One for the physics experts - Stopping distances at different speeds

Baggsy

Baggsy

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Apparently doubling your speed quadruples your stopping distance. So im guesstimating 50% of the original speed to double. 100km = 150km to double stopping distance.
 
sqrt(2)*100=141.4kph. At this speed your stopping distance doubles assuming your brakes, air resistance,.. all stay the same. At 120kph, your stopping distance is increased by 44 percent. (1.2)^2=1.44
 
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There are a ton of factors to take into consideration depending how in depth you want to go. Here's just a few I can think of: contact patch size (size of front and rear), cefficients of friction between what ever tire compound you have (will need to call manufacturer) and the asphalt (use industry average) wind drag, brake pad to rotor pressure, tire pressures, tire composition, tread type, condition of the road surface, % of moisture present and slope of the road. I'm sure there are more...

But if you want a generalized equation for stopping distance use this and solve for distance (this will by no means be accurate it will be ball park):

Vfinal^2 = Vinitial^2 + (2 * acceleration * distance)

Cheers,

- T -








 
Tam905 said:
There are a ton of factors to take into consideration depending how in depth you want to go. Here's just a few I can think of: contact patch size (size of front and rear), cefficients of friction between what ever tire compound you have (will need to call manufacturer) and the asphalt (use industry average) wind drag, brake pad to rotor pressure, tire pressures, tire composition, tread type, condition of the road surface, % of moisture present and slope of the road. I'm sure there are more...

But if you want a generalized equation for stopping distance use this and solve for distance (this will by no means be accurate it will be ball park):

Vfinal^2 = Vinitial^2 + (2 * acceleration * distance)

Cheers,

- T -
Click to expand...

+1
also coefficient of friction between brake pads and rotors
 
One thing that is mind boggling is how stopping distance are perceived at very high speeds. I was on the Autobaun driving a Porsche Boxster S at (real) speeds between 260 and 270 kph (GPS speed) Thats about 160 to 170 mph. When travelling at those speeds your brain doesn't initially understand what the distance squaring to stop means. When you see brake lights ahead you need to brake NOW. It takes way longer than you think to stop as you cover so much distance even braking at around 1g. Porsche's have AMAZING brakes (F1 cars use Porsche Monoblock Brake technology) and fading is not an issue at any speed.. it just takes a lot of distance to stop.

..Tom
 
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Tam905 said:
There are a ton of factors to take into consideration depending how in depth you want to go. Here's just a few I can think of: contact patch size (size of front and rear), cefficients of friction between what ever tire compound you have (will need to call manufacturer) and the asphalt (use industry average) wind drag, brake pad to rotor pressure, tire pressures, tire composition, tread type, condition of the road surface, % of moisture present and slope of the road. I'm sure there are more...

Click to expand...

Assuming you don't have brake fade and are using the full abilities of your brake system then essentially all those things will cancel out. It will take about four times the distance to stop if you are going twice as fast.

..Tom
 
Tam905 said:
There are a ton of factors to take into consideration depending how in depth you want to go. Here's just a few I can think of: contact patch size (size of front and rear), cefficients of friction between what ever tire compound you have (will need to call manufacturer) and the asphalt (use industry average) wind drag, brake pad to rotor pressure, tire pressures, tire composition, tread type, condition of the road surface, % of moisture present and slope of the road. I'm sure there are more...

But if you want a generalized equation for stopping distance use this and solve for distance (this will by no means be accurate it will be ball park):

Vfinal^2 = Vinitial^2 + (2 * acceleration * distance)

Cheers,

- T -
Click to expand...

Why complicate things? Just assume the same rate of deceleration as in the original braking test.
 
Ugur Dinch said:
How is that ?
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Because threshold braking power is dictated by the CofG of the bike and rider relative to the front tire contact patch.

If ridden properly, any modern sport bike will flip over the front before the theoretical Fmax (as calculated with combined rider and bike mass and coefficient of friction between the tire and road or brake pads and rotors) is reached.
 
caboose56 said:
If ridden properly, any modern sport bike will flip over the front before the theoretical Fmax (as calculated with combined rider and bike mass and coefficient of friction between the tire and road or brake pads and rotors) is reached.
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You mean if ridden improperly ?

You said "it's all about the COG and forces etc." - It would be true if you came to that max force at an instant, without delays and other interactions, but there is first the weight transfer, second the braking power, which is a non-linear variable depending on the temp of rotors and pads as TAFB mentioned. Sure it may effect the results only within the decimal points, but we're talking about the complexity of the problem here. So, IMO, your statement of "all those things don't matter, because ....." is not correct.
 
Ugur Dinch said:
You mean if ridden improperly ?

You said "it's all about the COG and forces etc." - It would be true if you came to that max force at an instant, without delays and other interactions, but there is first the weight transfer, second the braking power, which is a non-linear variable depending on the temp of rotors and pads as TAFB mentioned. Sure it may effect the results only within the decimal points, but we're talking about the complexity of the problem here. So, IMO, your statement of "all those things don't matter, because ....." is not correct.
Click to expand...

I'm sorry you don't understand physics but my statements are true.

Max braking power on a sport bike is limited by the geometry of the bike. Not the brand of brake pads, rotors, tire temp, etc.
 
caboose56 said:
I'm sorry you don't understand physics but my statements are true.

Max braking power on a sport bike is limited by the geometry of the bike. Not the brand of brake pads, rotors, tire temp, etc.
Click to expand...

Ultimately this would be true but only if everything else is working well enough.


If the road was slippery you aren't going to generate enough braking for CoG to matter. If the tires were cold and didn't grip enough same thing. If brake pads were worn out, hydraulic line had a bubble... bah blah blah... then you might not get anywhere near enough brakes.

But regardless of all that it will still take four times the distance to stop when you travel twice as fast (ignoring decimal points.)

..Tom
 
V-Tom said:
Ultimately this would be true but only if everything else is working well enough.


If the road was slippery you aren't going to generate enough braking for CoG to matter. If the tires were cold and didn't grip enough same thing. If brake pads were worn out, hydraulic line had a bubble... bah blah blah... then you might not get anywhere near enough brakes.

But regardless of all that it will still take four times the distance to stop when you travel twice as fast (ignoring decimal points.)

..Tom
Click to expand...

Correct.

I'm only talking about normal circumstances. When you start introducing all these 'what ifs' you make the solution mathematically impossible.
 
Bonus questions:

How much does my approximate stopping distance go up or down when I increase or decrease my speed by 10 percent?

Does the percentage scale across different speeds? e.g. 100 kph, 50 kph.
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