# Any way to keep rim brakes from wearing down rims?

Discussion in 'Cycling Equipment' started by Strelok, Nov 27, 2011.

1. ### alienator Well-Known Member

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Threads evolve, and besides, it's not really off topic yet, although it has taken a turn toward the ridiculous with the "turn the bicycle 90 degrees to the direction of travel..." strategy. I take it you have difficulty grasping Newtonian mechanics.

2. ### dhk2 Active Member

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I can only imagine how much skill it would take to pull that off at speed, say 20 mph. Would be fun to calculate the "lean away angle" it would take to produce that instant stop without high-siding though. But believe you'd still be subject to the sliding friction of the tires as a limit (assuming you could land it correctly). Of course, the contact patch would be wider sideways, at least for the period of time the tires stayed on the rims. Have seen kids on bmx bikes stunt like this at low speeds, but never on a road bike at normal speeds we ride. Do you practice this often yourself, and if so, do you have any videos?

Mere mortals like me are probably better off just learning to control the brakes in a straight line. On rare panic stops, I've had problems with locking of the back wheel during hard stops. Probably need to practice my panic stops more often, but I still find them a bit scary.

3. ### alienator Well-Known Member

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In terms of the contact patch, what matters is the size of the contact patch, and that's going to be the same whether you're moving forward or "flipping the bike 90Â°."
Remember the force generated by friction is just the (coeff. of friction)*(area of contact patch)*(component of rider/bike weight normal to the patch). The ol' flippin' the bike thing then adds a moment to those patches, which further reduces available braking power. Braking efficacy is further limited by all that time when the bike is in the air. The technique would be an effective way to crash.

4. ### davereo Well-Known Member

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Off topic is more like a girl friend I had in my teen years. Although it can be related to this thread somewhat. Quite often the back seat activity would begin with here softly moaning no-don't- stop and lead into her climaxing with No! Don't! Stop! on the top of her lungs.

5. ### alienator Well-Known Member

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That's not off-topic. This is off topic:

6. ### classic1 Well-Known Member

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Just buy new rims you tightarse. What are you, communist or something?

7. ### cheetahmk7 Well-Known Member

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If you hold up a flat blade against the braking surface you can see some light come through the centre, but there is still years of life in them.

8. ### An old Guy1 New Member

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I knew people who could do it at speed.

I always found it easier to avoid situations where I had to rely on the limits of my abilities of braking or steering. If you are riding "fast" a foot behind someone where somehting bad happens to him, you are going to be part of that badness. I don't care how good your skills at steering or braking are. You are going to be part of that badness.

When I was young, I would ride 2-4 seconds behind the fast locals all day long. Now, I am old. I ride 2-4 seconds behind a slower class of riders.

9. ### dhk2 Active Member

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Now that I'm old, I need to ride 2-4 inches behind the fast wheels. Can only let the gap build a few times and catch back on a few times before I'm toast. It does take a bit of trust though. Fortunately, have a group of experienced guys on the Sunday rides who don't do "bad stuff" in the paceline.....at least they haven't in the years we've been riding together weekly.

10. ### alfeng Well-Known Member

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FWIW. IMO, regardless of the rider's skill, a bunny hop is probably the worst way to stop a bicycle quickly ...

At ~20 mph (the previously suggested speed), a bunny hop of a half-second (or, longer!?!) will result in the bike & rider probably traveling 5 +/- yards BEFORE the tires make contact with the pavement, again ...

Based on the few time when I feel that I've needed to make a hard stop from ~20mph to Zero, I reckon it takes at least 30 feet (i.e., 10 yards if you want to use the same measuring gauge ... INPUT different values you don't think mine are correct) in a controlled, hard stop to go from ~20mph to Zero ... so, a bunny hop could mean that the final stopping point would probably have about 15 +/- feet beyond where I would have wanted to have been when the bike was stopped!?!

FYI. To truly stop suddenly the way to kill the forward momentum you want to lock the front wheel with the tire firmly ON THE GROUND and then allow the rear of the bike to swing around ... the odds are that the momentum will lift the rear wheel slightly off the ground even if you don't want it to ... the key is to keep the lift to a minimal amount ... regardless, your upper body controls the bike's forward swing.

A variation of drifting can be used, too, where you are almost dismounting from the bike ... basically, the equivalent of a "hockey stop" with your bicycle ... very hard on your tires.

And yes, I have first hand experience in using both methods for executing hard stops WITHOUT crashing ...

IMO, the skill (for want of a better term) in stopping quickly is more in knowing when you cannot wait any longer to lock the front wheel from rolling any further forward and then knowing how to transfer your weight appropriately (if necessary).

11. ### dhk2 Active Member

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alfeng, agree the bunny hop manuever is a waste of time, even if it could be accomplished from speed. To sharpen your numbers a bit, using 22 mph makes the math easy, since that speed is 32.2 ft per sec, the same number as 1 G acceleration (different units of course). Since change in velocity = acceleration x time, plugging in 1 G as a max limit for braking then results in a stopping time of exactly one second. The distance traveled in that second is 1/2 at**2, or simply 1/2a, or 16.1 feet. That distance seems impossibly short to me, nothing I could achieve which probably just underscores my belief that we cyclists can't stop nearly as fast as modern cars.

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