Brakes, Cables and Detanglers

U-brakes and how they work

The mechanism is worth understanding because it explains every setup problem people have with them.

Each arm is a lever. The bottom end pivots on a post. The pad sits partway up the arm against the rim. The top end carries one end of the straddle cable. The two arms lean inward across each other, so the left arm's top end is over on the right and the right arm's top end is over on the left. When the main cable lifts the yoke, both ends of the straddle cable are pulled up and inward, and the arms rotate about their posts to close the pads onto the rim.

Because the pull is upward on a triangle of cable, the shape of that triangle changes how hard the brake bites. A shallow, wide straddle gives more mechanical advantage and less pad travel. A tall, narrow straddle gives less advantage and more travel. That is why two identical brakes can feel completely different, and why a brake that feels wooden is often a straddle length problem rather than a pad problem.

Pad position is adjustable in more directions than most people use. The pad can move up and down the arm, in and out from the rim, and rotate to match the rim's braking surface. The pad face wants to sit fully on the braking surface, not hanging over the top edge of the rim and not touching the tire. A pad that has been running half on the tire is a used part that gets binned rather than sold.

Brake mounts and the 990 name

The name 990 is used in listings and in conversation to mean the standard BMX U-brake mount, and a brake described as a 990 brake is a U-brake meant for those posts. It began as a model designation and became a generic term the way a lot of BMX vocabulary does, and the section below traces which model. Both the post spacing and the post diameter are given in the table further down, sourced.

The identification job on a used frame is straightforward. Look at the seat stays just above the rear wheel, and at the chain stays just behind the bottom bracket. Posts on the seat stays mean the brake sits above the wheel. Posts on the chain stays mean it sits below and behind the bottom bracket, tucked out of the way, which was a common arrangement on frames built for riders who wanted a brake but not a brake in the way. Some frames have both. Many modern frames have neither.

Cantilever posts are not U-brake posts, even though they look similar at a glance. Both are a pair of posts either side of the wheel and both take a brake arm that pivots on them. The spacings differ, and the arms are shaped for different geometry, so a U-brake on cantilever posts will not sit where it should even if the arms slide on. The safe check is to measure the posts on your own frame and compare against what a brake is actually sold for.

Those first two rows look like a contradiction and are not. Frame builders aim at 82 to 85mm, wide enough to clear modern 32 to 38mm rims. The wider 75 to 85mm figure is the span a U-brake can actually cope with, because the orbital washers behind the pads take up the difference. So one number is what frames are built to and the other is what brakes tolerate, which is why general bicycle references quote the wide range and BMX makers quote the narrow one.

That resolves something this guide had wrong. An earlier pass published only the 75 to 85mm range and read the width as the absence of a standard. There is a standard, and the range is the accommodation around it.

Front and rear are the same. 990 posts on a fork and on a rear frame use the same 9.0mm post and the same M6 bolt at the same spacing, across every source checked, so a U-brake bought for one end will fit the other.

Where the name comes from

990 is a Dia-Compe model number. The AD-990 and FS-990 freestyle U-brakes arrived around 1986 to 1988, and their post dimensions were adopted so widely that the model name became the name of the standard. Sourced to Dia-Compe's own history and MOMBAT. Confirmed

So when a frame is described as having 990 mounts, it is being described by a brake that stopped being current decades ago, and nobody minds. Worth knowing when a seller uses the term without being able to say what it means.

Removable brake mounts

The reason these exist is a genuine design tension. Riders who never run a rear brake do not want two lumps welded to the seat stays that catch clothing on a barspin and collect grind damage. Riders who do run a brake need somewhere solid to bolt it. Removable mounts settle it by putting threaded bosses on the frame and letting the mounts themselves bolt on and off.

The practical consequence is that a used frame advertised as "brake compatible" may or may not come with the parts that make it so. Look for the threaded bosses, not for the mounts. If the frame has bosses with bolts or plugs in them, the frame can take a brake and you need to source the mounts. If it has smooth stays with nothing on them at all, the frame is brakeless and no amount of hardware changes that.

Mounts are usually specific to the frame or at least to the manufacturer, because the boss spacing and the mount shape have to agree. A set of removable mounts off one frame is not automatically a set of mounts for another. That is worth saying plainly, because these turn up sold loose all the time.

The other thing a brakeless frame often lacks is a cable stop or cable guide on the top tube or seat stay. Without somewhere for the outer cable to terminate, a brake cannot be routed properly even with the mounts on.

Detanglers and how they actually work

Here is the problem it solves. On a bike with a rear brake and no detangler, one cable runs from the lever on the bars down to the brake on the frame. Spin the bars and that cable wraps around the head tube. A couple of rotations and the cable binds, the brake drags, and the bars stop turning. Anyone doing barspins, tailwhips, or an x-up past a certain point runs out of cable.

The detangler splits that cable in two. The upper cable comes down from the lever and terminates on the upper plate. The lower cable leaves the lower plate and goes on to the brake. The two plates sit one above the other on a bearing race centred on the steering axis, and they are coupled so that pulling the top one lifts the bottom one. Crucially they are not coupled rotationally: the top plate turns with the bars, the bottom plate stays with the frame, and the bearing lets them slip past each other indefinitely.

Because the split is at the steering axis, the upper cable never wraps around anything. The bars can rotate as many times as you like and the brake stays live throughout.

Two cables run down from the top plate on most designs, one to each side, so the pull is balanced and the plate is not dragged sideways when the lever is squeezed. That is why gyro cable sets come as an upper pair plus a lower single, and why an incomplete set is useless.

What goes wrong with them

A gyro adds friction to the brake system, and a badly set up one adds a lot. The usual faults on a used bike are worth knowing.

Bearing seized or gritty. The plates should rotate against each other with almost no resistance. If turning the bars pulls the brake on slightly, the bearing is binding and the brake will drag on every barspin.

Cable tension wrong. The upper cables have to be tensioned evenly. If one is tighter than the other the plate cocks over, the bearing loads unevenly, and the whole thing gets stiff.

Wrong cable lengths. A gyro needs cables cut for a gyro. A standard brake cable will not do the job of the upper pair, and cables that are too long loop badly and rub the bars.

Missing hardware. The small parts (the spacers, the mounting bolts, the cable ends) are what disappear when a gyro is pulled off a bike and thrown in a box.

Gyro tabs

The tabs exist because the lower plate has to be held to the frame. If it turned with the bars there would be no detangling. Two tabs on the front or back of the head tube give the lower plate two fixing points and stop it rotating.

On a used frame, tabs are easy to spot and easy to miss: they are small, they sit close to the head tube, and on a brakeless build they are often ground off and painted over. Look carefully at the head tube from the side. Grinding marks in the paint where two tabs used to be is a common find on a street frame.

If a frame has no tabs, the options are a clamp-on adapter that grips the head tube, or a gyro design that mounts differently. Neither is as clean as tabs, and a clamp-on adapter has to fit your head tube, which brings you back to measuring.

Gyro tab spacing is standardised, which is the answer that makes a gyro a safe separate purchase. Any gyro will bolt to any tabbed frame, and the tabs themselves are held on with M4 hardware.

One distinction worth being careful about, because it is easy to state wrongly. The 38.1 and 45.0mm figures above are head tube outer diameters, which is what a clamp-on gyro plate grips. They are not steerer diameters. A 1 1/8 inch steerer is 28.575mm, as set out on the headsets guide. Head tube, steerer and bearing are three different diameters in the same few inches of bike, and mixing them up is how people order a part that cannot fit.

Cable lengths are quoted separately for upper and lower and sold as matched sets, so buy the set rather than assembling one from halves.

Straddle cable and yoke

The yoke is a tiny part doing a lot. The main cable from the lever ends at the yoke. The straddle cable passes through the yoke or clamps into it, and its two ends anchor to the tops of the brake arms. When the lever is pulled, the yoke is lifted, and the straddle triangle closes, and the arms come in.

Where the yoke sits on the straddle changes the brake's feel, as described in the U-brake section. Raising it makes the triangle taller and narrower, which gives more pad travel and a softer bite. Lowering it does the opposite. This is a genuine adjustment, not a fiddle, and it is the first thing to try on a brake that feels wrong.

Some setups use a quick release at the straddle, where one end of the straddle cable unhooks so the arms can spring apart and let a fat tire pass between the pads. That is the difference between changing a rear tire in two minutes and letting the air out to get the wheel past the brake. On used brakes the quick release fitting is often the part that went missing.

When buying a used U-brake, the checklist is: two arms, two pads with fixing hardware, the straddle cable, the yoke, the springs, and the mounting bolts. A brake sold as "complete" that is missing the yoke is not complete, and a yoke is not a universal part you can grab off a shelf.

Brake levers

The important distinction is pull length. A U-brake needs a certain amount of cable pulled for a certain amount of pad movement, and a lever is designed around that. A linear pull (V-brake) lever pulls considerably more cable for the same hand movement. Put a linear pull lever on a U-brake and the pads slam onto the rim in the first part of the stroke, which feels grabby rather than powerful and leaves you no modulation. Put a U-brake lever on a linear pull brake and the opposite happens: there is not enough cable to close the arms, so the pads never properly reach the rim and the lever comes back to the grip. That second one is the dangerous direction, because the parts fit together and the brake looks right until you need it. This is covered in full under cable pull ratio.

Reach is the separate comfort question. Riders with smaller hands want the blade closer to the bar. Many levers have a reach adjustment screw that limits how far out the blade sits at rest. On a used lever, check that the screw is present and that it is not seized.

Two other used-lever checks. The cable adjuster barrel at the lever should turn freely and not be rounded off, because it is how you take up pad wear without touching the brake. And the lever clamp has to suit your bar diameter, which is a cockpit question rather than a brake question.

A U-brake needs a short-pull lever, and now the page can say why rather than just saying so. A linear-pull lever moves about half as much cable again per stroke. Put one on a U-brake and the pads reach the rim early in the stroke with roughly a third less force behind them, so the lever goes hard and the brake still will not stop you. It feels like a fault in the brake and it is a mismatch in the lever.

Lever clamps are 22.2mm, matching the bars, so any BMX lever fits any BMX bar.

On cable, the dimensions are ordinary and the construction is not. Inner wire and housing are the same 1.5 to 1.6mm and 5.0mm as any bicycle. But a freestyle bike with a gyro needs linear strand compressionless housing rather than the coiled housing used on most bikes, because coiled housing binds and locks the brake when the bars spin. So you can buy the cable anywhere, but you cannot buy just any cable.

V-brakes and calipers on BMX

The difference from a U-brake is the force path. A U-brake pulls upward on a straddle triangle. A V-brake pulls sideways: the cable outer stops on a noodle on one arm and the inner cable crosses to the other arm, so squeezing the lever pulls the two arm tops directly toward each other. No yoke, no straddle, more cable pulled per unit of pad movement, and hence the need for a linear pull lever.

Where you actually meet V-brakes on BMX is on some cruisers, on race bikes, and on complete bikes that borrowed a mountain bike part. If you have one and it works, there is nothing wrong with it. Just remember it needs its own lever and it needs posts that suit it.

Caliper brakes (the single-pivot or dual-pivot type that bolts through a single hole in the frame or fork) turn up on old school BMX and on some cruisers. They need a drilled bolt hole rather than posts, so they are not an alternative to a U-brake on a frame with mounts. On a used old school bike, a missing caliper is a real problem, because the hole in the frame is the only thing you have to work with.

Cable routing and quick release

The routing rules are practical rather than dimensional.

Nothing crosses the steering axis without a detangler. If a cable runs from the bars to the frame and there is no gyro, the bars can only turn so far. On a bike meant to be ridden with a rear brake and no barspins, that is acceptable, and a cable stop on the head tube keeps it tidy. On a bike meant to spin, it is not.

Cable outers want gentle curves. A tight bend in the outer casing adds friction that never goes away, and on a used bike you can often feel it: the lever pulls fine but the pads come back slowly. Kinked outer casing is a five minute fix and worth doing before blaming the brake.

The lower cable has to clear the tire and the frame. On a rear brake, the run from the head tube back to the yoke passes over the top tube or along it, and it should be secured. On a fork brake, the cable comes down the front and needs to clear the tire at full compression.

The quick release at the straddle earns its place here. Undo it and the arms swing out far enough for a fat tire to pass between the pads. Without one, getting a wheel out means either deflating the tire or slackening the whole cable at the yoke and setting the brake up again afterwards. Check for one before you buy a used brake set.

A keyed washer shows up in this area more than people expect. On some detangler and dropout arrangements a washer with a tab locates in a slot to stop a part rotating under load. They are small, they are easy to lose, and a setup missing one will work loose. If a used brake or gyro comes with odd little tabbed washers, keep them.

Brakeless frames and what gets left behind

Most used BMX frames come to a shop brakeless, because most street riding is done that way. That leaves a predictable set of traces, and knowing them tells you what the frame will and will not do.

No mounts, no bosses, clean stays. The frame was built brakeless. It will never take a rear brake without a clamp-on adapter, and clamp-on rear brake adapters are a compromise nobody enjoys.

Threaded bosses, no mounts. The frame takes removable mounts. Find the correct mounts and you have a brake bike. This is the good outcome.

Welded posts, no brake. Everything is there and the previous owner just took the brake off. Fit any U-brake that suits the posts.

Ground-off posts. Someone removed welded mounts with a grinder. This is common and it is a judgement call: the frame is now brakeless permanently, and how much material came off the stay in the process is worth a very close look, because a stay ground thin at the old post location is a structural question rather than a cosmetic one.

Ground-off gyro tabs. Same story at the head tube, with less consequence, because the tabs are not carrying load in the way a seat stay is.

Cable stops still in place with no cable. Harmless, and a useful signal that the frame was built to take a brake.

On the front end, a fork with no brake mounts is very common on modern forks and completely normal. See Q019 below for what to look for.

Keyed washers

You meet it on threaded headsets and on older loose ball hubs. The job is small and specific. The adjusting cup or cone below sets bearing preload. The locknut above holds the setting. Without something between them that cannot turn, tightening the locknut drags the adjustment round with it and you end up chasing a headset that is either tight or loose and never right.

A missing keyed washer is a genuinely common problem on used parts, because it is a flat unremarkable component that gets lost during a rebuild and nobody notices. If a threaded headset will not hold an adjustment no matter how carefully it is set, check whether the washer is there and whether its tab is still intact before assuming the bearings are worn.

Questions

990 mounts

The posts are the two cylindrical studs the brake arms pivot on, either welded to the seat stays or chain stays, or threaded into bosses so they can be removed. If a frame has them, a 990 brake fits. If it does not, no amount of hardware will put a rear brake on it, which is covered under brake mounts and removable brake mounts.

What matters when buying used is which of the two you have. Welded posts are permanent and always there. Threaded posts unscrew, which is how a frame gets a clean brakeless look, and it is also how the posts go missing. A frame advertised as having mounts may have empty threaded holes, and the bosses are frame-specific enough that finding the right ones later is not guaranteed.

The spacing and post diameter that define the pattern are published above under brake mounts, sourced to Odyssey and Dia-Compe. Those are the numbers to check a frame against.

That one is worth explaining rather than leaving blank. Retailers selling replacement bosses list them variously as M6 and as M8, which are not close enough to be a rounding difference, and no manufacturer publishes a figure to settle it. Note that this is the thread of the boss itself, not the M6 x 1.0 brake mounting bolt in the table above, which is a different fastener and is sourced. Measure the boss on your own frame before ordering, and if you are buying bosses to suit a frame you do not have in front of you, ask the seller to measure.

Straddle cable hanger

Drop the hanger closer to the tire and the straddle wire runs at a wider angle. That increases the mechanical advantage: more braking force at the rim for the same pull at the lever, at the cost of the pads moving less far for the same lever travel. Raise it and the opposite happens, giving a firmer lever with less power.

That trade is the whole reason the part is adjustable, and it is the first thing to change on a used brake set that feels weak. Before deciding a brake is worn out, look at where the hanger is sitting.

Two practical limits. The hanger and straddle wire have to clear the tire through the brake's full travel, which matters more on a wide modern tire than the setup was probably designed around. And a hanger set very low leaves so little pad travel that the pads drag on a rim that is even slightly out of true.

Cable pull ratio

This is the reason a mountain bike V-brake lever does not work properly on a BMX U-brake, and it is a more common mistake than it sounds, because the lever will physically clamp to the bar and the cable will physically connect. Nothing about the fit tells you the geometry is wrong.

A linear pull brake needs a lever that pulls roughly twice as much cable as a lever made for cantilever and U-brake arms. Put a short-pull lever on a V-brake and the pads never reach the rim. Put a long-pull lever on a U-brake and the lever hits the grip before the pads bite hard.

On a used bike sold as a complete brake setup, the lever and brake usually match because they came off the same bike. Buying the two separately is where it goes wrong. If a brake feels wrong in a way that no cable adjustment fixes, the lever is worth checking before the brake.