30MPH - The Magic Speed

[RedPepper64]

Originally Posted by gfloyd2002

According to the Hurt Report a predominating factor in car/motorcycle accidents is the car failing to yield right of way. "Sorry, I didn't see you." This is most often a car pulling out in front (or into) a motorcycle at an intersection. This also coincides with what most motorcyclist find in their personal experience. So how do we reduce the risks from this type of accident? According to the Motorcycle Stopping Distance Calculator, it takes 150 feet to stop from 60mph, 66 feet to stop from 40mph, and 37 feet to stop from 30MPH. Think about those numbers for a minute -- small increases in speed have a huge impact in the distance we can stop.

And when one compares those stopping distances with the width of a standard intersection, an easy solution presents itself. A typical small intersection is 48 feet wide. Slowing down from 40 mph takes 66 feet, but slowing down from 30 mph takes only 37 feet. So how do we improve our chances of avoiding the most common type of motorcycle accident? Slow down to 30 approaching an intersection where a car might pull out in front of you, and give yourself the space to stop in time.

I know this is an old, old thread but it was brought up in a newer one I was just reading. Thank you for this information. I already find myself slowing at intersections but to have a greater understanding with the physics as to why and a "magic number" to shoot for.

[LNasty]

Originally Posted by nickjpass

No magic speed in this accident

Link to original page on YouTube.

this is why i don't stop directly behind cars, I pull to the side of them, pretty much in between the two lanes so that if someone is not going to stop they hit the car and hopefully not me

[Motofool]

Originally Posted by LNasty

this is why i don't stop directly behind cars, I pull to the side of them, pretty much in between the two lanes so that if someone is not going to stop they hit the car and hopefully not me

Correct !!!

And even if the car hits you, you will not be crushed in between cars.

Another advantage is that, having one mirror at the correct visual angle, you have a much better view of what is approaching from behind.

There is not a single reason to stop in the position that the video shows.

[gfloyd2002]

Originally Posted by RedPepper64

I know this is an old, old thread but it was brought up in a newer one I was just reading. Thank you for this information. I already find myself slowing at intersections but to have a greater understanding with the physics as to why and a "magic number" to shoot for.

I'm really happy this helped, thanks for the kind words.

[deeno415]

Originally Posted by Motofool

Correct !!!

And even if the car hits you, you will not be crushed in between cars.

Another advantage is that, having one mirror at the correct visual angle, you have a much better view of what is approaching from behind.

There is not a single reason to stop in the position that the video shows.

Riding in the city traffic, stopping in between cars is what I tend to do. I do try to ride all the way up to the crosswalk, but learned not to always be the 1st out of the gate (as there are some drivers trying to beat the red in cross traffic). I don't care that some drivers get upset at me (which some do). I just say, "I'm trying to avoid getting rear-ended by someone that might not see me... " and then they usually understand.

I try to ride paranoid all the time, especially in a city where cagers are so impatient and try to get moving at any given chance. These safety methods/practices are necessary for any rider!

[AlanDog]

Originally Posted by gfloyd2002

As for bike weight mattering, it is a myth. While counter-intuitive, heavier bikes still have (for the most part) sufficient braking power to lock up the wheels. Heavier weight just means you need more pressure on the brakes to achieve the same stopping power. This is because heavier weight bikes offer increased friction that balances out the increased momentum of the bike. (F = mu*N where F = frictional force (braking force between the tyres and the road), mu = friction co-efficient (changes based on road/tyre condition), N = normal force (force acting directly downwards with gravity i.e., the mass of the bike and rider). .

Okay, counter-intuitive for sure: say my bike weighs 5,000 lbs. My front tire is still 100 mm wide. won't the heavier bike just have a ton more momentum than a lighter bike? Isn't the tire contact patch skidding at the limit of tire traction, so why would a 5000 pound bike stop just as quick as a 500 lb bike?

[Motofool]

Originally Posted by AlanDog

Okay, counter-intuitive for sure: say my bike weighs 5,000 lbs. My front tire is still 100 mm wide. won't the heavier bike just have a ton more momentum than a lighter bike? Isn't the tire contact patch skidding at the limit of tire traction, so why would a 5000 pound bike stop just as quick as a 500 lb bike?

Both momentum and traction depend linearly on the weight on the contact patch; therefore, the heavy bike and the light bike can stop at the same distance and time.

When you are at maximum braking effort and your rear tire is starting to lift up, all those 5,000 lbs go vertically to ground through the front contact patch.

The heavier bike has as much forward momentum as heavier it is respect to another bike: your 5,000 lbs bike has 5/4 more momentum than a 4,000 lbs bike, if both are moving at the same speed and decelerating at the same rate.

http://www.engineeringtoolbox.com/fr...nts-d_778.html

Rolling or skidding, the force resisting the forward movement at the contact patch is 90% or 70% of the weight that goes vertically to ground through the front contact patch.