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I'm big on bicycle repairs and whipping vintage bikes back into shape. As such, I took an old Raleigh road bike frame and made it into my very first fixed gear bike. So far, loving the experience of ridind fixed gear, however I've been trying to learn to skid for a few weeks now and I can't seem to do it, I push really hard but the bicycle just jerks to a stop, no skidding except in rainy weather (but then my skids whip all over the place). I'm running a 42/12 gear ratio (I know, it's huge but it's all I could scavenge parts-wise) and a 700×28c slick michelin back tire, and I'd like to know if it's normal that I'm having such a hard time skidding? Also everyone on the internet seems to be running much lower gear inches than me, yet I personally find it fine for my daily commutes in the city. Should I just invest in a new chainring and cog?

Thanks to all those who take the time to answer!

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    Firstly, why would you want to skid? If your brakes are stopping well then that's the idea of them. If you really want to burn through rear tires you want most of your weight over the front wheel. DO SO AT OWN RISK... – Dan K Nov 24 '19 at 14:18
  • "Jerks to a stop' makes it sound like most of your braking occurs with the front wheel. The front wheel will not skid, except on slippery surfaces. You'll go "head over" (not good) instead. – Daniel R Hicks Nov 24 '19 at 14:33
  • 42/12 is very high gear. The common combination for all around use is more like 42/16. – ojs Nov 24 '19 at 14:48
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    Are you using the "vintage " brake pads? As pads age they get hard the hard pads tend to slip on the wheel resulting in less than optimal braking. Swap out the pads and see if there is any difference. – mikes Nov 24 '19 at 17:05
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    FYI, 42/12 is a bad choice of gearing on a fixie because you only get two skid patches, or 4 if you can do it with either foot backward. bikecalc.com/skid_patch_calculator is helpful for this. – Criggie Nov 25 '19 at 9:14
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I assume that by "riding fixie" you also mean "riding brakeless", or at least trying to master stopping without using brakes, only using your legs and modulating your weight. Changing the gearing is also a way to make it easier or possible, as I will try to show below.

To make any wheel skip, the friction between a tire and ground must exceed a critical value for friction force Fgliding. Below that value, friction force is defined by equation F = µN, where µ is a constant coefficient depending on the surfaces in contact, and N is reaction force, which is equal to the weight loading that wheel.

Under the drivetrain braking, the deceleration force (== tire/ground friction) will be equal to the force your legs apply to pedals multiplied to a constant. This constant is defined by your front and rear teeth count, cranks length and wheel radius. Basically your drivetrain is a lever, to which you apply one force at one end and get a transformed force value at another end. You cannot load your pedals more than you weight (more or less). Thus, your legs and body can create a limited amount of equivalent stopping force at the rear wheel, let's say Flegs.

If Flegs < Fgliding, you will be unable to skid. You'll still be decelerating, but the rear wheel will be turning while rolling (not sliding) over the ground. Because it is a fixie, pedals will also be turning "against your will", forcing your legs to obey their rotation.

To make it skid, you'll need to either 1) increase Flegs, or 2) decrease Fgliding. To affect the former value, gain weight or change your gearing combination. To affect the latter, you can either reduce µ, which is exactly what happens in the rain, or reduce N by unloading your rear wheel.

To illustrate it a bit differently, if you throw your rear wheel just high enough so that it does not contact the ground at all (N= 0), your will be easily able to stop its rotation with legs while it is in the air: a wheel's inertia is obviously less than that of the whole bicycle + yourself. When the wheel lands back to earth (N > 0), it will be stationary (relatively to the frame), while the ground at the contact point will be moving relatively to it (because the whole bike has not yet stopped). This non-zero relative speed between the ground and the wheel's contact point is the definition of skidding: instead of rolling, the wheel slides. The re-obtained non-zero friction will cause the wheel to gradually start gaining its rotational momentum again; but that cannot happen instantaneously. It will continue skidding until the speed of its contact point becomes zero, at which point it has again regained its full rolling contact and speed.

This is exactly what happens when braking a mountain bike over an uneven terrain. The bike "flies" for short moments from root to root, stone to stone. Wheels' rotation, while in air, is halted/slowed down by applied brakes. Upon landing wheels produce this "skrrt-skrrt" sound of spinning back while skidding for short periods.

Given that there is a small negative gap between values of rolling and sliding frictions, this state of the wheel gliding can be made "stable", not temporary. If you find a sweet spot where the rear wheel is loaded just enough so that Flegs = µN, you will be able to skid continuously until a full stop.

Sheldon Brown describes the same physics behind the process, while giving a bit of details on how to place your body and pedals to unweight the wheel: https://www.sheldonbrown.com/fixed.html#skip . Pay special attention to health warnings he gives about doing it on that page: "Heavy-duty resisting is widely reputed to be bad for your legs, and to be counterproductive for building up muscles and coordination for forward pedaling."

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