cityrider86

I built a bike out of spare parts, and during the test run, I did a stopping test. Only having front brakes, of course, leads me with only one way to brake, the front ones.
During the test, it took 53 feet to to slow from 25mph. To me, that's too slow. The only parts i bought were brake pads.

The brakes I have on the bike are single pivot side pulls. I'm not sure of the brand. The brake pads, Also, were second hand, but are in good condition.

My rim is a dull gray with uneven scratching on them. Should I replace the rims before i replace my calipers/pads?

This may be excessive, but I have my rims trued every 2 weeks, so the rim being true is not an important factor. They don't get more than 3cm out of true or round. Yes, in 2 weeks my wheels get out of true. I ride my bikes into the ground...literally.

Bezalel

If you need to true your wheels that often, the spokes aren't tight enough. A properly tensioned wheel won't go out of true even if abused (although with enough abuse the rim will get damaged before a spoke loosens).

bikeman715

clean your rims with rubbing alcohol and if needed lightly sand the pads for better braking power . as stated above the wheels shouldn't go out of true so often , they need to be re-tension . for braking ,faster you go more room you need to stop.

dsbrantjr

Are the brake handles the correct pull length for your caliper brakes? V-brake levers and caliper brake levers pull different amounts of cable with different levels of mechanical advantage and need to be matched to the brake type to work properly.

cityrider86

Yea, they are. They came off of the same bike, which was bought at the LBS good 10 years back.

As for the rim, i just put that wheel and took out the new racing wheel, off of a (almost) new mountain bike. I will try sanding the pads, just to see if that will improve anything.

fishymamba

If the pads are old, get new ones. Rubber gets hard over time and creates less friction.

echo

Agreed, this makes a world of difference if the pads are old, also make sure to get decent quality pads.

FastJake

FYI - this is totally irrelevant.

The front brake should be strong enough to lift the rear wheel (sending you into an endo if you go too far) on clean dry pavement. If it cannot do this, it needs work. Along with that, a proper front brake will have enough power to lock up the wheel in the wet, or in gravel, dirt, etc.

Can it do these things, if you squeeze hard enough?

cityrider86

Nope it cannot. Can't even lock the front wheel without a person on the bike.

dsbrantjr

If the brake pads are from 10years ago they definitely need to go. Kool Stop Salmon pads are frequently mentioned and might make a world of difference. Like Fast Jake observed, if you can't lift the rear wheel you have work to do to have a safe ride.

Drew Eckhardt 

I had to true a front wheel I built once in 13-15 years (bent the 400 gram rim on a road obstacle after I grew to 200 pounds and had to replace it) and rear wheel twice (put a flat spot in the original 400 gram rim then rebuilt it, crashed the heavier rim and then rebuilt it).

If you ever need to true a wheel you have one or more of

- Insufficient spoke tension. For aluminum box section rims you can generally use Jobst Brandt's method of alternately increasing tension and stress relieving (squeeze near parallel spokes on each side of the wheel together or wind them about each other at the outer crossing using something softer like a brass draft, plastic screw driver handle, or old left crank) until the wheel deforms in waves at which point you back-off half a turn, true, and be happy. For deeper rims or less time tensioning Park's TM-1 tension meter is affordable for the home mechanic (about $50 if you shop around).

- A bent rim which precludes getting all of the spokes tight enough. Too many bent rims suggest that you've chosen one that's not beefy enough.

- Excessive windup. Lubricate your spoke threads and rim sockets when building wheels (I use anti-seize applied with an acid brush with half the bristle length cut off) and when dealing with a wheel of unknown origins (3-in-one oil) to limit this. Stick a tape flag on a representative spoke in each group (front, rear drive side, rear non-drive side) at the hole(s) next to the valve stem hole and compensate correctly (varying from negligible to over 1/4 turn).

Kimmo

Single-pivot sidepulls simply aren't strong enough for the front.

Mind you, if you can't even lock the front wheel without being on the bike, something's up.

FastJake

Some are, most aren't. My 600 tri-color single pivot can endo the bike at will.

+1

Something is really wrong. Lookup how to setup these brakes and go through the process. What kind of MTB has caliper brakes anyway? Is this an xmart bike?

bradtx

cityrider86, Guessing the brakes worked well on the other bike so the differences are the replacement brake pads and the rim you're using. You could put the other pads on for a quick check and also use a rough red Scotchbrite pad followed by a finer white Scotchbrite pad to dress the braking surface.

Brad

blamp28

You need to find a better wheel man than you are using. A correctly tensioned wheel will last much longer. I have ten year old MTB wheels used in XC racing that are .5mm true. If you are truing your own wheels, you would be able to spend far less time working on the bike and more time riding it by learning about tensioning properly. A wheel will last longest when it has a uniform tension near the upper end of the specification for the rim. A general rule of thumb is 100 - 110 kgf for front spokes and drive side rears with the non drive sides coming in around 60 - 65% of that.

gyozadude

I'm a bit confused here. You said it takes 53 feet to brake with front brake from 25 mph to 0 mph. That's 37 ft/sec to 0 ft per second in a few seconds. That's not bad. Yet, you say you can even get the front brake to to lock even if no one is riding it. Really? and yet you stopped in 53 feet going 37 ft/sec?

+1 for me on the posts that suggest new pads and cleaning/surfacing the rims if you can. That might be sufficient after you decide on what to do about truing the wheel.

SBinNYC

It's a good thing you are using side pulls because they track wobble in a rim. It's unlikely that any other type of calibre brake would perform as well with a 3cm wobble.

As others have suggested, there is something basically wrong with a wheel that gets that much out of true within 2 weeks - especially a front wheel. There's much discussion in this forum about wheels that get 1mm out of true. Your wheel problem is probably due to undertensioned spokes, as others have suggested.Just how much distance do you think a bike should take coming to a complete stop from a speed of 25mph?

Assuming your reaction time is 3/4 second, your bike has travelled 27.6 ft before you apply the brakes. This means your stopping distance is 25.4 ft with the brakes applied. This works out to 1.4sec stopping time with an deceleration of 18 mph/sec or approximately 0.83g (where g is gravitational acceleraton).

Others have noted that your rear wheel will leave the ground, when too much braking power is applied. The point where this happens is due to the rider's weight distribution. On level ground the deceleration where the rear wheel starts lifting off the ground is equal to Lg/H, where H is the height of the rider's center of mass above the ground and L is the distance of the rider's center of mass behind where the front wheel touches the ground. The L/H ratio is usually around 0.6 for most bikes. The rider's center of mass is in the lower abdomen, which lies slightly above the seat at a distance roughly equal to the location of the bottom bracket.

Therefore your "slow" deceleration that caused you to stop in 53ft, is about all you should expect from bicycle brakes. Get used to it and avoid panic stops. Otherwise it may be your body that you run into the ground, not your bike.

FastJake

Huh? The front brake *should* be able to lift the rear wheel. I have one bike that won't do it, but that's because it's a hodgepodge of 2 old Huffys and the brake pads are about 20 years old. It's my commuter and I just dont care at the moment..

If the brakes are just fine as is, how well will they work when the rims are wet, or in other non-ideal conditions? Based on the OP's description there is clearly something wrong with them. If it won't even lift the rear wheel with no rider. Measured stopping distance means nothing.

There's no such thing as too much braking power, however poor modulation may lead to wheel lock-up. Learning to use the front brake correctly in a panic stop situaiton is an important step in becoming a safer cyclist.

SBinNYC

What does lifting the rear wheel with no rider show?

As I mentioned braking rates in excess of Lg/H will result in the rear wheel lifting. It's a question of weight distribution not total weight. The center of mass with the rider on the bike is essentially the center of mass of the rider. The center of mass of a riderless bike lies approximately half way between the front and rear axles (L) and about level with the hubs (H). The height distance (H) for the riderless bike is roughly half that with the rider on, while the length distance (L) are approximately the same. This means that braking rate for lifting a riderless rear wheel is roughly twice that with the rider on. The riderless wheel lifting test merely shows that the brake has twice the braking rate required to throw the rider over the handlebars. This is certainly sufficient braking power to stop a bike, however it is far in excess of what is required to stop the bike safely (with rider on).
The rear wheel lift test is an easy qualitative test to show that the brakes have more than enough stopping power to stop a bike. The OP believes that new brakes would make the bike stop in a shorter distance (with the rider still attached to the bike). They won't. He is as close to being thrown over the handlebars with his current brakes as is prudent to get.There's a contradiction there. Proper modulation means reducing braking power from what is available so as not to flip over or skid a wheel (lock-up). One can easily add more braking power but one cannot use this added braking power because the bike will flip over and throw the rider over the handlebars.The technique for adding more braking power in a panic stop is to increase the L dimension by moving one's center of mass as far back as possible. This is usually accomplished by straight arming when applying the brakes.

FastJake

No. You're ignoring the weight of the rider. Take a bike with good brakes, walk along side it, and jam the front brake. The bike just about flips over. Now try this when you're on the bike. Not as easy to do. Now find someone who weighs +300# and try the test again. Unless you have very good brakes I bet you won't be able to lift the rear wheel.

It's not just the moment of inertia of the bike + rider, but also the amount of force at the rim. If the brake cannot provide enough force, then the brake is the limiting factor.

Captain Blight

Set clean brake pads very close to a very true, clean rim, with no slack in the cable, and all your brake problems go away.

Kimmo

That's a nice brake... and I've swapped the front one for a dual-pivot on two bikes. A single-pivot just doesn't have the leverage to provide the best braking without either fancy pads and/or using way too much effort hauling on the lever. I'm only 65kg and they're not enough for me.

But it's a sweet brake all right... it's my preferred rear, had it on three bikes.