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View Full Version : Fixie mythbusting


superstator
06-29-2007, 11:38 AM
I've had this idea rolling around in my head for some time now, and every time there's a headline here about brakeless fixies etc., it bubbles back up. I always hesitate to bring it up, because I'm way to lazy to do it myself. But what the hell:

Plenty of people have good logical explanations as to why braking with a fixed rear wheel only isn't as effective as other means. Plenty of people have anecdotal evidence that it is. What I haven't seen anybody come up with is any actual empirical data one way or the other. Maybe its time we did it ourselves? We could get a selection of bikes: a "street" fixed with a lower gear ratio, a "real" track bike, a road bike, an average hybrid/commuter, and maybe a 50lbs beach cruiser with a coaster brake. Then find a handful of riders from different backgrounds: a messenger, a racer, a regular commuter, etc. Have them all do a series of stops, using different combinations of brakes & back pedaling, and see how far off they each really are. To make it truely mythbustery, of course, we'd need some over the top braking method to test as well - a boat anchor, or maybe retrorockets.

Or maybe somebody has already done this, and I just haven't heard about it?

toddistic
06-29-2007, 11:55 AM
you should submit the idea to mythbusters! ok nvm bad idea

wsbob
06-29-2007, 01:16 PM
I'm inclined to think that some fixie riders may be able to stop very effectively in traffic situations. Realistically though, for the greater range of people that ride bicycles, I'd bet that the ability to do so requires a lot greater experience and skill than it does to stop a hand-brake equipped bike.

That's not to say that fixie fans should necessarily be prohibited from legally riding their unique machines in traffic. Probably, people that want to ride fixies should be obliged to take a test to determine their ability to stop safely in traffic, and be issued a permit verifying this.

I saw a guy up in the South Park Blocks doing some wonderfully agile things on his fixie. They just didn't seem like the kind of things average people could pick up very easily. That back-pedal/lock-up/skid technique seems like it could be very scary and unreliable for a lot of people. With fixie bikes being so extraordinarily trendy right now, this seems like it could represent some problems with novice riders jumping on them just to be 'with it'.

I didn't read some of the earlier threads about fixies, so sorry if I'm repeating a point that's already been made and discussed.

mizake
06-29-2007, 01:25 PM
I volunteer my fixed breaking ability for this research.

superstator
06-29-2007, 02:13 PM
I'm halfway serious about doing this, if others are interested and we can figure out the details. I imagine having each rider stop in the following scenarios:

Big Fixed Gear, skidding without brake
BFG front brake & controlled (no skidding) backpedal
BFG front brake only
Little Fixed Gear, skid w/o brake
LFG front brake & controlled backpedal
LFG, brake only
Roadie, skid rear
Roadie, front only
Roadie, front & rear controlled
Commuter, same tests as roadie
Coaster, skid and controlled

Not sure how fast you'd want to stop from - 15mph, maybe? 20 is faster than most people are going to be riding, 10mph is pretty slow. You'd want a camera or two to record the stops and make sure people were stopping from the same point and speed. And you'd need somewhere to do it that was straight, flat, and traffic free. Alpenrose seems a little cramped, and PIR seems like overkill. Maybe an unused parking lot somewhere?

Rixtir
07-02-2007, 12:37 PM
I'm inclined to think that some fixie riders may be able to stop very effectively in traffic situations. Realistically though, for the greater range of people that ride bicycles, I'd bet that the ability to do so requires a lot greater experience and skill than it does to stop a hand-brake equipped bike.You've hit on who the laws address-- the lowest common denominator. The laws don't take into account (nor should they) superstars like this guy:

I saw a guy up in the South Park Blocks doing some wonderfully agile things on his fixie. They just didn't seem like the kind of things average people could pick up very easily. That back-pedal/lock-up/skid technique seems like it could be very scary and unreliable for a lot of people. With fixie bikes being so extraordinarily trendy right now, this seems like it could represent some problems with novice riders jumping on them just to be 'with it'.

brock
07-02-2007, 01:33 PM
There are sooo many additional variables to consider:

- width of tire
- psi of tire
- make of tire
- type of handbrake - canti, sidepull, disk
- type of brake pad
- wear on all of the above
- operator skill and weight, of course.

... it just goes on like that.

Good luck with your experiment. I'd be interested in hearing the results, but as someone with a fairly firm working understanding of physics, I'll go ahead and say that a mediocre bike handler with any type of front brake will beat the best fixie rider in a braking contest under similar conditions.

Attornatus_Oregonensis
07-02-2007, 01:41 PM
This is a great idea. I always like to see empirical questions answered empirically, rather than with a bunch of speculation. I used to design human factors experiments for a living, so I think I could design an experiment that controls extraneous variables and that gives us a valid experimental estimate of the answers we seek. I see two related questions: (1) Does a fixie without a separate braking mechanism typically comply with the OR brake law as currently written? As proposed (adding a specific stopping distance)? (2) Does a fixie without a separate braking mechanism stop as quickly as a bike with a separate braking mechanism?

The best way to do this experiment would be to control for rider effects by using a within-subjects design, in which the same rider rides in both the fixie and non-fixie groups and the order in which s/he rides in each is varied.

Jeff Wills
07-02-2007, 09:01 PM
I'm halfway serious about doing this, if others are interested and we can figure out the details. I imagine having each rider stop in the following scenarios:

Big Fixed Gear, skidding without brake
BFG front brake & controlled (no skidding) backpedal
BFG front brake only
Little Fixed Gear, skid w/o brake
LFG front brake & controlled backpedal
LFG, brake only
Roadie, skid rear
Roadie, front only
Roadie, front & rear controlled
Commuter, same tests as roadie
Coaster, skid and controlled

<snip> Maybe an unused parking lot somewhere?

You forgot "recumbent, short wheelbase" and "recumbent, long wheelbase". Since I have one of both, I'll help.

FWIW: I found that my SWB recumbent is the best braking bike I've ever owned. It's possible to mash the front brake to the point you can hear the front tire rippling- but it never loses traction. Trying the same on my LWB results in a skidding front wheel- scary.

Jeff

superstator
07-03-2007, 09:05 AM
The best way to do this experiment would be to control for rider effects by using a within-subjects design, in which the same rider rides in both the fixie and non-fixie groups and the order in which s/he rides in each is varied.

For sure. The goal would be to be as rigorous as possible without making it too hard for a bunch of amatuers to get done.

Any thoughts on how to do the timing/recording parts? Stopping distance is easy enough to do with measured chalk marks & a video camera, but incoming speed is just as important.

Attornatus_Oregonensis
07-03-2007, 09:14 AM
The best way (obviously) is to get a radar gun. Maybe we could get one from one of the racing organizations, or maybe a friendly cop.

The alternative would be to mark off a (pre-stopping) space and precisely time how long it takes the rider to cover that distance. Then we could calculate speed ourselves. We could give people as many practice runs as necessary to see about how fast they need to be going to be at 10mph, 15 mph, etc. And speed is easy to calculate, so we could only count the runs where the rider was on-speed.

DJoos
07-03-2007, 10:57 AM
I think this would be a good idea, if scheduling allows I would be happy to help out.

rubbish heap
07-03-2007, 05:45 PM
I'll volunteer for the 'brakeless track bike, high gear' category.

You forgot skip stopping by the way.

I'd wager that the brakeless skid will stop about as fast (maybe a little faster) than the coaster brake.

There's faster ways to stop than a skid. For example, instead of just stopping the rotation of the pedals, you'd do a series of skips with the same foot forward (stopping the rotation of the pedals at 3/9 o clock, resting for a fraction of a second to let them go forward, than pull up on the pedals again to put them back at 3/9 o clock and repeating until stopped as to make the back tire brush over and over again).

Mofopotomus
07-05-2007, 01:16 PM
This sounds like a ton of fun. I'll volunteer my fixed gear (with front and rear brakes) for the mythbusting (man I love that show)

pdxtex
07-06-2007, 06:28 PM
i'll be your commuter with brakes.

vseven
07-06-2007, 06:59 PM
A.O. has a history of not showing up as promised !

bonny790
07-07-2007, 04:29 PM
Great idea, I'd love to contribute my roadie commuter loaded with school books!
Toby

superstator
07-10-2007, 12:18 PM
Stay tuned, then. I'm trying to come up with a good location - suggestions are welcome.

endform
07-14-2007, 08:49 PM
I am totally baffled as to how this ever became a debate. It is so obvious if you have ever ridden a well adjusted bike and know how to use a front brake to its potential using a front brake only stops so much more quickly than rear wheel only braking.


But enough with the anecdotes, here's some science! And yes the scientific method is based upon using empirical evidence to back up theory but this is going to be a lot easier to do than arrange a bunch of people.

Sorry to spoil the party, but here's a look see at our little section entitled "Rear-wheel-only braking."

"Let us see what braking distance we may expect if the same rider and bicycle studied earlier, starting from 9 m/s (20.1 mph), brake with the rear brake only to the limit of tire adhesion. We assume that the rear brake is strong enough to lock the wheel if desired, and that the coefficient of friction (mu) between the tire and road surface is 0.8. Then the maximum retarding force is 0.8 x Fv,r, where Fv,r is the perpendicular reaction force at the rear wheel [the normal force]. This rear wheel reaction force Fv,r is somewhat less than the value during steady level riding or when the bicycle is stationary, because deceleration results in more reaction's being taken by the front wheel. Let us take the moments of forces about point 3 in figure 7.5 Under the assume static conditions the machine is in equilibrium.

[that figure shows some center of mass stuff]

Fv,r * 1067 mm + mu*Fv,r*1143mm = 873 N x (1067 - 432) mm,

[1067 mm is the wheel base, 1143 mm is total height of the system, 432 mm is the distance between the rear contact point and the center of mass in the horizontal dimension, 873 N is the total weight of rider and bike]

Fv,r = 279.8 N *62.9 lpf) for mu = 0.8.

Then the deceleration (a) as a ratio of gravitational acceleration (g) is given by Newton's law:

F = (m*a)/g, therefore a = (F*g)/m = (-mu*Fv,r*g)/m,

(a/g) = ( -mu*Fv,r*g ) / (m*g) = (0.8 * 279.8 N) / 873 N = 0.256.

So the retardation with rear braking is less than half the value at which, using the front brake to the maximum safe limit, the rider would be about to go over the handlebars (0.56g)."

haggis
07-15-2007, 03:47 PM
Rats. When I joined this forum I didn't know there would be a test! Arghhh. Oh well, just like in school: I'll flunk the test but have fun in the labs :p