most of the momentum is in the spinning of the wheel not chain or cranks. How loaded the bike is makes no difference in how much momentum there is in the drivetrain.

Huh? Now I'm confused.

Aren't the wheel and drivetrain directly linked?

You're wheel isn't being stopped by a concrete wall, it's being stopped by your leg. There are order(s) of magnitude of difference between those respective impulses. Your legs are giant shock absorbers compared to a concrete wall.

The last statement is inaccurate because you're talking about a wheel that's lost traction, and therefore has no connection to the momentum of the bike & rider. The force bringing the already sliding wheel to a stop isn't dependent on the wheel's momentum, it's a mostly independent variable. If the wheel is heavy and has a lot of momentum, it just won't stop as quickly. So it all comes back to the question of torque and how much force you can apply to the wheel.

How did this thread turn into a physics d1ck measuring contest? The answer has already been given really, the effects of skidding are worn tires and an increase in awesomeness!:D

nah dude, your tires wear down from the awesomeness skidding creates.

Even a wheel that has "lost traction" still has force transferred to it from the ground. The only difference is that it's kinetic friction rather than static friction, and kinetic friction is always less.

Momentum, normal force, they all still play a big role. A 200 lbs guy going 20 mph and locking up the rear wheel is going to be putting a lot more stress on his drivetrain than a 130 lbs guy doing the same at 10 mph.

Now if you balls-to-stem it and get your center of gravity as far away from that rear wheel is possible, then the differences become less significant. But you also won't stop very quickly.

I think we need to see some mathematical equations to really figure this out. I do NOT claim to be able to do these equations, but I think someone should try and quantify the claims that are being made.

It just seems odd to me that so many people are able to strip their lockring threads if skidding doesn't exert any additional force on your bike. It also seems odd that skidding hurts your knees more than regular cycling. I, for example, had been riding my road bike for a long time before I got a fixed gear, and I notice a lot of unusual soreness in my knees from riding fixed (presumably from practicing my skids).

Which claims would you like to see equations for, specifically?

OK.

Start at the foot. The foot is exerting a force a certain distance from the bottom bracket.
Because the pedal pivots around the bottom bracket it is rotational and is called a moment of force or torque. Torque is measured in units like foot*pounds and newton*meters. We can tell by the units that torque is a force acting at a distance from the pivot point. To be even more precise, it is the component of the force which is tangential to the pivot.

Let's calculate the torque of me standing on a pedal in the horizontal position. I weigh about 80kg and my cranks are 0.172 meters. Therefore I create a torque of about 14 newton meters. I can't create more than that just by standing, but I could create a bit more if I jump up and down or pull up with the other foot (this requires bracing myself with my arms). Depending on my strength there is a finite amount of torque I can create and this can either go into accelerating or deaccelerating. I contend you can mash as hard as you can when you skid. Why couldn't you? Same muscles, same weight, same position...

Now, that 14NM torque has to go somewhere. If it goes to a chainring with the same diameter as the crank length then to resist it, the chain has to pull in an equal and opposite amount (Newton's second law). So with a chain ring 165mm the chain has a pull on it equal to my 80kg. If the chain ring is half that diameter then the force has to double to maintain the same torque. So it would be under 160kg of tension. The smaller the chainring the higher the chain tension.

You can do the math - Torque = Force x Distance from the pivot
(Your weight x crank length) + (Whatever force you are exerting tangential to the bottom bracket by gripping the handlebars x distance from handlebars to bottom bracket) = Maximum human generated torque = Chainring radius x chain tension

Smaller chainring radius = greater chain tension
Longer cranks = greater chain tension
Heavier/stronger rider = greater chain tension


It is easy to see that the pull on the chain - which is the only thing stressing the bottom bracket and frame ends - is higher with a small granny gear than it is with a typical fixed gear chain ring.

Does that prove it?

what you forget is that this force varies with cadence which is also a function of ring size.

I don't think he forgot that, it's just not relevant to finding the maximum chain tension.

Work (Force x Distance) definitely varies with cadence. Force varies with crank position but is modified by the efforts of the rider. But the point is that any human has a maximum amount of force that they can apply. And that force translates to forces on the bottom bracket and axle and hub and frame ends via chain tension. And chain tension is proportional to gear ratio. And the typical mountain bike or even most road bikes are equipped with gear ratios which can provide much more force than a typical fixed gear.

Therefore, skidding stresses the frame less than using a low gear. Also skidding results in less bending moment on the seat stays than braking does.

It really is the increase in awesomeness which is the major effect of skidding and we all know Newton wrote how awesomeness wears out tires faster.

no, the maximum amount of force that a given human can apply is also a function of cadence.

wears out the hub faster too

the rocking motion induced by going from one direction to the other when you skid causes the spokes to wear the hub faster by making them wear grooves into the spoke holes unless you've got a hub with straight pull spokes

As for lower gearing like with a mtb torque is multiplied by gearing while power is divided so yes you can easily using mechanical advantage of gearing generate more force or torque at the hub with lower gears although with much less power. Difference with a fixed gear though is the sudden change of direction of the power, you tend to go from full power one way to the other causing sudden stress on the hubs and spokes and rim.

I've worn out two hubs so far from riding fixed, both cracked thru the hub flange at a spoke hole.

horsepower = (torque * rpm)/5250 where torque is measured in foot-pounds........and humans are similar power wise to electric motors with max torque at or close to 0 rpm and max power closer to max rpm. Not hard to plug in some numbers in that equation and work it out even with the multiplication of torque or division of torque created by gearing. My powertap readouts as do most I would think will give you charts of torque and power measured in watts and it varies from 5-10 ft-lbs cruising along to up around 300 ft-lbs from a dead stop depending on gearing. But that is measured at the hub, at the cranks the torq is divided while hp is multiplied due to gearing having the cranks spin slower than the hub. On a mtb where you can have the cranks spinning slower than the hub the difference swings the other way where at the cranks the hp is divided and the torque is multiplied in relation to the hub.

Joined the forum because I am building a fixed gear for bar hopping in the neighborhood.

Posting because it is obvious that there are three kinds of people who ride fixed gears:

1. those who understand physics and have a front brake because they understand they can stop faster with one.
2. those who do not understand physics and do not have a front brake, and do not understand they can stop faster.
3. those who do not care.

It is pretty simple:

take a corner, do you want your tires to have traction, or not?

you can apply a lot more load through a tire if it has traction with the ground than if it does not, this is true for stopping, starting, maintaining speeds, cornering, any other loads you can generate.

take a car, turn off abs, do a panic stop and lock the brakes, measure the distance it took to stop. do the same thing, but this time, brake only to the point right before you lock up the brakes, you will stop quicker.

no need for physics, no need for math, no need for chainring counts, no need for anything else. there should be no discussion as it is a fact.

the heat and wear goes directly into the tire, which is obvious by looking at the patches. it doesn't go into the rest of the bike.

If you don't believe me (and I wouldn't as it is my first post) research it on your own, as I care enough about your safety to corroborate the posts of others, but not enough to go out and research what I learned in high school just to show you what you either slept through, skipped through, forgot or haven't taken yet.

Sorry for such an abrasive post as my number 1, but I guess we all have to make an entrance some way. I'll go back to lurking now.

that should happen with disc brakes too but I'm still not convinced. The majority of force on the spokes comes from supporting your weight and that cycles with every wheel rotation which is faster than you can shift from full forward force on the pedals to full backwards unless the bike is stationary. I agree with the firstpart but I think you are considering the max rpm you usually ride at maybe even the max the powertap records rather than absolute max. I'm willing to bet you can easily spin around 200rpms but highly doubt your max power is anywhere near that.

meh, smoothly and actually make some power not measured in milliwatts, 160rpm give or take for me personally

anyway I was just pointing out the nature of the human powerband.......max torq at low rpm and it decreases with rpm while power is close to 0 at low rpm and climbs with rpm

anyway, put a few hundred thousand miles on a fixed gear dewtret and see how things wear, if what your saying is true the front hub would wear pretty much like the rear, not the case in real life

About your knee problems, it could very well be caused by the fact that you aren't fit. You gotta have some muscles around your knees. Thighs are important, but you have got to take away some pressure away from your knees.

I am not a doctor, but I used to be a runner and I can tell you pumping up all your QUADs solve about a half of your problems. You know, leg extension exercise. It is probably one of the most infernal exercise in the gyms, but you gotta do it. (The leg exercises I hate most were squads and cleans. Oh lawdy. They are super dangerous and super embarrassing. Don't do those without supervision.)

However, in rare case, you may be in a deep deep problem. Feel your knee caps and see if they move smoothely as you bend and straighten your legs. If you feel resistance or sandy, you better go see doctors. This could mean you have deposits in your knee cap. This could only become worse and cost you a lot of money.

Hey take care, OK? Knee pains are extremely serious. Some people are permanently screwed and had to wear knee supporter for the even easiest of daily chors.

T