More weight doens't make longer stopping

[dantama]

This is a continuation of a conversation with Blown Dodge about weight and stopping distances that we're spinning off of the car tire thread.


BD,

Here are portions of a topic from another site, I'll post a link at the end.

There seems to be a continuing belief amongst many of us that stopping distance increases as a direct function of increased vehicle weight. I would like to try again to put this issue to rest.

While it is true that a heavier vehicle requires more energy to brake to a stop than does a lighter vehicle, (there is, after all, more mass involved), that does NOT mean the heavier vehicle takes more time or more distance to stop....
Since you know that you can lock a wheel while the bike is still moving, you know that the braking energy you apply to your brakes is NOT WHAT LIMITS HOW FAST YOU CAN STOP! That limit is determined by the amount of traction your tires have.

Further, since it takes more braking energy to stop (lock) a spinning wheel than to merely slow it down, and because a sliding tire (the result of locking your brake) has less traction than one that is not sliding, your normally functioning brakes are NOT WHAT LIMITS YOUR STOPPING DISTANCE! That limit is also determined by the traction of your tires.

Traction, as we have discussed before, increases with weight. Thus, adding weight decreases your ability to slide the tire and, as a result, gives you the ability to stop more quickly while at the same time increasing the energy that must be converted to heat by your brakes in order to slow down. In effect, adding weight makes it harder to slow at the same time it makes it more possible to do so.

If you so severely overload your bike that the brakes are no longer powerful enough to cause a skid, then you know that the increase in traction gained by that added weight has finally overwhelmed the ability of your brakes and, thus, your brakes then become what limits your stopping ability (time and distance.)

Weight affects your ability to stop in TWO ways:

* It takes more energy (braking) to slow a heavier weight


* Traction INCREASES as a result of added weight such that more braking can be used without starting a skid.


Thus, adding weight essentially CANCELS itself out as an impact on stopping distance. All that you need to do is apply your brakes harder in order to TOTALLY compensate for added weight....

You know this already, of course. Else, for example, how could a car EVER stop as quickly as a motorcycle? Or, how could a heavy Valkyrie or an Ultra Classic Tour Glide EVER stop as quickly as a little 250 cc street bike?...

But, generally speaking, weight makes no difference in stopping distance because the brakes are more than adequate to handle any normal range of weight for that bike....


Taken from http://www.msgroup.org/Tip.aspx?Num=125&Set=


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A bike moving, say, 50 mph has four times as much kinetic energy as that same bike moving at 25 mph. In either case in order to skid your tire you need to achieve, say, 1.0 g's of deceleration. You maintain that the bike's brakes cannot do that at 50 mph because of the total amount of kinetic energy involved?

My experience is that unless you are moving at a speed that is beyond the capabilities of your brakes to handle in terms of dissipation of heat your brakes are more than adequate to LOCK your wheel which requires more braking energy than slowing down the speed of rotation.

Indeed, your brakes are more than adequate, assuming the coefficient of friction between your tires and the roadway is adequate, to scrub 20 mph of speed every second you use them. That is pretty close to 1.0 g's and to do so without burning up in the process.

Of course they get hotter when doing so at 50 mph than at 25 mph as they convert that amount of kinetic energy to heat in less time.

You could put the brakes from a Mack truck on a motorcycle and still only have deceleration rates of up to about 1.0 g's of deceleration capability as that is limited not by the brakes, but by the coefficient of friction available. Put the tiny squeeze brakes from a bicycle on a motorcycle and try that and they would fail - they are not adequate to the task. But *your* motorcycle brakes are.

http://www.msgroup.org/forums/mtt/to...TOPIC_ID=10818

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In a brief post he states,
The size of your contact patch does NOT determine traction. It is how much weight is being carried by each square inch of that contact patch that does that. A bigger contact patch merely means less weight per square inch as the weight of the bike does not change.

http://www.msgroup.org/forums/mtt/to...62&whichpage=2
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I didn't read through this one today, but I think it's the one where he got the MSF to change their position.

http://www.msgroup.org/forums/mtt/to...p?TOPIC_ID=918

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I don't like giving the guy props here, because he's and ass. But he seems to know physics.

[skiman]

I'd like to see him prove it in the real world

[phenrichs]

Dan in looking at the photos in the other thread I think the difference was obvious. It was the rubber compound that caused them to stop in an equal distance. We all know that MC tires are almost always of a softer compound which would naturally get heated more quickly and soften to increase traction which aided in the stopping distance. I would imagine that while the CT was a firmer compound and did not react with the same heat and sticky properties it did have more of a contact patch which probably evened out the tape measure test. I believe had the contact patch been equal the MC tire would most likely outperform the CT.

[dantama]

Originally Posted by OP

Dan in looking at the photos in the other thread I think the difference was obvious. It was the rubber compound that caused them to stop in an equal distance. We all know that MC tires are almost always of a softer compound which would naturally get heated more quickly and soften to increase traction which aided in the stopping distance. I would imagine that while the CT was a firmer compound and did not react with the same heat and sticky properties it did have more of a contact patch which probably evened out the tape measure test. I believe had the contact patch been equal the MC tire would most likely outperform the CT.

OP, the bike tire did outperform the car tire. The contact patch on the bike tire picture was much more pronounced than on the car tire. The one on the car tire was pretty faint by comparison.

For this thread, I think the best way to think of it is if the brakes are able to dissipate the heat adequately, and the tire gets maximum braking without losing traction, adding weight does not cause the bike to take longer to stop.

We can prove it by one of you guys hoping on the back of my bike for some brake testing :)

[phenrichs]

If weight is the pivotal variable I nominate Todd. :)

[Jared]

Here's something interesting. From what I have read, a MC tire is NOT softer than a CT.

http://mcdarksiders.forumotion.com/gener....ig ht=durometer

[blowndodge]

But todd is still overweight!

[Todd]

Originally Posted by OP

If weight is the pivotal variable I nominate Todd. :)

I feel so.....so.....so honored

[Todd]

Originally Posted by Blowndodge "Darksider"

But todd is still overweight!

DUFFUS !!! ;) :) ;) :)

[macmac]

Is this thread 'continuation' a real new thread and the other semi dead now?
It was getting long. I just read it and found it interesting. I enjoy the debate too.

I think Pablo has run a Ct the longest here, I think I am next and BD is next after me. We all ran something different.

More or less we all found out these same things and with the tire new and un-broken had odds and ends of problems. Somewhere after 200 to 500 miles most of the typical problems dissolved, and any new mc tire tore has it's problems too.

One reason for me to post is I read Dan had his new Ct step out braking. i don't test like that but i did locl or semi lock I hjope both brakes in order to sane my hide and my wifes hide.

Dressed in riding gear we may weight just barely over 300 pound combined. The night of the incident I was dressed light, just boots, socks, jeans and a tank top, with gloves. My wife was in hiking boots, jeans, a top of some sort, bra, panties, light leather coat and a full face lid. My jacket was in the saddle bags and I can't recall what else was..

The details above are for weights.

In any case the speed was about 45 in a 50 mph zone, and a jeep hit a moose, and i was aware that it would a milisencond before it did. I hit both brakes at the same time and instant before the jeep hit the moose. I could just tell something wasn't right about the jeeps lights ahead of it, and in moose country that means it's a moose.

The pic shows one patch of rubber to the right of mine, and mine appear broken, because i let up a bit, but the whole time I could hear rubber screeching at the same level. I let up because i thought maybe the wheels were not turning at all. Then I added more pressure again.

The rubber is a measured 81.5 feet, which wasn't quite enough to save my ft fender. But i was able to borrow a carpenter hammer and pry the fender off the tire, and rode till two weeks ago with that damaged ft fender.

I think I needed 3 to 12 more feet distance. I feel that the ct saved my life since hitting a jeep cherrokee with my noggin wouldn't have as good an ending. it was 98 degrees at 9pm that night and dark.
No stepping out at all as you can see, and my riding skills are much less than Dan's.

[dantama]

Mac, this thread is just to spin off the braking and contact patch part of the discussion.

It was the motorcycle tire that was stepping out at 25-30mph. So I dropped down to 15mph to get a good straight one.

The Car tire was also straight at 15mph, and I didn't try it any faster.

Too bad you didn't have another couple of feet of stop in you.

[Dave]

All this talk about coefficient of friction and other numbers reminds me of the following:

[skeeter]

I really need one of those cryptonaptic bowling shims.

Thanks Dave! Lots of info.

[glwilson]

I keep hearing everyone say that the CT has to be "worn-in" before it works as good as one would expect.

What has to be "worn-in" on it? Is it the sidewalls that have to "relax" or is it something else?

Just wondered...

[skiman]

Here's my take on the subject maybe in fact you could stopped just as fast fully loaded and you could with just a rider on the bike but how many of us in the real world want to grab the binders that hard on a weekend cruise with the wife on back and some luggage so for practical everyday real world riding you need more room to stop unless you just love braking so hard that the bike feels like it's diving in to turn 2 at Bristol motor speedway every time you stop. Bottom line leave extra space and live to ride another day