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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 12:05 am 
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Joined: Thu Dec 12, 2013 9:48 pm
Posts: 72
Ability to lock the front wheel doesn't mean that you have to do it. You just use less force on levers and modulate the brake to you needs.


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 Post subject: Re: 140 front disc rotor
Posted: Tue Nov 07, 2017 12:05 am 


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 2:06 am 
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Posts: 2221
Location: Houston, Texas
Cemicar wrote:
Some forks are actually 160mm only. BMC's Teammachine SLR01, for instance.

Is that new for 2017? I faced the flat mounts on the fork of a friends BMC as the calipers would not align properly, and his bike was equipped with the BR-RS805 calipers and the front had the 140/160 adapter!

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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 3:36 am 
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Joined: Sun Jul 10, 2011 7:48 pm
Posts: 330
Location: NorCal/SoCal
uraz wrote:
You are right, road disc brakes are not weak like for example u-brake on wet carbon clincher rim but they are not as strong as they could be.

Of course in normal day-to-day use I don't have to pull levers with unhumane force, they perform well (on pair with best conventional brakes setup or slightly better), but when I want/need all the braking power that my tires can handle, sram red hydroR can't deliver it. I had high hopes for road hydraulics and maybe this is a reason I'm not so satisfied with them. I like brakes that can generate force bigger than I would ever want to use. Front wheel locking point at 80% of total capability (something like shimano xt), not at 120%.

PS> I would be totally satisfied with 40% greater maximum power. Supposedly new sram hydroHC are stronger.



I love me disc brakes and think they are considerably better feeling and more powerful than the rim brakes I've had (carbon and alloy) even in the dry, but I have to agree with you. I too feel like I could use more power and I have not found dry grip to be something that is holding back the braking. I have Deore group on my Giant Seek kid carrier bike and they are really good. I think it's down to the fact that the pads are smaller on road disc, and perhaps there is more leverage with flat bar mount levers (?). I had been checking out the Hope RX4 4-piston calipers, but waiting to hear feedback. But I am fine with what I have for now.


To the topic, I started with 140 f/r and weighed about 84kg and they were fine and had no issues, but made the switch to 160/140 (and then 160/160) and I thought the braking was much better. I'm now down to 72.5kg and don't feel like the 160's are overkill at all (per above). I have been recommending going to 160mm front to my friends regardless of weight.



.



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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 4:29 am 
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Posts: 278
At 73kg, I have been using 160 at the front and never tried 140. More disc material means more capacity to brake (to convert kinetic energy to heat). More headroom is always welcome.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 1:28 pm 
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Joined: Fri Feb 18, 2005 6:50 pm
Posts: 224
Location: Wilmington, DE
MoPho wrote:
ITo the topic, I started with 140 f/r and weighed about 84kg and they were fine and had no issues, but made the switch to 160/140 (and then 160/160) and I thought the braking was much better. I'm now down to 72.5kg and don't feel like the 160's are overkill at all (per above). I have been recommending going to 160mm front to my friends regardless of weight.


Perhaps the difference does come down to the pads. TRP HY/RD calipers use a MTB size pad (M525/M515) though I have no idea how that compares to a newer Shimano MTB pad in terms of size. I'm also use Shimano IceTech RT-99 rotors which are probably some of the best rotors out there in terms of stopping power. I'll have to actually give front 140mm a try some day and see if it really makes for a noticeable decrease in stopping power. I just can't see how it would given the power of the 140mm rear.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 3:43 pm 
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Joined: Thu Mar 09, 2017 5:35 am
Posts: 131
uraz wrote:
Ability to lock the front wheel doesn't mean that you have to do it. You just use less force on levers and modulate the brake to you needs.

Yep, I'm with you Uraz. Locking or not the front wheel has as much as how you pull the lever rather than the total power available. If you yank the heck out of any decent brake, it should lock and send you over the bars, it's the ability to apply that linear squeeze to utilize the power that's important.

I find road Avid BB7 and MTB Avid BB7 pretty much lacking in power compared with Shimano XT, SLX and even Deore MTB brakes. I think I'd be pretty disappointed if I couldn't do a one finger endo with medium finger power on road disk hydros.

I hope they aren't dumbing down road disks power in an attempt to protect people from themselves or they'll be a whole bunch of people switching out their front calipers for MTB calipers.

To the original discussion I've had 140, 160 and 180 on various MTBs and CX bikes. Extra size really helps with stopping power, but if you can deal with asymmetry it could be wise to run 140mm on the rear and save some weight.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 5:11 pm 
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Joined: Sun Jul 10, 2011 7:48 pm
Posts: 330
Location: NorCal/SoCal
Lewn777 wrote:
[
I hope they aren't dumbing down road disks power in an attempt to protect people from themselves or they'll be a whole bunch of people switching out their front calipers for MTB calipers.



I think they're dumbing it down to make it smaller and lighter.
I'm no engineer, but I imagine that they are also limited by the road bike brake levers in how they can design the master cylinder.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 6:17 pm 
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Joined: Thu Dec 12, 2013 9:48 pm
Posts: 72
Smaller pistons, smaller brake pads - no doubt they are making road discs weaker on purpose. I think that the main reason is that road bike community is over paranoid about discs in general and they don't want to risk some sunday warrior flying over the bar :P

As mentioned before being unable to do a stylish stoppie is unforgivable.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 7:09 pm 
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Joined: Tue Sep 10, 2013 12:47 am
Posts: 32
why does a smaller brake pad = less stopping power. i was taught in physics that frictional force is independent of surface area.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 11:22 pm 
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Joined: Wed Sep 20, 2006 1:03 am
Posts: 507
Location: Madison, WI USA
uraz wrote:
Smaller pistons, smaller brake pads - no doubt they are making road discs weaker on purpose.

Braking power is independent of brake pad area. The area of the caliper's piston matters, but not on its own; the hydraulic leverage ratio is determined by the ratio of the areas of the master cylinder piston and the caliper piston. Power is determined by the leverage ratio and the pad's coefficient of friction against the disc. It sounds counterintuitive, but pad area doesn't affect braking power.

rpowell wrote:
why does a smaller brake pad = less stopping power. i was taught in physics that frictional force is independent of surface area.

Ding ding ding! We have a winner! A smaller brake pad doesn't equal less stopping power. Somewhere, a physics teacher is smiling.

Two brakes with the same leverage ratio and the same pad material will have the same power. It doesn't matter if one caliper piston is 20mm in diameter and the other is 10mm in diameter; as long as the lever pistons are sized proportionally, the brakes will produce the same power.

Hydraulic systems are essentially liquid levers. If a 20mm MTB caliper is driven by a 10mm lever piston (areas in a ratio of 4:1), then the 10mm road caliper would need to be driven by a 5mm lever piston to produce the same power (these areas also have a ratio of 4:1). A 10-pound force on either lever piston would produce a 40-pound force at their respective calipers. The difference is that the fluid would be under considerably more pressure in the road system than the MTB system for a given braking force. The smaller pad would also be generating the same amount of heat with a smaller mass, so it would heat up faster and reach a higher peak temperature than the larger pad.

Let's say these two brakes happen to have pads the same diameter as their caliper piston. The 20mm-diameter pad has four times the surface area (314 mm^2) of the 10mm pad (78.4 mm^2), but they produce the same frictional force. Strange but true.

Essentially, friction has nothing to do with contact area. It's self-canceling: if you double the contact area, you cut the contact pressure in half. So why do race cars and racing motorcycles have giant brakes with six-piston calipers? It's not to get more braking power; tires and their coefficients of friction dictate that. Race cars and motorcycles use big brakes not to stop quickly but to dump heat quickly. Six-piston calipers are mostly about packaging efficiency: they let you spread the heat generation over a large pad area while still managing to fit the caliper and disc between the hub and the car rim.

This is why it's silly to get a big brake kit for your street car: if you're not on a track, you're never using the increased heat-dumping capability those kits bring. Sure, the brakes feel different, but that's due to woven steel hoses, pads with a higher coefficient of friction and generous dollop of the placebo effect.

Some people point out that sports cars generate more grip by using wider tires. It's tempting to jump to the conclusion that the increased contact patch area is what improves adhesion, but it's not true. (Also, wider tires have the same contact patch area as narrow tires unless you lower the pressure). Sports/ race cars use wide tires essentially to manage heat and to improve the shape of the contact patch. A narrow car tire tends to take all the cornering loads on its outside shoulder, and this leads to a failure mode called "chunking" or "cording." Wider tires mean you can distribute heat over a larger area, which staves off tire failure. Race car tires are often heated above the boiling point of water, so heat management is a huge deal. In fact, racers use infrared imaging (and tire pyrometers) to ensure that the tire is heating up evenly across its width. They'll adjust tire pressure to even out the heat contours, avoiding tire failure due to overheated (and chunked) outside edges.

So yeah, mountain bikes don't have more stopping power because their tires are bigger. Once your brake is strong enough to lift the rear wheel--on road or off--max stopping power is dictated by where your center of mass is. Low and far behind the front wheel is what you want here...this is why tandems can stop faster than single bikes.

Smaller discs mean it takes more hand force to produce the same amount of power. That's one reason 140mm rear rotors make a ton of sense for both road bikes and XC bikes (though not for tandems). Time will tell whether 140mm can dissipate enough heat If the leverage ratio is different between two brakes, the power they produce per unit of hand force will be different. You can't just compare caliper diameters between brakes because caliper diameter doesn't matter: caliper piston diameter doesn't matter unless you also know the master cylinder diameter. Even then, you'll need to know the ratio between lever travel and master cylinder displacement.

The upshot is that it's really hard to compare the power of mountain bike brakes vs. road bike brakes unless you know a lot about each brake. The easiest thing to do is just to ride both and compare that way. There's a huge margin of error, but perceived power is what people actually care about anyway.

Physics aside, I've ridden Sram's Rival 1 hydraulic brakes, and they're certainly less powerful than my mountain bike's XTR Race brakes. I too would like the ability to do 1-finger stoppies on the road...I'm curious to try Etap HRD and the new Dura-Ace hydraulic discs to see if they're actually down on power like the SRAM brakes I tried.


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 Post subject: Re: 140 front disc rotor
PostPosted: Tue Nov 07, 2017 11:49 pm 
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Joined: Mon Apr 03, 2017 2:54 pm
Posts: 56
youngs_modulus wrote:
[Snip]


Coming from a physicist, excellent post! I'd just like to emphasize a few points. First of all, the diameter of the rotor will affect the stopping power of a braking system, all else being equal. This is because the same force applied farther from the hub will result in a greater torque and thus slow you down quicker for the same pull at the lever. (assuming you don't manage to lock up the wheels)

Secondly, while it is generally true that the force due to friction is independent of surface area, it is not always true. As you approach the point where the material itself deforms the friction becomes nonlinear and eventually falls off. The analogy I like to use is imagine you're trying to push a big pink (1x2") eraser across a table top. With just a little bit of weight on it, it is extremely difficult to get it to move. But if we apply the same conditions to the little eraser on the end of a pencil, it deforms and then skips and slides across the surface. That being said, the brake pad compounds used on bikes and cars are sufficiently hard to nearly completely negate this failure mode, so it's not particularly relevant to this discussion.

Finally, a point that many proponents of rim brakes fail to grasp is that the coefficient of friction between your tire and the tarmac is far greater when the wheel isn't locked up and the tire sliding. So the fastest way to stop your bike is to get your weight as far back on the bike as possible, and brake as hard as you can without the wheels locking up and without you going over the bars. This is one of the real advantages of disks. The modulation they provide makes it easier to approach that point where the wheel locks up without going too far. This is especially beneficial in poor conditions where the line between slowing down effectively and locking up the wheels is very fine. (This effect is very clearly shown in cars if you do a panic stop (full application of brakes) on a car with and without ABS engaged. The ABS will stop the car far, far faster than just locking up the wheels and sliding.)

Edit: In the name of full disclosure, I used to ride a 2011 Scott CR1 with 105 5800 rim brakes and now ride a Felt F3X cross bike with 685 shifters and 785 calipers on 160mm RT-99 rotors front and rear.


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 Post subject: Re: 140 front disc rotor
PostPosted: Wed Nov 08, 2017 12:48 am 
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Joined: Wed Sep 20, 2006 1:03 am
Posts: 507
Location: Madison, WI USA
CallumRD1 wrote:
youngs_modulus wrote:
[Snip]

Coming from a physicist, excellent post!

Thanks! I'm an applied physicist, so if you found serious problems in my post, that would say troubling things about my competence. ;)

CallumRD1 wrote:
I'd just like to emphasize a few points. First of all, the diameter of the rotor will affect the stopping power of a braking system, all else being equal.
<snip>
Secondly, while it is generally true that the force due to friction is independent of surface area, it is not always true.
<snip>
That being said, the brake pad compounds used on bikes and cars are sufficiently hard to nearly completely negate this failure mode, so it's not particularly relevant to this discussion.


All absolutely true; thanks for pointing them out. Rotor diameter (and tire outer diameter) are both part of the overall braking power equation. Regarding friction vs. surface area, I left out the tiny corner cases where friction does correlate with surface area because I thought they'd be more confusing to describe than to omit. They exist, yes, and as you suggest, they're not germane to the topic at hand.

CallumRD1 wrote:
Finally, a point that many proponents of rim brakes fail to grasp is that [μs > μk].


Yes; I find it much easier to modulate my CX bike's hydraulic discs than I do my road bike's rim brakes...it's easier to stay on the right side of the static/kinetic friction divide. I'm hardly skidding all over on my rim-braked bike, but I can comfortably get closer to skidding (and stay closer) on my bike with hydraulic discs.


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 Post subject: Re: 140 front disc rotor
PostPosted: Wed Nov 08, 2017 4:31 am 
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Joined: Mon Oct 02, 2017 11:35 pm
Posts: 3
I'm a big guy, 6'2 230lbs and ride a Giant TCR Advanced Pro 0 Disc on a set of Enve 5.6 Discs. The bike came stock with 140mm rotors on both the front and rear and I haven't experienced any issues and the bike had no problem stopping me on some pretty fast descents. I did experience some very temporary rubbing from the rotor and pads due to heat deformation after long brakes


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 Post subject: Re: 140 front disc rotor
PostPosted: Wed Nov 08, 2017 4:48 am 
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Joined: Tue Jun 30, 2015 3:30 am
Posts: 278
fast descents = better cooling and less continuous brake dragging... maybe that's why.


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 Post subject: Re: 140 front disc rotor
PostPosted: Wed Nov 08, 2017 1:12 pm 
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Posts: 2667
Location: Islip, NY
youngs_modulus wrote:
So yeah, mountain bikes don't have more stopping power because their tires are bigger. Once your brake is strong enough to lift the rear wheel--on road or off--max stopping power is dictated by where your center of mass is. Low and far behind the front wheel is what you want here...this is why tandems can stop faster than single bikes.


I'm not saying that you meant this, but it's worth pointing out.

The difference between wider tires on a car and wider tires on a bike is typically bike tires have increased volume as well. Wider tires on performance cars tend to have lower profiles so the volume is similar or even less (not the case with Formula 1 tires). Resulting tire pressure is about the same, 35-40psi for most road cars.

I point that out because as a result, wider bicycle tires are run at a lower pressure indeed resulting in a larger contact with the ground. I run my disc road bike at 35psi (650X42) so I have more than double the rubber the typical 700cX23-25mm tire setup has. That results in more grip for braking, cornering or whatever combination of the two. That's why I prefer that setup this time of year when grip is generally lower due to lower temps, wet, and increased debris. It's not so much slower that I can't keep up on the group rides.

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 Post subject: Re: 140 front disc rotor
Posted: Wed Nov 08, 2017 1:12 pm 


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