CityCyclingEdinburgh Forum » Debate!

Is reverse pressure braking efficient on a fixie?

(24 posts)

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  1. ih
    Member

    I'm breaking out of the 'obvious' other thread for this question.

    I've never ridden a fixie - I'm too dependent on gears for hills - but is reverse pressure an efficient way of braking on a fixie? We know that from the stand still, bikes can accelerate quickly by leg power alone, quicker than cars. On level ground reverse pressure braking uses the same mechanism, so aside from the slight difference in leg geometry for the two actions, is the deceleration roughly comparable? Also, the feet are on the pedals ready to brake, whereas there will almost always have to be some movement and time to get the hand on the brake. I am not advocating that fixies shouldn't have a front brake, and I can see that if the bike were going downhill fast, then reverse pressure might not be able to overcome gravity, but for level riding?

    Posted 7 years ago #

  2. stiltskin
    Member

    Well for a start: using the rear wheel as a brake is far less efficient than using the front, no matter what method you use to slow the wheel.

    Posted 7 years ago #

  3. dougal
    Member

    I've never been able to do it. The angular momentum of a wheel spinning generated by a person going even along a flat is considerable. From what I can tell it's all about weight distribution and locking out the wheels when they're not in contact with the ground. But like I say I'm not speaking from experience: I have two rim brakes on my bike and use them both. :-)

    Posted 7 years ago #

  4. steveo
    Member

    I could never brake as quickly as I could accelerate. I suspect my enormous weight (pies not muscle) meant that once upto speed I had more kinetic energy than I could stop. I did have a front brake and used it, once I did nearly kill myself when was a bit heavy handed on the front brake kicked the rear wheel up while I was also resisting the back wheel which suddenly had no weight on it, I was alo turning quite quickly.

    On the other hand for precicely controlling speed, approaching lights etc, they are very good.

    Posted 7 years ago #

  5. dougal
    Member

    http://web.mit.edu/cwarner/www/FixedGearSkidSample.pdf

    Posted 7 years ago #

  6. ARobComp
    Member

    Agree with Steveo, I was never able to adequately slow or stop a fixie from a reasonable speed to stopped. I could however make really smooth adjustments to speed using this. I had a front brake. I did used to ride a very large gear, and am quite large. I wonder how much the weight of the person and the size of the gear affects things?

    Posted 7 years ago #

  7. urchaidh
    Member

    Pie coefficient of the rider would effect both acceleration and deceleration pretty much equally, so probably cancel out.

    Posted 7 years ago #

  8. Min
    Member

    And of course it goes without saying that should you lose your chain (a common occurrence on a bike) the stopping power is 0.

    Posted 7 years ago #

  9. Arellcat
    Moderator

    And, http://www.pcbrakeinc.com/epub/AIQWinter2008MCBrakes.pdf


    Braking forces diagram - Brompton by beqi, on Flickr

    Posted 7 years ago #

  10. dougal
    Member

    @Min http://www.naden.de/blog/bbvideo-bbpress-video-plugin -->

    [+] Embed the video | Ted Shred that baby!" target="_blank">Video DownloadGet the Flash Video

    Posted 7 years ago #

  11. ih
    Member

    @Min very good point, although I have never 'lost my chain' as a result of chain break, but I have sometimes lost it as a result of a poorly adjusted front mech that has thrown the chain off. Some other braking mechanism definitely required.

    Posted 7 years ago #

  12. Murun Buchstansangur
    Member

    There's no front mech on a fixie? But I assume "engine" braking is highly dependent on chain tension ie any slack would be disastrous

    Posted 7 years ago #

  13. wingpig
    Member

    Your big stamp-down acceleration legmuscles coupled with pulling on the handlebars are waaaay mightier than their opposites.

    I only briefly used the fixed cog on my singlespeed as I'm far too used to standing on the pedals and unweighting the bike over potholes etc.

    Posted 7 years ago #

  14. steveo
    Member

    I snapped my chain as a result of awesome power being a bit heavy footed starting off causing me to crash to the ground uncerimoniously. I bought a heavier chain.

    I never shed or snapped one under breaking however. I don't think a well set up fixed gear/single speed should shed a chain, the tension should be just so that there is no where for the chain to go. And as for snapping under breaking I suspsect that differnt torque forces would prevent my feeble self from doing so.

    Posted 7 years ago #

  15. ih
    Member

    @Murun yes, I was just relating an anecdote about my geared bike experience.
    @dougal interesting paper, but it seems to be based on braking by 'locking your knees and skidding the rear wheel'. I would have thought that more efficient and controlled braking would be achieved by making sure the rear wheel does not lock and skid.
    @Arellcat nice graph. I'm afraid I don't understand it though!

    Posted 7 years ago #

  16. remberbuck
    Member

    The proper point of efficiency on a fixed wheel bike is to go forward. This happens, as mentioned above to the inherent momentum in the rear wheel, plus to a lesser extent in the efficiencies in a clean chainlink.

    Braking ability is a secondary thought. A simple comparison would be with a bike equiped with a coaster brake. Such bikes are usually heavier, taking longer to slow down, this being matched by the fixed wheel's generally faster speeds, and lower ground contact with narrower wheels. Stopping distances aren't a million miles away.

    I very briefly toyed with the fixed side of my single speed and dropped the idea very quickly. Partly because I realised my calves wold not be up to it, even with the help of a front brake, but mainly because the inherent momentum would reduce drastically my thinking time and chances of action when needed.

    The few fixed wheels you see in Edinburgh usually travel at quite low speeds, certainly slower than road or cross bikes, doubtless because of this.

    Posted 7 years ago #

  17. rust
    Member

    "The few fixed wheels you see in Edinburgh usually travel at quite low speeds, certainly slower than road or cross bikes, doubtless because of this."

    That would be my experience of riding fixed. I reckon that because of the lower speeds (and front brake) my stopping distances would have been shorter riding fixed than they currently are on my cable disc road bike.

    What's the top comfortable speed on a fixie? Maybe 18mph? Compare that to my road bike where the 20mph zone can feel very slow (when drivers are obeying it).

    Posted 7 years ago #

  18. remberbuck
    Member

    Guess top comfortable speed is going to depend on your gearing. My freehub single speed is 46x16 which OK for 95% of Edinburgh hills and probably works out at 15mph around town.

    A fixed hub's probably going to get more acceleration out of a smaller gearing which of course reduces the maximum speed.

    Neither are going to be fun going down Edinburgh hills.

    Posted 7 years ago #

  19. Tulyar
    Member

    @Arellcat is that based on a 70-80Kg rider on 12Kg bike with long wheelbase frame?

    Presumably also takes rider weight and CoG remaining constant position relative to height above ground and position on saddle or off.

    Would a heavier/upright rider give a higher limit to rear wheel lock-up?

    Presumably the green curve displaced to right?

    Posted 7 years ago #

  20. Arellcat
    Moderator

    @Tulyar, yes(ish), yes, no, and no(ish). In the model, as long as the front-rear weight distribution is held constant, the combined weight of bike and rider is not a factor in the combined front and rear braking ability.

    As the rear weight bias approaches the total weight however, the green curve is displaced to the right: thus the front wheel normalised acceleration (the red lines) tends towards zero, because less friction is available, and the rear wheel normalised acceleration (the orange lines) approaches a maximum.

    Conversely, as the front weight bias approaches the total weight, the model correctly shows that decreasing rear braking effort is available because the lines of constant friction all tend towards zero, and the theoretical maximum front braking effort is in fact limited because the rear wheel experiences increasing vertical acceleration (of course).

    And, if you double the wheelbase of the vehicle or halve the height from the ground of the centre of gravity of the bike and rider, the green curve – that is, the optimum combinations of front and rear braking – extends both to the right and upwards, typically by 25%.

    Posted 7 years ago #

  21. dougal
    Member

    @ih I'm not sure what you mean. If you're travelling at <10mph it's possible to control the speed by pedalling slower; but anything faster than that and locking the wheel seems the only option. Variations like whip skids are available but it still amounts to the same thing with a wiggly back end.

    Posted 7 years ago #

  22. Broony84
    Member

    I was having a debate with a work colleague the other day about this. I argued that I can actually stop quicker on my fixed gear with just a front brake fitted than I can on my road bike with 2 brakes. I regretted saying it after as I couldn't back up my argument but having had to do numerous emergency stops on the fixed I do believe I can brake more effectively on it. The front brake slows me down quickly to a point where my legs can then start to resist the rear wheel at the same time. Going downhill I would always feather the front brake rather than trying to resist the momentum from the crank/pedals

    With regards to speed. I always seem to have higher average on my fixed than I do on my geared bike but I think that is mainly to do with my lack of finesse.

    Posted 7 years ago #

  23. neddie
    Member

    whip skids are available...

    As kids we used to practice doing whip skids (on an ordinarysafety bike) in the playground. No plastic hats required...

    I feel like I should re-learn / re-practice that skill in case of any tram-track-crossing-related-whoopsies-due-to-following/parked-angry-traffic-shenanigans

    Posted 7 years ago #

  24. dougal
    Member

    whip skids (on an ordinary bike)

    Well that's bold...

    Posted 7 years ago #

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