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I've been under the impression that bikes and motorcycles cannot stop as quickly as cars. I'm curious what factors have the largest impact on bike stopping distance.

What comes to my mind are:

  • Tire size (more rubber on the road means more stopping power), a higher diameter/lower-pressure tire should be able to apply more friction to stop
  • Bike weight (less weight is less intertia)
  • Brake types(?) Do rim vs. disc make a difference (it would seem they are negligible as both lock up the wheel when too much pressure is applied)

What impact do these factors have? What other factors affect braking? Which factors can be tweaked for greater stopping power--and at what trade-off?


Note: My assertion that cars can stop more quickly than bikes originated from my motorcycle rider's ed when I was getting my motorcycle endorsement. It was stated that cars are able to stop more quickly in general, and as such you must learn to follow further back than if you were driving, and also learn to swerve around stopped vehicles (in emergencies) rather than trying to brake in a straight line--as you are most likely going to end up on the bumper if you only brake without additional maneuvering.

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Do you have any citations/studies that back this up? I'd be interested to know how the typical braking distance compares between cars, motorbikes and bicycles. –  Mac Aug 16 '11 at 8:19
    
+1 @Mac Motorbikes have huge stopping power, combined with very light weight (hence less energy opposing braking), I'd really like to see some backing numbers for this claim. –  jv42 Aug 16 '11 at 8:55
    
Highway Code says "large vehicles and motorcycles need a greater distance to stop" [than cars]. Doesn't seem to say anything about bicycles. direct.gov.uk/en/TravelAndTransport/Highwaycode/DG_070304 –  Tom77 Aug 16 '11 at 9:40
    
Thanks for the input, I've updated the question to remove my false assumptions –  STW Aug 16 '11 at 11:56
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@lantius - 13.9 seconds sounds like too long to brake from 30km/h. I just did the sums and got 1.39 seconds in about 15 meters. Does that sound better? –  Mac Aug 17 '11 at 11:18
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3 Answers 3

up vote 6 down vote accepted

The biggest factor in bike stopping distance is the mass transfer towards the front of the vehicle when braking. On an upright bicycle, the limiting factor in how hard you can brake is the point at which the front wheel goes from slowing you to simply throwing you over the handlebars. As you brake, your center of mass shifts forward, lifting the rear wheel. Shifting your weight backwards and down while you brake dramatically increases your braking capability. A human-powered vehicle with a lower center of gravity such as a recumbent trike or recumbent low-racer can also typically stop more quickly.

A bicycle's braking limit is approximately 0.67g, while the limit for a typical passenger automobile is somewhere between 0.8g and 1.0g. Legal requirements in the US are even less stringent than these limits - typically a passenger automobile must be able to brake at 0.53g, while a bicycle is only required to demonstrate the ability to skid the rear wheel, providing at most 0.2g of braking capability! It's certainly the case that there are automobiles driving around with less braking capability than many bicycles.

The factors you mention aren't really significant in the limiting case - friction is related to the normal force over the size of the contact patch, so larger surface area doesn't really help, and even if you did increase friction (sticky tires) you'd still hit pitchover before the front wheel skidded. The bike's weight is usually a tiny fraction of the total system weight. Disc brakes help in the average case since the discs typically have a better interface between the rotor and brake, but clamping a dry and clean rim brake can easily pitch a rider over the front wheel.

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Right. If you "panic" brake entirely with the rear wheel then weight lifts off of it and you skid fairly easily. If you "panic" brake entirely with the front wheel then you go head-over. It's pure physics, and determined mostly by the geometry/weight distribution of the bike. –  Daniel R Hicks Aug 16 '11 at 11:38
    
Wow, I always assumed that a bike could stop quicker than a car, so I was never concerned with how much space I left in front of me. Thanks for the linked article. I'm going to have to leave more space in front of me from now on. Cheers! –  Mac Aug 17 '11 at 0:45
    
Disc brakes are also easier to modulate (i.e. apply varying levels of force and hence braking power to get closer to max braking force), and their braking doesn't degrade as easily with water. So while they won't be better than a rim brake under perfect conditions it will improve the average case (as you say). –  Ben W Feb 11 at 0:30
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  • Tire size: not significant because on paved road any tire should provide enough friction (stopping force) to get you to the critical/tipping point.
  • Bike weight: not significant compared to your weight (the difficulty is to stop the rider)
  • Brake types: I've found disc brakes reliable in wet and snow on hills and in traffic.

The critical factor is the angle between your centre of mass and the point where your front tire meets the road. Being lower and further back reduces that angle, so push your bum off behind the back of the seat.

You also need enough arm strength or alignment to stop you: beware being high with your arms downwards when you hit the brake. Positioning yourself as far back and as low as possible gets you behind (not above) the handle bars, against which you push to brace yourself as you come to a stop.

The rear wheel skids before the front (as the deceleration pitches you forward, the weight goes onto the front wheel and off the rear wheel, and friction is proportional to weight). On the other hand, it's skidding the front wheel that would cause a loss of control. One possibility then is to brake with both wheels simultaneously, and to ease up on both sets of brakes when the back wheel starts to skid (treat it as a ABS system)..

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The front wheel won't skid, except on very slick surfaces. The usual advice, though, is to do as you suggest and brake with both wheels (about twice as much force on front as rear) and let up if the rear wheel begins to skid. (I've suggested before that a disk brake system could be set up as a true ABS system, using the rear brake to activate the front.) –  Daniel R Hicks Aug 17 '11 at 12:43
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Yep... what the others said. Your deceleration reaches its max limit when the rear wheel becomes totally unloaded and the bike is about to flip over by pivoting on the front axle.

Yeah, you can mitigate the situation significantly by lifting your butt off the seat and pushing your weight backwards as you're braking. Having more of your weight further behind the pivot point (the front axle) will give you even more deceleration before you flip.

These considerations, however interesting, are pretty much just academic.

The REAL determinants of being able to avoid a collision is NOT the theoretical limit of deceleration but whether or not the rider is paying enough attention to not get into a panic braking situation in the first place, the reaction time and judgment of the rider, the physical ability of the rider to properly use "body english" and bike handling to either stop the bike in time, maneuver around the obstacle, or just crash in a way that avoids the dreaded "face-plant."

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Seriously, any bike with halfway decent brakes will stop pretty damn fast. What I am trying to say that the best brakes and tires in the world won't help you more than merely paying attention to what is in front of you and practicing some handling skills. –  Angelo Aug 17 '11 at 18:12
    
I would guess you didn't get downvoted for the practical advise, but rather because your advice doesn't do much to answer the question. –  STW Aug 17 '11 at 20:13
    
I see, that's cool, but there just isn't much that can be tweaked design-wise to optimize stopping power. As long as your brakes can skid the rear wheel, you're most of the way there. Beyond that, its all about the rider. –  Angelo Aug 17 '11 at 21:30
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