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Discussion Starter #1
I recently got an Ovaltech triple crank for my all-Suntour equipped touring bike at quite a decent price. I was ready to ditch the half-step and go 28-38-48, and I enjoy ovalized rings for certain steady-cadence applications. I have Q-Rotor rings on my TT bike as well, and there's even more radically ovoid rings like Osymetrical out there, proving that these are indeed successful products when installed correctly. They don't increase the mechanical efficiency of a bike, but they have been proven to increase the physiological efficiency of the rider. But, they have to be at the correct position, and I thought I would share my experiences working to achieve that with my Ovaltechs.

I read a while ago about people who took their Biopace rings off the crank arms and rotated them, since they were installed on the crank at an offset that proved unproductive. I also came across a wonderful website of a company owned by an engineer in the UK that manufactured oval chainrings for over 25 years. I'll link it below, since it has a great explanation of how & why these are advantageous when installed correctly. The basic idea is that when the long axis of the oval is perpendicular to the ground (up and down), the crank arm should be somewhere between 2 and 3 o'clock, where the power stroke portion of the cadence takes place. This essentially creates a higher gear by making the front chainring effectively taller in relation to the chain axis. Then, when the crank rotates and the long axis of the oval becomes parallel to the ground, the crank arm ends up around 6 o'clock, where the power stroke ends and enters a dead spot until the other leg picks up its power stroke around 3 o'clock. That creates a lower gear for the dead spot when the oval is parallel to the ground and has the equivalent diameter of a smaller chainring.

1 powerstroke.jpg

The term "offset" has to do with the position of the crank arm relative to the chainring when the long axis is perpendicular to the ground, and how far away from horizontal the crank arm is mounted without moving the axis. There's a forumula for figuring that out, along with a bunch of other useful explanations, here.

The ideal offset can vary but should be close to 0, or horizontal. A good offset for racing is just a few degrees under zero (or slightly above 3 o'clock), and a good offset for touring is usually around -20 degrees, or 20 degrees above 3 o'clock (which pretty much ends up being 2 o'clock). Ovaltech rings were attached to the crank arms at a 90 degree offset, and Biopace rings were attached to the crank arms at a 50 degree offset. So, I had to find a way to re-orient my rings, because they were the complete opposite of what was wanted; as you see in the picture below, the crank arm is completely inline with the long axis of the oval. So, the oval would be at its shortest when I entered the power stroke and then at its tallest once I entered the dead spot.

Before 1a.jpg

I lucked out that the sticker marks 12 o'clock, so I always knew when the long axis of the oval was vertical. The trouble, though, with rotating the crank around the chainrings when you need to keep the long axis of the oval rings vertical in order to place the crank arm at, say, 2 o'clock, is that the crank arm can only be bolted in one of 5 positions. I could only rotate it in increments of 72 degrees (1/5 of 360), and while there was a chainring bolt right at 2 o'clock, the crank pointed right in between the bolt positions and couldn't line up where I needed it. Then I had the palm-to-forehead moment where I realized that the drive side crank arm didn't have to be the one at 2 o'clock; the non-drive side arm could, and the drive side crank arm would simply be 180 degrees opposite the desired location. That allowed me to point the it right at the 8 o'clock mark, landing the non-drive side arm right at 2. Since these rings are symmetrical, when the drive side crank arm comes halfway around, it will hit 2 o'clock when the long axis is vertical again. Voila, each foot has the high gear of the tall oval from 2 to 5 o'clock, and then it tapers off to the low gear of the short part of the oval.

Here's a side-by-side before and after pic with the crank arm pointed down on both sides so you can see that how oval is turned, and I'll attach a picture of how the crank arms ended up in position when I returned the long axis of the oval to vertical.

Before After vertical.jpg

Now, the drive side crank arm is at 8 o'clock and the non-drive side is at 2, and they'll simply trade places after a half revolution:

After 2aa.jpg

Here's the link to Highpath Engineering's homepage with lots of good info, from calculating ovality to proper installation, as well as a chart showing the percentage of ovality and offset of different ovalized and elliptical chainrings made over the last 120 years on the History page:

Highpath Engineering : EGGring oval cycle chainrings - page 1

A more difficult project would be doing this with Biopace, because their shape is actually a rounded rhombus and not an oval, which makes the transition coming out of the power stroke somewhat more curved and less linear (or so they claimed, amongst other things). I might tackle this next. Chris, the "Oval Engineer" at Highpath states that the crank offsets of Biopace rings orientated them back-to-front on upright bicycles (as opposed to recumbents), giving a higher gear to push through the dead-spots instead of a lower one, making matters worse, not better. Thus, I've seen people not only rotate Biopace rings on the crank, but turn them over as well to change the orientation of the transitions in or out of the corners of the rhombus. It reportedly works pretty well, and I'm looking forward to finding out.

Long live Suntour!
 

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Discussion Starter #3
As a follow-up, I decided to take a couple pics of a 42t SR Ovaltech and a 42t 1st generation Biopace ring on top of each other to show how exactly similar they are, but with different offsets. I always read that the Biopace was more a rounded-out rhombus than an oval, and that Ovaltech and Cycloid were oval; however, this shows they're exactly the same in shape, but that they are laid out differently on the crankbolts, thus emphasizing the importance of analyzing and rotating any ovalized ring on the crank arms according to the long axis. These rings mounted on the same crank arm would have completely different dead spots and power stroke areas- when you line up the teeth perfectly, the bolt holes don't line up at all.

The first pic is the 42t Ovaltech on top of the 42t Biopace. The second is the same thing flipped upside down. You can see that the teeth line up exactly, but the contour of the inner part of the ring differs slightly, and when aligned according to long axis, they bolts are around 16 degrees or so different from each other. And just for the nay-sayers who say there's a larger ring hiding the teeth underneath and that's why you only see one set of teeth on top, I slid one ring a quarter inch to the side.

PC261232-Optimized.JPG

PC261234-Optimized.JPG

PC261238-Optimized.JPG
 
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