qondamibuey

Hello,

I'm just got back from a errand and I was out biking on the streets in Chicago, where I live. I don't know if it's because of the heat on the pavement from the really hot day today (about 90 degrees in a big ass city that traps heat),or what. But the roads are absolutely insane today, and I have had to consistently do some quick braking to avoid the various ******* drivers who honestly don't care if you live or die, barring they are liable. I digress.

I was biking on an extremely bust street, clark st, which is insane today because of the weather and proximity to the beach. I was going along and a guy took a left hand turn right in front of me. I tries to brake hard and I slid right into the front of his car. Luckily, the immense 700c wheels I have bounced off his license plate and I was fine.

I tested them out afterwards when I got home and even when walking with my bike and applying the rear brake I could literally walk with it as it just slid along. When applying the front brake I couldn't.

The point is: What the hell is wrong with my rear wheel.

I have SUN Rhino Lytes, recently installed, perfect tension and true.

The brakes are cantilever and perfectly aligned. They are kool stop brake pads and are tensed correctly and lock the rim into place.

Now...my suspicions. The tire looks a little worn down but is only 2 months old!!

It's a Schwinn Kevlar comfort/hybrid tire. Is this true that it is the tire that is ****ty? It just slides along the pavement...and when I inspect the area that I slid it on it has a little worn streak.

Do I need a new tire or is it something else I'm overlooking?

FBinNY 

Nothing is wrong with your rear wheel or tire. Your problem is more one of physics than mechanics.

When you do a hard braking, there's a weight shift to the front which reduces the force holding the rear wheel down and therefore the traction. The more braking G's you generate, the lower the rear wheel traction, until the wheel skids and you cannot get any more Gs in the system. The same happens with cars which is why car brakes are proportioned with 60% or more braking power sent to the front wheels, and even then rear wheels always lock up and skid first.

In order to stop in the shortest distance you must bring the front brakes into play, but the physics here are trickier. Increased Gs increase the front wheel load and traction, making more Gs possible until the weight transfer is enough to flip you over the bars. So the key is to use both brakes and modulate them, to prevent rear wheel skids and/or endos. It's easily learned, but even very experienced riders can have an adrenaline rush and punch down the front brake causing a front end flip.

When I lived and commuted in Manhattan, I got so good at this that in an emergency stop (like behind a taxi cab) I could lock the front wheel, lifting the rear wheel, swing a leg over the handle bar, and step off the bike catching it behind me with my right hand. It looked spectacular when done perfectly, but I've also landed on a few trunks in my life.

The only thing I can suggest is to practice hard braking, until you can feel the point at which a flip is about to start, then ease off the front brake enough to prevent it. Do this at low speed and work your way up to the critical point by degrees, so you don't injure yourself learning to prevent injury. (the Catch-22 of learning good bike handling). It's important that good brake modulation is drilled so that it's totally automatic because you don't have time to thing about it during a 3 millisecond emergency.

dsbrantjr

Locking the rear wheel without using sufficient braking on the front is the classic novice motorcycle rider pitfall. The same applies to bicycles. Try to get into the habit of using your front brake only for most of your riding; use the rear brake only as a "drag brake" to control speed on long downhills. In this way you will tend to react correctly in an abrupt stop situation, that is, using maximum front braking until you nearly lift the rear wheel. If you habitually use the rear brake you will tend to use it in an emergency as well. According to Sheldon Brown: "The fastest that you can stop any bike of normal wheelbase is to apply the front brake so hard that the rear wheel is just about to lift off the ground. In this situation, the rear wheel cannot contribute to stopping power, since it has no traction. "Here's his full article article about braking: https://sheldonbrown.com/brakturn.html

Another tactic which many folks do not use is to turn away from the danger while braking rather than just hit the brakes, continue to go straight and hope for the best. You may be able to turn and avoid the collision altogether or mitigate the impact by striking the object at a shallow angle rather than head-on. It is rare that you will be totally cut off and you may be able to scoot through a narrow opening once you have slowed enough.

Regarding your rear brake locking when you are walking along, remember that the wheel has only the weight of the rear of the bike on it; when you are riding it has about 2/3 of your full weight added to that so it will have much more force to generate traction.

cny-bikeman

FB is of course correct that there is nothing wrong with your rear brakes (at least not just because you can make the tire skid) but there is more involved than just modulating the front brake. The key is to use the rear wheel as an indicator, and to release pressure on the front brake when the rear wheel skids. But shifting one's body to the rear can greatly increase the braking rate without the rear wheel skidding. I'll not go into the full explanation of an "Instant Stop" here, as it's already done well at this link:
https://floridabicycle.org/chapter/firstcoast/?p=516

Please note that the above article and just about every other one that is similar owe much to John Forester, the originator of Effective Cycling. There are other very useful techniques and traffic strategies that commuters especially may find useful. I had the great fortune to take his Effective Cycling Instructor Course back in the late 70's. I also credit him with teaching me a more "effective" way to change a tire, even though I had been a mechanic for several years.

Sheldon also has some interesting views about brake usage at https://sheldonbrown.com/brakturn.html

qondamibuey

I mean I understand the principal of sliding when sudden and tight pressure is put on a brake. I've experienced this in a car as a teenager and on my bike before hand. But the issue that is happening that I should have clarified more is the fact that it is literally sliding when I brake even at slow speeds. For example, I was inching behind a car at a stop light, and with hardly any speed I of course began to slide and had to dismount in order not to tap their rear bumper. In my head I was like, what the fu*k is this? I have never experienced this sensation of sliding on ice on perfectly good pavement and a new tire. The only thing I can honestly deduce is the tire because I have never experienced this and it's not like my riding style all of a sudden changed in a day. What I'm asking is if the tire being usually worn is a possibility. I'm not saying that I could do a better job at braking, and the way I ride, but today I got that, "I shouldn't even be on my bike in these conditions," feeling. I mean, it will virtually slide at almost any (harder) pulling I do of the brakes. I definitely do not think this is a normal phenomenon.

I may just be going crazy and be completely wrong here, but is it a possibility it's the tire?

Jeff Wills

New tires often have a coating of release agent left over from the molding process. It's possible that this is the problem. The coating wears off pretty quickly. If you want to accelerate the process, a little soap and water helps.

cny-bikeman

It is certainly understandable that you are concerned if you are convinced that you are doing the same thing with different results. However, walking the bike and being able to put on the rear brakes and drag/skid the tires is normal.

Tire wear is not a factor, as the tread on a bike tire has essentially no positive effect on traction when on pavement. You may even get better traction with a little wear. Assuming you are doing nothing different in technique and there is no significant weight change over the rear wheels, then the difference would have to be the pad/rim interface. I would think that the only thing there that would cause the problem is the brake pads grabbing too agressively due to some foreign matter. Try cleaning the pads with fine sandpaper, rinsing thoroughly afterward with something like Fantastic, 409 or similar. You could also try switching front and rear pads.

No matter what, it is a good idea to practice panic/instant stops before you need to stop quickly, and to practice in general using your front brake much more than the rear.

Myosmith

Video link, please

Kimmo

Ever noticed how cars and motorbikes have smaller brakes on the rear...?

This is why dual-pivot brakes on the rear of road bikes are silly. Single-pivot brakes are perfect for the back, being lighter and having less leverage, meaning it's easier to avoid skidding and control exactly.

With cantis, you can alter the geometry of the straddle cable to achieve the same effect. It sounds like your rear straddle cable is set too low, giving the brake way too much power.

Awesome. I'd love to see some vid of someone pulling that move.

When I first got a DP on the front, my first emergency stop sent me over the bars.

The next, my back wheel stayed in the air for a few metres, no problem. It was automatic once I realised I had as much braking as I wanted... can't do endos like that at will, though. Like I said, automatic...

noglider 

Don't follow so closely. Anticipate people cutting you off. Be prepared to stop if people do inconsiderate and stupid things. Don't push your bike to its limit. And don't get too angry at the people who do stupid things.

Not every problem has a mechanical solution.

Francis, the description of your technique is impressive, but it would take a lot of practice. I don't mean to recommend against it; I'm just saying it's not for everyone.

DannoXYZ 

Try rolling the bike backwards and doing the following:

1. applying front-brake
2. applying rear-brake

Please summarize the physics lesson you just learned.

Kimmo

Heh... nice one, Danno