world, so going into the amount of detail presented here (while impressive, i admire all of your manipulation of physics and mathematics to achieve your slightly different answers) really isn't necesarry. A rough estimate should due for all purposes other than hypothetical.

...on the other hand, it is fun to see how much of my physics i've forgotten! good thing i'm taking it next semester!