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Rim, Mechanical Disc, Hydraulic Disc, LOCKUP!

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Rim, Mechanical Disc, Hydraulic Disc, LOCKUP!

Old 07-19-20, 12:32 PM
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taylorgeo
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Rim, Mechanical Disc, Hydraulic Disc, LOCKUP!

What does it mean to have a brake inadvertently lockup a wheel, potentially causing one to fall off a bike?

Are there certain braking systems that are more prone to this happening?
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Old 07-19-20, 12:36 PM
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Exactly what it sounds like. Wheel stops completely while the bike is still in motion, causing the tire to skid across the ground. Skidding tire provides far less control than a rolling tire, potentially leading to a crash or fall.

All good braking systems are capable of locking the rear wheel while in motion. Physics typically prevents a front wheel lockup on dry pavement. On wet pavement or loose surfaces, a front wheel lockup can occur and be VERY dangerous, almost always leading to a crash. Rear wheel lockups are easily controlled and don't often result in a crash.

Bear in mind something like 80 to 90% of your braking potential comes from your front wheel, because as you decelerate, most of your combined human+bike mass loads up onto the front wheel. This provides far greater traction for braking, and subsequently leads to far less traction available on the rear tire, which makes the rear tire much easier to lock up (but less dangerous when it happens).

Last edited by General Geoff; 07-19-20 at 12:43 PM.
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Old 07-19-20, 01:11 PM
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Originally Posted by taylorgeo View Post
What does it mean to have a brake inadvertently lockup a wheel, potentially causing one to fall off a bike?
It means "user error." They don't lock w/o input from the rider. Become familiar with your system. I expect many of us have locked a rear wheel during moments of inattention or panic, no need for it to cause a crash.
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Old 07-19-20, 01:11 PM
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Originally Posted by General Geoff View Post
Exactly what it sounds like. Wheel stops completely while the bike is still in motion, causing the tire to skid across the ground. Skidding tire provides far less control than a rolling tire, potentially leading to a crash or fall.

All good braking systems are capable of locking the rear wheel while in motion. Physics typically prevents a front wheel lockup on dry pavement. On wet pavement or loose surfaces, a front wheel lockup can occur and be VERY dangerous, almost always leading to a crash. Rear wheel lockups are easily controlled and don't often result in a crash.

Bear in mind something like 80 to 90% of your braking potential comes from your front wheel, because as you decelerate, most of your combined human+bike mass loads up onto the front wheel. This provides far greater traction for braking, and subsequently leads to far less traction available on the rear tire, which makes the rear tire much easier to lock up (but less dangerous when it happens).
But what actually causes a wheel to lockup? Are there certain braking systems that are more prone to wheel lockup, thus making them more dangerous? I can't decide on rim, mechanical disc, or hydraulic disc.

I just don't want my 350 lb. body flying over the handlebars!
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Old 07-19-20, 01:28 PM
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A wheel locks up when the applied braking force is greater than the traction available to the tire. Every good braking system is readily capable of locking up the rear wheel at least.

the only way to avoid locking up good brakes is to moderate the pressure you apply to the brake lever with your hand. Use less pressure on wet or loose surfaces. Get in the habit of using both brakes to slow you down, but apply a bit more pressure to the front brake than the rear.

Hydraulic disc brakes will require less force to lock up than mechanical discs. In my experience, mechanical discs require a bit more force to lock up than well-adjusted rim brakes, but are also more easily modulated. ANY of them should be able to lock up the rear wheel at speed with a hard grab.

As I mentioned before, front wheel lockups are nearly impossible on dry pavement. On wet pavement or loose surfaces you want to allow yourself extra distance to stop so you can apply less force to the front brake, which will prevent a lockup.
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Old 07-19-20, 01:40 PM
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I don't think you need to worry.

Old style cantilevers had the potential to drop a cable across the tire under tension, causing an immediate tire lockup. This only happens if a cable breaks and a backup like a hanger or reflector isn't there.

Aside from that, I don't know of any catastrophic lockups.

A crooked wheel might lock it up but would slow you to crawl, not throw you.

A derailleur into the spokes could lock you up and cause expensive damage.

Be careful of hoodlums putting sticks between the spokes.

Generally, mal adjusted brakes drag, and make you slow. Or do nothing and you hit a mail truck. They don't throw you off the bike.
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Old 07-19-20, 01:48 PM
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Originally Posted by taylorgeo View Post
Are there certain braking systems that are more prone to this happening?
Dry (non lubricated) drum brakes - those would work pretty much on/off. However, drum brakes aren't used on bicycles anymore, AFAIK, and were quite rare to begin with.

A wheel locking up is usually understood as loss of traction due to braking, which when happening to front wheel makes going over the bars impossible (but can be very dangerous nonetheless).

To avoid going over the bars when applying front brake, learning to apply it correctly is pretty much the only way, short of having a weak front brake. A suspension fork can work well in this regard too, but finding one that works with your weight could be not worth the trouble.
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Old 07-19-20, 01:54 PM
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Originally Posted by taylorgeo View Post
But what actually causes a wheel to lockup?
If the friction occuring between brake pads and moving surface is greater than the friction generated between tire and road, the wheel will lock up.
Alternately, if the brake can generate more torque than what can be generated by the rider acting as a counterweight, the bike will topple over forward.
In short, user error. Braking too hard for what the surface or rider position allows.
Originally Posted by taylorgeo View Post
Are there certain braking systems that are more prone to wheel lockup...?
In a sense. The more effort it takes at the lever to create a given amount of braking, the more difficult it becomes to lock up a wheel. Stainless rims, single pivot caliper brakes, rain would make it hugely unlikely to lock up a (front) wheel. Even at a white-knuckle effort.
At the other end of the scale you find DH-rated hydraulic brakes. These will happily lock a wheel at a very moderate 2-finger effort.
Originally Posted by taylorgeo View Post
Are there certain braking systems that are more.... dangerous?
Barring some quite rare accidents, no brake locks up entirely by itself. Some rider engagement is required.
Personally, Iíve always felt more able to get the braking Ējust rightĒ if I donít need to apply a white-knuckle effort.
Iíll happily trade the potentially greater risk of misuse for the greater performance.
Accidents donít have to happen. The longer braking distance is always there.
Originally Posted by taylorgeo View Post
I can't decide on rim, mechanical disc, or hydraulic disc.
There must be hundreds of posts, if not threads, discussing the merits f different systems.
Originally Posted by taylorgeo View Post

I just don't want my 350 lb. body flying over the handlebars!
Spend some time doing deliberate brake practice. Learn to scoot your body back, behind the saddle.
I prefer to apply a little bit of rear brake as a warning system. When the rear locks up, I know the rear is getting close to lift off the ground. Time for some caution.
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Old 07-19-20, 02:37 PM
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Originally Posted by taylorgeo View Post
But what actually causes a wheel to lockup?
There's no mysterious mechanism at play, if that's what you're asking. Aside from a hard mechanical blockage like throwing a stick into your spokes, wheels don't just "lock up."

If you brake hard enough that a tire loses traction with the ground, the tire begins to skid. Since there's no longer static friction between the ground and the tire, the ground's ability to force the wheel to spin is diminished, and so the brake is able to decelerate the wheel until it stops rotating. That's the "lock up." The skidding isn't caused by the wheel locking up: instead, the brake was able to halt the wheel because the tire started skidding.

A lot of people get it backwards, thinking that there's some braking force threshold where the brakes will suddenly "lock" the wheel, and that this this causes skidding or other shenanigans to ensue. This is not how it works. As long as a wheel maintains traction with the riding surface, braking force will not "lock" it up.
This is why you basically never see anyone skid a front wheel on clean dry pavement. The more braking force is applied, the more the rider's weight and momentum get forced onto the front wheel, and therefore the more traction there is over the wheel. Since the front tire is being pressed extremely hard against the ground, it never begins to skid, so the wheel never "locks."
On looser or slipperier surfaces, the weighting of the front wheel under hard braking may not increase traction adequately to prevent the tire from skidding, allowing the front brake to "lock" the front wheel when the tire begins to skid.

The common phenomenon of new riders "going over the bars" when they grab their brakes is often attributed to "brake lockup", but it's actually usually caused by people not bracing their arms and body appropriately against the braking forces.
The bicycle begins to decelerate after the brakes are applied, but the rider's body does not. When the rider is moving faster than the bicycle, "going over the bars" is the inevitable consequence.
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Old 07-19-20, 02:43 PM
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I have BB7s on both my MTB and gravel bike...The rear is easy to lock up and skid on any surface including pavement. The front won't lock up on pavement but it will lock up on slippery loose terrain such as gravel, snow and dirt.
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Old 07-19-20, 02:54 PM
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Originally Posted by taylorgeo View Post
But what actually causes a wheel to lockup? Are there certain braking systems that are more prone to wheel lockup, thus making them more dangerous? !
The surface that you're riding on has a lot to do with it. Also the position of your body on a bike. For example: if you stand up on the pedals and lean forward and brake hard it could cause you to fly over the bars. Riding down very steep terrain and braking hard can also throw you over the bars. That's why mountain bikers shift their weight back a lot when riding in steep rough terrain , it prevents them from flying over the bars...Hydraulic brakes are more powerful than mechanicals or rim brakes and more likely to cause a lock up.
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Old 07-19-20, 03:07 PM
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It is entirely possible to lock up the front wheel of a bicycle by hard braking on dry pavement. I've personally seen it done. The result is an over the handlebar crash.

Bicycle component manufacturers in the early days of linear pull brakes devised various control systems with a spring of some kind to damper braking effort. I haven't seen one of those in a while so I assume they were either ineffective or deemed unnecessary. My advice is whenever you decide to use a lot of front brake, shift your body weight backward by sliding your butt back behind the saddle.
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Old 07-19-20, 03:46 PM
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Originally Posted by Retro Grouch View Post
It is entirely possible to lock up the front wheel of a bicycle by hard braking on dry pavement. I've personally seen it done. The result is an over the handlebar crash.
My advice is whenever you decide to use a lot of front brake, shift your body weight backward by sliding your butt back behind the saddle.
Your advice works because the issue you're solving isn't wheel lockup. If there was some sort of instant-stiction phenomenon happening between the pads and rim (or pads and rotor), shifting the weight backwards would do nothing to prevent the resulting violent crash.

It looks like "wheel lockup" when someone goes over the bars from hard braking because a bicycle by itself is a fairly lightweight object that can be extremely rapidly decelerated by brakes. When a rider gets their weight back and firmly holds that position, they're forcing the deceleration of the bicycle to also decelerate the rider.
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Old 07-19-20, 03:51 PM
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Don't worry, they're all fine; just practice braking gently at low speed somewhere safe, just to get to know what your new brakes feel like.
All of the advice about braking technique, above, is great too (obviously).

Good luck and welcome to cycling!
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Old 07-19-20, 04:48 PM
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On my Bike Friday* The BB7 Mountain had little modulation making the bike stop out from under me @ a corner
had me seeking a different front brake.. TRP Hy Rd (avid speed dial levers have worked on both)

*on a 20 " wheel , a 160 is relatively huge disc ..

Note: A bunch of gentle braking will bed the new pads to the rim, too..





...

Last edited by fietsbob; 07-19-20 at 05:04 PM.
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Old 07-19-20, 05:00 PM
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Uneducated or not having direct involvement to gain knowledge of the mechanical operation to a general bicycle's brake system leads to a lock up situation while under normal conditions. In the event of an unusual situation, locking the brakes up may occur & that may be related to a lacking cognitive skill or poor instinct decision making.
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Old 07-19-20, 08:23 PM
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Originally Posted by taylorgeo View Post
But what actually causes a wheel to lockup? Are there certain braking systems that are more prone to wheel lockup, thus making them more dangerous? I can't decide on rim, mechanical disc, or hydraulic disc.

I just don't want my 350 lb. body flying over the handlebars!
At your weight, you definitely WANT strong braking power. Any type of brakes should be sufficient, as long as they are set up properly and you learn to modulate them safely. It's not difficult. Just take it easy down the hills, at first, and you will get the "feel" of the brakes.
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Old 07-19-20, 08:35 PM
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Originally Posted by subgrade View Post
To avoid going over the bars when applying front brake, learning to apply it correctly is pretty much the only way, short of having a weak front brake. A suspension fork can work well in this regard too, but finding one that works with your weight could be not worth the trouble.
The rider can also move his or her CoG backwards by moving the butt to the rear end of the saddle or even shortly before braking to further prevent going over the bar during hard / emergency braking with greater force applied to the front brakes.

I admit, it's going to be fairly challenging to do for a 350 lb rider. Perhaps, avoid coming to a situation, ride at less speeds so you don't need to use as much braking force to stop.
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Old 07-19-20, 08:35 PM
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Originally Posted by taylorgeo View Post
But what actually causes a wheel to lockup?
Having properly working brakes, and squeezing the lever too hard.
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Old 07-19-20, 08:56 PM
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Looks like I'm getting the Kona Lana'i. It's $600 bucks, my max budget. Mechanical Disc brakes.

At 350 lbs., I think I should be fine just riding around the neighborhood on paved (and bumpy) concrete, then hitting some flat dirt trails. I just worry about hitting that unexpected pothole or tree branch and losing control of the bike.
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Old 07-20-20, 01:07 AM
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Originally Posted by taylorgeo View Post
I just worry about hitting that unexpected pothole or tree branch and losing control of the bike.
Never ride beyond your ability. Never ride beyond your sight line (i.e. if you can't see a clear path, slow the heck down until you can). Stay in your comfort zone and always be aware of your surroundings and the road surface.
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Old 07-20-20, 02:14 AM
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Originally Posted by cubewheels View Post
The rider can also move his or her CoG backwards by moving the butt to the rear end of the saddle or even shortly before braking to further prevent going over the bar during hard / emergency braking with greater force applied to the front brakes.
True, but I view that as a part of the correct procedure altogether. It won't happen if it isn't learned to the degree that it's done instictively, there's never enough time to do it consciously.
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Old 07-20-20, 03:09 AM
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Originally Posted by Retro Grouch View Post
Bicycle component manufacturers in the early days of linear pull brakes devised various control systems with a spring of some kind to damper braking effort. I haven't seen one of those in a while so I assume they were either ineffective or deemed unnecessary.
They are unnecessary for skilled enough riders who usually hate them anyway because they make the brake feel spongy. However, for a beginner they serve their purpose. I still see some on commuter bikes; my ex-gf had such a damper too on the front brake of her bike. I reckon they make sense on rental bicycles.
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Old 07-20-20, 03:27 AM
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Originally Posted by taylorgeo View Post
I just worry about hitting that unexpected pothole or tree branch and losing control of the bike.
Go slow then. If you're slow enough and you meet that unexpected pothole, you'll be able to stop safely with less braking force.

Always scan the road for hazards, road damage, people near or on the road, parked vehicle, anything that might cross your path, anything that can flat your tire, etc.. If the road is straight and scanned the road and no hazards exist, then you can speed up. Coming up to a blind corner? Then you need to slow down so if the unexpected shows up, you can safely stop your bike and avoid the hazard.
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Old 07-20-20, 05:10 AM
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Originally Posted by Retro Grouch View Post
Bicycle component manufacturers in the early days of linear pull brakes devised various control systems with a spring of some kind to damper braking effort. I haven't seen one of those in a while so I assume they were either ineffective or deemed unnecessary. My advice is whenever you decide to use a lot of front brake, shift your body weight backward by sliding your butt back behind the saddle.
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