braking distance car vs bike

marcw

Well-Known Member
having discussed this on another thread though better to move. Here are the results of some highly scientific Googling....

energy from forward momentum is converted to heat due to friction between braking surfaces.

max braking power is ammount of friction that can be applied before losing traction.
having lost traction energy is still converted to heat but at a lower rate, hence increasing braking distance.

bike - low contact area with road limits max braking power as traction is lost easily. High centre of gravity means rear wheel can lift hence less traction.

car - heavy so needs large brakes to convert forward momentum into heat without melting/ wearing out quickly. low centre of gravity and four wheels means traction should be pretty constant.


Couldn't find much in the way of figures.....


Stopping distance in highway code is 23m at 30mph
this can be reduced with;
wider/ bigger tyres to increase traction + bigger brakes to increase braking power i.e a VW Golf GTI will stop faster than a stock Golf of the same weight with smaller wheels and brakes.


Stopping distance for a bike at 30mph according to http://www.exploratorium.edu/cycling/brakes2.html is 10.4m.


If the figures above are correct, the bike has enough braking power to brake in half the distance of a 'standard' car.
 

DavieB

MIA
Location
Glasgow
Highway code figures are not accurate anymore (they also include thinking distance). Your average car now has abs and discs all round and some have massive tyres, compared to no abs and drum brakes back when that was written. I reckon cars can slow faster than me when im on my road bike.

Im no expert but I reckon cars can stop faster.
 

mark barker

New Member
Location
Swindon, Wilts
The only problem with your theory is using the stats from the highway code! Those figures are well out of date, and I doubt any modern car would need anything like that distance to stop.
 

dondare

Über Member
Location
London
If the car is in front of you then it can stop faster and you'll slam into the back if you're too close.
If the car is behind then the driver won't be able to tell that you're stopping 'cos of bikes having no break lights and he'll slam into the back of you.
 

Davidc

Guru
Location
Somerset UK
Highway code figures are not accurate anymore (they also include thinking distance). Your average car now has abs and discs all round and some have massive tyres, compared to no abs and drum brakes back when that was written. I reckon cars can slow faster than me when im on my road bike.

Im no expert but I reckon cars can stop faster.
Not to mention no braking servo assist!

I'm certain you're right that modern cars can stop quicker than either of my bikes.

Like most people I'm used to a car with abs, servo assist, discs all round, and modern large radial ply tyres. I recently drove a classic car of the sort typical when those figures were produced - a 1961 Ford Anglia with the slope inwards back window - with none of those things. It was terrifying. With my foot trying to go through the floor it semed as if it wasn't going to stop, the cornering was just as scary, small car but without power steering .... , and its owner was quite happy with my driving of the thing which I wasn't!

I'm just as sure that my bikes can stop and corner much better than that Ford Anglia.
 

BentMikey

Rider of Seolferwulf
Location
South London
I seem to recall reading that most two wheel vehicles, bikes and motorbikes, can only brake at around 0.5g, and cars closer to 0.83g. Wouldn't mind finding that source again!
 

summerdays

Cycling in the sun
Location
Bristol
Thing is how often am I going at 30 miles on my bike ... my usual is slower and how often are the cars sticking to the speed limit!
 

DavieB

MIA
Location
Glasgow
Thing is how often am I going at 30 miles on my bike ... my usual is slower and how often are the cars sticking to the speed limit!
I think th emain reason for the thread is for when the cyclists are up the cars exhaust pipe whatever the case if the car anchors fast when your too close your hitting it. reuguardless of the 30mph bit its more a guideline speed. Without a doubt my tyres would lock up behind a car doing an emergency stop (if i was too close)
 

pedallingpasty

New Member
Location
Derbyshire
<FONT size=2>Interesting topic. Hope i don't bore you with the following
Not sure what happened there, will write again.
Anyway, what hasn't been mentioned is the ability to apply a force in the opposite direction of motion.
Remembering back to my apprentice days, it depends on the frictional force that can be generated between the surfaces of contact. This can be worked out with the following formulae F=uR, where F ( newtons) is the frictional force that acts parallel to the surfaces in contact and in a direction to oppose motion that is occuring.
u is the frictional coefficent of the surfaces in contact, in our case rubber and asphalt )( a bit simplistic i know). This will be an average of 0.65 in the dry and 0.5 in the wet. Concrete is slightly higher of about 0.7 and 0.6 respectively.
R is the normal reaction force, in our case its the weight which is mass ( kgs) multiplied by acceleraton due to gravity ( 9.8m/s2).

So when you skid, the normal reaction force is greater than the frictional force.

So to increase the frictional force, either the mass or the frictional coeffient will have to be increase. So it is assumed a car will stop quicker because its frictional force is higher because its mass is higher than a cyclist.
But does this follow that a big bugger like myself can stop quicker than a svelte athelete on the same bike?
Its also interesting that area doesn't have an effect on the frictional force that can be generated. I remember the argument we had with the lecturer on that one, it's easier to push a sheet of steel on its edge rather than on its face. So wider tyres may not have much effect on stopping distance.
But of course this doesn't take into account of your inertial energy, does it?


Going to have a lie down now.
 

Dan B

Disengaged member
Force is Mass x Acceleration - so although the retarding force you can exert through friction is proportional to your mass, the retarding force you need to exert to slow down at the same rate is also proportional to your mass. Basically it comes out in the wash.

It should also be noted that F=uR is at best an approximation, and doesn't take account of things like the rubber composition changing as the tyre heats up or any of that kind of stuff
 

pedallingpasty

New Member
Location
Derbyshire
Force is Mass x Acceleration - so although the retarding force you can exert through friction is proportional to your mass, the retarding force you need to exert to slow down at the same rate is also proportional to your mass. Basically it comes out in the wash.

It should also be noted that F=uR is at best an approximation, and doesn't take account of things like the rubber composition changing as the tyre heats up or any of that kind of stuff
Nice one coruskate, good explaination there. Didn't think of that one.
Think we only really used F=uR to show the importance of lubrication and how to shift a stationary object, can see the problems of using it to show how to slow something down.
But the formulae is quite good in showing why we cyclists are always going to lock up the back wheel at first when emergency stopping due to the weight shift forward over the front wheel ( have only locked the front wheel when going backwards, don't ask ). So this raises the thought as to wether it best to shift your body slightly over the rear wheel when slamming on your brakes to increase frictional force, so we can increase the braking on the rear wheel slightly to stop in a shorter distance?
Will also have the advantage of moving your face slightly further away from the obstruction.
Think a bike could be ok against Davidc's Ford Anglia, but could really compare against a modern car due to its abs etc.
 
OP
marcw

marcw

Well-Known Member
which leads to the question of whether a front suspension mountain bike will brake better as the weight will shift forwards and down rather than arching over the front wheel.
 
Are you taking into account reaction time at all?

Two wheelers have brakes (or at least one in the case of motorbikes) covered with their hands, which can react faster than lifting, moving and pushing a foot onto a car brake.
Four wide tyres on a light car (caterham?) would probably outbrake a bike from 30, but add in the reaction time and I am not so sure.
 
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