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 Motorcycle Safety
 Physics and the theoretical
 Single Brake vs Dual Brake
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commonground
Male Standard Member
155 Posts


Windsor, PA
USA

Yamaha

V Star 1300

Posted - 12/02/2015 :  5:47 AM                       Like
I frequent a chat site upon which this subject arose and I thought that I would offer it for your review and comments.

The premise of one individual was that, having only one brake disc on the front wheel causes the front wheel to pull in the direction of the disc under a hard braking situation. Therefore, dual discs offer a more controlled stop by keeping the bike straight.

Your thoughts

scottrnelson
Advanced Member
6881 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 12/02/2015 :  8:16 AM
Unless you have weak forks that can twist due to brake pressure, that is impossible.

I have a single-disc bike out in the garage with long forks (and about twelve inches of suspension travel) that would be most likely to twist if that were possible. I can actually hold the front tire between my knees and push on the bars and get about 1/4" of twist at the front edge of the front tire if I push real hard. But if you look at where the braking forces go into the fork leg, which is above the front axle, pushing forward, it doesn't have anywhere near as much leverage as that at maximum braking.

Maybe this is something for Mythbusters, but I've never felt the slightest pull to one side or the other during braking on bikes with one brake disc.

The only real disadvantage that I know of is that a single disc will heat up more during heavy use than a dual disc will. But I've never used enough brake long enough to have that issue either.
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greywolf
Male Moderator
1483 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 12/02/2015 :  10:53 PM
I've owned many single front disc bikes. Braking does not cause steering input. The assemblies are too stiff to allow that. As was mentioned, braking is about converting kinetic energy into heat energy. Dual discs allow smaller, lighter discs and calipers to be used because two assemblies are dissipating heat.
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 12/05/2015 :  7:34 AM   Join poster on Facebook as Friend  
It seems to me, that if having one disc mounted on one side of a wheel, and one caliper mounted on one fork leg, generated any kind of twisting force, it would turn the whole front end. It would be easier to turn the forks running in bearings, than twist the forks.

But that doesn't happen. It seems to me if there was a steering force from one brake, the manufacturers wouldn't offer them, and if they did anyway, the regulations would prohibit it.
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scottrnelson
Advanced Member
6881 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 12/05/2015 :  7:42 AM
Even back in the days of drum brakes in front, the braking pressure always went to into one fork leg, not both. At least that's how it was on all of the drum brake bikes that I owned. It would have had the same twisting forces as disc brakes, except that the forks were usually much more spindly back then and more likely to twist.
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 12/05/2015 :  8:43 AM   Join poster on Facebook as Friend  
Didn't even think about that Scott. Yeah, the drum was secured from rotating on one side, by a brake stay arm to one fork leg.
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Horse
Senior Member
257 Posts


Newbury, Berkshire
United Kingdom

BMW

R850RT

Posted - 12/06/2015 :  1:42 PM
Opposite way round, on my old R100RT I could hear the drilled discs whirring as the forks twisted during cornering . . .
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scottrnelson
Advanced Member
6881 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 12/06/2015 :  5:51 PM
quote:
Originally posted by Horse

Opposite way round, on my old R100RT I could hear the drilled discs whirring as the forks twisted during cornering . . .

Are you sure it wasn't tire tread?

I don't have to lean very far on either of my bikes to get additional tire tread noise. But I wear earplugs if I'm going far and don't hear distractions like that.
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 12/06/2015 :  7:44 PM   Join poster on Facebook as Friend  
Never heard my discs on cornering on my old R90S, but those fork tubes were both skinny, and long. Small diameter axle, and the top triple clamp was a light piece of sheet metal. Mine came with an aftermarket fork brace already installed, I bet it helped a LOT.

Funny too on a side note. The bike became ancient in my hands, no rebuild kits for the calipers. One caliper became froze up, so I eliminated the hydraulic line to that side, and removed the pads.

I braked just as well with one disc, and didn't twist/turn the wheel on braking. I think lack of one disc was offset by more hydraulic pressure to the working disc. ( I did not test repeated braking down any mountain that way though, to test for fading.)
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commonground
Male Standard Member
155 Posts


Windsor, PA
USA

Yamaha

V Star 1300

Posted - 12/07/2015 :  5:29 AM
quote:
Originally posted by rkfire

I think lack of one disc was offset by more hydraulic pressure to the working disc.


How come there is more hydraulic pressure?
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scottrnelson
Advanced Member
6881 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Posted - 12/07/2015 :  9:48 AM
quote:
Originally posted by commonground

How come there is more hydraulic pressure?

Rather than having to push two sets of slave pistons outward, it only has to push one set, so you have only half of the surface area which should result in twice the pressure applied there.
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commonground
Male Standard Member
155 Posts


Windsor, PA
USA

Yamaha

V Star 1300

Posted - 12/07/2015 :  3:58 PM
In rkfire's situation: you removed one brake and blocked the hose end, the following is my logic. Maybe it is flawed.

1. The hydraulic pump (pump in the brake reservoir)is still the same.
2. The hydraulic pressure in the closed system is still the same if the same pressure is exerted on the brake lever.
3. The brake caliper piston is still the same size. Therefore, it exerts the same force on the brake pads.
4. The overall friction and/or stopping power is half what it was before with the same brake lever input.

It seems to me, that, you would have to increase the pressure on the brake lever to increase the hydraulic pressure in the system to get the more braking action. The remaining brake would get hot more quickly with the additional friction if additional pressure was applied.

Edited by - commonground on 12/07/2015 4:07 PM
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 12/07/2015 :  5:43 PM   Join poster on Facebook as Friend  
3. The caliper piston size is effectively half the size, since one is removed from the system.
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commonground
Male Standard Member
155 Posts


Windsor, PA
USA

Yamaha

V Star 1300

Posted - 12/08/2015 :  5:05 AM
So with that logic, if you had a vehicle with a four wheel hydraulic disc brake system and you applied 20 lbs of pressure to the system. All brake units would feel 20 lbs of pressure. Then with the stated logic, if you remove the brake units one at a time the hydraulic pressure goes up. If you got down to one brake the hydraulic pressure would be very high. Yet the pump still only puts out the original 20 lbs of pressure.
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rkfire
Advanced Member
1688 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 12/08/2015 :  1:24 PM   Join poster on Facebook as Friend  
I'll take a stab, but am no hydraulics engineer. I think it's less movement on the brake lever for the same force to the one caliper instead of 2.

On that old BMW, the regular models got only one disc, but the same disc size, same caliper, same master. Obviously that worked fine, for the era. Two discs were more about surface area and fade.

The pump is the hand at the brake lever. I think, removing a disc decreased the lever movement needed. In the end, I had one disc as the regular BMW had all along, and it worked fine.

The only other possibility is, one less set of hoses to expand with pressure, may have added a small amount of efficiency to the remaining brake.
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commonground
Male Standard Member
155 Posts


Windsor, PA
USA

Yamaha

V Star 1300

Posted - 12/08/2015 :  2:09 PM
Your quote: "I think it's less movement on the brake lever for the same force to the one caliper instead of 2."

I can buy into that. Moving one piston instead of two pistons takes less fluid.

I just can't wrap my head around the change in hydraulic pressure.
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Horse
Senior Member
257 Posts


Newbury, Berkshire
United Kingdom

BMW

R850RT

Posted - 12/09/2015 :  1:35 AM
quote:
Originally posted by scottrnelson

quote:
Originally posted by Horse

Opposite way round, on my old R100RT I could hear the drilled discs whirring as the forks twisted during cornering . . .

Are you sure it wasn't tire tread?

I don't have to lean very far on either of my bikes to get additional tire tread noise.




Hmm don't know. That bike went 20-odd years ago, so no chance of checking. I did have tread on the strip of tyre which was usually on the ground, though :)

[Road tyre too, not traily]
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