# braking distance car vs bike



## marcw (13 Jul 2010)

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.


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## DavieB (13 Jul 2010)

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.


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## mark barker (13 Jul 2010)

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 Those figures are well out of date, and I doubt any modern car would need anything like that distance to stop.


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## dondare (13 Jul 2010)

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.


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## Davidc (13 Jul 2010)

DavieB said:


> 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.


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## dondare (13 Jul 2010)

Ford Anglia has a 3 speed gear box I believe.


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## BentMikey (13 Jul 2010)

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!


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## summerdays (13 Jul 2010)

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!


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## DavieB (13 Jul 2010)

summerdays said:


> 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)


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## pedallingpasty (13 Jul 2010)

<FONT size=2>Interesting topic. Hope i don't bore you with the following


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## pedallingpasty (13 Jul 2010)

pedallingpasty said:


> <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.


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## Dan B (13 Jul 2010)

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


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## pedallingpasty (13 Jul 2010)

coruskate said:


> 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.


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## marcw (13 Jul 2010)

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.


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## 2Loose (13 Jul 2010)

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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## pedallingpasty (13 Jul 2010)

marcw said:


> 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.



Reckon you could be right there. When i first got my hardtail mtb with cantilevers and ali rims, tested brakes ( as you do) on tarmac and was surprised how hard i could use front brakes whilst still under good control due to front suspension compression. Have now fitted hydraulic discs to same mtb, tested them and bottomed out on suspension. Would have looked quite good if the rear wheel didn't lift, swing around and hit garage door 
So maybe a disc equiped hardtail could do quite well against a modern car, without denting his door of course.


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## summerdays (13 Jul 2010)

pedallingpasty said:


> 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.



Err the technicalities of this conversation is going over my head. As regards to braking I was taught that to stop in an emergency you should shift your weight backwards.


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## pedallingpasty (13 Jul 2010)

summerdays said:


> Err the technicalities of this conversation is going over my head. As regards to braking I was taught that to stop in an emergency you should shift your weight backwards.



Yes you are right, its all getting a bit nerdy.
You must have had a good tutor.
So all we need now is a person willing to ride a disc equiped hardtail mtb at 30 mph with their ass hanging over the back wheel to prove our theory.


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## marcw (13 Jul 2010)

pedallingpasty said:


> Yes you are right, its all getting a bit nerdy.
> You must have had a good tutor.
> So all we need now is a person willing to ride a disc equiped hardtail mtb at 30 mph with their ass hanging over the back wheel to prove our theory.



Chuck a recumbent in there too to see if the extra weight over the back wheel and lower CofG means they can stop quicker.


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## Davidc (13 Jul 2010)

summerdays said:


> I was taught that to stop in an emergency you should shift your weight backwards.



So was I - cycling proficiency circa 1959! Was also told to only use the front one to stop fast.


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## mr_hippo (14 Jul 2010)

Forget theory, formulae, the coefficient of this, the cosine of that and the price of fish, practical demos are much more fun!
*Experiment* 
Car brakes v bicycle brakes
*Equipment:* 
Long, long stretch of road with a reasonable surface
Average family saloon e.g. Astra, Escort
Average bicycle - road, hybrid or mountain
Measuring device
Cycling helmet - optional
A 'start point' 
Driver
Cyclist
Referee
*Method:* 
Drive the car towards the start point at a steady 30mph, At the start point, apply full braking force for an emerbency stop. When the car comes to a complete stop, apply handbrake and exit vehicle. 
Cyclist will then ride at 30mph to the start point and then apply full braking force for an emergency stop.
*Possible Outcomes:
*1) The cyclist stopping distance is shorter, he stays in complete control and upright then puts his feet on the ground,
2) The cyclist stopping distance is longer and the rear of the car is 'customised' by rider and machine; if this happens then the referee will call 999 for assistance.
3) Rider and machine are in an unstable position and part company; again the referee's assistance is needed*

*.*Follow up experiment: 
*How does a cyclist's BMI affect his coefficient of bounce?


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## gaz (14 Jul 2010)

If my brakes are tweaked right, i recon i could out brake some cars easily. Just like i can out accelerate some cars (to a certain speed)


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## marzjennings (14 Jul 2010)

If you get your weight all the way behind your saddle and low to the back wheel, pushing all your weight through your pedals and not the bars, you can pull on both brakes equally, coming to a stop a lot quicker. Bit of a mountain bike technique, but also works on the road.

[media]
]View: http://www.youtube.com/watch?v=-RNAYR3KPIg&feature=player_embedded[/media]


Watch the guy to the right at about 19sec and 61sec. Put's himself right behind his saddle, coming to stop quick enough to avoid the pile up.


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## smeg (14 Jul 2010)

lol are you going to volunteer yourself for that braking test Mr Hippo? I Don't fancy that myself, 30mph is bloody fast!


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## mr_hippo (14 Jul 2010)

smeg said:


> lol are you going to volunteer yourself for that braking test Mr Hippo? I Don't fancy that myself, 30mph is bloody fast!


Who, me? I know I can be a bit daft at times but not that daft! However, I can volunteer a few few forum members for the job.


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## DavieB (14 Jul 2010)

So what we are saying in on paper it should be possible. And we have a video of a professional cyclist stopping very fast on a £10000 sports bike lets compare that to a sports car...

I still dont think I or the average cyclist can stop faster than the average car. 

As for the ABS being slower to stop you than not having it, that was true 15 years ago modern abs systems are extremely good at what they do.


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## mark i (14 Jul 2010)

We could now really complicate the matter and add in the wet scenario. I think that one is a no brainer. Modern disc systems "wipe" the discs quite frequently to keep them dry-ish. I need a couple of meters to "wipe" my rims dry enough to brake in the wet. End result is I leave lots of space! 

I think that the braking performance is not so disimilar, I think that the variability in reaction time is a bigger issue, along with the ability to press the brake pedal / lever to it's limit of grip!


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## marcw (14 Jul 2010)

agreed there are many variables. Reaction times alone mean that if you are only a metre away from a vehicle and it brakes unexpectadly you're likely to hit it unless you can get down the sides, the back of buses should be covered in sponge to aid drafting.


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## HJ (14 Jul 2010)

I have never had a problem with stopping quicker that the vehicle I have been drafting, I am more worried by car try to wheel suck me, I know I can stop faster and I don't have brake lights so they are slower to realise them I am stopping...


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## Number14 (14 Jul 2010)

DavieB said:


> 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.




Mmmm, that's a presumption for some drivers.


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