JonnyVain

I'm usually pretty good at geometry and mechanical type stuff, but I can't wrap my head around the oval chainring. It seems like they're using basically an optical illusion to convince you it has power variance.

Bottom line though, to vary power output, it would need to vary the gear ratio. Look at these things online. Count the teeth. No matter what, the chain will always be in contact with half the ring.

Count the rings. If you're on the long half or short half, it's the same amount of teeth. Think of it this way. The chain will either be in contact with 2*1/2 the short side + a long side, or 2*1/2 long side, and a short side. Both equal a full long and full short side contact.

Same gearing ratio.

Make sense or no?

I've got one in my cart from Works, but feeling like it's a bad decision. Might change the 32T oval for 30T round.

Rollfast

Yes. Like chlorophyll.

prathmann

If you're looking at real oval/elliptical rings like:

then, yes, they change the effective mechanical advantage (gear) as they turn. When the chain engages the teeth on the long axis of the ring the effective gear inches will be given by:
Gear inches = long axis radius/rear cog radius * diameter of driven wheel
whereas when the chain engages the teeth on the short axis that's the measurement that would be used. Note that the amount of chain pulled forward by a given angular rotation of the chain ring is greater when engaging the teeth near the long axis. So at that point in the rotation it's equivalent to a round chain ring with a radius equal to the long axis radius.

Note that the Shimano 'Biopace' rings were kind of an odd shape without that much variance in the effective radius of the ring at different points of the rotation. So there were some minor variations in effective gear ratio but not as pronounced as with a very ovalized ring.

Rollfast

I have no trouble with my Schwinn Hornet's round one, the front tire has already gone flat though

JonnyVain

Absolute green?

HerrKaLeun

I think you don't understand power. Assuming the human in question puts out equal power at any rpm, power stays the same regardless of gear. I think you mean torque at the wheel, which changes with gear ratios.
How many teeth actually engage also doesn't matter. The effective lever length (distance from chainring center to top) matters for the gear ratio. Adn that changes based on clock position of your feet.

I upgraded my fatbike from round to oval. You don't get more power, but smoother torque delivery. In snow or other slippery conditions my wheel was between being stuck and slipping depending on the pedal position. So when my foot was at 3óçlock, it slipped, and at 12óçlock i was stuck in deep snow. the oval chainring smoothed that out (averaged torque if you will) and riding through deep snow and mud was better. Not more power, just more even delivery. If you have normal traction (like a road bike), that advantage may not appear. On my fatbike it also works well when going fast, but I can't say it is better than round. For the above mentioned deep snow/mud oval is really better. not more power, but better.

I have an absoluteblack chainring, BTW.

JonnyVain

I think when you say "long axis", that doesn't exist, or mean anything. The chain isn't being pulled by one link, it's being rotated. Long or short doesn't matter, it's overall radius, averaged over an oval. The overall radius is always the same because the chain is always over half the oval.

Would be interesting to test myself, but didn't wan't to spend the money.

Rollfast

Don't know what's in that, chlorophyll is the substance in plant leaves than converts nutrients via sunlight. It was in chewing gum in the 50s even, the sister of 'atomic', 'electric' and 'radio'.

HerrKaLeun

Speed of the chain depends on angular velocity of chainring times the length where the chain engages. If that wasn't the case, the chain would skip. The chain travels at exact the same speed as that chainring tooth where it engages, not more, not less.
with an oval chainring your feet move slower and faster (assuming we ignore micro speed changes of the chain through each revolution and the bike travels at constant speed throughout the entire revolution and doesn't go fast-slow-fast-slow in one crank revolution).

How many other teeth of the chainring are engaged doesn't matter. It just happens to be about half (180°) for the chainring. But for the cassette you have less than 180° engagement because the RD idler pulleys. Again, not relevant how many other tooth are engaged.

prathmann

Actually it is being pulled by one link at a time. The other links are just sitting idly on the ring. Think of the distance the chain has to move when you rotate the crank by a small fixed amount - say 10 degrees. That 10 degree rotation pulls a greater length of chain when the oval is standing on end than the same angular rotation does when the oval is horizontal. For small angles of theta:
Chain distance = sin(theta) x radius at that point of the oval (i.e. distance from the BB axis to the edge of the chain ring where the chain first touches it)

Try it with a chain ring that has a pronounced oval shape such as the one illustrated or the ones made by Roger Durham many years ago and the change in effective gear ratio is immediately obvious as the cranks speed up when the oval is horizontal and slow down when it's vertical. OTOH, I never really felt the effect with Shimano's Biopace. Whether that has any benefits is another question. I never rode any significant distance with oval rings - just tried out a friend's bike with them. Certainly felt different but not really better.

JonnyVain

It was a joke. Absolute black chainrings...

Yeah you get plenty of chlorophyll if you eat your veggies.

JonnyVain

You know, it would probably vary power output if the axle was not centered to the gear. Look at the 2nd picture down:

https://en.wikipedia.org/wiki/Non-circular_gear

But that doesn't work with a bike because we pedal with two legs.

prathmann

Oval and elliptical chain rings work just fine as they are with a centrally located axle. At least in the sense that they make for a varying effective gear ratio that is largest when the oval is vertical and smallest when it's horizontal. Whether they actually result in a faster or more efficient ride is debatable - some riders like them but they've never gained much of a following.

Clearly you're not willing to actually think about either my or HKL's arguments based on the geometry of the situation nor accept the experience of those who have ridden with such chain rings. Envision the distance that a single tooth of the ring moves for a given angular rotation of the crank. If that tooth is near the long axis of the oval it moves farther than if the tooth is near the short axis. The chain engages that tooth and therefore has to move exactly as far as the tooth does. If you still can't see it then I suggest you find someone with oval rings and borrow their bike. Ride at a reasonably constant speed and you'll immediately feel the variation in the angular rotation speed of the cranks.

JonnyVain

Oval chain rings are the "airplane on a treadmill" of the bike world.

The whole problem with plane on a treadmill was that people got hung up on the wrong thing: namely, that if there is no airflow over the wings, there is no generated lift. They would gloss over the premise of the question. That is, does the airplane actually move. The nature of the question (or I should say the human brain) caused people to assume no motion. Some would even argue that it's stated in the question that the plane won't move, because of how strongly their minds bought into the assumption. So they keep arguing that the plane won't have lift, while those who are correct try to bring the debate back to the velocity, or rather the question on where the force is applied. (it takes off btw)

With oval chain rings, you're assuming that the distance the chain travels is greater, that the torque (angular force) is varying, based on angle of the chain ring - all based on the assumption that it is "longer" because visually hey, it IS looks like it would be. Take a step back and question those assumptions.

Prove that the chain varies speed with consistent crank speed. Put a drill with a hex bit in your crank and video in slo mo on your iphone or go pro. Mark a link or two, put a yard stick above it, and count inches per frame (it won't speed up).

Prove that the force varies based on chain ring angle - measure with chain ring installed correctly, then measure with it rotated 90 deg. Measure at the rear wheel, not a single point on the chainring (that would be structuring the test around your assumptions) (it won't change).

Prove that the ring is longer (edit: bigger gear ratio - longer is incorrect nomenclature and feeds your assumptions) in certain orientations. Does the derailleur move to take up the slack as you rotate the crank? Or remove the derailleur altogether and shorten chain to fit. If ring size varies, it won't rotate all the way around (but it will). EDIT: you stated the ring changes gear ratio

You think I'm coming off as hard headed, but I'm not hard headed. I'm actually convincing myself as I'm typing as this is the first time I've thought it through. I'm reading what you're typing, but it only makes sense based on your assumptions, which are incorrect (or so I'm currently convinced). The comment about being willing to think about an argument goes both ways. I mean in the end I don't care, I have nothing to prove. I don't sell round chain rings, and Works had them for the same price so it's not like I'm ashamed of my poorness... I hate losing traction on a climb as much as the next guy, and I do a lot of climbing. I would benefit from them if they worked, I just don't see how they would.

JonnyVain

Btw, if they actually do work, they would indeed not increase power or speed (ideal). They would actually decrease peak power and increase low power. This would benefit in two ways.

1. Decrease risk of getting in a low-torque-stop/higho-torque-skip situation on a climb.
2. Take pressure off the knee at the top of the pedal stroke when the knee is at its most flexed position.

prathmann

Hardly. I've never seen any airport runways constructed out of treadmills, but I've seen a number of cyclists using oval chain rings and tried them myself at one time. And they've been around for many decades. My test ride was back in the '70s and it was a very old idea at that time.
BTW, the rear derailleur cage moves very slightly back and forth as the cranks turn but it's almost insignificant. OTOH, the change in effective gear ratio is quite pronounced. As I've suggested before, try it yourself - the oval chain rings are not hard to find.

JonnyVain

Is that due to a change in gear ratio, or chain slap? The chain is whipping up and down twice per rotation.

Same number of teeth per half.

Image screenshot from this video:

Text is theirs. If you count the teeth, it's 30 both directions.

Kapusta

The leverage on the chain that goes from the top of the chainring back to the cogs changes as the chain engages at different points on a non-circular chain-ring, including on oval one. This is a fact, and simple high school level physics.

That Wiki-page you linked to actually explains this. And you see the concept put into practice all around you. Brake levers that change the rate of cable pull throughout the stroke use this concept. So do many workout weight machines that use belts going over pulleys that are not circular.

I am afraid I can't quite follow where you are going amiss here, but I do notice a few things you seem confused about:

You are getting distracted by the fact that the same number of teeth are engaging the chainring. That is a red herring. Think about it: in the way you describe this is also true (well, sort of, see next paragraph) of the non-circular gear on the Wiki page you linked to and refereed to, yet you accept the fact that the leverage changes in that instance. The mechanical advantage of a gear has zero to do with how many links are engaged.

Also, while it is not even relevant, I should point at that the chain is NOT always over half of ANY chainring (round or otherwise). It depends on the size of the cog in the back, and the orientation of the tension pulley on the rear der. (these two factors change the angle at which the chain meets the chainring, and therefor the location on the ring). And as the oval ring is oriented with the long orientation horizontal, it will have less chain engaged compared to when it is in a vertical orientation (again, because of the angle of the chain). The difference is small, but it is there. BUT..... this is all irrelevant, and you may want to examine why you thought it WAS relevant, as that might be a clue as to where your reasoning is going awry..

The issue of whether the tension on the RD cage changes is also a red-herring. It is irrelevant.

Also, there is no "overall radius" in an oval. There is one for the long axis, and one for the short axis, and theoretically an infinite number in between. You are confusing radius with circumference. It is true that the circumference of a 32t round and 32t oval ring are essentially the same. But that is not what determines leverage of a gear on a chain (though in a round gear the circumference and radius always change together in a linear fashion).

The leverage (mechanical advantage) that a gear has is determined by it's radius, not it's circumference. this wiki page talks about this https://en.wikipedia.org/wiki/Mechanical_advantage

Circumference vs radius is a very important distinction here, because it is the RADIUS that determines mechanical advantage. Specifically, the radius where the chain is being picked up.

It can be a little confusing because cyclists always talk about tooth count of sprockets (which is a measure of circumference) when discussing gear ratios, but the reason this works is because for round rings, the radius is consistent, and varies linearly with circumference.

Also, you keep confusing "power" and "leverage". Oval rings DO vary the leverage throughout the rotation. That is an objective fact, backed by simply physics. And subjectively, you definitely notice it. Whether they do anything in terms of your power output is a bio-mechanical question and a very complex one at that, and whether they actually HELP you is subjective.

Good luck getting a handle on this, but I can give you some advice, you will save yourself a lot of time if you focus on what you are not understanding correctly, rather than trying to convince others that the basic laws of physics don't apply here.

What IS true is that the the OVERALL gearing throughout the entire crank revolution is the same in the sense that you go just as for in one crank revolution with a 32t round or 32t oval ring. What changes is the leverage throughout the crank revolution. At some points it is higher than a round, and at others lower.

HerrKaLeun

That is exactly what they claim to do and is my experience.
No transmission increases power (small inevitable friction losses actually decrease usable mechanical power). If you could increase power, you'd have a perpetuum mobile. I think you are mixing up power and torque.... whatever it is, it doesn't help your case against oval chainrings.

JonnyVain

I just bought a cheap oval chainring from amazon. Probably won't last on the trail but I'll be able to do some testing with it.

Kapusta

JonnyVain

Yeah that's the point i was missing, that the gear ratio change happens within and only within each 90 degrees rotation.

I'm honestly surprised i didn't think of that myself given that it's essentially a sine wave.

I'll analyze per 10 degrees and get back.

JonnyVain

Yep. I calculated 2 teeth difference over 90 deg, which is exactly what it's advertised as (Absolute Black and Works).

Well I was right about one thing, this was an airplane on a treadmill type argument. Except I was the one not getting lift...

Thanks for entertaining me. I'll get some sleep and hopefully be smarter in the morning.

Kapusta

Rollfast

You'll need Retsyn afterward