Breaking distance 1

[krugerblue]

This is a question that bothers me for some time. Since I am not capable of getting an answer myself, google has redirected me to an old closed thread in this forum, related to breaking distance. The question is, which one between a motorcycle and a car stops better in the same ideal conditions ? If someone is willing to make some calculations for me, I would appreciate it very much. The data is:

The car:

1000 kg, four tyres, no ABS, tyre dimensions are: 165/75ZR13


The motorcycle:


280 kg, two tyres, no ABS, tyre dimnensions are: FRONT:

130/70ZR16 REAR:

180/55ZR17


I do not think that the speed is important, but I really need this answer very much. I have given the details above, taking in consideration a pretty usual and common care, and the same for the motorcycle, with the rider's weight added



Thank you

 

[MintJulep]

Both cars and motorcycles are pretty reliable these days.

My car for example, is 11 years old and has never had any problems with parts breaking.

 

[krugerblue]

Mint, I was not talking about reliability, but thank you anyway for your answer. I was talking about a comparison between vehicles on 2 wheels and on 4 wheels, regarding their ability to stop faster.

 

[CoryPad]

Then perhaps you ought to spell correctly - the word is braking.

The real information needed for calculation is brake system pressure and piston diameter. This will give the braking force, which can be used to calculate deceleration.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[krugerblue]

I understand. Pardon me, but I am not a native English speaker.

In my opinion (not scientifically, but experience based) cars usually stop faster than motorcycles. Does anybody know otherwise ?


Thank you

 

[NickE]

braking distance is mainly controlled by when the tires skid, and weight.

Considering that if the contact patch and tire compound were the same (per wheel) (they're not) the mass of the car is 4x the mass of the motorcycle. obviously the motorcycle is going to stop way faster.

(Anecdotally I have always been told to be particularly carefull about following distances w/ motorcycles. In real life I would expect that they also can stop faster.)



Nick
I love materials science!

 

[MikeHalloran]

Absent ABS, the braking distance also depends on driver/ rider skill.

And in the particular case of car/motorcycle interaction, it greatly depends on whether the car driver was actually trying to stop, and when/if (s)he even noticed the motorcycle.

So the braking distance for the car is probably shorter if the motorcycle has no, or vestigial, mufflers.



Mike Halloran
Pembroke Pines, FL, USA

 

[krugerblue]

Thank you very much for your answers.

 

[BrianPetersen]

The answer is "it depends".

The tire size and the light weight and lack of ABS on the subject automobile suggest that it is some sort of economy car, and it is unlikely that tires of that size on a car of that type would be a "performance" type with particularly outstanding grip.

The front tire size of the subject bike is an oddball. For performance bikes, 16" front wheels are obsolete years ago and 130mm nominal width was not normally found in combination with those. Most common front tire size on performance bikes nowadays is 120/70-17. But the rear tire size is a very common one on current 600cc sport models. So, I don't know what to make of this. More information is needed in both cases.

One must make a further assumption about the surface on which one is braking. Dry pavement is one thing, loose gravel is quite another as the rider of the bike would have his time simply staying upright at all, unless it is a dirt bike (but that would not have the tire sizes quoted).

On dry pavement, with good warmed-up performance tires, IF we are talking about a supersports bike, AND if the rider knows what they are doing, the limiting factor with stopping performance will not be the grip but rather the tendency for forward weight transfer to lift the rear wheel off the ground. For this, one needs the wheelbase, the height of the combined center of gravity of the combined rider and bike, and the fore/aft weight distribution of the combined rider and bike. The rear brake and tire will obviously be completely out of the picture in this situation. (I don't use the rear brake on my roadrace bike when the track is dry, and scarcely when it is wet.) Typically this situation will out-stop an economy car with cheap tires and no ABS. BUT, the situation changes completely if the rider does not know what they are doing.

 

[patprimmer]

The other factors are:-

You can push a car closer to its limits as it does not fall over if you actually lock a front wheel.

You have infinite control over the front rear brake bias on a bike wheresa you do not with a car.

The typically higher centre of gravity of a bike transfers more weight to the front wheel, which typically is the smaller tyre.

Cars typically have same size tyre all round and share the weight more evenly when braking.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[thundair]

As all have said the reaction time, the confidence and skill of the the one braking all make a difference.
Autos have done well to improve the brakes over the years My older Z06 will stop @ 60-0 in 100' which is the same as my Ducati monster. While the Z06 is 3100# and the Ducati is 395#, there is no way to come up with a formula for comparison as the 4 wheels will allow some modulation close to lock up that will scrub off a lot of speed.
While my motorcycle under hard stops has only the front wheel on the tarmac..
Also if you consider the complete braking process the thought plus reaction plus braking is probably around 240' for 60-0 with reaction in the bikes favor, just because of the location of the front brake lever..

http://www.msf-usa.org/imsc/proceedings/a-Green-ComparisonofStoppingDistance.pdf

Cheers

I don't know anything but the people that do.

 

[Rob45]

Kruger:
Among other things, you need to know the brake swept areas of the car and the motorcycle, and how those figures relate to the overall mass of the vehicles.

If you look in any recent issues of car magazines (e.g., Road & Track, Carp & Drivel, etc.) you can see there are tremendous variations in stopping distances in otherwise similar automobiles.

 

[RossABQ]

Back in the '60's, on 60 Minutes (an American weekly show) they showed a comparison between a Harley, a Vespa-type scooter, and a typical American car for braking. The engineers interviewed claimed that the primary factor for production vehicles' braking performance (remember this was in the '60's) was tire contact area. Thus the Harley out-stopped the much-lighter Vespa (by a huge margin at 40 mph) and the sedan stopped a couple lengths shorter than the Harley. (of course, comparison with a semi wouldn't have worked out)

Not directly to your question, but it mirrors my experience on modern bikes of all sizes (from 120cc to 1200cc) and many different sizes/types of modern cars.

I get the feeling this question relates somehow to a lawsuit?

 

[BrianPetersen]

Trouble there is that 1960's Harley brakes = garbage (some would argue that they're still garbage and I wouldn't dispute it), 1960's Vespa brakes = garbage (think they're still like that, too), 1960's American car brakes = garbage.

A premium sport bike (IF that is what the original poster was discussing - see previous comment about the oddball front tire size) will out-stop the econobox that the original poster is talking about if the rider knows what they're doing, and won't if the rider doesn't.

 

[Junior33]

in my mind, the difference in tire size isn't particularly important. The friction force will be determined by the coefficient of friction and the weight of the vehicle. The differences in weight will balance as that much extra mass must be stopped and it is proportional. Therefor, the brakes ability to bring the tires to the point of locking will be the deciding factor.

if at the same coefficient of friction. it doesn't matter if it's a 20 ton vehicle with 100 wheels, or a 1 wheeled vehicle that weighs 10lbs. Nor does it matter what size the tires are. The reason (in my understanding) that you use a larger tire on a performance car, isn't necessarily traction, it's life. A smaller tire would be unable to take that amount of forces across such a small contact area for any length of time.

So really, the difference comes down to aerodynamic downforce, and braking force in proportion to force necessary to overpower the tire. And most importantly, coefficient of friction of the tire.

given the sizes listed, I'd bet good money that the motorcycle is going to have the stickier tires and will blow the car out of the water, by a wide margin. The tire sizes also suggest that it's an economy car (as stated above) and as such, the bodywork will most likely generate life, further compromising the vehicles ability to stop quickly. However at the peak levels of racing (Formula 1 and MotoGP) the cars have such a high amount of downforce, that they can brake much harder than the motorcycles (without downforce) can.

that said, this is far outside my area of expertise and as always, a simplification. And frankly, I'd be much more comfortable if someone would either support or deny these conclusions, however this is what my preliminary math indicates.

 

[Yawing]

If you had to take an educated guess (as that is all it is with the info provided). Id say the car should be able to pull up quicker. even with assuming the bike has stickier rubber, the bike effectively only uses the front tyre to stop at max decel (much higher CG to Wheelbase ratio hence weight transfer), and given that tyre is not square, it is heavily curved so the contact area is more equivalent to a car tyres with a width of say 80mm at best.

So looking at approx 80mm for 280kg compared to 660mm for 1000kg. car ends up having about twice the tyre contact area for each kg of mass.

Now this assumes a LOT of things. roughly equivalent tyre corner stiffnesses, assuming limiting factor isnt the brakes (which for say a singular 100km/h stop shouldnt be for either of these vehicles),etc.

 

[GregLocock]

" coefficient of friction "

Not a very helpful concept for tire performance in this context.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.