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DaVinci 34t timing chainrings

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DaVinci 34t timing chainrings

Old 12-01-10, 09:14 AM
  #1  
tiggermaxcocoa
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DaVinci 34t timing chainrings

Hi everyone,

I'm building up my first tandem and am currently trying to decide which crankset to get. I think I have settled on the DaVinci, but I'm not sure about the timing chainring setup. They recommended that I go with their 34t spider/chainring combo because they guarantee those rings to be perfectly concentric. I'm new to this stuff, but I had always thought that it was best to use timing chainrings around 42t because of efficiency and chainwear issues. I found much of this info on a discussion from earlier this year:

https://www.bikeforums.net/showthread...ming+chainring

DaVinci's response to my concerns is that due to the perfectly concentric rings, I will be able to set the chain with proper tension, which improves efficiency over a typical 42t setup with slightly ovalized rings. In addition, they said they machine their chainrings so that the chain wears slower, therefore extending the life.

It seems to me that the best solution to this would be to use a 42t timing chainring that IS perfectly concentric. Does anyone have any thoughts or experience that could help me out with this?
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Old 12-01-10, 09:49 AM
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jnbrown
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Plenty of people here using the DaVinci crankset with 34T rings so I would not worry about it. Its pretty lightweight and there is a wide variety crankarm lengths to choose from. My old tandem had Shimano 42T rings and they were out of round which did hinder the tension adjustment. The new one with FSA 42T rings doesn't have that problem. You will also save a small amount of weight due to the chain being shorter.
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Old 12-01-10, 10:40 AM
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TandemGeek
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It is what it is... smaller rings = higher tension under load. However, as jnbrown notes, there are a lot of folks out there using daVinci's 34t timing rings and I don't ever recall hearing of any systemic issues with accelerated or unusual bottom bracket failures. The chains would, in theory, wear faster on the smaller 34t rings, but I suspect other factors such as chain cleanlinss and proper lubrication or the lack thereof will do more to accelerate sync chain wear than the increase in tension alone.

I would guess that we've put over 50,000 miles on three different road tandems fitted with the daVinci 34t rings and the only set of rings that wore out quickly were an early set that weren't fully heat treated. The others have all held up quite well and, again, chain wear hasn't seemed to be all that bad; about 1:4 ratio compared to the drive chain replacement interval if it let it go to 1/8" of chain elongation. However, I keep my chains pretty clean and relubricate on a regular basis.

Don't overthink some of this stuff; it will just hurt your head and won't make your riding experience all that much better.
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Old 12-01-10, 12:54 PM
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WebsterBikeMan
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Of course the only problem with "concentric" - meaning circular, with the centre centred on the crank axle - chainrings is you can't use Bill McCready's recommended method of setting the eccentric:
Originally Posted by BillMcCready
First, rotate the cranks to the loosest part of the rotation (where the chain droops the most). After loosening the two eccentric grub screws one full turn each, use a pin-wrench to rotate the eccentric and snug the chain. Finally, backpedal a half dozen complete revolutions and then retighten the two grub screws. Because a too-tight chain will rotate a loose-but-not-wobbly eccentric, backpedaling will automatically rotate the eccentric to its optimal position.
Actually I'm only half-joking. I tried this method before I knew how round these chainrings are, and eventually shrugged my shoulders and gave up.

Extract taken from Gear To Go Tandems' web site.
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Old 12-02-10, 07:58 PM
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Have used crossover chainrings (TA) as small as 28T way back when in the 70s on our Assenmacher tandem. They lasted well over 20,000 miles.
Currently using FSA 38T crossovers with 30,000 miles on them.
So you are worried about 'wear'???
How particular you are about drivetrain cleanliness is key to longevity.
Have never used 42T as crossovers in over 35 years of extensive tandeming. Yes, am a bit of cleanliness nut.
Just our expereince.
Pedal on TWOgether!
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Old 12-08-10, 03:55 PM
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The DaVinci design is quite adaptable. Won't they let you specify arms with 110 or 130 BCD spiders on the timing side? That is how we spec'd our Bilenky way back in 1998. I do see value in larger, standard BCD chainrings vs. DaVinci's proprietary rings: Less boom tube flex (in theory, anyway), greater replacement 'ring availablility, and the ability to change to odd-tooth 'rings (e.g., 39) if that will result in better front BB axle location for your unique fit needs/wants on your particular frame.
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